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Priorities in Space Science Enabled by Nuclear Power and Propulsion Executive Summary In 2002, NASA initiated a program to explore the use of nuclear power and propulsion systems for both human and robotic activities. By 2004, this activity, Project Prometheus, had acquired five tasks: Developing a new generation of radioisotope power systems (RPSs); Conducting advanced studies of nuclear power and propulsion systems; Initiating development of the first Prometheus flight program, the Jupiter Icy Moons Orbiter, and its nuclear-electric propulsion (NEP) system; Studying nuclear power systems as a means 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 Exploring the use of much larger nuclear power systems to support thermal or NEP systems for human exploration activities beyond the Earth-Moon system. ORIGIN AND ORGANIZATION OF THE STUDY Against this backdrop, NASA asked the National Research Council (NRC) to undertake a two-task study. The first task was to identify high-priority space science objectives that could be uniquely enabled or greatly enhanced by the development of advanced spacecraft nuclear power and propulsion systems. The second was to make recommendations for an advanced technology development program for future space science missions employing nuclear power and propulsion capabilities. In response to NASA’s request, the Committee on Priorities for Space Science Enabled by Nuclear Power and Propulsion—consisting of a steering group and three science panels—was established to address the charge. This Phase I report addresses the first task only. As a starting point for its scientific deliberations, the committee used the goals, priorities, and recommendations from the NRC’s decadal surveys for solar system exploration (SSE),1 solar and space physics (SSP),2 and astronomy and astrophysics (AAp).3 Although these reports predate the initiation of Project Prometheus, the community consensus they embody makes them compelling guides to the identification of high-priority science activities in their respective disciplines. Although none of the missions identified in these decadal survey reports as priorities for implementation in the coming decade explicitly require NEP, these reports are not entirely silent
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Priorities in Space Science Enabled by Nuclear Power and Propulsion on the need for and uses of nuclear power and propulsion systems. In addition to calling for the reopening of RPS production lines, both the SSE and SSP decadal survey reports recommended that NASA assign a high priority to the development of advanced propulsion systems, including NEP.4,5 The SSE decadal survey explicitly included NEP in its list of recommended technology developments, and it pointed to several missions that “are enabled or enhanced by NEP” and that naturally follow on from missions recommended as priorities for the decade 2003–2013.6 The 2001 AAp decadal survey makes no recommendations concerning the use of nuclear power and propulsion systems. It was specifically 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. But because the committee was charged to identify high-priority objectives, it used selection criteria that are broadly consistent with those used by the three most recent decadal surveys. That is, priorities are determined by consideration of intrinsic scientific merit and a combination of other issues, including technical readiness, programmatic balance, availability of necessary infrastructure, and budgetary impact.7 In practice, this approach implied that the committee’s primary task was the identification of a series of promising mission concepts to help define where the availability of space nuclear systems could have a major scientific impact. It also implied that a necessary secondary task was the identification of a variety of technological, programmatic, societal, and budgetary caveats that might impact the potential space science applications of nuclear power and propulsion systems. It is the committee’s hope that, these caveats aside, the conceptual missions discussed in this Phase I report will be studied by NASA and the wider scientific community and, if found to have sufficient merit and potential, will then be considered for prioritization in future decadal surveys. CONTRIBUTIONS OF NUCLEAR POWER AND PROPULSION TO THE SPACE SCIENCES Solar and Space Physics and Solar System Exploration The material presented in Chapters 4 and 6 clearly demonstrates that the availability of nuclear power and propulsion technologies has the potential to enable a rich variety of solar and space physics and solar system exploration missions. A particularly exciting prospect for the solar system exploration community is the likely availability of a new generation of RPSs that will enable missions ranging from long-lived surface landers to deep atmospheric probes. Similarly, the solar and space physics community is intrigued by the possible uses of nuclear power and propulsion systems to enable complex, multidisciplinary exploration activities in the outer solar system and the local interstellar medium. Of the various nuclear technologies considered, RPSs directly enhance or are enabling for missions identified as priorities for the coming decade in the SSP and SSE survey reports. RPSs can also enhance and enable missions mentioned in the respective survey reports that are likely to be candidates for consideration as priorities in subsequent decades. These and additional mission possibilities exist, but because of the lack of detailed studies, it is not possible at this time to say whether or not these missions are uniquely enabled or greatly enhanced by nuclear power and propulsion systems. Rather, the discussions in Chapters 4 and 6 center on the identification of a number of promising mission concepts that could plausibly be enhanced or enabled by RPS technologies and/or NEP. The mission concepts selected by the committee as particularly promising include the following (in heliocentric order): Solar Coronal Cluster—an NEP-class mission designed to deploy multiple RPS-powered subsatellites in the inner heliosphere to study the origins of space weather (see Box 4.2); Long-Lived Venus Lander—an RPS-powered lander designed to conduct seismic and other observations on the surface of Venus for at least 1 month (see Box 6.1); Long-Lived Mars Network—a network of RPS-powered probes designed to conduct seismic, meteorological, and other observations on the surface of Mars for an extended period (see Box 6.2); Jupiter Magnetosphere Multiprobe Mission—an NEP-class mission designed to deploy multiple RPS-powered subsatellites to study the global dynamics of the jovian magnetosphere (see Box 4.4);
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Priorities in Space Science Enabled by Nuclear Power and Propulsion Cryogenic Comet Sample-Return Mission—an RPS-powered spacecraft designed to perform extensive remote-sensing and in situ observations of a cometary nucleus prior to collecting and returning a sample, maintained at cryogenic temperatures, to Earth (see Box 6.3); Titan Express/Interstellar Pioneer—an NEP-class mission designed to deploy an RPS-powered aerobot in Titan’s atmosphere and then continue on to perform a secondary mission in the outer heliosphere (see Box 6.4); Neptune-Triton System Explorer—an NEP-class mission designed to perform a comprehensive study of Neptune and Triton by deploying atmospheric probes and landers (see Box 6.5); Solar System Disk Explorer—an NEP-class mission equipped with RPS-powered subsatellites designed to study the collisional evolution of the solar system by conducting complementary observations of dust and Kuiper Belt objects (see Box 4.3); and Interstellar Observatory—an NEP-class mission equipped with multiple RPS-powered subsatellites designed to conduct a comprehensive multidisciplinary study of the particles, fields, and dust environments encountered as it traverses the heliosphere and penetrates into interstellar space (see Box 4.1). All these mission concepts and the others mentioned in Chapters 4 and 6 (and Appendix C) have to be studied in much more detail before their feasibility and priority can be determined. Exploiting the capabilities of nuclear power and propulsion systems will require ancillary technical developments in a variety of areas, including communications, radiation-hardened electronics, radiation-tolerant detectors, contamination mitigation procedures, and multispacecraft systems. Astronomy and Astrophysics The prospective contributions of nuclear technologies in astronomy, astrophysics, and fundamental physics are, as shown in Chapter 8, not very promising. Nuclear power and propulsion systems are not enhancing or enabling for any of the current high-priority goals of astronomy and astrophysics as defined in the decadal surveys or subsequent priority studies.8 Most envisaged missions work as well at 1 AU from the Sun as anywhere and have power requirements of <10 kWe. Thus, their power and propulsion requirements can be met most readily with photovoltaic arrays and chemical (or, if needed, solar-electric) propulsion systems, respectively. The one major exception where nuclear technologies appear to have some promise is in the area of infrared imaging. For this application, a case can be made between deploying a relatively large telescope in the high-zodiacal-light background at 1 AU versus deploying a smaller telescope in the lower-zodiacal-light background at ≥3 AU. Nuclear power in the form of RPSs or a small reactor might be attractive for such a mission, but there are serious issues—e.g., the effect of high-energy particles, gamma rays, and waste heat from nuclear reactors on sensitive astronomical detectors—that would have to be addressed. Other possibilities considered—e.g., the use of nuclear power to support astronomical facilities on the Moon—do not appear to offer clear advantages over other means of obtaining the same scientific observations. The lunar surface as an observatory site, for example, does not offer any enabling advantages over free space and has the disadvantages of gravity and, potentially, dust. Free space offers the same vacuum as the lunar surface does, and although the lunar polar craters are naturally very cold, passive cooling strategies—e.g., deployable sunshades—can achieve similarly low temperatures in free space. In addition, a nuclear-powered observatory on the lunar farside is not a uniquely enabling solution to the problem of terrestrial radio interference, because a free-flying fleet of solar-powered dipole receivers is likely to be easier to implement than is a similar array of dipoles deployed on the Moon’s farside. There do appear to be some more exotic astronomy and astrophysics mission possibilities that might be enhanced or enabled by the use of nuclear technologies—e.g., the Binary-Star Gravitational Telescope and the Solar Gravitational Telescope—but they do not uniquely address high-priority goals of the astronomy and astrophysics community. Similarly, there are a variety of interesting missions—e.g., the Gamma-Ray Burst Locator, the Infrared Background/Zodiacal Light Mapper, and the Microlensing Parallax Mapper—that also do not address major, high-priority questions in astronomy and astrophysics but that might be considered as cost-effective add-ons to missions to the outer solar system and interstellar space. Finally, missions such as the Interstellar Observa-
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Priorities in Space Science Enabled by Nuclear Power and Propulsion tory or the Titan Express/Interstellar Pioneer may allow direct measurement of the properties of the local interstellar medium beyond the heliosphere. Although such measurements are of astronomical interest, they are not high-priority goals enunciated in either the latest AAp decadal survey or more recent priority studies. Of particular concern to the astronomical community is the operation of nuclear reactors in Earth’s magnetosphere. This practice is well documented as causing significant interference to balloon-borne and orbiting gamma-ray observatories. The effect that operating space reactors might have on other scientific activities should be carefully studied. PRIMARY FINDINGS AND RECOMMENDATIONS If nuclear propulsion is developed and demonstrated, then it can enable radically new missions capable of conducting activities of a scope never before contemplated by the space science community. Thus NASA and its partners in other federal agencies have taken some courageous and undoubtedly important first steps in what will be a long-term program to harness nuclear power and propulsion for the benefit of space exploration. Despite the promise of these technologies, however, it is essential to be clear about their positive and negative aspects. Nuclear propulsion systems will give researchers access to previously inaccessible objects and destinations and enable them to conduct comprehensive studies of a type and with a flexibility not previously contemplated in the history of space exploration. Yet spacecraft nuclear propulsion is in its infancy and will require a great deal of technological development. As described in Chapter 2, NASA’s parametric studies of candidate NEP missions reveal a significant gap in performance (in terms of, for example, transit time) between what appears to be currently feasible and what is desirable from a scientific perspective. The committee is concerned that NASA’s current nuclear propulsion research and development activities may be too narrowly focused on a single technology—NEP—and believes that NASA’s efforts might benefit from a broader consideration of other technological approaches. Spacecraft using nuclear propulsion systems, regardless of the exact technologies employed, will be very large, very heavy, very complex, and, almost certainly, very expensive. The development and deployment of such technologies may proscribe the diversity of space science missions. But it is difficult to imagine that space science goals for the period beyond 2015 will still be addressed with the power and propulsion technologies of the Mariners, Pioneers, and Voyagers. At the same time, though, it is equally difficult to imagine how it will be possible to transition smoothly from an era of Cassini, Mars Exploration Rovers, New Horizons, Discovery, and Explorers to a time when the mix of activities will be just as diverse but will also include super-flagship-class NEP missions. Finding: Nuclear power and propulsion technologies appear, in general, to have great promise and may in some sense be essential for addressing important space science goals in future decades. This is particularly true for the fields of solar and space physics and solar system exploration, and especially so with respect to near- to mid-term applications of radioisotope power systems. Nevertheless, the committee has significant reservations about the scientific utility of some of NASA’s current nuclear research and development activities, about NASA’s current technological approach to the implementation of nuclear propulsion, and about the agency’s ability to integrate a new class of large and potentially very expensive nuclear missions into its diverse and healthy mix of current missions. This finding is elaborated on below, and specific recommendations are offered. Radioisotope Power Systems Finding: Radioisotope power system technologies will enable varied and rich space science activities. RPSs have a long history of enabling science investigations. For maximum scientific utility, RPSs must be able to operate in a number of modes and settings (e.g., on orbiters, landers, and rovers) and environments (e.g., in extremes of hot and cold, in the vacuum of deep space, or in planetary atmospheres). RPSs may be useful on a
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Priorities in Space Science Enabled by Nuclear Power and Propulsion variety of different classes of missions, and their use on small, principal-investigator-led missions, such as Mars Scout and Discovery, warrants serious consideration. NASA and its partners in other federal agencies (e.g., the U.S. Department of Energy) are to be commended for supporting the future use of RPS technologies. Of particular interest to the space science community in the near term is the ongoing development of two new types of RPS—the so-called multi-mission radioisotope thermoelectric generator (MMRTG) and the Stirling radioisotope generator (SRG). The committee notes that both of these new RPSs are less efficient in terms of their specific mass (i.e., kg/kWe) than the devices they are replacing. Further, the SRG has moving parts that may limit its lifetime as well as cause vibrations and electromagnetic interference. The committee is concerned that interruptions in the production, supply, or packaging of the plutonium isotope (238Pu) fuel for RPSs could have an impact on future mission plans. A steady and reliable source of 238Pu is required if the scientific potential of missions enhanced and enabled by RPS technologies is to be realized. Recommendation: NASA should expand the development and application of radioisotope power system (RPS) technologies. Advances in these technologies should be pursued for such purposes as reducing specific mass, minimizing electromagnetic and other forms of contamination, and developing systems that can work in a variety of environments, from the surfaces of diverse planetary bodies to orbiters to the outer solar system. Attention should be given to the development of new types of RPSs that have smaller and, possibly, larger electrical power outputs than those currently in use or under development. Nuclear Propulsion Systems Finding: Nuclear propulsion technologies will likely be used initially for moving relatively large scientific payloads (~1,000s kg) to destinations in the outer solar system and beyond and extremely large payloads (~10,000s kg) in support of human exploration activities in the inner solar system. But it is necessary to investigate nuclear propulsion technologies more thoroughly to determine if they can provide fast, affordable access to the outer solar system and beyond and can move large payloads in the inner solar system cost-effectively and efficiently. NASA’s parametric studies of the potential applications of the NEP system being developed by Project Prometheus indicate that numerous desirable missions—e.g., a Neptune orbiter and an interstellar probe—will require a transit time of more than 10 years. Transit times of a decade or more create problems for sustaining continuous operation of systems, maintaining public support, and ensuring systems’ reliability. Long transit times also mean that a scientific payload may be obsolete by the time it reaches its destination. The committee was not convinced that adequate work has been done to demonstrate that a viable NEP system with wide scientific applicability can be developed. Alternative technologies, such as nuclear-thermal propulsion (NTP) and bimodal systems, may provide a more cost-effective, faster means of transport to the outer solar system and beyond. Determining the benefits of nuclear propulsion requires level-playing-field trade-off studies that compare such metrics as the cost, initial mass in low Earth orbit, launch-vehicle requirements, and transit time of various propulsion options. Missions recommended for future study are described in the text boxes in Chapters 4 and 6. Other promising robotic mission candidates (e.g., those resulting from NASA’s so-called Vision Missions competition), together with human missions to the Moon and Mars, should also be studied. Assessments of trade-offs among chemical propulsion, solar-electric propulsion, solar sail, NEP, NTP, and bimodal systems should be completed for missions with requirements for a wide range of velocity changes. Trade-off studies should also consider the impact of system reliability when determining suitable space reactor system designs and operational profiles, especially for those systems designed to operate continuously without maintenance or repair for extended periods of time. For example, the current NEP concepts being considered for missions to the outer solar system are required to function without human intervention for durations between 10 and 20 years. However, no high-power-density reactor has ever been operated on Earth, without maintenance shutdowns, for any period longer than one order of magnitude or more below such a duration.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion Recommendation: NASA should commission detailed, comprehensive studies—supported by external independent reviews and the broad participation of the space science and space technology communities—to examine the feasibility of developing space nuclear propulsion systems with reduced transit times and costs, in order to determine which nuclear propulsion technologies should be pursued at this time, and to ensure that investments in advanced propulsion technologies yield the greatest benefit for the NASA community. SECONDARY FINDINGS AND RECOMMENDATIONS The Decadal Surveys and Program Balance Finding: Program balance is critical to the long-term health of the space science enterprise. An important aspect of a balanced program is a flight program encompassing a range of flagship missions combined with moderate and small missions. The most recent decadal surveys have placed high importance on overall program balance and have reiterated the need for a mix of more frequent, principal-investigator-led small- and medium-class missions combined with less frequent, more costly, larger missions. This overall program balance is considered practical, given the size of NASA budgets, and is necessary for nurturing a healthy scientific and engineering community; it promotes the achievement of progress and discovery on a broad front. The development of nuclear propulsion systems is seen as offering many possible advantages for future science missions. However, the cost associated with their development may have a dramatic impact on near-term mission capability. If NASA’s science program is required to cover the development of nuclear power and propulsion systems, the result will be a substantial decline in the diversity and scope of space science activities. Recovery from such a decline will not occur quickly. Recommendation: The cost of developing advanced power and propulsion technologies, and of implementing missions employing such technologies, must not be allowed to compromise the diversity of the space science missions recommended by the decadal surveys, because these missions address the most important scientific questions in solar and space physics, solar system exploration, and astronomy and astrophysics and are thus essential to maintaining the long-term health and vitality of the entire space science enterprise. Given the level of resources required to implement NEP-class missions, these super-flagship endeavors will have to be extraordinarily capable of addressing a broad-based, cross-cutting range of truly interdisciplinary scientific activities, if such missions are to provide a science return commensurate with the investment made in them. Public Acceptance and Launch Approval Finding: Previous launches of nuclear-powered spacecraft have raised concerns among members of the public. If such concerns were to intensify, it could seriously affect any planned use of nuclear technology on space science missions. The perceived risk of nuclear power plants and the associated hazards posed by the disposal of long-lived radioactive waste have led to a significant fraction of the U.S. public resisting the development of new nuclear systems. The experience with space nuclear systems has been somewhat different. Opposition to the launch of RPS-powered spacecraft has been visible and vocal, but not necessarily widespread, and not ultimately successful in preventing the launch of these systems. Nevertheless, the potential for public opposition to nuclear power development exists and must be considered by NASA in planning the development of nuclear space power and propulsion systems. Denial of risk and neglect of possible impacts have led to the demise of otherwise potentially beneficial nuclear technologies, such as nuclear power generation for civilian uses. Nuclear space reactor technology has seen very limited development and undoubtedly poses a number of reliability and safety questions that
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Priorities in Space Science Enabled by Nuclear Power and Propulsion have to be fully understood and addressed by the technical and scientific community, and made comprehensible to the public at large. Recommendation: It is essential that NASA communicate clearly and openly with the public regarding the potential benefits of and challenges posed by the use of space nuclear power and propulsion systems. The agency and its partners must avoid the denial of risks and neglect of impacts, as well as the perception thereof. NASA should adopt a very proactive stance and role in the management and integration of current (e.g., National Environmental Policy Act and interagency launch-approval procedures) and future foreseeable processes of assessment and decision making that will undoubtedly influence public opinion about the general environmental and safety risks associated with the use of nuclear power and propulsion systems in space. Human Exploration Activities Finding: Fission reactors are likely to be useful in providing long-term power for human activities on the surface of the Moon and Mars. Surface power systems are, in practice, likely to be very different from shipboard reactors and will require separate development programs. Nuclear systems could provide the large amounts of electricity necessary to power astronauts’ life-support systems and to support surface science and exploration activities. Surface and space power systems are likely to be different, and it is not clear whether the NEP-class reactor currently under study by Project Prometheus is adequate for either application. NTP systems may be better able to provide the thrust needed to send astronauts to Mars. Nuclear reactor power and propulsion systems for human exploration missions, however, must be qualified to a level of reliability much higher than that for power and propulsion systems for robotic missions, and will have to be validated for reliable operation at full power and mission lifetime. Recommendation: NASA should reexamine the technology goals of Project Prometheus to assess the benefits (in terms of cost, schedule, and performance) of using a technology that can support the propulsion requirements of both human and robotic missions. Heavy-Lift Launch Vehicles Finding: NEP-class spacecraft are inherently massive and, as such, will require either in-space assembly following multiple launches of components on the largest launch vehicles currently available, or a single launch on a new heavy-lift booster. A heavy-lift launch capability would potentially enable new classes of space science missions. Recommendation: Studies of trade-offs comparing propulsion options should take into account the complexities and cost of launching NEP-class missions. Technical, Programmatic, and Infrastructure Issues Finding: Attention has to be paid to a variety of technical and programmatic issues that can affect the scientific utilization of NEP-class missions. These issues include the fraction of a spacecraft’s launch mass dedicated to the science payload; high-bandwidth communications; onboard data processing; the capacity of the Deep Space Network, Planetary Data System, and research and analysis programs to handle increasing volumes of data; the availability of radiation-hardened components and radiation-tolerant detectors; and mitigation of contamination. Failure to make allowance for some or all of these factors can lead to hidden costs that will impact the implementation of NEP-class missions.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion Considerations relating to these technical and programmatic issues include the following: Fraction of launch mass for science—On typical planetary missions flown over the last 30 years, the ratio of science payload mass to total mass has varied between 0.09 and 0.17. The science payload mass ratios for Cassini and JIMO are ≥0.1 and ≤0.06, respectively. The much larger masses necessitated by large NEP systems should offer the opportunity for much larger science payloads. The Deep Space Network and the Planetary Data System—The ability to accommodate the extremely large volume of data returned by NEP missions will require some combination of advanced onboard data processing, high-bandwidth communications, and improvements in the Deep Space Network. The Planetary Data System and other data repositories will have to be expanded, and data-analysis programs will have to be established and/or augmented to meet the needs of future missions and ensure that the data returned are fully analyzed. Radiation-hardened components and radiation-tolerant detectors—The development of new, more capable, radiation-hardened electronic components, together with new detector materials and detection concepts, will enhance measurement capabilities. Contamination mitigation for instruments—To enhance or enable scientific measurements from spacecraft equipped with nuclear reactors, power and propulsion systems must be “clean” and “stable” in terms of transient magnetic and electric fields, chemical contamination, radiation and charged-particle levels, and vibration. In addition, nuclear reactors should not be operated within Earth’s magnetosphere unless it can be demonstrated that interference to other spacecraft caused by primary and secondary gamma rays, electron bremsstrahlung, and positron-annihilation radiation will not occur. Recommendation: Determination of the cost of NEP-class missions should take into account the cost of necessary associated technologies and programs. Particular emphasis should be placed on studies of the means to maintain or, if possible, increase the fraction of launch mass allotted to science payloads above that typical for current space science missions. REFERENCES 1. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, Space Studies Board, The National Academies Press, Washington, D.C., 2003, pp. 202–205. 2. National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, Space Studies Board, The National Academies Press, Washington, D.C., 2003. 3. National Research Council, Astronomy and Astrophysics in the New Millennium, Board on Physics and Astronomy–Space Studies Board, National Academy Press, Washington, D.C., 2001. 4. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, Space Studies Board, The National Academies Press, Washington, D.C., 2003, pp. 202–205. 5. National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, Space Studies Board, The National Academies Press, Washington, D.C., 2003, pp. 10–11 and 85. 6. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, Space Studies Board, The National Academies Press, Washington, D.C., 2003, p. 205. 7. See, for example, National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, Space Studies Board, The National Academies Press, Washington, D.C., 2003, pp. 176–177 and 189. 8. See, for example, National Research Council, Connecting Quarks with the Cosmos—Eleven Science Questions for the New Century, Board on Physics and Astronomy, The National Academies Press, Washington, D.C., 2003.
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