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 questions 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 missions 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 recommendations 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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