6
Applications of Nuclear Power and Propulsion in Solar System Exploration: Missions

PRIORITIES ENHANCED OR ENABLED BY NUCLEAR POWER AND PROPULSION

What high-priority solar system exploration objectives could be uniquely enabled or greatly enhanced over the next 25 years by the development of advanced spacecraft nuclear power and propulsion systems? The answer depends on the assumptions. Given the technical uncertainties surrounding space applications of nuclear power—e.g., the United States has flown only one space-reactor experiment, and that was 40 years ago—budgetary vagaries, and the distant time horizon of this study, very broad assumptions are necessary if any progress is to be made in answering this question. The basic assumptions used are as follows:

  • The science priorities as laid out in the solar system exploration (SSE) decadal survey1 have been unchanged by recent discoveries;

  • NASA continues to follow a reasonable, steady, and well-justified course based on sound science strategic planning exercises, e.g., the priorities and recommendations of the SSE decadal survey;

  • The current administration’s new exploration initiative does not have a negative impact on the long-term stability of NASA and its budget lines; and

  • A heavy-lift launch vehicle and/or on-orbit assembly is developed to accommodate intrinsically massive nuclear-electric propulsion (NEP) systems.

Project Prometheus and the capabilities it may enable were not public knowledge at the time the SSE decadal survey was undertaken. Thus, in light of this new potential, the committee identifies here opportunities that merit new studies of feasibility and science return. The committee has neither the expertise nor the resources to undertake trade-off studies of alternative ways of doing these missions; that is, it has not performed a detailed quantitative analysis of the relative merits of alternative power and propulsion technologies, nor has it made a comparison of alternate means of achieving the same scientific goals. The goal has been to indicate where studies are needed, not what the mission priorities should be.

SSE Decadal Survey Missions That Do Not Need Radioisotope Power Systems or Nuclear-Electric Propulsion Technologies

A variety of the missions given high priority in the SSE decadal survey do not have any need for the various



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Priorities in Space Science Enabled by Nuclear Power and Propulsion 6 Applications of Nuclear Power and Propulsion in Solar System Exploration: Missions PRIORITIES ENHANCED OR ENABLED BY NUCLEAR POWER AND PROPULSION What high-priority solar system exploration objectives could be uniquely enabled or greatly enhanced over the next 25 years by the development of advanced spacecraft nuclear power and propulsion systems? The answer depends on the assumptions. Given the technical uncertainties surrounding space applications of nuclear power—e.g., the United States has flown only one space-reactor experiment, and that was 40 years ago—budgetary vagaries, and the distant time horizon of this study, very broad assumptions are necessary if any progress is to be made in answering this question. The basic assumptions used are as follows: The science priorities as laid out in the solar system exploration (SSE) decadal survey1 have been unchanged by recent discoveries; NASA continues to follow a reasonable, steady, and well-justified course based on sound science strategic planning exercises, e.g., the priorities and recommendations of the SSE decadal survey; The current administration’s new exploration initiative does not have a negative impact on the long-term stability of NASA and its budget lines; and A heavy-lift launch vehicle and/or on-orbit assembly is developed to accommodate intrinsically massive nuclear-electric propulsion (NEP) systems. Project Prometheus and the capabilities it may enable were not public knowledge at the time the SSE decadal survey was undertaken. Thus, in light of this new potential, the committee identifies here opportunities that merit new studies of feasibility and science return. The committee has neither the expertise nor the resources to undertake trade-off studies of alternative ways of doing these missions; that is, it has not performed a detailed quantitative analysis of the relative merits of alternative power and propulsion technologies, nor has it made a comparison of alternate means of achieving the same scientific goals. The goal has been to indicate where studies are needed, not what the mission priorities should be. SSE Decadal Survey Missions That Do Not Need Radioisotope Power Systems or Nuclear-Electric Propulsion Technologies A variety of the missions given high priority in the SSE decadal survey do not have any need for the various

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Priorities in Space Science Enabled by Nuclear Power and Propulsion nuclear power and propulsion systems being developed under the aegis of Project Prometheus. These missions include the following: Cassini Extended. The logical extension to the observations of the saturnian system following the end of Cassini’s 4-year prime mission. By definition, an operating spacecraft does not require new power and propulsion technologies. Mars Upper Atmosphere Orbiter. Missions of this type do not, in general, require nuclear power or propulsion for their operation because solar power is adequate and cost-effective inside 2 AU. Kuiper Belt-Pluto Explorer. Currently being implemented as New Horizons, this is the first of the New Frontiers line of medium-class, principal-investigator (PI)-led missions. It is making use of the last remaining radioisotope thermoelectric generator (RTG) available from the Department of Energy (DOE) and therefore cannot directly benefit from Project Prometheus and related technological developments. NASA’s current ground rules for several continuing lines of small, PI-led missions specifically exclude the use of radioisotope power systems (RPSs). These lines, given high priority in the SSE decadal survey, include the following: Mars Scout. These missions are designed to address priorities outside the principal objectives of the Mars Exploration Program and, as such, provide the planetary science community with a means to respond to discoveries and technological advancement. Because these missions are selected via an open competition, the exact scope of each such mission is unknown at this time. If RPSs were allowed, the availability of a range of small to medium-size power sources (i.e., ones providing an electrical output in the range from ~10 mW to ~10 W) would likely enhance capabilities.2 Discovery. Missions in this line are suitable for exploration of Mercury, Venus, near-Earth and main-belt asteroids, comets, and the Moon. Some future missions could potentially benefit from use of RPSs if they were permitted. SSE Decadal Survey Missions Enhanced by RPS Technologies Currently viable missions described in the SSE decadal survey that might be significantly enhanced by RPS technologies include the following: Venus In Situ Explorer. This mission is designed to make compositional and isotopic measurements of the atmosphere (especially the lower atmosphere) and the surface of Venus to characterize the geochemistry, mineralogy, and past tectonic history of Earth’s sister planet. The battery-powered concept in the SSE decadal survey takes samples from the lower atmosphere and surface of Venus and lofts them to a higher and cooler altitude for further analysis. However, the high-temperature and high-pressure environment near the surface of Venus makes such a mission extremely challenging. Refrigeration using an RPS and pressure equalization of the Explorer while keeping it cool would greatly simplify the tasks of both obtaining the samples and analyzing them in situ. A concept for a refrigerated, long-lived Venus lander is described in a subsequent section. South Pole-Aitken Basin Sample Return. This mission is intended to collect rock and soil samples from one or more locations in the largest and deepest impact basin in the solar system. The capability of this cost-capped, New Frontiers mission to select optimal samples will be limited by its reconnaissance capabilities and by the design of the landers.a If this mission is less rigorously cost capped, one or more rovers with drilling capability could be added to carry out long-term exploration of this very large region. For elaboration of such a mission concept see the discussion of the Lunar Polar Rover/Driller in Appendix C. a   Moonrise, an implementation of this mission concept, was the runner-up in NASA’s Phase-A competition for the second New Frontiers launch opportunity.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Mars Long-Lived Lander Network. This network will monitor ground seismic activity and ground-level atmospheric chemistry and dynamics for at least 1 martian year in multiple locations. Although solar power is adequate for missions lasting weeks or months near the equatorial regions, solar-powered network stations at higher latitudes are not viable because of inadequate solar flux. Martian dust and ultraviolet irradiation will also reduce the efficiency of solar cells over time. The multiple stations would benefit immensely from the availability of continuous power from RPSs. The Long-Lived Mars Network, a possible RPS implementation of this mission concept, is described in a subsequent section. Mars Science Laboratory. This mission is currently scheduled for launch in 2009. It is designed to perform in situ studies of a water-modified site to provide ground truth for orbital data and to test hypotheses for the formation and composition of water-modified environments. It may also test and validate sample-handling and other technologies required for a sample-return mission. Long-term operation and long-range roving are clearly enhanced by the availability of RPSs. For information on a more advanced mission of this type, see the discussion of the Mars Advanced Science Laboratory in Appendix C. Mars Sample Return. Collection of samples from Mars’s surface and crust requires significant roving and drilling capabilities, which are enhanced by the use of RPSs. A more advanced concept, the Mars Cryogenic Sample Return mission, is outlined in Appendix C. Jupiter Polar Orbiter with Probes. This New Frontiers concept is representative of a class of mission that will study the inner magnetic field, determine the size of the cores, and provide in situ measurements of water and other volatiles in the deep atmospheres (at depths exceeding 100 bars) of the giant planets.b The use of RPS potentially enables a more capable mission. Comet Surface Sample Return. This mission is designed to collect material from one or more sites from the surface of a comet and transport those samples back to Earth for detailed analysis. Both ices and organic materials are of great interest in order to understand the initial processes that led to the formation of the solar system. An attempt to land on the surface of a comet is best undertaken while the comet is still in the outer solar system and is not outgassing violently. The mission outlined in the SSE decadal survey is enhanced by RPS technologies, but more advanced RPS- and NEP-enabled concepts are possible. For an example of the former, see the description of the Cryogenic Comet Sample Return mission in a subsequent section. SSE Decadal Survey Missions That Require Prometheus RPS Technologies Europa Geophysical Explorer Only one of the priority missions for the period up to 2013 described in the SSE decadal survey absolutely requires the RPS technologies that are being developed as part of Project Prometheus. This mission, the Europa Geophysical Explorer, is the SSE decadal survey’s highest-priority large endeavor. It is designed to confirm the presence of Europa’s interior ocean, characterize the satellite’s ice shell, and understand its geological history. The intensity of the jovian radiation belts precludes the use of photovoltaic arrays and limits the operational lifetime of a spacecraft in orbit about Europa to about 1 month only. The potential availability of Prometheus-derived RPS and NEP technologies was not known at the time the SSE decadal survey was drafted. Because there were only limited nuclear options available at the time (i.e., the spare Cassini RTGs), the survey’s authors focused on keeping the spacecraft’s payload small, and therefore selected only relatively simple sets of science objectives. However, the availability of nuclear power and propulsion technologies enables a mission with a much more comprehensive payload and expanded exploration goals encompassing both Europa and the other icy satellites of Jupiter. For a description of an NEP implementation of a Europa mission, see the details on the Jupiter Icy Moons Orbiter in a subsequent section. b   Juno, a probe-less implementation of this mission concept, was the winner of NASA’s Phase-A competition for the second New Frontiers launch opportunity.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion New Mission Concepts Enhanced or Enabled by RPS and NEP Technologies What missions, besides those mentioned in one form or another in the SSE decadal survey, might be uniquely enabled or greatly enhanced by nuclear power and propulsion technologies? Providing a definitive answer is not possible given the limited scope of this study. Determining whether or not a particular concept is uniquely enabled or just somewhat enabled, as opposed to significantly enhanced or just somewhat enhanced, will require extensive additional study. Rather, the committee has attempted to identify missions that, in its collective experience, can be plausibly enhanced or enabled by the use of nuclear technologies. To promote a broad range of science objectives, to stimulate discussion in the planetary science community, and in particular to further identify the technological challenges in bringing forward nuclear power and propulsion capabilities, the committee has developed more detailed descriptions of a subset of five missions. Boxes 6.1 through 6.5 describe the scientific objectives of these concepts and the means by which they might be addressed in the context of missions utilizing nuclear power and propulsion systems. The committee strongly emphasizes that it does not prioritize these missions above any other missions considered here or elsewhere. The missions selected as examples are organized into three broad categories, based on the principal power and propulsion technologies they will employ and a rough assessment of the scientific or technical issues that might determine if they are ready for initiation in the period 2015 to 2020 or after 2020. These categories are as follows: Missions envisioned as possible in the period 2015 to 2020 that would be enhanced or enabled by RPSs, but would be unlikely to require NEP or other fission-reactor-based propulsion systems. Missions envisioned as possible in the period 2015 to 2020 that would be enhanced or enabled by NEP and/or the substantial power provided by a fission reactor. These missions might also require an auxiliary RPS to accomplish their objectives. Missions envisioned as possible in the period 2015 to 2020 that would use reactor-based power and/or propulsion systems for which there are scientific or technical considerations suggesting deferral of flight until after 2020. RPS-Enhanced or RPS-Enabled Missions Envisioned by 2020 The development of a new RPS capability is extremely important for enabling solar system exploration. With the era of quick reconnaissance over, more detailed examination requires additional power, whether for heaters, refrigeration, or longer life. A variety of mission types are enabled by the RPS technologies being developed by Project Prometheus. Individual missions are listed in each category to illustrate the richness of the science questions that can be addressed. The order listed here does not imply priority. Long-Lived Landers. Solar cells and batteries enable many types of surface-based science operations. However, they are not the solution for experiments that require sustained power for lengthy periods of time ranging from months to years. When the solar flux is too low, the Sun is not visible for long periods of time, or photovoltaic cells deteriorate, experiment life is limited with solar power sources. The monitoring of seismic or atmospheric properties of a planet benefits from the extended availability of power. Although these experiments often do not need large amounts of power, they require that power be available continuously. The advanced RPSs being developed as part of the Prometheus project are a likely solution for such needs. Possible missions include the following: Venus Long-Lived Lander. Researchers are ignorant of the dynamics of Venus’s atmosphere and of the composition, structure, and activity of its surface and interior. The planet’s 740 K surface temperature and 90-bar pressure are significant challenges to surface exploration. Liquid-cooled pressure vessels, such as those used by the former Soviet Union’s Venera probes, can support only several hours of operation. The high power provided by an RPS can, potentially, provide long-term (months) refrigeration for Venus surface craft. More details of such a mission are described in Box 6.1.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion BOX 6.1 Long-Lived Venus Lander Mission Type: RPS-class Objectives: Pioneer new technologies to enable long-lived surface operations on Venus; Provide new insights into why the terrestrial planets have evolved so differently; Conduct seismic observations on the surface of Venus; Retrieve and analyze surface samples at high-priority locations, to address questions about the diversity of crustal geochemistry and mineralogy, and also surface/atmosphere interactions and processes; Analyze the atmosphere during descent (particularly the lower-most 20 km) and on the surface; and Determine the structure of the outer 10 to 20 cm of surface material. Implementation: Lander utilizing an active cooling system capable of surviving on the surface of Venus for a minimum of 1 month; Total landed mass of ~200 kg; Cooled volume of <1 cubic meter; and Power for instruments of between 20 and 60 W. Payload: Seismometer; Multispectral imaging system (0.3–2.5 μm) down to centimeter resolution; Surface chemistry package including x-ray fluorescence and subsurface sampling mechanism; Atmospheric chemistry package capable of determining abundances of the principal oxygen isotopes to 0.1‰); and Meteorology package. Other: Multiple lander packages enabling compositional measurements of several diverse terrains; and Three or more landers operating simultaneously as a seismic lander network to detect seismic activity and assess crustal thickness and internal structure (currently unknown). Some Questions for Additional Study: What is an appropriate data rate for the seismometer and how does the inclusion of this instrument drive other aspects of the mission? Additional long-lived lander concepts. Examples of such missions, described in Appendix C, include the following, in heliocentric order: Mercury Polar In Situ Explorer, Mars Deep Driller, Mars Polar Profiler, Io In Situ Explorer, Europa Astrobiology Lander, Icy Satellite Deep Driller, and Comet Nucleus and KBO Surface laboratories.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Rovers. Robotic laboratories mounted on rovers powered by RPSs can, in principle, explore large areas and operate in severe conditions. Equipped for in situ analysis and aggressive sampling techniques, they can characterize the geochemical and geophysical properties of a variety of solar system bodies. Use of RPSs allows for operation when there is no sunlight and temperatures are cold. Examples of possible missions of this type, described in Appendix C, include the following, in order of heliocentric distance: Venus Mobile Laboratory, Lunar Polar Rover/Driller, Mars Advanced Science Laboratory, and Titan Surface Laboratory. Global Networks. Networks of scientific stations that make coordinated, global measurements are of great interest for addressing a variety of scientific issues. Global networks can detect seismic activity and measure heat flow, providing constraints on internal structure, and, in the case of bodies with atmospheres, can make extensive synoptic measurements of the atmosphere and weather. These measurements by definition require multiyear observations. RPSs enable the longevity and continuous operation of these stations. Examples include the following: Long-Lived Mars and Venus networks. The highest-priority goals are seismological determination of internal structure, including the core, global sampling of a range of surface chemical and material properties, and extensive synoptic measurements of the atmosphere and weather. For Venus, the emphasis should be on understanding the nature of any tectonic activity and the planet’s lack of magnetic field as well as the processes relating to the interaction between the surface and the atmosphere. More details of a Mars network mission are contained in Box 6.2. Additional network concepts. Examples of other possible network missions are described in Appendix C, and include the following, in no particular order: Mercury and Lunar Long-Lived Networks, and Icy Satellite Long-Lived Networks. Sample-Return Missions. Ultimately, rovers are limited by their small size and low power. Bringing samples back to Earth-based laboratories allows for more sophisticated analysis, which can be done only with large, complicated equipment. In addition, curation allows for samples to be analyzed as techniques improve. Returned samples can take the form of rock, dust, gas, and even ices, with the last being the most difficult to return because of the need to keep the samples frozen during transport and reentry. Missions of this type include the following: Cryogenic Comet Sample Return. This mission would collect samples of a well-characterized comet nucleus from two or more selected sites, both from the surface and at a depth of about 1 m. To preserve the full suite of volatile materials, the samples would be actively maintained at a temperature below 150 K during the return to Earth for subsequent analysis. A mission of this complexity requires further technological developments, particularly for drilling and sample collection and for cryogenic preservation and return to Earth. Similarly, consideration will have to be paid to techniques for accomplishing this without bringing an RPS or similar device back into Earth’s atmosphere. See Box 6.3 for a more complete description of this concept. Additional sample-return concepts. Examples of possible missions, described in Appendix C, include the following, in no particular order: Mercury Sample Return, Venus Selected Sample Return, and Mars Cryogenic Sample Return.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion BOX 6.2 Long-Lived Mars Network Mission Type: RPS-class Objectives: Advance understanding of the formation and evolution of planets in general and of Mars in particular; Resolve questions concerning the size and other physical characteristics of the martian core; Determine the seismic properties of the martian mantle; Characterize the crustal structure and thickness; Conduct extensive synoptic measurements of the martian atmosphere and weather at ground level to address issues relating to atmospheric dynamics; Contribute to an understanding of the history and nature of the volatile inventory and distribution on Mars by studying surface/atmosphere interactions; Address issues pertaining to climate history (external forcing) as well as volcanic history and its interaction with climate; and Monitor the abundance and distribution of molecules of possible biological importance, including water and methane. Implementation: Deployment of a global network of small, identical sensor packages on Mars to monitor seismic activity and ground-level atmospheric dynamics and chemistry over a period of at least 1 martian year—the natural period for the atmosphere and also an appropriate time period for characterizing seismic activity; Operation of network stations at night and at some high-latitude and high-altitude sites; Separation of any two stations that should, ideally, not exceed the planetary radius (this implies more than 14 sites); and Individual instrument packages with the following characteristics: mass ~2 kg (total landed mass depends on delivery system); power ~80 mW; data rate ~10–100 kbit/day as a compressed dataset. Payload: Seismometer with large dynamic range and broadband frequency response, which can be coupled effectively to the martain surface; Temperature, pressure, humidity, atmospheric opacity, and ultraviolet sensors; Entry accelerometers; Wind velocity meter; Computer and software to manage these data (this is very important for the seismometer so that the data rate is manageable); and Other possibilities, including monitors of water vapor and methane partial pressures. Some Questions for Additional Study: What are the trade-offs between the number and capabilities of the stations? Are there any conflicting requirements between the meteorological/climatic and seismic experiments? What impact would a methane sensor have on a station’s mass, power, and data transmission rate requirements?

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Aerobots. Global surveys of a planet or satellite can be undertaken by orbiters. However, more detailed, regional surveys and, potentially, in situ studies of multiple dispersed sites are better suited to the capabilities of various types of aerial robots (aerobots), whether they are aircraft, balloons, blimps, or something else.3 Possible aerobot missions, described in Appendix C, include the following, in order of distance from the Sun: Venus Aviator, and Titan Aerobot Explorer. Deep Atmospheric Probes. The giant planets are the best local analog to currently known extrasolar planets. The highest-priority science questions address the internal structure and elemental composition of the gas giants (Jupiter and Saturn) and the denser, more remote ice giants (Uranus and Neptune).4 For all the giant planets, knowing the volatile abundances has high priority, both individually and comparatively (among the giant planets and as compared with the Sun). Experience with Galileo suggests that short-lived, single-probe experiments are not well suited for atmospheres with significant latitudinal and longitudinal diversity. Far more is learned scientifically from comparing three or more dispersed sites. Furthermore, determination of the more cosmogonically interesting atmospheric constituents requires penetration below the weather-driven upper layers and down to the well-mixed region. Cloud bases may be deep within the planet, and so the abundance of the upper atmosphere might reflect the saturation vapor pressure rather than the bulk abundance of H2O and other volatiles such as CH4, NH3, and H2S. Deep probes, combined with microwave remote-sensing observations, are needed to sample the well-mixed deep atmosphere for these compounds. These probes are envisioned to be long lived—possibly probe-aerobot hybrids that can actively control their descent and, if feasible, ascent—both to give more complete vertical profiling and to be able to transmit deep-atmosphere information from a less-challenging depth. Power requirements for shallow probes can likely be met with advanced batteries; however, communication with deep probes (at depths of ~100 bars) will probably require more power. RPS technologies are critical both to communications and to refrigeration for thermal protection. An example of a possible mission is as follows: Neptune Orbiter/Deep Multiprobes. This concept is designed to study the gravitational and magnetic fields of an ice giant planet. In addition, entry probes will obtain in situ measurements of chemical composition to constrain theories of solar system formation. Multiple flybys of Triton and other icy satellites will yield information on the interaction of the satellites with Neptune and its magnetosphere. Although this mission is envisaged as being accomplished by gravity assists and an RPS-powered spacecraft, a more comprehensive, NEP version of this mission is possible; see the description of the Neptune-Triton System Explorer in a subsequent section. NEP-Enhanced or Enabled Missions Envisioned by 2020 Missions requiring the use of NEP systems that may be ready for flight by 2020 include the following: Jupiter Icy Moons Orbiter. This concept combines the mission-enabling characteristics of NEP with the SSE decadal survey recommendation for a Europa Geophysical Explorer5 to create an exciting opportunity to study in unprecedented detail the jovian system and, in particular, the icy moons, Callisto, Ganymede, and Europa. Much more information about this mission is given in Chapter 1. Titan Express/Interstellar Pioneer. This mission is specifically intended to push the envelope of what is and is not possible with NEP. It attempts to marry Titan-exploration goals with space physics priorities for the exploration of the distant outer solar system. The latter goals are addressed by the inclusion of a secondary payload that responds to some but not all of the goals of the much more elaborate Interstellar Observatory mission (see Box 4.1 and Appendix B). The basic concept involves an NEP-powered bus accelerating through the saturnian system and releasing, without first decelerating, a high-priority payload directly into Titan’s atmosphere. The bus then continues on toward interstellar space while conducting space physics observations with a small instrument

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Priorities in Space Science Enabled by Nuclear Power and Propulsion BOX 6.3 Cryogenic Comet Sample Return Mission Type: RPS-class Objectives: Study the molecular, volatile, and refractory composition of cometary nuclei, including the structure of the ices; Compare the composition and structure of the material from the surface and from depth (1 m); Determine the global surface properties of the nucleus of the target comet by remote-sensing techniques, supplemented by in situ studies of one or more selected sites; and Determine the rotation state and heat transfer properties of the nucleus. Implementation: Rendezvous with a comet when it is at least 5 AU from the Sun. Perform an initial reconnaissance of the comet’s nucleus to pick the sample site or sites, with a full trade-off study of site safety versus site interest (for instance, vents might be dangerous to the spacecraft but landing at such a site would provide fresher material). The requisite remote-sensing instrument package (mass of ~200 kg and power of ~100–150 W) would need to return an initial dataset of ~100 Gbits (including full visible imaging coverage at a resolution of 1 m per pixel in 7 channels [≈13 Gbits]; full spectrometric imaging coverage in the infrared at a resolution of 5 m per pixel and 512 channels [≈39 Gbits] and in the ultraviolet at 5 m per pixel in 256 channels [≈19 Gbits], before compression and before error correction and packetization overhead—assuming a 5-km-diameter nucleus) to science team within a period of <30 days to enable the selection of potential landing sites (a Ka-band system with a radio-frequency power of 20 W through a 1.5-m high-gain antenna to the 34-m stations of the Deep Space Network [DSN] could send down about 100 Gbits in about 2 weeks of full-time coverage, or about 40 days of one DSN pass per day, at a distance of 5 AU. This is probably faster than is truly necessary, but it fits comfortably in the RPS-mission envelope). package that takes advantage of the abundant electrical power provided by the spacecraft’s reactor. More details about this mission can be found in Box 6.4. Neptune-Triton System Explorer. It will be challenging, even with the availability of NEP, to mount a comprehensive investigation of the Neptune system, including its complex magnetosphere; its dusty rings; its array of small, icy moons; and, not least, Triton, its large, geologically active moon. The feasibility of this mission hinges on providing an adequate science payload, an acceptable transit time from Earth to Neptune, the desired degree of orbital mobility, and a robust communication capability. A candidate payload and observing scheme for the Neptune-Triton System Explorer could be expanded from that flown on Cassini. Given Neptune’s distance from Earth, data transmission rates will be a significant issue and trade-off studies are needed on the balance between spacecraft transmitter power and Earth-based receiver aperture. Although it may be possible to use aerocapture to place a spacecraft into orbit about Neptune, orbiting Triton requires nuclear propulsion. Flexibility in orbit control would also be provided by NEP. Trade-off studies should involve efforts to reduce transit time and consideration of multiple craft. More details about this concept can be found in Box 6.5. Additional missions. The capabilities of NEP systems suggest several other types of mission. These possibilities are described in Appendix C and include the following: Saturn System Multiple Rendezvous; and Main-Belt, Trojan Asteroid, and Centaur Multiple Rendezvous.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Touch down at a selected site on the surface and obtain two samples: one from the surface and one at a depth of at least 1 m. Repeat sampling at a second selected site, if feasible. Transfer the samples to containers that can be maintained at no warmer than 150 K for transfer back to Earth. Acquire the samples and return them to Earth in such a manner that the ices are not compacted. Fly with the comet and monitor the surface of the nucleus, especially the sampling site or sites. Follow the comet through the onset of activity and back to 1 AU. Return the sample back to Earth for analysis in the laboratory. Ensure that samples contain at least 1,000 cm3 of material each. Payload: Remote-sensing package: Optical camera with resolution of at least 1 m/pixel pair; Near-infrared spectrometer; Ultraviolet spectrometer; and Other possibilities, including neutral and ion gas mass spectrometer and dust impact analyzer, and a near-infrared mapping spectrometer. In situ package: Surface sample collection tool; Depth sample collector (which must not compact sample); Materials strength tester; and Other possibilities including thermal inertia detector (via microwave radiometry or an in situ instrument). Some Questions for Additional Study: What are the realistic mass and power estimates for the in situ instrument package? What are the power requirements of the refrigeration system? How do the mass and power requirements of the refrigeration system scale with the total sample mass and volume? Are the power requirements of the refrigeration system consistent with the use of an RPS? NEP-Enhanced or NEP-Enabled Missions Envisioned After 2020 Missions Deferred for Scientific Reasons Until After 2020. Although certain missions are plausibly enabled by nuclear power and/or propulsion systems, scientific arguments can be made for delaying their launch until after 2020. Examples of such missions, described in Appendix C, include the following: Titan Surface Sample Return, Uranus System Explorer, and Multiple Kuiper Belt Object Rendezvous. Missions Deferred for Technical Reasons Until After 2020. Although many missions are plausibly enabled by nuclear power and/or propulsion systems, the necessary technologies may not be available until after 2020. Examples of such missions, described in Appendix C, include the following: Icy Moons Subsurface Sample Return; and Main-Belt, Trojan Asteroid, and Centaur Multi-Sample Return.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion BOX 6.4 Titan Express/Interstellar Pioneer Mission Type: NEP-class Objectives: Conduct an extended close study of Titan’s surface, subsurface, and lower atmosphere: its geomorphology and meteorology, as well as the identification of sites of astrobiological interest; Study the composition and distribution of organic compounds and the processes and energy sources resulting in the creation, modification, and destruction of organic compounds; Conduct in situ chemical analysis of selected surface sites; Explore the interstellar medium and its implications for the origin and evolution of matter in our galaxy and universe; Explore the outer solar system for clues to its origin and to the nature of other planetary systems; Explore the influence of the interstellar medium on the solar system and its dynamics and evolution; and Explore the interaction between the interstellar medium and the solar system as an example of how a star interacts with its local galactic environment. Implementation: An NEP-class spacecraft travels to Saturn under continuous acceleration to minimize delivery time, and releases an aerobot (inside an entry shield) directly into Titan’s atmosphere as the spacecraft flies by. The aerobot is an RPS-powered airship that inflates during the parachute descent after entry. The aerobot would conduct a 6- to 12-month mission, using an RPS of ~100 watts of electrical power (We). It would communicate directly to Earth at ~1 kbps. The total mass of the aerobot system Maero (floating mass plus deployment/inflation) is nominally 400 kg, but if the system design is not sensitive to delivered mass, consider also Maero = 1,000 kg to 2,000 kg. The entry shield is assumed to have a mass that depends on entry speed, as follows After the delivery of the aerobot to Titan, the trajectory of the carrier spacecraft is directed as close as is feasible toward the nose of the heliosphere and into interstellar flow. The Interstellar Pioneer’s payload mass is <100 kg, its power requirement is >100 W, and it has a bit rate of >1,000 bps. The desired flight time to a distance of 200 AU is more than 15 years but less than 30 years. Payload: Titan Aerobot: Side- and down-looking imagers, with some spectral and/or fluorescence capability for identifying organic deposits;

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Small subsurface radar sounder; In situ surface chemistry package; and Environment sensors (meteorological and also radiation detectors for radiocarbon). 2a. Interstellar Pioneer (in situ): Magnetometer; Plasma and radio wave detectors; Instruments for studying solar-wind plasma and electrons; Pickup and interstellar ion mass spectrometer; Interstellar neutral atom mass spectrometer; Suprathermal ion mass spectrometer; Anomalous and galactic cosmic-ray element/isotope spectrometer; Cosmic-ray electron and positron detectors; Gamma-ray-burst detector; Dust composition analyzer; and Other possibilities, including an instrument for studying the charge states of suprathermal ions, molecular analyzer for organic material, and cosmic-ray antiproton detector. 2b. Interstellar Pioneer (remote sensing): Infrared spectrometer; Energetic neutral atom imager; Ultraviolet spectrometer; and Other possibilities, including a small Kuiper Belt object detector and an infrared background/zodiacal light mapper. Additional Questions for Study: How sensitive is the system to Titan delivery time? Is the mass of the aerobot plus its entry system so small compared with the mass of the delivery spacecraft that additional entry mass is not a significant factor and there can be therefore a large Maero? How does the delivery system change if the mass delivered to Titan is 1,500 kg, 2,000 kg, or 2,500 kg? Is there anything to be gained in Mentry by having a coast or retropropulsion phase prior to Titan delivery, or should the propulsion be continuous to simply get there as fast as possible, given propulsion capabilities? What are the direction and trip-time capabilities for the interstellar portion of the mission, given the Titan delivery requirement? Does a gravity assist from Saturn contribute significantly to modifying direction, or would this be a complication better avoided? How do direction and time to heliopause vary with launch date? Is continued propulsion (after Saturn encounter) worthwhile?

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Priorities in Space Science Enabled by Nuclear Power and Propulsion BOX 6.5 Neptune-Triton System Explorer Mission Type: NEP-class Objectives: Explore differences between the ice giants and the gas giants. Measure the elemental atmospheric composition; determine the constraint on the planet’s interior structure; and investigate the processes that control the distribution of gases, clouds, temperatures, and winds. Investigate Triton, a large captured object (perhaps a KBO). Measure the composition of Triton and the global distribution of volatiles; investigate processes that control Triton’s orbital history, surface morphology, and internal structure; determine if a subsurface water layer exists; characterize the diverse cryovolcanic features on Triton’s surface, including its geyser-like features; and study the diffuse atmospheric haze (possibly condensed hydrocarbons) and the discrete polar clouds (likely N2). Study Neptune’s small satellites and the vertical and radial structure of its rings. Measure the composition, size, and dynamical properties of ring particles; explore how Neptune’s satellite-ring interactions control ring structure and evolution. Probe Neptune’s magnetosphere. Measure structure, composition, density, and dynamics; sample the magnetosphere in latitude, longitude, altitude, and local time; and determine how the magnetosphere interacts with other Neptune-system elements and the solar wind. Pioneer new technologies to explore the outer solar system within a decade. Implementation: Desired flight time of 10 years or less from Earth to the Neptune system; Orbital insertion at Neptune; Delivery, deployment, and support of Neptune miniprobes with a mass of 100 kg (excluding propulsion system, if needed); power of 100–150 W (may be higher for depths below 200-bar level); data rates of ~200 bps (above 10-bar level) down to ~10 bps (very deep); and delta-V (probes released after orbit insertion might need delta-V capability). Science Categorization of New Mission Concepts The SSE decadal survey defined a set of 12 fundamental science questions to be addressed by solar system exploration missions.6 Table 6.1 presents a cross-matrix of the new mission concepts discussed in this report relative to these 12 questions. Further prioritization of these concepts will require further understanding and study of the ability and applicability of nuclear power and/or propulsion to achieve the respective science objectives. TECHNOLOGY ENHANCEMENTS AND ISSUES Of the 17 important technologies identified by the Aldridge Commission as enabling the new exploration initiative,7 eight technology areas are particularly relevant to solar system exploration missions: Advanced power and propulsion; Cyogenic fluid management; High-bandwidth communications;

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Delivery, deployment, and support of a Triton lander with a mass of ≥500 kg; power of 200–300 W; data rate of ~100–300 bps; and delta-V (needs to be determined). Multiyear tour of Neptune system supported by in situ and remote-sensing instruments on the carrier spacecraft (instrument mass of ~300 kg assuming post-Cassini instrument development; power of ~500 W and more if radar is carried; and data rates of >30 kbps). Payload: Carrier instruments: Multispectral imaging system (0.3 to 2.5 μm) down to 100-m resolution; Microwave radiometer; Telecommunications system incorporating a two-frequency coherent transponder for radio science/celestial mechanics (gravitational field measurement); Ultrastable oscillator for radio science occultations; Subsurface radar profiler (for Triton, possibly other satellites as well); and Space physics package (e.g., magnetometer, ion and neutral mass spectrometer, plasma wave spectrometer, charged-particle spectrometer). Atmospheric Miniprobe: Gas chromatograph/mass spectrometer; Temperature, pressure, and acceleration sensors; Ultrastable oscillator for Doppler-wind experiments; and Nephelometer. Triton lander: Multispectral imaging system (0.3 to 2.5 μm) down to centimeter-level resolution; Surface chemistry package including x-ray fluorescence and subsurface sampling mechanisms; and Meteorology package. Issues for Additional Study: Does the requirement for a transit time of 10 years translate into a technically feasible value for the specific mass parameter α? Autonomous systems and robotics; Scientific data collection/analysis; Entry, descent, and landing; Affordable heavy-lift capability; and Automated rendezvous and docking techniques. Detailed discussions of the other nine technologies (i.e., advanced structures; high-acceleration, high-life-cycle, reusable in-space main engine; large-aperture systems; formation flying; closed-loop life support and habitability; extravehicular activity systems; biomedical risk mitigation; transformational spaceport and range technologies; and planetary in situ resource utilization) are beyond the scope of this chapter. However, some of these additional technologies are highly relevant to astronomy and astrophysics (e.g., large-aperture systems) and solar and space physics (e.g., formation flying). Table 6.2 provides a summary description of Prometheus-driven technologies whose development is needed to significantly enhance or enable future missions discussed above.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion TABLE 6.1 Science Questions Addressed by New Mission Concepts SSE Decadal Survey Science Questionsa Missions 1 2 3 4 5 6 7 8 9 10 11 12 RPS Missions Envisioned by 2020 Venus Long-Lived Network       X   X X   X   X   Lunar Polar Rover/Driller       X X X         X   Mars Advanced Science Laboratory       X X X X X X   X   Titan Aerobot Explorer       X X X X       X   Neptune Deep Multiprobes X X   X X X X X       X Comet Nucleus Surface Laboratory X     X X         X X X Science Suggests >2020 Mercury Polar In Situ Explorer X     X   X     X   X   Venus Long-Lived Network X     X   X     X   X   Venus Aviator       X X X     X   X   Lunar Long-Lived Network X               X X X   Mars Long-Lived Network X     X   X X   X X X   Europa Astrobiology Lander       X X X X X     X   Io Observer       X   X         X   Titan Surface Laboratory       X X X X       X   Technology Suggests >2020 Mercury Long-Lived Network X         X     X   X   Venus Mobile Laboratory       X   X X   X   X   Venus Selected Sample Return X     X   X X   X   X   Mars Polar Profilerb       X X X X X X   X   Mars Deep Drillerb     X X X X X X X   X   Mars Cryogenic Sample Return       X X X X X X   X   Icy Satellite Long-Lived Network X     X X X X       X   Icy Satellite Deep Drillerb X     X X X X X     X   Io In Situ Explorer X     X   X     X   X   Cryogenic Comet Sample Return X     X X         X X X KBO Surface Laboratory X     X X         X X X Prometheus Propulsion Missions Envisioned by 2020 Jupiter Icy Moons Orbiter X   X X X X X X     X X Saturn System Multiple Rendezvous X X X X X X X       X X Titan Express/Interstellar Pioneer       X X X X       X   Main-Belt/Trojan/Centaur Multiple Rendezvous X     X X         X X   Neptune-Triton System Explorer X X X X X X         X X Science Suggests >2020 Titan Surface Sample Return X     X X X X X     X   Uranus System Explorer X X X X X X         X X Multiple KBO Rendezvous X     X X         X X X Technology Suggests >2020 Icy Moons Subsurface Sample Return X   X X X X X X     X   Main-Belt/Trojan/Centaur Multi-Sample Return X     X X         X X   aSSE Decadal Survey Science Questions: 1. What processes marked the initial stages of planet and satellite formation? 2. How long did it take the gas giant Jupiter to form, and how was the formation of the ice giants Uranus and Neptune different from that of Jupiter and its gas-giant sibling, Saturn? 3. How did the impactor flux decay during the solar system’s youth, and in what way(s) did this decline influence the timing of life’s emergence on Earth? 4. What is the history of volatile compounds, especially water, across the solar system? 5. What are the nature and the history of organic material in the solar system?

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Priorities in Space Science Enabled by Nuclear Power and Propulsion 6. What global mechanisms affect the evolution of volatiles on planetary bodies? 7. Where are the habitable zones for life in the solar system, and what are the planetary processes responsible for producing and sustaining habitable worlds? 8. Does (or did) life exist beyond Earth? 9. Why did the terrestrial planets differ so dramatically in their evolution? 10. What hazards do solar system objects present to Earth’s biosphere? 11. How do the processes that shape the contemporary character of planetary bodies operate and interact? 12. What does the solar system tell us about the development and evolution of extrasolar planetary systems and vice versa? For more details, see National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, pp. 177–188. bMay require a surface reactor, depending on the speed and depth of drilling. CONCLUSIONS The availability of nuclear power and propulsion technologies has the potential to enable a rich variety of solar system exploration missions. A particularly exciting prospect for the planetary science 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. If nuclear propulsion is developed and demonstrated, then it will enable radically new mission concepts capable of conducting activities of a scope never before contemplated by planetary scientists. This prospect has both positive and negative aspects. NEP missions, such as the Jupiter Icy Moons Orbiter (JIMO), will be able to reach previously inaccessible objects and conduct comprehensive studies with a degree of flexibility unprecedented in the history of solar system exploration. The potential scientific return from a single such mission is difficult to appreciate. But spacecraft nuclear propulsion is in its infancy and will require a great deal of technological development. By today’s standards, the spacecraft using these technologies will be very large, very heavy, very complex, and, almost certainly, very expensive. The question, then, is to what extent the development and deployment of such technologies will interfere with the diversity of solar system exploration missions. Mission diversity in solar system exploration gives scientific breadth and depth to these pursuits and is essential for the long-term health and vitality of the solar system exploration activities. It is difficult to imagine that the planetary-science goals of one, two, or three decades hence will still be addressed with the power and propulsion technologies of the Mariners, Pioneers, and Voyagers. It is equally difficult to imagine, however, how to transition smoothly from an era of Cassini, Mars Exploration Rovers, New Frontiers, and Discovery to one whose mix of activities will be as diverse but will now also include JIMO-class missions. With the launch of JIMO now delayed to beyond 2017, or later, there is a lack of flight opportunities to the outer solar system. Exploration of the outer solar system is the area where nuclear power systems have their greatest potential but, ironically, the lack of maturity of NEP or hybrid systems, combined with the current emphasis on lunar and martian exploration, may cause such long delays that an effective program cannot be sustained. The travel times, costs, and complexities, which are increasing the time to flight of the currently studied NEP system envisaged for JIMO, are making it unattractive to the science community. Further studies of nuclear thermal propulsion or bimodal systems may at least alleviate the concerns with trip times, but the concerns with cost and complexity will remain. Given the limited scope of this study, the various RPS and NEP mission concepts developed in this chapter appear to be capable of addressing expanded exploration of the inner solar system and providing limited support to robotic and human missions to the Moon and Mars, as well as increasing the flexibility for exploring the outer solar system. Nevertheless, the capabilities of competing technologies need to be studied in much more detail

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Priorities in Space Science Enabled by Nuclear Power and Propulsion TABLE 6.2 Technologies Enabling the New Mission Concepts Category (Heliocentric Order) Space Reactors Planetary Environment Reactorsa Radioisotope Power (vacuum) Radioisotope Powera (atmosphere) Radioisotope Heater Units Active Cooling Low-Power Electronics Radiation-Hard Electronics High-Thrust and Isp Nuclear Propulsion A: Enhanced/Enabled by Prometheus Power Sources (Envisioned by 2020) Mercury Polar In Situ Explorer     X   X   X     Mercury Long-Lived Network     Xb   X   X     Venus Long-Lived Lander       X Xc X       Venus Long-Lived Network       X   Xc X     Venus Aviator       X   Xc X     Venus Mobile Laboratory       X   Xc X     Venus Selected Sample Return       X   Xc X     Lunar Polar Rover/Driller   Xd Xd   X   Xd     Lunar Long-Lived Network     Xe   X   X     Mars Advanced Science Laboratory       X X   X     Mars Long-Lived Network       Xe X   X     Mars Polar Profiler   Xd   Xd Xd   Xd Xd,e   Mars Deep Driller   Xd   Xd Xd   Xd     Mars Cryogenic Sample Return   Xd   Xd X Xf Xd     Neptune Deep Multiprobes       X   Xc X     Io Observer     X   X   X X   Io In-Situ Explorer     X   X   X X   Europa Astrobiology Lander     X   X   X X   Titan Aerobot Explorer       X X   X     Titan Surface Laboratory       X X   X     Icy Satellite Long-Lived Network     X   X   X Xe   Icy Satellite Deep Driller     Xd   Xd   Xd Xd,e   KBO Flyby with Giant Planet Gravity Assist     X   X   X     Comet Nucleus Surface Laboratory     X   X   X     Cryogenic Comet Sample Return     Xg   Xf   X     KBO Surface Laboratory     X   X   X     B: Enabled by Prometheus Propulsion (Envisioned by 2020) Jupiter Icy Moons Orbiter X   Xh   Xh   Xh X   Saturn System Multiple Rendezvous X   Xh   Xh   Xh X Xi Titan Express/Interstellar Pioneer X     X X   X    

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Main-Belt/Trojan/Centaur Multiple Rendezvous X       X X X   X Neptune-Triton System Explorer X   Xh   Xh   Xh X   C: Enabled by Prometheus Propulsion and/or Power (Scientific Rationale Beyond 2020) Titan Surface Sample Return X Xg Xg   Xg Xf Xg X Xg Uranus System Explorer X       Xh   Xh X   Multiple KBO Rendezvous X       X   X X   D: Enabled by Prometheus Propulsion and/or Power (Capability Beyond 2020) Icy Moons Subsurface Sample Return X       Xg Xf Xg X X Main-Belt/Trojan/Centaur Multi-Sample Return X   Xh   Xh   Xh X Xg a Design factors include atmosphere compatibility (temperature, corrosion), gravitational effects on structure, coolant transport, planetary protection. b Part of network may be viable with solar power, but full coverage needs RPS. c Refrigeration power required to maintain vehicle systems cool in a hot environment. d Reactor may be required for rapid, deep drilling. Lower-capability (slower/shallower) mission may be possible with RPS/RHUs and low-power electronics. e All missions require some radiation tolerance. Severe radiation issues exist for missions to Europa and Io, or if a close Jupit er flyby is needed en route to another destination. f Refrigeration power required to maintain sample in pristine cryogenic conditions during return. g Depends on mission architecture: land entire vehicle or have small subvehicle to retrieve sample. h RPS required if mission includes surface element (e.g., Triton lander, Enceladus, Europa, Oberon, etc.). i Higher thrust than JIMO ion propulsion needed for full ring access or larger small body landing. NOTE: Definitions of column heads. Space reactors—In-space power generation by fission systems. Previously flown systems, the configuration envisaged for the Jupiter Icy Moons Orbiter (JIMO), NTP, and bimodal systems are examples. Planetary environment reactors—Space reactors as defined above may be unworkable in a number of environments on, near, or in planetary bodies. Particular complications may include gravity that imposes structural requirements on the radiators and the heat transfer implications of an atmosphere or su rface. The challenges and requirements vary from body to body; an asteroid surface is little different from deep space, while the venusian surface would be particularly difficult. Radioisotope power (vacuum) refers to radioisotope (nonfission) power systems, including existing and planned RPSs and Stirling converters. Heat rejection is via radiators. Radioisotope power (atmosphere) refers to RPSs designed to operate within a planetary atmosphere. Efficiency of heat rejection and corrosion resistance are par ticular issues. Radioisotope heater units are heat sources without energy converters. Although little or no technology development appears required, they are listed here because the demand for them has implications for the radioisotope inventory. Active cooling refers to power-consuming heat management techniques such as Stirling coolers or other refrigeration, either for maintaining sy stems under operating conditions in Venus’s near-surface environment, or for maintaining a sample under refrigerated or even more stringent cryogenic conditions for retu rn to Earth. Low-power electronics—Specific components may be required to permit systems to operate at milliwatt power levels for decades from miniature RPSs. This may also include the ability to operate in harsh environments without active thermal control. Radiation-hard electronics—Electronic systems suffer prompt (“bit-flip” and latchup) and cumulative (total dose) damage from radiation, both natural and from any nearby nuclear power sources. Specific fabrication materials and architectures for components are required to survive high-radiation environments. High-thrust and Ispnuclear propulsion—The JIMO mission assumes fission-powered ion propulsion, which has high specific impulse (Isp) but very low thrust. Some missions require higher thrust levels (e.g., asteroid landing, hovering above Saturn’s ring plane etc.) that would ideally also have high Isp—an example of such a system is the magnetoplasmadynamicarcjet.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion before the priority of specific approaches for implementing these missions can be adjudicated. There is a particularly strong need for trade-off studies of large NEP-powered craft versus other modes for implementing the same science goals. The development of nuclear technologies must go hand in hand with ancillary developments in such areas as communications, data entrapment, and spacecraft subsystems. Planetary exploration is a rich field that allows public involvement and generates student interest in engineering and science. A mix of exploration capabilities should be developed that will allow diverse pursuits that will enhance this capability and ensure the long-term health and vitality of the planetary exploration enterprise. REFERENCES 1. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003. 2. R.B. Abelson, ed., Enabling Exploration with Small Radioisotope Power Systems, JPL 04-10, Jet Propulsion Laboratory, Pasadena, Calif., 2004. 3. Space Studies Board, National Research Council, A Scientific Rational for Mobility in Planetary Environments, National Academy Press, Washington, D.C., 1999, pp. 42–43. 4. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, p. 96. 5. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, p. 196. 6. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, pp. 177–188. 7. President’s Commission on Implementation of United States Space Exploration Policy, A Journey to Inspire, Innovate, and Discover, U.S. Government Printing Office, Washington, D.C., 2004, p. 28.