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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity 2 New Frontiers Mission Options The National Research Council’s (NRC’s) 2003 solar system exploration decadal survey, New Frontiers in the Solar System: An Integrated Exploration Strategy,1 specified five mission candidates and ranked them according to priority: Kuiper Belt Pluto Explorer, South Pole-Aitken Basin Sample Return, Jupiter Polar Orbiter with Probes, Venus In Situ Explorer, and Comet Surface Sample Return. To date there have been two New Frontiers missions selected—the New Horizons mission to Pluto and the Kuiper Belt and the Juno mission to orbit Jupiter. Three missions remain from the original decadal survey list of potential New Frontiers missions: South Pole-Aitken Basin Sample Return, Venus In Situ Explorer, and Comet Surface Sample Return. The committee believes that all three of these remaining missions are viable candidates for the New Frontiers Program; the committee also recommends expanding the list of mission candidates to include the five additional medium-size missions that were mentioned in the decadal survey but not ranked: Network Science, Trojan/Centaur Reconnaissance, Asteroid Rover/Sample Return, Io Observer, and Ganymede Observer. 1 National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity For consideration in the next New Frontiers Announcement of Opportunity, the committee offers descriptions of eight candidate missions and an innovative mission option. The eight mission options, described below in the same order as they appeared in the decadal survey, are followed by a description of the innovative mission option. No science prioritization is implied by their order. SOUTH POLE-AITKEN BASIN SAMPLE RETURN The South Pole-Aitken Basin Sample Return mission, as described in the decadal survey, is an inner-solar-system mission to study basin-forming processes and impact chronology by returning samples from the deepest, most heavily cratered—and, hence inferred to be the oldest—impact structure preserved on the Moon (Figure 2.1). Another goal of the mission is to use these returned samples to understand the nature of the Moon’s deep crust and upper mantle and the planetary processes that produced these features. These goals are to be accomplished through the intensive study of the returned materials in Earth-based laboratories. Heavy bombardment in the very early history of the solar system is a paradigm established from analysis of the samples returned by the Apollo and the Soviet robotic Luna missions. Careful site selection and the study of new samples of the Moon will result in detailed verification and extension of this central concept for the formation and early history of the terrestrial planets and its implications for the earliest appearances and evolution of life. In particular, this mission would allow a test of theories that have been proposed about the early impact history of the inner solar system, notably the “lunar cataclysm” model. FIGURE 2.1 South Pole-Aitken Basin. The basin is clearly visible in both the topography and iron projections, illustrating how a large impact affected the Moon. The black area at the pole was not imaged. SOURCE: Courtesy of Clementine Science Group, Lunar and Planetary Institute.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity Melting of planetary surfaces (magma oceans) during the early accretion process of planetary bodies in the inner solar system is an important concept resulting from detailed analysis of the Apollo and Luna samples in Earth-based laboratories. The detailed analysis of samples from the South Pole-Aitken Basin Sample Return mission should verify and extend this central concept for the differentiation of the early planetary body into crust and mantle. Sample return allows samples to be analyzed with the most sophisticated instruments on Earth (many of which cannot be transported to the sampling location). And it has other benefits as well, providing the ability to share samples with many research teams for broad-based experimentation and the archiving of samples for analysis in the future when better instrumentation will exist. It is possible, for example, to conduct far more sophisticated analysis of Apollo samples today than it was when they were first returned to Earth. Background A South Pole-Aitken Basin Sample Return mission would directly address the following crosscutting themes and key questions identified in the decadal survey (numbering is taken from the decadal survey):2 The First Billion Years of Solar System History What processes marked the initial stages of planet and satellite formation? 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? Processes: How Planetary Systems Work How do the processes that shape the contemporary character of planetary bodies operate and interact? In addition, the decadal survey identified this mission as providing scientific return in three categories: highly significant scientific return, very useful scientific return, and supporting scientific return:3 Highly Significant Scientific Return Past: What led to the unique character of our home planet? What are the bulk compositions of the inner planets and how do they vary with distance from the Sun? Determine elemental and mineralogic surface compositions. Determine interior (mantle) compositions. What is the internal structure and how did the core, crust, and mantle of each planet evolve? Determine compositional variations and evolution of crusts and mantles. What were the history and role of early impacts? Determine large-impactor flux in the early solar system and calibrate the lunar impact record. Investigate how major impacts early in a planet’s history can alter its evolution and orbital dynamics. Very Useful Scientific Return Past: What led to the unique character of our home planet? What is the internal structure and how did the core, crust, and mantle of each planet evolve? Determine major heat-loss mechanisms and resulting changes in tectonic and volcanic styles. 2 New Frontiers in the Solar System, p. 3, Table ES.1. 3 New Frontiers in the Solar System, pp. 56-57, Table 2.1.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity What were the history and role of early impacts? Determine the global geology of the inner planets. Present: What common dynamic processes shape Earth-like planets? How do active internal processes shape the atmosphere and surface environments? Determine absolute ages of surfaces. Future: What fate awaits Earth’s environment and those of the other terrestrial planets? What are the resources of the inner solar system? Assess mineral resources. Supporting Scientific Return Past: What led to the unique character of our home planet? What are the bulk compositions of the inner planets and how do they vary with distance from the Sun? Measure oxygen isotopic ratios of the unaltered surface and atmosphere. What is the internal structure and how did the core, crust, and mantle of each planet evolve? Determine horizontal and vertical variations in internal structures. What is the history of water and other volatiles and how did the atmospheres of inner planets evolve? Determine the composition of magmatic volatiles. Present: What common dynamic processes shape Earth-like planets? How do active external processes shape the atmosphere and surface environment? Quantify regolith processes on bodies with tenuous atmospheres. Future: What fate awaits Earth’s environment and those of the other terrestrial planets? How do varied geologic histories enable predictions of volcanic and tectonic activity? Assess the distribution and age of volcanism on the terrestrial planets. What are the consequences of impacting particles and large objects? Determine the recent cratering history and current flux of impactors in the inner solar system. Developments Since the Decadal Survey Significant advances have been made in modeling the early history of our solar system, in particular the timing of accretion of the large planets in the outer solar system and the dynamical effects of their subsequent orbital evolution. Recent work has shown that the earliest crust on Earth may have formed very early, potentially providing a foothold for the early development of life. However, the intense bombardment during the earliest history of the solar system may have prevented or delayed the development of life until more quiescent times. It has long been known that the major basins on the Moon that have been dated are all around 4 billion years in age. A major question is whether all major basins formed in a 200-million-year time period—a “lunar cataclysm”—or whether the basins that have been dated simply represent the end of a declining flux of large impacts starting around 4.5 billion years ago. Argon-argon dating of impact-produced glasses in lunar meteorites, which plausibly sample the entire lunar surface, suggests that there was a lunar cataclysm. If this is correct, some mechanism must be found that would explain how asteroids were dislodged from the main belt 500 million years after solar system formation. Two models that are not necessarily incompatible have been proposed. One model for the evolution of the outer solar system postulates that the eccentricities of Jupiter and Saturn were pumped up as they passed through 2:1 orbit:orbit resonances, sweeping resonances through the main belt and dislodging main belt asteroids. These asteroids then produced cataclysms on all terrestrial planets and satellites, including the Moon.4 4 See R. Gomes, H.F. Levison, K. Tsiganis, and A. Morbidelli, Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial plants, Nature 435:466-469; K. Tsiganis, R. Gomes, A. Morbidelli, and H.F. Levison, Origin of the orbital architecture of the giant planets of the solar system, Nature 435:459-461, 2005; and A. Morbidelli, K. Tsiganis, A. Crida, H.F. Levison, and R. Gomes, Dynamics of the
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity The other model notes that the size-frequency distribution of the highland craters on the Moon is the same as that in the main belt and distinct from the modern population of near-Earth asteroids. The near-Earth asteroids seem to be responsible for cratering the lunar maria, i.e., more recently than 3.9 billion years ago.5 Strom et al. postulate that Neptune, and possibly Uranus, were formed 500 million years after the formation of the other planets, around 4 billion years ago.6 The resultant migration inward of Jupiter caused resonances to sweep through the main belt with the same consequences. Conclusions The committee concludes that the South Pole-Aitken Basin Sample Return mission remains a very scientifically important mission that should be considered for the New Frontiers Program. Although the committee is concerned that NASA should not be too specific in defining how New Frontiers missions should be conducted, it has concluded that in this case, given the maturity of the science questions and the precise design of the mission as stated in the decadal survey, studying new samples from the Moon is a reasonable and irreducible requirement of the mission. Furthermore, the South Pole-Aitken Basin is the preferred lunar region to target for this mission. Other sample return sites may exist that can address the preponderance of the objectives for this mission; however, it is the responsibility of the proposer to convincingly defend the merits of an alternative site. After exhaustive and extended laboratory analysis of the Apollo and Luna lunar samples and meteorites from the Moon, no evidence of water in any form has been found in lunar rocks or soils.7 While the search for water on the Moon is not a science objective for this mission, returned samples from the deep crust or upper mantle may contain trace water. Discovery of water in returned lunar samples, even in the minutest quantities, would constitute a major scientific discovery. Mission-Specific Recommendations A South Pole-Aitken Basin Sample Return mission is tenable under the New Frontiers Program and can address many decadal survey objectives. The committee recommends that the South Pole-Aitken Basin Sample Return mission as described in the decadal survey remain a high priority for the New Frontiers Program. The committee has no changes to the decadal survey’s scientific objectives or engineering implementation of this mission. However, the committee recommends that NASA not be overly prescriptive about specific approaches to address the scientific objectives. Instead, NASA should allow proposers to develop their own innovative approaches. The committee believes that the following science goals, not in priority order, should be established for this mission: Elucidate the nature of the Moon’s lower crust and/or mantle by direct measurements of its composition and of sample ages; Determine the chronology of basin-forming impacts and constrain the period of late, heavy bombardment in the inner solar system, and thus, address fundamental questions of inner solar system impact processes and chronology; giant planets of the solar system in the gaseous protoplanetary disk and their relationship to the current orbital architecture, The Astronomical Journal 134:1790-1798, 2007. 5 See R.G. Strom, R. Malhotra, T. Ito, F. Yoshida, and D.A. Kring, The origin of planetary impactors in the inner solar system, Science 309(5742):1847-1850, 2005. 6 See R.G. Strom, R. Malhotra, T. Ito, F. Yoshida, and D.A. Kring, The origin of planetary impactors in the inner solar system, Science 309(5742):1847-1850, 2005. 7 As the committee was finishing its report, it learned of a presentation at the fall meeting of the American Geophysical Union that may indicate the presence of significant water in lunar volcanic glasses. See A.E. Sael, E.H. Hauri, M. Lo Cascio, J. Van Orman, M. Rutherford, and R. Cooper. Volatiles in the lunar volcanic glasses: Evidence for the presence of indigenous water in the Moon’s interior, AGU Fall 2007 Meeting.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity Characterize a large lunar impact basin through “ground truth” validation of global, regional, and local remotely sensed data of the sampled site; Elucidate the sources of thorium and other heat-producing elements to understand lunar differentiation and thermal evolution; and Determine the age and composition of farside basalts to determine how mantle source regions on the Moon’s farside differ from the basalts from regions sampled by Apollo and Luna. VENUS IN SITU EXPLORER A Venus In Situ Explorer mission would address fundamental unanswered questions of the history and current state of Venus through a characterization of the chemical composition and dynamics of the atmosphere of Venus, and/or measure surface composition and rock textures. While it is unlikely that all of the objectives delineated in the decadal survey could be addressed within the New Frontiers cost constraints, a mission that addresses a subset of these objectives would provide critical information about the present state and history of Venus. The current European Space Agency (ESA) Venus Express mission has greatly expanded knowledge of the upper atmosphere and exosphere of Venus, and has contributed to understanding of regions of the atmosphere nearer to the planet’s surface. However, characterization of the noble-gas and isotopic signatures of the well-mixed lower atmosphere would greatly expand understanding of the formation and evolution of the atmosphere of Venus, illuminate important elements of the current climate, including the drivers for the Venus greenhouse effect, and potentially provide insight on the early tectonic evolution of the planet. Prior landed missions of Soviet Venera and Vega spacecraft (Figure 2.2) have provided some information on crustal compositions and textures, but they have been confined to lowland areas composed of basaltic lava flows. Landed missions in highland regions or on older terrains could answer questions related to presence of silicic rock compositions or earlier phases of tectonism, but they present significant technological challenges. FIGURE 2.2 Image taken of the surface of Venus by Venera 13 in 1982. The Soviet Union successfully conducted several Venus lander missions with 1980s era technology. These images depict the distorting effects of the thick Venusian atmosphere. SOURCE: C.M. Pieters, J.W. Head, W. Patterson, S. Pratt, J.B. Garvin, V.L. Barsukov, A.T. Basilevsky, I.L. Khodakovsky, A.S. Selivanov, A.S. Panfilov, Y.M. Getkin, and Y.M. Narayeva, The color of the surface of Venus, Science 234:1379-1383, 1986. Reprinted with permission of AAAS.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity Background The science questions targeted by a Venus In Situ Explorer (VISE) mission directly address the following crosscutting themes and key questions identified in the decadal survey:8 Volatiles and Organics: The Stuff of Life What global mechanisms affect the evolution of volatiles on planetary bodies? The Origin and Evolution of Habitable Worlds Why have the terrestrial planets differed so dramatically in their evolutions? Processes: How Planetary Systems Work How do the processes that shape the contemporary character of planetary bodies operate and interact? As noted in the decadal survey, a New Frontiers VISE mission should address a number of the following objectives, which were not prioritized:9 Science mission objectives for VISE are as follows: Determine the composition of Venus’s atmosphere, including trace gas species and light stable isotopes; Accurately measure noble gas isotopic abundance in the atmosphere; Provide descent, surface, and ascent meteorological data; Measure zonal cloud-level winds over several Earth days; Obtain near-infrared descent images of the surface from 10-km altitude to the surface; Accurately measure elemental abundances and mineralogy of a core from the surface; and Evaluate the texture of surface materials to constrain weathering environment. The mission objectives are provided in the decadal survey in the context of atmospheric and surface science objectives:10 Atmospheric Science Objectives: The composition of the lower atmosphere of Venus is unknown. Without this knowledge, comparisons of the factors that affect climate on Earth and on Venus, including photochemistry, clouds, volcanism, surface-atmosphere interactions, and the loss of light gases to space, are impossible. VISE will measure the abundance of trace gas species in the lower atmosphere of Venus to parts per million accuracy, enabling an understanding of how these processes affect terrestrial planetary climates. A fundamental quest is to understand how and why Venus, roughly the same size, composition, and distance from the Sun as Earth, has evolved to such a different state. The record of planetary atmospheres is contained in the isotope ratios of the most inert gases—xenon, krypton, argon, and neon. Are planetary atmospheres the remnants of gases that were originally solar in composition but then suffered massive hydrodynamic escape, or did they require atmospheres from volatiles that had already been differentiated? What was the role of impacts on the ultimate compositions and evolution of the terrestrial planets? Discrimination 8 New Frontiers in the Solar System, p. 3, Table ES.1. 9 New Frontiers in the Solar System, p. 58. 10 New Frontiers in the Solar System, p. 59.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity between these events for each of the inner planets is possible if noble gas isotopic ratios can be measured with a state-of-the-art neutral mass spectrometer. Previous spacecraft measurements have been inadequate to address these issues. VISE will determine the noble gas abundances and isotope ratios to sufficient accuracy to distinguish between hypotheses of the origin and evolution of Venus’s atmosphere. A meteorological package will measure atmospheric pressure and temperature profiles down to the surface, and pressure, temperature, and winds at the surface. Cloud-level winds will be determined by tracking the ascent balloon during its 3.5-day lifetime, providing improved data on atmospheric dynamics and the origin of Venus’s mysterious atmospheric superrotation. Surface Science Objectives: The former Soviet Union’s Venera landers returned basic elemental chemistry and images of four sites on the surface, and Magellan data provided evidence of possible evolved volcanic deposits. However, we lack sufficient information on surface elemental abundances and mineralogy to determine the degree of crustal evolution on Venus. The VISE mission would measure elemental compositions at a surface site complementary to those of the Veneras. Mineralogy of a surface sample core will be obtained for the first time, allowing analysis of any weathered layer and testing for depth of alteration and occurrence of unaltered material. Textural analysis of the sample using a microscope imaging system would provide information on the formation and nature of surface rocks. These data will be used to constrain questions outlined above. Despite global radar coverage of Venus by Magellan, little is known of the surface morphology at scales of 1 to 10 m. Without such information, it is difficult to determine how the plains formed and to understand the nature of mobile materials on the surface. A descent camera on the lander will provide the first broadscale visible images of the surface, with images returned from about 10 km altitude to the surface. These images will enhance interpretation of the Magellan radar images by providing ground-truth data on the surface texture of the lava flows that make up Venus’s plains. The morphology and texture of these flows can be related to emplacement rate, volatile content, and rheology, which are needed in order to understand the role of volcanism in shaping the atmosphere and surface of Venus. Images of Venus’s surface will also be returned from the lander, with filters chosen to provide compositional information. These images will help to determine the recent geological history of Venus and will resolve differences in the interpretation of Venus’s resurfacing history. Developments Since the Decadal Survey Since the decadal survey, NASA’s Venus Exploration Analysis Group (VEXAG) has worked with the Venus science community to develop the following Venus exploration goals, with prioritized objectives.11 Goal 1: Origin and Early Evolution of Venus: How did Venus originate and evolve? The highest priority objectives are: Determine the elemental and isotopic composition of the atmosphere to identify earlier epochs of Venus’s history, and clues to Venus’s origin, formation and evolution. Map the mineralogy and chemical composition of Venus’s surface on the planetary scale for evidence of past environmental conditions and for constraints on the evolution of Venus’s atmosphere. 11 The committee has reproduced only the top three VEXAG goals but notes that the VEXAG committee has produced a valuable document that can be used as a reference on Venus science objectives. This document, Venus Exploration Goals, Objectives, Investigations, and Priorities: 2007, is available at http://www.lpi.usra.edu/vexag/vexag_goals_2007.pdf.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity Characterize the history of volatiles in the interior, surface and atmosphere of Venus, including volatile additions due to cometary impacts, degassing and atmospheric escape, to understand the planet’s geologic and atmospheric evolution. Goal 2: Venus as a Terrestrial Planet: What are the processes that have shaped and still shape the planet? The highest priority objectives are: Constrain the coupling of thermochemical, photochemical and dynamical processes in Venus’s atmosphere and between the surface and atmosphere to understand radiative balance, climate, dynamics, and chemical cycles. Constrain the resurfacing history of Venus, and the nature of the resurfacing processes, including the role of tectonism, volcanism, impacts of asteroids or comets, sedimentation/erosion, and chemical weathering. Constrain the nature and timing of volcanic activity on Venus, including thermal evolution, current and past rates of volcanic activity, and the effects of outgassing on atmospheric and interior processes. Goal 3: What does Venus tell us about the fate of Earth’s environment? The highest priority objectives are: Search for evidence of past global-climate changes on Venus, including chemical-and-isotope evidence in the atmosphere, as well as rock chemistry and characteristics of surface weathering. In particular, seek evidence for the presence or absence of past oceans. Search for evidence of past changes in interior dynamics, volcanics and tectonics, including possible evolution from plate tectonics to stagnant-lid tectonics, which may have resulted in significant changes in the global climate pattern. Characterize the Venus greenhouse effect, including the interplay of chemistry, dynamics, meteorology, and radiative physics in the atmosphere, especially in the clouds. In addition, ESA’s Venus Express has entered Venus orbit and has returned new data since the decadal survey. Venus Express has expanded understanding of the upper atmosphere and exosphere and has contributed to knowledge of the mid- to lower atmosphere. However, most of the science objectives from the decadal survey require in situ measurements that are beyond the capabilities of an orbital mission such as Venus Express.12 Conclusions The committee concludes that a VISE mission remains a very scientifically important mission that should be considered for the New Frontiers Program. The VEXAG goals and objectives align well with the New Frontiers Venus mission objectives, further validating the selection process in the decadal survey. Although these objectives address fundamental science themes for Venus exploration, it is unlikely that they can be fully addressed in a single mission. Cost and technology risk factors may preclude a single VISE mission proposal from addressing all of the objectives. Consequently, a mission that addresses a major subset of the objectives would be consistent with the recommendations of the decadal survey. For example, a successful mission might not have to include a landed component if it addressed the major atmospheric objectives. In addition, an interpretation of the decadal survey’s science objectives should prescribe the important data to be collected, rather than dictate measurement techniques or mission scenarios. While no attempt is made here to prescribe or define implementation strategies, potential challenges related to the Venus environment—such as high temperatures, high pressures, and a corrosive atmosphere in the near-surface environment—may require the use of nontraditional (though previously demon- 12 New Frontiers in the Solar System, p. 58.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity strated) mobility systems such as balloons (a technology that also has some applications on other atmospheric bodies). The committee also notes that most of the technologies required to address the decadal survey objectives have been demonstrated on prior missions. For instance, Soviet-era Venus missions not only successfully reached the surface, but also operated there for up to an hour, proving that surface missions are possible. In the decadal survey, the VISE mission concept was discussed in terms of what it could contribute to a future flagship-class Venus sample return mission. While such an approach has significant merit, the committee warns that placing a technology demonstration for a future mission in the critical path of VISE mission success is unwise, particularly given the technical challenges for Venus sample return. Nonetheless, future Venus exploration beyond a VISE mission would require major technology development and demonstration, so that the inclusion of demonstration technologies in a VISE mission on a non-interference, non-critical-path basis is justified. Mission-Specific Recommendations The committee concluded that a VISE mission that addresses a significant number of the decadal survey objectives is tenable. Such a mission would make use of technologies that have been successfully demonstrated in prior missions to the Venus surface and near-surface environment. The committee also concluded that several of the VEXAG goals should be included with the goals established in the decadal survey, particularly the VEXAG goals concerning understanding the thermal balance of the atmosphere and gathering global mineralogic data. The challenges associated with landing in a region not previously sampled, collection of a sample, and lofting to a more clement altitude are the source of greatest technology and cost risk. Consequently, the New Frontiers announcement of opportunity should not preclude a mission that addresses the major goals for chemical sampling of the mid- to lower atmosphere on Venus and characterizing atmospheric dynamics, but lacks a surface sampling component. On the other hand, a mission that only addressed surface sampling would not be acceptable. The science goals for this mission, which are not in priority order, should be to: Understand the physics and chemistry of the atmosphere of Venus through measurement of its composition, especially the abundances of sulfur, trace gases, light-stable isotopes, and noble-gas isotopes; Constrain the coupling of thermochemical, photochemical, and dynamical processes in the atmosphere of Venus and between the surface and atmosphere to understand radiative balance, climate, dynamics, and chemical cycles; Understand the physics and chemistry of the crust of Venus, for example, through analysis of near-infrared descent images from below the clouds to the surface and through measurements of elemental abundances and mineralogy from a surface sample; Understand the properties of the atmosphere of Venus down to the surface through meteorological measurements and improve understanding of zonal cloud-level winds on Venus through temporal measurements over several Earth days; Understand the weathering environment of the crust of Venus in the context of the dynamics of the atmosphere and the composition and texture of surface materials; and Map the mineralogy and chemical composition of the surface of Venus on the planetary scale for evidence of past hydrologic cycles, oceans, and life and constraints on the evolution of the atmosphere of Venus. COMET SURFACE SAMPLE RETURN Scientific community interest in a Comet Surface Sample Return (CSSR) mission has been very high for many years. The advantages of such a mission have been stated in many documents including the decadal survey. Flyby missions to comets are fairly simple, and the Deep Space-1, Stardust, and Deep Impact missions have produced remarkable data. Rendezvous missions such as the ESA’s Rosetta mission (Figure 2.3) are more challenging, and a sample return mission can take twice as long as a rendezvous mission, thereby increasing cost and risk. The decadal survey concluded that bringing back a warm (i.e., non-cryogenic) sample was within a New Frontiers mission budget. While cometary science goals make the return of a cryogenic core sample highly desirable, such
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity FIGURE 2.3 The European Space Agency’s Rosetta mission to a comet. This is a rendezvous and landing mission. Sample return would double the length of such a mission and add additional risk. SOURCE: Courtesy of European Space Agency. a mission may not fit within the fiscal limits and programmatic timescale of the New Frontiers Program. The science yield from a warm sample return mission will have to be strongly defended by proposers. Background The decadal survey recommended that a comet mission be included in the New Frontiers Program:13 The Comet Surface Sample Return (CSSR) mission will collect materials from the near surface of an active comet and return them to Earth for analysis. These samples will furnish direct evidence on how cometary activity is driven. 13 New Frontiers in the Solar System, p. 6.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity Optical properties Reaction rates/kinetic information What are the processes currently affecting organic-rich surfaces? Cryovolcanic processes Tectonic processes Understanding Dynamic Planetary Processes What are the active interior processes and their relations to tidal heating, heat flow, and global patterns of volcanism and tectonism? Interior structure Although the decadal survey outlined the ways in which an Io mission could contribute to important questions about planetary satellites, it did not detail Io-specific science goals for an Io orbiter mission.56 Similarly, although the decadal survey lacked a detailed mission description, it did summarize the mission concept. The mission concept for Io could involve either a Jupiter orbiter dedicated to multiple close flybys of Io or a multirole mission with part of the mission and payload being devoted to magnetospheric space physics goals and/or atmospheric and auroral observations. The assumption that this mission could achieve the stated goals within the New Frontiers cost category rests partially on an assumption that heritage from the Europa Geophysical Explorer would allow significantly reduced costs. Although the Europa Geophysical Explorer was not pursued, significant studies of the Jupiter radiation environment were performed as part of the Jupiter Icy Moons Orbiter program, and some radiation-hardened electronics have been developed in the interim. Nevertheless, an Io Observer spacecraft would definitely benefit from future studies and technology development, including work currently underway for the Juno mission. More Io-specific goals can be drawn from the white paper provided to the decadal survey by the Io community.57 Developments Since the Decadal Survey The decadal survey report was written mostly in 2001 after most of Galileo’s Io flybys. Remote analyses of Io have continued, using Earth-based assets. Near observations of Io have been limited to the few, but spectacular, views provided during the New Horizons flyby of Jupiter on its way to Pluto in February 2007. These additional views provide insights into the time variations in and extent of the eruptions on Io. Study of volcanic activity on Io—the presumed prime driver of phenomena throughout the Jovian magnetospheric system—thus has consequences for understanding the dynamics and evolution of the Jovian system on time scales stretching from mere hours to the age of the solar system. The New Horizons Io images (see Figure 2.11) underscore the importance of temporal coverage, and they open a new window on plume dynamics by showing how plume structure can be tracked to reveal rapid motions, while ground-based adaptive optics images underscore the importance of long-term temporal coverage in tracking volcanic eruptions. Continuing work on Io’s atmosphere, such as the recent identification of sodium chloride, hints at the atmosphere’s likely chemical complexity, and its physical complexity is revealed in the large regional variations in atmospheric density revealed by new Hubble Space Telescope and ground-based observations, and global circulation patterns made available by disk-resolved millimeter-wave atmospheric observations. New Horizons’ exploration of Jupiter’s magnetotail also sheds new light on the processes of magnetospheric volatile loss from Io. Observations from the ground and from spacecraft such as Hubble, Cassini, and New Horizons can help pro- 56 Other top-level notes for such a mission are included in Table 5.3 and Table 7.1 of the decadal survey, New Frontiers in the Solar System, pp. 144 and 176. 57 J.R.Spencer et al., The future of Io exploration, community white paper for the Planetary Decadal Survey in The Future of Solar System Exploration, 2003-2013, M. Sykes, ed., Astron. Soc. Pac. Conference Series 270:201-216, 2002.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity FIGURE 2.11 Image of Io taken during Jupiter flyby in February 2007. Io has a diameter of 3,642 kilometers. SOURCE: Courtesy of NASA. vide clues to the Jovian system. Juno will also provide magnetic field data when it reaches Jupiter in 2015. But the lack of dedicated, targetable observations makes any approach to understanding the system extremely piecemeal. Resolution, or even significant advances in understanding, of the processes at work on Io is problematic without nearby, focused observations. There have been technological developments since the decadal survey that may make an Io Observer mission more feasible than it was 5 years ago. In particular, radiation-hardened electronics have been developed that would be vital to an Io mission. The 2007 flagship-class mission studies for the Europa Explorer and Jupiter System Observer (JSO) demonstrate the spacecraft longevity that is possible with modern radiation-hardened electronics—which is especially important in the intense Jovian radiation environment. In particular, Figure 4.4-4 of the JSO report shows that a spacecraft would accumulate only 10 percent of its allowable radiation dose during its first three Io flybys, suggesting that a dedicated Io mission could survive a large number of Io flybys.58 In addition, the Juno mission demonstrated the feasibility of using radiation shielding and a solar-powered satellite at Jupiter’s distance from the Sun. Conclusions The Galileo mission to Jupiter provided relatively limited information on Io for several reasons: Galileo had very limited ability to provide high-resolution spatial coverage of Io because of the low data rate, and its instrumentation was limited (e.g., with almost no ability to study Io’s molecular atmosphere, and very limited spatial coverage possible during each flyby). Galileo was also limited to seven Io flybys (many of which were late in the 58 National Aeronautics and Space Administration, Jupiter System Observer, Mission Study: Final Report, November 1, 2007, available at http://solarsystem.nasa.gov/multimedia/downloads.cfm.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity extended mission and were compromised by spacecraft problems), which was not nearly enough to characterize Io’s internal structure and determine if it has a magnetic field or to investigate temporal variability of the surface with high spatial resolution. Several new technology developments have occurred which make an Io mission more feasible than it was 5 years ago. The committee, like the decadal survey, envisions a possible mission concept involving a Jupiter orbiter in eccentric orbit with multiple Io flybys and extensive monitoring at other times in its orbit. Mission-Specific Recommendations An Io Observer mission that addresses fundamental goals for solar system exploration may be possible. Consequently, an Io Observer mission should be included in the suite of possible missions included in the next New Frontiers announcement of opportunity. The mission should address some of the following science questions, which are not listed in order of priority. The committee, however, acknowledges that there are more objectives here than can be included in a single New Frontiers mission and leaves it to potential competitors to choose their science goals and defend their choices. Science objectives that could be addressed with an Io Observer mission can include the following: Determine the magnitude, spatial distribution, temporal variability, and dissipation mechanisms of Io’s tidal heating. Determine Io’s interior structure, such as whether it has a magma ocean. Determine whether Io has a magnetic field. Understand the eruption mechanisms for Io’s lavas and plumes and their implications for volcanic processes on Earth, especially early in Earth’s history when its heat flow was similar to Io’s, and elsewhere in the solar system. Investigate the processes that form Io’s mountains and the implications for tectonics under high-heat-flow conditions that may have existed early in the history of other planets. Understand Io’s surface chemistry, including volatiles and silicates, and derive magma compositions (and ranges thereof), crustal and mantle compositions and implications for the extent of differentiation, and contributions to the atmosphere, magnetosphere, and torus. Understand the composition, structure, and thermal structure of Io’s atmosphere and ionosphere, the dominant mechanisms of mass loss, and the connection to Io’s volcanism. These objectives are probably best addressed within a New Frontiers budget by a Jupiter-orbiting spacecraft with multiple Io flybys. It is possible that such a mission may exceed the New Frontiers cost cap—considering the results of NASA’s 2007 billion-dollar-box study of missions to Titan and Enceladus, which found that a Saturn orbiter with Enceladus flybys would probably cost well in excess of a billion dollars.59 Nevertheless, innovative approaches might be able to circumvent these problems and enable a capable New Frontiers Io Observer mission. GANYMEDE OBSERVER Large icy satellites may hold the key to answering many fundamental questions about the solar system, and Jupiter’s largest moon, Ganymede, is of particular interest because of its unique internal magnetic field and its interaction with Jupiter. Ganymede is the only icy body in the solar system known to generate its own magnetic field, thus providing a unique window into its interior and, moreover, shedding light on how internal magnetic fields are generated elsewhere in the solar system. Ganymede also provides a laboratory for the study of plasma effects on satellite surfaces: 59 See K. Reh, J. Elliott, T. Spilker, E. Jorgensen, J. Spencer, and R. Lorenz, Titan and Enceladus $1B Mission Feasibility Study Report, JPL D-37401 B, Jet Propulsion Laboratory, Pasadena, Calif., January 30, 2007.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity FIGURE 2.12 Ganymede, Jupiter’s largest moon, which has a mean radius of 2,631.2 kilometers. Like Europa, which was the highest-rated outer planets priority in the decadal survey, Ganymede is also believed to have a large subsurface ocean. SOURCE: Courtesy of NASA. the decadal survey notes that “Ganymede’s magnetic field is strong enough that it creates a mini-magnetosphere of its own in Jupiter’s magnetosphere, partially shielding the satellite from plasma bombardment. The interaction between Ganymede’s magnetosphere and Jupiter’s magnetosphere is similar to the interaction between Earth’s magnetosphere and the solar wind, where magnetic reconnection plays a key role.”60 Ganymede also exhibits evidence for a subsurface ocean. In contrast to Europa, an ocean in Ganymede may be bounded both above and below by ice rather than rock; nonetheless, it is likely to illuminate processes that may produce habitable environments elsewhere in the solar system (or maybe on Ganymede itself). Ganymede’s surface suggests a complex geologic history (see Figure 2.12) with similarities to those of Miranda and Enceladus. Moreover, some of its geologic terrains may be analogous to terrestrial features, thereby providing a bridge between silicate and icy bodies that could well provide fundamental information regarding the behavior of ice in geologic processes. Ganymede’s geologic activity and magnetic field are probably powered by tidal heating. The decadal survey states that “Ganymede’s differentiated interior and actively convecting core (required to generate its magnetic field) may be a consequence of its passage into resonance, while Callisto has not experienced this history” (pp. 121-122). Thus, better understanding of Ganymede could provide information about the tidal history of the entire Jovian system. 60 New Frontiers in the Solar System, p. 129.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity Background The decadal survey identified top-level, crosscutting science themes. While the survey did not specifically address how these crosscutting themes and key questions would be addressed by a Ganymede Observer mission, it is possible to map such a mission against many of these themes:61 The First Billion Years of Solar System History What processes marked the initial stages of planet and satellite formation? 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? 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? Volatiles and Organics: The Stuff of Life What is the history of volatile compounds, especially water, across the solar system? What global mechanisms affect the evolution of volatiles on planetary bodies? The Origin and Evolution of Habitable Worlds What planetary processes are responsible for generating and sustaining habitable worlds, and where are the habitable zones in the solar system? Processes: How Planetary Systems Work How do the processes that shape the contemporary character of planetary bodies operate and interact? The following excerpts from Table 5.2 of the decadal survey62 indicate questions that would be addressed by a Ganymede Observer: Questions on which a Ganymede Orbiter could offer a breakthrough-level advance Origin and Evolution of Satellite Systems How do conditions in the protoplanetary nebula influence the compositions, orbits, and sizes of the resulting satellites? Characterization of magnetic fields in satellites What does the magnetic field of Ganymede tell us about its thermal evolution, and is Ganymede unique? Internal magnetic fields Origin and Evolution of Water-Rich Environments in Icy Satellites What combination of size, energy sources, composition, and history produce long-lived internal oceans? Intrinsic magnetic field (past/present) Understanding Dynamic Planetary Processes What are the active interior processes and their relations to tidal heating, heat flow, and global patterns of volcanism and tectonism? Secular variations of magnetic field 61 New Frontiers in the Solar System, p. 3, Table ES.1. 62 New Frontiers in the Solar System, pp. 140-143.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity What are the complex processes and interactions on the surfaces and in volcanic or geyserlike plumes, atmospheres, exospheres, and magnetospheres? Atmospheric loss (fields and particles) Questions on which a Ganymede Orbiter could offer a major advance Origin and Evolution of Satellite Systems What affects differentiation, outgassing, and the formation of a thick atmosphere? (Why is Titan unique?) Production/loss rates To what extent are the surfaces of icy satellites coupled to their interiors (chemically and physically)? Geologic processes/ history (including impacts) Tectonics/volcanism Map surface composition Subsurface sounding Origin and Evolution of Water-Rich Environments in Icy Satellites What is the chemical composition of the water-rich phase? Remote and in situ composition observations What is the distribution of internal water, in space and in time? Geology/stratigraphy Subsurface sounding Internal structure What combination of size, energy sources, composition, and history produce long-lived internal oceans? Composition Internal Structure Can and does life exist in the internal ocean of an icy satellite? Characterization of surface radiation environment Characterization of chemistry of surface and ocean Life in extreme environments (Earth analogues) Transport processes Exploring Organic-Rich Environments What is the nature of organics on large satellites? Composition (elemental, isotopic, and molecular), remote and in situ Understanding Dynamic Planetary Processes What are the active interior processes and their relations to tidal heating, heat flow, and global patterns of volcanism and tectonism? Interior structure Heat flow and tidal heating Global volcanism and tectonism Questions on which a Ganymede Orbiter can offer a significant advance Origin and Evolution of Satellite Systems How do conditions in the protoplanetary nebula influence the compositions, orbits, and sizes of the resulting satellites? Interior structure and composition of (major) satellites Secular variation of orbital parameters
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity What affects differentiation, outgassing, and the formation of a thick atmosphere? (Why is Titan unique?) Atmospheric composition Interior structure and composition Characterization of internal heat sources To what extent are the surfaces of icy satellites coupled to their interiors (chemically and physically)? Transport processes How has the impactor population in the outer solar system evolved through time, and how is it different from the inner solar system? Observation of craters (on many different bodies) Geology/modification What does the magnetic field of Ganymede tell us about its thermal evolution, and is Ganymede unique? Plasma/ionospheric observation of external field Transport processes Origin and Evolution of Water-Rich Environments in Icy Satellites What is the distribution of internal water, in space and in time? Elemental and isotopic composition What combination of size, energy sources, composition, and history produce long-lived internal oceans? Geology Can and does life exist in the internal ocean of an icy satellite? Search for evidence of biology and organic compounds at surface and in the deeper interior Exploring Organic-Rich Environments What is the nature of organics on large satellites? Production/loss (radiation, degassing, escape, lightning, and exogenic/endogenic) Physical state Optical properties Reaction rates/kinetic information What are the processes currently affecting organic-rich surfaces? Impact processes Tectonic processes Chemical (and radiation) processes Understanding Dynamic Planetary Processes What are the currently active endogenic geologic processes (volcanism, tectonism, diapirism) and what can we learn about such processes in general from these active worlds? Observations of dynamic processes with high spatial and temporal resolution Composition of recent surface deposits, plumes or geysers, and atmospheres Search and discovery of new types of activity What are the complex processes and interactions on the surfaces and in volcanic or geyserlike plumes, atmospheres, exospheres, and magnetospheres? Dynamic of plumes, geysers, atmospheres, exospheres, and magnetospheres History of volatiles
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity Developments Since the Decadal Survey No white paper on Ganymede was submitted for the decadal survey; however, the science definition team for the Jupiter Icy Moons Orbiter mission (which was canceled in 2005 but included a long stay in Ganymede orbit) discussed Ganymede science goals in some detail.63 More recently, an extensive review of the science that could be accomplished at Ganymede by a flagship-class Ganymede orbiter has been published in the Jupiter System Observer Science Definition Team’s 2007 report, which established several goals relevant to a Ganymede Observer mission that are reprinted below:64 Goal, Magnetospheres: Understand the magnetospheric environments of Jupiter, its moons and their interactions Objective A. Moon Interior Structure. Establish internal structure of icy moons including presence and properties of putative conducting layers, measurement of higher harmonics and secular variations of Ganymede’s magnetic field and set limits on intrinsic magnetic fields for Europa and Callisto Objective B. Ganymede’s Intrinsic Magnetosphere. Investigate the magnetic field, particle populations, and dynamics of Ganymede’s magnetosphere Objective C. Moon-Magnetosphere Interactions. Determine the effect of the Jovian magnetosphere on the icy moons. Understand effects of the moons on the magnetosphere and Jupiter’s auroral ionosphere Goal, Satellites: Understand the mechanisms responsible for formation of surface features and implications for geological history, evolution, and levels of current activity Understand geologic history, potential for current activity, and the implications for Jupiter’s satellite system Understand the processes responsible for the observed geologic features Understand heat balance and tidal dissipation Goal, Satellites: Determine the surface compositions and implications for the origin, evolution and transport of surface materials Understand composition, physical characteristics, distribution, and evolution of surface materials Goal, Satellites: Determine the compositions, origins, and evolution of the atmosphere, including transport of material throughout the Jovian system Understand the sources (sublimation, surface sputtering) and sinks (freezing out, plasma pickup/sputtering, thermal escape) of atmospheric components Understand the temporal, spatial, and compositional variability of the atmosphere Goal, Interiors: Determine the interior structures and processes operating in the Galilean Satellites in relation to the formation and history of the Jupiter system and potential habitability of the moons. The study also established several goals at the investigation level for the interiors section: Characterize the formation and chemical evolution of the Jupiter system Place bounds on the orbital evolution of the satellites Determine the sizes and states of the cores of the moons Determine the presence and location of water within these moons Determine the extent of differentiation of the three icy satellites Establish the presence of oceans Characterize the extent and location of water (including brines) in 3D within Europa, Ganymede and Callisto Determine the thickness of the ice layer for all Icy Satellites Characterize the operation of magnetic dynamo processes in the Jovian system and their interaction with the surrounding magnetic field Globally characterize Ganymede’s intrinsic magnetic field and search for temporal variability in the field 63 R. Taylor, Prometheus Project Final Report, 982-R120461, NASA Jet Propulsion Laboratory, Pasadena, Calif., 2005. 64 National Aeronautics and Space Administration, Jupiter System Observer, Mission Study: Final Report, November 1, 2007, available at http://solarsystem.nasa.gov/multimedia/downloads.cfm.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity Characterize the interaction of Ganymede’s magnetosphere with Jupiter’s magnetosphere Identify the dynamical processes that cause internal evolution and near-surface tectonics of all four moons Determine the extent of differentiation of all four satellites Characterize the near-surface tectonic and volcanic processes and their relation to interior processes The committee notes that there have also been technological developments since the decadal survey that may make such a mission more feasible now than it was 5 years ago. In particular, radiation-hardened electronics have been developed that would be vital to a Ganymede mission, and the radiation environment in the Jovian system is much better understood now that data from Galileo have been fully analyzed. Also, the Juno mission to Jupiter demonstrates the feasibility of using a solar-powered satellite at Jupiter’s distance from the Sun, and NASA’s development of a Stirling engine could also help enable this mission. Conclusions The Galileo mission to Jupiter provided relatively limited information on Ganymede’s internal structure and magnetic field. Numerous fundamental questions about Ganymede remain—questions that bear on essential scientific objectives identified in the decadal survey. Thus, a mission to explore Ganymede in depth has great potential for substantial science return. Furthermore, the results from Galileo’s radiation data and the Jupiter System Orbiter demonstrate the spacecraft longevity possible at Ganymede’s distance from Jupiter; therefore, a platform located at Ganymede could also provide potential for long-term monitoring of other high-priority targets in the Jovian system. Finally, the success of the Juno mission illustrates the feasibility of solar power at Jupiter. A Ganymede orbiter was identified as a potential medium-class mission in the decadal survey, which stated: “No detailed studies are yet available, and the assumption that this mission could achieve the stated goals within this cost category rests partially on assuming that the lesser radiation environment and heritage from the Europa Geophysical Explorer mission would allow significantly reduced costs.”65 The development of the Juno mission and the more recent NASA Science Definition Team investigation of the flagship-class Jupiter System Observer could produce a mission that ultimately would achieve orbit around Ganymede, characterizing its surface in detail as well as its gravity and magnetic fields, thereby accomplishing a multitude of science objectives. However, the committee is concerned whether a spacecraft orbiting Ganymede would be feasible under New Frontiers budgetary constraints, given the results of NASA’s billion-dollar-box study in 2007.66 Nonetheless, such a rich array of fundamental science questions can be addressed at Ganymede that a New Frontiers mission that focuses on answering a subset of these questions would be very worthwhile. The committee concluded that a spacecraft going into Ganymede orbit may not be required. If a significant number of such questions can be addressed without the spacecraft going into Ganymede orbit, significant cost savings may ensue, which would more easily accommodate the New Frontiers cost caps. In addition, such a mission would enable broader goals within the Jovian system. Mission-Specific Recommendations A Ganymede Observer mission that addresses fundamental goals for solar system exploration may be possible and would also enable broader goals within the Jovian system. Consequently, such a mission should be included in the next New Frontiers announcement of opportunity. Because the Ganymede Observer was not described in significant detail in the decadal survey, the committee chose to list science objectives that such a mission could address, but stresses that this list should not be exclusive. In no case should these science questions be considered to be mission requirements—they are merely options for such a mission. This list includes far more science than can be included in a single New Frontiers mission and the committee stresses that it fully expects those proposing 65 New Frontiers in the Solar System, p. 133. 66 See K. Reh, J. Elliott, T. Spilker, E. Jorgensen, J. Spencer, and R. Lorenz, Titan and Enceladus $1B Mission Feasibility Study Report, JPL D-37401 B, Jet Propulsion Laboratory, Pasadena, Calif., January 30, 2007.
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity such a mission to choose among these science objectives. It will be up to the proposers to make the case as to why some science objectives are more important than others. These objectives, which are not prioritized, include: Understand Ganymede’s intrinsic and induced magnetic fields and how they are generated, and characterize their interaction with Jupiter’s magnetic field. Determine Ganymede’s internal structure, especially the depths to and sizes or thicknesses of the probable metallic core and deep liquid water ocean, and the implications for current and past tidal heating and the evolution of the Galilean satellite system as well as ocean chemistry. Understand Ganymede’s endogenic geologic processes, e.g., the extent and role(s) of cryovolcanism, the driving mechanism for the formation of the younger, grooved terrain, and the extent to which Ganymede’s tectonic processes are analogs for tectonics on other planetary bodies (both icy and silicate). Document the non-ice materials on Ganymede’s surface and characterize in detail the connection between Ganymede’s magnetosphere and its surface composition (e.g., polar caps). Document the composition and structure of the atmosphere, identifying the sources and sinks of the atmospheric components and the extent of variability (spatial and/or temporal). Under a New Frontiers budget it is likely that the most feasible way to address these objectives is by a Jupiter-orbiting spacecraft with multiple Ganymede flybys—in other words, the spacecraft may not have to enter Ganymede orbit. Even so, it is possible that such a mission may exceed the New Frontiers cost cap. Nevertheless, innovative approaches might be able to circumvent these problems and enable fundamental Ganymede science under New Frontiers constraints. INNOVATIVE MISSION OPTIONS During the course of this study, the committee was impressed with the abilities of those competing in both the Discovery and New Frontiers programs to develop innovative ideas about how to accomplish their proposed missions. Missions that were considered nearly impossible less than two decades ago—like a solar-powered spacecraft at Jupiter or a Mercury orbiter—can now be done because clever solutions have been developed by principal investigators. The committee believes that this is a strength of the competitive process that will increase the probability that NASA will receive New Frontiers proposals that are realistic and doable within the constraints of the program. The committee was also impressed with arguments it heard about the importance of innovation and the risks of being overly specific regarding how to accomplish the goals of the decadal survey. Thus, in addition to the eight missions identified by the committee in this report, NASA should offer an additional option for other missions in the same size class that offer compelling answers to high-priority science questions from the decadal survey. The committee heard of several proposals for missions in the New Frontiers class that were not explicitly drawn from the decadal survey. Although the committee is not recommending any of these missions for the next New Frontiers announcement of opportunity, it is unwilling to explicitly rule them out. In order for the New Frontiers Program to remain healthy over the long run, it must maintain an influx of new ideas and a growing applicant pool for new missions. Finally, as the sections above on the eight proposed missions demonstrate, scientific understanding of the solar system has continued to advance since the decadal survey. Thus, there may be new science to be explored that was not included in the survey and may be viable as the basis for a New Frontiers mission. Thus, the committee concluded that NASA’s next New Frontiers announcement of opportunity should not be strictly limited to the eight mission options discussed in detail above, but should also be open to proposals with extraordinary justification and inventiveness. This was the foundation for the committee’s third top-level recommendation: Recommendation 3: NASA should consider mission options outside the three remaining and five additional medium-size missions described in the decadal survey that are spurred by major scientific and technological developments made since the decadal survey. As with any New Frontiers mission, these proposals must
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Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity offer the potential to dramatically advance fundamental scientific goals of the decadal survey and should accomplish scientific investigations well beyond the scope of the smaller Discovery Program. Both mission-enabling technological advances and novel applications of current technology could be considered. However, NASA should limit its choices to the eight specific candidate missions unless a highly compelling argument can be made for an outside proposal.