Sending humans to destinations beyond low Earth orbit (LEO) is a technologically, programmatically, and politically complicated endeavor. As discussed elsewhere in this report, there are frequently cited but difficult-to-substantiate arguments that, despite the cost and risk to human life, such endeavors are justified by various national and more general societal benefits. In any resource-constrained environment, it is responsible to attempt not just to identify the potential benefits but also to understand the level of proposed or required investments and the difficulty of the proposed tasks.
Given the complexity of human space exploration—that is, human spaceflight beyond LEO—and the fact that U.S. goals in human spaceflight have changed on timescales much shorter than the time it would take to accomplish the goals, it makes sense to decompose a human spaceflight program into smaller building blocks. This would allow the building blocks to be assembled in various configurations that allow the changed goals to be addressed without analysis ab initio.
A “capability-based” approach to space exploration focuses research and technology development resources on systems and capabilities that are expected to be of value in the future with no particular mission or set of missions in mind. The process of selecting future missions would then tend to favor those missions that could make use of the systems and capabilities that have been developed. The Asteroid Redirect Mission (ARM) is an example of this process in action. A “flexible-path” approach is a more sophisticated version of capability-based planning that considers what destinations might be desirable. In contrast, a “pathway-based” approach would commit the U.S. human spaceflight program to a pathway with a specific sequence of missions, usually of increasing difficulty and complexity, that target specific exploration goals that are typically tied to various destinations that humans may explore. A pathways approach would facilitate continuity of development of required systems for increased capability and efficiency.
NASA is developing modular and general-purpose systems. Although NASA has characterized this approach as capability-based, the systems may or may not be supportive of a pathway-based approach. Instead of pursuing a “capability-based” or “flexible path” approach where no specific sequence of destinations is specified. The committee’s prioritized recommendations (see Chapter 1), if adopted, would lead to a NASA commitment to design, pursue, and maintain the execution of an exploration pathway beyond LEO that leads toward a clear “horizon goal” (that is, the most distant destination that is considered feasible in the foreseeable future). The committee is
not recommending one pathway over another, but it does recommend (see Chapter 1) that NASA “maintain long-term focus on Mars as the ‘horizon goal’ for human space exploration.”
The rest of this chapter contains three major sections:
4.2 Technical Requirements
4.3 Technology Programs
4.4 Key Results
The Technical Requirements section begins by defining possible destinations for human spaceflight. For the foreseeable future, limitations of human physiology and space exploration technology limit potential destinations to the Earth-Moon system, Earth-Sun Lagrange points, near-Earth asteroids, and Mars. Design reference missions (DRMs) to these destinations have been prepared by NASA, the International Space Exploration Coordination Group (ISECG), and others.
This chapter assesses three specific, notional pathways as examples to illustrate the various tradeoffs among schedule, development risk,1 affordability, and decommissioning date of the International Space Station (ISS). (The analysis presented in this chapter was completed before the administration announced its intention to extend the operation of the ISS to 2024; the bounding cases of 2020 and 2028 presented in this report nonetheless well illustrate the impact of the ISS decommissioning date on human spaceflight beyond LEO.) Although other pathways are possible, the ones chosen for exposition here span the likely programmatic space well enough to provide insight into affordability and technical difficulty.
All three of the pathways terminate with a human mission to the most challenging destination that is still technically feasible: the Mars surface. Depending on practical factors, an actual human spaceflight program might have to take an off-ramp to an intermediate destination before the final destination is reached.
Each pathway to Mars includes three to six different DRMs, as follows:
- — ARM
- — Martian moons
- — Mars surface
- — Lunar surface sortie
- — Lunar surface outpost
- — Mars surface
Enhanced Exploration pathway
- — Earth-Moon L22
- — Asteroid in native orbit
- — Lunar surface sortie
- — Lunar surface outpost
- — Martian moons
- — Mars surface
Completing any of the above pathways would require a variety of mission elements. For example, a human mission to the Mars surface would require development of 11 primary mission elements, such as heavy-lift launch vehicles, deep-space habitats, and pressurized surface mobility systems. Eight additional mission elements would be necessary to complete all six of the other DRMs that appear in one or more of the roadmaps. Three of the additional mission elements are transitional in that they contribute directly to the development of one of the 11 primary
1 Development risk is the risk that a program will encounter large increases in cost and/or schedule during the program’s development phase.
2 As described later in the chapter, the Earth-Moon L2 point is a particular point in space defined by its position relative to the Earth and Moon.
mission elements. For example, the lunar orbital outpost is not needed for a Mars surface mission, but it would contribute directly to the development of the deep-space habitats that would be needed. There are also five dead-end mission elements. Although necessary for completing one or more of the DRMs, the advanced capabilities of these mission elements have little or no applicability to the Mars surface mission.
Requirements for completing the pathways can also be framed as capabilities. A wide range of capabilities was assessed in terms of the technical challenges, capability gap, regulatory challenges, and cost and schedule challenges that would need to be overcome for their development to be completed. This assessment determined that current research and development programs would need to address the 10 capabilities below as a high priority, with particular emphasis on the first three:
- Mars entry, descent, and landing (EDL)
- Radiation safety
- In-space propulsion and power
- Heavy-lift launch vehicles
- Planetary ascent propulsion
- Environmental control and life support system
- Extravehicular activity (EVA) suits
- Crew health
- In-situ resource utilization (ISRU) (with the Mars atmosphere as a raw material)
For a pathway to be affordable, its cost profile must fit within the projected human spaceflight budget profile. Given the uncertain nature of federal budget projections and to show the impact of budget profile on other pathway characteristics, the affordability of each pathway is assessed in terms of three scenarios:
- A schedule-driven affordability scenario, in which the pace of progress is determined by the rate at which necessary development programs can be completed. This scenario would require rapid growth in NASA’s human spaceflight budget.
- A budget-driven affordability scenario, in which the pace of progress is limited by a human spaceflight budget that grows with inflation. This scenario would result in an operational tempo (in terms of the overall launch rate of crewed and cargo missions and/or the frequency of crewed missions in particular) that is far below historical norms.
- An operationally viable affordability scenario, in which the pace of progress reflects a compromise between the human spaceflight budget and operational tempo. This scenario would require a human spaceflight budget that increases faster than inflation but not as fast as for the schedule-driven scenario. It would also result in an operational tempo that is below historical norms but not nearly as much as the budget-driven scenario.
Having examined mission elements, capabilities, and the affordability of the pathways, the Technical Requirements section concludes with an assessment of each pathway in terms of six desirable properties:
- The horizon and intermediate destinations have profound scientific, cultural, economic, inspirational, or geopolitical benefits that justify public investment.
- The sequence of missions and destinations permits stakeholders, including taxpayers, to see progress and develop confidence in NASA’s ability to execute the pathway.
- The pathway is characterized by logical feed-forward of technical capabilities from one mission to subsequent missions.
- The pathway minimizes the use of dead-end mission elements that do not contribute to later destinations on the pathway.
- The pathway is affordable without incurring unacceptable development risk.
- The pathway supports, in the context of the available budget, an operational tempo that ensures retention of critical technical capability, proficiency of operators, and effective use of infrastructure.
The Technology Programs section briefly summarizes noteworthy programs and plans related to human spaceflight that are under way by NASA, industry, the Department of Defense, and the international community. A later discussion of robotic systems describes the importance of evolutionary improvements in robotic capabilities to the future of human spaceflight and the possibility that rapid improvements in robotics in the coming decades will open new pathways to Mars and beyond.
The Key Results section contains seven statements that flow from the panel’s assessment of the following topics:
- Feasible destinations for human exploration
- Pace and cost of human exploration
- Human spaceflight budget projections
- Potential cost reductions
- Highest-priority capabilities
- Continuity of goals
- Maintaining forward progress
The chapter finishes by emphasizing that without a strong national (and international) consensus about which exploration pathway to pursue and without the discipline needed to maintain course over many administrations and Congresses, the horizons of human existence will not be expanded beyond LEO—at least not by the United States.
NASA’s vision includes “expanding human presence” in the solar system. This concept was first made part of U.S. national space policy by President Reagan in the classified Presidential Directive on National Space Policy.3 All but one of the succeeding administrations have articulated similar goals, and various committees and commissions outlined proposed pathways and projected budgets for achieving the goals.4,5,6,7,8 President Obama commissioned the Review of United States Human Spaceflight Plans Committee9 (also known as the Augustine Committee), which concluded in 2009 that “the ultimate goal of human exploration is to chart a path for human expansion into the solar system” but noted the need for both physical and economic sustainability. The report of the Augustine Committee detailed various options, such as “Mars First,” “Moon First,” and “Flexible Path,” potentially involving missions to lunar orbit, Lagrange points, near-Earth objects, the moons of Mars, the surface of the Moon, and the surface of Mars. Ultimately, President Obama declared that the near-term goal for U.S. human spaceflight beyond the Earth-Moon system would be exploring a near-Earth asteroid, which would lead to human
3Presidential Directive on National Space Policy, February 11, 1988, available in NASA Historical Reference Collection, History Office, NASA, Washington, D.C., http://www.hq.nasa.gov/office/pao/History/policy88.html.
4 NASA, Report of the 90-Day Study on Human Exploration of the Moon and Mars, November 1989, available in NASA Historical Reference Collection, History Office, NASA, Washington, D.C., http://history.nasa.gov/90_day_study.pdf.
5 Advisory Committee on the Future of the U.S. Space Program, Report of the Advisory Committee on the Future of the U.S. Space Program, U.S. Government Printing Office, Washington, D.C., December 1990.
6 Executive Office of the President, “President Bush Announces New Vision for Space Exploration Program: Remarks by the President on U.S. Space Policy, NASA Headquarters,” Washington, D.C., January 14, 2004, http://history.nasa.gov/Bush%20SEP.htm.
7 NASA, The Vision for Space Exploration, NASA, Washington, D.C., February 2004, http://history.nasa.gov/Vision_For_Space_Exploration.pdf.
9 Review of U.S. Human Space Flight Plans Committee, Seeking a Human Spaceflight Program Worthy of a Great Nation, October 2009, http://www.nasa.gov/pdf/396093main_HSF_Cmte_FinalReport.pdf.
orbital missions to Mars and a Mars landing thereafter.10 In contrast, the Clinton administration’s space policy committed the United States only to establish a permanent human presence in Earth orbit, where “the International Space Station will support future decisions on the feasibility and desirability of conducting further human exploration activities.”11 Thus, since 1988, except for the Clinton administration, presidential space policy has advocated expanding human presence in some form throughout the solar system; however, since President Reagan, the United States for practical purposes has executed the Clinton administration space policy.
Congressional guidance for NASA has, in general, been less expansive than the executive branch’s human spaceflight vision. The appetite for ambitious, Apollo-style goals beyond LEO and the attendant budgets has been notably lacking. Even so, the National Aeronautics and Space Administration Authorization Act of 2010,12 which mandated this report, called explicitly for the development of a heavy-lift launch vehicle capable of supporting human spaceflight beyond the ISS and LEO, with a focus on cislunar space in the near term. Indeed, this congressional direction constitutes an important boundary condition for the report’s analysis, which is based on the assumption that the Space Launch System (SLS) would be the primary launcher that enables exploration beyond LEO.
It is critically important for stakeholders of U.S. human spaceflight to understand, however, that currently understood physiological limitations of human beings to endure the space radiation and zero gravity of space for long periods, the inadequacy of foreseeable technological and medical countermeasures in addressing many of those limitations, and the performance of future in-space propulsion systems, severely limit the destinations in the solar system to which humans may travel. For example, using traditional standards for lifetime cancer risk, the NASA Human Research Program has established that the risk of cancer induced by galactic cosmic radiation (GCR) exceeds current guidelines for missions longer than 615 days.13,14 This is for the most optimistic case with 55-year-old men who have no previous radiation exposure and have never smoked, assuming complete engineered protection (such as shielding crew quarters with water) from solar particle events (SPEs). For female astronauts, for astronauts younger than 55 years old, for astronauts who have had previous radiation exposure, and for missions that do not take place during solar maximum (when the Sun’s magnetic field provides substantial protection from GCR), the permissible durations are much shorter, approaching 6 months in many cases. It is also probable that other factors—such as noncarcinogenic effects of GCR, musculoskeletal degeneration in zero g, ocular impairment, and psychosocial effects—could further limit permissible mission durations. Although NASA’s Human Research Program is working efficiently to quantify the health risks of deep-space exploration, remaining uncertainties are considerable. In addition, the number of potential health problems associated with human spaceflight continues to increase. Ocular impairment and increased intracranial pressure were identified as a potentially serious problem only in 2011,15 and the potential for microgravity to have adverse effects on the development of endothelial cells, which line the interior of blood vessels, was identified only in 2013.16 Apart from risks that might limit permissible cruise durations, astronauts will face, for example, such environmental factors as dust or perchlorates from lunar or martian soil, respectively. The impact of psychosocial factors on long-duration spaceflight has also been an issue of concern for some time.17 Finally, it is worth noting that the number of evidence books that the
10Remarks by the President on Space Exploration in the 21st Century, John F. Kennedy Space Center, Florida, April 15, 2010, http://www.whitehouse.gov/the-press-office/remarks-president-space-exploration-21st-century.
11 National Science and Technology Council, Presidential Decision Directive/NSC-49/NSTC-8, September 14, 1996, available at http://www.fas.org/spp/military/docops/national/nstc-8.htm.
12 Public Law 111-267, October 11, 2010.
13 National Research Council, Managing Space Radiation Risk in the New Era of Space Exploration, The National Academies Press, Washington, D.C., 2008.
14 Steve Davison, NASA Headquarters, “Crew Health, Medical, and Safety: Human Research Program,” briefing to NRC Committee on Human Spaceflight Technical Panel, March 27, 2013.
15 J. Fogarty et al., The Visual Impairment Intracranial Pressure Summit Report, NASA/TP–2011-216160, NASA Johnson Space Center, October 2011.
16 S. Versari et al., The challenging environment on board the International Space Station affects endothelial cell function by triggering oxidative stress through thioredoxin interacting protein overexpression: The ESA-SPHINX experiment, Journal of the Federation of American Societies for Experimental Biology 27:4466-4475, 2013.
17 Institute of Medicine, Chapter 5 in Safe Passage: Astronaut Care for Exploration Missions, The National Academies Press, Washington, D.C., 2001.
Human Research Program has accumulated (more than 30) highlights the complexity of assessing the safety of long-duration human spaceflight.18
For distant destinations, limitations on the duration of human exploration missions impose minimum spacecraft velocities that will be hard to achieve. Example mission designs for Mars suggest that chemical propulsion (the only technology that has been used for human spaceflight) might enable getting humans to Mars orbit and back if substantial progress is made in storing cryogenic propellants for long periods with minimal loss. Other types of propulsion systems—such as solar electric propulsion (SEP), nuclear electric propulsion (NEP), and nuclear thermal propulsion (NTP)—could be used to support a human mission to the Mars surface. However, developing high-power operational systems that use any of these concepts would be a major undertaking.
The development challenges associated with any solar system destinations beyond the Earth-Moon system, Earth-Sun Lagrange points, near-Earth asteroids, and Mars are profoundly daunting, involve huge masses of propellant, and have budgets measured in trillions of dollars.19
With an initial reconnaissance of the Moon completed, the maturation of the ISS, and robotic exploration of Mars supporting NASA’s search for signs of past or present life elsewhere in the solar system, human exploration of Mars seems to be a logical goal for U.S. human spaceflight given its prominence in the national space policies of multiple administrations, including the current one. To achieve a human mission to the Mars surface, advanced capabilities and new technologies will be needed. NASA has outlined the challenge of a Mars surface mission with the Mars Design Reference Architecture (DRA) 5.0, which lays out the major mission requirements for a Mars surface landing and exploration.20 The DRA 5.0 report summarizes the tradeoffs among energy expenditure, surface time, and cruise duration for a human mission to the Mars surface. However, Mars is not the only potential destination beyond LEO. A number of scientifically and technically interesting destinations exist between Earth and the surface of Mars. Missions to such intermediate destinations may also demonstrate capabilities and improve technologies needed for a Mars surface mission. NASA and others have examined the potential destinations,21,22 and reasonable mission concepts have been formulated.
DRMs are point designs that provide an overview of how a mission goal could be achieved. DRMs serve two purposes during initial conceptual design. First, they provide an overview of the mission and mission needs, so that mission requirements can then be used to generate system requirements. Second, they provide a benchmark that can be used for comparison with alternative mission concepts for achieving mission goals. This study used a set of representative DRMs to define the three pathways and to assist in evaluating the challenges of expanding human spaceflight beyond LEO as far as the “horizon goal” of a human mission to the Mars surface. DRMs to other feasible destinations, such as the Earth-Moon L1 point and the Earth-Sun L1 and L2 points,23 could also have been used, but pathways that use the selected DRMs are sufficient for assessing the full scope of the technical and affordability challenges faced by human space exploration. The definitions of the DRMs are based on recent
18 Evidence books are collections of risk reports and journal articles that address a particular human health issues. A list of Human Research Program evidence books appears at http://humanresearchroadmap.nasa.gov/Evidence/, accessed April 20, 2014.
19 R. McNutt et al., Human missions throughout the outer solar system: Requirements and implementations, Johns Hopkins APL Technical Digest 28(4):373-388, 2010.
20 B. Drake, “Design Reference Architecture 5.0,” http://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf, 2009.
22 Chris Culbert and Scott Vangen, NASA Human Space Flight Architecture Team, “Human Space Flight Architecture Team (HAT) Technology Planning,” NASA Advisory Council briefing, March 6, 2012, http://www.nasa.gov/pdf/629951main_CCulbert_HAT_3_6_12=TAGGED.pdf.
23 Lagrangian points, also referred to as L points or libration points, are five relative positions in the co-orbital configuration of two bodies, of which one has a much smaller mass than the other. At each of the points, a third body with a mass that is much smaller than either of those two will tend to maintain a fixed position relative to the two. In the case of the Earth-Moon system, L1 is a position between the Moon and Earth where a spacecraft could be placed, and L2 is a position beyond the Moon that would be similarly fixed in orientation to the Earth and Moon. See “The Lagrange Points,” http://wmap.gsfc.nasa.gov/mission/observatory_l2.html.
publications from NASA (for the ARM,24 Lunar Surface Outpost,25 Asteroid in Native Orbit,26 Mars’s Moons,27 and Mars Surface28 DRMs) and the ISECG Global Exploration Roadmap29 (for the Earth-Moon L2, Lunar Surface Sortie, and Lunar Surface Outpost DRMs).30
The DRMs presented below are notional and representative of possible missions, but they are neither comprehensive nor final designs. The DRMs for each major destination beyond LEO demonstrate the challenges associated with various pathways to human exploration of Mars.
The DRMs below can be divided into two groups: cislunar and deep-space missions. Cislunar missions include missions to the vicinity of the Moon as well as lunar surface missions. Cislunar missions have the advantage of remaining close to Earth. This reduces mission risk by allowing abort contingencies with a relatively quick return to Earth.31 Cislunar missions would also cost substantially less and be more affordable than deep-space missions. Autonomy requirements are also reduced because of short time delays in communications between spacecraft and Earth. Cislunar missions that do not include a lunar landing also minimize propulsion requirements by avoiding interactions with strong gravitational bodies. Lunar surface missions are a good analogy for Mars surface operations and some of the associated constraints on hardware and human physiology that will need to be overcome. Deep-space missions include the horizon goal of a human mission to the Mars surface, missions to asteroids in their native orbits, and missions to the moons of Mars. The asteroid and Mars Moons missions would allow demonstration of spacecraft vehicles and systems and validate the ability to sustain human health during long-duration missions that are similar in scale to a Mars surface mission.
220.127.116.11 The Space Launch System and the Design Reference Missions
The SLS is a heavy-lift launch vehicle that is being developed by NASA to support human space exploration beyond LEO. The need for a heavy-lift launch vehicle has been noted in many blue ribbon studies of human spaceflight. The planned SLS payload capacity of 70-130 metric tons (MT) depending on the version of the SLS, and the large shroud would reduce the number of launches required for human exploration missions beyond LEO. The Falcon Heavy launch vehicle, which is being developed by SpaceX, will have a payload capacity of up to 53 MT. The Augustine study suggested that human exploration could be accomplished with a 50-MT launch system if the architecture relied on advanced capabilities, such as in-space refueling and expandable habitats.32 Smaller alternatives to the SLS would require more launches, more time in orbit, and more docking events, which might reduce mission reliability. The effect on costs is hard to predict. On the one hand, the increased launch rate and the potential for commonality with other launch systems that use smaller vehicles might reduce development and operational costs for the launch vehicle. On the other hand, the development cost for other mission elements would increase because of the need for additional technologies and interface hardware. In addition, increasing the amount of in-orbit assembly would tend to increase operational costs. For simplicity and consistency of presentation, the analysis of all the DRMs in this report assumes the use of the SLS as the launch vehicle.
24 NASA, “Asteroid Initiative Related Documents,” http://www.nasa.gov/content/asteroid-initiative-related-documents/.
25 L. Toups and K. Kennedy, “Constellation Architecture Team-Lunar Habitation Concepts,” in AIAA Space 2008 Conference and Exposition, American Institute of Aeronautics and Astronautics, 2008, http://arc.aiaa.org/doi/abs/10.2514/6.2008-7633.
26 B.G. Drake, “Strategic Considerations of Human Exploration of near-Earth Asteroids,” paper presented at the Aerospace Conference, 2012 IEEE, March 3-10, 2012.
27 D. Mazanek et al., “Considerations for Designing a Human Mission to the Martian Moons,” paper presented at the 2013 Space Challenge, California Institute of Technology, March 25-29, 2013.
28 B. Drake, “Design Reference Architecture 5.0,” 2009, http://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf.
30 Detailed analyses of the DRMs that in some cases added to or modified the information from these sources were performed to ensure that each DRM was based on a consistent set of assumptions concerning, for example, the performance of major system elements. As indicated, the Lunar Outpost DRM is based on information from NASA and the ISECG.
31 For some emergencies, such as a subsystem failure that would doom a Mars-bound crew, a safe return from cislunar space might be possible. For other emergencies, such as an acute medical emergency, the quicker return from cislunar space would still be problematic.
32 Review of U.S. Human Space Flight Plans Committee, Seeking a Human Spaceflight Program Worthy of a Great Nation, 2009.
The business model and schedule for the SLS are almost totally driven by the projected costs and the flat budget profile established for the SLS program. A system that, like the SLS, is derived from mature systems could reach full operational capability in less time and at less total cost than currently planned if the funding profile resembled that for a normal development (that is, if funding ramped up as the program progressed from program initiation into advanced development and production). Much of the necessary design work was done under the Constellation program. Before that, vehicles derived from the space shuttle had been proposed several times, and considerable preliminary design work had been done. Nonetheless, the designs for the SLS were announced in September 2011 with a projected development cost (including ground-based infrastructure) of $12 billion through first flight in late 2017 and an additional $6 billion for development of the Orion Multi-Purpose Crew Vehicle.33,34 Orion is the crew capsule being developed in concert with the SLS to support human space exploration beyond LEO. Since then, NASA’s budget uncertainty has increased, and this has put both the launch date and the cost at high risk.35
18.104.22.168 Asteroid Redirect Mission
The ARM envisions a crew of two briefly interacting with a 7- to 10-m asteroid while avoiding the longer travel times in deep space required to reach an asteroid in its native orbit. A precursor robotic mission using advanced SEP would be sent to an as-yet-unspecified near-Earth asteroid with appropriate mass and orbital properties. Depending on the mission concept ultimately selected, the robotic spacecraft would either capture a small asteroid in entirety or retrieve a boulder from the surface of a larger asteroid36 and then transport its payload to an orbit in the lunar vicinity. After the multiyear asteroid redirect phase, the crew would be launched on the SLS with the Orion capsule, rendezvous with the asteroid in its new orbit, and dock to the attached robotic spacecraft. The mission is ultimately constrained by the current capability planned for the Orion capsule, but a two-person crew could spend up to 6 days at the asteroid, collecting samples during EVA before returning to Earth.
22.214.171.124 Earth-Moon L2
The Earth-Moon L2 reference mission would demonstrate long-term human habitation and operations in deep space. To achieve this goal, a habitation module and supporting systems would be developed, built, and launched by an SLS. The spacecraft would transit to either an Earth-Moon Lagrange point or a stable lunar orbit. This minimal space station would be capable of supporting a crew of four or more for at least 6 months. The crew would reach the habitat using the SLS and Orion systems. The primary goal would be to develop technologies and techniques that could enable crews to survive and function on long-duration deep-space missions while maintaining a mission-abort capability with a quick return to Earth. Other activities, such as observations of the far side of the Moon or support of lunar surface activities, could occur. This mission would require a substantial improvement in the reliability and sustainability (operation without resupply) of environmental control and life support systems (ECLSS) compared to the ECLSS on the ISS. In addition, radiation exposure from SPEs during extended stays outside Earth’s magnetosphere would have to be mitigated without exceeding mass and volume constraints. (No mitigation of GCR currently appears to be practical.) The ISECG has proposed a mission of this type.37
126.96.36.199 Lunar Surface Sortie
The Lunar Surface Sortie reference mission would leverage substantial prepositioned assets in lunar orbit, and it is similar to the proposal for lunar exploration in The Global Exploration Roadmap recently updated by the
33 NASA, “NASA Announces Design For New Deep Space Exploration System,” Press Release 11-301, September 14, 2011.
35 M.K. Matthews, “New NASA rocket faces delays,” Orlando Sentinel, September 6, 2013 (quoting NASA Deputy Administrator Lori Garver).
36 NASA is studying two mission concepts for robotic the ARM mission. The reference robotic mission concept would capture a small asteroid. The alternative mission would capture a boulder.
37 ISECG, The Global Exploration Roadmap, 2013.
ISECG.38 This mission would sustain a crew of four on the lunar surface for 28 days. Predeployed pressurized lunar surface mobility units would be positioned using SLS launch vehicles and reusable lunar ascent-descent vehicles. The crew would be launched using SLS and Orion and then rendezvous with a permanent lunar orbital facility. This facility would serve as the staging point for the crew and the ascent-descent stage, which would be augmented by a low-cost disposable deceleration stage. Scientific exploration would be conducted using surface mobility units that also operate as habitats. The exploration range would be limited to a reasonable return distance to the ascent-descent stage. Beyond the attendant lunar surface science, this DRM would demonstrate surface operations, surface habitation, and surface mobility required for partial-gravity environments where dust and other potential contaminants are present.
188.8.131.52 Lunar Surface Outpost
The Lunar Surface Outpost DRM is an extension of the Lunar Surface Sortie mission and requires the deployment of long-duration surface assets. These assets would be delivered using a similar architecture of SLS launch vehicles, reusable lunar ascent-descent vehicles, a staging orbital facility, and disposable propulsion stages. The additional assets would extend the surface mission duration from 28 days to as much as 6 months. The mobile assets would allow extended sortie missions from the outpost to scientifically diverse sites, while the outpost itself would be used for scientific experimentation and testing of Mars-focused technologies, such as long-life high-capacity power generation systems and operations planning for long-duration surface stays.
184.108.40.206 Asteroid in Native Orbit
The Asteroid DRM is a deep-space mission beyond cislunar space for a crew of four to a near-Earth asteroid. The asteroid for the mission would be selected on the basis of scientific interest (or relevance to planetary defense) and Earth-asteroid alignment to allow the crew to transfer to and from the asteroid within a 270-day total mission duration. An Orion vehicle, a deep-space propulsion unit, a deep-space habitation module, and a space exploration vehicle would be launched on SLS vehicles and rendezvous in LEO before the transfer to the asteroid. On arrival at the asteroid after a 60- to 130-day transit, the crew would transfer to the space exploration vehicle and perform close-proximity operations and EVAs to collect samples and perform experiments on the asteroid. After a 14-day mission at the asteroid, the crew would return to Earth in the space habitat on a 70- to 160-day journey before performing a direct entry using the Orion vehicle. This DRM features deep-space habitation capability for more than the 180 days needed for a transit to or from Mars, deep-space navigation, low-gravity foreign-body exploration, and potentially important scientific returns.
220.127.116.11 Mars Moons
The Mars Moons DRM is similar to the Asteroid DRM in that it is an exploration of a low-gravity body in deep space using space exploration vehicles and EVAs for crewed exploration. The major distinguishing factor is the location of the low-gravity body. A crewed mission to Phobos and Deimos in Mars orbit would include many elements of a crewed mission to Mars but without the challenge of EDL and ascent from Mars. After departure from Earth, the mission would attain Mars orbit insertion and then use orbital maneuvering units to spend up to 60 days at Phobos and Deimos. The mission length would increase from less than 1 year for the Asteroid in Native Orbit mission to almost 2 years for the Mars Moons mission. The increases in mission duration and propulsion requirements result would require an advanced in-space propulsion system. The current design baseline is NTP, although NASA was still examining the propulsion trade space as of the end of 2013. Two space exploration vehicles and small propulsion stages for martian orbital maneuvering would be predeployed at Mars using an advanced propulsion stage. The crew would rendezvous with their long-duration, deep-space habitat and advanced propulsion stage in LEO before transferring to Mars. The habitat would need to protect the crew from the deep-space environment
38 ISECG, The Global Exploration Roadmap, 2013.
FIGURE 4.1 Design Reference Architecture 5.0 Human Landing on Mars. SOURCE: NASA, Human Exploration of Mars Design Reference Architecture 5.0, 2009.
throughout the 700-day mission (except for the 2 months spent in Mars orbit). The in-space duration for this mission is more than 3 times that expected for the Mars surface mission, and deep-space habitation and logistics for a 700-day in-space mission may not be feasible, depending on the challenges of GCR.
18.104.22.168 Mars Surface
The horizon goal for human spaceflight is the human exploration of the Mars surface. Numerous concepts for surface exploration missions have been described in various documents; the analysis in this report is based on NASA’s Mars DRA 5.0. The mission is based on sending three different vehicles to Mars, as shown in Figure 4.1. Multiple SLS launches would be required to place both the cargo and crewed portions of the mission in LEO.
The cargo portion of the mission would use two vehicles to carry support equipment and travel to Mars during a planetary alignment before the crew transit.39 This would allow verification that the support equipment had arrived successfully and that the crew’s ascent vehicle has landed on Mars with sufficient time to use in-situ resources to produce propellant. The mission concept relies on the crew landing on Mars close to the predeployed ascent vehicle and its support equipment. The cargo missions would use a minimal-energy 350-day trajectory from Earth, and they would enter Mars orbit using aerocapture technology. One of the predeployed vehicles would then perform aero-assisted EDL and initiate preliminary robotic efforts to prepare the landing site for the crewed mission. The second predeployed vehicle would wait in Mars orbit for the arrival of the crewed vehicle.
39 Transit times and propulsion requirements for missions to Mars are minimized when Earth and Mars are favorably aligned in their orbits. Such alignment occurs every 26 months or so.
The crewed portion of the mission would depart in a long-duration habitat from a LEO staging point using NTP on a 6-month transfer to Mars orbit. The crewed system would be propulsively captured into Mars orbit and rendezvous with the predeployed vehicle. The crew of six would then transfer to the predeployed vehicle that contains the surface habitat, which would transfer them to the planet’s surface. The EDL system would land the habitat close to the predeployed assets, and the crew would then be able to conduct mobile scientific exploration of Mars. The Mars surface mission would last for about 500 days, and the crew would then board the ascent stage and return to the deep-space habitat and propulsion system that remained in Mars orbit. The surface assets would continue autonomous missions and data collection for possible use by future Mars exploration crews. On crew transfer to the deep-space habitat, which would have been in standby mode, the crew would jettison the ascent vehicle and return to Earth on another 6-month transfer and a direct Earth entry using the Orion vehicle.40
The crew and cargo portions of the mission would be sufficiently massive to require advanced propulsion stages, currently modeled using NTP, for the transfer to Mars orbit from a LEO staging point. Because NTP may not be feasible because of technical, financial, and/or political factors, NASA is still evaluating other advanced in-space propulsion options. The Mars DRA 5.0 study suggests that an NTP-based system would result in the lowest life cycle cost and mission risk, assuming a sustained campaign of many Mars missions. Other propulsion options, such as SEP and advanced cryogenic chemical propulsion, may have lower cost if only a few missions are planned. The Mars surface elements would require a highly reliable power source that is capable of generating a total of 30 kW or more. Mars DRA 5.0 concluded that a fission reactor would have about one-third the mass of a solar-based surface power system and that deploying large solar arrays robotically in the high wind and dust environment of Mars would be very challenging.
Now that the various stepping-stone destinations and associated mission concepts have been highlighted, potential pathways, all ending with a Mars surface mission, can be defined. As described above, each of the three pathways is a series of human spaceflight missions to various destinations. The first pathway (ARM-to-Mars) is essentially the current administration’s proposed U.S. human spaceflight program. The Moon-to-Mars pathway makes use of the Moon as a testing and development destination to mature Mars-oriented technology, while revisiting the lunar environment for more in-depth scientific study than was possible during Apollo. This pathway is also consistent with the goals of the United States’ traditional international space partners and the ISECG, of which NASA is a member.41 Finally, the Enhanced Exploration pathway essentially exhausts the potentially feasible classes of destination through a Mars surface landing, and it allows exploration of essentially all destinations that humans can explore, given the current understanding of physiology and technology.
The three pathways are used to compare and illustrate the challenges of sending humans as far as the surface of Mars. Although the specific destinations on the journey to Mars are few and each requires development and demonstration of hardware elements of various categories (such as propulsion systems, power systems, and habitation systems) as well as research related to human health issues, NASA, in concert with other international and domestic organizations, could further define, mature, and analyze a broad range of detailed conceptual pathways to Mars.
Table 4.1 and Figure 4.2 define and illustrate the three representative pathways to the horizon goal of a human mission to the Mars surface. Table 4.1 defines the specific DRMs of each pathway, while Figure 4.2 illustrates each of the stepping-stone destinations and the three pathways to the Mars surface. The first pathway, ARM-to-Mars, leverages the initial demonstration of the SLS and Orion systems in cislunar space via the ARM and then proceeds directly to activity in the Mars vicinity by focusing on exploring the moons of Mars, followed by a Mars landing.
The second pathway, Moon-to-Mars, first focuses on missions in the lunar vicinity and surface to demonstrate longer-duration in-space habitats and complex propellant staging in lunar orbit. These missions would also develop
40 The DRA 5.0 architecture outlined here is for the long-surface-duration, so-called conjunction mission. Given the orbital mechanics of Earth and Mars, a second, short-stay “opposition” class mission is also possible. This alternate mission has a transit time to Mars of about 200 days, a surface stay of about 60 days, and a return transit of 400 days.
41 ISECG, The Global Exploration Roadmap, 2013.
TABLE 4.1 Pathway Definition and Associated DRMs
|ARM TO MARS|
|MOON TO MARS|
|PATHWAY||DESIGN REFERENCE MISSION (DRM)|
NOTE: The sources used to characterize the DRMs are listed above in the section “Design Reference Missions.”
critical partial-gravity surface habitats, both fixed and mobile, and long-term, reliable power generation systems to maintain these assets. The focus in this pathway is to develop the required assets and techniques for martian surface exploration using the nearest and most easily accessible celestial body, relatively short return times, and open launch windows. To proceed to Mars as soon as is practical, after an appropriate time for examining hardware, operational, and human health issues, the lunar assets would be retired from government service and optionally maintained and leveraged by future commercial endeavors. Alternatively, if the actual crewed exploration of Mars became infeasible for financial, technical, or crew health reasons, the Moon-to-Mars pathway would constitute a natural off-ramp, leaving the United States to lead global exploration and exploitation of the Moon.
The third pathway, Enhanced Exploration, presents a potentially lower risk than the other pathways, but it is also a longer-duration pathway, exploring several destinations while slowly increasing the capability of key mission elements needed for a Mars surface mission. It begins by focusing on the challenges of a long-term in-space habitat with a mission to Earth-Moon L2 and a native asteroid. That would be followed by a focus on the Moon to develop partial-gravity surface operations capabilities. Finally, there is the development and use of the advanced in-space propulsion systems with missions to the Mars moons followed by Mars surface operations. Human health issues would also be investigated during this pathway as more challenging missions were completed.
Each of the three pathways is described in more detail and assessed below.
Each DRM that appears in one or more of the three pathways discussed above requires the development of key mission element groups to support the specific mission. These element groups are launch, in-space transportation, habitation, EDL and ascent, and destination systems. A brief description of the performance requirements that would drive the development of these groups, with a focus on the Mars surface mission as the horizon destination or goal, is presented below. Although the groups encompass most of the basic needs for successful missions
FIGURE 4.2 Pathway stepping-stone destinations.
beyond LEO, other assets, such as improved communication networks and advanced mission operations, will probably need to be developed.
Launch system requirements for human space exploration are driven by the total payload mass required in a specific orbit (typically LEO), payload diameter and volume, and reliability requirements. State-of-the-art U.S. launch systems can place 23 MT of payload in LEO with a 5-m-diameter fairing, but no U.S. launch systems are human-rated. In any case, a much larger launch system would be needed for the human surface mission in Mars DRA 5.0. This mission would require payload diameters of about 10 m with total lift capability to LEO of 105-130 MT. NASA is developing the SLS, which eventually will be human-rated, to meet the Mars surface mission requirements. The initial version of the SLS will have only a 70-MT capability, but planned development includes 105-MT and 130-MT variants. Although the total mass needed for a Mars surface mission could be launched with more launches using lower-capability vehicles, the associated increase in launches, system complexity due to assembly, and operational complexity may reduce total mission reliability to below an acceptable level.
TABLE 4.2 Approximate Initial Mass in Low Earth Orbit (IMLEO) Required for the DRMs That Appear in One or More of the Pathways
|DRM||Approximate IMLEO (metric tons)|
|Asteroid in native orbit||200|
NOTE: For each of the destinations shown, the values include all launch mass needed up to and including the first human launch in accordance with the relevant DRMs. For the lunar missions, this includes a habitat in lunar orbit that would serve as a waypoint and docking station for a reusable lunar lander. An IMLEO for a Mars mission is shown as a range of values; the actual value would depend on the propulsion systems chosen for different mission elements (such as nuclear thermal, solar electric, and chemical). For comparison, the mass of the ISS, which is in LEO, is 420 MT.
An analysis of the DRMs that appear in one or more of the pathways allows the determination of the total initial mass in LEO required to accomplish each mission (see Table 4.2). As discussed above, the payload capacity requirements of the launch vehicles used for these missions could be traded off with factors such as risks of in-orbit assembly or fueling. In principle, none of the missions beyond ARM can be accomplished using a single SLS launch. Either multiple SLS launches would be required or one or more SLS launches could be replaced by multiple launches of smaller rockets that could carry the human crew, smaller subsystems, and/or bulk consumables.
22.214.171.124 In-Space Transportation
Human spaceflight missions would require in-space transportation systems to perform major propulsion burns that are not provided by the launch vehicle, often at multiple points during a mission. These transportation systems would consist of high-performance, multiple-restart engines with large fuel tanks that are capable of long-term propellant storage and management. In-space transportation systems with these characteristics do not exist on the scale necessary for human spaceflight missions beyond cislunar space.
For a space propulsion system, thrust is directly proportional to specific impulse (Isp)42 and mass flow rate of engine exhaust gases. Current chemical storable propellants have low propulsion efficiency (that is, low Isp), resulting in high mass. Cryogenic fuels for deep-space missions would require advanced thermal control and boil-off management.
SEP offers much higher Isp, but it is limited by relatively low propellant flow rates, which results in lower thrust, lower velocities, and longer trip times. For crewed missions, this would increase the exposure of the crew to space radiation en route. As a result, SEP is not considered as a viable option for crewed missions, although it may be suitable for cargo vessels that would preposition food, water, propellant, and/or landing and return vehicles in Mars orbit. However, the SEP systems needed for an appropriately sized cargo vessel would require massive solar arrays with a generating capacity at least 3 times the power of the ISS arrays to achieve sufficient thrust.
NTP offers a relatively modest increase in Isp while maintaining higher thrust capability and shorter flight durations. Nuclear thermal rockets were developed as part of Project Rover from 1955-1972. After NASA was formed in 1958, Project Rover was administered by NASA, with the nuclear reactor portion falling under the Atomic Energy Commission. Ultimately, several NTP systems (collectively known as NERVA—Nuclear Engine for Rocket Vehicle Application) were successfully ground tested. The NERVA program was canceled by the Nixon
42 Isp is the force produced by a propulsion system divided by the mass flow rate of the fuel that is consumed to produce that force. The higher the specific impulse, the lower the propellant mass flow rate required to achieve a given level of thrust. Thus, in some sense, Isp is a measure of propulsion-system efficiency.
administration in 1973, in part because of environmental and hazard concerns but primarily because the intended application, a crewed Mars mission, was considered too expensive.43
NEP systems would avoid problems associated with the large solar arrays of SEP systems, but they face the complications associated with development, production, and operation of nuclear systems. In addition, NEP systems appropriate for space travel have never been developed.
The tradeoff among in-space transportation options—namely chemical propulsion, SEP, NTP, and NEP, alone or in combination—is complex and ongoing. Total life cycle cost considerations are heavily influenced by the total number of flights to the various destinations and by the time and cost needed for development and testing. Substantial advances in technology are required for the use of any of these technologies for the aforementioned pathways, and the final selection of one technology over the others will probably be driven by many factors over the next several decades.
Human presence aboard the ISS and its predecessors has demonstrated in-space habitation capabilities since the 1970s. The ISS has been consistently occupied for more than a decade, and individual stays have reached 180 days or longer. Some Russian cosmonauts were on the Russian space station Mir for more than a year. One astronaut and one cosmonaut will undertake a year-long stay on the ISS beginning in 2015. Unlike space stations in LEO, habitats for deep-space missions will not be able to take advantage of periodic resupply, the removal of waste products, and the radiation protection provided by Earth’s magnetosphere. The Mars DRA 5.0 surface mission includes two periods of travel in deep space of 6 months or more with a crew of six. The deep-space habitat must meet stringent volume and mass constraints for launch and must provide a highly reliable ECLSS with closed or near-closed loops for air, water, and food. The habitat will need to accommodate all crew needs so that they arrive at their destinations physically and mentally fit for their exploration tasks and are subsequently returned to Earth in a similar state. The habitat will need to protect the crew from SPEs. As discussed below in section 126.96.36.199.2 (“Radiation Safety”), protection from GCR by the habitat is impractical and most likely will be accomplished primarily by limiting mission duration. The in-space habitat will also need to operate in a dormant state with no crew during the 500-day surface mission. A mission to Mars that does not include a stay on the surface of Mars, such as the Mars Moons DRM, will need an in-space habitation system that can maintain crew health continuously for 2 years or longer. Such extremely long missions may require habitats to have artificial gravity generated through centripetal acceleration to maintain crew health.
New habitation elements will also be needed when the crew is on the surface of the Moon. In addition to the needs of in-space habitats, surface habitats for the Moon or Mars will need to operate in partial gravity environments and have mitigation strategies for additional hazards, such as potentially toxic dust. Mars crews will need to live on the surface of Mars for more than 1.5 years, spending most of their time in stationary habitats with excursions of weeks or longer in mobile habitats.
188.8.131.52 Entry, Descent, Landing, and Ascent
To conduct a human mission to the Mars surface, the landing system must be capable of placing individual payloads of about 40 MT with accuracy to within hundreds of meters of the targeted landing point. This is well beyond current capabilities at every stage of EDL. The recent landing of the Mars rover Curiosity demonstrated the ability to land a payload of about 1 MT with a landing error ellipse of tens of kilometers. In addition, the EDL approach used for Curiosity applied g-loads to the payload that are inconsistent with human passengers. Thus, the Curiosity EDL system cannot simply be scaled up for human missions. In addition, NASA DRA 5.0 calls for the crew to land very close to the ISRU plant predeployed to the Mars surface, so the error ellipse achieved by Curiosity is far large for a human mission. The size and mass of the payloads for a Mars surface mission demand
43 B. Fishbine et al., “Nuclear rockets: To Mars and beyond,” in National Security Science, Issue 1, LALP-11-015, 2011, http://www.lanl.gov/science/NSS/issue1_2011/story4full.shtml.
a more advanced thermal protection system, more advanced hypersonic and supersonic deceleration systems, and more advanced terminal landing systems to survive the passage through the thin martian atmosphere. Each of the EDL phases also represents potential single point failure opportunities with no abort options. Such a system would most likely rely on a combination of aerodynamic and propulsive deceleration techniques and may require rapid changes in configuration during the flight sequence. An advanced aerodynamic shield would also be required to perform an aerocapture maneuver to place spacecraft into Mars orbit.
The first human landing on Mars will be a monumental occasion for expanding human presence to another planet, but equally important is returning the crew to Earth at the end of the mission. Propellant for the ascent from the planetary surface and return of the crew to Earth requires either the production of resources on the surface or the deployment of a fully fueled system from Earth. The crew ascent vehicle would be predeployed and prefueled and probably launched from Earth 26 months in advance of the crew. This would allow in situ production of the oxygen portion of the propellant by separation from the martian carbon-dioxide atmosphere. Alternatively, the complete ascent vehicle and all its associated resources could be launched from Earth, although this would require additional launch assets and more capable EDL systems. The ascent vehicle carrying a crew of six would need to launch from the Mars surface to orbit and rendezvous with the waiting deep-space habitat.
The entry capsule, not used since the crew launched from Earth approximately 30 months earlier and thus requiring long-duration standby abilities, would return the crew to Earth. The entry from a Mars mission would have to survive the highest-velocity crewed entry ever attempted, at more than 13 km/s. The entry shield would have to withstand temperatures of at least 3,000°C, whereas a spacecraft on a lunar return trajectory would experience peak temperatures of about 2,750°C.44
184.108.40.206 Destination Systems
Not since the Apollo lunar landings has a spaceflight crew performed surface exploration. Most current space-based tools and human interface systems are focused on maintenance and repair of the ISS and crew and cargo transportation systems to support the ISS. Surface exploration missions will require specialized suits, tools, and vehicles that need to be developed for the destinations of interest. Deep-space space suits and surface spacesuits would be necessary for the EVAs during space and surface operations. Surface EVA suits would require much greater dexterity than any suits built to date. Large rovers capable of carrying multiweek habitation units would be essential for expanding the range of exploration outside the near vicinity of the stationary habitat. Robotic assistants may be required to support the crew by performing tasks and going into environments for which the crew is not suited. Destination systems need to be developed for the environments of specific DRMs while keeping in mind the potential for reuse of the design or function during subsequent missions in a particular pathway.
Mars surface systems would require a continuous supply of 30 kW or more of power. Power would need to be supplied throughout the year, day and night, and during dust storms. The power generation system would need to be predeployed with the ascent stage to support ISRU operations. A nuclear fission reactor is used as the baseline surface power system in DRA 5.0 because of its expected lower mass, reduced volume, and greater reliability compared to the alternative of a surface solar array. On the Moon, due to the 14 days of darkness during surface eclipse, nuclear fission reactors are also the most practical method of providing power to a lunar outpost unless it is in specific locations near the lunar poles.
Additional excursion vehicles would be used for missions to bodies that have near-zero gravity (e.g., an asteroid in its native orbit or the moons of Mars). The excursion vehicles would have the short-term habitation systems of the surface pressurized rovers mated to a zero-gravity mobility system. The vehicles would offer the crew greater protection than an EVA suit when working in close proximity to an asteroid or the moons of Mars. They would also allow the in-space transportation and deep-space habitats to be kept a safe distance from any debris generated by the crew during interactions with celestial bodies.
44 NASA, “To the Extreme: NASA Tests Heat Shield Materials,” February 3, 2009, http://www.nasa.gov/mission_pages/constellation/orion/orion-tps.html.
FIGURE 4.3 Primary mission elements for a DRA 5.0 human mission to the Mars surface along with transitional mission elements and dead-end mission elements and their associated icons.
To achieve the horizon goal of a human mission to the Mars surface, all the aforementioned element groups are necessary. More specifically, 11 individual mission elements required for the DRA 5.0 Mars surface mission have been identified. These 11 are the primary mission elements shown in Figure 4.3. Only two of the 11 are currently funded for development of operational flight hardware (SLS and Orion). DRMs to destinations other than the surface of Mars might not directly use the primary mission elements but instead use elements that are in the technological development path of a primary element. These are the three transitional mission elements identified in Figure 4.3. Also shown in Figure 4.3 are five dead-end mission elements. Beneficial technology may arise from the development of the dead-end mission elements, but for the most part they have requirements other than what is needed for a Mars surface mission and are extraneous to the direct goal of landing humans on Mars. To understand how each mission builds toward the horizon goal, the progression of element use in each pathway is presented below. It is important to note that the element development for each pathway highlights the location of major jumps in capability between various DRM stages and shows the need for nonessential mission elements that may take additional funding away from the future Mars surface horizon goal.
The buildup of mission critical elements for the ARM-to-Mars pathway is shown in Figure 4.4. The figure shows the mission elements used for each of the three DRMs in this pathway, starting with the ARM, then the Mars
FIGURE 4.4 Buildup of mission elements for the ARM-to-Mars pathway. Mission elements whose icons are grayed out are not required for the relevant missions.
Moons mission, and finally the Mars Surface mission. The ARM mission uses only three of the key Mars surface elements, and each element must be further improved before the Mars Moons mission. Advanced EVA capabilities may be needed for the ARM mission, and the Mars Moons mission may leverage some of the capabilities. Because of the assumption of an NTP-based advanced propulsion system for the Mars Surface mission, the ARM robotic asteroid-redirect vehicle is considered a dead-end mission element, inasmuch as its advanced SEP capabilities are not leveraged in future missions as currently envisioned. In the event that NTP in-space propulsion is not used to transfer cargo for a Mars Surface or Mars Moons mission, SEP could be used for this function. However, the 50-kW SEP system required for ARM is about an order of magnitude below what is required to carry cargo to Mars for the Mars Moons and Mars Surface missions.
The Mars Moons mission would demonstrate all the in-space elements required for a Mars Surface mission and would require major advances in in-space habitation and propulsion. In fact, the in-space habitation requirements for the Mars Moons mission vastly exceed those for the Mars Surface mission. The additional habitat requirements and the need for the space exploration system and orbital maneuvering elements, needed to reach the Mars moons from Mars orbit, lead to additional dead-end mission elements and development. This pathway leaves the development of the six critical surface elements until its final step in the pathway without previous transitional development.
FIGURE 4.5 Buildup of mission elements for the Moon-to-Mars pathway.
The element buildup for the Moon-to-Mars pathway is shown in Figure 4.5. The Lunar Sortie mission requires the development of a number of elements, most of which provide some advance in capabilities that will be needed for the Mars Surface mission. The cryogenic propulsion system will advance propellant management and storage for an NTP-based in-space propulsion system. The reusable lunar lander will advance terminal landing and ascent operations. The disposable descent stage does not provide any significant advance in technology that supports the Mars Surface mission, so it is shown as a dead-end mission element.
The Lunar Outpost mission requires the development of many more of the mission elements needed by a human mission to the Mars surface than any of the other missions included in the pathways.45 It entails long-term surface operations in a dust-prone partial-gravity environment and would demonstrate habitation, robotic augmentation, and nuclear power generation technologies and systems that would be required for the Mars Surface mission.46
45 Three core primary mission elements are part of every mission: the heavy-lift launch vehicle (SLS), the crew command and service module (Orion), and advanced EVA. The lunar-outpost mission requires the development of and provides the opportunity for operational demonstrations of four additional primary mission elements. In contrast, the DRMs for the other five intermediate destinations would demonstrate no or one primary mission element in addition to the core three.
46 Certainly, there are important differences between the surface environments of the Moon and Mars with regard to, for example, atmosphere (or the lack thereof), gravity, day-night cycle, and dust properties. Nevertheless, the Moon provides the best opportunity to test surface systems and human physiology in a partial-gravity environment before committing astronauts to a Mars surface mission.
FIGURE 4.6 Buildup of mission elements for the Enhanced Exploration pathway.
Important advances can be made in short-term habitation in lunar orbit, but the longer-term habitat would have to be demonstrated as part of the Mars Surface mission. The Moon-to-Mars pathway would also leave the major efforts of in-space propulsion and EDL until the Mars Surface mission.
The Enhanced Exploration pathway element buildup shown in Figure 4.6 illustrates the more gradual pace and lower risk of this pathway, with each of the six DRMs incrementally developing required hardware. The in-space habitation capabilities begin with a short-duration capability that uses a more efficient and more reliable ECLSS
FIGURE 4.7 Comparison of the mission-element buildup for each pathway.
starting with the Earth-Moon L2 mission, with increasing duration and volume for the Native Asteroid mission, and culminating in the Mars Moons mission. The surface exploration capabilities are matured on the lunar missions, leaving the advanced in-space propulsion system as the only significant development for the Mars Moons mission. The only completely new development for the Mars Surface mission is the one capability that cannot be demonstrated anywhere else—Mars EDL.
A simplified summary of the critical element progression for each of the three pathways is shown in Figure 4.7. Although the full interaction of the technology advances and precursor missions required to achieve the capability to land humans on the Mars surface is complex, a simple count of the critical elements for each DRM in each of the three pathways shows how they bound the problem.
All pathways have the goal of landing humans on Mars, which is consistent with the DRA 5.0 architecture, so each pathway in Figure 4.7 indicates a cumulative total of 11 primary mission elements (which are depicted by green squares). The ideal progression would be a smooth transition, with minimal jumps, from a few to the 11 final green elements. This progression would minimize both technical risk and the need for major, temporary increases in human spaceflight funding, and it would spread major technical challenges out in time. For efficiency, there should be few, if any, red or dead-end mission elements.
Developing the capabilities needed for a human mission to the Mars surface will require considerable resources and technological innovation in many disciplines to accommodate the environments to be encountered in space and during surface operations.47,48,49 Technology has made huge leaps since the early days of human spaceflight, as has understanding of the risks and challenges posed by the space environment. The ISS has proved to be an essential platform for investigating and enhancing the ability of humans to survive most of the hardships of space exploration. However, some unknowns remain, particularly with regard to the long-term effects of space radiation and the partial gravity present on the Moon and Mars. Enabling humans to land on the surface of Mars and return safely requires advanced capabilities in many areas, including the following:
- Launch vehicles capable of placing large masses in LEO at minimum cost.
- Reliable power generation for deep-space and surface operations.
- Efficient in-space propulsion to increase transit time and payload capacity, while reducing human exposure to the deep-space environment.
- Habitats, systems, and procedures to ensure the health of human explorers and preserve their physical and mental capabilities during long stays in space and on the surface of Mars.
- EDL systems to land very large payloads on Mars and subsequently return them to Earth.
- Vehicles and systems for landing large masses on planetary bodies, lifting an ascent vehicle back into Mars orbit, and returning the crew to Earth.
- Systems for surface operations, including science instruments, robotic vehicles, crewed rovers, spacesuits, and ISRU systems to produce oxygen and, if possible, other consumables and materials.
Making the necessary advances in some of the above areas will be more challenging than in others. To determine which capabilities should have the highest priority for current research and development programs, the Technical Panel assessed a wide variety of capabilities in terms of four parameters:
- Technical challenges
- Capability gap
- Regulatory challenges
- Cost and schedule challenges
Mission need was not used as an evaluation parameter because it did not help in differentiating capabilities. A great many technical capabilities—not just those ranked as having high priority—are essential for the success of a human mission to the Mars surface.
Based on the expertise of the Technical Panel members and additional information reviewed by the panel, the difficulty of making needed advances for each capability was ranked as high, medium, or low for each of the four parameters. The criteria for assigning these rankings are listed in Figure 4.8.
47 NRC, NASA Space Technology Roadmaps and Priorities: Restoring NASA’s Technological Edge and Paving the Way for a New Era in Space, The National Academies Press, Washington, D.C., 2012.
48 NRC, Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies, National Academy Press, Washington, D.C., 2000.
49 NRC, Recapturing a Future for Space Exploration: Life and Physical Sciences Research for a New Era, The National Academies Press, Washington, D.C., 2011.
FIGURE 4.8 Capability assessment criteria.
The capability assessment ranked the following capabilities as a high priority:
- Mars EDL
- Radiation safety
- In-space propulsion and power50
- — Fission power
- — In-space cryogenic propulsion
- — NEP
- — NTP
- — SEP
- Heavy-lift launch vehicles
- Planetary ascent propulsion
- EVA suits
- Crew health
- ISRU (Mars atmosphere)
Advances in many other capabilities will be essential for human exploration beyond LEO. These are addressed below in section 220.127.116.11 (“Additional Capabilities”).
In 2010, the National Research Council issued a report that assessed and prioritized the space technologies that were included in a comprehensive set of draft roadmaps prepared by the NASA Office of the Chief Technologist.51 All but one of the high-priority capabilities listed above are closely linked to one or more of the high-priority technologies identified in the roadmaps report; the lone exception is heavy-lift launch vehicles.
50 As development of NEP, NTP, SEP, and cryogenic propulsion technologies proceeds, a down-selection will be required.
51 NRC, NASA Space Technology Roadmaps and Priorities, 2012.
FIGURE 4.9 Relationship of mission elements to high-priority capabilities.
FIGURE 4.10 Relationship of missions to high-priority capabilities.
o For a given capability, a substantial development effort would be required to execute a particular mission, at which point the development effort would need to continue essentially unabated to prepare for a Mars surface mission.
• For a given capability, a substantial development effort would be required to execute a particular mission, at which point minimal additional development would be needed to prepare for a Mars surface mission.
18.104.22.168 High-Priority Capabilities
This section summarizes the assessments of the high-priority capabilities. Although all these capabilities were ranked as a high priority, the first three—Mars EDL, radiation safety, and in-space propulsion and power—are in a class by themselves. In particular, the cost and schedule of developing a Mars EDL capability will be extraordinary because of the physical size of the operational system, the need to test at least one operational system in the atmosphere of Mars, the narrow windows to launch test systems, and the extreme cost and schedule delays that
would result if the operational test failed. In addition, an approach for overcoming the technical challenges of Mars EDL for a crewed mission remains to be determined. Radiation safety is unique in that conventional approaches to reducing radiation exposure (i.e., shielding) would increase radiation exposure to astronauts while they are in transit and on the surface of Mars, and the efficacy of unconventional technical and biological solutions remains speculative. As with Mars EDL, the large size of in-space propulsion systems greatly increases the cost of developing, manufacturing, and testing operational systems. That cost will be further increased for systems that include nuclear power. The cost of developing chemical in-space propulsion systems will be modest by comparison, but the cost of using chemical propulsion systems on a per mission basis will be so high that it would threaten the sustainability of the Mars program. The only other options for in-space transportation are NEP and SEP, but because of the inherent limitations of electric propulsion NEP and SEP may not be feasible for crewed vehicles. The cost of in-space propulsion and power capability will be further increased by the need to develop a surface power system. All 10 of the high-priority capabilities are essential, and all will be challenging to develop, but none is comparable either with Mars EDL and in-space propulsion and power in terms of cost or with radiation safety and Mars EDL in terms of technical challenges and capability gap.
22.214.171.124.1 Mars EDL
On August 6, 2012, the world watched as the Mars Science Laboratory (MSL) completed an autonomous landing on Mars. The fate of the most complex machine that humans had ever sent to another planet rested on an innovative 7-minute landing sequence that had been years in the making. EDL encompasses mission design, software, systems development, operations, and integration processes. For MSL, hundreds of people worked for about 8 years to get the job done. During EDL, the MSL spacecraft had to autonomously perform six configuration changes, complete 76 pyrotechnic events, and execute approximately 500,000 lines of computer code with almost no margin for error.
Prior to MSL, the United States had successfully landed six robotic systems on Mars. Those systems all had landed masses of less than 600 kg and landing errors of hundreds of kilometers in diameter.52 MSL raised the bar considerably, safely delivering a 900-kg rover to the Mars surface within a landing error of just 20 km. Even so, compared with the capability required to deliver humans to the Mars surface, MSL was a baby step.
A human mission to the Mars surface could require landing payloads of 40-80 MT in close proximity (tens of meters) to pre-positioned assets.53 Because existing EDL technologies do not scale up to payloads of this size, the EDL systems required for a Mars surface mission would have little resemblance to those in use today. Multiple EDL concepts for a human surface mission have been proposed, including supersonic retropropulsion, slender-body aeroshells, inflatable aerodynamic decelerators, and advanced thermal protection systems. However, an EDL system that incorporates such advanced concepts would be extremely complex, and the feasibility of these concepts remains to be proved.
EDL technologies are highly interdependent and are generally validated in the context of the sequence of events associated with a particular mission. Before a human mission to the Mars surface, Mars EDL systems would probably require precursor flight tests in the atmospheres of both Earth and Mars. Developing new EDL technologies and systems and flight tests on Earth and at Mars would require substantial resources and time. NASA’s Entry, Descent, and Landing Roadmap, published in 2012, concluded that for a human landing on Mars in the 2040s the United States would need to begin EDL technology development within the next few years.54
Advances in EDL technology that are developed to support a human mission to the Mars surface would likely facilitate robotic missions to Venus, Titan, or the gas giants (Jupiter, Saturn, Uranus, and Neptune). The development of Mars EDL systems would also facilitate the development of EDL systems for spacecraft returning to Earth from remote destinations in the solar system on high-velocity trajectories.
The assessment of EDL systems needed for a human mission to the Mars surface is summarized in Figure 4.11. Technical challenges are ranked high because the technologies needed for a Mars EDL system that would be
52 R.D. Braun and R.M. Manning, “Mars Exploration Entry, Descent and Landing Challenges,” paper presented at the Aerospace Conference, 2006 IEEE, March 4-11, 2006, doi:10.1109/AERO.2006.1655790.
53 Braun and Manning, “Mars exploration entry, descent and landing challenges,” 2006.
FIGURE 4.11 Assessment of EDL systems for a human mission to the Mars surface.
capable of handling large payloads have yet to be identified. The capability gap is ranked high because the necessary payload capacity of the EDL systems is far beyond the capability of existing EDL systems. Regulatory challenges are ranked low because no regulatory changes are needed. Cost and schedule challenges are ranked high because extraordinary resources and time would be needed to identify suitable technologies, scale them up to the requisite size, and conduct flight testing in the atmosphere of Earth and/or Mars to build confidence that they are safe enough for use on a crewed mission.
126.96.36.199.2 Radiation Safety
Space radiation in the form of ionizing radiation, SPEs, and extremely high-energy GCR would be a serious threat to crew health on long-duration missions beyond LEO.55 Overcoming this threat would require advances in all aspects of radiation safety: prediction, risk assessment modeling, total exposure monitoring, and protection. Radiation safety systems reduce or counteract the effects of radiation, and associated standards limit the total dose of a given type of radiation that people are authorized to accumulate over a given period. Shielding incorporated into the design of vehicles and habitats could effectively reduce the exposure of astronauts to SPEs. However, because of secondary radiation produced when primary penetrating particles interact with spacecraft structures, shielding, or other materials, conventional shielding has not been shown to be an effective countermeasure for GCR. In addition, spacesuits do not effectively shield astronauts from SPEs or GCR during EVAs.56 Shielding astronauts using spacecraft-generated electromagnetic fields has been proposed, but such systems would carry severe power and mass penalties, new health issues could arise from the exposure of the crew to powerful electromagnetic fields, and the systems are well beyond the technology horizon considered in this report.57
The overall goal of radiation safety research is to reduce radiation exposure to acceptable levels with as little impact as possible on spacecraft and habitat mass, cost, complexity, and so on. Adequate technical, biological, and/or pharmacological solutions have yet to be identified, and there is a large gap between current capabilities and what is needed to provide adequate safety.
The ISS is substantially protected from space radiation (especially SPEs) by Earth’s magnetic field, so the space radiation environment in which the ISS operates is more benign than it is above LEO, although passage through the South Atlantic Anomaly increases the crew’s radiation exposure. With longer ISS tours planned and because radiation effects are cumulative, crew radiation exposure on the ISS is becoming a matter of greater concern. Because the ISS does not provide an environment typical of deep space, NASA’s Human Research Program
56 NRC, NASA Space Technology Roadmaps and Priorities, 2012.
57 NRC, Managing Space Radiation Risk in the New Era of Space Exploration, 2008.
TABLE 4.3 LEO Exposure Limits in Sieverts
|LEO 10-Year Career Whole-Body Effective Dose Limits (Sv)|
SOURCE: National Council on Radiation Protection and Measurement (NCRP), NCRP-132: Recommendations of Dose Limits for Low Earth Orbit, Bethesda, Md., 2000.
conducts radiation studies using animal models in ground particle accelerators, but these do not achieve the energies of some of the GCR particles that are of concern for astronauts.
The National Council on Radiation Protection and Measurements has recommended career exposure limits for astronauts in LEO (see Table 4.3). The limits are based on the probability of a 3 percent excess cancer mortality for the type of radiation experienced in LEO. For longer-duration missions outside LEO, it may be necessary to re-evaluate the limits on the basis of tradeoffs between the difficulty of meeting existing limits, the applicability of the limits to the deep-space radiation environment, the potential health risks and regulatory challenges associated with modifying the limits, and a better understanding of the noncarcinogenic effects of GCR, such as cumulative neural degeneration, which may prove to be more limiting than carcinogenic effects.
Based on current estimates of the space environment, existing radiation limits would likely be exceeded after about 600 days in space, even with the most permissive crew composition (never-smoking men more than 55 years old with no previous radiation exposure) and with the assumption that carcinogenesis is the only radiation risk that needs to be controlled. More conservative but realistic assumptions might lead to considerably shorter permissible durations. However, it may be possible to increase the above limits safely by reducing current uncertainties, such as the risk of adverse biological effects and the efficacy of possible radiation countermeasures. More accurate predictions of SPEs and solar storms associated with intense periods of ionizing radiation would provide more time to prepare for these events and reduce false alarms and thereby improve mission effectiveness.58 Finally, astronaut selection and mission assignment may ultimately involve consideration of individual susceptibility to radiation as inferred from genome analysis.59 Note, however, that use of personal susceptibility information to inform mission assignment would appear to violate the Genetic Information Nondiscrimination Act of 2008, from which only the military is exempt.
The assessment of the ability to ensure radiation safety for a human mission to the Mars surface is summarized in Figure 4.12. Technical challenges are ranked high because a suitable approach for providing adequate radiation safety has yet to be identified.60 The capability gap is ranked high because the ability to provide the level of radiation safety required for a human mission to the Mars surface is so far beyond the state of the art. Regulatory challenges are ranked medium because part of the solution may be to relax current radiation exposure limits (based on greater knowledge of the human health effects of the radiation environment in space and on the Mars surface and/or a reconsideration of the level of acceptable risk). Cost and schedule challenges are ranked medium because the time and resources necessary to develop adequate radiation safety systems are substantial—although not of the same order as, for example, those required to develop Mars EDL systems.
188.8.131.52.3 In-Space Propulsion and Power
Once a spacecraft is launched into Earth orbit, in-space propulsion systems are used to move it to its intended destination and to return it to Earth (for crewed missions and robotic sample return missions). Most of the bio-
58 NRC, NASA Space Technology Roadmaps and Priorities, 2012.
59 M.R. Barratt and S.L. Pool, Principles of Clinical Medicine for Space Flight, Springer, New York, 2008. p. 67.
60 NRC, Managing Space Radiation Risk in the New Era of Space Exploration, 2008.
FIGURE 4.12 Assessment of radiation-safety systems and capabilities for a human mission to the Mars surface.
medical and life support risks posed by human exploration missions to distant destinations would be greatly mitigated by advanced high-thrust in-space propulsion systems that substantially reduce transit times to and from the destination and thereby reduce exposure to zero-g, space radiation, and psychosocial stress.
For exploration missions, a large portion of the launch mass is the fuel and oxidizer required for in-space propulsion. Therefore, the efficiency of in-space propulsion systems (in terms of Isp) is of key importance. High thrust is also important to reduce transit time. Despite decades of research and study, it remains to be seen what type of advanced in-space propulsion system would provide the best combination of Isp and thrust for future exploration missions. The four technologies of greatest interest are cryogenic propulsion, NEP, NTP, and SEP. Each of these options is discussed below along with fission power systems that could be adapted for operation in space (to power NEP systems) or to provide power on the surface of the Moon or Mars.
As the technologies for in-space propulsion are developed and matured, there will probably need to be a down-selection among the four options because of the high development costs required for each one. SEP and NEP provide the highest Isp, but the megawatts of power needed to provide sufficient thrust for a crewed exploration mission creates high development risk. NTP delivers Isp that is double that of cryogenic systems but comes with a high development risk due in large part to difficulties of safely ground testing an open-cycle nuclear fission system. Cryogenic propulsion is a more mature technology than the other options, but it offers lower performance and poses additional technical challenges, primarily in connection with low-loss, long-term storage and in-space transfer of cryogenic fuels and oxidizers.
184.108.40.206.3.1 Fission Power
Robotic spacecraft typically require only a few hundred watts, whereas larger satellites can require a few kilowatts. The highest-power satellites are geosynchronous communication systems; they require about 20 kW of electric power.
Human space missions require much more power than typical uncrewed spacecraft. The space station has a 100-kW capability, and concepts for long-term missions to the surface of the Moon or Mars typically call for 50 100 kW of installed power. Solar and nuclear fission systems are currently the only viable options for providing long-term power at those levels both on the surface of the Moon or Mars and in space as part of an SEP or NEP system.
Nuclear fission reactors are fundamentally a long-term source of thermal energy. A complete space nuclear power system converts the thermal energy produced by the reactor into electricity. The nuclear reactors that would be used for space applications would be similar in some ways to low-power nuclear fission systems, such as research reactors, currently used on Earth. However, substantial modifications would be required. For example, a space nuclear reactor system must be designed to minimize risk to the public during launch and to operate safely and reliably in the intended environment (in space or on the surface of the Moon or Mars).
FIGURE 4.13 Assessment of a 100-kW fission power system.
The United States has launched just one fission reactor, the SNAP-10A in 1965. The Soviet Union was the only other country to operate fission reactors in space, and those systems were also launched decades ago. The SP-100 program—which was jointly funded by NASA, the Department of Defense (DOD), and the Department of Energy—was developing fission reactors for a surface power system until it was canceled in 1992. U.S. space reactor designs were based on the use of thermoelectric systems to convert heat into electricity. In the early 1990s, DOD also investigated the feasibility of developing a space reactor with a thermionic energy conversion system that used unfueled components of “Topaz-2” space reactors purchased from Russia, but this effort was terminated without such a system being launched by Russia or the United States.61 Most of the key people and facilities from prior programs are no longer available. A new program to design, test, and produce space nuclear power systems would be a substantial undertaking. Allowing for increased regulatory complexity, it would cost billions of dollars and take at least a decade to develop a fission power system capable of producing 50-100 kw.62 Existing federal regulations specify the approval process needed to develop and launch fission reactors. The approval process, however, is very time-consuming and costly, and political opposition to the launch of a nuclear reactor may arise during the approval process.
The assessment of fission power systems needed for a human mission to the Mars surface is summarized in Figure 4.13. Technical challenges are ranked medium because of extensive experience with reactor technologies although some new technologies would be needed to provide reliable, long-term operation in space and on the surface of Mars. The capability gap is ranked medium because, despite past accomplishment in nuclear power technology in general and space nuclear power in particular, it has been almost 50 years since a U.S. space nuclear power program succeeded in conducting a flight test of a fission power reactor. Regulatory challenges are ranked medium because of the difficulty of completing the regulatory process that has been established to obtain launch approval. Cost and schedule challenges are ranked high because of the extraordinary resources and time that would be required to develop an operational reactor system and obtain the necessary launch approvals.
220.127.116.11.3.2 In-Space Cryogenic Propulsion
Given the long and successful history of engines such as the RL-10 and J-2, cryogenic engines for in-space propulsion systems are well defined. In fact, the cryogenic propulsion option for in-space propulsion described in DRA 5.0 proposes the use of existing RL-10-B2 engines for all three of the major in-space propulsion modules: the trans-Mars injection module for the trip to Mars, the Mars orbit insertion module, and the trans-Earth injection
61 NRC, Priorities in Space Science Enabled by Nuclear Power and Propulsion, The National Academies Press, Washington, D.C., 2006, p. 114.
62 V.C. Truscello, SP-100, The U.S. Space Nuclear Reactor Power Program. Technical Information Report, Jet Propulsion Laboratory Report 1085, November 1, 1983, Pasadena, Calif., http://www.osti.gov/scitech/servlets/purl/10184691.
FIGURE 4.14 Assessment of in-space cryogenic propulsion systems that could be used for a human mission to the Mars surface.
module for the trip home.63 New technologies are needed, however, to enable the use of these engines for a human mission to Mars. Key requirements include low-loss, long-term storage and in-space transfer of cryogenic fuels and oxidizers. In addition, existing RL-10 engines have operational lifetimes measured in hours. The propulsion modules used for a human Mars mission would need to be stored in LEO for perhaps 4-6 months during vehicle assembly, and the Mars orbit insertion module and trans-Earth injection module would need to operate reliably after being exposed to the space environment for years. Currently, chemical propulsion systems are the only option available for human exploration missions, but chemical propulsion has a lower Isp (450 seconds in vacuum for the shuttle main engine, which uses liquid hydrogen and liquid oxygen) than other in-space candidates. As a result, the use of chemical systems would require large amounts of fuel to be launched from Earth and carried to Mars.
NASA has plans to fly in-space experiments to advance cryogenic propellant storage capabilities with the goal of improving lifetime from hours to months. New technology would still be needed, however, to close the capability gap between the performance of experimental systems now being developed and the performance of operational systems for a Mars mission. Developing the propulsion modules needed for a human mission to the Mars surface and demonstrating the ability to assemble them in space and operate them reliably after the long transit to Mars would also be an expensive and lengthy undertaking.
The assessment of in-space cryogenic propulsion systems that could be used for a human mission to the Mars surface is summarized in Figure 4.14. Technical challenges are ranked medium because, although high-performance in-space cryogenic propulsion systems are already operational, new technologies are needed for in-space fuel-handling and long-term storage. The capability gap is ranked medium because of the improvements needed to extend the in-space storage and operational lifetime of existing systems. Regulatory challenges are ranked low because no regulatory changes are needed. Cost and schedule challenges are ranked high because of the long time that will likely be required to develop the ability to store cryogenic fuel in space for years at a time.
18.104.22.168.3.3 Nuclear Electric Propulsion
Electric propulsion systems are used extensively on Earth-orbiting satellites and on some robotic science missions. Electric propulsion systems accelerate ions to very high velocities. The resulting Isp is in the range of 3,000-6,000 seconds, which is higher than that of any other in-space propulsion technology. As a result, NEP systems would use about 10 percent of the fuel that a cryogenic propulsion system would use to produce an equivalent change in spacecraft velocity. NEP systems would be powered by a fission power system. To date, all electric propulsion systems have been powered by solar energy (see the discussion of SEP systems, below), and that will remain the case until fission power systems for space are developed. Unlike SEP systems, the power available to an NEP system is constant regardless of the distance from the Sun to the spacecraft.
63 NASA, “Human Exploration of Mars Design Reference Architecture 5.0,” 2009, http://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf.
FIGURE 4.15 Assessment of megawatt-class NEP systems for a human mission to the Mars surface.
The major shortcoming of all electric propulsion systems (NEP and SEP) from the standpoint of human transportation is that the governing physics result in accelerations that are very low compared with the alternatives for plausible combinations of system characteristics and power levels. Research to increase the thrust of electric propulsion engines is under way. NASA is actively working on scaling engines to the 60-kW range. However, the technology developed by these efforts will still leave electric propulsion systems far short of the thrust required for crew transport vehicles travelling to Mars or other distant destinations. This would require megawatt-class electric-propulsion technologies, which are not being developed.
SEP and NEP systems both offer the potential to preposition cargo and habitation subsystems. The lack of human passengers would accommodate the low acceleration of these systems. However, the DRA 5.0 architecture for a Mars surface mission uses NTP (see below) for both crewed and cargo missions to reduce the number of separate technology-development programs needed. It would probably not be economical to develop a propulsion system for cargo that could not also be used for crewed vessels.
The assessment of megawatt-class NEP in-space propulsion systems for a human mission to the Mars surface is summarized in Figure 4.15. The cost and schedule challenges, regulatory challenges, and technical challenges are driven largely by the challenges associated with the fission power system (see Figure 4.13) that lies at the heart of the NEP system. A megawatt-class NEP system, however, faces a higher capability gap than the 100-kW fission system discussed above because of the higher power levels that the fission power and electric propulsion engines would be required to meet relative to the state of the art.
22.214.171.124.3.4 Nuclear Thermal Propulsion
NTP systems generate high thrust by using a nuclear fission reactor to heat a propellant (typically hydrogen) to very high temperatures. The propellant gases are then expanded through a nozzle to produce thrust. The resulting Isp (800-1,000 seconds) is higher than that of chemical propulsion systems but lower than the Isp of NEP or SEP systems.
NTP is the baseline in-space propulsion system for the Mars DRA 5.0 mission. However, other than some low-level technology research efforts, NTP technologies are not being developed. A full-scale system, NERVA, was built and ground tested about 40 years ago. The NERVA program built and tested 23 reactors and engines with peak power up to 4,000 MW, system operation up to 1 hour, and an Isp of 850 seconds.64 Key facilities and personnel from the NERVA program are no longer available, and current environmental regulations are much more stringent. It would be difficult to produce a test facility that could contain the propulsion exhaust of a full-scale NTP system, and the political and regulatory opposition to the construction of such facilities could be a problem. However, a concept for subsurface active filtering of exhaust coupled with adsorption of exhaust in a bed of metal
64 S.D. Howe, High energy-density propulsion—Reducing the risk to humans in planetary exploration, Space Policy 17(4):275-283, 2001, doi:10.1016/S0265-9646(01)00042-X.
FIGURE 4.16 Assessment of NTP systems for a human mission to the Mars surface.
hydride was viewed as a workable approach during the Strategic Defense Initiative Office’s Project Timberwood, which sought to develop NTP well after the NASA NERVA program.65
The assessment of NTP systems needed for a human mission to the Mars surface is summarized in Figure 4.16. Technical challenges are ranked medium because the NERVA program developed most of the technologies that would be needed by an operational NTP system. The capability gap is ranked medium; the NERVA program tested a full-scale system, but the state of the art has degraded somewhat during the ensuing 40 years. Regulatory challenges are ranked high because it would be technically and politically difficult to develop test facilities for a large nuclear rocket program. Cost and schedule challenges are ranked high because it would be extraordinarily expensive and time-consuming just to repeat the NERVA work of the past, let alone proceed with development of an operational system.
126.96.36.199.3.5 Solar Electric Propulsion
All electric propulsion systems to date have been powered by solar energy, and that will remain the case until space nuclear power systems are developed, as discussed above. SEP is commonly used in orbital spacecraft, and SEP systems have been used on the Deep Space 1 and Dawn scientific spacecraft, which were launched in 1998 and 2007, respectively. Both spacecraft used 2-kW ion thrusters and SEP systems. Most of the ongoing activity in support of SEP is focused on scaling up the thrust levels. As with an NEP system, thrust levels needed for a nominal 6-month human trip to Mars would require scaling up existing systems to megawatts. This would require tremendous advances in both electric engines (see the discussion of NEP system, above) and solar power systems. As with a megawatt-class NEP system, developing a megawatt-class SEP system would require a long development program supported by substantial resources to overcome the capability gap between required performance levels and the existing state of the art.
The assessment of megawatt-class SEP systems that could be used for a human mission to the Mars surface is summarized in Figure 4.17. Technical challenges are ranked low because SEP systems are well developed and have a long history of operation in space. The capability gap is ranked high because the power level of state-of-the-art systems is far below the power level needed for a crewed spacecraft transiting to and from Mars. Regulatory challenges are ranked low because no regulatory changes are needed. Cost and schedule challenges are ranked high because extraordinary resources and time would be needed to close the capability gap.
An SEP system with a power level of hundreds of kilowatts, which would be suitable for transporting uncrewed cargo vessels to Mars or other distant destinations, would face medium cost and schedule challenges and a medium capability gap. However, as noted above, it would be preferable to have a single in-space propulsion system for both cargo vessels and crewed vehicles.
65 Final Environmental Impact Statement (EIS) for the Space Nuclear Thermal Propulsion (SNTP) Program, Sanitized Version, September 19, 1991, http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA248408.
FIGURE 4.17 Assessment of megawatt-class SEP systems for a human mission to the Mars surface.
188.8.131.52.4 Heavy-lift Launch Vehicles
Heavy-lift launch systems (that is, launch systems with a payload capability of about 50 MT or more to LEO) would reduce the number of launches required for human exploration missions beyond LEO. Launch vehicles that can accommodate payloads with large mass and volume enable the launch of large mission elements as single units, which reduces or eliminates the cost, time, and technical risk associated with in-orbit assembly. Two heavy-lift launch systems are under development in the United States: the NASA SLS and the SpaceX Falcon Heavy.
184.108.40.206.4.1 Space Launch System
The NASA Authorization Act of 2010 directed NASA to develop the SLS. The system design selected by NASA retains many of the characteristics of the heavy-lift vehicle that was under development as part of the Constellation program, which was canceled in 2010. The Block 1 SLS will have a payload capacity of 70 MT to LEO. Two major upgrades are planned, to increase payload capacity to 105 MT and, subsequently, to 130 MT.
The core stage of the SLS is based on the space-shuttle external tank, stretched and modified to house the main propulsion system at the aft end and an interstage structure at the forward end. The core stage is characterized by NASA personnel as the “long pole” that is pacing SLS development toward the first flight in 2017.
As currently planned, the propulsion system will consist of space-shuttle main engines (RS-25s) left over from the Space Shuttle Program. As with the space shuttle, during the first 2 minutes of flight the RS-25 liquid propulsion system will be augmented by two solid rocket boosters mounted on either side of the core stage. The boosters for the Block I SLS will be modified space-shuttle solid rocket boosters. Boosters for the Block IA and Block II vehicles will have higher performance. They will be procured after a source selection that will be open to both liquid-fuel and solid-fuel systems.
The Block I SLS will use a Delta IV upper stage for the first two missions: the uncrewed Exploration Mission (EM)-1 and the first crewed mission, EM-2. An upper stage developed for the SLS will be used for later flights that require more than the 70-MT payload capacity available with the Block I configuration. The upper stage for Block IB (which will have a payload capacity105 MT) will use four RL-10A-4-2 engines, which are used on the Centaur upper stage for the Atlas V Evolved Expendable Launch Vehicle. Higher-capacity SLS vehicles (130 MT or more) will use J-2X engines, which will be upgraded versions of the upper-stage engines used during the Saturn program.
No technological breakthroughs are required to complete SLS development. In fact, the SLS was intentionally configured so that each key component could be derived from systems with a long heritage of successful flight, such as the external tank, main engine, and solid rocket booster used on the space shuttle and upper-stage engines used for other launch vehicles. Nonetheless, given the physical size of the SLS, its development will be a major undertaking. In fact, the cost of developing SLS and related systems (the Orion Multi-Purpose Crew Vehicle and the ground systems) are so high relative to the budget of the Human Exploration and Operations Mission Director-
FIGURE 4.18 Assessment of the SLS heavy-lift launch vehicle.
ate that the schedule has been stretched to accommodate available funding. As a result, the currently planned time between SLS launches is much greater than in past human spaceflight programs: the first two SLS flights (EM-1 and EM-2) will be launched 4 years apart, in 2017 and 2021.
220.127.116.11.4.2 Falcon Heavy
The Falcon Heavy launch system, now being developed by SpaceX, is a heavy-lift variant of the Falcon 9 launch system. The Falcon 9 system uses Merlin 1D rocket engines fueled by kerosene and liquid oxygen on both its first and second stages (nine engines on the first stage and one on the second). The Falcon Heavy configuration uses a standard Falcon 9 core with two additional Falcon 9 first stages strapped on as boosters. The heaviest-lift variant of the Falcon Heavy will have a payload capacity of 53 MT. Falcon Heavy is designed to tolerate the loss of thrust from several engines and still complete its mission, thus enhancing mission reliability.
The Merlin 1D engine is a higher-thrust version of the flight-tested Merlin 1C engine. Ground testing of the Merlin 1D was completed in June 2012, and SpaceX announced that the engine had achieved flight qualification in March 2013. The successful first launch of a Falcon 9 version 1.1 (with Merlin 1D engines) took place at Vandenberg Air Force Base during September 2013 and was followed by a second successful launch in December 2013.
When launching heavy payloads, the Falcon Heavy uses propellant cross-feed from the side boosters to the center core. As a result, the center core still has most of its fuel after the side boosters separate, and this increases its maximum payload capacity. This unique feature is being implemented through the innovative adaptation of existing technologies.66
As with NASA’s SLS, no technological breakthroughs are required to complete development of the Falcon Heavy. The first test flight is scheduled for 2015, and the first two operational flights are scheduled for 2015 (for the U.S. Air Force) and 2017 (for Intelsat).
The assessment of the SLS heavy-lift launch vehicles is summarized in Figure 4.18. Technical challenges and the capability gap are ranked low because the SLS was designed to avoid the need for either new technologies or substantial improvements to existing technologies. Regulatory challenges are ranked low because no regulatory changes are needed. Cost and schedule challenges are ranked high because of the very high cost that NASA has projected to complete development and flight testing of the SLS and the long period before the first operational flight.
The assessment of the Falcon Heavy is summarized in Figure 4.19. It is the same as Figure 4.18 for the SLS except that the cost and schedule challenges are ranked low because the Falcon Heavy development program is much closer to completion and a Falcon Heavy launch would cost less than an SLS launch. However, given the smaller payload capacity of the Falcon Heavy, use of the Falcon Heavy for a Mars surface mission might increase cost, technical challenges, and capability gaps for other elements of a Mars program. A smaller launch system would
66 Erik Seedhouse, SpaceX: Making Commercial Spaceflight a Reality, Springer-Praxis Books, New York, N.Y., 2013.
FIGURE 4.19 Assessment of the Falcon Heavy heavy-lift launch vehicle.
require more launches, more time in orbit, more docking events, smaller spacecraft modules, and more time for orbital assembly and checkout of the larger number of modules. As a result, mission reliability might be reduced.
As noted above, the Augustine study suggested that human exploration could be accomplished with a 50-MT launch system.67 The business case for the SLS versus that for multiple launches of smaller rockets depends primarily on the number of missions to be accomplished and the operational tempo. At present, the crossover point at which an SLS-based approach would be more economical than an approach using smaller launch vehicles has yet to be determined. Regardless, the business case for developing the SLS is weakened if the United States is not committed to a robust program of human exploration, large robotic spacecraft, or other high-mass missions (such as large-scale optics). China appears to be examining the tradeoffs among heavy-lift launch vehicles of various capacities. The Long March 9, which is still under study, would have a LEO payload capacity of 130 MT.68 Russia is developing a new family of launch vehicles. Proposed variants would have LEO payload capacities up to 41.5 MT (the Angara A7V).
18.104.22.168.5 Planetary Ascent Propulsion
The required characteristics of an ascent propulsion system are absolute reliability after a long period of dormancy, high-to-medium thrust levels, and high efficiency. The Mars DRA 5.0 study did not include detailed analysis of the Mars ascent vehicle. However, based on prior studies, the DRA 5.0 ascent propulsion system would be fueled by liquid methane and liquid oxygen. One mission concept would reduce launch mass on Earth and the mass of landed systems on Mars by relying on ISRU systems to produce liquid oxygen on Mars; liquid methane would be brought from Earth. NASA is not working on an ascent propulsion system that is scalable for human transportation.
Development of a planetary ascent propulsion system will be similar to the development of the propulsion system for a new launch vehicle, except that the ascent system will need to operate from a remote location with no ground crew, it must be integrated into a vehicle that descends to the surface of Mars, and it must operate reliably after a long period of dormancy during the transit from Earth and while on the surface of Mars before launch. In addition, new technology will be needed to develop cryogenic propellant storage capabilities that can store cryogens for years with little or no boil-off.
The assessment of planetary ascent systems needed for a human mission to the Mars surface is summarized in Figure 4.20. Technical challenges are ranked medium because experience with lunar ascent engines and existing in-space propulsion systems provide a solid foundation for developing the technologies needed for Mars. In addition, new technologies are needed for long-term storage of cryogenic fuels. The capability gap is ranked medium
67 Review of U.S. Human Space Flight Plans Committee, Seeking a Human Spaceflight Program Worthy of a Great Nation, 2009.
68 B. Perret, Launcher Leap, Aviation Week & Space Technology, pp. 22-23, September 22-23 (reporting Chinese mission and launcher concepts from the International Astronautical Congress in September 2013).
FIGURE 4.20 Assessment of planetary ascent propulsion systems for a human mission to the Mars surface.
because of the improvements needed to advance available technologies enough to provide the power needed for ascent from Mars. Regulatory challenges are ranked low because no regulatory changes are needed. Cost and schedule challenges are ranked high because of the long time that will probably be required to develop the ability to store cryogenic fuel in space for years at a time.
22.214.171.124.6 Environmental Control and Life Support System
A reliable closed-loop ECLSS is needed for spacecraft, surface habitats, and EVA suits to enable long-duration human missions beyond LEO. For missions to Mars and other missions without an early-return abort option, the ECLSS must be highly reliable and easily repairable. The U.S. and Russian ECLSS on the ISS have demonstrated rates of hardware failures that would be unsustainable on a Mars mission.
The ECLSS maintains a safe atmosphere by monitoring and controlling partial pressures of nitrogen, oxygen, carbon dioxide, methane, hydrogen, and water vapor; maintaining total cabin pressure; filtering out particles and microorganisms; and distributing air. The ECLSS also provides potable water and performs habitation functions, such as food preparation and production, hygiene, collection and stabilization of metabolic waste, laundry services, and trash recycling. ECLSS waste management subsystems safeguard crew health, recover resources, and protect planetary surfaces. Key functions include reducing the mass and volume of consumables, including food; controlling odors and the growth of microorganisms; and recovering water, oxygen, other gases, and minerals.69
The ECLSS in use today requires constant repair and a large store of spares and uses too many consumables to be practical for missions lasting more than a few weeks. Some progress is being made to improve the reliability and performance of in-space ECLSS, and the ISS is an excellent platform for testing in-space ECLSS subsystems. However, there is still a large gap between current capabilities and the performance that would be needed for long-duration missions in space. It remains to be seen how soon that gap can be closed and what new research capabilities and technologies will be needed. In addition, little effort is being made to develop an ECLSS that is tailored for operation in the partial-gravity environments found on the surface of the Moon or Mars.
The assessment of ECLSS for a human mission to the Mars surface is summarized in Figure 4.21. Technical challenges are ranked medium because ECLSS technologies and systems are already operational. The capability gap is ranked high because of the substantial improvements that are needed to extend the lifetime and increase the reliability of existing technologies and systems. Regulatory challenges are ranked low because no regulatory changes are needed. Cost and schedule challenges are ranked high because extraordinary resources and time would be needed to develop and validate the performance of closed-loop ECLSS that would operate reliably over long periods in space and on the surface of Mars.
69 NRC, NASA Space Technology Roadmaps and Priorities, 2012.
FIGURE 4.21 Assessment of ECLSS for a human mission to the Mars surface.
All human missions to space require a pressurized and safe environment in which crews can live and work productively. Habitats of interest include short-term in-space habitats, such as the Orion Multipurpose Crew Vehicle; long-term in-space habitats, such as the ISS and the transit habitats for long-duration missions; and surface habitats for missions to the surface of the Moon or Mars. All types of habitats for space exploration have some common requirements and other distinct requirements, based on the environments in which they operate, mission duration, the number of crew, and mission goals.
Conventional habitats are large, complex, and heavy. NASA and private industry have supported development of expandable habitats in recent years that could be used for in-space and surface habitats. NASA is scheduled to attach an expandable habitat developed by Bigelow Aerospace to the ISS in 2015. This will provide an opportunity to characterize its performance under constant loads, the extent of degradation in the space environment, resistance to meteorite impact, and so on.
Key habitat systems include ECLSS and radiation safety systems (discussed above), thermal management, power generation and distribution, and micrometeorite protection. Each of these systems has been constantly improved. However, none has been designed or tested for long-duration missions with no possibility of resupply, with no options for a quick return to Earth in case of mission abort, and with constant exposure to the high-radiation environment of deep space.
The major differences between in-space habitats and surface habitats are the presence of dust and partial gravity (1/6 g on the Moon and 3/8 g on Mars). Lunar dust is highly abrasive and detrimental to mechanical systems, and it could pose a health hazard during long-duration missions if proper isolation cannot be achieved. The martian dust is not as well characterized as lunar dust, but the soil of Mars is known to be toxic because of high concentrations of perchlorates. In addition, the thin atmosphere and high winds on Mars will increase the diffusion of dust over all surface systems. Effective dust mitigation and control technologies and systems for both the Moon and Mars would be essential.
The assessment of habitats that are needed for a human mission to the Mars surface is summarized in Figure 4.22. (This summary pertains to habitat systems other than ECLSS and radiation safety systems, which are addressed separately; see Figures 4.21 and 4.12, respectively.) Technical challenges are ranked medium because NASA has extensive experience in designing and building habitats in LEO, culminating with the ISS. The capability gap is ranked medium because substantial improvements are needed to extend the lifetime and increase the reliability of existing technologies and systems and to assure that habitat systems work as expected in the partial gravity of the Moon or Mars. Regulatory challenges are ranked low because no regulatory changes are needed. Cost and schedule challenges are ranked medium because substantial resources and time would be needed to upgrade and validate the performance of habitat systems that would operate reliably over long periods in space and on the surface of Mars.
126.96.36.199.8 Extravehicular Activity Suits
EVA suits can be viewed as individualized spacecraft. Key performance characteristics of EVA suits include mobility, pressurization, environmental protection (protection from heat, radiation, and micrometeoroids), portable life support (oxygen supply and CO2 removal), ease of donning and doffing the suit, emergency capabilities, range of sizing, operational reliability, durability, sensory capabilities, data management, adaptability, level of articulation, and the forces and torques that an astronaut must apply to conduct assigned tasks. Given that current EVA suits represent incremental changes to suits that were developed more than 30 years ago, potentially substantial increases in performance are possible. EVA suits are needed for operations in microgravity (during in-space operations) and partial gravity (on the surface of the Moon or Mars). The microgravity environment in LEO is extremely well understood, and there is vast experience in performing EVAs during the past 50 years. However, since the end of the Apollo program, little research has addressed EVA suits for surface operations. EVA suits for the Mars surface will need to accommodate the effects of long-term exposure to the deep-space environment en route to Mars and EVA operations on Mars that will be much more extensive than the Apollo EVA operations on the Moon. Key issues include the effects that the partial gravity on Mars could have on gait, posture, and suit biomechanics; extending suit operational life; reducing suit mass; reducing suit maintenance; and reducing the effects of dust on bearings, seals, and closure mechanisms. It will be important for EVA suits to be designed to integrate easily with the design of rovers, habitats, and robotic assist vehicles during surface operations. It would also be beneficial to improve the mission duration, reliability, and maintainability of the portable life support systems incorporated into EVA suits while reducing system mass.70
The assessment of EVA suits needed for a human mission to the Mars surface is summarized in Figure 4.23. Technical challenges are ranked low because there is substantial research and experience with EVA suits in space and, to a lesser extent, on the surface of the Moon. The capability gap is ranked medium because of the advances needed to accommodate the long duration of a human mission to the Mars surface during transit and on the surface. Regulatory challenges are ranked low because no regulatory changes are needed. Cost and schedule challenges are ranked medium because substantial resources and time would be needed to close the capability gap.
188.8.131.52.9 Crew Health
The ability to maintain crew health during long-duration exposure to the space environment is critical for the success of human missions to Mars and other distant destinations. Both physiological and psychosocial issues present medical threats to crew well-being during extended missions.
NASA and the international community have a basic understanding of physiological problems associated with long exposure to microgravity, and they have been executing a methodical plan to reduce the effects of identified
70 NRC, NASA Space Technology Roadmaps and Priorities, 2012.
FIGURE 4.23 Assessment of EVA suits for a human mission to the Mars surface.
issues (most notably, bone loss, muscular and cardiovascular deconditioning, and neurosensory decrements) and to screen for as-yet-unidentified problems that may exist. The plausibility of such as-yet-unidentified problems is bolstered by the recent discovery of new problems, such as the ocular impairments experienced by some astronauts. There is considerable individual variability in physiological responses to microgravity, but all astronauts are affected to some degree. One of the universal effects is bone loss, which is caused by a rapid increase in bone resorption and a decrease in bone formation during space missions. Physical countermeasures (specialized exercises) and pharmaceuticals have been studied, but bone loss is not yet manageable for long-duration space missions.71 Ongoing research plans include testing of astronauts during and after extended stays of up to 12 months on the ISS. However, because of the small number of potential test subjects available on the ISS and the high degree of variation between individuals in both susceptibility and recovery, it takes a long time to accumulate datasets that are large enough to support general conclusions about human health effects.
The extent to which long-term exposure to partial gravity on the Moon or Mars may be a problem remains to be determined. If no adverse effects occur in the 3/8 g on Mars, minimal additional efforts (beyond those necessary to enable extended stays on the ISS) would be needed to counteract the effects of weightlessness during the full extent of a human mission to the Mars surface, including the transit times to and from Mars. Managing the effects of weightlessness for a human mission to the Mars surface would be further eased if the partial-gravity environment on the surface of Mars allowed astronauts to recover from at least some of the effects of weightlessness encountered during the transit to Mars.
Apart from the effects of weightlessness, crew physiology would be threatened by other factors, such as space radiation, illness, and injuries. Radiation safety is addressed separately above. Highly capable diagnostic and treatment equipment, including surgical facilities designed for operation in space and on the surface, would reduce the threats posed by injuries and illnesses, but this is a difficult challenge given that (1) the types of injuries and illnesses that might be experienced cannot all be anticipated and (2) the mass and volume of medical facilities on spacecraft and in ground habitats will be limited.
Psychosocial issues could affect the behavior and performance of astronauts on long-duration exploration missions. Studies of personnel in isolated and confined extreme environments (crews of nuclear submarines, groups wintering over in Antarctic research stations, and astronauts) suggest that psychosocial issues can substantially reduce crew performance, health, and well-being. The effects of psychosocial issues on the crews of deep-space missions could be more severe than those documented in the studies above because of reduced prospects for escape and safe return to Earth in case of spacecraft emergencies, the ineffectiveness of real-time two-way communication with friends and family because of time delays, and the longer duration of the mission, which exacerbates all the
71 E.S. Orwoll, R.A. Adler, S. Amin, N. Binkley, E.M. Lewiecki, S.M. Petak, S.A. Shapses, M. Sinaki, N.B. Watts, and J.D. Sibonga, Skeletal health in long-duration astronauts: Nature, assessment, and management recommendations from the NASA bone summit, Journal of Bone and Mineral Research 28:1243-1255, 2013.
FIGURE 4.24 Assessment of crew health systems for a human mission to the Mars surface exclusive of the effects of space radiation, which are assessed separately (see Figure 4.12).
stresses associated with living and working in a confined space with minimal privacy, barely adequate facilities for personal hygiene, the physiological effects of weightlessness, and so on.72
Psychosocial stresses affect individuals and interpersonal relationships. Individual effects include changes in personality during and after the mission, anxiety, depression, insomnia, reduced productivity, cognitive impairment, psychosis, and psychosomatic illness. Interpersonal effects include a greater desire for privacy, increased tension and conflict, and loss of cohesiveness among the flight crew and between the flight crew and ground personnel. Individual and interpersonal effects tend to increase over the course of a mission. The effects of psychosocial issues could be reduced by research that identifies countermeasures that could be taken before, during, or after long-duration missions. Research of interest includes ground-based simulations of long-duration missions, bed-rest experiments of various durations, and studies conducted on the ISS.73–79
The assessment of crew health systems needed for a human mission to the Mars surface is summarized in Figure 4.24. (This summary pertains to crew health systems other than radiation safety systems, which are addressed separately; see Figure 4.12.) Technical challenges are ranked medium because final solutions of many physiological and psychosocial threats to crew health have yet to be identified. The capability gap is ranked medium because solutions to some issues are rather well defined although others still require substantial research. Regulatory challenges are ranked medium because new standards may be needed as research into physiological and psychosocial issues continues, particularly given the results of a recent report on ethical issues associated with human spaceflight.80 Cost and schedule challenges are ranked medium because substantial resources and time would be needed to overcome the technical and regulatory challenges and to close the capability gap.
72 Institute of Medicine, Health Standards for Long Duration and Exploration Spaceflight: Ethics Principles, Responsibilities, and Decision Framework, The National Academies Press, Washington, D.C., 2014.
73 G.G. De La Torre, B. van Baarsen, F. Ferlazzo, N. Kanas, K. Weiss, S. Schneider, and I. Whiteley, Future perspectives on space psychology: Recommendations on psychosocial and neurobehavioural aspects of human spaceflight, Acta Astronautica 81(2):587-599, 2012.
74 N. Kanas, Psychological, psychiatric, and interpersonal aspects of long-duration space missions, Journal of Spacecraft and Rockets 27(5):457-463, 1990.
75 N. Kanas, From Earth’s orbit to the outer planets and beyond: Psychological issues in space, Acta Astronautica 68(5-6):576-581, 2011.
76 N. Kanas, G. Sandal, J.E. Boyd, V.I. Gushin, D. Manzey, R. North, G.R. Leon, et al., Psychology and culture during long-duration space missions, Acta Astronautica 64:659-677, 2009; N. Kanas et al., Erratum to “Psychology and culture during long-duration space missions,” Acta Astronautica 66(1-2):331, 2010.
77 L.A. Palinkas, Psychosocial issues in long-term space flight: Overview, Gravitational and Space Biology Bulletin, 14(2):25-33, 2001.
78 M.P. Paulus, A neuroscience approach to optimizing brain resources for human performance in extreme environments, Neuroscience and Biobehavioral Reviews 33(7):1080-1088, 2009.
79 Institute of Medicine, Safe Passage: Astronaut Care for Exploration Missions, The National Academies Press, Washington, D.C., 2001, Chapter 5.
80 Institute of Medicine, Health Standards for Long Duration and Exploration Spaceflight, 2014.
FIGURE 4.25 Assessment of ISRU systems designed to produce consumables from the Mars atmosphere.
184.108.40.206.10 In Situ Resource Utilization (Mars Atmosphere)
ISRU refers to the use of natural resources found on the Moon, Mars, or an asteroid to support space science or exploration missions. Resources of interest include water, volatile substances implanted by solar wind in surface rocks, metals, minerals, and the atmosphere (for missions to Mars). ISRU systems can potentially transform these resources into materials needed for life support, propellant, manufacturing, and construction. To the extent that the mass of the materials produced by ISRU systems exceeds the mass of the ISRU system itself, ISRU capabilities offer the potential to reduce the launch mass and cost of space missions.
Without an ISRU capability, a human mission to the Mars surface would need to carry all the propellant, air, food, water, radiation shielding, and so on from Earth. This would increase the launch mass from Earth by about 10-15 percent and the landed mass on Mars by about 25-30 percent compared to a mission scenario that includes ISRU capability.
Highly advanced ISRU systems could conceivably use lunar regolith or the soil on Mars to produce a wide variety of materials. As a first step, however, the ISRU system specified for the Mars DRA 5.0 mission would use only the atmosphere of Mars as its raw material. The primary output of this system would be oxygen, which would be used for life support and as the oxidizer for the ascent propulsion system (in the form of liquid oxygen). The ISRU system would convert the CO2 in the Mars atmosphere into oxygen and carbon monoxide and then vent the carbon monoxide back into the atmosphere. The ISRU plant would also separate and collect nitrogen and argon from the Mars atmosphere for use as buffer gases for crew breathing. In addition, the ISRU system could be designed to produce water by reacting hydrogen brought from Earth with oxygen produced on Mars. This water would be used to replace water lost during crew and EVA operations.81
The assessment of ISRU systems designed to produce consumables from the Mars atmosphere is summarized in Figure 4.25. Technical challenges are ranked low because technologies to achieve the ISRU capabilities described above have been demonstrated on Earth. The capability gap is ranked high because there is a large gap between the capabilities of the small-scale experiments that have been completed and the development of a full-scale operational system capable of reliable operation during long-term exposure to the partial gravity, dust, atmosphere, and radiation environment on the surface of Mars. Regulatory challenges are ranked low because no regulatory changes are needed. Cost and schedule challenges are ranked medium because substantial resources and time would be needed to close the capability gap.
81 NASA, “Human Exploration of Mars Design Reference Architecture 5.0,” 2009, http://www.nasa.gov/pdf/373665main_NASASP-2009-566.pdf, p. 40.
220.127.116.11 Additional Capabilities
In addition to the high-priority capabilities described above, advances in many other capabilities will be essential for a human mission to the Mars surface or to some of the other DRMs that appear in the pathways. Examples of the additional essential capabilities include the following:
- Autonomous systems.
- EDL systems for return to Earth.
- In-space operations, including assembly of large structures and propellant storage and transfer.
- ISRU systems capable of using lunar regolith or the soil of Mars to produce materials for manufacturing, construction, or repair of systems.
- Mission operations and communication.
- Planetary protection (to minimize the biological contamination of explored environments and to protect Earth from biological contamination in case life is encountered by human exploration missions).
- Surface mobility.
- Surface operations.
Capabilities such as these are not included in the list of high-priority capabilities. Advances in these areas are not urgent because they are not needed for early missions in any of the pathways and/or they can be achieved more quickly and with fewer resources than the high-priority capabilities. For example, the state of the art for EDL systems for Earth return, for mission operations and communication, and for autonomous systems is sufficient for missions in cislunar space; surface mobility systems for lunar sorties could be procured without having to overcome any major technical challenges; crewed surface mobility systems for Mars could be developed in time for a mission to Mars in the 2030s or 2040s even without substantial research and technology development in the near term; ISRU systems that can process lunar regolith are not essential for lunar surface missions; and ISRU systems that can process Mars soil are not essential for the first generation of Mars surface missions.
18.104.22.168 Summary of Challenges in Developing High-Priority Capabilities
The four parameters used to assess each of the high-priority technical capabilities discussed above in section 22.214.171.124 (“High Priority Capabilities”) are summarized in Figure 4.26. Without belaboring the point, the relative paucity of green in this summary highlights the difficulty and cumulative scale of technology development required to achieve the horizon goal of a human mission to the Mars surface, whatever the intermediate destinations along the pathway. This technology development challenge bears directly on the next major section of this chapter, which addresses the affordability of a human spaceflight program over the decades required to extend human presence beyond LEO and make meaningful progress in addressing the enduring questions (see Chapters 1 and 2).
The biggest challenge in implementing pathways of human exploration beyond LEO may be financial rather than technical. A pathway is affordable if the costs of all flight programs and associated development fit within the resources available. Any proposed pathway to Mars must be affordable by the U.S. taxpayer (and international partners) to be sustainable. Recent history has shown that budget constraints have dictated the pace of development of exploration systems, and while it is thought that it is feasible to overcome the technical and development challenges to landing humans on Mars, the technical challenges are daunting, and substantial development effort is required.
126.96.36.199 Potential Budget Available to Human Spaceflight Beyond LEO
Total annual spending by NASA on human spaceflight programs, adjusted for inflation to 2013 dollars, has fluctuated over the past 30 years, but the trend has been flat with an annual budget around the current level of
FIGURE 4.26 Summary of the assessments of the high-priority capabilities. High, medium, and low are defined for each of the assessment areas in Figure 4.8.
approximately $8 billion in FY 2013 dollars (see Figure 4.27).82 The most recent presidential budget request83 shows $7.9 billion for the Human Exploration and Operations Mission Directorate (HEOMD) for FY 2014. The budget request proposes continuation of $7.9 billion for FY 2015 and then annual increases of 1 percent through the budgeting horizon of FY 2019, which represents a decreasing budget in constant dollars. This report uses the NASA FY 2014 presidential budget request, which projects NASA’s budget through FY 2018, as a departure point for projections beyond 2018. Future developments in NASA’s human spaceflight program will also attempt to leverage work done by the Space Technology Mission Directorate (STMD), which was funded at about $0.6 billion in FY 2014 and is proposed to increase to $0.7 billion in FY 2015.
NASA’s human spaceflight program has four main areas: operations, research, support, and development, broken out by approximate percentage of the proposed annual FY 2018 budget in Figure 4.28. The operations budget, 41 percent, is dominated by the ISS, including its transportation costs. Research, at 11 percent, includes the STMD’s Exploration Technology Development Program (ETDP) and lays the foundation for future developments by advancing technologies and reducing knowledge gaps. Research funding is spread across many competing technologies with the goal of developing enhanced capabilities that are relevant to a variety of potential missions but without a generally accepted guiding roadmap for what is specifically required for future human spaceflight beyond LEO. The development of some critical capabilities, such as in-space transportation and EDL, are funded at low levels across broad trade spaces; decisions as to where to focus efforts have not been made. This spreading of resources poses a serious challenge to progress in human spaceflight. Differential investment is one of the few tools that program leadership can use in highly constrained situations, but differential investment is extremely
82 Figure 4.26 shows the NASA funding approved specifically for human spaceflight programs does not fully account for funding provided to cross-agency elements that indirectly support human spaceflight.
FIGURE 4.27 Historical funding of NASA human spaceflight programs in constant FY 2013 dollars. SOURCE: NASA FY 2014 President’s Budget Request Summary, http://www.nasa.gov/pdf/750614main_NASA_FY_2014_Budget_Estimates-508.pdf, accessed January 24, 2014; NASA Historical Data Books, SP-4012, Volumes 2-7, http://history.nasa.gov/SP-4012/cover.html.
FIGURE 4.28 Approximate distribution of NASA’s FY 2018 proposed human spaceflight budget, which is used as the basis for projecting the cost of current human spaceflight programs beyond 2018. NOTE: The research funding depicted in this figure is a lower bound. STMD funds several research and technology programs. One, the Exploration Technology Development Program, is focused on exploration, and its budget is included above. The funding for other STMD programs that support all three NASA mission directorates is not included above, because it is not clear how much of this funding supports the HEOMD as opposed to the Science Mission Directorate and the Aeronautics Research Mission Directorate. SOURCE: President’s Budget Request for FY 2014.
difficult to implement in highly bureaucratic organizations in which level-of-effort funding practices are the status quo. Support costs, 15 percent, are needed to maintain the required infrastructure, such as manufacturing facilities, and other supporting efforts, such as the deep-space communication networks for future missions. Development, 33 percent, funds the design and testing of next-generation systems. Major systems now being developed are the commercial cargo and crew systems, the Orion spacecraft, the Block 1 SLS, and their associated ground systems. When completed, the latter three systems will provide the basic transportation systems and infrastructure needed for some missions in cislunar space. The Orion capsule as currently planned limits mission duration to about 21 days for a crew of four. To proceed with longer-duration missions beyond LEO, substantial development will be required in longer-duration in-space systems.
Projecting beyond the current budget horizon provides insight into the availability of funds for future development and flight operations. For this study, a few key assumptions were made to facilitate understanding of potential future scenarios. Human spaceflight budgets through FY 2018 are assumed to be those specified in the NASA FY 2014 presidential budget request. For budgets beyond FY 2018, this analysis assumes a lower bound of a flat budget for human spaceflight beyond FY 2018 (with no increase for inflation) and an upper bound of a human spaceflight budget increasing with inflation, projected to be approximately 2.5 percent per year in NASA’s 2013 new start inflation index.84Figure 4.29 is a projection of the current human spaceflight program of record relative to these upper and lower bounds (see also Box 4.1). Budget uncertainty is indicated with a solid black line to represent the lower bound (a flat budget) and a dashed line to represent the upper-bound budget (increasing with inflation). The projection includes funding for ISS operations to its previous baseline decommissioning date of 2020 and a possible extension of operations through the current engineering estimated limit of 2028.85 Also included is funding for two performance upgrades of the SLS (the latter occurring in the late 2030s), which would increase payload capacity to LEO to 130 MT. The light and dark turquoise areas under the upper and lower budget bounds indicate funds available for new projects beyond LEO.
Using the budget uncertainty projection bounded by flat and inflation-adjusted growth, the total available funding for development and operations for new projects of potential future human spaceflight systems and pathways can be projected. Figure 4.30 illustrates the cumulative funds available from FY 2015 to FY 2065 for new development and flight operations beyond that required for fixed infrastructure, ISS operations, and development of Orion and the 130-MT SLS. Previous studies have provided a wide range of estimates for the costs of developing and operating the critical elements for a Mars surface mission, most of which are in the hundreds of billions of dollars.86,87 The four available funding cases shown in Figure 4.30 include those with (turquoise) and without (purple) an extension of ISS operations from 2020 to 2028 for both a flat budget projection (solid) and a budget increasing with inflation (dashed) at a projected annual rate of 2.5 percent. Two observations can be drawn from this figure:
- The lower bound of the budget uncertainty, or flat human spaceflight budget projected from 2018 guidance in then-year dollars (not adjusted for inflation), accumulates a total of less than $100 billion in then-year dollars that can be used to develop, build, and operate the new critical elements—such as in-space habitats, new in-space propulsion systems, and landers—that will be required to venture and explore beyond cislunar space. Although the exact effects of the fixed support and infrastructure costs are unknown and must be evaluated in more detail, the combination of a flat budget for the extended future and the prerequisite large infrastructure costs associated with the current NASA plan would prevent NASA from proceeding very far down any human exploration pathway to Mars.
84 NASA, “2013 NASA New Start Inflation Index for FY14,” http://www.nasa.gov/sites/default/files/files/2013_NNSI_FY14(1).xlsx, accessed March 11, 2014.
85 In January 2014, the administration expressed a commitment to continue to operate the ISS to 2024. At the time of writing of the present report, the international partners and Congress had not committed to operations beyond 2020. In this chapter, the analysis considers the bounding cases of 2020 and 2028.
86 M. Reichert and W. Seboldt, “What Does the First Manned Mars Mission Cost? Scientific, Technical and Economic Issues of a Manned Mars Mission,” DLR–German Aerospace Research Establishment. Cologne, Germany, presented at the 48th International Astronautical Congress, October 6-10, 1997, Turin, Italy, IAA-97-IAA.3.1.06.
87 M. Humboldt, Jr., and K. Cyr. “Lunar Mars Cost Review with NASA Administrator, NASA Exploration Initiative,” October 31, 1989.
FIGURE 4.29 Projected available budget and costs of the currently planned human spaceflight program.
The figures in this report that show notional projections of annual costs and available funding for human spaceflight as a function of time are commonly referred to as sand charts. The sand chart that shows projected available budget and costs of the currently planned human spaceflight program (Figure 4.29) was derived as follows: Near-term costs are based on the NASA FY 2014 presidential budget request,a which projects budget requests through FY 2018. For years after 2018, the costs of operations, support, and research projects are held at their proposed 2018 funding levels adjusted for inflation (using the NASA new start index). The exceptions to this are ISS costs, which are held at proposed FY 2018 funding levels until the year of termination and then followed by 2 years of ramping down budgets to cover termination-related costs. By 2018, SLS and Commercial Crew will have reached their initial operation capability (IOC) leaving the Orion spacecraft as the only unfinished currently funded major development project. Orion’s costs after 2018 are ramped down until a steady state is reached in the 2022 timeframe corresponding to the predicted IOC. Planned upgrades of the SLS would increase its capability beyond that provided at IOC and are included in the projected SLS fixed costs.
Projections of the annual budget available for human spaceflight in all the sand charts are shown for two scenarios: a flat budget at FY 2015 levels and a budget that increases with inflation.
a NASA FY 2014 President’s Budget Request Summary, http://www.nasa.gov/pdf/740512main_FY2014%20CJ%20for%20Online.pdf, retrieved January 24, 2014.
FIGURE 4.30 Projected cumulative then-year dollars available for new projects for human spaceflight beyond LEO, with inset showing detail through 2030.
- For any given cost estimate for a pathway to a Mars surface mission, Figure 4.30 indicates when, for a budget increasing with inflation, a landing could be achieved. For example, a $400 billion cost for a pathway to Mars cannot be achieved before roughly 2060. An alternative way to look at this information is that to achieve a landing before 2050 and still be affordable, the pathway to Mars would have to be technically feasible and cost less than about $220 billion.
The scale of the government investment required to send humans to Mars is, to a rough order of magnitude, equivalent to:
- The cost of perhaps 75-150 “flagship class” robotic exploration spacecraft (assuming an average cost of $1 to $2 billion each).
- Twice as much as the National Science Foundation budget over the corresponding period.
- Two to four times the U.S. investment in the ISS, which amounted to roughly $150 billion, including launch costs.
Barring unforeseen changes such as significant budget increases, substantial increases in operational efficiency, or technological game changers, progress toward deep-space destinations will be measured on time scales of decades. Policy goals that state shorter time horizons cannot change this reality. Thus, it would be well to note that 20 years before Apollo 11 landed on the Moon, it would have been difficult to anticipate the technological progress in many area—from computing to guided missile technology—that would enable Apollo. Many, perhaps most, of those technology advances resulted from research outside NASA. The architectures imagined for travel to the Moon by visionaries of the late 1940s bore little resemblance to the path ultimately followed. Thus, the committee treads cautiously in noting the difficulties associated with human space exploration beyond LEO, based primarily on the reference architectures developed by NASA.
188.8.131.52 Pathway Cost Range Methodology
Now that the range of available resources, or budget, for human spaceflight has been established, affordability can be assessed. The first step is to determine the likely cost range of the three representative pathways. Projecting the cost of the development, production, and operations of the exploration elements required to land humans on Mars over the next several decades involved a high degree of uncertainty, so all cost assessments in this report are notional.
New developments and operations beyond those currently funded are projected using a combination of historical analogies (such as the space shuttle and Apollo) and results of previous exploration studies (such as the Space Exploration Initiative88 and Mars DRM 389). The use of historical analogies is preferred, previous studies are used when there are few or no valid historical analogies. These sources lead to cost projections that align with NASA’s traditional ways of doing business in human spaceflight and assume no significant international cost savings. New ways of doing business are not assumed except where explicitly noted (such as for the Commercial Crew and Cargo program). The funding needed to advance required technologies to the point where project development can begin is not included, because it is assumed that technology will be advanced to sufficient levels through technology incubation programs, such as NASA’s ETDP and Exploration Research and Development program, which are continued at their proposed FY 2018 funding levels (adjusted for inflation). Notional cost projections are generated for development of new systems and for their production and operation. Development costs are adjusted to account for potential heritage from previous developments. For example, the projected cost of a deep-space habitat to support a Mars Moons DRM is reduced if a deep-space habitat was developed for the Asteroid in Native Orbit DRM. Projected development costs are spread over development times (nominally 8-10 years) using a beta curve distribution. Cost for production and operations are split into fixed and variable costs for systems that are sustained over extended periods, and the variable costs associated with each mission are spread over 2- to 3-year procurement periods.
Many variables affect the mission rates and development schedules for the various DRMs, pathways, and affordability scenarios. In all cases, the missions are scheduled using an optimistic assumption of no catastrophic development or mission failures. The crewed mission timelines assumed full operational capability in the first flights. The missions are planned to minimize the time before the first human mission to the Mars surface subject to available budget or imposed restrictions on mission rate. Each new DRM in a pathway requires a substantial amount of development time and funding and is phased to meet the budgetary guidelines of the specific scenario being considered. Specific mission operations are placed between the peaks in development funding, and there is an attempt to add as many missions as possible given the budget available in each scenario. The projected costs
88 NASA, Report of the 90-day Study on Human Exploration of the Moon and Mars, November 1989, available in NASA Historical Reference Collection, History Office, Washington, D.C., http://history.nasa.gov/90_day_study.pdf.
89 NASA, “Reference Mission Version 3.0 Addendum to the Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team,” NASA/SP-6107-ADD, 1998, http://ston.jsc.nasa.gov/collections/trs/_techrep/SP-6107-ADD.pdf.
of predeployed cargo missions (such as prepositioning surface outposts) are included, but precursor technology demonstrators are not, under the previously noted assumption that those costs are wrapped into either the development costs or the technology incubation programs. Human missions to Mars are constrained by orbital mechanics to occur only every 2 years or so, depending on mission type. In the actual figures, the costs for each DRM for a particular pathway are time-phased and stacked in order above the projections of fixed infrastructure costs (gray area) to create the sand chart that indicates the annual cost projections until the first human landing on Mars. The SLS and Orion marginal mission costs for both crew and cargo are included with the specific DRM; their fixed infrastructure and upgrade costs are held in the gray area because they support all the DRMs.
Uncertainty in the projection of funding requirements is evident in all cost modeling. Trying to project the costs of systems that will not be designed for decades adds to the uncertainty. As already noted, many technological advances will be required just to initiate the design of some mission elements. For each mission element, a range of possible costs was generated to account for various levels of margin added to the historically derived numbers as well as variable growth rates in fixed infrastructure costs. The margin accounts for incomplete knowledge of the system requirements; low to minimal design maturity; uncertainties about the ability to scale up the capabilities of current technologies, such as SEP, by orders of magnitude; and historical cost growth of NASA projects. In general, 50 percent margins were used for developmental efforts, and 25 percent margins for production and operations.90 Although these margins are reasonably consistent with historical patterns at NASA, averaged over many programs, some programs experience cost overruns that greatly exceed historical averages. In addition, the degree of challenge for some of the technologies needed for travel to Mars is much greater than that faced by NASA in recent decades. Thus, if anything, the cost projections generated for this study are optimistic. This effort is intended both to produce a notional cost range for each pathway so that this range can be compared with future funding levels and to evaluate affordability and sustainability in terms of flight frequency and the required time span to land humans on Mars. More detailed cost analysis that could be used for budget planning will have to be carried out by NASA and other independent groups once decisions are made to pursue particular goals.
184.108.40.206 Cost Implications of International Collaboration
An additional uncertainty to consider is the effect of international collaboration. It is generally accepted that landing humans on Mars will require substantial international cooperation for many reasons. With respect to affordability, it is unclear how the goals of the various spacefaring nations will translate to budget commitments and how potential hardware contributions will effectively and efficiently contribute to overall pathway design. The ISS would not exist today without strong international cooperation and participation. However, only 17 percent of the total ISS cost has been covered by non-U.S. stakeholders.91 The ISS is also an example of the typical increases in complexity and inefficiency associated with multinational endeavors. In fact, independent cost models often add an international complexity factor that increases projected costs for international programs. Considering increased complexity and likely additional cost elements to satisfy specific and unique political goals of an international partner, international partnerships with cost-sharing on the scale required for human spaceflight tend to be cost-neutral relative to the cost of the program without international partners. If large enough, international contributions could overcome the cost increases associated with management complexity, but that was not the case with the ISS. To make the Mars pathways affordable to the United States, international contributions would need to be large enough to cover the costs of increased complexity and additional cost elements introduced by the partnership with enough excess to cover the gap between the projected human spaceflight budget and the projected costs of each pathway, which are described in the sections that follow.
International partnerships can be highly effective when interfaces are clearly defined and each partner has specific responsibilities for providing one or more particular mission elements. However, the amounts and types of contributions from partner nations for exploration efforts beyond LEO are unknown, and potential international
90 D.L. Emmons, M. Lobbia, T. Radcliffe, and R.E. Bitten, Affordability Assessments to Support Strategic Planning and Decisions at NASA, Proceedings of IEEE Aerospace Conference, 2010.
91 Claude LaFleur, Costs of U.S. piloted programs, The Space Review, March 8, 2010.
partners with the technical capability and budget commitment may also have views that differ from those of the United States when it comes to selecting a pathway to Mars. A variety of possible international contributions—such as cost support, innovation, and stability—are outlined in the Global Exploration Roadmap,92 and those contributions may be important. Nonetheless, NASA is still likely to carry most of the financial burden, especially if the United States wants to have a strong influence on pathway and technology direction.
The pathway cost profiles in this report do not reflect the value of international contributions to human space exploration, nor do they include any increase in costs associated with the international complexity factor.
220.127.116.11 Schedule-Driven Affordability Scenarios
To understand the more detailed aspects of the three representative pathways, it is helpful first to determine what an unconstrained, NASA-only program might look like if it is desirable to land on Mars as early as possible. The three previously described representative pathways have been modeled using optimistic development times and flight rates with minimal gaps between major pathway developments. In the history of U.S. human spaceflight, the longest flight gap was the transition from the Apollo to the Space Shuttle Program. That gap was initially expected to be 4 years, but it stretched to 6 years because of unplanned delays in space shuttle development. The Apollo Program launched 11 crewed missions at an average of one launch every 4 months. The Space Shuttle Program sustained an average flight rate of one launch every 3 months over a span of 3 decades. The time between shuttle flights ranged from 17 days to 32 months (after the loss of Challenger). The time between the first two shuttle flights was 7 months. In contrast, the planned time between the first two SLS launches is 4 years.
In some cases, in particular the Moon-to-Mars and Enhanced Exploration pathways with an ISS retirement date of 2020, the flight rates modeled in this scenario are representative of sustainable, historical flight rates. However, as noted above, an optimistic bias with respect to program margin is included in all the affordability analyses. In addition, it is assumed that each developmental system is available when it is needed for operations and that there are no mission failures (with associated inquiries and program delays, as happened for Apollo 1 and in the loss of Challenger and Columbia). Also ignored are programmatically necessary robotic precursor missions. For example, the SLS will be launched just once with no crew before the first human flight (EM-2). One can scarcely imagine that a Mars EDL system will not have been extensively tested robotically in the atmospheres of Earth and Mars before a human crew depends on it for survival. Thus, for all affordability analyses shown below, estimated dates of first landing on Mars are highly optimistic.
As a representative example, the cost profiles for each of the stepping-stone DRMs of the Enhanced Exploration pathway are presented in Figure 4.31a. This figure breaks out the cost profiles associated with the development and operation of the payloads for the individual DRMs, as well as basic infrastructure and support. The costs shown are representative of medium point cost projections of new systems and mission operations (with margins based on historical cost growth) on top of the baseline costs. Figure 4.31a presents the total human spaceflight then-year annual budget required where the fixed infrastructure costs and the development costs for Orion and SLS have been grayed out and the required pathway developmental and operational costs (including marginal SLS and Orion costs) for each DRM are shown in the colors indicated. The extension of the ISS to 2028 is also shown. Because of the notional nature of the cost projections in this study, the vertical cost axes in Figure 4.31a and similar figures are not marked with dollar values. Even so, the committee is confident that the cost projections that are summarized in these figures provide a sound basis for making relative comparisons among the pathways and between the pathways and budget projections.
Figure 4.31a shows that a large increase in the human spaceflight budget would be required to use the Enhanced Exploration pathway to land on Mars prior to 2040. The schedule-driven results for all three pathways with and without the ISS extension to 2028 are shown in Figure 4.31b. The total annual cost to NASA for the Enhanced Exploration pathway and the extension of the ISS to 2028 (Figure 4.31a) is represented as a thick purple line and is compared with the other pathway schedule-driven scenarios. In these schedule-driven scenarios, the estimated dates of the Mars surface landing are driven primarily by technology development timelines and the associated
92 ISECG, The Global Exploration Roadmap, 2013.
FIGURE 4.31 (a) Schedule-driven cost profile of the Enhanced Exploration Pathway in then-year dollars. (b) Schedule-driven cost profile comparison in then-year dollars of three representative pathways with and without the ISS extended to 2028. (c) Schedule-driven crewed mission timeline assumptions for three representative pathways with and without the ISS extended to 2028.
increase in funding to achieve the assumed dates. Figure 4.31c shows the planned crewed launch dates for each scenario with lunar surface missions occurring twice a year, cislunar and asteroid missions occurring once a year, and Mars-based missions occurring every 26 months because of planetary launch constraints. In all those cases, the final destination of the pathway is reached in the 2030s, but, as seen in Figure 4.31b, the projected costs for all three pathways extend well above the projected human spaceflight budgets and are not considered affordable. To fund such programs fully, the human spaceflight budget would have to increase at a rate 2-4 times that of inflation for the next 15 years, depending on the specific pathway and the status of the ISS.
The schedules shown here are considered optimistic in that they assume a fully funded and success-oriented program, and a landing on Mars may be delayed, depending on the successful demonstration of the required technologies.
18.104.22.168 Budget-Driven Affordability Scenarios
Budget-driven affordability scenarios are based on the assumption that the three representative pathways to Mars are constrained by the human spaceflight budget increasing with inflation. The lower bound of the budget uncertainty, or flat budget, was not considered, because this condition cannot sustain any pathway to land humans on Mars. To achieve pathway cost scenarios constrained to trend with the human spaceflight budget increasing with inflation, the year in which humans first land on Mars must slip to the right, flight gaps between DRMs must increase, and crewed flight rates will have to be lowered to below historical rates. Figure 4.32a indicates that each DRM of the Enhanced Exploration pathway can be delayed until sufficient budget is available to proceed after the extension of the ISS to 2028 and that a Mars landing would occur in about 2054. However, to achieve this budget-constrained scenario, the number of crewed flights must be greatly reduced and the mission rate lowered to just two crewed flights per 5-year period, as shown in Figure 4.32c.
Figures 4.32b and 4.32c compare the three representative pathways with respect to the budget-driven scenarios. With respect to timeline, as expected the ARM-to-Mars pathway potentially could lead to a human mission to the Mars surface as early as 2037-2046, depending on the ISS extension. The Moon-to-Mars pathway could yield a landing between 2043 and 2050. All of these landing dates are likely optimistic in that delays will inevitably occur as developmental challenges and potential failures delay the specific pathway schedule. Such issues will be exacerbated by tight budget constraints and the limited ability of project managers to react to unexpected issues and concerns. These budget-constrained scenarios result in unrealistic and unsustainable mission rates well below any historical precedent. For the ARM-to-Mars pathway, there are only five crewed missions in the 18 years between ISS retirement in 2028 and landing on Mars. For the Moon–to-Mars pathway, there are only six crewed missions to the Moon for both the Lunar Sortie and Lunar Outpost DRMs with planned periods of up to 7 years with no flights. Only once missions to the vicinity of Mars are undertaken do the flight rates for SLS increase substantially. For each of the pathways, the number of SLS launches in any year prior to 2030 varies from zero to four. Conversely, missions to Mars (either the moons of Mars or the Mars surface) will require salvos of SLS launches for each mission. (In both cases, some of the launches will take place in the launch opportunity prior to the launch of the crew to preposition resources for their use and sustainment.) Even when SLS launch rates are in a sustainable range,93 the mission rates in the budget-driven scenario are much lower than in previous experience in the U.S. human spaceflight program.
The serious potential programmatic and operational risks attendant on the low operational tempo of the budget-driven scenarios led the committee to consider augmentation of the mission rate beyond what is strictly necessary to effect each of the pathways, based primarily on the professional experience of the Technical Panel and those members of the committee experienced in spaceflight programs and associated engineering. This is discussed in the following section.
93 According to NASA leadership in a recent presentation to the NASA Advisory Council’s Human Exploration and Operations Committee, that rate is once a year.
FIGURE 4.32 (a) Budget-driven cost profile of the Enhanced Exploration Pathway in then-year dollars. (b) Budget-driven cost profile comparison in then-year dollars of three representative pathways with and without the ISS extended to 2028. (c) Budget-driven crewed mission timeline assumptions for three representative pathways with and without the ISS extended to 2028.
22.214.171.124 Operationally Viable Scenarios
Examination of the schedule-driven and budget-driven affordability scenarios for each pathway indicates, independently of the ISS extension, that the pathways using historical mission rates are not affordable, and affordable pathways based on a human spaceflight budget increasing with inflation are not sustainable. For each pathway, the purpose of this section is to illustrate optimistic but potentially sustainable mission rates with the minimum budget possible. The average time between crewed missions during the period from ISS retirement until the first Mars mission would be 19-28 months, depending on the combination of pathway and ISS retirement date. Some gaps in crewed missions would occur between major mission operations to allow hardware development and predeployment of mission assets. Additionally, robotic missions to test some key systems, such as Mars EDL, would need to be completed before crews could be committed to a Mars surface mission.
Figures 4.33a-c indicate the budget required for the three representative pathways to achieve an operationally viable mission rate. Assuming that the ISS is extended to 2028 and that the human spaceflight budget is increased by 5 percent per year (twice the rate of inflation), the earliest that a crewed surface mission to Mars is likely to occur is about 2040-2050. Again, these dates are probably optimistic in that delays would inevitably occur as developmental challenges and failures require design modifications and schedule delays. If the exploration budget grows at 5 percent per year, the benefit of terminating the ISS in 2020 is not that great from an affordability perspective, in that a human landing on Mars may be advanced by just 2-4 years, depending on the pathway and the associated risk.
Many possible pathways can be conceived, even given the relative paucity of feasible destinations for humans, because of the combinatoric complexity of ordering destinations and because of the large number of DRMs that have accumulated for each of them. However, it is possible to articulate desirable properties of pathways that can guide choices for the nation’s human spaceflight program. Six such desirable properties follow:
- The horizon and intermediate destinations have profound scientific, cultural, economic, inspirational, and/or geopolitical benefits that justify public investment.
- The sequence of missions and destinations permits stakeholders, including taxpayers, to see progress and develop confidence in NASA’s ability to execute the pathway.
- The pathway is characterized by logical feed-forward of technical capabilities.
- The pathway minimizes the use of dead-end mission elements that do not contribute to later destinations on the pathway.
- The pathway is affordable without incurring unacceptable development risk.
- The pathway supports, in the context of available budget, an operational tempo that ensures retention of critical technical capability, proficiency of operators, and effective use of infrastructure.
The committee is not recommending any particular pathway, but the pathways outlined above are assessed in the following sections in terms of these desirable properties.
126.96.36.199 Significance of the Pathway Destinations
Any of the pathways, if executed to completion, would rate highly in terms of the significance of the pathway destination because they all include a human mission to the surface of Mars. This is the most challenging destination for human exploration in the context of foreseeable technology. In addition, Mars has high scientific value in the context of the evolution of terrestrial planets and the possible origins of life.
The Moon-to-Mars and Enhanced Exploration pathways arguably rate higher on this property because they include the Moon and near-Earth asteroids large enough to be scientifically interesting. Although some have dismissed the Moon as no longer interesting because humans have visited it before, this is similar to considering
FIGURE 4.33 (a) Operationally viable cost profile of Enhanced Exploration Pathway in then-year dollars. (b) Operationally viable cost profile comparison in then-year dollars of three representative pathways with and without the ISS extended to 2028. (c) Operationally viable crewed mission timeline assumptions for three representative pathways with and without the ISS extended to 2028.
the New World to have been adequately explored after the first four voyages of Columbus, whereas the continued exploration and exploitation of the New World had profound cultural, economic, and geopolitical impact on the Old World. The New World in Columbus’s time was discovered to be verdant and populated. Clearly, neither adjective is relevant to the Moon, but some of the most significant effects of the New World on the Old were as a result of the systematic extraction of mineral wealth, principally in the form of gold, which had been mined by the indigenous peoples and later by European colonists. That mineral wealth was not apparent after the initial reconnaissance of the New World by Columbus. It was discovered later during more complete exploration. Similarly, as a result of further exploration (in this case, robotic exploration) of the Moon, it is thought that the Moon probably contains “mineral wealth” worth much more than its weight in gold in the form of water in permanently shadowed craters. That water could be a critically enabling resource for human exploration and perhaps a space-based economy as a source of oxygen, fuel, and potable water that would not require expensive transport from the bottom of Earth’s gravity well. The characterization of near-Earth asteroids, although almost certainly most cost-effectively explored robotically, is also important because of the implications for planetary defense and perhaps ultimately their potential for exploitation in a space-based economy.
188.8.131.52 Sequence Shows Progress with Intermediate Destinations
As has been noted earlier, human spaceflight beyond LEO would take place over the course of many decades. Given current budget realities, it is tempting to suggest that the “best” pathway to Mars is the shortest and least expensive, that is, the ARM-to-Mars pathway. However, even with the decommissioning of the ISS in 2020, a budget-constrained schedule means that the earliest conceivable human presence in the vicinity of Mars would occur in the late 2030s, assuming that all technological challenges are met without setbacks. Until then, the only human spaceflight missions would be trips to the ISS (until 2020) and voyages in cislunar space without landings on any naturally occurring celestial body. Thus, without a considerable increase in human spaceflight funding for NASA, the ARM-to-Mars pathway presents the prospect of a long period of technology development during which NASA’s stakeholders do not see human exploration missions taking place. This problem poses one of the most serious challenges to program sustainability that the study’s Technical Panel identified.
ARM has failed to engender substantial enthusiasm either in Congress or in the scientific community. Support for such voyages and the extensive and expensive technology development needed to take the next step in this pathway (to the moons of Mars) may not be sustainable in the context of such distant goals.
In contrast, the Moon-to-Mars and Enhanced Exploration pathways would allow Congress and the public at large to see an expanding horizon of human activity with intermediate milestones that are distinctly different. Such pathways may be more sustainable, even though they would cost more than the ARM-to-Mars pathway.
184.108.40.206 Logical Technological Feed-Forward
The Moon-to-Mars and Enhanced Exploration pathways have a relatively steady pace of enabling system development. They lack the “cliff” present in Figure 4.7 for the ARM-to-Mars pathway, in which only a few primary mission elements are developed in the near term to practice human spaceflight in cislunar space, and more than half of the total number of primary mission elements must be developed to take the last step from the moons of Mars to the surface of Mars. This need for such a large increment of capability poses a very high development risk. One of the best motivators for successful technology development is to put that technology development into a program that requires it in a specific time frame. This tends to prevent technology development from falling into a mode of self-perpetuation that may ultimately have little or no value. As noted earlier, NTP was developed over a period of almost 20 years with multiple variants. Although significant progress was made technologically, the lack of a program that defined actual requirements ultimately doomed these efforts, and all the resources dedicated to the effort led to essentially no payoff.
220.127.116.11 Minimizing Dead-End Mission Elements
The ARM-to-Mars pathway has four dead-end mission elements: the asteroid capture vehicle and three elements needed to support human crews for long periods on or near the moons of Mars. The Enhanced Exploration pathway also has four dead-end mission elements: the three needed for the Mars Moons mission and one (a large storage stage) for missions to an asteroid in its native orbit. The Moon-to-Mars pathway has only a single dead-end mission element, the disposable descent stage for lunar sorties. This analysis shows how one could improve pathway affordability. Deleting the moons of Mars from the Enhanced Exploration pathway would substantially reduce the number of dead-end mission elements while still allowing exploration of an asteroid in its native orbit. Such tradeoffs must take place in the context of the value of the Mars Moons missions in a program of human exploration and the effect that eliminating these missions would have on other pathway characteristics, such as program development risk. Of course, the nation is unlikely to adopt any of the pathways exactly as they are presented in this report. The goals of “minimizing dead-end mission elements” are to maximize the logical feed-forward of systems and to make the best use of constrained resources. Only in the context of a government-consensus pathway can one ultimately determine whether a given mission element is a dead end.
18.104.22.168 Affordability and Development Risk
Table 4.4 summarizes the affordability, development risk, and operational tempo for every combination of pathway and affordability scenarios examined in this report. The table also shows the number of crewed flights (including the first Mars landing) and the earliest year possible for the Mars landing for both ISS scenarios, that is, ISS decommissioning in either 2020 or 2028.
The total number of crewed flights and the earliest possible year for a human mission to the Mars surface vary widely for the various combinations of pathways and affordability scenarios. The ARM-to-Mars pathway generally has the fewest crewed flight missions (no more than nine). It may be difficult to engage the public’s interest with so few missions, but if it is able to overcome the exceedingly high development risk, this pathway would yield the earliest possible landing on Mars. The Enhanced Exploration pathway tends to have the largest number of crewed flights, and it includes missions to the largest number of destinations. It would probably enhance public interest, but it tends to delay the first human mission to the Mars surface relative to the other pathways.
Affordability and operational tempo are determined by the choice of affordability scenario. The schedule-driven scenarios are not affordable, because the cost of executing them far exceeds any feasible increases in the NASA human spaceflight budget. (These scenarios would require the human spaceflight budget to increase at 4 times the rate of inflation for at least 15 years.) The budget-driven scenarios are the most affordable, but they have unacceptably low operational tempos. The operationally viable scenarios are all marginal both in affordability (they would require the human spaceflight budget to increase at twice the rate of inflation) and in operational tempo (the mission rate would still be well below historical precedents, although not as low as in the budget-driven scenarios).
The development risk is determined by the choice of pathway. As discussed above in section 4.2.5 (“Contribution of Key Mission Elements to the Pathways”), the ARM-to-Mars pathway has three destinations, of which the first two (the ARM mission and Mars Moons mission) develop just 5 of the 11 primary missions elements needed for a mission to the Mars surface (see Figure 4.7). In addition, missions to the first two destinations include minimal time at the destinations (about 10 days for the ARM mission and about 60 days for the Mars Moons mission), and neither would provide any data on human health or system performance in an environment that has substantial partial gravity. During the Mars surface mission, however, astronauts would be on the surface for approximately 500 days. A tremendous development effort and major advances in capabilities would be needed for the subsequent Mars surface mission. Thus, the ARM-to-Mars pathway has exceedingly high developmental risk.
The Moon-to-Mars pathway develops 7 of the 11 primary mission elements during lunar missions, which have greater mission-abort capability than missions beyond cislunar space. Compared to the ARM-to-Mars pathway, the Moon-to-Mars pathway features a smoother progression of developing mission elements required for the Mars surface mission. In addition, several transitional mission elements are developed during the lunar missions, which would further reduce development risk for the Mars surface mission. Even so, it would be a big step to go from lunar missions to a Mars surface mission, and the Moon-to-Mars pathway carries very high development risk.
TABLE 4.4 Summary of Pathway Affordability, Development Risk, and Operational Viability
|ARM TO MARS||MOON TO MARS||ENHANCED EXPLORATION|
|TOTAL # CREWED FLIGHTS||ISS 2020||ISS 2028||ISS 2020||ISS 2028||ISS 2020||ISS 2028|
|Schedule driven:||9||9||Schedule driven:||17||17||Schedule driven:||20||20|
|Budget driven:||7||9||Budget driven:||7||7||Budget driven:||11||14|
|Operationally viable:||9||9||Operationally viable:||9||8||Operationally viable:||14||14|
|EARLIEST POSSIBLE YEAR MARS LANDING||ISS 2020||ISS 2028||ISS 2020||ISS 2028||ISS 2020||ISS 2028|
|Schedule driven:||2033||2033||Schedule driven:||2035||2035||Schedule driven:||2039||2039|
|Budget driven:||2037||2046||Budget driven:||2043||2050||Budget driven:||2050||2054|
|Operationally viable:||2037||2041||Operationally viable:||2041||2043||Operationally viable:||2048||2050|
|OPERATIONAL TEMPO SATISFACTORY||Schedule driven:||NO||Schedule driven:||YES||Schedule driven:||YES|
|Budget driven:||NO||Budget driven:||NO||Budget driven:||NO|
|Operationally viable:||MARGINAL||Operationally viable:||MARGINAL||Operationally viable:||MARGINAL|
||Schedule driven:||NO||Schedule driven:||NO||Schedule driven:||NO|
|Budget driven:||YES||Budget driven:||YES||Budget driven:||YES|
|Operationally viable:||MARGINAL||Operationally viable:||MARGINAL||Operationally viable:||MARGINAL|
|DEVELOPMENT RISK||Schedule driven:||EXCEEDINGLY HIGH||Schedule driven:||VERY HIGH||Schedule driven:||HIGH|
|Budget driven:||EXCEEDINGLY HIGH||Budget driven:||VERY HIGH||Budget driven:||HIGH|
|Operationally viable:||EXCEEDINGLY||Operationally viable:||VERY HIGH||Operationally viable:||HIGH|
a In the “Affordable” column, “Yes” means that the scenario can be executed with budget increases equal to inflation, “Marginal” means that the scenario requires human spaceflight budgets to increase at twice the rate of inflation (that is, about 5 percent per year), and “No” means that the human spaceflight budget would need to increase at 4 times the rate of inflation (that is, about 10 percent per year).
The Enhanced Exploration pathway shows a long incremental growth of capability (see Figure 4.7) while avoiding multiple concurrent developments of major mission elements. Transitional elements are used to advance technology gaps. This incremental growth in capabilities implies that the Enhanced Exploration pathway has lower development risk than either the ARM-to-Mars or Moon-to-Mars pathway. However, given the level of technological advances required to develop the 11 primary mission elements, supporting systems, and associated capabilities, the Enhanced Exploration pathway still has high development risk.
22.214.171.124 Operational Tempo
Operational tempo is assessed by examining crewed mission rates and SLS flight rates in the context of historical norms for successful programs. The first two SLS flights are scheduled for 2017 and 2021. Table 4.5 shows average time between SLS launches from 2022 until the first launch of an SLS for a human mission to the moons of Mars or the Mars surface. Once Mars missions are under way, the timing of launches will be driven by the synodic period of Mars, with launch windows opening an average of once every 26 months.
The rate of crewed launches will be driven by the ISS until its retirement. Table 4.5 shows the average time between crewed missions from the retirement of the ISS until the first launch of a crewed mission to the moons of Mars or the Mars surface. As with SLS launches, once Mars missions begin, the timing of launches would be driven by the synodic period of Mars.
TABLE 4.5 Operational Tempo: SLS Launches and Crewed Missions
|Year of ISS Retirement||2020||2028||2020||2028||2020||2028|
|Average time between SLS flights from 2022 until first launch for Mars Moons or Mars surface mission (months)|
|• Schedule-driven scenario||16||16||4||4||3||3|
|• Budget-driven scenario||24||32||9||12||9||8|
|• Operationally viable scenario||18||29||6||7||5||5|
|Average time between crewed missions from ISS retirement through first Mars moons or Mars surface mission (months)|
|• Schedule-driven scenario||17||12||11||7||11||7|
|• Budget-driven scenario||30||40||46||44||33||29|
|• Operationally viable scenario||16||18||28||23||22||19|
After the ISS is retired, crewed mission rates would be equal to or lower than SLS flight rates for the same period, so crewed mission rates are the key factor in assessing operational tempo. The lower the operational tempo, the more difficult it is to retain critical technical capabilities, to retain operator proficiency, and to use personnel and infrastructure effectively. Action needed to overcome these risks would tend to increase costs. As the disparity between historical and proposed launch rates increases, past experience becomes less relevant as a basis for planning future programs, and at some point the increased cost and risk make pathways unsustainable. Accordingly, it is unrealistic to assume that very low SLS flight rates or crewed mission rates are compatible with an exploration pathway that would span decades.
The operational tempo of each of the pathways depends on whether the pathway is executed in a schedule-driven, budget-driven, or operationally viable scenario and on the retirement date of the ISS. Because of budget constraints, all the pathways in the budget-driven scenario present mission rates much lower than previous successful U.S. programs, which is a significant program risk. Operational tempo in the schedule-driven scenario is in general excellent except for the ARM-to-Mars pathway, where the technology development “cliff” imposes a long delay in advancing from the ARM mission to the Mars Moons mission. The operationally viable scenario constitutes a marginal case in that the crewed mission rate is lower than previous U.S. experience, but it might be achievable.
This section describes ongoing efforts to develop new technologies and capabilities to support future human spaceflight endeavors. The discussion is a summary of key exploration technology programs being conducted by NASA, industry, DOD, and foreign governments. This is not intended to be a rigorous survey or assessment, but it does indicate the breadth of potentially relevant programs and the magnitude of investment, in NASA and elsewhere, that is available to address human spaceflight challenges.
The new capabilities that enable future human spaceflight will be drawn from a wide variety of sources. Foremost among them are the technology programs managed by NASA that respond directly to the needs of NASA’s human spaceflight program. These technology programs bring to bear the results of NASA-funded research to inform related engineering efforts. Similarly, the mission requirements should guide researchers in focusing on mission-enabling capabilities. Important new capabilities may also be derived from commercial activities that are pursuing analogous or related goals for their own purposes, including traditional aerospace activities, such as launch vehicle or satellite development, as well as nascent industries, such as space tourism. Other government agencies, principally DOD, also produce new technologies that can be leveraged into the human spaceflight program. Finally, many activities in non-U.S. space programs are producing capabilities of interest to NASA, and international partnerships would allow the United States to draw on these capabilities.
NASA’s development of new and advanced exploration systems and technologies is funded by the exploration budget of the Human Exploration and Operations Mission Directorate (HEOMD) and STMD. HEOMD development activities are organized into three theme areas:
- Exploration System Development
- — Orion Multi-Purpose Crew Vehicle (MPCV) Program
- — SLS Program
- Exploration Ground Systems Program
- — Exploration Research and Development
- — Human Research Program
- Advanced Exploration Systems Program
- — Commercial Spaceflight
- — Commercial Crew Program
STMD manages one program, the Exploration Technology Development Program, which is wholly dedicated to the development of exploration technology, and several other programs that support the development of technology for exploration as well as other applications.
The status of HEOMD and STMD exploration technology programs is summarized below.
126.96.36.199 Exploration System Development
The SLS heavy-lift launch vehicle, Orion MPCV, and related ground systems are being developed with the goal of restoring the ability of the United States to conduct human spaceflight beyond LEO. The development of these systems is not directed at any particular destination. Rather, they are part of NASA’s effort to enhance human spaceflight capabilities that could support missions to a variety of destinations in anticipation of a future commitment to initiate a flight program to one or more specific destinations.
The first flight of an SLS launch vehicle (EM-1) is scheduled for 2017, with a second flight (EM-2) to follow in 2021.94 Both of these vehicles will have a payload capacity of 70 MT to LEO. Upgrades to the SLS could increase the payload capacity to 130 MT or more. Additional information on the SLS program appears above in section 188.8.131.52.4 (“Heavy-lift launch Vehicles”).
The main components of the Orion MPCV are the launch abort system, the crew module, and the service module. Orion will have a crew capacity of four, and it will serve as the crew vehicle for launch and return to Earth. Its capabilities will include rendezvous, docking, and EVA. Orion’s maximum mission length will depend on the capabilities of the service module, which will provide consumables needed to sustain the crew. The first uncrewed test flight is scheduled to take place in 2014 using a Boeing Delta IV Heavy launch vehicle. The second flight will take place in 2017 as part of the first SLS flight, EM-1. The first crewed flight of the Orion will take place in 2021 on SLS flight EM-2. The service module for the first Orion flight will be provided by NASA. The service module for the second Orion flight (EM-1) will be provided by the European Space Agency (ESA) as part of its cost-sharing obligation to the United States as an ISS partner. The design of the ESA service module will be based on the design of the Automated Transfer Vehicle that has flown to the ISS several times.
The Exploration Ground Systems Program is developing new and refurbished launch site infrastructure to support integration, launch, and recovery operations for the SLS and Orion MPCV flight systems. NASA’s Kennedy Space Center has established a single office to manage the Exploration Ground Systems Program and the 21st Century Ground Systems Program. The latter is intended to improve NASA’s ability to support non-NASA launch customers in government and industry.
94 These dates appeared to be in flux as this report was in its final preparation but are those reported by NASA in January 2014.
184.108.40.206 Exploration Research and Development
NASA’s Exploration Research and Development Program consists of the Human Research Program and the Advanced Exploration Systems Program.
The Human Research Program is concerned with investigating, understanding, preventing, and mitigating risks to human health and performance that are associated with long-duration space missions. Areas of particular interest include exposure to space radiation, emergency medical care in space, psychosocial issues associated with the effects of confinement and isolation, and the effects of microgravity on the human body. This program makes extensive use of the ISS to conduct experiments and collect data on astronaut health in the microgravity environment. The Space Radiation NASA Research Announcement engages the external scientific community to provide better understanding and risk reduction of the space radiation hazard faced by spaceflight crews on exploration missions. This research announcement and associated NASA research are critically aligned to U.S. deep-space exploration objectives.
The Advanced Exploration Systems Program is developing new capabilities and operational concepts to improve safety, reduce risk, and lower the cost of human spaceflight missions beyond LEO. The scope of this program includes crew mobility, habitat systems, vehicle systems, and operations robotic technology for precursor missions. Activities include the demonstration and evaluation of prototype systems during test on the ground and in space on the ISS. The program is also cooperating with the Science Mission Directorate to develop instruments and concepts for robotic precursor missions that would collect data on potential destinations for human exploration missions.
220.127.116.11 Commercial Spaceflight
Anticipating the termination of space shuttle operations in 2011, in 2006 NASA began the process of partnering with industry to foster the development of commercial systems to transport cargo and crew to and from the ISS. The development of commercial cargo launch systems is complete, as evidenced by successful demonstration and operational flights to the ISS by SpaceX and Orbital Sciences Corporation. SpaceX and Orbital are under contract to provide a total of 20 cargo flights to the ISS, and funding for commercial cargo activities has been transferred from exploration to operations in the NASA HEOMD budget. As of April 2014, SpaceX had completed three ISS resupply missions, using its Falcon 9 launch vehicles and Dragon capsules. Orbital completed its first resupply mission in January 2014, using its Antares launch vehicle and Cygnus capsule. NASA is continuing to support the development of commercial crew capabilities (to transport astronauts to the ISS and return them to Earth) with three industry partners: Boeing, SpaceX, and Sierra Nevada. NASA has financial agreements with these companies to provide a total of up to $1.1 billion for all three companies over a 2-year period. Subsequent agreements are expected to result in crewed flights to the ISS starting no earlier than 2017. The main effect of the commercial crew program on NASA is to mitigate the United States’ complete dependence on Russia for crew transport. It is too early to know if this approach will reduce costs. What is certain, however, is that the U.S. industrial base capable of supporting human spaceflight has been stabilized and expanded by the commercial cargo and commercial crew programs. This will most probably provide NASA with additional capabilities in the future. In addition, it may enable the establishment of a space-based economy that is not completely dependent on NASA, much as the commercial imaging and communication satellite industries are no longer dependent on NASA. It is well to note, however, that establishment of a commercial space-based economy with human spaceflight as a major component is highly speculative.
18.104.22.168 Space Technology Mission Directorate
The Exploration Technology Development Program is developing technologies to support human exploration beyond Earth orbit with a focus on advanced technologies that have a long development time. The scope of this program includes advanced technologies for SEP, ECLSS, ISRU, EDL, storage and transfer of cryogenic fluids in space, and robotic systems to improve crew safety and effectiveness, and it consumes about one-third of STMD’s budget.
In addition to the Exploration Technology Development Program, STMD also funds three other programs:
- Crosscutting Space Technology Development Program.
- Small Business Innovation Research/Small Business Technology Transfer Program.
- Partnership Development and Strategic Integration Program.
Each of these three programs supports HEOMD, the Science Mission Directorate, and the Aeronautics Research Mission Directorate.
The Crosscutting Space Technology Development Program develops technology that is broadly applicable to the needs of future NASA science and exploration missions as well as the needs of other federal agencies and industry. Various elements of this program investigate technologies across a wide range of maturity levels, from conceptual studies through flight demonstrations. Areas of interest include advanced manufacturing technologies, nanotechnology, and synthetic biology. This program also conducts technology demonstration missions to validate selected technologies that have successfully completed ground testing. These missions are currently being developed for low-density supersonic decelerators, laser communications, deep-space atomic clocks, and solar sails.
The Small Business Innovation Research/Small Business Technology Transfer Program is intended to increase the participation of small businesses in NASA research and technology development and to facilitate the commercial application of NASA research results. The 2012 solicitation for this program had a broad scope, encompassing topics such as the reduction of airframe noise and drag; the development and use of launch vehicles, in-space propulsion systems, and space habitats; and advanced space telescopes for astrophysics and Earth science.
The Partnership Development and Strategic Integration Program coordinates technology development activities throughout NASA and takes the lead in NASA’s technology transfer and commercialization activities.
22.214.171.124 NASA Infrastructure
The United States had a fairly rudimentary space program when the Apollo program started. As a result, in addition to developing the required technology, the supporting infrastructure for testing, assembly, launch, and so on had to be created ab initio. Some of this infrastructure, such as the Vehicle Assembly Building and the Pad 39 Complex (A and B) at Kennedy Space Center, was repurposed after Apollo. Even so, NASA carries a very large set of aging infrastructure. NASA’s Office of the Inspector General (OIG), the Government Accountability Office, and the National Research Council have each reported on NASA infrastructure and offered worrisome observations.95 NASA’s OIG determined that about 80 percent of NASA facilities are more than 40 years old, that maintenance costs for these facilities amount to more than $24 million a year, and that continuing shortfalls in maintenance are adding to an already substantial backlog in deferred maintenance.96 Carrying such costs constitute latent threats to the development of newer infrastructure that will be needed to support human exploration beyond LEO. For example, some of the infrastructure needed to develop and test fission surface power and NEP systems will undoubtedly be extremely challenging to plan, permit, fund, and construct.
A wide range of corporations are developing advanced launch vehicles, capsules, and space habitats to support traditional customers, such as the commercial satellite industry, NASA, and DOD. However, some of the new initiatives to provide orbital and suborbital flight capabilities are focused on new markets such as space tourism or transportation to a privately developed space station, should one be constructed. Some of the new transportation systems are being developed by aerospace companies, such as Boeing and Orbital Sciences Corporation, which
95 A.D. McNaull, House Space Subcommittee discusses aging NASA infrastructure, FYI: The AIP Bulletin of Science Policy News, Number 145, October 4, 2013.
96 NASA Office of Inspector General, “NASA’s Efforts to Reduce Unneeded Infrastructure and Facilities,” February 12, 2013, http://oig.nasa.gov/audits/reports/FY13/IG-13-008.pdf.
have substantial spaceflight experience. Others are being developed by relatively new ventures such as Bigelow Aerospace, Blue Origin, SpaceX, Virgin Galactic, and XCOR Aerospace. In some cases, the development of new vehicles has been aided by NASA (to support the development of commercial cargo and crew space transportation systems, for example). In other cases, development programs have relied entirely on private funding. In all cases, however, these companies have invested heavily in making commercial human spaceflight profitable, and market-based evidence indicates that there is a sustainable path forward for at least some of these initiatives. In fact, many of the companies are making good progress in developing new systems; as noted above, the Orbital Sciences Antares launch vehicle and the SpaceX Falcon 9 launch vehicle have entered commercial cargo service, and a space habitat developed by Bigelow Aerospace will be docked to the ISS in 2015 for a 2-year evaluation period.
Looking beyond LEO, some new ventures have been established with the goal of launching privately funded crewed missions to the Moon or Mars and uncrewed missions to asteroids. However, none of these more ambitious endeavors has evidenced substantial funding or vehicle development activities. This is not surprising because exploration missions beyond LEO would be extremely expensive, they would need to overcome substantial technical risks, and the time to produce a positive return on investment would be well beyond the normal time horizon of corporate business plans.
The largest of the new commercial vehicles under development, the SpaceX Falcon Heavy, would be able to launch robotic missions to Mars, but it would have limited capabilities to support human spaceflight beyond LEO unless the United States committed itself to in-orbit assembly, replenishment, fuel depots, and so on. Given the congressional mandate to develop the SLS, that seems unlikely. Smaller launch vehicles now in service or under development would be less suitable. This is not surprising given the small market for such capabilities and the fact that NASA is developing its own launch vehicle, the SLS, for human spaceflight beyond LEO.
Research and development of new technology and capabilities in government agencies other than NASA could benefit future human spaceflight programs. Most prominently, DOD is highly invested in improving the capabilities and reducing the costs of its space-related activities. Launch costs are of particular interest. The U.S. Air Force recently announced that it will allow new launch vehicle providers, such as Space X, to pursue certification to launch national security payloads. In contrast, there has been little overt DOD interest in the SLS. DOD research agencies such as the Defense Advanced Research Projects Agency (DARPA), the Air Force Research Laboratory, and the Naval Research Laboratory are also advancing space technologies at various levels of maturity, from basic building blocks, such as advanced materials and power generation technologies, to complex in-space demonstrations of systems such as automated rendezvous and docking. Specific examples include the DARPA Fast Access Spacecraft Testbed, which facilitated the development of solar arrays that have higher power and less mass; the DARPA Orbital Express missions, which demonstrated automated rendezvous and docking as well as the transfer of noncryogenic fluids between two spacecraft; and the Air Force Research Laboratory’s research into advanced thermal protection systems.
Research focused on terrestrial applications can also contribute to future human spaceflight missions. For example, some of the medical and behavioral advances to improve the performance and safety of soldiers on the battlefield may help assure the health of astronauts on long-duration missions, and advances in robotics technology for military applications may contribute to the development of advanced robotics by NASA. In addition, technology developed for the Army’s Human Universal Load Carrier could enable NASA to develop advanced spacesuits that reduce the forces and torques that a wearer would need to exert during EVA operations.
A growing number of nations have substantial human exploration programs, including system or technology development programs that could possibly contribute directly to future NASA human space activities. The programs of greatest note are being conducted by Russia, ESA, Japan, and China. In addition, Canada is an active partner in the ISS program, and India has a long-term plan for human space exploration. Altogether, 38 nations have had their astronauts fly in space. As noted in Chapter 2, human space exploration enhances international
stature and national pride, and this seems to be a key motivation for emerging space powers to enter and expand human spaceflight activities.97
Like the United States, Russia has focused its current human spaceflight program on supporting the ISS. Russia has a long-term space policy, and, as discussed in the mass media, it has the capabilities to develop technologies and systems in support of human exploration programs beyond LEO, particularly with regard to nuclear propulsion, power systems, and human exploration. Russian nuclear development facilities, however, would likely require major refurbishment before they could support a new system development effort. As in the United States, nearly all Russia’s work in the development of space nuclear systems took place decades ago.
ESA’s human spaceflight program is focused on supporting the ISS. ESA is conducting cargo flights to the ISS and exploring options for a crewed spacecraft, and it is also committed to developing a service module for the Orion MPCV. The service module will incorporate technology from ESA’s Automated Transfer Vehicle, which has been used for ISS resupply.
Chinese astronauts (also known as yŭhángyuán or taikonauts) are viewed domestically as symbols of Chinese cultural and technological prominence. China’s human spaceflight program is focused on LEO missions. China intends to develop a modular space station and a crewed lunar exploration program, although details of the technologies and systems being developed for these missions are not publicly available. NASA is prohibited by law from bilateral cooperation with the Chinese, so the U.S. and Chinese space programs are proceeding independently. This policy, driven by congressional sentiment, denies the United States the option to partner with a nation that will probably be capable of making truly significant contributions to international collaborative missions. It may be time to reexamine whether this policy serves the long-term interests of the United States.
India and Japan are separately considering options for developing a crewed spacecraft to be launched on indigenous rockets.98
The 2013 The Global Exploration Roadmap99 authored by the International Space Exploration Coordination Group (ISECG) includes a single reference mission scenario that leads to a human mission to the Mars surface. This scenario “reflects the importance of a stepwise evolution of critical capabilities which are necessary for executing increasingly complex missions to multiple destinations.…The roadmap demonstrates how initial capabilities can enable a variety of missions in the lunar vicinity, responding to individual and common goals and objectives, while contributing to building the partnerships required for sustainable human space exploration.”100 The 12 space agencies and countries that created the ISECG The Global Exploration Roadmap continue to support human exploration beyond LEO.
126.96.36.199 Robotic Science and Exploration
Robotic space exploration, which is driven by science objectives, has greatly expanded knowledge of the solar system, including its origin and evolution. Robotic missions involving orbiters, landers, and rovers have discovered extensive evidence of past warm and wet conditions on Mars. The evidence includes lake systems, alkali flats, and volcanic vents.101 The possibility that robotic and/or human missions may discover evidence of past life on Mars is a driving scientific reason for continued interest in exploring and understanding the red planet. Other targets of interest for human and/or robotic expeditions include asteroids, Earth’s Moon, and the two moons of Mars, Phobos and Deimos. Analysis of lunar material brought back by Apollo and by robotic surface missions launched by the Soviet Union, along with the results of subsequent robotic orbital missions to the Moon, indicate that it
97 D.A. Mindell, S.A. Uebelhart, A.A. Siddiqi, and S. Gerovitch, The Future of Human Spaceflight: Objectives and Policy Implications in a Global Context, American Academy of Arts and Sciences, Cambridge, Mass., 2009.
99 ISECG, The Global Exploration Roadmap, 2013.
101 J.P. Grotzinger, D. Y. Sumner, L.C. Kah, K. Stack, S. Gupta, L. Edgar, D. Rubin, et al., A habitable fluvio-lacustrine environment at Yellowknife Bay, Gale Crater, Mars, Science 343(6169), doi:10.1126/science.1242777.
was created when a Mars-size planetesimal collided with Earth early in its history.102 Earth-based observations and robotic exploration of asteroids show a plethora of evolutionary states, from primitive bodies to remnants of iron-nickel cores of impact-disrupted objects.103 Phobos and Deimos are particularly interesting because these two small bodies appear to be composed of very primitive materials that are quite different than the inferred average composition of Mars.104 Given the high scientific interest in Mars, the Moon, asteroids, and the moons of Mars, there will be continued interest in additional robotic missions to the Moon and beyond.
Robotic science missions provide information about the environmental, terrain, and surface properties and other information that is essential for the safety and effectiveness of crewed missions. For example, radiation data collected by the MSL mission en route to Mars and on the ground (by the Curiosity rover) are providing NASA with more accurate information about the nature and magnitude of radiation that astronauts would encounter during a mission to the Mars surface.
Evolutionary improvements in the capabilities of robotics would enable robotic servicing of spacecraft and robotic precursor missions to conduct ground operations at a landing site before a crewed lander arrives. Experiments on the ISS are testing the ability of robotic systems to refuel and repair spacecraft. Surface operations on the Moon and Mars could include exploration of potentially hazardous terrains and environments near landing sites, and a robotically operated ISRU system on the surface of Mars could generate needed supplies of oxygen, some of which could be converted to water using hydrogen brought from Earth. After the crew arrives on the surface, robotic systems could relieve the crew of menial tasks, such as housekeeping. In addition, surface exploration could be conducted as a cooperative effort involving humans and robotic systems. For example, robots capable of traversing hazardous terrain might be used to explore the safest paths for humans to sample strata on the walls of canyons and crater rims, or they could be commanded to conduct reconnaissance and sampling if the terrain proved to be too difficult for humans to explore.
188.8.131.52 A Game Changing Vision of Robotics
As observed elsewhere in this chapter, visionaries in the late 1940s correctly presumed that humans would eventually travel to the Moon, but the approaches that they envisioned (a single huge rocket that would make the round trip) was not the approach ultimately used. That was in part because more practical approaches were developed in the constrained resource environment of a real program and in part because there had been tremendous technological advances unforeseen by the earlier visionaries, particularly with regard to digital computers.
Although the fundamental technology of chemical propulsion has hardly advanced in the past 50 years, the capability of computers has continued to advance at an exponential pace with no end in sight. The capabilities of sensor technologies have also increased dramatically. Inexpensive consumer devices now have more computing power, sensor resolution, and programmability than were available to NASA in the midst of Apollo.
Robotics is a manifestation of the exponential increase in computational capability, sensor quality, and mechatronics (which combines mechanical, electric, and control technologies). Industrial robots are commonplace. Self-driving cars, which in the recent past were the subject of highly speculative cutting-edge experiments, are now being developed commercially. Robots are seriously contemplated as an answer to demographic crises in Western societies, where eldercare will require highly autonomous systems that are capable of complex interactions with humans.
The most common response to the criticism that human spaceflight is extremely expensive is that humans’ capacity for contextual reasoning and problem-solving is indispensable in exploration scenarios. Nonetheless, much more has been learned about our solar system by relatively primitive robots than by human explorers. Similarly, oceanographic research, until recently in decline because of the prohibitive costs of ship-based oceanography, is undergoing a revolution through the use of autonomous systems that can gather data in larger quantities and much less expensively than conventional ocean expeditions. Even in the culturally resistant domain of warfare, where
102 D.J. Stevenson, Origin of the Moon—The collision hypothesis, Annual Review of Earth and Planetary Sciences 15(1):271-315, 1987.
103 H.Y. McSween, Jr., Meteorites and Their Parent Planets (2nd ed.), Oxford University Press, 1999.
104 A. Fraeman, S.L. Murchie, R.E. Arvidson, R.N. Clark, R.V. Morris, A.S. Rivkin, and F. Vilas, Spectral absorptions on Phobos and Deimos in the visible/near infrared wavelengths and their compositional constraints, Icarus 229:196-205, doi:10.1016/j.icarus.2013.11.021.
it has been assumed that human presence is mandatory, uncrewed aircraft, uncrewed submersibles (to detect and destroy mines), and other robotic systems are increasingly taking the place of humans for dangerous missions. In coming decades, advances in robotic capabilities may likewise present new options for space exploration and open new pathways that are technologically achievable and can be implemented affordably without unacceptable developmental risk.
The Technical Panel’s technical analysis and affordability assessment yielded the following key results.
- Feasible Destinations for Human Exploration. For the foreseeable future, the only feasible destinations for human space exploration are the Moon, asteroids, Mars, and the moons of Mars. A human mission to the Mars surface is feasible, although doing so will require overcoming unprecedented technical risk, fiscal risk, and programmatic challenges. Mars is humanity’s horizon destination.
- Pace and Cost of Human Exploration. Progress in human exploration beyond LEO will be measured in decades with costs measured in hundreds of billions of dollars and significant risk to human life.
Human Spaceflight Budget Projections. With current flat or even inflation-adjusted budget projections for human spaceflight, there are no viable pathways to Mars.
- a. A continuation of flat budgets for human spaceflight is insufficient for NASA to execute any pathway to Mars and limits human spaceflight to LEO until after the end of the ISS program.
- b. Even with a NASA human spaceflight budget adjusted for inflation, technical and operational risks do not permit a viable pathway to Mars. c. The currently planned crewed flight rate is far below the flight rate of past human spaceflight programs.105 d. Increasing NASA’s budget to allow increasing the human spaceflight budget by 5 percent per year would enable pathways with potentially viable mission rates, greatly reducing technical, cost, and schedule risk.
Potential Cost Reductions. The decadal timescales reflected above are based on traditional NASA acquisition. Acceleration might be possible with substantial cost reductions resulting from
- a. More extensive use of broadly applicable commercial products and practices.
- b. Robust international cost sharing (that is, cost sharing that greatly exceeds the level of cost sharing with the ISS).
- c. Unforeseen significant technological advances in the high-priority capabilities.
Highest-Priority Capabilities. The highest-priority capabilities that are needed to enable human surface exploration of Mars are related to
- a. Entry, descent, and landing for Mars.
- b. Radiation safety.
- c. In-space propulsion and power.
- Continuity of Goals. Frequent changes in the goals for U.S. human space exploration (in the context of the decades that will be required to accomplish them) dissipate resources and impede progress.
- Maintaining Forward Progress. Within the current budget scenario, there are only a few actions, all of which would be difficult to take, that can ensure that the U.S. human exploration program keeps moving forward during the next decade following the pathways laid out in this report. These options include the following:
105 Late in the production of this report, NASA leadership told the NASA Advisory Council’s Human Exploration and Operations Committee that inasmuch as a “repetitive cadence is necessary” for a viable SLS program, the SLS would launch every year. Although that statement is consistent with the findings of the present committee, the “assumed long-term SLS manifest” shown to the Human Exploration and Operations Committee did not identify payloads past the EM2 mission currently planned for 2021. It has been suggested that military, commercial, or science missions will take up the slack, but no commitments have been announced. The SLS heavy-lift capacity suggests that the cost of payloads appropriate for its use will provide a substantial barrier to increasing the SLS launch rate to once per year.
- a. Aggressively divesting NASA of nonessential facilities and personnel to free up resources for future-oriented programs.106
- b. Focusing the use of the ISS to support future exploration beyond LEO, including the use of the ISS as a testbed for technologies and human health. (However, for technical reasons and because the operation of nuclear fission systems in LEO is prohibited,107 the ISS cannot be used to support development of any of the highest-priority capabilities identified in key result 5.)
- c. Making substantial research and technology investments in the highest-priority capabilities.
- d. Maintaining a robust planetary and space science program to engage and maintain the interest of the public in deep-space exploration, to advance technological capabilities relevant to human exploration operations in space and on the surface of the Moon and/or Mars, and to better understand space and surface environments.
- e. Investing in people: developing a workforce with hands-on experience in flight programs.
- f. Continuing to leverage commercial products and practices to strengthen the industrial base and increase NASA’s efficiency.
- g. Strengthening international relationships and partnerships with the goal of reducing duplication, exploiting worldwide expertise, and substantially reducing the cost of the total program to the United States.
Human spaceflight is an extraordinarily challenging endeavor that is fraught with daunting technical, political, and programmatic hurdles and carries a high degree of physical risk to the explorers who push forward the boundaries of human presence. The scale of U.S. investment in this endeavor is substantial and has remained so for decades despite periodic financial strictures and without broad and deeply committed public support. The frequently cited rationales for human spaceflight are self-evident to some and unconvincing to others, and there are unlikely to be novel rationales that are more potent.
The United States and a group of international partners have succeeded in building and operating the most ambitious space engineering project ever: the ISS. This required a substantial investment by multiple countries, and it is natural to attempt to amortize that investment and retain the capability. Programmatic and budgetary factors led to the completion of the ISS just as the main launch system that enabled its construction was retired from service, and Congress mandated the development of a new heavy-lift launcher, the SLS. In addition, national leadership has called for the United States to venture beyond LEO, where more distant destinations beckon. The SLS is but one part of the technology that needs to be developed to enable even a return to the Moon, let alone human visits to near-Earth asteroids or to the vicinity or surface of Mars. Although national leadership has sustained operations in LEO, it seems disinclined to increase the level of investment in human spaceflight substantially. Thus, the human spaceflight program faces a dilemma. Maintaining the ISS and developing the SLS leave precious little budgetary maneuvering room to plan the next steps beyond LEO. However, the affordability analysis summarized above has shown that with currently projected human spaceflight budgets it will take decades to achieve the next significant human spaceflight milestone even with optimistic assumptions about costs, technology development, and programmatic stability.
Probably the most significant factors in progressing beyond LEO are the development of a strong national (and international) consensus about the pathway to be undertaken and sustained discipline to maintain course over many administrations and Congresses. Without that consensus and discipline, it is all too likely that the potential of the SLS will be wasted, human spaceflight to LEO will be increasingly routine (although still with risk to life), and the horizons of human existence will not be expanded—at least not by the United States. With such a consensus, however, and with strict adherence to the pathways approach and principles outlined in this report, the United States could maintain its historical position of leadership in space exploration and embark on a program of human spaceflight beyond LEO that, perhaps for the first time in the more than half-century of human spaceflight, would be sustainable.
106 The political difficulty of taking this action suggests that it may require an extraordinary process, akin to NASA’s “real property assessment” of 2004 but using an independent, bipartisan assessment commission and later an up-or-down vote by Congress.