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Priorities in Space Science Enabled by Nuclear Power and Propulsion 1 Introduction and Background Two of the most constrained resources on all spacecraft are propulsion power and electrical power. Chemical systems have been used to propel spacecraft since the dawn of the Space Age. Similarly, photovoltaic arrays, batteries, and fuel cells have been the principal source of electric power in space. The increasing demands for higher power, shorter trip times, and greater maneuverability at the target destination have led to numerous suggestions and recommendations calling for the development of nuclear reactors for space-based power and propulsion systems.1 Chemical rockets have been the only available option for launch into Earth orbit or for any other task requiring large amounts of thrust. Even if powerful nuclear propulsion systems are developed in the future, safety and environmental considerations will prevent them from being operated until they are in space, which means that spacecraft will have to be launched using a chemical propulsion system. Photovoltaic and other potential solar-power systems are well suited to long-duration applications because they eliminate the need for large stores of chemical fuels and associated spacecraft structures. However, solar-power systems are not generally suitable for high-power applications, and photovoltaic systems in particular suffer from other drawbacks such as degradation caused by solar particles and ultraviolet radiation. Moreover, the output capacity of all types of solar-power systems drops off dramatically as the distance between the spacecraft and the Sun increases. On the other hand, photovoltaics are technologically mature and unit cost ($/watt of electrical power) is well understood (Figure 1.1). Radioisotope thermoelectric generators (RTGs) and other radioisotope power systems (RPSs)a are the systems of choice when low levels of electric or thermal power are needed for extended durations. RPSs are also ideally suited for use by spacecraft far from the Sun, or in locations where solar energy is available only intermittently (e.g., on the lunar surface) or not at all (e.g., at the lunar poles). Except for one short-lived experimental reactor—the SNAP-10A—launched in 1965, no U.S. space mission has used a nuclear reactor as a source of electric power, and none has used a reactor-based propulsion system.b Instead, space exploration missions have been designed within the power and energy envelope defined by the capabilities of chemical, solar, and radioisotope power and propulsion systems (see Figure 1.2). This is not a a The RTG is a particular technological implementation of a more generic class of devices, the RPS. Thus, this report uses the latter term unless the specific RTG technology is implied. b Details of past U.S. space nuclear power and propulsion systems can be found in Appendix A.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion FIGURE 1.1 A schematic showing the approximate cost ($) per watt of electrical power (We) required for radioisotope power systems (RPSs), photovoltaic systems at various distances from the Sun, and fission reactors of the type that might be used in nuclear-electric propulsion (NEP) systems. Most current space science missions have relatively modest power requirements, typically in the range of 0.1 to 1 kilowatts of electricity (kWe). For spacecraft operating at or near 1 AU from the Sun, photovoltaic systems have the advantages of cost-effectiveness and reliability. At greater distances from the Sun, RPSs become the favored option for satisfying modest power requirements. Fission reactors may fill a niche for supplying the large amounts of electrical power required by NEP systems and new types of power-hungry scientific instruments or to support human exploration activities. The plot is based on data supplied by the Boeing Company and is courtesy of Michael Kaplan. serious handicap for space missions limited to Earth orbit, short visits to the Moon, or robotic missions to Mars. But the capability of missions to the outer planets (Jupiter, Saturn, Uranus, Neptune, and Pluto) and their moons is restricted if they must rely on only chemical, solar, and/or radioisotope systems for electric power and propulsion. While RPSs are inherently low-power systems, most space science experiments have only low power demands. Thus, highly capable missions such as Cassini, which is equipped with a dozen instruments, can manage sufficiently on some 800 watts of electricity. The outer planets are so far from the Sun, however, that the output of solar-power systems at such distances is minuscule. Also, the duration of missions to the outer planets is so long that the average power available to a spacecraft over its lifetime from a chemical power system would also be minuscule, and the total energy available from a chemical system would be limited by the need to carry along the requisite fuel. Low power and energy limits are problems because power is needed to operate science instruments, communications systems, and propulsion systems. Nuclear reactor systems, which can provide relatively high power over long periods, make it possible to design missions with more numerous and more capable science instruments, high-bandwidth communications systems, shorter transit times, and greater flexibility to change the course and speed of spacecraft enough to: Conduct extended investigations (rather than brief flybys) of bodies of interest;
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Priorities in Space Science Enabled by Nuclear Power and Propulsion FIGURE 1.2 A schematic showing the relative applicability of various space-based sources of electrical power. Courtesy of George Schmidt, Project Prometheus, NASA. Visit multiple bodies much more easily; and Significantly alter a spacecraft’s trajectory in response to information collected during a particular mission. Nuclear reactors have the potential to overcome limitations associated with low energy and power. They do this by providing electricity and propulsion over a wide range of power levels for extended periods (years to decades), including during both transit and surface operations, without regard to the availability of either solar energy or large quantities of chemical fuel. Nuclear reactor systems, however, are expensive to develop, and their potential will be realized only if key technology issues can be overcome. PROJECT PROMETHEUS In 2002, NASA initiated the Nuclear Systems Initiative, within the Office of Space Science (now the Science Missions Directorate), to explore the use of nuclear power and propulsion systems for both human and robotic activities. According to NASA, the initiative was begun in response to identified limitations of the current paradigm for solar system exploration missions. In particular, solar power constrains power budgets and is of limited use in the outer solar system, and chemical propulsion limits spacecraft maneuverability and mission destinations. The following year, the Nuclear Systems Initiative was renamed Project Prometheus and given three tasks: To develop a new generation of RPSs; To conduct advanced studies of nuclear power and propulsion systems; and To initiate development of the first Prometheus flight program, the Jupiter Icy Moons Orbiter (JIMO). In February 2004, responsibility for Project Prometheus was transferred to NASA’s newly established Office of Explorations Systems (now the Explorations Systems Mission Directorate). The Office of Space Science,
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Priorities in Space Science Enabled by Nuclear Power and Propulsion however, retained managerial control of the development of RPSs and the science requirements for JIMO. The initiation, at about the same time, of the Vision for Space Exploration—NASA’s new program of coordinated robotic and human exploration of the Moon, Mars, and beyond (Box 1.1)—greatly expanded the scope of the type of nuclear power and propulsion systems under consideration by Project Prometheus. In addition to RPS technology and the fission reactor to power JIMO’s NEP system and high-powered instruments, Prometheus was now responsible for research and development of the following: Nuclear power systems to supply auxiliary power for spacecraft in transit (i.e., to operate, for example, life-support and other spacecraft systems) and to supply power for surface activities on the Moon or Mars; and Much larger nuclear power systems to support thermal or NEP systems for human exploration activities beyond the Earth-Moon system. BOX 1.1 The Vision for Space Exploration The major activity that now focuses many if not all of NASA’s programs, including Project Prometheus, is the human and robotic exploration initiative known as the Vision for Space Exploration. On January 14, 2004, President George W. Bush announced a new civil space policy that was soon named the Vision for Space Exploration.1 Not only did NASA quickly start several new projects as part of this new policy, but the agency also began a major restructuring of its organization. Significantly, NASA was one of only a few non-defense agencies to receive a budget increase in the 2005 fiscal year, and the reason for this was the Vision for Space Exploration. Origins of the Vision for Space Exploration The new space policy owes its origins to the Columbia accident in February 2003. Prior to that, NASA’s overall policy goals—as outlined in the national space policy annunciated by the Clinton administration in 1996—included completing the International Space Station (ISS), maintaining a continuous robotic presence on Mars, undertaking a concerted effort to search for and characterize extrasolar planets, and conducting a long-term program of Earth remote sensing.2 But NASA’s leadership had explicitly declared that the agency would pursue space technology development to enable future efforts, rather than establish a specific destination to explore, and lunar exploration was not part of the agency’s future plans. At the beginning of 2003, NASA also had no plans to retire the Space Shuttle in the foreseeable future. The Columbia disaster in February 2003 changed everything. The Columbia Accident Investigation Board (CAIB) declared in August 2003 that if the Space Shuttle were to continue to operate past 2010 (the planned completion date for the core version of the ISS), the space agency should “recertify” it.3 The CAIB also declared that one of NASA’s institutional problems was the lack of a clear programmatic focus, which led to constantly changing plans for the Shuttle fleet. As the CAIB conducted its investigation in the spring and summer of 2003, staffers in the White House, including the National Security Council and the Office of Science and Technology Policy, began discussing 1 See, for example, National Aeronautics and Space Administration, The Vision for Space Exploration, NP-2004-01-334-HQ, NASA, Washington, D.C., 2004. 2 National Science and Technology Council, National Space Policy Fact Sheet, September 19, 1996, Executive Office of the President, Washington, D.C., 1996, available online at <http://history.nasa.gov/appf2.pdf>. 3 Columbia Accident Investigation Board, Report, Volume 1, August 2003. Available online at <http://www.nasa.gov/columbia/home/CAIB_Voll.html>.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion The basic characteristics of the nuclear systems relevant to Project Prometheus are shown in Table 1.1, and NASA’s notional timeline for the development of these technologies is indicated in Figure 1.3. In August 2004, NASA and the Department of Energy’s (DOE’s) Office of Naval Reactors signed a memorandum of understanding relating to the development, design, delivery, and operational support of civilian space nuclear reactors within NASA’s Project Prometheus. The following month, the Jet Propulsion Laboratory (JPL) selected Northrop Grumman Space Technology as the prime contractor for developing a preliminary design for the JIMO spacecraft. The contract, valued at approximately $400 million and relating to activities through mid-2008, also covers the development of hardware, software, and test activities for the design of the non-nuclear portion of the spacecraft, and the interfaces for the spacecraft, reactor, and science instruments. the future of the civilian space agency.4 These discussions gained momentum after the August release of the CAIB report, which contained a scathing critique of NASA. The internal deliberations continued through the autumn, gaining increased momentum from the CAIB report. Discussions on the future of NASA were not confined to the White House. In the waning days of 2003, Congress, the Space Studies Board (SSB),5 the media, and other interested parties all began to pay more attention to the goals of, and prospects for, human spaceflight activities. Then, in January 14, 2004, President Bush announced the new policy at NASA Headquarters. Scope of the Vision for Space Exploration The Vision established exploration as the primary goal for the space agency. President Bush called for humans to return to the Moon no later than 2020, leading to the eventual human exploration of Mars. The Vision for Space Exploration also called for greater use of robotic probes “to maximize our understanding of the solar system and pave the way for more ambitious manned missions.” The new policy also addressed several existing programs. Under the new plans, the Space Shuttle would be retired by 2010, after the completion of the ISS. NASA would develop a new human exploration vehicle to explore “beyond our orbit to other worlds” and replace the Space Shuttle. This new craft would be known as the Crew Exploration Vehicle (CEV). Later, the CEV was declared to be part of a broader human exploration effort called Project Constellation. Project Prometheus, the effort to develop space nuclear power and related technologies for various missions, predated the Vision for Space Exploration, but was incorporated into it. Budgetary Impact An important aspect of the Vision for Space Exploration was that it would not require substantial increases in the NASA budget over the next 15 years. Retiring the Space Shuttle in 2010 and curtailing operations on the ISS around 2016 should free up substantial amounts of money that could be applied to the new initiative. These savings will not, however, be realized until early in the next decade, creating a potential cash shortage in the latter part of this decade. Compounding NASA’s near-term budgetary situation are a number of non-Vision-related issues, including the cost of returning the Space Shuttle to flight status, a Hubble Space Telescope servicing mission, and an ever-growing number of congressional earmarks in the agency’s budget. 4 C. Stadd and J. Bingham, “The U.S. Civil Space Sector: Alternate Futures,” Space Policy 20:241–252, 2004. 5 National Research Council, Issues and Opportunities Regarding the U.S. Space Program—A Summary of a Workshop on National Space Policy, The National Academies Press, Washington, D.C., 2004.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion TABLE 1.1 Characteristics of Nuclear Systems of Relevance to Project Prometheus Type Approximate Power Range (kWe) Approximate Mass Range (kg) Shielding Issues Launch Approval Issues Radioisotope power systems <0.3 <50 to 100 Alpha particles, gamma rays On-pad accident with solid rocket boosters; reentry NEP reactor system 100 to 500 4,000 to 15,000 Neutrons, gamma rays No inadvertent criticalitya Auxiliary power reactor system 10 to 100 400 to 4,000 Neutrons, gamma rays No inadvertent criticalitya NTP reactor system 20,000 to 4,000,000 1,000 to 8,000 Neutrons, gamma rays No inadvertent criticalitya NEP human transport reactor system 6,000 to 20,000 60,000 to >200,000 Neutrons, gamma rays No inadvertent criticalitya Moon/Mars surface reactor system 20 to 100 800 to 6,000 Neutrons, gamma rays, regolith activation, scatter No inadvertent criticalitya aRelease of radioactive material from launch or reentry accidents involving a reactor is relatively minor if the reactor is not powered and accidental criticality is prevented by design. Adaptation of current National Environmental Policy Act and launch-approval risk assessment processes may be adequate to validate/verify system safety characteristics and assumptions. NOTE: NEP, nuclear-electric propulsion; NTP, nuclear-thermal propulsion; kWe, kilowatts of electric power. SOURCE: A. Newhouse, Project Prometheus, NASA. RADIOISOTOPE POWER SYSTEMS RPSs have been used extensively on solar system exploration missions and to a somewhat lesser extent on space physics missions. The continued availability of these devices has been of great concern to the space science community. Plutonium-238, whose decay provides the RPSs’ thermal power, is no longer manufactured in the United States and supplies have to be purchased from Russia. DOE has, however, recently announced its intention to open a new 238Pu production facility at the Idaho National Laboratory.2,3 The solar system exploration (SSE) decadal survey completed in 2002 was drafted in part under the assumption that there was a significant possibility that no additional RPSs would be available for use on planetary exploration missions other than the spare Cassini RTG now allocated to the New Horizons mission to Pluto and the Kuiper Belt.4 The commitment of NASA and its partners at DOE to development of new RPS technologies is welcome. NASA’s current RPS activities are focusing on the development of two alternative technologies: the multi-mission radioisotope thermoelectric generator (MMRTG)—baselined for use by the Mars Science Laboratory in 2009—and the Stirling radioisotope generator (SRG). Although both are designed to supply approximately 110 to 120 watts of electrical power (We), the former makes use of eight 238Pu general-purpose heat sources (GPHSs), whereas the latter’s more efficient power-conversion system requires only 2 GPHSs. Unlike the RPSs used on Cassini and Galileo, the MMRTG and SRG can function equally well in a vacuum or in a planetary atmosphere. It is important to note, however, that when compared in terms of their mass per unit of electrical power output, the MMRTG and SRG are significantly less efficient than the Cassini-class GPHS-RTG. The characteristics of both new systems and the Cassini-class RPS are shown in Table 1.2 A related development of great potential interest to the space science community involves the nascent activities by NASA and DOE to develop RPSs much smaller than the devices discussed above. These activities are
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Priorities in Space Science Enabled by Nuclear Power and Propulsion FIGURE 1.3 The potential applications of the various technologies of interest to Project Prometheus are outlined in a notional p rogression. The chart is intended to indicate the evolutionary relationship between the technologies being developed for the JIMO propulsion system and more advance d nuclear systems that would be applicable to human exploration or space science missions. The chart was generated by NASA to guide internal studies and reflec ts the agency’s thinking as of November 2004. Since then, there have been significant changes in NASA’s plans, including the deferment of JIMO until beyond 2017. Note that the 75-kilonewton NTP system mentioned would require a fission reactor with a thermal power output of approximately 330 megawatts. Courtesy of Project Prometheus.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion TABLE 1.2 Characteristics of Different Radioisotope Power System Technologies RPS Type Power (We) Mass (kg) Specific Mass (kg/kWe) 238Pu Usage Notes Beginning of Mission End of Mission Beginning of Mission End of Mission MMRTG 123 ~100—deep space 40 325 400—deep space ~5 kg Developed from SNAP-19 RTG used on Viking and Pioneer 10 and 11 121 ~100—Mars 331 400—Mars 8 GPHSs SRG 112 ~94—deep space 34 303 361—deep space ~1 kg Stirling-cycle power converter is four times as efficient as the MMRTG’s thermocouples 110 ~90—Mars 309 378—Mars 2 GPHSs GPHS-RTG 290 ~250—deep space 55.5 191 222—deep space ~11 kg Used on Galileo, Ulysses, Cassini, and New Horizons; design incompatible with operation in the martian atmosphere 18 GPHSs NOTE: MMRTG, multi-mission radioisotope thermoelectric generator; SRG, Stirling radioisotope generator; GPHS-RTG, general-purpose heat source radioisotope thermoelectric generator. End of mission is arbitrarily defined as 10 years after launch. SOURCE: G.R. Schmidt, R.L. Wiley, R.L. Richardson, and R.R. Furlong, “NASA’s Program for Radioisotope Power System Research and Development,” Space Technology and Applications International Forum—STAIF-2005, M.S. El-Genk, ed., American Institute of Physics, Melville, N.Y., 2005. focusing on developing an RPS whose heat source is either one GPHS, a fraction of a GPHS, or multiple radiosotope heater units (RHUs).5 The potential characteristics of such systems are indicated in Table 1.3. An area not actively pursued currently is the development of either a replacement for the GPHS-RTG or something with an even larger power output. In other words, it is possible to conceive of missions needing ~1 kWe—e.g., spacecraft using a radioisotope-electric propulsion system—but the only way to provide such power at the moment is to gang ~10 MMRTGs or SRGs. TABLE 1.3 Characteristics of Possible Small Radioisotope Power Systems Small-RPS Class Approximate Power (We) Approximate Mass (kg) Specific Mass (kg/kWe) 238Pu Packaging Example of Possible Use Large 10 to 20 ~5 ~250 to ~500 1 GPHS Europa Surface Science Package Medium 0.1 to <10 ~0.5 to 1.5 ≥150 to ~15,000 Multiple RHUs Magnetospheric Microsatellite Constellation Small >0.01 ≤0.5 ≤50,000 1 RHU Mars Meteorological Network NOTE: RHU, radioisotope heater unit; GPHS, general-purpose heat source. SOURCE: R.D. Abelson, ed., Enabling Exploration with Small Radioisotope Power Systems (JPL-Publication 04-10), Jet Propulsion Laboratory, Pasadena, Calif., 2004.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion THE JUPITER ICY MOONS ORBITER JIMO was conceived as an ambitious mission designed to send a spacecraft to orbit three of the Galilean moons: Callisto, Ganymede, and Europa. The spacecraft would explore the moons and investigate their makeup, their history, and their potential for sustaining life in the vast subterranean oceans believed to exist under these moons’ icy surfaces. It was intended to serve as the flight test of a fission reactor power system, an advanced ion propulsion system, and a new generation of high-power scientific instruments. Science Goals The Galileo spacecraft that orbited Jupiter during the latter 1990s discovered evidence that Europa had an icy surface that probably covers a huge liquid water ocean up to 100 kilometers thick. The SSE decadal survey, New Frontiers in the Solar System, ranked the scientific exploration of Europa as a top priority for NASA’s planetary exploration program and recommended the development of a Europa Geophysical Explorer. It is worth noting that although the Europa Geophysical Explorer is envisaged as an RPS-powered spacecraft with a limited instrument complement,6 the report’s authors did consider the possibility of a nuclear-electric mission that would sequentially orbit the three outer Galilean satellites. This possibility was, however, deemed inappropriate until after there was confirmation of a subsurface ocean on one of the Galilean satellites.7 At the same time that the NRC was conducting its review of solar system exploration, NASA was actively considering a demonstration mission for Prometheus that would orbit Jupiter and its large Galilean moons. In 2003, NASA formally established the JIMO project to explore not only Europa but also Ganymede and Callisto, all of which are believed to exhibit evidence of subterranean liquid water. The initiation of JIMO was essentially concurrent with the SSE decadal survey, rather than, strictly speaking, flowing from its recommendations. Funding for JIMO was first included in NASA’s budget for fiscal year 2003.c The goals of the JIMO mission are as follows: To determine the evolution and present state of the Galilean satellite surfaces and subsurfaces and the processes affecting them; To determine the interior structures of the icy satellites and the potential “habitability” of the moons; To search for signs of past and current life; and To determine how the components of the jovian system operate and interact. JIMO was, however, intended to be a technology-demonstration mission as well as a mission of scientific exploration. Spacecraft Design As conceived by NASA, the basic JIMO spacecraft would weigh ~25,000+ kilograms and would consist of three main components (Figure 1.4): the reactor and power-conversion systems, a long boom equipped with radiators to disperse waste heat, and the propulsion module and science instrument platform. The reactor—probably a fast, moderator-less design equipped with external neutron reflectors to control criticality and using a gas, heat-pipe, or liquid-metal cooling system—is capable of generating ~500 kilowatts of thermal energy (kWth). It is mounted several tens of meters from the propulsion module to protect the avionics and the scientific payload from radiation.d The reactor’s thermal energy is converted into ~100+ kWe using an as-yet-to-be-determined power-conversion system; possibilities under consideration include thermoelectric, Stirling- c JIMO was first proposed in NASA’s budget request for fiscal year 2004. But the first funding was actually included in NASA’s budget for fiscal year 2003, which was not finally approved by Congress until after the 2004 request had been announced. d NASA’s so-called TB2.5 design for JIMO stretched some 36 meters (m) from the leading edge of the reactor to the trailing edge of the ion thrusters. The design proposed by the Northrop Grumman Corporation was some 75 m long.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion FIGURE 1.4 A schematic of one possible configuration for the Jupiter Icy Moons Orbiter. Also indicated are the various technologies that are under consideration for its various major subsystems. Courtesy of NASA/Jet Propulsion Laboratory. cycle, and Brayton-cycle systems. The electricity from the reactor would be used to power an ion propulsion system much larger than that employed on previous spacecraft such as NASA’s Deep Space 1, the European Space Agency’s SMART 1, or the Japan Aerospace Exploration Agency’s Hayabusa. Although the exact details are highly subject to assumptions made, a notional JIMO mission would involve a 5- to 8-year transit time from Earth to Jupiter and a 4- to 6-year-duration tour of the outer three Galilean satellites.8 On arrival at Jupiter, JIMO would sequentially go into high-inclination (>70°) orbits about Callisto, Ganymede, and finally Europa. The sequence is dictated by the spacecraft’s propulsion capabilities and the hazards posed by the jovian radiation environment. It is worth noting that the Earth-Jupiter flight time of the Europa Orbiter mission, studied by NASA in the late 1990s, was a little over 3 years, followed by an additional 2 years or so of maneuvering into orbit about Europa.9 Following the selection of Northrop Grumman Corporation (NGC) as JIMO’s prime contractor in October 2004, elements of NASA’s TB2.5 design for JIMO were merged with elements of the NGC design to create the so-called Prometheus Baseline (PB) 1 concept (Figure 1.5). This basic spacecraft design would be descoped or augmented as necessary to undertake a variety of missions in the outer solar system.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion FIGURE 1.5 Artist’s impression of the so-called Prometheus Baseline 1 spacecraft proposed by NASA and the Northrop Grumman Corporation as the basic design to be used for the Jupiter Icy Moons Orbiter and, in descoped and enhanced versions respectively, for Prometheus 1 and possible JIMO follow-on missions to Saturn and other destinations in the outer solar system. The proposed spacecraft would stretch some 58 m from the tip of the nuclear reactor (top) to the end of the boom (bottom). Courtesy of NASA.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion Launch Issues JIMO would be the largest robotic spacecraft ever launched (for comparison, the launch mass of the Cassini-Huygens spacecraft was some 5,800 kg). The size of the spacecraft presents special challenges for launch. Its mass is close to, or exceeds, the launch capability of the largest rocket in the current U.S. inventory. Mission designers have stated a strong desire to boost the entire spacecraft onto an Earth-escape trajectory prior to turning on its reactor and engaging its ion propulsion system. This stems from several considerations, including the following: Operating the reactor in low Earth orbit (LEO) will build up an inventory of fission products, which would present a hazard in the event of an unplanned reentry into Earth’s atmosphere. The radiation from the reactor could interfere with other spacecraft. This is particularly problematic for reactors located inside Earth’s magnetosphere because they generate charged particles that remain trapped in the geomagnetic field for relatively long periods (up to several minutes). Using the ion propulsion system to spiral out from LEO would likely add several years to the flight time. Protracted immersion in Earth’s Van Allen belts during the spiral out will expose the spacecraft’s sensitive instruments and avionics to a significant radiation dose. As a result, the spacecraft has to be boosted to escape velocity with a chemical propulsion system. Lifting both the spacecraft and this propulsion system into space would require either a single launch of a new heavy-lift launch vehicle or two or more separate launches of large, existing launch vehicles, followed by their rendezvous and mating in Earth orbit. Neither capability currently exists. Scientific Payload In 2004, the JIMO Science Definition Team strongly recommended increasing the spacecraft’s science payload from a nominal 600 kg to 1,500 kg (including scan platforms, booms, etc.).10 For comparison, Cassini’s total science payload (including Huygens and booms) is approximately 600 kg. The team also recommended that JIMO include a Europa Surface Science Package—a relatively simple lander designed to conduct a limited set of geophysical, geochemical, geological, and astrobiological studies. The lander would make a soft landing on the surface and would have to weigh approximately 375 kg, or roughly 25 percent of the science payload mass. The electrical power available to JIMO’s scientific instruments would far exceed that available on any previous scientific spacecraft. Instruments such as high-power, ground-penetrating, and synthetic-aperture radar systems; laser-ablation spectrometers; and active plasma sounders could in principle be accommodated.11 To begin development of such instruments, NASA initiated the High Capability Instruments for Planetary Exploration grants program. JIMO Deferred Because of its size and complexity, JIMO would be significantly more expensive than any previous planetary exploration mission. In late 2004, NASA began to investigate a simpler, less expensive mission, designated Prometheus 1 (see next section), which could be implemented more rapidly than JIMO. The agency also evaluated the application of the baseline JIMO spacecraft to a variety of follow-on missions. In February 2005, the President’s proposed budget for the 2006 fiscal year deferred additional work on JIMO until after 2017 at the earliest, and Project Prometheus focused its flight-development efforts on a less complex NEP mission designated Prometheus 1 (see next section). PROMETHEUS 1 The ambitious scope of JIMO, its advanced technology, and its considerable size make it an expensive spacecraft.12 In the second half of 2004, NASA’s Prometheus program office at JPL began the so-called Analysis
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Priorities in Space Science Enabled by Nuclear Power and Propulsion of Alternatives (AoA) process to identify a less ambitious, nearer-term mission that could demonstrate the capabilities of NEP, high-power instrumentation, and high-capacity communications systems. By fall 2004, NASA personnel were actively seeking an NEP technology demonstration mission that could meet several key criteria, including the following: Launch using a single, existing expendable launch vehicle (or a derivative thereof) by approximately 2014; A 3-year mission duration; Operation in a more benign environment than JIMO; and Significantly less complexity than JIMO, but still using the JIMO-class reactor (possibly at a lower mission power level). The goal was to demonstrate technology as soon as possible that could be used later on a JIMO-class mission. Concepts suggested for this so-called Prometheus 1 mission were as follows: A technology-demonstration mission would carry a minimum of science instruments and would be intended primarily to demonstrate successful operation of the fission power system in deep space. A lunar geophysical orbiter would be placed in a high-inclination, low-altitude orbit about the Moon. Its instrument complement would include a topographic mapping radar, a scanning lidar system, and a high-resolution imager. A next-generation Mars telecommunications station would test high-power communications systems in Mars orbit, to support very-high-data-volume Mars missions. A near-Earth-object mission would sequentially rendezvous with and study several near-Earth objects. A Venus orbiter would be designed to produce very-high-resolution radar maps of the planet’s surface. There would be an unspecified astrophysics mission. In addition, JPL scientists began to reconsider the possibility of launching an RPS mission to Europa in 2012. This mission would be somewhat akin to the SSE decadal survey’s Europa Geophysical Explorer or to the somewhat simpler Europa Orbiter that NASA was considering in the late 1990s. A detailed study of such a mission would not only be relevant to achieving the primary science goal of any Europa mission—i.e., conclusively demonstrating whether or not the satellite has a liquid-water layer beneath its icy surface—but it would also serve as a technological baseline against which other programmatic options could be compared. It would also be a means of focusing the development of technology for, and subsequent demonstration of, the radiation-hardened avionics and sensors that would be required for an eventual JIMO-class mission. By early 2005, NASA had significantly expanded the AoA process to define possible mission options for Prometheus 1 and other nuclear-electric vehicle concepts. Presentations to the NASA Nuclear Systems Strategic Roadmap Team in April 2005 made passing reference to AoA concepts with names such as Heliostorm and Solar Polar, but without providing any specific details of what any of these concepts entailed. Without having access to details of any of the missions considered in the AoA process, it is not possible for the committee to say if any of these missions are uniquely or even plausibly enabled by nuclear power and propulsion technologies. At about the same time the AoA process was under way, NASA’s planning for the Prometheus 1 mission was being subjected to close congressional scrutiny.13 NUCLEAR POWER AND PROPULSION IN THE DECADAL SURVEYS AND THE INITIATION OF THIS STUDY Although the NRC decadal strategies exist to guide NASA on scientific priorities in space science,14-16 all of them were drafted prior to the start of Project Prometheus. This does not mean, however, that the decadal surveys were silent on the need for and uses of nuclear power and propulsion systems. In addition to calling for reopening the RPS production lines, the solar and space physics (SSP) survey also recommended that NASA assign a high priority to the development of advanced propulsion systems, including NEP.17 The solar system exploration (SSE)
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Priorities in Space Science Enabled by Nuclear Power and Propulsion survey echoed the need for a ready supply of RPSs and also included NEP systems in its list of recommended technology developments.18 Moreover, the latter report pointed to several missions “which are enabled or enhanced by NEP” and which naturally follow on from missions recommended as priorities for the decade 2003–2013.19 These missions include a Neptune orbiter, the Titan Explorer, and the Saturn Ring Observer. Against this backdrop, NASA asked the NRC in late 2003 to identify high-priority space science objectives that could be either uniquely enabled or greatly enhanced by development of advanced spacecraft nuclear power and propulsion systems of the type being developed under the aegis of Project Prometheus.20 In response to NASA’s request, the Space Studies Board (SSB) and the Aeronautics and Space Engineering Board (ASEB) jointly organized a study that was formally initiated in the spring of 2004. The study committee, divided into a steering group and three science panels, focused on the task of identifying space science objectives and possible missions that could be enabled in the time frame beyond 2015 by the development of advanced spacecraft nuclear power and propulsion systems. It was not the task of the committee to reprioritize the decadal surveys, to set priorities for the period beyond the time horizons of the respective surveys, or to draft a formal review of Project Prometheus. The committee was, however, specifically charged to identify high-priority space science objectives. As such, it is important to be specific about the criteria used to select the priorities. In the absence of specific instructions to do otherwise, the committee chose criteria broadly consistent with those used in the three most recent space science decadal surveys. In other words, the priorities are determined by consideration of intrinsic scientific merit and a combination of more practical issues, including technical readiness, programmatic balance, availability of necessary infrastructure, and budgetary impact.21 In practice, this meant that the committee saw its primary task as identifying a series of promising mission concepts to help define where the availability of space nuclear systems could have a major scientific impact. The practical aspects of the prioritization criteria imply that consideration of scientific issues alone is insufficient to be fully responsive to the charge from NASA. Thus, the committee saw its secondary task as including some minimal discussion of technological, programmatic, societal, and budgetary caveats that might impact potential space science applications of nuclear power and propulsion. It is the committee’s hope that, these caveats notwithstanding, the conceptual missions will be studied by NASA and the wider scientific community and, if found to have sufficient merit and potential, will then be prioritized in the context of future decadal survey activities. ORGANIZATION AND APPROACH OF THIS REPORT Chapter 2 is based on material drafted, in part, by a subset of members of the committee’s Steering Group and is designed to fill three roles: (1) to be the focus of the discussion on the applications of nuclear power and propulsion to human exploration activities; (2) to discuss the performance characteristics of the NEP system on which Project Prometheus is currently focusing its attention; and (3) to preview the specific technical issues to be discussed in Phase 2 of this study. The bulk of the remainder of the report (Chapters 3 through 8) was drafted based on material supplied by the committee’s three science panels. Chapters 3, 5, and 7 summarize the existing science and mission priorities from the decadal surveys conducted by the solar and space physics, solar system exploration, and astronomy and astrophysics communities, respectively. These chapters also discuss important scientific developments since the surveys were issued and provide important background information on how these different scientific communities address their priority goals. Chapters 4, 6, and 8 discuss possible areas where nuclear power and propulsion systems may have an impact in solar and space physics, solar system exploration, and astronomy and astrophysics, respectively. These chapters also, where appropriate, outline promising mission concepts and briefly discuss important community-specific technology issues. The final chapter was drafted by the Steering Group and outlines the report’s conclusions and recommendations. Basic Assumptions The Steering Group and its panels worked under several assumptions. These can be summarized as follows:
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Priorities in Space Science Enabled by Nuclear Power and Propulsion The starting point for the scientific deliberations in this study are the goals, priorities, and recommendations in the NRC’s decadal surveys for solar system exploration (SSE), solar and space physics (SSP), and astronomy and astrophysics (AAp). NASA’s exploration initiative—the Vision for Space Exploration—has caused significant changes in the structure of NASA, and in some cases, scientific priorities and strategies of the sub-disciplines have been reexamined.22-24 Some observers might argue that the use of the science priorities in the decadal surveys is nothing more than maintaining the status quo or protecting entrenched interests. Nothing could be further from the case. The three decadal surveys represent the community consensus that emerged from extensive consultations with, and deliberations among, hundreds of individuals in the respective communities. The committee believes strongly that this consensus is compelling. Thus, it is an exercise in due diligence to require that the science objectives and high-priority missions within those surveys retain their priority unless new scientific priorities have emerged with community-wide consensus in the time since the decadal surveys were completed. Important issues relating to the launch from Earth, or assembly in low Earth orbit, of massive JIMO-class spacecraft are assumed to be solvable, or will be addressed by another and more appropriately constituted committee. Project Prometheus is a moving target. When the committee held its first meetings in spring of 2004, JIMO, for example, was a priority activity and NASA was actively studying follow-on missions. Six months later, however, when the committee held its final meeting, JIMO was being deferred in favor of an undefined, simpler, and less expensive NEP mission, Prometheus 1. Initial Concerns Although the committee was specifically charged to identify high-priority space science objectives that might be enabled or enhanced by nuclear technologies, it could not, as indicated above, do this in a vacuum. From its very first briefings on Project Prometheus, the committee was highly concerned about a variety of practical issues, including the possible impact of expensive, long-lived, data-rich missions—as exemplified by the now-deferred JIMO—both on the critical infrastructure supporting NASA’s space science activities and, indeed, on the rest of NASA. In other words, missions deriving from Project Prometheus, whether Prometheus 1, JIMO, or possible follow-ons, will coexist with other NASA programs and activities. The question is whether this coexistence, complicated by possible launch-approval and public-acceptance issues, is feasible within any plausible budgetary and implementation scenario. These concerns, in no specific order, include the following: Are issues relating to the safety of and the launch-approval process for radioactive materials, whether packaged as RPSs or fission reactors, sufficiently well understood so that they do not present insuperable obstacles to the expanded use of such devices? Is a new class of super-flagship, NEP-enabled missions programmatically conceivable while preserving a balance of mission classes and targets? Can the existing Deep Space Network handle the data-return rates from missions such as JIMO? Can existing data archives, such as the Planetary Data System, accommodate the large volumes of data returned by NEP-class missions and ensure that the wider scientific community has ready access to these? To what extent will data analysis programs and other related activities have to be augmented to meet the needs of JIMO-type missions and to ensure that the research base for follow-on activities has a sound foundation? Additional concerns of the committee relating to the capabilities of nuclear power and propulsion systems included the following: Are the decadal surveys an appropriate starting point for deliberations on the scientific opportunities opened up by nuclear technologies, given that these reports were drafted on the basis of particular sets of assumptions concerning the likely power and propulsion technologies available to support priority missions? In other
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Priorities in Space Science Enabled by Nuclear Power and Propulsion words, are the priorities in the decadal surveys still valid if their technological assumptions are significantly perturbed by the introduction of radically new capabilities such as nuclear power and propulsion? Has there been sufficient foundational work in the form of mission and trade-off studies so that a quantitatively informed assessment can be made of what scientific opportunities are or are not enhanced or enabled by nuclear technologies? These issues are considered in subsequent chapters of the report. REFERENCES 1. See, for example, National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, pp. 202–203. 2. W.J. Broad, “U.S. Has Plans to Again Make Own Plutonium,” The New York Times, June 27, 2005, pp. A1 and A13. 3. See, for example, Office of Nuclear Energy, Science, and Technology, U.S. Department of Energy, Draft Environmental Impact Statement for the Proposed Consolidation of Nuclear Operations Related to Production of Radioisotope Power Systems, DOE/EIS-0373D, U.S. Department of Energy, Washington, D.C., June, 2005. Available at <www.consolidationeis.doe.gov/>, accessed February 2, 2006. 4. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, p. 203. 5. G.R. Schmidt, R.L. Wiley, R.L. Richardson, and R.R. Furlong, “NASA’s Program for Radioisotope Power System Research and Development,” Space Technology and Applications International Forum—STAIF-2005, M.S. El-Genk, ed., American Institute of Physics, Melville, N.Y., 2005 6. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, p. 196. 7. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, p. 204. 8. R. Greeley, T.V. Johnson, et al., Report of the NASA Science Definition Team for the Jupiter Icy Moons Orbiter (JIMO), National Aeronautics and Space Administration, Washington, D.C., 2004. 9. See, for example, National Research Council, A Science Strategy for the Exploration of Europa, National Academy Press, Washington, D.C., 1999, pp. 11–12. 10. R. Greeley, T.V. Johnson, et al., Report of the NASA Science Definition Team for the Jupiter Icy Moons Orbiter (JIMO), National Aeronautics and Space Administration, Washington, D.C., 2004, p. 49. 11. M.J. Hart, Jupiter Icy Moons Orbiter High Capability Instrument Feasibility Study, Aerospace Report No. TOR-2004(2172)-3231, The Aerospace Corporation, El Segundo, Calif., 2004. 12. See, for example, Congressional Budget Office, A Budgetary Analysis of NASA’s New Vision for Space Exploration, Congress of the United States, Washington, D.C., 2004, p. 22. 13. United States Government Accountability Office, NASA’s Space Vision: Business Case for Prometheus 1 Needed to Ensure Requirements Match Available Resources, GAO-05-242, Congress of the United States, Washington, D.C., 2005. 14. National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001. 15. National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, The National Academies Press, Washington, D.C., 2003. 16. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003. 17. National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, The National Academies Press, Washington, D.C., 2003, pp. 10–11 and 85. 18. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, pp. 202–205. 19. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, p. 205. 20. Letter from Edward J. Weiler, Associate Administrator of Space Science, to Lennard Fisk, Chair of the Space Studies Board, October 14, 2003. 21. See, for example, National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, pp. 176–177 and 189. 22. National Research Council, Solar and Space Physics and Its Role in Space Exploration, The National Academies Press, Washington, D.C., 2004. 23. National Research Council, “Review of Progress in Astronomy and Astrophysics Toward the Decadal Vision: Letter Report,” The National Academies Press, Washington, D.C., 2005. 24. National Research Council, Science in NASA’s Vision for Space Exploration, The National Academies Press, Washington, D.C., 2005.
Representative terms from entire chapter: