This chapter identifies the technologies that enable exploration missions to the Moon, Mars, and elsewhere, as well as the foundational research in life and physical sciences. Science and technology development areas are recommended to support near-term objectives and operational systems (i.e., prior to 2020) and objectives and operational systems for the decade beyond 2020. While technologies and operational systems may be near term (prior to 2020) or longer term (2020 and beyond), it is anticipated that supporting research will be conducted in the coming decade. In addition to defining research in science, this chapter, where appropriate, includes discussions of establishing the technological know-how required to ensure the orderly transition of new technology into space exploration systems.
The National Aeronautics and Space Administration’s (NASA’s) future exploration missions are likely to include long durations, microgravity and partial-gravity environmental conditions, and extreme thermal and radiation environmental conditions. The specific environmental conditions needed to successfully perform the required research are identified. To provide information that can be utilized in the most flexible manner possible, the specific schedules or timetables by which these research objectives should be achieved are not specified; instead, the relative temporal sequences are described.
An initial assessment of science and technology needs is based on an update of topics found in the National Research Council’s (NRC’s) 2000 report, Microgravity Research in Support of Technologies for Human Exploration and Development of Space and Planetary Bodies.1 Other major sources of information include NASA’s project plans for the Exploration Technology Development Program,2 NASA’s report, Technology Horizons: Game-Changing Technologies for the Lunar Architecture,3 and the space exploration options discussed in the final report of the Review of U.S. Human Spaceflight Plans Committee (also known as the Augustine Commission or Augustine Committee).4 Current NASA planning documents, such as the lunar architecture, were used to illuminate the possible need of future exploration activities. Science and technology needs were categorized into seven topic areas: space power and thermal management; space propulsion; extravehicular activity (EVA); life support; fire safety; space resource extraction, processing, and utilization; and planetary surface construction.
This chapter describes necessary scientific research and technology development in each of these seven areas and categorizes the research recommendations in one of two time frames: either “Prior to 2020” or “2020 and Beyond.” The two time periods are used to indicate when, given a best-case scenario, an operational system or exploration activity is likely to be implemented. Placing the implementation time either prior to or after 2020 can be stated with greater certainty than trying to establish when the research to support the implementation of these
activities would need to commence. The research required to support implementation will have to be initiated well in advance of the desired implementation date; however, the exact time required to accomplish the needed research is uncertain due to technical reasons, as well as to budget and political factors, all of which are beyond the ability of the panel to predict with any precision. The strategic and tactical decisions regarding if and when a research program should be undertaken appropriately fall to those responsible for the implementation of the various missions under consideration. Since NASA’s exploration mission schedule has been notional at best for decades, the “Prior to 2020” period should be viewed as representing near-term activities (e.g., near-Earth human exploration activities), while the “2020 and Beyond” period represents activities that enable longer-term exploration goals (e.g., human exploration of the lunar surface, planetary surfaces, or deep-space missions).
While there is always more that can be learned to further the understanding critical to enabling future exploration, prioritizing the various areas of potential study allows the critical needs to be translated into a plan that can be implemented successfully. Since NASA’s missions, budgets, and priorities cannot be predicted, selecting one particular technology over another would be premature. Instead, this chapter describes the attributes of and development issues associated with viable technology options and explains the critical research gaps that need to be addressed if a particular option is taken. The chapter therefore divides technologies and their associated research challenges into two categories: “required” and “highly desirable.” A required technology is one NASA needs to achieve an exploration objective. A highly desirable technology is one that offers a significant benefit in performance, efficiency, cost savings, or likelihood of mission success. The rating as either required or highly desirable applies directly to the technology or operational system; research challenges in the life and physical sciences are listed with the technology or system they enable. The committee’s prioritization process took the following factors into account:
• The relative importance of the research to its topic area,
• The topic area’s impact on overall exploration efforts,
• The interdependencies among topic areas and how knowledge in one area could be an enabler or prerequisite for advancing knowledge in another area, and
• Whether the topic area’s knowledge needs were unique to NASA’s exploration requirements such that they would be left unaddressed were NASA not to pursue them.
Table 10.3 at the end of the chapter summarizes the technologies (and their associated research challenges) required for implementation prior to 2020. Table 10.4 similarly summarizes the technologies (and their associated research challenges) required for implementation in 2020 and beyond.
The Integrative and Translational Research for the Human Systems Panel considered the realities and challenges of transitioning new technology to enable or greatly improve systems unique to NASA and critical to space exploration. The transition process requires that engineers understand and apply the research results of scientists and that research scientists work within the parameter space of specific mission categories. Successful transition from research results to implemented technology requires that program managers, engineering leaders, and research leaders create an environment where scientists interact with engineers on the specifics of system requirements. NASA leadership will know that a success-oriented environment has been achieved when they no longer hear the familiar refrains: “the engineers are not talking to the scientists,” “the scientists are not working in the regimes of interest to the engineers,” and “the program managers are risk-averse and not qualifying improved components/subsystems for flight.” Chapter 12’s section “Linking Science to Mission Capabilities Through Multidisciplinary Translational Programs” contains an in-depth discussion of the issues associated with creating organizations that can successfully translate research into technologies and technologies into exploration systems.
NASA’s power generation, energy storage, and heat rejection technology needs in the coming decades are driven by three major and diverse categories of missions: (1) platforms for near-Earth science, resources (such
as orbiting cryogenic propellant depots), and communications; (2) lunar and planetary surface missions; and (3) deep-space exploration probes. These categories give rise to a spectrum of future power and thermal management requirements ranging from a few watts (e.g., for microsatellites) to tens of kilowatts and perhaps megawatts (e.g., propulsion systems for exploration missions or for a permanent lunar or Mars presence), and durations ranging from a few hours or days (e.g., planetary rovers) to perhaps tens of years (e.g., permanent surface presence, deep-space missions). These power demands will be met by a variety of evolutionary and revolutionary technologies for providing prime energy sources, energy conversion, energy storage, thermal management and control, and heat rejection. Prime energy sources include insolation,* radioisotopes, and nuclear fission. Energy conversion technologies applicable to this broad range of power needs include those for both “direct” energy conversion (such as photovoltaic and thermoelectric systems, fuel cells, primary and secondary batteries, and/or alkali metal thermal-to-electric conversion approaches) and “indirect” conversion or heat engine approaches (Stirling, Brayton, or Rankine cycles).
Energy storage is accomplished principally by chemical or thermal energy storage (e.g., electrochemical batteries, fuel cells, or thermal energy reservoirs). Thermal management will be accomplished by thermal heat acquisition, transport, and rejection methods including radiators, evaporators, sublimators, and thermal storage techniques (e.g., thermal wadis†), as well as advanced environmental control refrigeration cycles. The mission environment also influences and in some cases dictates the technology selection. Deep-space and extrasolar missions (extending beyond the orbit of Jupiter, such as Voyager) can today only be performed using nuclear energy sources. Near-Earth missions, such as the International Space Station (ISS), can be conducted using photovoltaic, chemical, or thermal methods. Outer planet missions may combine photovoltaic, chemical, thermal, or nuclear sources. Extended lunar and planetary presence missions are greatly enhanced by nuclear power, often referred to in this context as fission surface power. The emerging area of in situ resource utilization (ISRU) will require more capable power systems than previously deployed. Novel power and thermal solutions that increase performance (e.g., increase efficiency) and/or reduce mass will enable human exploration on the surfaces of other solar system bodies. Figure 10.1 shows the power and duration-of-use regimes for different space power systems operating at 1 AU from the Sun. The region of photovoltaic operation shrinks as a mission moves farther from the Sun.
Although NASA faces a wide range of challenging power and thermal energy technology requirements, it does have partners in meeting some of these needs. The Department of Defense (DOD), the commercial satellite industry, and the international space community will need many of the same technologies that NASA will need. Partnering, formally and informally, will reduce NASA’s research, development, and manufacturing production costs. Nevertheless, there remain NASA-unique power system technology needs for which NASA must bear the full cost, most notably nuclear technology—although the Department of Energy (DOE) would be a partner in the future development of nuclear systems, just as it is currently a partner in the program for radioisotope power systems (RPSs); the advanced Stirling radioisotope generator; and the fission surface power system technology development effort).‡
Prime power system mass for exploration missions to the Moon and Mars is a major fraction of the total mission mass to be transported from Earth. Thus, gains in efficiency and lifetime directly reduce mission cost. In
* Insolation is the solar radiation energy received on a given surface area in a given time. The term “insolation” is a contraction of “incoming solar radiation.”
† Thermal wadis are engineered sources of stored solar energy using modified lunar regolith as a thermal storage mass.
‡ DOE has a statutory responsibility “for the conduct of research and development activities relating to … production of atomic energy, including processes, materials, and devices related to such production” (Atomic Energy Act of 1954, as amended, Sec. 31). In addition, DOE, “as agent of and on behalf of the United States, shall be the exclusive owner of all [nuclear] production facilities” (Atomic Energy Act of 1954, as amended, Sec. 41). There are some exceptions regarding the ownership requirements, but they would not apply to the production of nuclear material to fuel space nuclear power systems. For example, under the existing memorandum of agreement between NASA and DOE regarding RPSs, DOE’s responsibilities include the design, development, fabrication, evaluation, testing, and delivery of RPSs to meet NASA system-performance and schedule requirements. DOE also provides nuclear risk assessments; specifies minimum radiological, public-health, and safety criteria and procedures for the use of RPSs; provides safeguards and security guidance for NASA facilities and services; supports NASA operational plans, mission definition, environmental analysis, launch approval, and radiological contingency planning; affirms the flight readiness of RPSs with respect to nuclear safety; participates in the nuclear launch approval process; jointly investigates and reports nuclear incidents; and assumes legal liability for damage resulting from nuclear incidents and accidents involving RPSs.
addition, several of the enabling technologies for more affordable space exploration, such as small modular reactors, have the potential to transition to important and timely applications on Earth.
Power Generation Systems
Solar Power Systems
Photovoltaic power generation has an extensive heritage in terrestrial and space applications. Space-based systems ranging from a few watts (“nanosats”) to many tens of kilowatts (space station) are currently in use.5 Arrays have been developed for use in some of the most extreme temperature and radiation environments in space (e.g., the Mercury orbiter MESSENGER and the Jupiter orbiter Juno), although high radiation levels still degrade system performance over time and therefore limit mission lifetime.
Typical current spacecraft solar arrays achieve areal power densities of about 200-300 W/m2 at specific powers ranging from 20 to 150 W/kg (using 27 percent efficient triple-junction solar cells). The efficiency of solar cells for space applications is projected to reach 37 percent by 2020. The ISS solar array produces 250 kW of power and is the largest space solar array to date. Concentrator arrays have been flown to distances of 1.5 AU from the Sun and have demonstrated 2.5 kW of output power (at 300 W/m2 and 45 W/kg). Arrays capable of achieving up to 220 W/kg have been demonstrated on the ground.
Solar cells for terrestrial applications using concentrators have now demonstrated energy conversion efficiencies of 38.5 percent; efficiencies of 45 percent are on the near horizon and will enable further reductions in array area. Terrestrial systems capable of producing tens of megawatts are planned; such surface-based systems would have application to the exploration of planetary bodies with or without atmospheres.
TABLE 10.1 Solar Irradiance and Operating Temperatures for Different Environments
|Earth Orbit||Lunar Surface||Martian Surface|
|Solar irradiance (W/m2)||1,368±41||1,368±41||590a|
|Temperature range (°C)||−100 to +100||−180 to +250||−140 to +20|
aAverage value from G.A. Landis and J. Appelbaum, Design considerations for Mars photovoltaic systems, pp. 1263-1270 in Conference Record of the Twenty First IEEE, Photovoltaic Specialists Conference, Kissimmee, Fla., May 21-25, 1990; actual irradiance varies with location and season on Mars.
Table 10.1 shows the solar irradiance and operating temperatures for Earth orbit and for lunar and martian surfaces; solar array size (and mass) requirements depend on these environmental parameters.
The total mass of a photovoltaic-based power generation system capable of producing tens of kilowatts on the lunar or martian surface will be driven by the energy storage components (e.g., batteries, regenerative fuel cells [RFCs],§ or thermal energy reservoirs), power management electronics, and thermal management system, as well as the size of the solar array. The design of surface power systems will also need to consider the native dust environment, plasma arcing issues,6 cosmic radiation, and the performance of associated thermal management components. Thermal management technologies should also be adapted to local environmental conditions.
Photovoltaic power generation on the surface of Mars requires an array approximately three times larger in area than needed in Earth orbit or on the lunar surface, due to the increased distance from the Sun. Energy storage requirements are far less demanding on Mars than on the Moon because of the much shorter martian “night” (12 h versus 2 weeks); a battery system on the surface of the Moon will require 28 times the capacity (and roughly the same increase in mass) as one on Mars to support the same load. But the effects of dust, dirt, and wind on the martian surface require additional mitigations that will increase the mass of any surface solar array. However, the net result is that photovoltaic power systems on Mars require much less mass to produce the same amount of steady-state power as a system on the Moon.
While the Juno mission will use photovoltaic power at Jupiter (at about 5.5 AU), the current practicality of using solar power diminishes with greater distances from the Sun, due to a combination of the fall-off in solar intensity and colder operating temperatures.7
Concentrating solar-thermal power systems are also in development for terrestrial applications and are potentially valid for space applications.8 Typically, these consist of a solar concentrator such as a parabolic trough unit (which can produce heat at temperatures up to about 500°C) or a parabolic dish unit (which can produce heat at temperatures up to 1,000 to 2,000°C) combined with a heat engine (such as a Stirling cycle engine). For extraterrestrial surface applications, concentrating solar-thermal power systems can be manufactured from ultralight reflective foils creating deployable booms that can supply thermal or electricity power, or both, for ISRU.9
Lightweight solar arrays can be a low-mass, efficient source of power for near-Earth applications and can support future science missions that use solar electric power, such as visits to asteroids, cargo transport to the Moon or Mars, or possibly outer planet missions.
It is likely that the evolutionary development of multijunction solar cells will reach its practical achievable performance potential in the coming decade and additional modest performance gains will be found from technology development in concentrators. Next-generation photovoltaic technologies, such as nanotechnology-based solar cells or quantum-dot solar cells, have potential advantages of lower mass, lower cost, and/or higher efficiencies over existing photovoltaic technologies. Though these technologies are unlikely to be mass-competitive with RPSs for deep-space applications in the next 10 years, factors such as cost and availability may make them attractive alternatives for inner solar system missions in the future.
§ All fuel cells utilize a fuel and an oxidizer. Fuel cells combine reactants to produce electrical power and waste heat. RFCs can either combine or produce reactants to produce or store power.
Nuclear Power Systems
Space nuclear power systems are among the NASA-unique technology needs. These include RPSs (Pu-238 energy source) and fission reactors as a primary energy source.
RPSs continue to be a high priority for NASA because they provide reliable, long-term power where solar power is not feasible. More than 26 NASA and DOD spacecraft have used radioisotope power since 1961. A recent NRC report10 documents the current catastrophic lack of Pu-238, as well as lack of plans to produce a supply. The merits of Pu-238 include the low emitted-radiation shielding requirements, long half-life, and high energy density. Pu-238 production is a complex process including nuclear reactor irradiation and radiochemistry processing. Neither the United States nor any other country currently has the capability to produce Pu-238. Both the United States and Russia have small stockpiles of Pu-238; planned missions will consume it.¶ Russia has stopped selling Pu-238 to the United States.
Re-establishing Pu-238 production capability is critical to sustaining a deep-space mission capability and is a crosscutting enabler for research and development (R&D) as well as science missions. Congress has not approved DOE’s fiscal year (FY) 2010 and 2011 budget requests to begin re-establishing a domestic Pu-238 production capability. While Pu-238 production is a technology/policy issue rather than an area of research, the issue is noted here because of its importance to deep-space science mission spacecraft.
Nuclear reactor power systems could supply substantially more electrical power than current prime power systems (solar cells or RPSs). NASA flew the System for Nuclear Auxiliary Power–10A (SNAP-10A), a 500-We thermoelectric fission reactor, in 1965 but has not flown a nuclear reactor since. The 1983 to 1994 100-kW SP-100 project and the last decade’s Prometheus project were both terminated before reactor development could be completed.** Fission surface power systems are “an attractive power option for some lunar and Mars mission scenarios,”11 and NASA has identified nuclear power reactors as one of 19 game-changing technologies for the lunar exploration architecture.12 NASA has identified nuclear power as a high-priority technology because it releases many other exploration technologies from severe constraints on power use.13 The recent confirmation of water at the lunar poles and on Mars underscores the need for ample power to enable ISRU for propellant production and life support.14,15 The availability of nuclear reactor power systems would make it possible to relax stringent power constraints, thus reducing development costs across the entire lunar exploration architecture—except for the cost of the power system itself. Development of a nuclear power reactor for lunar missions will likely be a long and expensive effort. Looking back, efforts by both the SP-100 program and Prometheus Program to develop space nuclear reactors were terminated prematurely as support for these expensive projects dwindled over the years in the face of tight budgets and new agency priorities. Furthermore, even if space nuclear reactor systems were successfully developed, the cost of manufacturing each system (including the nuclear fuel) would be so high that such systems would be suitable only for very large missions that could afford a power system costing billions of dollars to develop.
NASA, in cooperation with DOE, is currently supporting a fission surface power system (FSPS) technology effort, but the magnitude, scope, and goals of this effort are quite modest in comparison to the total effort required to develop an operational reactor system. The current goal of the FSPS is to “generate the key products to allow Agency decision-makers to consider FSPS as a preferred option for flight development.”16 Continuation of NASA’s FSPS program is desirable, and it would be advantageous to the project if the prototyping of the test unit were
¶ The NRC study committee wrote, “The total amount of 238Pu available for NASA is fixed, and essentially all of it is already dedicated to support several pending missions—the Mars Science Laboratory, Discovery 12, the Outer Planets Flagship 1 (OPF 1), and (perhaps) a small number of additional missions with a very small demand for 238Pu. If the status quo persists, the United States will not be able to provide RPSs for any subsequent missions” (NRC, Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration, The National Academies Press, Washington, D.C., 2009). The nominal launch date for OPF 1 is 2020 (see http://opfm.jpl.nasa.gov/), at which point the stockpile will be depleted.
** Project Prometheus was terminated in 2005 after it became clear that it would cost at least $4 billion to complete development of a spacecraft reactor module and at least $16 billion in total to develop the entire spacecraft and complete the mission, not counting the cost of the launch vehicle or any financial reserves to cover unexpected cost growth (Jet Propulsion Laboratory, Project Prometheus Final Report, 982-R120461, Jet Propulsion Laboratory, Pasadena, California, available at http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38185/1/05-3441.pdf, 2005, p. 178).
accelerated and carried out in parallel with testing to confirm the reactivity characteristics of the material assembly. Supporting physical science research includes high-temperature, low-weight materials for power conversion and materials for high-temperature radiators. New system geometries can be investigated further to take advantage of possible inherent shielding of the generators. Extending the current technology effort to develop an FSPS system could enable a power-rich lunar architecture, accelerating research on the lunar surface in all other areas. However, the hardware systems being assembled for testing do not include any nuclear fuel, and the current project does not include fuel development, which would be very costly (for DOE) and is unlikely to happen until NASA makes a firm commitment to deploy an FSPS. Once the current FSPS technology project is complete, NASA may decide to make fission surface power systems a priority, but that has not happened yet.
In conclusion, nuclear reactor systems have much to offer to the exploration of space, but they would also be very expensive to develop. History cautions against underestimating how difficult it would be to complete development of space nuclear power reactor systems on a scale large enough to support future U.S. space exploration missions.
Thermal Energy Conversion
Three dynamic energy conversion cycles are Stirling, Brayton, and Rankine. Stirling power conversion for space applications uses sealed gas/piston-linear alternator components that can operate at relatively high efficiency with comparatively small heat source-sink differential temperatures. Brayton power conversion uses a closed cycle version of gas turbine alternator technology. Rankine cycle systems use a space-adapted version of terrestrial two-phase steam power plants having a turbine and alternator. Stirling and Brayton systems, because they use gas as the working fluid, have gas heat transfer coefficients that are typically smaller than two-phase flow heat transfer coefficients. This necessitates larger heat transfer surfaces than Rankine cycle systems but eliminates issues related to zero gravity, freeze-thaw cycles, and two-phase flow. Thus, at higher powers, Rankine systems typically have smaller components than Stirling or Brayton systems because of the smaller heat transfer area required. Further, the Rankine cycle condenser (radiator) operates at a higher temperature and in an isothermal mode, and so the size/mass of the heat rejection system is much smaller.
Figure 10.2 compares various thermal energy conversion systems for producing electrical power as a function of specific mass (kilograms per kilowatt-electric) and power (kilowatts-electric). It shows that at low power, static conversion technologies such as thermoelectric and thermionic generally have lower specific mass, whereas at higher powers Stirling, Brayton, and finally, Rankine cycles have lower specific mass. Existing space power systems have been at relatively low power (e.g., the ISS, with a lifetime average electrical power of ~75 kWe produced from 260-kW solar arrays). However, future power systems for habitats, life support, ISRU, and propulsion all trend toward higher powers, including up to ~100 MWe. Thermoelectric and Stirling power conversion technologies are not mass competitive at these high power levels, as indicated in Figure 10.2. Thus, for NASA’s projected high-power needs, the Brayton system may be a reasonable option to achieve required system mass performance. Figure 10.2 shows that there is little difference between a thermoelectric system and a Stirling system at low power levels. Therefore, at low powers, a new power conversion technology must be developed if performance gains are to be achieved. Thermophotovoltaic energy conversion, which operates at high temperatures and converts thermal power to electrical power using a photovoltaic-like technology, has the potential for reduced weight, high conversion efficiency, and operational simplicity.17 However, it is currently an immature technology that would require significant R&D before it would be a viable option for NASA.
Advanced energy storage technologies can offer an order-of-magnitude improvement over current technology. Lithium ion batteries offer a theoretical energy density of 700 W·h/kg and RFCs a theoretical energy density of 1,000 W·h/kg. Practical embodiments of these potential electrical energy storage devices will likely attain only about half the theoretical values, but this would nonetheless represent a 10-fold improvement over current
technology. Figure 10.3 shows the generally achievable energy and power densities for current and near-term electrochemical storage technologies.
NASA Glenn Research Center researchers and others have developed concepts for RFCs that would store energy on the ISS and on high-altitude balloons or high-altitude aircraft. They are now investigating RFCs for storing energy on the Moon or Mars.18 A unitized RFC (Figure 10.4)†† would use no electrolyzer; rather, it would regenerate water and store the hydrogen and oxygen as high-pressure gases directly through a single stack.
RFCs are considered viable options for solar energy storage in a wide variety of environments where the Sun eclipse period is several hours or longer. Reactant tankage size is proportional to the required stored energy (eclipse load power × eclipse time). For applications in which the stored energy requirement is modest, reactant tankage mass is also modest. However, for long eclipse time (14 days) and high eclipse power (tens of kilowatts or greater), such as for lunar surface power, the tankage mass becomes significant, perhaps as much as 5-10 percent of the overall mass. Thermal management, tankage mass optimization, and system mass versus operating pressure are also issues to consider. To use hydrogen and oxygen on the lunar surface or elsewhere on a large scale, safe, practical storage systems must be developed. Fuel cells, both primary and regenerative, have been in existence for years. However, the issues of dead-ended gas flow paths versus through-flow, cryogenic versus pressurized gas storage, thermal management, and reliable long-life operation (i.e., years) under reduced gravity and extreme temperatures all remain to be demonstrated for both planetary and orbital applications.
†† Unitized RFCs are a particular packing geometry.
With few exceptions, NASA spacecraft and planned planetary systems all require electrical energy storage. The ISS is currently using nickel metal hydride batteries, but lithium ion batteries with the requisite energy storage density and cycle life are under development. Currently, nickel hydrogen batteries are the workhorse spacecraft energy storage approach but are being supplanted by rapidly evolving lithium ion approaches, which offer significant (>2 to 5 times) increases in energy density. Li-ion approaches are now baselined for most NASA robotic explorer missions under development in the Science Mission Directorate. If nuclear power is not used, RFCs and/or thermal energy storage must be developed in order to satisfy mass, power, and energy requirements for lunar and martian bases, especially when ISRU is incorporated into the base.
The major design goals for any space thermal management system are high performance, reduced cost, reduced physical size, and high reliability. Earth-based system processes involving phase change and/or multiple phase flow have been shown to have the highest heat transfer coefficients.19 The benefits of two-phase flows are illustrated in Figure 10.5, in which boiling heat transfer coefficients exceed single phase heat transfer coefficients by multiples in all cases.
While the thermal management technology requirements for NASA’s different missions overlap, there are unique challenges posed by each environment. NASA requirements must ensure cooling for spacecraft on inner planet missions (which experience high insolation), on planetary surfaces (where dust from the lunar regolith
or martian soil degrades radiator surfaces), and on cis-lunar space missions. Further, high-power missions will ultimately require two-phase (single-component, vapor/liquid) thermal management technologies yet to be demonstrated in microgravity and in lunar and martian partial gravity. Gas and liquid phases in components such as boilers, condensers, and heat pipe radiators behave differently in other than Earth gravity due to the change in body force. An NRC report documented the research needs in two-phase thermal management technologies a decade ago,20 but little has been done to advance the technology readiness level (TRL) of the component technologies. Two-phase flow technologies offer reduced system mass and size because their high heat transfer coefficients
require smaller and therefore lighter hardware than single-phase systems with the same capacity. Two-phase systems are naturally isothermal.
Active two-phase flow thermal management technologies operating in reduced-gravity conditions are essential to enable high-power thermal management systems.21 However, such technologies require separation of the two phases of a single component. This need has been described in numerous NRC and NASA reports.22,23 If missions requiring high power are to be carried out efficiently, this technology must be brought to TRL 8 and demonstrated at a system level; currently the technology is at perhaps TRL 6. Microchannel devices for heat exchange and phase separation are also under investigation. Component- and system-level tradeoffs among channel sizing, interchannel stability, pumping power, and overall system mass are being studied.
Gaps in the demonstration of two-phase technology have led NASA to avoid system designs based on multiphase flow. For example, the main cooling loops (ammonia and water) for the ISS are single-phase fluid loops, even though using two-phase flow would have resulted in reduced system mass and power requirements. NASA needs to be able to scale such systems confidently and predict their performance and system reliability by quantifying failure. Until these gaps are filled, NASA will not deploy two-phase flow systems, regardless of the cost and/or mass performance advantages they promise. Hence, more research is needed on two-phase systems subjected to reduced gravity.
Liquid-solid phase-change devices are commonly used in spacecraft thermal control to regulate temperature during peak heat generation periods and/or to stabilize payload component temperatures during Sun eclipse. A variety of waxes and hydrated salts have been employed. Their drawbacks include heat transfer rate limits (unless encapsulated in a high-conductivity porous matrix), relatively low effective heat capacity per unit mass, and freeze/thaw volume changes. Novel encapsulated thermal energy storage microcapsules mixed in a single-phase fluid have received some development funding from NASA but remain in an early stage of development. The effect of such a “slurry” working fluid is to both increase the heat transfer properties of the pumped liquid and increase its effective specific heat.
Thermal energy storage using modified surface material is a promising new thermal tool that can use in situ resources. Energy storage can be provided through the application of thermal energy reservoirs, with the thermal mass being provided by either materials brought from Earth (such as phase-change materials) or processed space resources. For example, lunar regolith can be modified through thermal process methods to yield a material whose thermal diffusivity‡‡ is increased by approximately two orders of magnitude compared with untreated regolith. Thermal energy storage can also be directly integrated with solar concentrators for nighttime power generation. The placement of thermal wadis24 has been proposed as an engineered source of heat (and power) for the protection of rovers and other exploration assets on the lunar surface.
Summary of Enabling Science and Technologies for Space Power and Thermal Management
Power and thermal management improvements will become increasingly important for future NASA mission needs, especially for missions that include ISRU. Important exploration technologies in space power and thermal management that would benefit from near-term R&D include the following:
‡‡ A high thermal diffusivity is crucial for thermal energy storage because it allows a material’s heat capacity to be utilized. Thermal diffusivity, which describes the rate at which heat flows through a material, is the thermal conductivity of that material divided by its volumetric heat capacity. In SI units, thermal diffusivity is measured in m2/s.
Before 2020: Required
Two-Phase Flow Thermal Management Technologies. NASA would benefit from the potential advantages of low mass, small component size, and isothermality for future missions with high power requirements. Research should be conducted to address active two-phase flow questions relevant to thermal management. (T1)
2020 and Beyond: Required
Regenerative Fuel Cells. NASA would benefit from the enhanced flexibility in power and energy storage offered by regenerative fuel cells. The necessary research should be conducted to allow regenerative fuel cell technologies to be demonstrated in reduced-gravity environments, including research related to dead-ended gas flow paths versus through-flow, cryogenic versus pressurized gas storage, thermal management, and reliable long-life operation. (T11)
Energy Conversion Systems. NASA would benefit from additional energy conversion capabilities in the low-and high-power regimes, as shown in Figure 10.2. The development of thermal energy conversion technologies beyond the existing thermoelectric and Stirling systems is needed to enable higher-performance missions. Research should be done on high-temperature energy conversion cycles and devices coupled to essential working fluids, heat rejection systems, materials, etc. (T12)
Fission Surface Power. Fission surface power could be a valuable option to NASA in the future for missions with high power requirements. The continued development of supporting technologies and systems space nuclear reactor power would ensure that reactor power systems are a viable option for future space exploration missions. Areas of physical science research that enable the development of those systems include high-temperature, low-weight materials for power conversion and radiators. (T13)
Radioisotope Production. While there are no underlying science or technology gaps, re-establishing domestic production of Pu-238 is necessary to ensure the continued viability of deep-space missions.
To support future space exploration missions, an evolutionary space transportation architecture will need to deliver humans, surface habitats, and transportation systems for the purposes of (1) exploration for science discovery and (2) maturing technology for the next exploration destination. In addition to supporting precursor missions in the near term, smaller scale and even micro propulsion options will be required in the far future, as exploration sensors and payloads become smaller and nanotechnology matures.
NASA’s original Constellation space transportation architecture consisted of Earth-to-orbit launch vehicles for delivering humans and large cargos to orbit.25 Proposed launch vehicles such as Ares I and Ares V were to use derivatives of the legacy large-thrust propulsion technology base including space shuttle solid rocket motors, Delta IV RS-68s, Apollo second and third stage J-2s, and Apollo/space shuttle aluminum cryogenic tanks. Thus, minimal technology development for launch vehicles was envisioned for missions to the Moon and Mars.
The Augustine Review Committee offered several other approaches in addition to the Constellation architecture, including the development of one launch vehicle for both humans and cargo, the use of current Evolved Expendable Launch Vehicles (EELVs), and space shuttle-derived concepts,26 to improve affordability and involve commercial space enterprises. However, in all cases, little or no new propulsion technology was required for these launch vehicles because existing, or derivatives of existing, rocket engines were envisioned.
For small- to medium-lift launch vehicles (e.g., current EELVs), cryogenic propellant depots for on-orbit refueling could reduce cost, increase mission payload, and improve mission success.27-30Figure 10.6 illustrates the
principal operations of robotically managing cryogenic propellant storage and transfer. Propellant depots can be operated and filled through competitive commercial space launch companies, which, because of the cost advantage of even 4 to 6 EELVs over an equivalent heavy-lift launcher, would use multiple EELV launches to incrementally deliver propellant to a depot. This high-rate launch demand would provide a sustained competitive market for EELVs, with the potential to dramatically reduce the cost per kilogram to orbit compared with the alternative scenario of lifting all the propellant for a long-duration mission on a single launcher that also lifts the mission payload to orbit. Moreover, with depots, EELVs can deliver much heavier critical exploration transportation systems to orbit because only the inert mass of the systems would be lifted into orbit, improving DV§§ capability by 75 percent over a vehicle delivered to orbit with all its propellant. For lunar missions, the Ares V could be eliminated, and for Mars missions, the majority of Ares V launches could be eliminated.31 On-orbit refueling increases operational complexity and requires advanced cryogenic fluid transport and handling technology for reduced gravity, but 80 to 90 percent of the initial mass in low Earth orbit (IMLEO) propellant mass is decoupled from the delivery of the critical exploration transportation systems, providing an additional opportunity for tradeoffs to reduce costs.
A space exploration system’s mass, power requirements, and cost are driven by the mission requirements (including safety) and destination. A long-term lunar mission, such as a 180-day mission to an outpost at Shackleton Crater near the south pole of the Moon, would dramatically increase the system requirements over those of
§§ ΔV is the magnitude of the change in velocity.
the Apollo missions, which had only two-crew/several-day sorties near the Moon’s equator. For every kilogram of mass delivered to the Moon, 600 kg of IMLEO transportation mass is required. Thus, advances in the in-space propulsion system, which is at least 80 percent of the IMLEO, have a dramatic compounding impact on total exploration architecture viability and sustainability.
NASA’s existing plans for a lunar mission include the use of in-space propulsion based on derivatives of the Apollo oxygen/hydrogen J-2 for the Earth departure stage, Centaur oxygen/hydrogen RL-10 for the Altair lunar lander, and Delta II hypergolic storable AJ-10 for the lunar ascent stage and the Earth return service module. Higher-performance oxygen/methane engines were originally planned for the ascent stage and service module because of the synergy with the planned Mars descent and ascent modules,32 but they were dropped because the technology was not sufficiently mature. In addition, the even more energetic oxygen/hydrogen engine was not considered because of the lack of technology for long-term (180 days for the Moon and 1 year for Mars) cryogenic storage and engine ignition concerns. Current cryogenic tank multilayer insulation has a liquid hydrogen boiloff rate of approximately 3 percent per month for the Lunar Module.33 Thus, advanced active cooling techniques are desirable for lunar missions and necessary for the longer Mars missions or for long-term storage propellant depots for lunar missions.
For Mars missions in 2030 and beyond, the Mars Transfer Vehicle with Nuclear Thermal Rockets is projected to be two-thirds the IMLEO of a vehicle using oxygen-hydrogen propulsion, reducing the number of Ares V launches from 12 to 9.34 Both the nuclear and the chemical propulsion systems can be developed using the existing technology base, although unique test facilities have to be developed for the nitrogen thermal rocket (NTR). For long-duration missions, such as a 900-day Mars human mission, zero-boiloff technologies would be required for cryogenic propellant storage. In addition, as described in earlier chapters, such a lengthy mission would affect system reliability and crew health in terms of physical/physiological deconditioning of the crew and exposure of both vehicle systems and the crew to radiation. For such long-duration missions, more than half the transits would likely be for cargo transfer, for which low-thrust, high-specific-impulse options like solar or nuclear electric propulsion (SEP or NEP) systems should be considered. However, the high cost of an NTR or a NEP system makes them suitable only for relatively long, heavy-payload missions that cannot easily be supported by non-nuclear power or propulsion systems.
For Mars orbit insertion and atmospheric descent, significant performance benefits can be achieved with proposed drag devices, which could reduce descent engine performance requirements by 60 to 80 percent. For Mars ascent vehicles, oxygen-methane engines have been proposed,35 which could take advantage of ISRU of the carbon dioxide atmosphere and possibly the surface ice (water) of Mars to eliminate the need to supply ascent propellant from Earth.36 Other options for ascent stages from the surface of the Moon and Mars, for which the architecture includes both crewed and uncrewed vehicles, include noncryogenic or Earth-storable propellants such as hydrazine or advanced storable propellants. Requirements for the high performance and high reliability of propellants following long-term exposure to the harsh environments in space and, for some systems, the surfaces of the Moon or Mars, will demand greater understanding of propellant storage and handling.
Cryogenic Fluid Management
Advances in in-space cryogenic fluid management technology can improve the affordability and performance of orbiting cryogenic propellant depots and hence the feasibility of long-duration exploration missions.37 Research areas of particular interest include passive insulation and active cooling for zero boiloff; zero-gravity propellant transfer, including the automated coupling of cryogenic fluid lines; gauging the quantity of propellant in the tanks;38 and the role of capillary forces in propellant management.39 In addition, low-mass, cryogenic compatible, thermally insulated multifunctional materials for tank storage and fluid line transfer could potentially enhance safety and affordability over using multiple layers of material for each function. Micrometeroid protection for long-term propellant storage depots will require extensive materials interaction research to ensure that appropriate models are developed for depot replenishment schedules. Because some of the liquid could be dispersed as drops or globs floating in random parts of the container, the gauging of liquid quantity, both contained and transferred,
in low gravity presents new, important problems. This issue is exacerbated by the low density of most cryogens, which will require large storage facilities.
Propellant boiloff mitigation can be achieved through the use of both active and passive systems. Passive techniques such as multilayer insulation and vapor-cooled shields are more mature than active systems, but they cannot achieve zero boiloff. The current state of the art in passive insulation limits propellant loss of liquid hydrogen to approximately 3 percent per month, a level not sufficient for long-term propellant storage. Active thermal management systems use a refrigeration system, called a cryo-cooler, to keep the propellant below its vaporization temperature. Research and demonstrations are required to assess refrigeration versus reliquefaction of the cryogens with respect to minimizing system mass and power requirements.
For in-space cryogenic transfer, new technologies are required for leak-free connect/disconnect and fluid transfer. For fluid transfer, a number of technologies could be applied including linear acceleration (where the fluid is pushed toward the feedline) or angular acceleration (where the fluid is forced to the outside of the tank for collection). The in-space depot will be required to continuously supply vapor-free supercritical cryogenic liquids to an orbital transfer vehicle at an acceptable flow rate and pressure drop. Research and development is needed to characterize and develop a design database for fundamental screen wicking characteristics, surface tension data, stratification, and screen channel outflow performance with cryogenic fluids.40 A low-gravity mass gauging with an accuracy of better than 5 percent of fill tank will be required for the in-space depot. Although mass gauges available now lack the desired accuracy needed for low-gravity applications, two concepts—the compression mass gauge and the optical mass gauge—appear promising.
As noted above, some future space architectures will include propulsion using liquid oxygen as oxidizer with both hydrogen and methane engines. Although the technology for these engines is well known for Earth applications, critical technologies are needed for lunar and planetary descent and ascent in zero or reduced gravity, including engine start, combustion stability, and deep throttle. These technologies require research in cryogenic fluid management, propellant ignition, flame stability, and active thermal control of the injectors and combustors. The associated physical processes41 should be understood and predictable over the full range of gravitational variation, orientation, acceleration, temperatures, fluid phases, destratification, etc.
Noncryogenic (Earth Storable) Propellants
Earth-storable, hypergolic propellant engines for long-term placement of launch assets (uncrewed and crewed) on other moons and planets require a good understanding of the impacts of fluid migration, tribology for fluid handling devices, and chemistry (mixing or separation) in low-gravity environments. These technologies will be essential not only for propulsion but also for ISRU for relevant missions.
Supersonic Retro Propulsion and Aeroassist
For large-payload delivery to Mars and other planetary bodies with atmospheres, the use of large inflatable aerodynamic decelerators or rigid biconic/ellipsled aeroshells for atmospheric entry and descent deceleration, coupled with supersonic retro propulsion, could reduce the propulsive DV requirements by an order of magnitude relative to an all-propulsive descent system. The reduction in propulsive DV from aeroassist systems could significantly reduce IMLEO by factors of approximately 2 to 5 (depending on assumptions about system performance and mass), resulting in mission masses comparable to NEP or NTR systems.42 Similar aeroassist technologies could also be used for planetary braking, eliminating the orbit insertion burns required for planetary orbit capture. Inflatable aerodynamic decelerators are projected to have a 35 percent mass advantage over rigid systems and an 80 percent mass advantage over all-propulsive systems.43 However, the technology has not been demonstrated in a flight environment.
Current technology gaps include the materials and technologies to reduce the mass of inflatable and rigid
aeroshells; the dynamic stability and thermal response of an inflatable structure at high Mach numbers; control requirements; the integrated precision landing performance including parachute performance and atmospheric uncertainties; and thermal protection system requirements, materials, and safety. Basic research is needed for the development of high-strength, high-temperature inflatable materials and for understanding the system’s dynamic response in a planetary atmosphere. In addition, the flight environment is critical to the system design (including modeling and verification of stochastic atmospheres) and to the flow-field physics of the system (including chemical kinetics, boundary layer transition, and surface catalysis).
After aeroassist maneuvers decelerate the spacecraft from Mach 5 to Mach 2, supersonic retro propulsion completes the deceleration and landing. Unknowns include the interaction of the rocket plume with a planetary atmosphere at supersonic conditions; modeling of vehicle static and dynamic stability and control; the thrust and drag performance; the stability of the rocket nozzle; and the aerodynamic heating from the exhaust and flow field, including the radiation from the bow shock if one exists.
Nuclear Thermal Propulsion
NASA will benefit greatly from the development and demonstration of space nuclear reactors capable of supporting NTR44 and NEP systems for missions beyond Mars and/or to enhance Mars exploration transportation capabilities. Although expensive to develop, the NTR is one of the leading propulsion system options for human Mars missions.45 Such systems would have high thrust (≅ 50,000 N) and high specific impulse (Isp ≅ 825-971 s),46 a capability approximately twice that of today’s oxygen/hydrogen chemical rocket engines. Hydrogen is the only working fluid for competitive NTR systems. Demonstrated in 20 rocket/reactor ground tests in the 1960s during the Rover/NERVA (Nuclear Engine for Rocket Vehicle Applications) Programs, the NTR uses heat from a fission reactor to directly heat liquid hydrogen propellant for rocket thrust.47 NTR performance is limited by the temperature limits of the nuclear fuels and the regenerative cooling of the chamber.
Technology gaps include recapturing and updating the technology proven in the 1960s, ground tests, improving safety, reducing mass for affordability, and investigating long-life performance and reliability. Specific research and technologies include thermal control systems, efficient energy conversion and thermal transfer technologies, and lightweight/very high temperature thermal structures, along with safe and acceptable testing facilities. Enabling research includes the characterization of fissionable materials, development of a highly efficient, low-mass radiator, high-temperature superalloys and refractory metals for fuel cladding, and power conversion equipment to enable higher-temperature cycle operations and heat rejection. High-power (~0.5-MWe to multi-megawatt-class) helium-xenon Brayton or potassium Rankine power conversion units and efficient, long-life, high-power electric propulsion thrusters in the ~0.5- to 2-MWe size range are also desirable to contain the number of operational power conversion units and thrusters on the spacecraft. For bimodal systems providing both propulsion and modest (tens of kilowatts) power for life support and other subsystems, research and demonstration are required on the integration of a secondary closed He/Xe coolant loop into the engine design for the Brayton power cycle.
Nuclear Electric Propulsion
The NEP system is a low-thrust, high-specific-impulse option for transferring cargo for space exploration. It requires long transit times but significantly less propellant than systems with higher thrust. As a 2000 NRC report points out (at p. 41), “The energy source for NEP would be nuclear fission, with the generated heat transferred from the reactor to a suitable working fluid, rather than directly to a propellant gas as in the NTR. The working fluid would then be used to generate electric power through a thermodynamic cycle, and that electric power would drive a plasma or ion thruster…. [An NEP system would avoid] the temperature limitation on specific impulse that characterizes the NTR, and specific impulse can be thousands of seconds.”48 However, NEP systems would need high-burn-up/high-temperature nuclear fuels such as cermets, and high-temperature superalloys and refractory metals would be important for fuel cladding and power conversion equipment.
In addition to the nuclear technology needed to develop a high-power, long-life space nuclear power reactor, research gaps exist in the heat exchanger, thermal control, and scaling up of the electric thrusters in terms
of power and longevity under continuous instead of intermittent operation. A Rankine cycle power conversion system would require fluid handling equipment such as piping, valves, pumps, turbines (with associated bearing systems), seals, and controls, all of which tend to be especially complex for the Rankine cycle, and design for high reliability will be correspondingly important. According to the 2000 NRC report (at p. 42), “Start-up and shutdown in orbit poses complex issues of design and operation, especially for a Rankine cycle with a liquid-metal working fluid and a nuclear heat source. Thawing … [in reduced gravity] of the nuclear reactor is required and the entire system must, in a controlled way, be brought to an equilibrium appropriate to steady-state operation. Transient, intermittent, or variable operations generally must also be considered carefully, taking account of system dynamics and possibilities for system instabilities … the propellant material, … [whether] liquid metal or noble gas, must be stored and maintained for especially long times,” and the effects of NEP effluents on space design and operations must be defined.49,50
Several types of electric thrusters developed primarily by NASA have reached a high TRL and have been transferred successfully to commercial applications after decades of developments. These systems are used for small-scale intermittent applications (e.g., station keeping and orbit positioning).51 Electric propulsion engine technology with power levels up to about 100 kW per thruster has been demonstrated at TRL 3 to 4; development of a megawatt-class NEP may need to include investigation of megawatt nuclear power sources and condensable propellant options. Development of megawatt nuclear power sources is required for either clusters of moderate power thrusters or the parallel development of megawatt power thrusters. In addition, condensable propellant options would pose unique issues for propellant transfer and management in very low gravity. Ground-based experiments, advanced modeling, and in-flight demonstrations would advance the understanding of fundamental processes involved in electric propulsion, leading to thrusters with greater performance and longer life, thereby enabling or enhancing some future exploration missions.
Solar Electric Propulsion
SEP is another low-thrust option appropriate for transferring cargo to the Moon or Mars. Unlike NEP, SEP can operate at a small scale, and a SEP system could also support small robotic exploration missions at an affordable cost. The feasibility of prospective SEP systems has been greatly enhanced by recent advances in solar cell efficiency and packaging. While low-power (kilowatt-class) SEP is currently widely applied on communications satellites, high-power systems would require significant additional development. A critical step for high-power SEP is the development of solar arrays with specific power in the 200-300 W/kg range. Based on recent demonstrations,52 such arrays can be made operational during the next decade. They also can operate at very high voltages and are radiation resistant. Mission analysis studies53 showed that a SEP freighter could deliver 10,000 to 20,000 kg to the lunar surface once per year for at least 5 years at a saving of >$2 billion compared to chemical propulsion systems. SEP is thus a viable and critical technology for future science missions.
A key issue presently unaddressed by supporting R&D is a space-based architecture using high-efficiency cargo transfer and significantly higher specific power and associated structural dimensions. Space-basing requires automated flight operations well beyond the state of the art, including formation flying, precision pointing, reducing physical disturbances in the arrays during continuous operation of the thrusters, and control of large space structures. The size, structural characteristics, and other attributes of the solar power array of a SEP system imply more complex operations than were being addressed54 for the Orion element of the Constellation program, though they have much in common with current ISS and higher power geosynchronous satellite operations. The mass of the large structures would be minimized to increase payload capacity and/or reduce transit times. This would require validated knowledge of the structural behavior of very large, low-strength systems and innovative, likely dynamic, modes of control during flight.55 With the present level of theoretical understanding, high-fidelity validation of flight operations and control for a high-power SEP will likely require high-fidelity ground testing as well as in-flight, ISS-captured, or free-flying demonstrations at an appropriate scale.
For robotic and specialized remote sensing applications, electric micropropulsion would be required. This would involve either scaled-down versions of thrusters or thrusters fabricated with micro-electromechanical
systems. Micropropulsion could be used to enhance and enable microspacecraft (mass less than 100 kg) or pico-spacecraft (mass less than 10 kg).
High-power (megawatt) SEP is an alternative propulsion option to enable large payloads and short transit times for crewed space exploration missions.56 Representative high-thrust SEP concepts include magnetoplasmadynamic and pulsed-inductive thrusters.57 Research includes the scale-up from currently demonstrated kilowatt thrusters to megawatt class, with the associated requirements for lightweight power, efficient thermal control systems, long-life components, and high-efficiency/low-cost propellants similar to those for NEP systems.
Advanced Propulsion and Propellants
Magnetohydrodynamic (MHD) fluid accelerators are an example of a high-thrust, mid-TRL propulsion system. MHD fluid accelerators could potentially double or triple the specific-impulse performance of in-space cryogenic propellant rockets if adequate electrical power were available. This performance increase is accomplished by using the Lorentz force produced by crossed electric and magnetic fields to accelerate charged particles in the propellant. Conventionally, rocket performance is restricted to approximately 3,000 K because of material and active-cooling constraints; however, by direct infusion of power MHD accelerates the flow without increasing the temperature.58,59,60
The technology gaps for MHD are the large mass of the magnets and the high electrical power (and high currents) needed to drive the magnets. Research is required to develop highly conducting, low-mass magnets. Power could be supplied by nuclear fission or large photovoltaic collectors (for missions close enough to the Sun); however, the mass of the spacecraft power system could be greatly reduced through the use of microwave or direct concentrated solar beaming. Although the concept of power beaming to space has received attention since the 1980s, convincing flight tests or proof-of-principle demonstrations are yet to be conducted. Critical gaps still exist in advanced methods of systems dynamics and flexibility and the controls for precise alignment. At best, success requires an optimal location for a large power source with a stable line of sight to the spacecraft. The power requirements of MHD fluid accelerators may be comparable to the power requirements of high-power thrusters that would be needed for a large SEP or NEP system.
In addition to improving propulsion performance through power advancements, advanced propellants also have the potential of high specific impulse, ease of storage, and improved safety. Metallized-gelled propellants have shown dramatic payload improvement with large increases in bulk density and modest improvements in specific impulse.61
Summary of Enabling Science and Technologies for Space Propulsion
In-space propulsion systems represent 50 to 90 percent of the mass that has to be delivered to low Earth orbit, and therefore presents an opportunity for reducing the cost of the Exploration Program. Advances in propulsion performance (specific impulse, efficiency, thrust-to-weight ratio, propellant bulk density), reliability, thermal management, power generation and handling, and propellant storage and handling are key drivers to dramatically reduce mass, cost, and mission risk. The following recommendations address exploration technologies in space propulsion that would benefit from near-term R&D.
Before 2020: Required
Zero-Boiloff Propellant Storage. Research and technology is needed to enable zero-boiloff propellant storage for orbiting depots, on-orbit refueling, and long-duration exploration missions. To support this capability, physical sciences research should be conducted on advanced insulation materials, active cooling, multiphase flows, and capillary effectiveness. (T2)
Cryogenic Handling and Gauging. NASA would benefit from enhanced cryogenic fluid management for in-flight refueling, propellant depots, and long-duration planetary missions. Research in cryogenic handling and gauging
under low-gravity conditions is needed. Research in in-space cryogenic fluid management includes active and passive storage, fluid transfer, gauging, pressurization, pressure control, leak detection, and mixing destratification. (T3)
2020 and Beyond: Required
Ascent and Descent Technologies. NASA will require lunar and planetary descent and ascent propulsion technologies, including engine start after long quiescent periods, combustion stability at all gravity conditions, and deep throttle. Areas of research include cryogenic fluid management, propellant ignition, combustion stability, and active thermal control of the injectors and combustors. (T14)
Inflatable Aerodynamic Decelerators for Bodies with Atmospheres. The availability of inflatable aerodynamic decelerators would reduce propulsive mass requirements. To develop these systems, physical science research is required in high-strength, low-density, high-temperature flexible materials; dynamics, stability and control; and aeroelasticity in the flight environment. (T15)
Supersonic Retro Propulsion System. A combination of a supersonic retro propulsion system with an aerodynamic decelerator completes the deceleration and landing and would reduce mass requirements. To enable development of such a system, physical sciences research is needed on flow-field interactions of the rocket plume with the atmosphere, aerothermodynamics of the flow, and the dynamic interactions and control of the vehicle. (T16)
Space Nuclear Reactors. The development and demonstration of space nuclear reactors capable of supporting nuclear thermal rockets (NTRs) are required for missions beyond Mars and/or to enhance Mars exploration transportation capabilities. Required technologies include thermal control systems, efficient energy conversion and thermal transfer technologies, and lightweight/very high temperature thermal structures, along with safe and acceptable testing facilities. Enabling physical science research has been identified in the section “Space Power and Thermal Management.” Additional physical science research is required on liquid-metal cooling under reduced gravity, thawing under reduced gravity, and system dynamics. (T17)
Solar Electric Propulsion (SEP) Technologies. SEP is an important option for the efficient transfer of propellant and cargo to distant locations. To support the development of such systems, advances are needed in understanding the complex behavioral modes of very lightweight large space structures so as to enable development of innovative control methods for such structures during flight. In addition, research is needed in condensable propellants and in propellant transfer and management in very low gravity. (T18)
2020 and Beyond: Highly Desirable
Nuclear Electric Propulsion (NEP) Technologies. NEP will enable the very efficient transfer of propellant and other cargo to extended outposts on Mars and beyond. Areas of research, in addition to those summarized above under Space Nuclear Reactors (T17), include propellant management under reduced gravity and flow processes in electric thrusters.
A new vision for EVA systems is emerging for exploration missions—one that encompasses spacesuits, rovers, and robotic assistants working collaboratively during mobile exploration sorties. In the past, EVA has enabled complex work outside a crewed space vehicle or lunar module, contributing a supporting operational role (repair, maintenance, observation, etc.). However, since the end of the Apollo missions, EVA has not typically served a primary mission role (excluding the EVAs on the Hubble Repair Missions). For space exploration missions to
planetary surfaces (Moon/Mars/Flexible Path¶¶), EVA resumes a primary, required mission role that is critical to enabling successful mission operations, gaining new scientific knowledge, and gaining experience with exploration systems.
History of Extravehicular Activity in NASA Programs
NASA and Russian EVA systems documentation62,63,64 provides necessary background, information on development, and knowledge about EVA capabilities. NASA’s experience with EVA dates to the Gemini Program in 1965. As Jordan et al. have pointed out, “The capability for humans to work outside the spacecraft [has] proved invaluable time and again. For example, Skylab astronauts performed 12 contingency EVAs to fix unanticipated problems, repeatedly saving Skylab from abandonment. However, despite the advantages of EVA capability, early space shuttle designs did not include the means to perform EVA.”65
Historically, an EVA system has consisted of the following components: a spacesuit; a portable life support system that provides the suit with a breathable atmosphere while removing carbon dioxide, water vapor, and trace contaminants; suit subsystems providing pressurization, mobility, temperature control, power, communications, and data systems, as well as protection from radiation and particle impacts; rovers and mobility aids; and tools (including robotic tools) that enable the EVA crew member to accomplish necessary mission tasks. NASA developed pressure protection during the Mercury program and took a clean-sheet approach to spacesuit design for each of the Gemini, Apollo, and space shuttle programs. An Apollo spacesuit was adapted for Skylab operations, and the space shuttle Extravehicular Mobility Unit (EMU) was enhanced and certified to meet the requirements for the ISS.
Because of indecision about developing a shuttle spacesuit, EMU development lagged behind the shuttle by approximately 4 years, which resulted in a very compressed development phase and certification and flight testing carried out in parallel with early shuttle flights. The most extensive information on the EMU can be found in Balinskas and Tepper’s contractor report66 and the Hamilton Sundstrand EMU Data Book,67 as well as in conference proceedings.68-78
Existing NASA EVA systems include (1) a launch, entry, and abort (LEA) suit or an advanced crew escape suit (ACES), used inside a spacecraft or in emergencies, and (2) the EMU for microgravity EVA. The launch and entry suit, a partial-pressure suit, became operational for space shuttle flights after the Challenger accident and was used until 1998. The ACES, a full-pressure suit introduced in 1994, is the current space shuttle suit worn for launch and re-entry. The ACES technology derives from high-altitude pressure suits (used for advanced vehicles such as the SR-71, U-2, and X-15) and includes parachute bailout capability and a self-contained emergency oxygen system consisting of two 120-cubic-inch oxygen bottles that can provide at least 10 min of oxygen.79
The EMU is essentially a self-contained miniature spacecraft in the sense that it provides all of the functions necessary to sustain life in a human-sized, mobile form while enabling useful work in space. The EMU has undergone evolutionary development to meet the needs of microgravity spacewalks for the space shuttle as well as the ISS. The system is optimized for a microgravity environment and enables crew members to perform complex tasks safely. It incorporates a closed loop, portable life support system (PLSS). As Jordan et al. have observed, “That the EMU was initially designed as a limited capability suit to satisfy minimal mobility and operational requirements is astounding in light of the fact that it has subsequently been used to repair satellites, construct a massive space structure, and maintain the ISS…. [In fact] the EMU, like most complex engineering systems, has faced considerable uncertainty during its service life. Changes in the technical, political, and economic environments have often caused changes in requirements, which in turn necessitated design changes.”80 Initially, a new, advanced spacesuit to service the ISS and to serve as a test bed for planetary exploration technologies was planned. The 1989 decision not to build a space station-specific suit and instead to modify the EMU for construction and operation of the ISS resulted in 10 system requirements changes and 23 design/procedural changes.
¶¶ The “flexible path” architecture to inner solar system locations, such as lunar orbit, Lagrange points, near-Earth objects, and the moons of Mars, followed by exploration of the lunar surface and/or martian surface, was put forward by the Augustine Commission (Review of U.S. Human Spaceflight Plans Committee, Seeking A Human Spaceflight Program Worthy of a Great Nation, Office of Science and Technology Policy, Washington, D.C., October 2009).
TABLE 10.2 Summary of U.S. Extravehicular Activity Duration by Program (1965 to 2009)
|Program||Total EVA Duration (hours)||Suit Used|
|International Space Station||835:02||EMU|
bD.S.F. Portree and R.C. Trevino, Walking to Olympus: and EVA Chronology, Monographs in Aerospace History Series #7, 1997.
Table 10.2 shows that, in the 40-year history of human spaceflight, NASA astronauts have logged approximately 3,000 h of total EVA time, the vast majority in EMUs on the space shuttle or the ISS. The total EVA duration for each new generation of spacesuit design has increased by more than an order of magnitude: 13 h for the G-4/G-5 series, 248 h for the A7L series, and 2,729 h for the EMU. A new paradigm is emerging for exploration missions in the next decades: with the “mountain of EVA” (Figure 10.7), another 10-fold increase in EVA operational hours is projected for exploration missions to the Moon or Mars. This large increase in EVA hours is relevant for any future mission to the Moon or Mars and is not dependent on a particular mission or architecture.
The literature and past scientific and technical recommendations regarding EVA systems81-84 are assumed as baseline knowledge and are not recapitulated here. The focus here is on issues relevant to NASA in the next decade for achieving a translational portfolio to enable exploration missions to meet research and operational objectives in the life and physical sciences.
Future Extravehicular Activity Needs
The requirements for future EVA systems include (1) crew safety and mobility during LEA, (2) contingency microgravity EVA capability utilizing an umbilical, and (3) surface EVA capability.85 The vision of mobile exploration EVA encompasses astronauts, rovers, robotic assistants, and possible mobile laboratories or bases in the next decade. Future EVA system concepts might include two spacesuit configurations. Suit 1 would provide an initial EVA capability for LEA and contingency microgravity EVA; Suit 2 would be designed for surface EVA capability. The two configurations would be modular designs incorporating many shared design elements and components. However, NASA has yet to develop a plan and processes to achieve this vision. NASA should further its interaction with systems engineering experts in industry, academia, and the DOD to leverage their knowledge and experience in fielding related systems with shared, modular components.
In addition to incremental design concepts that extend current EVA systems, bold design innovations should be encouraged, such as the Bio-Suit originally developed at the Massachusetts Institute of Technology under a NASA Institute for Advanced Concepts contract.86,87
EVA suit elements include the pressure garment subsystem; life support subsystem; and power, communications, avionics, and informatics (PCAI) subsystem. The capability to conduct EVA surface operations is a primary and critical component of future surface exploration missions, regardless of exploration destination. NASA has appropriately defined system capabilities and technologies for the Suit 1 concept, and the objectives for that concept are well understood. Current NASA and contractor knowledge and experience seem sufficient to implement the Suit 1 concept, which relies on existing and proven technologies; requirements definition and vehicle interface design reviews for it are ongoing. However, the panel did identify a research gap in NASA’s Suit 1 concept in the area of joint mobility. EVA suits should have detailed and accurate specifications for mobility, joint torque, and joint range of motion requirements for both intravehicular activities and EVAs. Torque specifications based
on prototype suits should be verified and refined using precise torque measurements that account for realistic, multijoint motions that an astronaut would naturally make if unrestricted by a suit. Further research is needed to improve EVA suit joint flexibility and conformal fit while considering anthropomorphic differences between men and women to prevent injuries to crew and enhance safety during lunar and planetary explorations.
The current EMU glove has caused problems with astronaut finger tip trauma. Improving space suit glove design is a suit-independent design challenge and represents a crosscutting, multidisciplinary research question.88-91
Developing a new EVA system to support anticipated surface operations on the Moon, Mars, or asteroids presents significant technical challenges, especially with regard to providing “mobility equal to that of an Earth-based geologist.”92 The engineering and biomedical requirements for pressure production, a breathable atmosphere, thermal control and ventilation, carbon dioxide removal, and waste management for surface EVA systems are understood and well specified in the current EVA system plans. However, critical elements of Suit 2 still require additional research and significant technological advances to provide enhanced exploration EVA capabilities. In particular, research and technology development is vital in the surface PLSS, the PCAI and information technology, and the interaction of the EVA suit with the surface environment.
A previous examination of the Exploration Technology Development Program stated that promising planetary PLSS technology enhancements include new heat-rejection technologies, variable pressure regulation, a rapid cycling amine swing bed for CO2 control, and metabolic temperature swing absorption designs, even though some of these technologies are still at a low TRL.93 The PCAI systems for surface operations will require additional capabilities. The DOD has made numerous advances in battery technology, displays, and speech and audio communications;94 many of these technologies could be incorporated into or adapted to suit the EVA PCAI subsystem.
The aggregate risk from micrometeoroids and orbital debris during EVA over the life of the ISS has been doubled by the extension of the ISS mission to 2020. Thus, possible suit penetration risk due to micrometeoroids or orbital debris remains an ongoing concern.
For surface operations, protection from environmental hazards becomes a significant concern. Currently there are gaps in our understanding of and technological capabilities to mitigate harmful effects of continual dust exposure and possible micrometeroid impacts on EVA systems. The effects of dust on the functionality and operational life of equipment are well known.95,96 All of the Apollo missions were adversely affected by lunar dust. Significant effects included (1) visual obscuration from blowing dust while landing, (2) false instrument readings, (3) dust coating and contamination, (4) clogging of mechanisms, (5) abrasion, (6) thermal control problems, (7) seal failures, and (8) irritation from dust inhalation.97 These problems probably could have been reduced if, early in the Apollo program, NASA had developed a better simulation environment including better simulants, higher vacuum, correlated simulations, and more realistic thermal and illumination environments.98 Astronaut Richard Gordon reported:
The cabin atmosphere was okay. On the way out, it was clean. On the way back, we got lunar dust in the command module. The system actually couldn’t handle it; the system never did filter out the dust, and the dust was continuously run through the system and throughout the spacecraft without being removed.99
Knowledge gaps in dust mitigation still exist in the areas above.
Research in dust mitigation techniques and technologies will need to consider the specific requirements of the different surface environments. On the lunar surface, the dust transport phenomena are due to thermophoresis and electrostatics forces in vacuum, whereas on the martian surface, dust is transported primarily by CO2 gas convection in the martian atmosphere. The strong martian winds, which are driven by the seasonal and diurnal insolation cycles combined with the effects of planetary rotation, cause the dust to suspend in the martian atmosphere for a long time.100 The structure of dust particles is also different: martian dust particles are likely to be more rounded than lunar dust. The electrostatic charge is greater on lunar dust than on martian dust.
Requirements do not currently exist for thermal control and radiation protection for EVA systems, rovers, and habitats in the partial-gravity environment. The gaps in our understanding of thermal control and radiation biology are discussed in Chapter 7. Accumulation of surface electrical charge during long-duration EVAs has to be investigated within the context of Earth, the Moon, or martian environments. Plasma interactions with astronauts
during EVAs in proximity to space structures with high-power, high-voltage solar arrays are also of particular importance.101 Plasma interactions with suits should be further investigated.
For long planetary exploration missions, it will be important to reduce overall system mass (on-back and total mass, including expendables) beyond those attainable with current technologies and to expand the exploration radius from the lander/habitat by increasing system robustness and fault tolerance.
Finally, from a systems engineering perspective, it is important that EVA capabilities be integrated into the rest of systems and operations development from the earliest stages, in order to reduce life cycle and operational costs. Biomedical and EVA systems relationships and synergies should be identified for exploration missions. EVA technologies, human performance, and life and physical science phenomena in partial gravity need to be better understood. Design principles that include EVA systems with all other lunar/Mars surface systems and operations should be followed to produce an overall integrated mission architecture.
Summary of Enabling Science and Technologies for Extravehicular Activity
The following exploration technologies that are important to EVA would benefit from near-term R&D.
Before 2020: Required
EVA Suit Mobility Enhancements. NASA can enhance human performance and mobility in the next generation of EVA systems by minimizing astronaut injury and improving comfort. Significant mobility enhancements are also required. Research should be conducted to minimize joint torques, improve suit comfort, improve suit glove design, provide the equivalent of shirt-sleeve mobility, develop trauma countermeasures, and improve physical systems performance across the spectrum of gravity environments. Areas of biological and physical science research that will enable the development of these enhancements include suited astronaut computational modeling, biomechanics analysis for partial gravity, robot-human testing and quantification of advanced spacesuit joints and full body suits (joint torque characterization), and musculoskeletal modeling and suited range-of-motion studies as input for garments designed for comfort, protection, minimizing injury, and enhancing mobility. (T4)
Dust and Micrometeoroid Mitigation Systems. Benefits would arise from improved durability and maintainability of suits and EVA systems by developing technology to mitigate effects of dust and mitigate risks from micrometeoroids, orbital debris, and plasma. Physical science research should address the impact mechanics of particulates, the design of outer layer dust garments, advanced material and design concepts for micrometeoroid mitigation, possible magnetic repulsive technologies, and the quantification of plasma electrodynamic interactions with EVA systems. (T5)
2020 and Beyond: Required
Radiation Protection Systems. Define requirements for thermal control and radiation protection for EVA systems, rovers, and habitats and develop a plan for radiation shelters. (T19)
2020 and Beyond: Highly Desirable
Suit/Helmet Modular Information Technology Architecture. NASA would benefit from an enhanced architecture that maximizes exploration footprint and scientific return. Areas of biological and physical science research that enable the development of such an architecture include human factors engineering, display and information technology system design, and human-machine interactions with an emphasis on planning and real-time navigation.
PCAI Systems. NASA should leverage technology advances made in other agencies and in industry to improve its own PCAI capabilities. Advances in battery technology, displays, and speech and audio communications by DOD and industry should be incorporated into NASA’s EVA PCAI subsystem. Areas of biological and physical
sciences research that enable the development of these systems include advanced biomedical sensors, physiological monitoring, and health advising capabilities.
Planetary PLSS Technology. PLSS enhancements should include new heat-rejection technologies, variable pressure regulation, rapid cycling amine swing bed for CO2 control, and metabolic temperature swing absorption designs.
A life support system (LSS) is necessary for space vehicles, rovers, and EVA systems. LSS functions include pressure control; atmosphere revitalization (removing carbon dioxide, water vapor, and trace contaminants); temperature and humidity control; waste collection; and fire prevention, detection, and suppression. Fire safety is discussed in the next section of this chapter. Dust mitigation and radiation protection are significant challenges as well. The literature and past scientific and technical recommendations regarding life support systems and technologies are assumed as baseline knowledge for this report. (Please see findings and recommendations on LSS in NRC reports published in 1997, 2000, and 2008.102,103,104) The focus here is on issues relevant for NASA in the next decade to achieve a translational portfolio to enable exploration missions to meet research and operational objectives in the life and physical sciences.
A range of atmosphere compositions are in use or proposed for various aspects of space exploration. Humans can survive only a few minutes without oxygen. Many biological responses are dependent on gas partial pressure, and total gas concentrations influence design tradeoffs in atmosphere selection, especially for space vehicles, habitats, rovers, and EVA systems.105 At sea level on Earth, O2 concentration is 21 percent and the normal O2 partial pressure is 21 kPa. While human physiological needs set the requirement for partial pressure of O2, fire risk increases as the absolute concentration of O2 increases, and the level of engineering difficulty increases as the total pressure increases (for example, on joints and seals). Thus, the selection of the oxygen environment involves a tradeoff between engineering and fire safety issues. The atmosphere inside space vehicles and habitats can range from pure oxygen at low pressure to oxygen/nitrogen mixtures approximating Earth’s atmosphere in both concentration and pressure. The current plan for future space vehicles is to use an oxygen-nitrogen atmosphere with up to 34 percent O2. By contrast, the NASA EMU spacesuit supplies the astronaut with 100 percent oxygen, which is why the suit can be operated at a low pressure of 29.6 kPa (4.3 psi). Generation of O2 can itself be a source of fire or explosion, as was the case in the Mir space station fire, so the method of O2 generation should also consider fire safety implications.
Future Life Support System Needs
The LSS must provide adequate thermal control to maintain a suitable internal temperature regardless of internal activities (by astronauts and equipment) and the external environment. The LSS must also remove water vapor released by crew members and collect waste (fluids and solids). The space shuttle and the ISS provide a shirt-sleeve working environment for astronauts and various life support equipment. The ISS uses a pumped single-phase thermal bus to collect waste heat and transport it to heat rejection radiators.106 For permanent settlements on the lunar or martian surface, environmental control of habitat, rovers, and in situ resource processing factories will be required, analogous to terrestrial HVAC systems, as well as a thermal bus.
NASA has appropriately defined LSS capabilities and technologies to meet the objectives for planned missions to the ISS and the Moon, including a new space vehicle, rovers, EVA systems, and surface habitats.107 For missions to the ISS and initial short-duration (approximately 7-day) missions to the Moon, the LSS for space vehicles, including the lunar lander, would include a water loop to supply and store potable water; collection of human waste, which is then discarded; an oxygen loop to store oxygen; and a rapid cycling amine bed to collect CO2, which is then vented overboard. The significantly enhanced LSS for a more permanent lunar presence would include continuous habitation for a crew of perhaps four. The current specifications and tradeoffs for this LSS include moving toward full closure of the water and oxygen loops, which necessitates wastewater recovery, brine
recovery, and solid waste drying; O2 generation from H2O; and CO2 reduction (to CH4 or C). If lunar ISRU oxygen becomes available, full closure would no longer be necessary and the CO2 reduction step might be eliminated. Capabilities and technologies that need to be further investigated and matured include electrolysis (for the Environmental Control and Life Support System, ISRU, and energy storage applications); CO2 removal for habitats, pressurized rovers, and spacesuits; and compatibility with CO2 reduction capabilities. Fluid system components (pumps, valves, sensors, etc.) need to be investigated and assessed for functionality in partial-gravity environments, in the context of system performance. Knowledge gaps in two-phase flow in partial gravity need to be filled.108
The Constellation Program’s lunar architecture specifically emphasizes the goal of improving reliability and functionality of EVA and LSS.109 Dust mitigation and thermal control on the lunar surface should be high priorities, but they had been omitted from NASA’s prior lunar architecture. In the longer term, advanced bioregenerative life support technologies could support self-sufficient human habitation of space, as detailed in Chapter 4 of this report.
Environmental factors that need to be considered when designing an LSS for the lunar surface are (1) dust, (2) extreme temperatures and high temperature gradients, (3) radiation, (4) micrometeoroid impact, (5) low pressure, and (6) different gravity from Earth. As described in the EVA section above, significant knowledge gaps still exist in dust mitigation. Effects of extreme temperature and high temperature gradients on equipment, systems, and habitats are known.110 Researchers111 have suggested coupling ISRU mining and habitat construction as a way to control temperature swings and protect crew and equipment from cosmic radiation. Balasubramaniam et al.112 performed computer simulations that show the advantage that thermal wadis can provide as a way to reduce the temperature swings. However, none of these technologies have been demonstrated in a relevant environment. Closed-loop air revitalization and closed-loop water recovery have been NASA goals since the 1990s. The developmental systems for neither capability have yet achieved a sufficiently high TRL.113,114,115
Cryogenic (<120 K) refrigeration of fluids stored in the liquid state and cryo-cooling of superconductors and infrared sensors will also be required. The development and adaption of cryogenic technology for life support systems would lead to more efficient storage of life support consumables (such as oxygen), provide superconductor technology for life support equipment, and lead to thermoelectric refrigeration for the storage of food supplies. These needs require adaptation of refrigeration and heat pump technology to the harsh surface temperatures, dust, and reduced gravity of exploration targets.
Finally, the panel notes that NASA should enhance its collaborations with ongoing international life support experiments and simulations (i.e., the European Space Agency, Japan, and Russia).
Summary of Enabling Science and Technologies for Life Support
NASA should undertake an incremental addition of advanced life support technologies and systems into existing designs in such a way as to establish a reliable performance track record before they are relied on as the primary LSS. The following exploration technologies that are important for LSS would benefit from near-term R&D.
Before 2020: Required
Fluid and Air Subsystems. NASA should undertake partial-gravity characterization and testing of fluid and air subsystems of new life support systems. Required research includes heat and mass transfer in porous media under low and microgravity conditions. (T6)
Before 2020: Highly Desirable
Environmental Atmospheric Sensors. Long-life atmosphere monitors should be developed and demonstrated to identify major atmosphere constituents, as well as trace contaminants that may be harmful to humans in a closed habitat.
2020 and Beyond: Required
Radiation Protection Systems. As described in Chapter 7, NASA would benefit from additional work in radiation shielding, countermeasures, and mitigators. NASA should continue to develop radiation protection and mitigation technologies and demonstrations. (T19)
Thermoregulation Technologies. NASA should develop and demonstrate technologies to support thermoregulation of habitats, rovers, and spacesuits on the lunar surface. Areas of biological and physical science research that could help enable the development of those technologies include understanding the human response characteristics to extreme physical conditions (temperature, pressure, oxygen level, etc.) and developing materials and methods to protect humans from these extreme conditions. (T20)
Closed-Loop Air Revitalization. NASA could achieve a significant savings in resupply/consumables by closing the air loop. Technology needs include CO2 removal, recovery, and reduction; O2 generation via electrolysis with high pressure capability; improved sorbents and catalysts for trace contaminant control; and atmosphere particulate control and monitoring. Fundamental physical sciences research to support the development of these technologies includes understanding the effects of gravity on the coupling of electrochemical systems with multiphase flow physics. (T21)
Closed-Loop Water Recovery. As with closed-loop air revitalization, NASA would benefit from significant savings in resupply/consumables with closed-loop water recovery. Technology needs include water recovery from wastewaters and brines, pretreatments, biocides, low expendable rates, and robustness. More fundamental work is also needed to assess the effect of variable gravity on multiphase flow systems (i.e., water management and recycling). (T22)
Dust Mitigation Technologies. Dust has a well-known adverse effect on life support systems. NASA should develop technologies to mitigate or eliminate the effects of dust on systems and hardware. Dust may coat solar energy harvesting systems, contaminate supplies, scuff equipment, create thermal control problems, and cause seal failures. NASA should develop, demonstrate, and test dust mitigation technologies needed to overcome these problems. Fundamental physical science research is needed to support the development of these technologies, specifically to understand the coupling of electrostatic fields and dust particulates dynamics in any dust mitigation technology. (T23)
2020 and Beyond: Highly Desirable
Solid Waste Treatment. NASA should develop and demonstrate long-duration waste stabilization and water recovery from solid wastes. As with closed-loop air revitalization, NASA would benefit from significant savings in resupply/consumables with recovery of water from solid waste.
Food Production and Bioregenerative Life Support Systems. As described in the recommendations in Chapter 4 of this report, NASA should develop an incremental approach toward the development of a bioregenerative life support system.
Although they are relatively rare events, fires have occurred in space vehicles and habitats and will occur again. Fire safety is critical to any human space exploration because fires can have devastating consequences, including loss of life, loss of vehicle or habitat integrity, and mission failure. Historically, fire safety R&D has been treated as a subset of combustion research. While both basic and applied combustion research support fire safety research and it is essential to pursue deeper understanding of the combustion processes involved (see Chapter 9), many of
the operational aspects of fire safety need to be defined in a manner that does not require a full understanding of the underlying basic principles of combustion. Because fire is a complex phenomenon that simultaneously involves many aspects of chemistry and physics, including kinetics, reaction dynamics, fluid mechanics, and heat and mass transfer in multiple phases, a comprehensive understanding of fire at the fundamental level does not currently exist. Instead, much of the current understanding of fire behavior is phenomenologically based, but with some reference to the underlying combustion physics and chemistry. As a result, many of the research and development needs for fire safety (fire safety R&D), especially at the translational stage, require the development of phenomenologically based models and correlations for materials response to fires, fire detection, fire suppression, and recovery from fire (and explosions). While both basic and applied combustion research are invaluable to spacecraft fire safety, there are other practical aspects of fire safety issues outside the scope of combustion science that are part of fire safety systems and thus are mission-enabling. In order to have a complete safety program, additional funding should be allocated to fire safety research separate from and in addition to funds allocated for combustion research, and the proposals for this research should be subject to end-user oriented reviews.
Fire safety for space exploration is predicated on the principles of prevention, detection, and suppression.116 Little work has been done on post-fire clean-up or recovery, largely based on the false belief that fires are rare events and can be prevented, so clean-up and recovery from them are unimportant. However, although fires are rare, they are inevitable, and fires have occurred wherever human exploration has taken place. Furthermore, the potential for damage to mission-critical systems necessitates that strategies be developed and put in place for post-fire clean-up and recovery, especially for operations far from Earth, where terrestrial help for recovery is unavailable on any reasonable time frame. Unfortunately, much of fire safety to date has been predicated on assumptions about fire behavior in reduced gravity environments that have been called into question by recent research. This research has raised serious issues about the underlying assumptions for all three fire safety principles: prevention, detection, and suppression.
Materials Flammability and Toxicity
The cornerstone of fire safety prevention for space exploration since the tragic fire in Apollo 1 has been the qualification of materials used in space vehicles and proposed habitats. This qualification is based on NASA Standard 6001, which uses an upward flame spread test in 1 g as a screening test for materials flammability. This test is predicated on (1) the belief that upward flame spread represents a worst-case condition for any space exploration environment and (2) the desire to have a simple, 1-g-based test for materials flammability. Unfortunately, recently reported work has challenged the assumption that 1-g flame spread is the most severe. These results, although very limited, indicate that upward flame spread peaks somewhere in the range of 0.15 g to 0.5 g, i.e., the range of lunar and martian partial gravity.117,118,119 As a result, there is a critical need going forward to reassess the validity of NASA Standard 6001 by developing substantial data on flame spread for various materials under relevant levels of gravity from 1 g down to microgravity levels, in order to develop more comprehensive screening tests for flammability of materials to be used in spacecraft and habitats. Furthermore, past research120,121,122 has shown that the yield of toxic products of combustion can be many times higher in fire retardant materials than in non-fire retardant materials. Since toxic and/or corrosive agents can pose a serious threat to human life in space vehicles and habitats, control of the production of such agents should be included in a revised NASA 6001 Standard.
Finally, fire properties of various materials are affected by the oxygen environment. While the human need for oxygen depends on the partial pressure of oxygen, ignition and fire spread on materials depend on the absolute concentration of oxygen.123 Recent moves to environments with higher concentrations of oxygen conflict with the current understanding of fire behavior in these atmospheres. Thus, more work is needed to establish the comprehensive definition of material flammability for spacecraft and the validity of material screening as a “preventive” measure. Furthermore, materials flammability should include defining the potential environment generated by a fire, in order to evaluate the tradeoffs between fire safety and human life support in various atmospheres and enable the design of an integrated human life support/fire safety strategy.
Recent work has called into question some fundamental assumptions on current fire detection practices in space. Although the space shuttle uses ionization technology for fire detection, the ISS switched to use of photoelectric detection technology based on the belief that smoke particles in low-gravity environments would be large (>1 μm). This belief was based largely on fundamental combustion experiments carried out in microgravity, rather than on projects specifically designed to assess fire detection in reduced gravity. Since photoelectric detectors respond better than ionization detectors to particles over 1 μm in size, the assumption was that photoelectric detectors would provide earlier detection of fires in the ISS than ionization technology would. Recently reported results from Urban and coworkers124 from work conducted on the space shuttle in microgravity show that particle sizes from smoldering materials may actually be too small (<0.3 μm) to be detected by photoelectric smoke detectors. Additional R&D is needed to validate this work and to better understand fire signatures, including particle sizes from both flaming and smoldering fires in reduced-gravity environments. More important, fire detection systems need to be tested in environments that approximate the actual environments in which they will be installed, thus reproducing the actual “fire signatures” that the detectors could see.
The assumption that where there are particles, there is smoke, and where there is smoke, there is fire, must also be supplanted by the development of multisignature fire detection schemes and systems. In low-gravity environments, relying on smoke as the harbinger of fire is not a reliable assumption. Smoke detectors rely on the detection of a certain density of particles in a particular size range. Particles generated from many other sources that are free to float about in a microgravity environment have the potential to generate false alarms from such smoke detectors. Detection of other fire signatures, including carbon monoxide, HCl, HCN, or soot precursors, can potentially shorten times to fire detection while reducing false alarms.
Given the fact that sensors are ubiquitous in crewed spacecraft, there could be value added in treating fire signatures as an input variable for detection system design. Not only can combinations of sensors provide faster and more reliable detection but they also can provide information that will enable astronauts to minimize the impact of the fire. Adequate management of this information requires the use and adaptation of computer-based fire models to integrate sensor data, speed computations, and reduce uncertainty.125 Such a system would also likely require the study of improved training practices to enable more effective response.
Fire suppression presents a significant problem for future crewed space missions. Fire suppression includes the deployment of a suppression agent and the interaction between the suppression agent and (1) the combustion reaction, (2) the fuel generating surfaces (pyrolysis), and (3) the non-pyrolyzing surfaces (and the associated potential cooling effect). The overall effect is that a suppression/control system needs to be addressed as an ensemble.
Currently, there is inconsistency between suppression agents used by different countries involved in the ISS. While NASA currently relies on CO2 for the ISS (and halon for the space shuttle), the Russians rely on water-based agents.126 Although there are advantages and disadvantages to various suppression agents, there is no qualification test for agents or for suppression systems that is specific to operation in reduced-gravity environments or high-O2 atmospheres. While some recent work has shown the effectiveness of CO2 in extinguishing gaseous diffusion flames,127 NASA’s reliance on gaseous CO2 suppression systems for the ISS has been called into question by recent work from Sutula and coworkers.128 They conducted a study of the delivery of extinguishing concentrations of CO2 to areas similar to those found in electronic racks aboard the ISS. This research, which used Earth-based experiments coupled with low-gravity computer modeling, showed that it is unlikely that current CO2 fire suppression systems can deliver a sufficient concentration of CO2 to a fire for sufficient duration to extinguish the fire with no relight. Other recent work129,130 has shown that water mist might be effective in various gravities, but many unanswered questions remain about how droplet size, size distribution, and injection method affect its efficacy under different gravity environments.
This work highlights the need not only for fundamental understanding of suppression agents but also for development of a methodology based on physical principles for qualifying the overall performance of fire suppression systems for relevant gravity levels and O2 environments.
Computer Fire Modeling
Since large-scale fire tests in reduced gravity environments are currently impractical, computer-based fire modeling is of high importance to fire prevention, detection, and suppression for human space exploration. One of the important goals of applied fire research in reduced gravity should be to provide the data necessary to develop and validate computer fire models. Computer fire models are used extensively for design of terrestrial fire-safe structures. Furthermore, such modeling is already being used to assess the location and response of spacecraft fire detectors to various fire scenarios.131 This modeling has already shown some weaknesses in the location of smoke detectors on the ISS and has identified problems associated with the effects of temporary storage of materials around the ISS. In addition, as discussed above, modeling by Sutula et al. has called into question the efficacy of CO2 extinguishing systems on the ISS. Although the Sutula et al. findings were validated against 1-g data, there is no comparable reduced-gravity data to use in validating their results.
Work should be done to determine which existing fire models can be successfully used in reduced-gravity environments and the limitations of these uses. For example, one of the conclusions of the Sutula work is that large eddy simulation models do not work well for predicting fire suppression in rack geometries. Thus, the Fire Dynamics Simulator, a large eddy simulation model used for the smoke detector work of Urban et al. discussed above, requires further validation. Adaptation of computer fire models from terrestrial use to space exploration will require both an in-depth examination of how the physics is modeled and comparison with appropriate reduced-gravity data for validation. It is important to emphasize that fire models are required to resolve large volumes, and so the emphasis is not on the detailed resolution of the combustion chemistry but on the definition of simplified combustion, suppression, and detection models that are adequate for a coarse resolution grid. Thus the problem presented by the system defines the nature of the modeling strategy.
Fires during human space exploration have occurred and will occur again, since fires have followed every endeavor by mankind. The catastrophic fire of Apollo 1, a serious fire on Mir in 1997, and several documented incidents of smoldering or charred electrical components on the space shuttle underscore the potential for fire in spacecraft systems.132,133 Thus, it is important that NASA not only prevent, detect, and suppress fires but also prepare for recovery from inevitable fires. Little research has been done to date on recovery from fires. Most of the efforts in fire safety have been directed toward prevention, detection, and suppression.
Some recent work has begun to examine what the toxic environment will be after a fire. One approach heats a composite disk (similar to a hockey puck) made of pellets of various materials that would be in a compartment, fused together in quantities representative of their proportions in the compartment.134 The toxic gases that are generated by the smoldering of this disk are then analyzed. This approach, while novel, has never been validated even for terrestrial compartment fires and ignores the fact that materials interact during fires in ways that are different than is represented by their overall relative quantities.
As discussed above, determination of the toxic gas generation of materials in both smoldering and flaming fires should be part of the basic standard for qualifying materials to be used in space. Once that basic characterization is completed, research into how materials interact during fires will be required. All of this work should be combined with computer fire models to provide predictions of the kind of environments that would occur in a compartment during a specified fire. Such predictions should include generation of heat and toxic materials from both the fire and the extinguishing agent(s) and should include those materials that may deposit on surfaces.
Once the types of environments that can be expected from various fire scenarios are determined, strategies for clean-up should be developed. Such strategies might include isolating the compartment and venting it to space or using filtration canisters to absorb toxics from the environment. In any case, monitoring technology that can determine the state of the post-fire environment should be developed and tested. In particular, a strategy should be developed for determining when the atmosphere is breathable after a fire. One approach that bears research is whether a single species such as CO can be used as a surrogate for all toxics such that, when its level is below some threshold value, the air is sufficiently clear of all toxics to be deemed breathable again. This approach has
been suggested by NASA and makes sense; however, substantial research and testing will be necessary to identify the right marker species and validate its critical level.
Work also needs to be done on assessing the impact of heat and products of combustion on mission-critical components and systems. For example, research should be conducted to determine the effects of heat and deposition of combustion products on electronic components and boards. This type of work will allow determination of what type of clean-up, if any, or replacement of electronics may be necessary following a fire to continue mission-critical operations.
Finally, since fire can cause a breach of a compartment, fire recovery should include strategies and equipment for repair of breaches in the integrity of a compartment. This might include patch kits or other materials and assemblies to seal a breach and to re-establish a habitable environment inside the compartment after a fire.
Explosion Mitigation and Recovery
Like fires, explosions can cause catastrophic loss of one or more compartments in a habitat or space vehicle. Hydrogen and oxygen will likely be on board any human exploration vehicle (and probably in habitats as well). High O2 environments and reduced gravity affect flammability limits; therefore, materials such as various types of hydrocarbon liquids (e.g., oils and greases) and some metals can become explosive. So, as with fires, there is a need to develop sensors that can detect a potentially explosive atmosphere before explosive conditions are reached. Mitigation strategies should be developed once a potentially explosive environment is detected. Such strategies might include the introduction of an explosion suppression agent or a method to remove one of the components from the environment. Again, as in the fire case, strategies and equipment should be developed to repair breaches in compartments as the result of an explosion.
Summary of Enabling Science and Technologies for Fire Safety
Required by 2020
Materials Standards. NASA should develop and implement new testing standards to qualify materials for flight. Research is necessary in materials qualification for ignition, flame spread, and generation of toxic and/or corrosive gases in relevant atmospheres and reduced gravity levels. (T7)
Particle Detectors. NASA would benefit from improved particle (smoke) detectors. Research is necessary in characterizing particle sizes from smoldering and flaming fires under reduced gravity. (T8)
Fire Suppression Systems. NASA should develop and implement a standard methodology for qualifying suppression systems in relevant atmospheres and gravity levels and with various delivery systems. Research is needed to characterize the effectiveness of various fire suppression agents and systems under reduced gravity so that the qualification is based on physical principles. (T9)
Post-fire Environment Strategies. NASA would benefit from strategies to characterize a safe post-fire environment and to clean up a post-fire environment, including strategies and equipment to repair breaches in compartments as a result of fire or explosion, to the extent that such repairs are likely to be practical in a deployed spacecraft or habitat. Biological science research is needed to assess the toxicity of various fire products under reduced gravity. (T10)
Highly Desirable by 2020
Multisignature Fire Detection Technologies. Fire detection technologies that rely on other fire signatures in addition to smoke particles are likely to be faster and more reliable and can be used as information sources for effective response. To enable the development of such multisignature systems, research is needed to characterize
fire signatures in addition to smoke particles in relevant atmospheres and at reduced gravity levels, as well as research on interpretation methods based on computational fire models.
Numerical Fire Modeling. NASA should develop and conduct experiments to provide validation data in relevant atmospheres and at reduced gravity levels.
Explosion Sensors. To identify a potentially explosive environment before explosive conditions are reached, sensors for incipient explosion detection need to be developed. Research to support this technology should address flammability limits under reduced gravity.
Explosion Mitigation Strategies. NASA should develop and test mitigation strategies to engage when potentially explosive conditions are reached. Research is needed on explosion suppression agents and/or methods to remove reactant components from a closed environment under reduced gravity.
Two other important recommendations for fire safety involve the organizational integration of fire safety R&D into operational fire safety for space vehicles and habitats. First, there appears to be no specific office in NASA that is responsible for fire safety per se. While all programs profess to have a high regard for fire safety, there do not appear to be specific personnel with the appropriate fire safety background who oversee the implementation of current fire safety knowledge in space vehicles and habitats. Second, there does not appear to be any specific mechanism for translation of fire safety R&D findings into the design and production of space vehicles and habitats. A NASA office specifically responsible for fire safety in vehicles and habitats would ensure that fire safety systems are based on the latest, best knowledge on fire prevention, detection, suppression, and mitigation. Furthermore, such an office would be able to inform the fire safety R&D community, both within NASA and outside, about specific needs for R&D on fire safety.
If humans are to undertake long-duration missions to the surfaces of other planetary bodies, beginning with the Moon and leading to Mars, utilization of local resources, or ISRU, will be an essential element. For reviews of many of the possibilities, see McKay et al.135 and Duke et al.136 For example, a number of modeling studies have demonstrated that, when all system elements are included (excavation, thermal and chemical processing, water electrolysis, product purification, power production, heat rejection, storage, etc.), the amount of oxygen that can be produced in a year from lunar resources exceeds by more than an order of magnitude the mass of equipment that must be brought from Earth to produce oxygen on the Moon. The system that has received the most study would use hydrogen brought from Earth to reduce lunar ilmenite (FeTiO3) in a system in which the hydrogen is recycled and oxygen is the product.137 These types of systems are beneficial to exploration missions because they offset the high cost of transporting equipment or propellant to planetary surfaces by using locally produced materials. They probably are essential for scenarios where permanent human presence is expected. The production of materials from resources found in the environments of the Moon, Mars, or other solid bodies can serve a wide variety of uses.
Initial ISRU applications would likely include propellants (H2, O2, CH4, or other hydrocarbons), energy storage (H2, O2, and thermal mass materials), and life support consumables (H2O, O2, N2). Unprocessed surface material will likely be used for radiation, meteoroid, or thermal shielding, particularly on the Moon. With experience gained from such efforts, more advanced ISRU systems in the future could potentially produce photovoltaic arrays for energy production,138 solid materials for fabrication of spare parts and construction components, materials for radiation shielding, etc. In advanced applications, ISRU may play a role in the construction of pressurized structures for planetary surface operations. The benefits of ISRU derive in part from the fact that relatively small systems can work over long periods of time to produce relatively large amounts of product. The production and use of planetary resources can lead to modified interplanetary and surface architectures, reduced cost, and reduced risk for long-term space exploration. Many of the elements found in some ISRU production systems, such as reactors, electrolyzers, gas purification systems, and filters, are common or similar in ISRU and life support systems.
The benefits of ISRU are great; however, among the significant challenges are the following:
• No end-to-end ISRU system has been demonstrated, on Earth or in space. Not enough work has yet been done to determine which are the best candidates for flight systems.
• ISRU systems tend to require machinery with moving parts, raising problems of useful lifetime, maintenance, and repair.
• Complex robotic systems have yet to be demonstrated. Progress with rovers for Mars exploration is promising,139 but ISRU systems will require more complex surface operations than moving over the surface and taking measurements.
• Excavating and processing materials from lunar or planetary surfaces will create a dusty environment, exacerbating issues with dust control and mitigation (as discussed above in this chapter).
The early results from recent robotic missions to the Moon are particularly encouraging for ISRU. From the Indian Chandrayaan-1 mission, we have learned that water and hydroxide molecules are mobile on the surface, with surface concentrations that vary through the lunar diurnal cycle.140,141,142 From the Lunar Crater Observation and Sensing Satellite (LCROSS) impact mission, we have learned that there is substantial water ice, perhaps in concentrations of a few percent, and other volatiles present at least in the targeted cold trap, Cabeus Crater.143 From the Diviner instrument on the Lunar Reconnaissance Orbiter (LRO),144 we have learned that the polar regions are perhaps 20°C colder than previously expected. These results support the prospect that water and other volatiles may be present in useful concentrations at depths of about 1 m outside craters in the polar regions. If the other volatiles are confirmed to be the same constituents as might be expected due to cometary impacts (CH4, other hydrocarbons, NH3, etc.), then all of the elements necessary for life and eventual settlement or colonization may be present on the Moon.
There are a number of potential synergistic interactions between ISRU and other subsystems of a human planetary outpost. The production of propellant from in situ resources can offset the need to bring propellants from Earth and thereby drastically reduce the scale of a propulsion system designed for round-trip journeys. Production of propellants from local resources demands the development of reusable (restartable, at least) space transportation elements. Creation of cryogenic propellant depots in space, supplied from propellant sources on the Moon, can change dramatically the architecture and economic cost of exploration activities beyond low Earth orbit.145
For Mars, production of propellant from atmospheric carbon dioxide by reduction, using hydrogen brought from Earth, has been proposed as a straightforward way to produce methane, which significantly reduces the propellant to be brought from Earth. Water ice, which could be a direct source for hydrogen/oxygen or (with atmospheric CO2) methane has been known for some time to exist at the martian poles.146 Recently, Mars robotic missions, particularly the Mars Polar Orbiter, have given evidence of the widespread occurrence of large amounts of subsurface ice at least down to mid-latitudes.147 These deposits may have formed during a previous wetter period on Mars and have been preserved by burial under dust or regolith. If such deposits can be found near places that are also targets for extended human exploration, they could become sources of propellant. The technology for extracting them would be similar to that required to mine lunar polar ice deposits (excavation, thermal extraction).
Some asteroids, including some near-Earth asteroids, have carbon-rich, water-rich compositions (up to 20 percent water by weight), judging from analyses of meteorites and remote spectroscopic analysis. Because it is unlikely that an asteroid would become a target for repeated or continuous human exploration, the development of asteroidal resources is likely only if a substantial, probably commercial, space products infrastructure develops. Lewis148 has advocated the mining of iron-rich asteroids for platinum-group metals for commercial use on Earth. This would be a longer-term possibility.
ISRU systems designed to produce materials by thermochemical or electrochemical processes require significant amounts of power and could benefit from systems with high power output, such as nuclear fission power systems. As many production systems work best in continuous modes and may “freeze-up” if the process is terminated, continuous power generation is a significant benefit. Conversely, ISRU systems potentially allow for the production of photovoltaic solar power systems from local resources.149 The production of H2 and O2 from local resources offers the potential to use them as fuel cell reactants. As another ISRU application to provide base power,
the thermal energy storage media for nighttime power generation at a crewed outpost150 could potentially be produced from locally available resources. There is also the potential for constructing thermal wadis: engineered sites providing heat and/or power, where rovers and other exploration hardware can hibernate while waiting out periods of darkness on a lunar or planetary surface.151,152,153 These ISRU chemical and thermal energy storage options are of particular interest for the Moon, where long periods of darkness would require substantial energy storage for the protection of surface assets and for nighttime power generation, compared to a location in space or on Mars.
ISRU systems will likely be highly automated and, for the Moon, may be teleoperated from Earth even when humans are present at the exploration base. For Mars, they will have to be run with limited attention from Earth during robotic phases of exploration. Development of automated interfaces between elements of the ISRU system will be needed, particularly for mechanical elements such as between excavation and hauling rovers and material extraction elements or for transfer of cryogens between production and storage elements. Development of sensors that monitor and measure important aspects of the operation and provide information to control systems will be essential. The current NASA architecture for the Moon envisions the production of 1 metric ton of oxygen per year, for use in replenishing losses from LSSs and providing the larger reservoirs of oxygen and water that make a surface LSS more robust. Such a system would be a step toward, and could be viewed as a pilot-scale plant leading to, a subsequent propellant production facility. An oxygen production system might be based on the potential for extracting water ice from lunar polar regions or could alternately use one of the systems, now under development, for producing oxygen by chemical reaction of lunar regolith.154
The development of ISRU capabilities traditionally has been a commercial enterprise. If NASA’s development of initial capability can solve key technical problems, commercial implementation of production systems could supply materials for continued government exploration and for other applications.
The development of ISRU capabilities would likely require a significant R&D thrust during the early stages of occupation of a lunar outpost. An ISRU laboratory module could focus on applications supporting outpost expansion, reducing operational risk and cost, and creating a more sustainable surface capability. These applications could include producing material for radiation and meteoroid shields, manufacturing spare parts, and constructing facilities using local materials (glass, composites, metals, concrete, etc.).
Recent NASA architectures for human exploration of Mars incorporate the use of martian oxygen and methane as propellant for lifting return flights from the surface of Mars.155 Although human exploration of Mars is beyond the time frame of this decadal survey, an active robotic exploration program is ongoing. Opportunities should be sought to clarify the distribution of resources and feasible propellant extraction processes in conjunction with the robotic scientific exploration program.
Known Effects and Knowledge Gaps
For early applications, sufficient knowledge now exists on the distribution of potential resources on the Moon, except for the question of polar resources. The regolith everywhere on the Moon provides a source of oxygen (by reduction of metal oxides and silicates) and potentially hydrogen (hydrogen is generally present in the regolith, but at low levels: just 50 to 100 parts per million), but economical recovery has not been demonstrated. For polar regions, where there is remote-sensing evidence for enhanced hydrogen concentrations and recent reports of the existence of water (present in the LCROSS ejecta plume at apparent concentrations of 1 to 2 percent) and other volatiles,156 surface exploration is needed to characterize the resource deposits (distribution, form, concentration, accessibility) before extraction techniques can be defined.
If it is determined that extraction of water from lunar polar regolith is feasible, three research questions become important: (1) How do the physical properties of icy regolith change between its ambient temperature and the temperature at which volatiles would be extracted (say, 0°C), and how do cold volatiles interact with higher-temperature environments? (2) How do flow processes work for granular solids at cryogenic temperatures in reduced gravity? (3) What are the behaviors of water-bearing granular materials at cryogenic temperatures, when mechanical friction heating is introduced? If sufficient water is located in shadowed polar regions, a substantial set of studies will be needed to learn to excavate and handle granular materials at cryogenic temperatures in the lunar vacuum.
For Mars, CO2 in the atmosphere can be a source of carbon for the production of methane or other hydrocar-
bons, using hydrogen brought from Earth, and martian water ice can be a source of H2O, H2, and O2. Although water ice is widespread on Mars,157 little is known about its accessibility for ISRU uses. The behavior of cold, subsurface ices when exposed to a warmer surface environment needs to be understood.
NASA’s ETDP for the past few years and its proposed ETDP project plan address production of oxygen on the Moon. Among the wide variety of potential ISRU processes that have been suggested,158 research has focused on chemical extraction of O2 by H2 and CH4 reduction159 and molten regolith electrolysis.160 Initial versions of oxygen extraction systems have been demonstrated in the field.161 Lunar regolith excavation techniques have been brought to the level of field demonstration.162 All unit operations associated with regolith excavation and processing have been demonstrated preliminarily at lunar analog test sites on Earth.163 These systems are not yet flight-like versions and have not been integrated operationally. Several issues, such as system mass, power requirements and delivery, adaptation to the vacuum environment of the Moon, and long-term operation, will have to be solved to bring this technology to the level needed to produce the targeted quantities of oxygen. Including the current emphasis on the ETDP’s laboratory and field development of lunar oxygen production, the following are key issues that must be addressed in a long-term exploration program that intends to incorporate ISRU into its strategy:
a. Reliable, long-duration operations in a dusty environment. The extraction techniques are operating on dusty materials, and so dust will be an ever-present problem. Because the techniques operate slowly over long periods of time, repeated or continuous feeding of dusty material from the lunar surface to and from reaction chambers will require development of new approaches to reliably and repeatedly seal the extraction systems. Problems that become research issues include the flow of solid materials in the vacuum environment, which will require a better understanding of how grain size, shape, and composition affect the transfer of granular materials between the surface and containment systems in reduced gravity, both in vacuum and in the pressurized volumes of extraction systems at elevated temperatures. Long-term behavior of materials from which excavation and extraction systems might be constructed are research concerns. See Chapter 9 for additional details on research issues in fluid physics, which applies to all areas of materials processing systems that involve the movement of materials on the lunar surface and within extraction systems.
b. Gas or cryogen storage and transfer techniques. Cryogenic storage of gases would provide higher performance (lighter tanks) than pressurized gas storage, but cryogenic storage is operationally more complex. Problems of reliably and repeatedly transferring O2, H2O, H2, and CH4 in dusty environments should be solved and long-term storage technology (e.g., zero-boiloff tanks) developed. To properly design equipment, the effects of reduced gravity on flow of cryogens should be understood.
c. End-to-end integrated tests should be carried out over suitable periods of time in thermo-vacuum chambers and in appropriate terrestrial field environments. It is especially important to demonstrate that oxygen of appropriate purity can be produced and its composition certified. It is also important to demonstrate that the system will continue to function through starts and stops that would be associated on the Moon with daytime operations and nighttime stand-downs, if nighttime power is not available.
d. An end-to-end robotic demonstration of oxygen production on the Moon (excavation, extraction, purification, storage, and transfer) is needed to validate operations of gravity-dependent processes in the 1/6-g lunar surface environment and to provide design requirements for a lunar outpost system. Physical modeling is needed of all elements of an end-to-end system that incorporates the relevant environments (vacuum, reduced gravity, static charging effects) on the flow of materials, thermal modifications of regolith materials (such as sintering), and heat transfer to support system design.
e. A wide variety of other materials could be accessible on the Moon as byproducts or via unique extraction pathways. Although ISRU processes for these materials are not included in early lunar outpost architectures, their development may improve the potential for development and sustainability of a long-duration lunar outpost and reduce the risks and cost for lunar-outpost growth phases. Some of these ISRU options could be developed into research tasks to be carried out by crew members in the lunar outpost. They might include production of consumables (C, H, N) from the lunar regolith; preparation of metals, ceramics, glasses, and concrete materials; and development and demonstration of forming and fabrication techniques. Research issues include identifying feasible processes and understanding the behavior of extraction systems in the lunar surface environment.
f. For ISRU on Mars, surface exploration will be necessary to define the properties of water ice deposits (location, extent, amounts of water contained, physical properties, etc.) that could be accessible for ISRU. This exploration could be carried out in conjunction with currently planned robotic missions or as a standalone Mars mission. A demonstration of CO2 extraction from the martian atmosphere on a robotic exploration mission is needed to validate production techniques for CH4 or other hydrocarbons.
ISRU Development and Test Environments
The environments on Earth in which materials production systems for ISRU are developed and tested must be chosen to allow all important variables to be tested before demonstrations are conducted on the Moon or Mars. For the most part, materials production, handling, and storage technologies (e.g., excavation, materials transportation and handling, chemical reactors) will look much like their terrestrial counterparts; significant advances in understanding systems for exploration applications have been gained by modeling based on terrestrial systems.164 The principal physical differences that must be accounted for in the extraterrestrial ISRU systems are lower gravity, low external pressure (Mars) or vacuum (Moon), different raw materials, low temperatures (as low as 80 K on the Moon) and high temperatures (120°C on the Moon), and the absence of water for chemical processing. Fortunately, during system development, these physical conditions can to a great extent be tested in isolation. For example, subsystems for excavation, thermal extraction, or electrolysis can be tested and demonstrated in high vacuum and Earth gravity and also at 1/6 g (e.g., in parabolic aircraft flights) and room pressure. Simulants of lunar and martian materials can be prepared that mimic with sufficient fidelity the characteristics of lunar or martian starting materials. The following are relevant development and testing environments:
• Laboratory bench-top experimentation. Only a limited number of ISRU subsystem concepts have been verified in the laboratory. Laboratory-based experiments should be continued that define alternative, potentially higher-efficiency extraction techniques; extend the life expectancy of machinery with moving parts; and mitigate surface dust issues.
• Terrestrial thermal-vacuum chambers to simulate lunar/Mars surface environments for end-to-end ISRU production tests. Vacuum chambers should be dedicated to ISRU experimentation because of the need to carry out long-duration testing of materials production systems. Operation of a “dirty” thermal vacuum chamber is a significant challenge in itself.
• Reduced-gravity aircraft flights. These flights will generally be suitable for testing specific subsystems to verify functionality in lunar or martian gravity environments.
• Analog field tests. Analog testing environments are needed to demonstrate system interactions, evaluate repair and maintenance needs, and demonstrate long-lived operations. Terrestrial analog field tests are essential to demonstrate long-term reliability of candidate systems and to develop operational protocols.
• Lunar robotic demonstrations to qualify systems and subsystems for lunar use; lunar robotic missions to explore polar regions for volatiles and nonvolatiles. A vigorous program of robotic exploration of the Moon should be used to verify conditions near the lunar poles, develop resource maps, and demonstrate ISRU system end-to-end operations in the lunar environment. Because of the strategic importance of ISRU to exploration system architectures, robotic missions may be more important if NASA were to choose near-term exploration paths not principally focused on scientific exploration. Such missions might make use of landers and compact rovers that are in development by private teams competing to win the Google Lunar-X Prize*** and might make use of lunar assets, such as thermal wadis comprising regolith-derived thermal mass materials, as platforms that enable rovers and other exploration hardware to survive periods of cold and darkness on the lunar surface. In the architecture assumed for this study, lunar robotic missions, including demonstrations of ISRU, should be included within the next few years, if resource maps are to be developed and if the goal of producing 1 MT/year of oxygen is to be achieved by the program.
*** The Google Lunar X PRIZE is a $30 million competition for the first privately funded team to send a robot to the Moon, travel 500 meters, and transmit video, images, and data back to Earth (see http://www.googlelunarxprize.org/).
• Mars ISRU demonstrations. Through Mars robotic missions, NASA should explore the potential for utilizing near-surface ice deposits. Likewise, robotic exploration missions to Mars should include capabilities, such as drills and surface geophysical sensors that can determine the amounts of ice and its accessibility to possible future human explorers. Demonstrating the feasibility of propellant production on Mars at an early stage in exploration can influence the later stages of robotic exploration. For example, robotic sample return missions could be made more effective if martian propellants were available.
• Sample return missions to carbonaceous near-Earth asteroids to define their potential for resource extraction and utilization. Prior to such missions, it is important to understand both the composition of the resources and the physical environment in which they exist.
Summary of Enabling Science and Technologies for Space
Resource Extraction, Processing, and Utilization
The following exploration technologies that are important to space resource extraction, processing, and utilization would benefit from near-term R&D.
2020 and Beyond: Required
Lunar Oxygen Extraction System. A lunar oxygen extraction system would replenish life support system consumables and would be a critical first step in local resource utilization for larger-scale uses, such as a propellant production facility. Key supporting technologies should be developed to support an oxygen production system, including technologies to enable excavation, fluid handling, cryogenic transfer, and zero-boiloff cryogenic storage. Research will be required to develop techniques to mitigate environmental challenges (dust, repeated transfers to and from vacuum). A lunar oxygen extraction system will require an integrated research program to develop needed technologies, test them in lunar analog facilities (vacuum, field analog facilities), and conduct robotic demonstration missions to the Moon, potentially as piggy-back experiments on science exploration missions. (T24)
ISRU Capability Planning. Exploration missions should be conducted to the polar regions of the Moon and to Mars to delineate the resources that can be available at potential landing sites in order to properly plan ISRU capability development. The distribution and accessibility of water or hydrogen near the lunar poles and the delineation of ice on Mars as potential resources are important ISRU objectives and should be coordinated with scientific exploration missions to these targets. Water extraction from the lunar polar regolith will require fundamental research on physical properties and flow processes of the water-bearing material at cryogenic temperature. In addition, exploration for resources and development and demonstration of ISRU technology should be incorporated, as feasible, in relevant robotic missions. Two examples are (1) a Mars sample return mission that includes demonstration of the extraction of propellant from the martian atmosphere, and (2) robotic exploration missions to characterize near-Earth asteroids and return samples that would demonstrate their potential resource value. Finally, expansion of technical capabilities such as improved power production and storage and development of in-space propellant depots will improve the potential for utilizing off-Earth resources. Fundamental research is needed to provide a sound basis for how grain size, shape, and composition affect the transfer of cryogenic granular materials into continuous-process systems in reduced gravity and pressurized reactors. The uniqueness of the materials being processed and their low pressure, reduced gravity, and other special conditions will require new predictive physical models. (T25)
This section covers the principal technologies and sciences necessary to plan, develop, deploy, construct, and maintain habitats, rovers, and other engineering systems related to construction on the surface of the Moon or Mars. Surface construction is a challenging task, given the harsh and isolated nature of the lunar and Mars environments, with their great temperature differentials, reduced gravity, partial or no atmospheric pressure, high
doses of radiation, and potential micrometeorite bombardment. The equipment, tools, materials, technologies, and methods used on Earth will have to be modified—and some new ones will have to be created—to build habitats on the Moon and or Mars.
Most of the construction that will take place at a Moon or Mars base will likely be assembly and deployment of modules and equipment (unless and until ISRU capabilities advance to the point of providing construction materials). Although the ISS has provided extensive experience with assembling a large and complex structure in space, experience constructing and assembling structures on the surface of the Moon is lacking, with the exception of small experiments during the Apollo program. Despite the small size of those experiments, and even though all the tasks were completed successfully, some of the work proved to be very challenging, given the limitations of 1960s-era EVA suits and tools. Work done in simulated environments by the Desert RATS††† teams has provided some additional experience, and NASA has done several deployments of inflatable structures: one has been deployed in the Antarctic as a partial simulation of the extreme environment of the Moon or Mars.
In most cases, some site preparation will be needed, particularly where a leveled field will be needed to install, deploy, or construct structures. Specific equipment will be needed to accomplish this. Although many conceptual studies have been proposed for these types of systems, few have been actually built and tested in simulated gravity fields and the extreme thermal and radiation environments of the Moon or Mars.
Equipment and Tools
Specific equipment and tools need to be developed to build, assemble, and deploy the different types of structures; these will be operated by robotic systems as well as by astronauts during EVA and intravehicular activities and in pressurized rovers. In some instances, these operations may be controlled from Earth. Although many tools and equipment were designed and fabricated for the ISS construction, few exist that were designed to operate on the surface of the Moon or Mars. Although the final construction equipment and tools will be designed specifically to the characteristics of the habitat and infrastructure to be deployed, an extensive list of possible tools and/or equipment (such as excavators, backhoe loaders, bulldozers, graders, or cranes) will need to be developed for lunar or martian habitat construction. From this list, a minimum set can be selected to perform the needed task(s). The issue of lunar and martian dust is of high priority when considering the survivability of construction systems and equipment. Research on materials and mechanisms to prevent equipment damage from planetary surface dust is critical.
Robotic systems will play a critical role in the construction of surface habitats. Robotics is a major gap not represented in the NRC’s previous report, Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies.165 Autonomous, semi-autonomous, or teleoperated robots may prepare a site for construction, unload equipment from a lander, and provide transport to temporary or permanent sites. Robotic systems may also be used for assembling, deploying, and constructing many of the systems and infrastructure of a surface outpost. For robotic systems to be used effectively in surface outpost construction and operation, close coordination is needed between NASA’s research programs in habitats and robotics.
Many robotic technologies and systems exist today in the manufacturing and commercial sectors that are applicable to planetary construction and assembly processes. Many of the technologies developed to operate the Mars rovers Spirit, Opportunity, and Phoenix are applicable to some construction-process robots (i.e., control
††† Desert Research and Technology Studies, or Desert RATS, is an annual field test led by NASA in collaboration with non-NASA research partners. The Desert RATS effort assesses preliminary exploration operational concepts for surface operation concepts, including rovers, EVA timelines, and ground support (see http://www.nasa.gov/exploration/analogs/).
systems, navigation cameras, hazcams,‡‡‡ communications systems, warm electronics,§§§ drives, motors, and mobility systems). Robotic systems that were designed for operations on Earth or on Mars will have to be adapted if they are to be applied to accomplishing tasks on the lunar surface.
Initially, most of the construction activities will encompass assembling modules and elements prefabricated on Earth. Some of the equipment and tools listed in the previous subsection will be required to unload these elements from a lander, transport them to a significantly distant site, and install them in their final or temporary location.
No large structures have been assembled to date in partial-gravity or planetary surface simulators. Existing NASA facilities could be modified to accomplish such simulations. Some unloading and loading of modules has been simulated during the Desert RATS activities using the ATHLETE¶¶¶ carrier. Most of the assembly knowledge to date is based on the ISS experience.
Precise mechanical berthing of modules with other modules, or with pressurized rovers, will be critical, given the proposed high frequency of these operations.
Continued research in advanced structures will be required to construct efficient, safe, and lightweight structures for habitats, rovers, and other surface infrastructure systems. Advanced structures will be required to reduce mass, enhance radiation protection, and operate in the extreme conditions of the Moon and Mars surfaces. Examples of such advanced structures and intelligent structures are self-healing structures, antimicrobial material coatings, carbon nanotube membranes for carbon dioxide capture, organic coatings, biomimetics structures, extreme temperature composites, and fabrics for inflatables. Different types of structures will be required to build the different elements of a surface outpost such as habitats, pressurized tanks for expendables, solar panels, radiators, rovers, robots, thermal insulation, and radiation and micrometeorite shields. The structures designed will be required to withstand the launch, landing, and operational loads of transportation, translation, and operation from Earth and on their final location on the planetary surface. In addition, they will have to endure the effects of radiation, extreme temperature differentials, and abrasiveness of the operational environment. Many types of structural systems are operating today in space. However, all these structures have been designed to operate in free space with minimal gravitational loads without the effects of landing, downloading, transportation, partial gravity, and dust present on the Moon or Mars. Planetary structures may include the following types:
• Pressurized—rigid, foldable, inflatable, metallic, composite, and hybrid;
• Trusses—space frames, foldable, telescopic, tensigrity, and quick assembly mechanisms;
• Tents and shields—frames, inflatable, rigid panels, fabrics, and films; and
• Excavated—walls, berms, trenches, caves, artificial craters, and foundations.
It is uncertain at this time what size and type of habitats would be needed for a surface base on the Moon or Mars. The initial number of crew, resupply period, mission duration, and purposes of outposts have not yet been defined. These basic parameters are critical to the final design of any surface infrastructure that will support human life. The basic elements of a habitat, no matter its scale and purpose, will need to house and address the fundamental needs of humans to remain alive and productive. One of the latest and most ingenious design con-
‡‡‡Hazcams (short for Hazard Avoidance Cameras) are photographic cameras mounted on the front and rear of NASA’s Spirit and Opportunity rover missions to Mars.
§§§The body of the Mars rover is called the warm electronics box, or “WEB” for short. The outer layer of the WEB protects the rover´s computer, electronics, and batteries from temperature extremes.
¶¶¶The All-Terrain Hex-Legged Extra-Terrestrial Explorer (ATHLETE) is designed to roll over undulating terrain and “walk” over extremely rough or steep terrain so that robotic or human missions on the surface of the Moon can load, transport, manipulate, and deposit payloads to a range of desired sites (see http://www-robotics.jpl.nasa.gov/systems/).
cepts includes the ability to move the outpost and the entire related infrastructure to several distant sites during the design life cycle of the systems. This “mobility” concept adds a significant level of complexity to the habitats previously conceived as static structures.
Any human habitat, whether on a planetary surface or for deep-space exploration, will be required to house the following systems and subsystems:
• Food—preparation, delivery, consumption, and long-term storage;
• Hygiene and waste management—water dispensing, sinks, toilet, and personal toiletries;
• Health maintenance—exercise equipment, medical care equipment;
• Sleeping accommodations—horizontal bunks, privacy, crew personal equipment;
• Operations center—wardroom activities, communication and control;
• Lighting—general, task, emergency, EVA operations;
• Furnishings—seats, bunks, restraint systems, working surface, and scientific equipment;
• Storage—refrigerated, frozen, ambient, dry, and wet;
• Acoustics—surface materials, geometry layouts, equipment design, fireproof fabrics;
• Airlocks—egress and ingress, EVA suit donning, dust control;
• Berthing ports—connection between modules and rovers in a dust environment;
• Dust control—laminar flow air systems, materials, flooring systems, vacuums;
• Radiation protection—water, ice, polyethylene and other low-density and high-hydrogen-content materials, equipment layout, in situ materials; and
• Mobility—drives, wheels, suspension, winches, skids, etc.
Power and cooling for planetary habitats are considered in the “Space Power and Thermal Management” section of this chapter. Although most of these subsystems have been developed for the ISS, there is no experience with most of them on the Moon or Mars, with all the related effects of such sites. Some of the most critical areas that need further research are dust control, radiation protection, and mobility systems.
Almost all systems and subsystems of a habitat will have to be maintained, serviced, and/or replaced during any long-term mission. Therefore, they will have to be designed to minimize the impact of these activities with respect to time, complexity, mass, power, and safety. The actual number and types of spare parts will be determined by the final design of all the subsystems. Attributes such as robustness, modularity, commonality, interchangeability, simplicity, and protection/shielding will need to be applied to all operating systems. However, without specific design requirements for the subsystems, it is difficult to anticipate the maintenance schedules and spare parts required.
Pressurized and unpressurized rovers greatly extend the range of exploration on planetary surfaces. Initially, uncrewed unpressurized rovers explore and survey sites, as Spirit and Opportunity have already done on Mars; in the future they will prepare sites for future human landings. Based on the latest designs of lunar surface systems architectures, pressurized rovers will play a prominent role as extensions to habitats and habitability support, as well as in mobility and exploration activities.
The Lunar Roving Vehicle, although unpressurized and very simple in design, gave the Apollo astronauts a wide range of mobility that would have been impossible without it. The Mars rovers have exceeded their design lives and primary mission requirements in terms of longevity and capabilities on the martian surface. However, many new technologies need to be advanced to deploy, operate, and maintain large habitable rovers for long-duration missions on the surface of the Moon or Mars. In particular, electromechanical systems are needed that can provide fine control and precise operation when performing complex simultaneous tasks.
Summary of Enabling Science and Technologies for Planetary Surface Construction
The following exploration technologies that are important to planetary surface construction would benefit from near-term R&D.
2020 and Beyond: Required
Construction: Teleoperated and Autonomous Construction. Robotic systems will play a critical role in the construction of surface habitats. An extensive, coordinated R&D program should be established to study human-robot interaction (including teleoperations) for the construction and operation of planetary surface habitats. (T26)
Construction: Regolith- and Dust-Tolerant Systems. Construction technologies are needed that include innovative designs for dust-tolerant mechanisms and fluid connectors, O-rings, and materials that can withstand the abrasive effects of regolith and provide a tight seal in its presence. To develop these technologies, a better understanding is needed of regolith mechanics, the behavior of ice-laden regolith, equipment traction, and the forces required to excavate, move, and compact regolith. There is a need to develop gravity-dependent soil models to better understand regolith strength, stiffness, and density on the Moon and Mars. (T27)
Habitats: Habitability Requirements. Surface habitability systems design requirements need to be developed. In particular, they should incorporate accurate lunar surface radiation modeling and simulation to predict crew dosage for long-duration missions within complex structures. (T28)
2020 and Beyond: Highly Desirable
Construction: Excavation Systems. NASA currently lacks excavation and earthmoving capabilities for the Moon or Mars. Research on human-controlled, semi-autonomous, and/or autonomous equipment to meet earthmoving and excavation requirements on the Moon or Mars is desired. Specific research on adaptation and downscaling of Earth-based subsystems is suggested.
Construction: Robots. The design and development of robots for the extreme environments of the Moon and Mars should include capabilities for the performance of the complex tasks required during construction.
Construction: Processes. Research is needed on construction and assembly processes under planetary simulated or scaled-down conditions.
Structures: Materials. NASA would benefit from structures and materials technology development for habitats, rovers, and other surface infrastructure systems that enable development of structures that have low mass and improved radiation protection capability and that can be deployed in extreme temperatures. Basic structural systems and materials research in reduced gravity and all other extreme planetary conditions is needed in several areas, including extreme temperature composites, alloys, fabrics for space suits, and inflatable structures.
Structures: Structural Systems. A number of structural systems will need to be developed and tested for the planetary environment, including pressurized structures, trusses, tents and shields, and excavated structures.
Maintenance: Subsystem Operations. Research should be conducted on maintenance strategies and operations of all subsystems and should be specific to the environment in which these systems will be operating.
Rovers. A number of technologies should be developed to enhance the use of rovers on a planetary surface, including mechanisms for mitigating the effects of extreme temperature (including highly reliable, lightweight thermal control, Sun shades, heat rejection systems, and radiator dust mitigation methods), mechanisms for mitigating the
TABLE 10.3 Current Research and Technologies Required to Support Objectives and Operational Systems up to 2020
|Recommendation||Research Topic||Current Gap||Critical Technology||Enabling Research||Environmental Constraints||Crosscutting Applications|
|T1||Space power and thermal management||Inability to utilize multiphase flow systems to increase performance||Two-phase flow thermal management technologies||Harness ability to use active two-phase flow thermal management in reduced gravity fields||Partial and microgravity||Space and surface operations, propellant systems, EVA, life support, habitats, power, ISRU|
|T2||Space propulsion||Inability to limit boiloff of cryogenic propellants to extend storage||Zero-boiloff propellant storage systems||Research in such areas as advanced insulation materials, active cooling, multiphase flows, and capillary effectiveness||Full gravity range||Space and surface operations, ISRU|
|T3||Space propulsion||Lack of knowledge of cryogenic propellant flow, handling, and gauging in microgravity||Cryogenic fluid management technologies||Research to enable microgravity propellant flow, handling, and gauging||Partial and microgravity||Enables propellant depots, ISRU|
|T4||EVA||Inadequate mobility for suited crew||EVA suit mobility enhancements||Research in suit comfort, trauma countermeasures, and joint mobility to provide crew the mobility to perform tasks over extended periods without injury||Partial and microgravity||Space and surface operations|
|T5||EVA||Lack of suit durability in on-orbit, lunar, and martian environments||Dust and micrometeroid mitigation systems||Research and test beds to deal with durability and maintainability issues of suits stemming from micrometeoroid and orbital debris damage, dust exposure, and plasma||Partial and microgravity, temperature extremes||Space and surface operations|
|T6||Life support systems||Lack of understanding of partial-gravity effects on life support systems (fluid/air)||Fluid and air subsystems||Design, test, and operation of highly reliable life support fluid and air systems in reduced gravity environments||Partial gravity||Propellant systems, habitats and rovers, ISRU|
|T7||Fire safety||Lack of knowledge regarding materials flammability and toxicity in various atmospheres and gravity fields; lack of adequate standards to determine acceptable materials based on flammability and toxicity||Materials standards||Research to describe the flammability and toxicity of materials with respect to ignition, flame spread, and toxic/corrosive gas generation in various environments and gravity fields||Partial and microgravity, various O2 atmospheres||Space and surface operations, space vehicles, habitats, rovers|
|T8||Fire safety||Current fire detection techniques lack reliability in reduced gravity fields||Particle detectors||Research to characterize particle sizes generated by smoldering and flaming fires; identification of other fire signatures that can facilitate fire detection||Partial and microgravity, various O2 atmospheres||Space and surface operations, space vehicles, habitats, rovers|
|T9||Fire safety||Effectiveness of fire suppression systems in reduced gravity environment is not well understood||Fire suppression systems||Research to describe the effectiveness of fire suppression agents and systems against various types of fires in various spatial configurations and gravity fields||Partial and microgravity, various O2 atmospheres||Surface and space operations, space vehicles, habitats, rovers|
|T10||Fire safety||Lack of knowledge of postfire environment||Post-fire environment strategies||Research to characterize post-fire environment and clean-up strategies including removal of toxic gases||Partial and microgravity, various O2 atmospheres||Surface and space operations, space vehicles, habitats, rovers|
TABLE 10.4 Current Research and Technologies Required to Support Objectives and Operational Systems for 2020 and Beyond
|Recommendation||Research Topic||Current Gap||Critical Technology||Enabling Research||Environmental Constraints||Crosscutting Applications|
|T11||Space power and thermal management||Need energy storage density improvements (factor of 10) beyond current battery and fuel cells||Regenerative fuel cells (RFCs)||Research to enable RFC demonstration, including research related to dead-ended gas flow paths versus through-flow, cryogenic or pressurized gas storage, thermal management, and reliable, long-life operation||Partial and microgravity, extreme low temperature||Habitat and rovers, ISRU, surface operations, power, space vehicles|
|T12||Space power and thermal management||Current energy conversion systems are not efficient for all power regimes||Energy conversion technologies for low- and high-power regimes||Research in high-temperature energy conversion cycles and devices coupled to essential working fluids, heat rejection systems, materials, etc.||Partial and microgravity||Surface operations, ISRU, habitats|
|T13||Space power and thermal management||Photovoltaic and RPS systems not always adequate for very high power and high power density; lack of technology demonstration exists for fission surface power||Fission surface power||Research in high-temperature, low-mass materials for power conversion and radiators||Partial and microgravity||Space and surface operations|
|T14||Space propulsion||Lack of lunar and planetary descent and ascent propulsion capabilities||Technologies to enable engine start after long quiescent periods, combustion stability at all gravity conditions, and deep throttle||Research into cryogenic fluid management, propellant ignition, flame stability, and active thermal control of the injectors and combustors over the full range of gravities, orientations, fluid phases, etc.||Partial and microgravity, extreme low temperature||Propulsion for crew rescue and emergency maneuvers|
|T15||Space propulsion||Lack of aerodynamic decelerators to reduce propellant required for heavy payload entry to Mars||Inflatable aerodynamic decelerators||Research into flexible materials to enable the use of lightweight flexible aeroelastic re-entry devices||Partial and microgravity, Mars atmosphere||Subsystem for innovative crew return system|
|T16||Space propulsion||Lack of lunar and planetary descent propulsion capabilities||Supersonic retro propulsion system||Research on flow-field interactions of the rocket plume with atmosphere, aerothermodynamics of decelerator flows, dynamic interactions on the vehicle, and control for safe flight||Partial and microgravity, Mars atmosphere||Avoiding optical and sensor blinding during thrusting maneuvers|
|T17||Space propulsion||Lack of propulsion systems with high specific impulse||Space nuclear propulsion||Research in liquid-metal cooling under reduced gravity, thawing under reduced gravity, and system dynamics||Partial and microgravity||Nuclear power|
|T18||Space propulsion||Lack of efficient propulsion for very high specific impulse and high thrust for shorter trip times||Solar electric propulsion||Research in high specific power, multiphase thermal control, long-life engine components, control of lightweight space structures, and low-cost/efficient propellants||Partial gravity||Thermal management, space power|
|T19||EVA and life support systems||Inadequate protection from ionizing radiation exposure for EVA, rovers, habitats||Radiation protection systems||Research to enable crew to survive in the anticipated ionizing radiation environment||Full gravity range||Space and surface operations, habitat and rovers|
|T20||Life support systems||Inadequate protection from temperature extremes||Thermoregulation technologies||Research into thermoregulation of habitats, rovers, and spacesuits on the lunar surface||Temperature extremes||Space and surface operations, EVA, ISRU, propellant systems|
|T21||EVA and life support systems||Lack of closed loop air revitalization (oxygen loop closure)||Closed-loop air revitalization||Research in CO2 removal, recovery, and reduction; O2 generation via electrolysis with high pressure capability; improved sorbents and catalysts for trace contaminant control; and atmosphere particulate control and monitoring||Partial gravity||Thermal management, environmental control, habitats|
|T22||EVA and life support systems||Lack of closed loop water recovery||Closed-loop water recovery||Research in water recovery from wastewaters and brines, pretreatments, biocides, low expendable rates, and robustness; more research to assess the effect of lunar gravity on two-phase flow systems||Partial gravity||Thermal management, environmental control, habitats|
|T23||EVA and life support systems||Dust mitigation techniques and technologies do not exist||Dust mitigation technologies||Development and testing of dust countermeasures to mitigate the effects of dust coating, contamination, and abrasion and to prevent thermal control problems, seal failures, and inhalation/irritation||Partial gravity||Habitat and rovers, ISRU|
|T23||EVA and life support systems||Dust mitigation techniques and technologies do not exist||Dust mitigation technologiester||Development and testing of dust countermeasures to mitigate the effects of dust coating, contamination, and abrasion and to prevent thermal control problems, seal failures, and inhalation/irritation||Partial||Habitat and rovers, ISRU|
|T24||Space resource extraction, processing, and utilization||Current extraction process and mechanical systems operations are not suited to the lunar environment, e.g., cryogenic conditions at lunar poles||Lunar water and oxygen extraction system||Research to identify and adapt excavation, extraction, preparation, handling, and processing techniques; mitigate durability problems (dust, repeated operations); and demonstrate long-term operations in lunar environment||Partial gravity, cryogenic granular materials, vacuum, extreme temperatures||Surface operations, habitat construction, water management, power|
|Recommendation||Research Topic||Current Gap||Critical Technology||Enabling Research||Environmental Constraints||Crosscutting Applications|
|T25||Space resource extraction, processing, and utilization||Lack of knowledge regarding physical and handling properties of in situ resources||ISRU capability planning||Research (including remote assay and sampling) to characterize specific resources available at planned lunar and martian surface destinations available for ISRU planning and extraction||Partial gravity, cryogenic granular materials, extreme temperatures||Surface operations, habitat construction, propulsion, life support|
|T26||Planetary surface construction||Lack of understanding how to effectively integrate human and robotic operations||Teleoperated and autonomous construction||Research to determine how to best utilize human and robotic resources for construction and other surface operations||Partial gravity, extreme temperatures||Surface operations|
|T27||Planetary surface construction||Lack of information regarding regolith mechanics and properties||Regolith- and dust-tolerant systems||Research to describe the physical and mechanical properties of regolith to facilitate surface operations, construction, and ISRU||Partial gravity, extreme temperatures||Surface operations|
|T28||Planetary surface construction||Habitability requirements for partial-gravity operations unknown||Habitability requirements||Research to define partial-gravity habitability requirements for surface operations on the Moon and Mars||Partial gravity, extreme temperatures||Surface operations|
abrasion on components and materials, suit docking/undocking systems, and dust-resistant seals. Improved modeling and simulation techniques for lunar thermal environments and dust characterization need to be developed.
Rovers: Windows. Large, lightweight windows with correct optical properties and protection from radiation/ultraviolet, micrometeoroid/orbital debris, blast effects, and scratching would enhance rover capabilities. Research is needed in new materials development, as well as in design.
Terrestrial Analogs. Several terrestrial analogs to space surface systems already exist and more are planned. These very useful facilities should be actively and systematically employed to assess designs, materials, and operation related to habitat design and construction.
The utility of a coherent plan that is appropriately resourced and consistently applied to enable exploration cannot be overemphasized. This is especially noteworthy in light of the frequent and large postponements and redirections that NASA’s exploration-related goals have experienced over the past several decades. NASA’s existing ETDP goals seem well aligned with the panel’s recommendations, with augmentation as specified in this chapter.
Transition of technology on schedule and within budget to meet mission needs is an intellectual challenge worthy of the attention of our nation’s best technologists. Usually it not treated as a job category. Rather, transition involves an ad hoc interplay among engineers transitioning the research findings, scientists continuing to advance the associated technology, and program managers assessing the risk, schedules, and budget. Seldom is the technological handoff a simple process. For example, advances in scientific understanding may be good enough to enable design and fabrication of a prototype of a new or improved major subsystem; nonetheless, research may continue on facets of the technology discovered during the prototyping. More attention should be given to understanding how to accomplish transition of technology within the NASA system. The goal is to reduce the uncertainty of the process for mission managers, thereby reducing unwarranted risk aversion and giving NASA the confidence to use tomorrow’s technology sooner. Transitioning technology on schedule and within budget is integral to mission management. Attention should be given to improving NASA’s confidence in predicting the transition of science to mission application, thereby improving projections of new systems to which NASA can aspire. When establishing major missions, NASA should ensure that program managers, engineers, and scientists will be true partners in transitioning the essential new technology. To improve the process, the specifics leading to successful transitions should be analyzed after the mission. In Chapter 12, the section “Linking Science to Mission Capabilities Through Multidisciplinary Translational Programs” describes more thoroughly the means to transition technology successfully within NASA.
The body of this chapter contains many recommendations. Tables 10.3 and 10.4 above summarize the research areas previously identified by the panel as required for prudent execution of the exploration program. Table 10.3 lists those topics for which information is required for activities that the exploration plan indicates will occur prior to 2020, and Table 10.4 lists research for the activities that are scheduled to occur in 2020 and beyond. Due to the uncertainty surrounding the funding that will be allocated to these various research topics, the panel did not factor in the lead time that would be needed for these research activities to provide answers to the questions they address. For example, planetary surface construction appears in the “2020 and Beyond” table, but it is essential that these activities be undertaken well in advance of 2020 to lead to operational systems and implementation in the 2020 time period. NASA can determine when to initiate a particular research project, based on the level of support and the state of knowledge that exists at the time the decision is made to pursue a future activity so that it will be ready at the appropriate time indicated in the tables. This approach implies that even topic areas listed in Table 10.4, “2020 and Beyond,” might require initiation of the enabling research well before 2020.
Finally, in order for the efforts recommended here to yield the greatest benefit, NASA needs to ensure that explicit and robust organizational mechanisms and structures are in place that promote interdisciplinary collaboration and sharing of knowledge so that successful research is efficiently translated into applications.
1. National Research Council. 2000. Microgravity Research in Support of Technologies for Human Exploration and Development of Space and Planetary Bodies. National Academy Press, Washington, D.C.
2. The following NASA Exploration Technology Development Program project plans were provided to the NRC Committee for the Decadal Survey on Biological and Physical Sciences in Space by the Exploration Technology Development Program, Advanced Capabilities Division, Exploration Systems Mission Directorate: FY10 Project Management Plan for Advanced Avionics and Processor Systems (AAPS), AAPS–PLAN-0001 (REV B), Document No. RHESE-PLAN-0001, October 08, 2009; Advanced Environmental Monitoring and Control Project Plan, Document No. ESTO FY10-01, Version 1.00, October 20, 2009; FY2010 Project Plan for Autonomous Landing and Hazard Avoidance Technology (ALHAT), Document No. ALHAT-1.0-001, September 21, 2009; FY 10 Project Plan for the Cryogenic Fluid Management (CFM) Project, October 1, 2009; Dust Management Project Plan, DUST-PLN-0001, Rev. B, October 27, 2009; Energy Storage Project Lithium-Ion Batteries and Fuel Cell Systems, Document ES08-105, Revision C, October 2, 2009; EVA Technology Development Project: Project Plan, CxP 72185, Annex 01, Rev. B, September 22, 2009; Exploration Life Support (ELS), Document No. JSC-65690 Rev B, October 15, 2009; Fire Prevention, Detection, and Suppression Project Plan, FPDS10-PP-001, Ver. 4.0, October 22, 2009; Project Plan (FY10) for Human-Robotics Systems (HRS), Document No. HRS1002, October 23, 2009; FY 2010 Project Plan for In-Situ Resource Utilization (ISRU), undated; Technology Development FY10 Project Plan Propulsion and Cryogenic Advanced Development (PCAD) Project, PCAD10_001, October 1, 2009; and FY 2010 Thermal Control System Development for Exploration Project Plan, October 20, 2009.
3. NASA. 2009. Technology Horizons: Game-Changing Technologies for the Lunar Architecture. NP-2010-01-237-LaRC. NASA Langley Research Center, Hampton, Va. September.
4. Review of U.S. Human Spaceflight Plans Committee. 2009. Seeking a Human Spaceflight Program Worthy of a Great Nation. Office of Science and Technology Policy, Washington, D.C. October.
5. Byers, D.C, and Dankanich, J.W. 2008. Geosynchronous-Earth-orbit communication satellite deliveries with integrated electric propulsion. Journal of Propulsion and Power 24(6):1369-1375.
6. Schneider, T., Mikellides, I.G., Jongeward, G.A., Peterson, T., Kerslake, T.W., Snyder D., and Ferguson, D. 2005. Solar arrays for direct-drive electric propulsion: Arcing at high voltages. Journal of Spacecraft and Rockets 42(3):543-550.
7. Benson, S.W., NASA Glenn Research Center. 2007. “Solar Power for Outer Planets Study,” presentation to Outer Planets Assessment Group, November 8. Available at http://www.lpi.usra.edu/opag/nov_2007_meeting/presentations/solar_power.pdf.
8. Piszczor, M.F., Jr., O’Neill, M.J., Eskenazi, M.I., and Brandhorst, H.W., Jr., 2006. The Stretched Lens Array Square Rigger (SLASR) for Space Power. Presented at the 4th International Energy Conversion Engineering Conference and Exhibit (IECEC). AIAA Paper 2006-4137. American Institute of Aeronautics and Astronautics, Reston, Va.
9. Block, J., Straubel, M., and Wiedemann, M. 2010. Ultralight deployable booms for solar sails and other large gossamer structures in space. Acta Astronautica 68(7-8):984-992.
10. National Research Council. 2009. Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration. The National Academies Press, Washington, D.C.
11. Warren, J., NASA Exploration Systems Mission Directorate. 2009. “Nuclear Power Systems for Exploration,” presentation to the Panel on the Translation to Space Exploration Systems of the Decadal Survey on Biological and Physical Sciences in Space, November 12. National Research Council, Washington, D.C.
12. NASA. 2009. Technology Horizons: Game-Changing Technologies for the Lunar Architecture. NP-2010-01-237-LaRC. NASA Langley Research Center, Hampton, Va. September.
13. NASA. 2009. Technology Horizons: Game-Changing Technologies for the Lunar Architecture. NP-2010-01-237-LaRC. NASA Langley Research Center, Hampton, Va. September.
14. Craig, D., NASA Exploration Systems Mission Directorate. 2009. “Current Concepts for Lunar Outpost Habitation,” presentation to the Panel on the Translation to Space Exploration Systems of the Decadal Survey on Biological and Physical Sciences in Space, November 12. National Research Council, Washington, D.C.
15. Warren, J., NASA Exploration Systems Mission Directorate. 2009. “Nuclear Power Systems for Exploration,” presentation to the Panel on the Translation to Space Exploration Systems of the Decadal Survey on Biological and Physical Sciences in Space, November 12. National Research Council, Washington, D.C.
16. Warren, J., NASA Exploration Systems Mission Directorate. 2009. “Nuclear Power Systems for Exploration,” presentation to the Panel on the Translation to Space Exploration Systems of the Decadal Survey on Biological and Physical Sciences in Space, November 12. National Research Council, Washington, D.C.
17. Kovacs, A., and Janhunen, P. 2010. Thermo-photovoltaic spacecraft electricity generation. Astrophysics and Space Sciences Transactions 6:19-26.
18. NASA Glenn Research Center. 2005. Fuel Cells: A Better Energy Source for Earth and Space. February 11. Available at http://www.nasa.gov/centers/glenn/technology/fuel_cells.html.
19. Sienski, K., Eden, R., and Schaefer, D. 1996. 3-D electronic interconnect packaging. Aerospace Applications Conference Proceedings, IEEE 1(3-10):363-373.
20. National Research Council. 2000. Microgravity Research in Support of Technologies for Human Exploration and Development of Space and Planetary Bodies. National Academy Press, Washington, D.C.
21. Lahey, R.T., Jr., and Dhir, V. 2004. Research in Support of the Use of Rankine Cycle Energy Conversion Systems for Space Power and Propulsion. NASA/CR-2004-213142. NASA Center for Aerospace Information, Hanover, Md. Available at http://gltrs.grc.nasa.gov/reports/2004/CR-2004-213142.pdf.
22. National Research Council. 2000. Microgravity Research in Support of Technologies for Human Exploration and Development of Space and Planetary Bodies. National Academy Press, Washington, D.C.
23. NASA. 2009. “Exploration Systems Mission Directorate: Life and Physical Sciences: Current Programmatic Content,” briefing charts provided to Panel on the Translation to Space Exploration Systems of the Decadal Survey on Biological and Physical Sciences in Space, August 19. National Research Council, Washington, D.C.
24. Wegeng, R., Mankins, J.C., Balasubramaniam, R., Sacksteder, K., Gokoglu, S.A., Sanders, G.B., and Taylor, L.A. 2008. Thermal wadis in support of lunar science and exploration. Presented at the 6th International Energy Conversion Engineering Conference. AIAA Paper 2008-5632. American Institute of Aeronautics and Astronautics, Reston, Va.
25. Creech, S., and Sumrall, P. 2008. Ares V: Progress toward a heavy lift capability for the Moon and beyond. Presented at the AIAA SPACE 2008 Conference and Exposition, San Diego, Calif., September 9-11. AIAA Paper 2008-7775. American Institute of Aeronautics and Astronautics, Reston, Va.
26. Review of U.S. Human Spaceflight Plans Committee. 2009. Seeking a Human Spaceflight Program Worthy of a Great Nation. Office of Science and Technology Policy, Washington, D.C. October.
27. Review of U.S. Human Spaceflight Plans Committee. 2009. Seeking a Human Spaceflight Program Worthy of a Great Nation. Office of Science and Technology Policy, Washington, D.C. October.
28. NASA. 2005. NASA’s Exploration Systems Architecture Study. Technical Memorandum NASA-TM-2005-214062. November. NASA, Washington, D.C.
29. Drake, B. 2009. Human Exploration of Mars Design Reference Architecture 5.0 Addendum. NASA/SP-2009-566-ADD. July. NASA, Washington, D.C.
30. National Research Council. 2000. Microgravity Research in Support of Technologies for Human Exploration and Development of Space and Planetary Bodies. National Academy Press, Washington, D.C.
31. Arney, D., and Wilhite, A. 2010. Orbital propellant depots enabling lunar architectures without heavy-lift launch vehicles. Journal of Spacecraft and Rockets 47(2):353-360.
32. NASA. 2005. NASA’s Exploration Systems Architecture Study. NASA-TM-2005-214062. November. NASA, Washington, D.C.
33. Martin, J.J., and Hastings, L. 2001. Large Scale Liquid Hydrogen Density Multilayer Insulation with a Foam Substrate. NASA-TM-2001-211089. NASA, Washington, D.C., p. 33.
34. Drake, B. 2009. Human Exploration of Mars Design Reference Architecture 5.0 Addendum. NASA/SP-2009-566-ADD. July. NASA, Washington, D.C.
35. Sullivan, T.A., Linne, D., Bryant, L., and Kennedy, K. 1995. In-situ-produced methane and methane/carbon monoxide mixtures for return propulsion from Mars. Journal of Propulsion and Power 11(5):1056-1062.
36. Sanders, G.B., and Duke, M. 2005. In-Situ Resource Utilization (ISRU) Capability Roadmap, NASA Workshop, Final Report, May 19. NASA, Washington, D.C.
37. Griffin, J.W. 1986. Background and programmatic approach for the development of orbital fluid resupply. Presented at the AIAA/ASME/SAE/ASEE 22nd Joint Propulsion Conference, Huntsville, Ala. AIAA Paper 1986-1601. American Institute of Aeronautics and Astronautics, Reston, Va.
38. Review of U.S. Human Spaceflight Plans Committee. 2009. Seeking a Human Spaceflight Program Worthy of a Great Nation. Office of Science and Technology Policy, Washington, D.C. October.
39. Hastings, L.J., Tucker, S.P., Flachbart, R., Hedayat, A., and Nelson, S.L. 2005. Marshall Space Flight Center In-Space Cryogenic Fluid Management Program overview. Presented at the 41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Tucson, Ariz. AIAA Paper 2005-3561. American Institute of Aeronautics and Astronautics, Reston, Va.
40. Howell, J.T., Mankins, J.C., and Fikes, J.C. 2006. In-Space Cryogenic Propellant Depot stepping stone. Acta Astronautica 59:230-235.
41. Chato, D.J. 2005. Low gravity issue of deep space refueling. Presented at the 43rd AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nev. AIAA Paper 2005-1148. American Institute of Aeronautics and Astronautics, Reston, Va.
42. NASA. 2005. NASA’s Exploration Systems Architecture Study (ESAS) Final Report. NASA-TM-2005-214062. NASA, Washington, D.C.
43. Drake, B. 2009. Human Exploration of Mars Design Reference Architecture 5.0 Addendum. NASA/SP-2009-566-ADD. July. NASA, Washington, D.C.
44. Borowski, S., and Schnitzler, B. NTR development strategy and key activities supporting NASA human Mars missions in the early-2030 timeframe. AIAA Paper 2010-6818. American Institute of Aeronautics and Astronautics, Reston, Va.
45. Review of U.S. Human Spaceflight Plans Committee. 2009. Seeking a Human Spaceflight Program Worthy of a Great Nation. Office of Science and Technology Policy, Washington, D.C. October, p. 102.
46. Humble, R.W., Henry, G.N., and Larson, W.J. 1995. Space Propulsion, Analysis, and Design. McGraw-Hill Companies, Inc., New York, p. 457.
47. Drake, B. 2009. Human Exploration of Mars Design Reference Architecture 5.0 Addendum. NASA/SP-2009-566-ADD. July 2009. NASA, Washington, D.C.
48. National Research Council. 2000. Microgravity Research in Support of Technologies for Human Exploration and Development of Space and Planetary Bodies. National Academy Press, Washington, D.C.
49. National Research Council. 2000. Microgravity Research in Support of Technologies for Human Exploration and Development of Space and Planetary Bodies. National Academy Press, Washington, D.C.
50. Mauk, B.H., Bythrow, P.F., and Gatsonis, N.A. 1993. Science plan for the Nuclear Electric Propulsion Space Test Program (NEPSTP). Presented at the 29th Joint Propulsion Conference, Monterey, CA, June 28-30. AIAA-93-1895. American Institute of Aeronautics and Astronautics, Reston, Va.
51. National Research Council. 2000. Microgravity Research in Support of Technologies for Human Exploration and Development of Space and Planetary Bodies. National Academy Press, Washington, D.C.
52. Brandhorst, H.W., Jr., Rodiek, J.A., O’Neill, M.J., and Eskenazi, M.I. 2006. Ultralight, compact, deployable, high-performance solar concentrator array for lunar surface power. Presented at the 4th International Energy Conversion Engineering Conference and Exhibit, San Diego, Calif., June 26-29. AIAA 2006-4104. American Institute of Aeronautics and Astronautics, Reston, Va.
53. Carpenter, C.B. 2006. Electrically propelled cargo spacecraft for sustained lunar supply operations. Presented at the 42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Sacramento, Calif., July 9-12. AIAA 2006-4435. American Institute of Aeronautics and Astronautics, Reston, Va.
54. National Research Council. 2008. A Constrained Space Exploration Technology Program: A Review of NASA’s Exploration Technology Program. The National Academies Press, Washington, D.C.
55. Woodcock, G.R. 2004. Controllability of large SEP for Earth orbit raising. Presented at the 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Fla., July 11-14. AIAA 2004-3643. American Institute of Aeronautics and Astronautics, Reston, Va.
56. Chang Diaz, F.R. Squire, J.P., Bering III, E.A., George, J.A., Ilin, A.V., Petro, A.J., and Casady, L. 2001. The VASIMR engine approach to solar system exploration. Presented at the 39th AIAA Aerospace Sciences Meeting and Exhibit, January 8-11. AIAA 2001-0960. American Institute of Aeronautics and Astronautics, Reston, Va.
57. Kodys, A., and Choueiri, E. 2005. A review of the state-of-the-art in the performance of applied-field magnetoplasmadynamic thrusters. Presented at the 41st Joint Propulsion Conference, Tucson, Ariz., July 10-13. AIAA-2005-4247. American Institute of Aeronautics and Astronautics, Reston, Va.
58. Litchford, R.J., Cole, J.W., Lineberry, J.T., Chapman, J.N., Schmidt, H.J., and Lineberry, C.W. 2002. Magnetohydrodynamic augmented propulsion experiment: I. Performance analysis and design. Presented at the 33rd AIAA Plasmadynamics and Lasers Conference/14th International Conference on MHD Power Generation and High Temperature Technologies, Maui, Hawii, May 20-23. AIAA 2002-2184. American Institute of Aeronautics and Astronautics, Reston, Va.
59. Schulz, R.J., Chapman, J.N., and Rhodes, R.P. 1992. MHD augmented chemical rocket propulsion for space applications. Presented at the AIAA 23nd Plasmadynamics and Lasers Conference, Nashville, Tenn., July 6-8. AIAA Paper 92-3001. American Institute of Aeronautics and Astronautics, Reston, Va.
60. Litchford, R.J., Bitteker, L.J., and Jones, J.E. 2001. Prospects for Nuclear Electric Propulsion Using Closed-Cycle Magnetohydrodynamic Energy Conversion. NASA/TP-2001-211274. NASA Marshall Space Flight Center, Huntsville, Ala.
61. Palaszewski, B., and Powell, R. 1994. Launch vehicle propulsion using metallized propellants. Journal of Propulsion and Power 10(6):828-833.
62. Harris, G.L. 2001. The Origins and Technology of the Advanced Extravehicular Space Suit. AAS History Series, Volume 24. American Astronautical Society, San Diego, Calif.
63. Young, A. 2009. Spacesuits: The Smithsonian National Air and Space Museum Collection. PowerHouse Books, New York, N.Y.
64. Clark, P. 1988. The Soviet Manned Space Program: An Illustrated History of the Men, the Missions, and the Spacecraft. Orion Books, New York, N.Y.
65. Harris, G.L. 2001. The Origins and Technology of the Advanced Extravehicular Space Suit. AAS History Series, Volume 24. American Astronautical Society, San Diego, Calif.
66. Balinskas, R., and Tepper, E. 1994. Extravehicular Mobility Unit Requirements Evolution. Contract No. NAS 9-17873. Hamilton Sundstrand, Houston, Tex.
67. Hamilton Sundstrand. 2003. NASA Extravehicular Mobility Unit LSS/SSA Data Book. Revision J, February. Hamilton Sundstrand, Houston, Tex.
68. Wang, J., Yuan, W., and Yuan, X. 2009. Research progress of portable life support system for extravehicular activity space suit. Space Medicine and Medical Engineering (Beijing) 22(1):67-71.
69. Ross, A., Aitchison, L., and Daniel, B. 2008. Constellation space suit system development status. Scientific and Technical Aerospace Report 45(26): Abstract.
70. Chase, T.D., Splawn, K., and Christiansen, E.L. 2007. Extravehicular mobility unit penetration probability from micrometeoroids and orbital debris: Revised analytical model and potential space suit improvements. Scientific and Technical Aerospace Report 45(20): Abstract.
71. Jordan, N.C., Saleh, J.H., and Newman, D.J. 2006. The extravehicular mobility unit: A review of environment, requirements, and design changes in the US spacesuit. Acta Astronautica 59(12):1135-1145.
72. Jones, R.J., Graziosi, D., Ferl, J., Splawn, W.K., Cadogan, D., and Zetune, D. 2006. Micrometeoroid and orbital debris enhancements of shuttle extravehicular mobility unit thermal micrometeoroid garment. Presented at the International Conference on Environmental Systems, July 2006, Norfolk, Va. SAE Technical Paper 2006-01-2285. SAE International, Warrendale, Pa.
73. Bell, E.R., Jr., and Oswald, D.C. 2005. Past and present extravehicular mobility unit (EMU) operational requirements comparison for future space exploration. Presented at Space 2005, Long Beach, Calif. August 30-September 1, 2005. AIAA Paper 2005-6723. American Institute of Aeronautics and Astronautics, Reston, Va.
74. Akin, D.L. 2004. Robosuit: Robotic augmentations for future space suits. Presented at the International Conference on Environmental Systems, July 2004, Colorado Springs, Colo. SAE Technical Paper 2004-01-2292. SAE International, Warrendale, Pa.
75. Shavers, M.R., Saganti, P.B., Miller, J., and Cucinotta, F.A. 2003. Radiation Protection Studies of International Space Station Extravehicular Activity Space Suits. NASA/TP-2003-212051. NASA Johnson Space Center, Houston, Tex., Chapter 1, pp. 1-18.
76. Wilde, R.C., Baker, G.S., McBarron II, J.W., Graziosi, D., Persans, A., and Stein, J. 2000. Evolving EMU for space station: Status of current changes. IAA Paper 00-10103. International Academy of Astronautics, Paris, France.
77. Newman, D.J., Schmidt, P.B., and Rahn, D.B. 2000. Modeling the extravehicular mobility unit (EMU) space suit—Physiological implications for extravehicular activity (EVA). SAE Technical Paper 2000-01-2257. SAE International, Warrendale, Pa.
78. Graziosi, D., Stein, J., and Kearney, L. 2000. Space shuttle small EMU development. SAE Technical Paper 2000-01-2256. SAE International, Warrendale, Pa.
79. United Space Alliance. 2005. Crew Escape Systems 21002. Available at http://www.nasa.gov/centers/johnson/pdf/383443main_crew_escape_workbook.pdf.
80. Jordan, N.C., Saleh, J.H., Newman, D.J. 2006. The extravehicular mobility unit: A review of environment, requirements, and design changes in the US spacesuit. Acta Astronautica 59(12):1135-1145.
81. National Research Council. 1997. Advanced Technology for Human Support in Space. National Academy Press, Washington, D.C.
82. National Research Council. 2000. Microgravity Research in Support of Technologies for Human Exploration and Development of Space and Planetary Bodies. National Academy Press, Washington, D.C.
83. National Research Council. 2003. Assessment of Directions in Microgravity and Physical Sciences Research at NASA. The National Academies Press, Washington, D.C.
84. National Research Council. 2008. A Constrained Space Exploration Technology Program: A Review of NASA’s Exploration Technology Program. The National Academies Press, Washington, D.C.
85. Blanco, R., NASA. 2009. “Current and Future EVA Capabilities,” presentation via teleconference call to Panel on the Translation to Space Exploration Systems of the Decadal Survey on Biological and Physical Sciences in Space, December 3. National Research Council, Washington, D.C.
86. Sim, Z., Bethke, K., Jordan, N., Dube, C., Hoffman, J., Brensinger, C., Trotti, G., and Newman, D. 2005. Implementation and testing of a mechanical counterpressure bio-suit system. Presented at the AIAA and SAE International Conference on Environmental Systems (ICES 2005), Rome, Italy, July. SAE Paper 2005-01-2968. SAE International, Warrendale, Pa.
87. Newman, D., Canina, M., and Trotti, G.L. 2007. Revolutionary design for astronaut exploration—Beyond the Bio-Suit. Presented at the Space Technology and Applications International Forum, STAIF-2007, Albuquerque, N.M., February 11-15. AIP Conference Proceedings 880. American Institute of Physics, College Park, Md.
88. Kosmo, J., Bassick, J., and Porter, K. 1988. Development of higher operating pressure extravehicular space-suit glove assemblies. Presented at the 18th SAE Intersociety Conference on Environmental Systems, San Francisco, Calif. July 11-13. SAE Technical Paper 881102. SAE International, Warrendale, Pa.
89. Wright, H.C. 1985. Enhancement of space suit glove performance. Presented at the 15th AIAA, SAE, ASME, AIChE, and ASMA Intersociety Conference on Environmental Systems, San Francisco, Calif., July 15-17. SAE Technical Paper 851335. SAE International, Warrendale, Pa.
90. Tanaka, K., Danaher, P., Webb, P., and Hargens, A.R. 2009. Mobility of the elastic counterpressure space suit glove. Aviation, Space, and Environmental Medicine 80(10):890-893.
91. Danaher, P., Tanaka, K., and Hargens, A.R. 2005. Mechanical counter-pressure vs. gas-pressurized spacesuit gloves: Grip and sensitivity. Aviation 76(4):381-384.
92. Blanco, R., NASA. 2009. “Current and Future EVA Capabilities,” presentation via teleconference call to Panel on the Translation to Space Exploration Systems of the Decadal Survey on Biological and Physical Sciences in Space, December 3. National Research Council, Washington, D.C.
93. National Research Council. 2008. A Constrained Space Exploration Technology Program: A Review of NASA’s Exploration Technology Program. The National Academies Press, Washington, D.C., p. 40.
94. National Research Council. 2008. A Constrained Space Exploration Technology Program: A Review of NASA’s Exploration Technology Program. The National Academies Press, Washington, D.C., p. 40.
95. Taylor, L.A., Schmitt, H., Carrier, W., and Nakagawa, M. 2005. The lunar dust problem: From liability to asset. Presented at the AIAA 1st Space Exploration Conference, Orlando, Fla., January 30-February 1. AIAA Paper 2005-2510. American Institute of Aeronautics and Astronautics, Reston, Va.
96. Gaier, J.R. 2005. The Effects of Lunar Dust on EVA Systems during the Apollo Missions. NASA/TM-2005-213610. March. NASA Glenn Research Center, Cleveland, Ohio.
97. Gaier, J.R. 2005. The Effects of Lunar Dust on EVA Systems during the Apollo Missions. NASA/TM-2005-213610. March. NASA Glenn Research Center, Cleveland, Ohio.
98. Gaier, J.R. 2005. The Effects of Lunar Dust on EVA Systems during the Apollo Missions. NASA/TM-2005-213610. March. NASA Glenn Research Center, Cleveland, Ohio.
99. Gaier, J.R. 2005. The Effects of Lunar Dust on EVA Systems during the Apollo Missions. NASA/TM-2005-213610. March. NASA Glenn Research Center, Cleveland, Ohio.
100. Forget, F. 2004. Alien weather at the poles of Mars. Science 306:1298-1299.
101. Koontz, S., Valentine, M., Keeping, T., Edeen, M., Spetch, W., and Dalton, P. 2003. Assessment and Control of Spacecraft Charging Risks on the International Space Station, Proceedings of the 8th Spacecraft Charging Technology Conference, October 20-24. Available at http://dev.spis.org/projects/spine/home/tools/sctc/VIIIth.
102. National Research Council. 1997. Advanced Technology for Human Support in Space. National Academy Press, Washington, D.C.
103. National Research Council. 2000. Microgravity Research in Support of Technologies for Human Exploration and Development of Space and Planetary Bodies. National Academy Press, Washington, D.C.
104. National Research Council. 2008. A Constrained Space Exploration Technology Program: A Review of NASA’s Exploration Technology Program. The National Academies Press, Washington, D.C.
105. Campbell, P.D. 2006. Recommendations for Exploration Spacecraft Internal Atmospheres: The Final Report of the NASA Exploration Atmospheres Working Group. NASA JSC-63309. January. NASA Johnson Space Center, Houston, Tex.
106. Science@NASA Headline News. 2001. Staying Cool on the ISS. March 21. Available at http://science.nasa.gov/headlines/y2001/ast21mar_1.htm.
107. Bagdigian, R., NASA Marshall Space Flight Center. 2009. “Environmental Control and Life Support in the Constellation Program,” presentation to Panel on the Plant and Microbial Biology of the Decadal Survey on Biological and Physical Sciences in Space, October 8. National Research Council, Washington, D.C.
108. Balasubramaniam, E.R., Kizito, J., and Kassemi, M. 2006. Two Phase Flow Modeling: Summary of Flow Regimes and Pressure Drop Correlations in Reduced and Partial Gravity. NASA/CR-2006-214085. National Center for Space Exploration Research, Cleveland, Ohio.
109. Bagdigian, R., NASA Marshall Space Flight Center. 2009. “Environmental Control and Life Support in the Constellation Program,” presentation to Panel on the Plant and Microbial Biology of the Decadal Survey on Biological and Physical Sciences in Space, October 8. National Research Council, Washington, D.C.
110. Thornton, J., Whittaker, W., Jones, H., Mackin, M., Barsa, R., and Gump, D. 2010. Thermal strategies for long duration mobile lunar surface missions. Presented at the 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, Orlando, Fla., January 4-7. AIAA-2010-0798. American Institute of Aeronautics and Astronautics, Reston, Va.
111. Baiden, G., Grenier, L., and Blair, B. 2010. Lunar underground mining and construction: A terrestrial vision enabling space exploration and commerce. Presented at the 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, Orlando, Fla., January 4-7. AIAA 2010-1548. American Institute of Aeronautics and Astronautics, Reston, Va.
112. Balasubramanian, R., Gokoglu, S., Sacksteder, K., Wegeng, R., and Suzuki, N. 2010. An extension of analysis of solar-heated thermal wadis to support extended-duration lunar exploration. Presented at the 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, Orlando, Fla., January 4-7. AIAA 2010-0797. American Institute of Aeronautics and Astronautics, Reston, Va.
113. Bockstahler, K., Funke, H., Lucas. J., Witt, J., and Hovland, S. 2009. Design status of the closed-loop air revitalization system ARES for accommodation on the ISS. SAE International Journal of Aerospace 1(1):543-555.
114. Tomes, K., Long, D., Carter, L., and Flynn, M. 2007. Assessment of the Vapor Phase Catalytic Ammonia Removal (VPCAR) technology at the MSFC ECLS Test Facility. Presented at the SAE International Conference on Environmental Systems, July 9-12. SAE Technical Paper 2007-01-3036. SAE International, Warrendale, Pa.
115. NASA Ames Research Center. Ames Technology Capabilities and Facilities. Advanced Life Support. Available at http://www.nasa.gov/centers/ames/research/technology-onepagers/advanced-life-support_prt.htm.
116. Ruff, G.A., Urban, D.L., Pedley, M.D., and Johnson, P.T. 2009. Fire safety. Chapter 27 in Safety Design for Space Systems (G.E. Musgrave, A. Larsen, and T. Sgobba, eds.) Elsevier Press, Oxford, U.K.
117. Ferkul, P., and Olson, S. 2010. Zero-gravity centrifuge used for the evaluation of material flammability in lunar-gravity. Presented at the AIAA 40th International Conference on Environmental Systems. July 11-15. AIAA 2010-6260. American Institute of Aeronautics and Astronautics, Reston, Va.
118. Sacksteder, K., Ferkul, P.V., Feier, I.I., Kumar, A., and T’ien, J.S. 2003. Upward and Downward Flame Spreading and Extinction in Partial Gravity Environments. NASA/CP-2003-212376/REV1. Available at http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040053567_2004055227.pdf.
119. Sacksteder, K., and T’ien, J. 1994. Buoyant downward diffusion flame spread and extinction in partial-gravity accelerations. Symposium (International) on Combustion 25:1685-1692.
120. Braun, E., Levin, B.C., Paabo, M., Gurman, J., Holt, T.H., and Steel, J.S. 1987. Fire Toxicity Scaling. NBSIR 87-3510. U.S. Dept. of Commerce, National Bureau of Standards, National Technical Information, Gaithersburg, Md.
121. Hshieh, F.-Y., and Beeson, H.D. 1995. Flammability testing of pure and flame retardant-treated cotton fabrics. Fire and Materials 19(5):233-239.
122. Levin, B.C., Paabo, M., Fultz, M.L., and Bailey, C.S. 1985. Generation of hydrogen cyanide from flexible polyurethane foam decomposed under different combustion conditions. Fire and Materials 9(3):125-134.
123. Lange, K.E., Perka, A.T., Duffield, B.E., and Jeng F.F. 2005. Bounding the Spacecraft Atmosphere Design Space for Future Exploration Missions. NASA/CR-2005-213689. NASA Johnson Space Center, Houston, Tex.
124. Urban, D.L., Ruff, G., Sheredy. W., Cleary, T., Yang, J., Mulholland, G., and Yuan, Z. 2009. Properties of smoke from overheated materials in low gravity. Presented at the 47th AIAA Aerospace Sciences Meeting, Orlando, Fla., January 5-8. AIAA-2009-0956. American Institute of Aeronautics and Astronautics, Reston, Va.
125. Cowlard, A., Jahn, W., Abecassis-Empis, C., Rein, G., and Torero, J.L. 2008. Sensor assisted fire fighting. Fire Technology 46:719-741.
126. Ruff, G.A., Urban, D.L., Pedley, M.D., and Johnson, P.T. 2009. Fire safety. Chapter 27 in Safety Design for Space Systems (G.E. Musgrave, A. Larsen, and T. Sgobba, eds.). Elsevier Press, Oxford, U.K.
127. Takahashi, F., Linteris, G., and Katta, V.R. 2008. Extinguishment of methane diffusion flames by carbon dioxide in coflow air and oxygen-enriched microgravity environments. Combustion and Flame 155:37-53.
128. Sutula, J.A. 2008. Towards a Methodology for the Prediction of Flame Extinction and Suppression in Three-Dimensional Normal and Microgravity Environments, Ph.D. Dissertation, University of Edinburgh, Scotland, U.K.
129. Butz, J.R., and Abbud-Madrid, A. 2009. Advances in development of fine water mist portable extinguisher. Presented at the 39th International Conference on Environmental Systems, Savannah, Ga., July 12-16. SAE Technical Paper 2009-01-2510. SAE International, Warrendale, Pa.
130. Takahashi, F., and Katta, V.R. 2009. Extinguishment of diffusion flames around a cylinder in a coaxial air stream with dilution or water mist. Proceedings of the Combustion Institute 32:2615-2623.
131. Urban, D.L., Ruff, G., Brooker, J., Cleary, T., Yang, J., Mulholland, G., and Yuan, Z. 2008. Spacecraft fire detection: Smoke properties and transport in low-gravity. Presented at the 46th AIAA Aerospace Sciences Meeting, Reno, Nev., January 7-10. AIAA-2008-0806. American Institute of Aeronautics and Astronautics, Reston, Va.
132. NASA. 1997. Small Fire Extinguished on Mir. News Release. Available at http://spaceflight.nasa.gov/history/shuttle-mir/history/h-f-linenger-fire.htm.
133. Friedman, R. 1996. Risk and Issues in Fire Safety on the Space Station. NASA/TM 106430. NASA Glenn Research Center, Cleveland, Ohio.
134. Urban, D.L., Ruff, G., Sheredy. W., Cleary, T., Yang, J., Mulholland, G., and Yuan, Z. 2009. Properties of smoke from overheated materials in low gravity. Presented at the 47th AIAA Aerospace Sciences Meeting, Orlando, Fla., January 5-8. AIAA-2009-0956. American Institute of Aeronautics and Astronautics, Reston, Va.
135. McKay, M.F., McKay, D.S. and Duke, M.B. 1992. Space Resources. NASA SP-509. NASA, Washington, D.C.
136. Duke, M.B., Gaddis, L.R., Taylor, G.J., and Schmitt, H.H. 2006. Developing the Moon. Pp. 597-655 in New Views of the Moon (B.L. Jolliff, M.A. Wieczorek, C.K. Shearer, and C.R. Neal, eds). Reviews of Mineralogy and Geochemistry, Volume 60. Mineralogical Society of America, Chantilly, Va.
137. Eagle Engineering. 1988. Conceptual Design of a Lunar Oxygen Pilot Plant. NASA/CR EEI 88-182. Contract NAS9-17878. NASA Johnson Space Center. Available at http://www.isruinfo.com//docs/LDEM_Draft4-updated.pdf.
138. Ignatiev, A., Kubricht, T., and Freundlich, A. 1998. Solar cell development on the surface of the Moon. IAA-98-IAA.13.2.03. International Astronautical Federation.
139. PBS News Hour. Mars Exploration Rovers. Available at http://www.pbs.org/newshour/indepth_coverage/science/marsrover/archive.html.
140. Pieters, C.M., Goswami, J.N., Clark, R.N., Annadurai, M., Boardman, J., Buratti, B., Combe, J.-P., Dyar, M.D., Green, R., Head, J.W., Hibbitts, C., et al. 2009. Character and spatial distribution of OH/H2O on the surface of the Moon seen by M3 on Chandrayaan-1. Science 326(5952):568-572.
141. Clark, R.N. 2009. Detection of adsorbed water and hydroxyl on the Moon. Science 326(5952):562-564.
142. Sunshine, J.M., Farnham, T.L., Feaga, L.M., Groussin, O., Merlin, F., Milliken, R.E., and A’Hearn, M.F. 2009. Temporal and spatial variability of lunar hydration as observed by the Deep Impact spacecraft. Science 326(5952):565-568.
143. Colaprete, K., Ennico, K., Wooden, D., Shirley, M., Heldmann, J., Marshall, W., Sollitt, L., Asphaug, E., Korycansky, D., Schultz, P., Hermalyn, B., et al. 2010. Water and more: An overview of LCROSS impact results. Presented at the 41st Lunar and Planetary Science Conference, The Woodlands, Tex., March 1-5. Abstract #2335. Lunar and Planetary Institute, Houston, Tex.
144. Paige, D.A., Greenhagen, B.T., Vasavada, A.R., Allen, C., Bandfield, J.L., Bowles, N.E., Calcutt, S.B., DeJong, E.M., Elphic, R.C., Foote, E.J., Foote, M.C., et al. 2010. DIVINER Lunar Radiometer Experiment: Early mapping mission results. Presented at the 41st Lunar and Planetary Science Conference, The Woodlands, Tex., March 1-5. Abstract #2267. Lunar and Planetary Institute, Houston, Tex.
145. Blair, B.R., Diaz, J., Duke, M.B., Lamassoure, E., Easter, R., Oderman, M., and Vaucher, M. 2002. Space Resource Economic Tool Kit: The Case for Commercial Lunar Ice Mining. Final Report to the NASA Exploration Team. Jet Propulsion Laboratory, Pasadena, Calif.
146. Beaty, D., Buxbaum, K., Meyer, M., Barlow, N., Boynton, W., Clark, B., Deming, J., Doran, P.T., Edgett, K., Hancock, S., Head, J., et al. 2006. Findings of the Mars Special Regions Science Analysis Group. Astrobiology 6(4):677-732.
147. Plaut, J.J., Holt, J.W., Head III, J.W., Gim, Y., Choudhary, P., Baker, D.M., Kress, A., and the SHARAD Team. 2010. Thick ice deposits in Deuteronilus Mensae, Mars: Regional Distribution from radar sounding. Presented at the 41st Lunar and Planetary Science Conference, March 1-5. Abstract #2454. Lunar and Planetary Institute, Houston, Tex.
148. Lewis, J.S. 1996. Mining the Sky: Untold Riches from the Asteroids, Comets, and Planets. Helix Books, Reading, Mass.
149. Ignatiev, A., Kubricht, T., and Freundlich, A. 1998. Solar cell development on the surface of the Moon. Presented at the 49th International Astronautical Congress. IAA-98-IAA.13.2.03. International Astronautical Federation, Paris, France.
150. Wegeng, R.S., Humble, P.H., Saunders, J.H., Feier, I.I., and Pestak, C.J. 2009. Thermal energy storage and power generation for the manned outpost using processed lunar regolith as thermal mass materials. Presented at the AIAA Space 2009 Conference, Pasadena, Calif., September 14-17. AIAA-2009-6534. American Institute of Aeronautics and Astronautics, Reston, Va.
151. Wegeng, R.S., Mankins, J., Taylor, L., and Sanders, G. 2007. Thermal energy reservoirs from processed lunar regolith. Presented at the AIAA 5th International Energy Conversion Engineering Conference, St. Louis, Mo., June 25-27. AIAA-2007-4821. American Institute of Aeronautics and Astronautics, Reston, Va.
152. Wegeng, R.S., Mankins, J., Balasubramaniam, R., Sacksteder, K., Gokoglu, S., Sanders, G., and Taylor, L. 2008. Thermal wadis in support of lunar science and exploration. Presented at the AIAA 6th International Energy Conversion Engineering Conference, Cleveland, Ohio, July 28-30. AIAA-2008-5632. American Institute of Aeronautics and Astronautics, Reston, Va.
153. Balasubramaniam, R., Gokoglu, S., Sacksteder, K., Wegeng, R., and Suzuki, N. 2009. Analysis of solar-heated thermal wadi to support extended-duration lunar exploration. Presented at the AIAA Aerospace Sciences Meeting, Orlando, Fla., January 5-8. AIAA-2009-1339. American Institute of Aeronautics and Astronautics, Reston, Va.
154. Larson, W.E., Sanders, G.B., Sacksteder, K.R., Simon, T.M., and Linne, D.L. 2008. NASA’s in-situ resource utilization project: A path to sustainable exploration. Presented at the 59th International Astronautical Congress. IAC-08-A3.2.B13. International Astronautical Federation, Paris, France.
155. Drake, B. 2009. Human Exploration of Mars Design Reference Architecture 5.0 Addendum. NASA/SP-2009-566-ADD. July. NASA, Washington, D.C.
156. Colaprete, K., Ennico, K., Wooden, D., Shirley, M., Heldmann, J., Marshall, W., Sollitt, L., Asphaug, E., Korycansky, D., Schultz, P., Hermalyn, B., et al. 2010. Water and more: An overview of LCROSS impact results. Presented at the 41st Lunar and Planetary Science Conference, The Woodlands, Tex., March 1-5. Abstract #2335. Lunar and Planetary Institute, Houston, Tex.
157. Boynton, W.V., Feldman, W.C., Squyres, S.W., Prettyman, T.H., Brückner, J., Evans, L.G., Reedy, R.C., Starr, R., Arnold, J.R., Drake, D.M., Englert, P.A.J., et al. 2002. Distribution of hydrogen in the near-surface of Mars: Evidence for subsurface ice deposits. Science 297:81.
158. Taylor, L.A., and Carrier III, W.D. 1994. Oxygen production on the Moon: An overview and evaluation. Pp. 69-108 in Resources of Near Earth Space (J.S. Lewis, M.S. Matthews, M.L. Guerrieri, eds.). University of Arizona Press, Tucson, Ariz.
159. Larson, W.E., Sanders, G.B., Sacksteder, K.R., Simon, T.M., and Linne, D.L. 2008. NASA’s in-situ resource utilization project: A path to sustainable exploration. Presented at the 59th International Astronautical Congress. IAC-08-A3.2.B13, International Astronautical Federation, Paris, France.
160. Larson, W.E., Sanders, G.B., Sacksteder, K.R., Simon, T.M., and Linne, D.L. 2008. NASA’s in-situ resource utilization project: A path to sustainable exploration. Presented at the 59th International Astronautical Congress. IAC-08-A3.2.B13, International Astronautical Federation, Paris, France.
161. Carlson, R.R., Bland, D., Fox, R., Hamilton, J., and Schowengerdt, F. 2009. Lunar surface equipment testing and demonstrations at the PISCES Lunar Analog Facilities. Presented at the 27th International Symposium on Space Technology and Science, Tsukuba City, Ibaraki Prefecture, Japan. ISTS 2009-f-04. International Symposium on Space Technology and Science, Tokyo, Japan.
162. Carlson, R.R., Bland, D., Fox, R., Hamilton, J., and Schowengerdt, F. 2009. Lunar surface equipment testing and demonstrations at the PISCES Lunar Analog Facilities. Presented at the 27th International Symposium on Space Technology and Science, Tsukuba City, Ibaraki Prefecture, Japan. ISTS 2009-f-04. International Symposium on Space Technology and Science, Tokyo, Japan.
163. Carlson, R.R., Bland, D., Fox, R., Hamilton, J., and Schowengerdt, F. 2009. Lunar surface equipment testing and demonstrations at the PISCES Lunar Analog Facilities. Presented at the 27th International Symposium on Space Technology and Science, Tsukuba City, Ibaraki Prefecture, Japan. ISTS 2009-f-04. International Symposium on Space Technology and Science, Tokyo, Japan.
164. Eagle Engineering.1988. Conceptual Design of a Lunar Oxygen Pilot Plant. NASA/CR EEI 88-182. Contract NAS9-17878. NASA Johnson Space Center. Available at http://www.isruinfo.com//docs/LDEM_Draft4-updated.pdf.
165. National Research Council. 2000. Microgravity Research in Support of Technologies for Human Exploration and Development of Space and Planetary Bodies. National Academy Press, Washington, D.C.
This page intentionally left blank.