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Executive Summary In recent years, planetary science has seen a tremendous growth in new knowledge. Deposits of water ice exist at the Moon’s poles. Discoveries on the surface of Mars point to an early warm, wet climate and perhaps conditions under which life could have emerged. Liquid methane rain falls on Saturn’s moon Titan, creating rivers, lakes, and geologic landscapes with uncanny resemblances to Earth’s. Comets impact Jupiter, producing Earth-size scars in the planet’s atmosphere. Saturn’s poles exhibit bizarre geometric cloud patterns and changes; its rings show processes that may help us understand the nature of planetary accretion. Venus may be volcanically active. Jupiter’s icy moons harbor oceans below their ice shells: conceivably Europa’s ocean could support life. Saturn’s tiny moon Enceladus has enough geothermal energy to drive plumes of ice and vapor from its south pole. Dust from comets shows the nature of the primitive materials from which the planets and life arose. And hundreds of new planets discovered around nearby stars have begun to reveal how the solar system fits into a vast collection of other planetary systems. This report was requested by the National Aeronautics and Space Administration (NASA) and the National Sci- ence Foundation (NSF) to review the status of planetary science in the United States and to develop a comprehensive strategy that will continue these advances in the coming decade. Drawing on extensive interactions with the broad planetary science community, the report presents a decadal program of science and exploration with the potential to yield revolutionary new discoveries. The program will achieve long-standing science goals with a suite of new missions across the solar system. It will provide fundamental new scientific knowledge, engage a broad segment of the planetary science community, and have wide appeal for the general public whose support enables the program. A major accomplishment of the program recommended by the Committee on the Planetary Science Decadal Survey will be taking the first critical steps toward returning carefully selected samples from the surface of Mars. Mars is unique among the planets in having experienced processes comparable to those on Earth during its forma- tion and evolution. Crucially, the martian surface preserves a record of earliest solar system history, on a planet with conditions that may have been similar to those on Earth when life emerged. It is now possible to select a site on Mars from which to collect samples that will address the question of whether the planet was ever an abode of life. The rocks from Mars that we have on Earth in the form of meteorites cannot provide an answer to this question. They are igneous rocks, whereas recent spacecraft observations have shown the occurrence on Mars of chemical sedimentary rocks of aqueous origin, and rocks that have been aqueously altered. It is these materials, none of which are found in meteorites, that provide the opportunity to study aqueous environments, potential prebiotic chemistry, and perhaps, the remains of early martian life. 1

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2 VISION AND VOYAGES FOR PLANETARY SCIENCE If NASA’s planetary budget is augmented, then the program will also carry out the first in-depth exploration of Jupiter’s icy moon Europa. This moon, with its probable vast subsurface ocean sandwiched between a potentially active silicate interior and a highly dynamic surface ice shell, offers one of the most promising extraterrestrial habitable environments in the solar system and a plausible model for habitable environments outside it. The Jupiter system in which Europa resides hosts an astonishing diversity of phenomena, illuminating fundamental planetary processes. While Voyager and Galileo taught us much about Europa and the Jupiter system, the relatively primitive instrumentation of those missions, and the low volumes of data returned, left many questions unanswered. Major discoveries surely remain to be made. The first step in understanding the potential of the outer solar system as an abode for life is a Europa mission with the goal of confirming the presence of an interior ocean, characterizing the satellite’s ice shell, and enabling understanding of its geologic history. The program will also break new ground deep in the outer solar system. The gas giants Jupiter and Saturn have been studied extensively by the Galileo and Cassini missions, respectively. But Uranus and Neptune represent a wholly distinct class of planet. While Jupiter and Saturn are made mostly of hydrogen, Uranus and Neptune have much smaller hydrogen envelopes. The bulk composition of these planets is dominated instead by heavier elements: oxygen, carbon, nitrogen, and sulfur are the likely candidates. What little we know about the internal structure and composition of these “ice giant” planets comes from the brief flybys of Voyager 2. The ice giants are thus one of the great remaining unknowns in the solar system, the only class of planet that has never been explored in detail. The proposed program will fill this gap in knowledge by initiating a mission to orbit Uranus and put a probe into the planet’s atmosphere. It is exploration in the truest sense, with the same potential for new discoveries such as those achieved by Galileo at Jupiter and Cassini at Saturn. The program described in this report also vigorously continues NASA’s two programs of competed planetary missions: New Frontiers and Discovery. It includes seven recommended candidate New Frontiers missions from which NASA will select two for flight in the coming decade. These New Frontiers candidates cover a vast sweep of exciting planetary science questions: the surface composition of Venus, the internal structure of the Moon, the composition of the lunar mantle, the nature of Trojan asteroids, the composition of comet nuclei, the geophysics of Jupiter’s volcanic moon Io, and the structure and detailed composition of Saturn’s atmosphere. And continuation of the highly successful Discovery program, which involves regular competitive selections, will provide a steady stream of scientific discoveries from small missions that draw on the full creativity of the science community. Space exploration has become a worldwide venture, and international collaboration has the potential to enrich the program in ways that will benefit all participants. The program therefore relies more strongly than ever before on international participation, presenting many opportunities for collaboration with other nations. Most notably, the ambitious and complex Mars Sample Return campaign is critically dependent on a long-term and enabling collaboration with the European Space Agency (ESA). To assemble this program, the committee used four criteria for selecting and prioritizing missions. The first and most important was science return per dollar. Science return was judged with respect to the key science ques- tions identified by the planetary science community; costs were estimated via a careful and conservative procedure that is described in detail in the body of this report. The second was programmatic balance—striving to achieve an appropriate balance among mission targets across the solar system and an appropriate mix of small, medium, and large missions. The other two were technological readiness and availability of trajectory opportunities within the 2013-2022 time period. To help in developing its recommendations, the committee commissioned technical studies of many candidate missions that were selected for detailed examination on the basis of white papers contributed by the scientific community. Using the four prioritization criteria listed above, the committee chose a subset of the studied mis- sions for independent assessments of technical feasibility, as well as conservative estimates of costs. From these, the committee finalized a set of recommended missions intended to achieve the highest-priority science identified by the community within the budget resources projected to be available. The committee’s program consists of a balanced mix of small Discovery missions, medium-size New Frontiers missions, and large “flagship” missions, enabling both a steady stream of new discoveries and the capability to address major challenges. The mission rec- ommendations assume full funding of all missions that are currently in development, and continuation of missions that are currently in flight, subject to approval obtained through the appropriate review process.

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3 EXECUTIVE SUMMARY SMALL MISSIONS Missions for NASA’s Discovery program lie outside the bounds of a decadal strategic plan, and so this report makes no recommendations on specific Discovery flight missions. The committee emphasizes, however, that the Discovery program has made important and fundamental contributions to planetary exploration and can continue to do so in the coming decade. Because there is still so much compelling science that can be addressed by Discovery missions, the committee recommends continuation of the Discovery program at its current level, adjusted for inflation, with a cost cap per mission that is also adjusted for inflation from the current value (i.e., to about $500 million in fiscal year [FY] 2015). And so that the science community can plan Discovery missions effectively, the committee recommends a regular, predictable, and preferably rapid (≤24-month) cadence for release of Discovery Announcements of Opportunity and for selection of missions. An important small mission that lies outside the Discovery program is the proposed joint ESA-NASA Mars Trace Gas Orbiter that would launch in 2016. The committee supports flight of this mission as long as the currently negotiated division of responsibilities and costs with ESA is preserved. MEDIUM MISSIONS The current cost cap for NASA’s competed New Frontiers missions, inflated to FY2015 dollars, is $1.05 bil- lion, including launch vehicle costs. The committee recommends changing the New Frontiers cost cap to $1.0 billion FY2015, excluding launch vehicle costs. This change represents a modest increase in the effective cost cap and will allow a scientifically rich and diverse set of New Frontiers missions to be carried out, and will help protect the science content of the New Frontiers program against increases and volatility in launch vehicle costs. Two New Frontiers missions have been selected by NASA to date, and a third selection was underway while this report was in preparation. The committee recommends that NASA select two additional New Frontiers missions in the decade 2013-2022. These are referred to here as New Frontiers Mission 4 and New Frontiers Mission 5. New Frontiers Mission 4 should be selected from among the following five candidates: • Comet Surface Sample Return, • Lunar South Pole-Aitken Basin Sample Return, • Saturn Probe, • Trojan Tour and Rendezvous, and • Venus In Situ Explorer. No relative priorities are assigned to these five candidates; instead, the selection among them should be made on the basis of competitive peer review. If the third New Frontiers mission selected by NASA addresses the goals of one of these mission candidates, the corresponding candidate should be removed from the above list of five, reducing to four the number from which NASA should make the New Frontiers Mission 4 selection.* For the New Frontiers Mission 5 selection, the following missions should be added to the list of remaining candidates: • Io Observer, and • Lunar Geophysical Network. Again, no relative priorities are assigned to any of these mission candidates. Tables ES.1 and ES.2 summarize the recommended mission candidates and decision rules for the New Frontiers program. * On May 25, 2011, following the completion of this report, NASA selected the OSIRIS-REx asteroid sample-return spacecraft as the third New Frontiers mission. Launch is scheduled for 2016.

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4 VISION AND VOYAGES FOR PLANETARY SCIENCE TABLE ES.1 Medium-Class Missions—New Frontiers 4 (in alphabetical order) Mission Recommendation Science Objectives Key Challenges Chapter Comet Surface Sample • Acquire and return to Earth for laboratory analysis a • Sample acquisition 4 macroscopic (≥500 cm3) comet nucleus surface sample Return • Mission design • Characterize the surface region sampled • System mass • Preserve sample complex organics Same as 2003 decadal surveya Lunar South Pole-Aitken Not evaluated by 5 Basin Sample Return decadal survey Saturn Probe • Determine noble gas abundances and isotopic ratios of • Entry probe 7 hydrogen, carbon, nitrogen, and oxygen in Saturn’s atmosphere • Payload requirements • Determine the atmospheric structure at the probe descent growth location Trojan Tour and Visit, observe, and characterize multiple Trojan asteroids • System power 4 Rendezvous • System mass Same as 2003 decadal surveya (and amended by 2008 NRC Venus In Situ Explorer Not evaluated by 5 report Opening New Frontiersb) decadal survey NOTE: On May 25, 2011, following the completion of this report, NASA selected the OSIRIS-REx asteroid sample-return spacecraft as the third New Frontiers mission. Launch is scheduled for 2016. a National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003. b National Research Council, Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity, The National Academies Press, Washington, D.C., 2008. LARGE MISSIONS The highest-priority flagship mission for the decade 2013-2022 is the Mars Astrobiology Explorer-Cacher (MAX-C), which will begin a three-mission NASA-ESA Mars Sample Return campaign extending into the decade beyond 2022. At an estimated cost of $3.5 billion as currently designed, however, MAX-C would take up a dispro- portionate share of NASA’s planetary budget. This high cost results in large part from the goal to deliver two large and capable rovers—a NASA sample-caching rover and the ESA’s ExoMars rover—using a single entry, descent, and landing (EDL) system derived from the Mars Science Laboratory (MSL) EDL system. Accommodation of two such large rovers would require major redesign of the MSL EDL system, with substantial associated cost growth. The committee recommends that NASA fly MAX-C in the decade 2013-2022, but only if the mission can be conducted for a cost to NASA of no more than approximately $2.5 billion FY2015. If a cost of no more than about $2.5 billion FY2015 cannot be verified, the mission (and the subsequent elements of Mars Sample Return) should be deferred until a subsequent decade or canceled. It is likely that a significant reduction in mission scope will be needed to keep the cost of MAX-C below $2.5 billion. To be of benefit to NASA, the Mars exploration partnership with ESA must involve ESA participa- tion in other missions of the Mars Sample Return campaign. The best way to maintain the partnership will be an equitable reduction in scope of both the NASA and the ESA objectives for the joint MAX-C/ExoMars mission, so that both parties still benefit from it. The second-highest-priority flagship mission for the decade 2013-2022 is the Jupiter Europa Orbiter (JEO). However, its cost as JEO is currently designed is so high that both a decrease in mission scope and an increase in NASA’s planetary budget are necessary to make it affordable. The projected cost of the mission as currently

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5 EXECUTIVE SUMMARY TABLE ES.2 Medium-Class Missions—New Frontiers 5 (in alphabetical order) Mission Recommendation Science Objectives Key Challenges Decision Rules Chapter Comet Surface Sample See Table ES.1 See Table ES.1 Remove if selected 4 Return for NF-4 Io Observer Determine internal structure of • Radiation None 8 Io and mechanisms contributing • System power to Io’s volcanism Lunar Geophysical Enhance knowledge of the • Propulsion None 5 Network lunar interior • Mass • Reliability • Mission operations Same as 2003 decadal surveya Lunar South Pole-Aitken Not evaluated by decadal survey Remove if selected 5 Basin Sample Return for NF-4 Saturn Probe See Table ES.1 See Table ES.1 Remove if selected 7 for NF-4 Trojan Tour and See Table ES.1 See Table ES.1 Remove if selected 4 Rendezvous for NF-4 Same as 2003 decadal surveya Venus In Situ Explorer Not evaluated by decadal survey Remove if selected 5 (as amendedb) for NF-4 NOTE: On May 25, 2011, following the completion of this report, NASA selected the OSIRIS-REx asteroid sample-return spacecraft as the third New Frontiers mission. Launch is scheduled for 2016. a National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003. b National Research Council, Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity, The National Academies Press, Washington, D.C., 2008. designed is $4.7 billion FY2015. If JEO were to be funded at this level within the currently projected NASA planetary budget it would lead to an unacceptable programmatic imbalance, eliminating too many other impor- tant missions. Therefore, while the committee recommends JEO as the second-highest-priority flagship mission, close behind MAX-C, it should fly in the decade 2013-2022 only if changes to both the mission and the NASA planetary budget make it affordable without eliminating any other recommended missions. These changes are likely to involve both a reduction in mission scope and a formal budgetary new start for JEO that is accompanied by an increase in the NASA planetary budget. NASA should immediately undertake an effort to find major cost reductions for JEO, with the goal of minimizing the size of the budget increase necessary to enable the mission. The third-highest-priority flagship mission is the Uranus Orbiter and Probe mission. The committee carefully investigated missions to both ice giants, Uranus and Neptune. Although both missions have high scientific merit, the conclusion was that a Uranus mission is favored for the decade 2013-2022 for practical reasons involving avail- able trajectories, flight times, and cost. The Uranus Orbiter and Probe mission should be initiated in the decade 2013-2022 even if both MAX-C and JEO take place. But like those other two missions, it should be subjected to rigorous independent cost verification throughout its development, and should be descoped or canceled if costs grow significantly above the projected cost of $2.7 billion FY2015. Table ES.3 summarizes the recommended large missions and associated decision rules.

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6 VISION AND VOYAGES FOR PLANETARY SCIENCE TABLE ES.3 Large-Class Missions (in priority order) Mission Recommendation Science Objectives Key Challenges Decision Rules Chapter Mars Astrobiology • Perform in situ science on • Keeping within Mars Science Should be flown 6 Explorer-Cacher descope Mars samples to look for Laboratory design constraints only if it can be evidence of ancient life or • Sample handling, conducted for a cost prebiotic chemistry encapsulation, and to NASA of no more • Collect, document, and containerization than approximately package samples for future • Increased rover traverse speed $2.5 billion (FY2015 collection and return to over Mars Science Laboratory dollars) Earth and Mars Exploration Rover Jupiter Europa Orbiter Explore Europa to investigate • Radiation Should be flown 8 descope its habitability • Mass only if changes to • Power both the mission • Instruments design and the NASA planetary budget make it affordable without eliminating any other recommended missions Uranus Orbiter and • Investigate the interior • Demanding entry probe Should be initiated 7 Probe (no solar-electric structure, atmosphere, and mission even if both MAX-C propulsion stage) composition of Uranus • Long life (15.4 years) for and JEO take place • Observe the Uranus satellite orbiter and ring systems • High magnetic cleanliness for orbiter • System mass and power EXAMPLE FLIGHT PROGRAMS: 2013-2022 Following the priorities and decision rules outlined above, two example programs of solar system exploration can be described for the decade 2013-2022. The recommended program can be conducted assuming a budget increase sufficient to allow a new start for JEO. It includes the following elements (in no particular order): • Discovery program funded at the current level adjusted for inflation, • Mars Trace Gas Orbiter conducted jointly with ESA, • New Frontiers Missions 4 and 5, • MAX-C (descoped to $2.5 billion), • Jupiter Europa Orbiter (descoped), and • Uranus Orbiter and Probe. The cost-constrained program can be conducted assuming the currently projected NASA planetary budget (see Appendix E). It includes the following elements (in no particular order): • Discovery program funded at the current level adjusted for inflation, • Mars Trace Gas Orbiter conducted jointly with ESA, • New Frontiers Mission 4 and 5,

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7 EXECUTIVE SUMMARY • MAX-C (descoped to $2.5 billion), and • Uranus Orbiter and Probe. Plausible circumstances could improve the budget picture presented above. If this happened, the additions to the recommended program should be, in priority order: 1. An increase in funding for the Discovery program, 2. Another New Frontiers mission, and 3. Either the Enceladus Orbiter mission or the Venus Climate Mission. It is also possible that the budget picture could be less favorable than the committee has assumed. If cuts to the program are necessary, the first approach should be to descope or delay flagship missions. Changes to the New Frontiers or Discovery programs should be considered only if adjustments to flagship missions cannot solve the problem. And high priority should be placed on preserving funding for research and analysis programs and for technology development. Looking ahead to possible missions in the decade beyond 2022, it is important to make significant near-term technology investments now in the Mars Sample Return Lander, Mars Sample Return Orbiter, Titan Saturn System Mission, and Neptune System Orbiter and Probe. NASA-FUNDED SUPPORTING RESEARCH AND TECHNOLOGY DEVELOPMENT NASA’s planetary research and analysis programs are heavily oversubscribed. Consistent with the mission recommendations and costs presented above, the committee recommends that NASA increase the research and analysis budget for planetary science by 5 percent above the total finally approved FY2011 expenditures in the first year of the coming decade, and increase the budget by 1.5 percent above the inflation level for each successive year of the decade. Also, the future of planetary science depends on a well-conceived, robust, stable technology investment program. The committee unequivocally recommends that a substantial program of planetary exploration technology development should be reconstituted and carefully protected against all incursions that would deplete its resources. This program should be consistently funded at approximately 6 to 8 percent of the total NASA Planetary Science Division budget. NSF-FUNDED RESEARCH AND INFRASTRUCTURE The National Science Foundation supports nearly all areas of planetary science except space missions, which it supports indirectly through laboratory research and archived data. NSF grants and support for field activities are an important source of support for planetary science in the United States and should continue. NSF is also the largest federal funding agency for ground-based astronomy in the United States. The ground-based observational facilities supported wholly or in part by NSF are essential to planetary astronomical observations, both in support of active space missions and in studies independent of (or as follow-up to) such missions. Their continued support is critical to the advancement of planetary science. One of the future NSF-funded facilities most important to planetary science is the Large Synoptic Survey Telescope (LSST). The committee encourages the timely completion of LSST and stresses the importance of its contributions to planetary science once telescope operations begin. Finally, the committee recommends expan- sion of NSF funding for the support of planetary science in existing laboratories, and the establishment of new laboratories as needs develop.

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