Grading NASA's Solar System Exploration Program: A Midterm Report (2008)

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FIGURE 2.1  Analysis of a particle from the Stardust mission. SOURCE: NASA. 

2 Science The National Research Council’s (NRC’s) decadal survey New Frontiers in the Solar System established an ambitious and comprehensive set of science objectives for solar system exploration. In the present midterm review, the Committee on Assessing the Solar System Exploration Program evaluates the progress being made in addressing these objectives, assessing NASA’s mission portfolio, concept studies, research and analysis programs, Earth-based observing programs, and supported laboratory science, and the degree to which each has addressed the decadal science objectives. For each science objective, the committee provides a grade based on current progress toward the science objective, along with an indication of the trend in progress foreseen over the remaining 5 years of the period (2003-2013) covered by the decadal survey. (The grades and trends are defined in Chapter 1.) Science Questions OVERALL ASSESSMENT: Grade: B Trend: ➜ Overall, NASA is making impressive progress toward many of the decadal survey science goals, with con- tinued gains foreseen over the next 5 years. NASA’s Mars program in particular has been highly successful, with a comprehensive and detailed plan of investigations aimed at high-priority science objectives. Also, tremendous progress has been made in understanding the primitive, nonplanetary bodies in the solar system. The committee gives this area a grade of B. Despite its evident importance, however, NASA has not significantly addressed the primary goal of astrobiol- ogy (life detection), and progress toward other objectives has been slowed by the steep reduction in astrobiology research and analysis funding. Also, although it is beyond the horizon of the decadal survey, the committee notes that there is a large and growing gap between missions to the giant planets once the Juno mission (currently scheduled for launch in 2011) is completed. Because of these reductions, the committee debated assessing the area of meeting the science goals of the decadal survey as having a downward trend, but concluded that “No change” is more appropriate for now. As presented in Summary Table S.1, the crosscutting themes of the decadal survey form the titles of the sec- tions below, and each key question is shown together with the grade and trend for activity in that arrea as assessed by the committee. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Wash- ington, D.C., 2003. 19 

20 GRADING NASA’S SOLAR SYSTEM EXPLORATION PROGRAM THE FIRST BILLION YEARS OF SOLAR SYSTEM HISTORY New Frontiers:  Key Question No. 1 Results of Midterm Review “What processes marked the initial stages of planet and satellite formation?” Grade: B Trend: ➜ (p. 3) NASA has made significant progress toward answering this question through a series of Discovery missions to comets; the missions include Stardust and Deep Impact (another Discovery mission, CONTOUR, or Comet Nucleus Tour, was lost in 2002). Missions such as Dawn and mission concepts to asteroids will extend the under- standing of the parts of the solar system that did not go through the planetary formation process. The Discovery mission of opportunity (MoO) EPOXI and Stardust New Exploration of Tempel 1 (NExT) will extend the utility of existing assets (Deep Impact and Stardust) to provide additional investigations into other small bodies or into surface processes on a recently impacted comet. Laboratory study of meteoritic material and Earth-based observations of primitive bodies in the solar system and of extrasolar planets and circumstellar nebulae provide constraints on the evolution of solids in the early solar system. The New Frontiers mission line will address this question of the processes marking the initial stages of planet and satellite formation through support of operations of the New Horizons spacecraft on its way to study the composition and dynamics of Pluto (arrival in 2015), and through the Juno mission to study the interior of Jupiter (launch in 2011). Upcoming MESSENGER flybys of Mercury will also add to the understanding of the planet by imaging areas of Mercury that have not yet been seen. Two of the future New Frontiers mission concepts, South Pole-Aitken Basin Sample Return from the Moon and Comet Surface Sample Return (CSSR), would both make major contributions to this objective. Finally, the Lunar Reconnaissance Orbiter (LRO) and Moon Mineralogy Mapper (M3, a Discovery MoO) would make significant observations of the structure and composition of the Moon’s surface. NASA’s mix of current and future missions has made significant progress toward achieving the goal outlined in the decadal survey and may meet or exceed the objective by the end of the decade (2013). New Frontiers:  Key Question No. 2 Results of Midterm Review “How long did it take the gas giant Jupiter to form, and how was the formation Grade: C Trend: ➜ of the ice giants (Uranus and Neptune) different from that of Jupiter and its gas giant sibling, Saturn?” (p. 3) This is a giant-planet-specific objective requiring missions to these bodies. The ongoing Cassini-Huygens flagship mission has contributed significantly toward the understanding of giant planets, and the approved 2-year extension (while shorter than requested by the decadal survey) will further the understanding of the composition and dynamics of Saturn. The Juno mission will take another important step by investigating the deep interior of Jupiter. The committee notes that all of the flagship missions currently under study are to the giant planets, and it encourages NASA to include characterization of the giant planets themselves in these studies, along with investiga- tion of the satellite targets. Some progress has been made toward understanding Saturn in particular, and Juno will enable a significant advance in the scientific understanding of Jupiter. But the relationship of the gas giants to the ice giants Uranus and Neptune has been left unexplored. The decadal survey identified a Neptune/Triton mission as being of great interest in the next decade, and the committee recommends that the next decadal survey address this mission concept. The flagship mission studies that NASA sponsored in 2006-2007 are highly encouraging, but any such mission is still at least a decade from launch, resulting in a significant (and growing) gap in time after the completion of the Juno mission before the next giant-planet mission can arrive. Recommendation:  The next solar system exploration decadal survey should address the objectives and merits of a Neptune/Triton mission. The acronym EPOXI combines Extrasolar Planet Observations and Characterization (EPOCh) and Deep Impact eXtended Investigation (DIXI). 

SCIENCE 21 New Frontiers:  Key Question No. 3 Results of Midterm Review “How did the impactor flux decay during the solar system’s youth, and in what Grade: B Trend: ➜ way(s) did this decline influence the timing of life’s emergence on Earth?” (p. 3) NASA is undertaking or studying many missions that will investigate the heavily cratered surfaces of Mercury (MESSENGER), the Moon (M3, LRO), asteroids (Dawn), and certain inactive bodies in the outer solar system (Cassini-Huygens, New Horizons, flagship mission studies) that reveal to varying degrees the early history of the impactor flux across the solar system. Lunar sample return (South Pole-Aitken Basin) will help improve understand- ing of the history of large impacts on the Moon and the impact process itself; the Moon also constrains the impact history on Earth. Visits to comets (Stardust, Deep Impact, EPOXI, Stardust NExT) and asteroids (Dawn) help characterize surviving members of the impactor population. Earth-based observations of near-Earth objects (NEOs) constrain the present population and provide constraints for theoretical studies of the dynamics and evolution of such populations. The effects of the impact processes on the origin and evolution of life have been investigated through the astrobiology research and analysis program. The mix of present and future missions has resulted in significant progress in characterizing the past and current impactor flux, with much more to be learned in the near future. But progress in understanding the effects of the impactor flux on life is threatened by cuts to astrobiology research and analysis funding. Continued research efforts in astrobiology will be needed to fully understand how impact processes would have influenced the origin of life on Earth. VOLATILES AND ORGANICS: THE STUFF OF LIFE New Frontiers:  Key Question No. 4 Results of Midterm Review “What is the history of volatile compounds, especially water, across the solar Grade: A Trend: ➜ system?” (p. 3) This objective is addressed very strongly by NASA’s mission portfolio for 2003-2013. The Mars program’s “Follow the water” strategy has led to a series of ongoing and planned missions (Odyssey, Express, Reconnais- sance Orbiter, Exploration Rovers, Mars Science Laboratory, the Phoenix Scout mission, the Analyzer of Space Plasma and Energetic Atoms [ASPERA]-3 Discovery MoO, and studies of the Astrobiology Field Laboratory, Mars Science Orbiter, and Mars Sample Return) investigating the processes that affect water. Missions to comets (Stardust, Deep Impact, EPOXI, NExT), asteroids (Dawn), and primitive bodies (New Horizons) constrain the original distribution of water and other volatiles through the solar system. Investigation of the outer planets and their icy satellites by Juno, Cassini-Huygens, and the flagship missions constrain the evolution of water and other volatiles during giant-planet and satellite formation. Missions to characterize the Venus atmosphere and surface, although currently only under study and not yet funded, can search for the signature of past water on Earth’s sister planet. The MESSENGER mission will seek evidence of ice at Mercury’s poles, while closer to home, planned lunar missions (M3, LRO) will address the presence of ice at the lunar poles. Earth-based observations of comets and Kuiper Belt objects provide constraints for theoretical models of the distribution and evolution of volatiles in the solar system. NASA has “followed the water” all over the solar system. The result has been a tremendous increase in the understanding of the distribution and evolution of water and other volatiles. New Frontiers:  Key Question No. 5 Results of Midterm Review “What is the nature of organic material in the solar system and how has this Grade: B Trend: ➜ matter evolved?” (p. 3) The NASA mission portfolio is well composed to address this objective. Significant contributions to the understanding of the nature, distribution, and evolution of organic material in the solar system have been and will continue to be made by missions to comets (Stardust, Deep Impact, EPOXI, NExT), asteroids (Dawn), and primi- tive bodies (New Horizons). Cassini-Huygens has provided valuable new data on the composition and dynamics 

22 GRADING NASA’S SOLAR SYSTEM EXPLORATION PROGRAM of Saturn, Titan, and the other Saturnian satellites including active Enceladus. The outer-planet flagship mission studies would also contribute to the understanding of organics in the outer solar system. A Mars mission has pos- sibly detected methane in the martian atmosphere, and future in situ measurements (Mars Science Laboratory and Phoenix) will be made of this and more complex organic compounds. Excellent progress has already been made, and NASA seems poised to make great strides toward and beyond this goal in the near future. However, the committee gives progress toward this recommendation a grade of B because neither Mars’s methane nor Titan’s organics have been well studied. New Frontiers:  Key Question No. 6 Results of Midterm Review “What global mechanisms affect the evolution of volatiles on planetary bodies?” Grade: B Trend: ➜ (p. 3) This is a broad goal that seeks to understand the processes that drive the evolution of volatiles on planetary bodies. Past, current, and planned missions are relevant to answering this question. Several missions have visited comets, providing constraints on the sources of planetary volatiles, while Dawn will be visiting an ice-rich asteroid (Ceres). Significant progress has been made in understanding the climate and hydrology of Mars, and the ASPERA- 3 instrument and the next Mars Scout selection (Mars Atmosphere and Volatile Evolution [MAVEN] mission or Great Escape) will study the processes of volatile evolution in the martian upper atmosphere. Cassini-Huygens has made great progress in contributing to understanding of the evolution of volatiles on icy satellites, including the astounding discovery of water geysers erupting from Enceladus (see Figure 2.2). MESSENGER will also add to knowledge about volatiles on Mercury. Not yet approved or funded, a mission to study the Venus atmosphere would extend knowledge of processes affecting the evolution of volatiles on terrestrial planets. THE ORIGIN AND EVOLUTION OF HABITABLE WORLDS New Frontiers:  Key Question No. 7 Results of Midterm Review “What planetary processes are responsible for generating and sustaining Grade: A Trend: ➜ habitable worlds, and where are the habitable zones in the solar system?” (p. 3) The Mars program has contributed substantially to answering this question with a comprehensive strategy of orbital, atmospheric, and in situ investigations. Expanding current understanding of the locations of habitable zones requires the study of other parts of the solar system. Cassini-Huygens is making progress toward this objective among the icy satellites of the giant planets. In particular, the Cassini-Huygens exploration of Titan and Enceladus has had an important impact on this question. The exploration of comets (Stardust, Deep Impact, EPOXI, NExT), asteroids (Dawn), and primitive bodies (New Horizons) increases the understanding of the building blocks from which Earth was constructed. NASA is currently conducting studies of flagship missions to the outer planets that could also contribute to addressing this question if a flagship mission is funded. The study of Venus would provide a compelling contrast to the generation of the habitable planet Earth. Astrobiology research and analysis is also a critical component of this objective, and recent cuts to that program threaten progress. The broad range of current and potential, but not yet approved, future missions addressing this objective meets the goals of the decadal survey for this decade and should continue into the next. 

SCIENCE 23 FIGURE 2.2  Saturn’s moon Enceladus as viewed by the Cassini spacecraft. Barely visible in this image is the plume of ice particles jetting from the tiny moon’s south pole. SOURCE: NASA. 

24 GRADING NASA’S SOLAR SYSTEM EXPLORATION PROGRAM New Frontiers:  Key Question No. 8 Results of Midterm Review “Does (or did) life exist beyond Earth?” (p. 3) Grade: C Trend: ➜ This objective is separated from the habitability objective and is narrowly focused on life and biosignature detection. Of all the missions within the decade 2003-2013, only the Mars Science Lander will attempt to measure biosignatures elsewhere in the solar system, although the Phoenix mission will also address this question through a search for complex organics. Mission studies of two future Mars missions (Astrobiology Field Laboratory and Mars Sample Return) will contribute to addressing this question in the next decade. No measurements elsewhere in the solar system are under study, and the lack of a Europa mission has also had a negative impact on the ability to answer this question. Instrument development for such measurements is essential, but the astrobiology instrument development programs (Astrobiology Science and Technology Instrument Development, or ASTID, and Astro- biology Science and Technology for Exploring Planets, or ASTEP) have experienced decreased funding, with no new selections since 2004. Some development of life and biosignature detection instrumentation has been funded through the Planetary Instrument Definition and Development Program (PIDDP) since then. But decreased fund- ing for ASTID and ASTEP will negatively impact future mission planning and delay progress in the next decade. Although the Mars program has an integrated strategy for achieving this objective at Mars in this decade and the next, there is need for a comprehensive strategy to address this question throughout the solar system. Recommendation:  NASA should return funding for the Astrobiology Science and Technology Instrument Development program and the Astrobiology Science and Technology for Exploring Planets program to at least their individual Planetary Instrument Definition and Development levels. However, this should not be accomplished to the detriment of the astrobiology research and analysis program, which has already suffered large cutbacks. New Frontiers:  Key Question No. 9 Results of Midterm Review “Why have the terrestrial planets differed so dramatically in their evolutions?” Grade: A Trend: ➜ (p. 3) The Mars program has significantly improved the understanding of that terrestrial planet, and MESSENGER is on its way to greatly increasing our knowledge of the smallest terrestrial planet, leaving Venus as the remaining terrestrial body to be investigated. The strong Mars program and the MESSENGER study of Mercury are the basis for the grade of A and the assessment of an upward trend. There is one Venus mission under study at present; if selected, it will close a gap in the knowledge of the terrestrial planets. A Venus mission is also a possible candidate for the next New Frontiers Announcement of Opportunity. The ASPERA instrument is currently carried on the European Space Agency’s Venus Express. If the NASA Venus missions are not funded, it would negatively affect the current trend in the ability to answer this question. Missions to the Moon (LRO) and the largest asteroids (Dawn) also inform us about processes that were important in the early history of the terrestrial planets. The Mars Long-Lived Lander Network (ML3N) study will address interior processes on that planet in a future decade. Significant progress has been made across the terrestrial planets by current missions, and a number of future missions to address this question are under study. The limited data on Venus must be augmented in order to allow an understanding of the origin and evolution of the terrestrial planets. If one of several Venus missions under study at present is selected, the program would significantly advance such studies. The terms “biosignatures” and “biomarkers” are used interchangeably throughout this report to refer to evidence of biological activity, past or present. 

SCIENCE 25 New Frontiers:  Key Question No. 10 Results of Midterm Review “What hazards do solar system objects present to Earth’s biosphere?” (p. 3) Grade: B Trend: ➜ The present hazards posed to Earth’s biosphere by solar system objects have been partially characterized by the ongoing NASA Near Earth Object Observing Program (Spaceguard). Additionally, investigations into recent lunar impacts by planned lunar missions (Moon Mineralogy Mapper, LRO) will constrain the impact flux in Earth’s neighborhood over the recent past. Many of the missions that address the early history of the impactor flux (the third in this series of science questions) contribute to this objective. Investigation of the effect of impacts on the biosphere has been slowed by the reduced funding for astrobiology research and analysis. But significant progress has been made, and Spaceguard and the planned lunar missions should allow NASA to meet the objective outlined in the decadal survey. This progress is the basis for the committee’s assessment of an upward trend. PROCESSES: HOW PLANETARY SYSTEMS WORK New Frontiers:  Key Question No. 11 Results of Midterm Review “How do the processes that shape the contemporary character of planetary bodies Grade: B Trend: ➜ operate and interact?” (p. 3) This is an extremely broad science question that extends across the solar system in time and space. NASA’s overall mission portfolio, extending from Mercury to Pluto to extrasolar planets and from the solar wind to primitive bodies to giant planets, is well constructed to address this question. The current missions have made significant progress, and the second half of the decade promises many more advances. Nonetheless, the part of this objective related to how processes interact requires comprehensive investigations with multiple mission architectures (for example, orbiters, landers, rovers) building on one another at multiple destinations. The Mars program provides an excellent example for this type of comprehensive investigation, but a solar-system-wide strategy, so vital for such a broad objective, is lacking. NASA’s broad array of research and analysis programs is well constructed to address such a broad question, but recent across-the-board funding cuts have affected progress. New Frontiers:  Key Question No. 12 Results of Midterm Review “What does the solar system tell us about the development and evolution of Grade: B Trend: ➜ extrasolar planetary systems, and vice versa?” (p. 3) The observational campaigns for detecting and characterizing exoplanets have provided a wealth of information and have made great progress toward this objective. Studies of extrasolar planetary systems have determined the distributions of giant-planet masses, orbital periods, and eccentricities. Theoretical analysis of these data is now illuminating the physical processes responsible for these observed distributions of system characteristics. Applica- tion of these physical processes to our own solar system is significantly improving the understanding of the early phases of solar system exploration. The collaboration between NASA and the National Science Foundation in this area is encouraging, but the committee notes the absence of a specific research and analysis program for extrasolar planet research. The Kepler Discovery-class mission scheduled for launch in 2008 will directly address this ques- tion by studying transiting extrasolar planets; this mission offers great hope for progress in the future. Scientists are making slower progress toward understanding our own planetary system, particularly the giant planets that are most like those which scientists detect around other stars. The committee is also concerned about the future, after the conclusion of the Juno mission in the second half of the next decade. The essential cancellation of the Stellar Interferometry Mission and the indefinite postponement of the Terrestrial Planet Finder are events that prevent the committee from declaring an upward trend in this area.