Vision and Voyages for Planetary Science in the - Solar System ...

hazard. Asteroid 433 Eros, one of the largest NEOs, has been visited by a low-cost Discovery rendezvous
mission (Near-Earth Asteroid Rendezvous-Shoemaker), and one of the asteroids most accessible by
spacecraft (25143 Itokawa) was visited by the Japanese Hayabusa sample-return mission. Delivered to
near-Earth space from the main belt and more distant reservoirs, NEOs encompass a stunning variety of
taxonomic types, sizes and histories representative of the solar system at large, including a fraction of
extinct comets. Future missions may take advantage of low-ΔV trajectories to visit bodies of more
uncommon and primitive spectroscopic type, both to study them geologically and to return samples of
such compositions that are unlikely to be represented by meteorites.
Notional ESMD robotic precursor mission to Mars have also been discussed. There has been
little mention of missions to the martian moons, Phobos and Deimos. It is clear that these two moons
could play important roles in the future exploration of Mars, especially if they turn out to be related to
volatile-rich asteroids, a possibility which has not been excluded by existing data and observations. If so,
they may be the surviving representatives of a family of bodies that originated in the outer asteroid belt or
further, and reached the inner solar system to deliver volatiles and organics to the accreting terrestrial
planets.
Investigation of Phobos and Deimos crosscuts disciplines of planetary science including the
nature of primitive asteroids, formation of the terrestrial planets, and astrobiology. Key science questions
are the moons’ compositions, origins, and relationship to other solar system materials. Are the moons
possibly re-accreted Mars ejecta? Or are they possibly related to primitive, D-type bodies?
These questions can be investigated by a Discovery-class mission that includes measurements of
bulk properties and internal structure, high-resolution imaging of surface morphology and spectral
properties, and measurements of elemental and mineral composition. A possible follow-up New
Frontiers-class sample return mission could provide more detailed information on composition. Because
Phobos and Deimos are potential staging areas and sources of resources for future human exploration of
Mars, missions to Phobos and Deimos would contribute to human exploration in a way unique among
missions to primitive bodies.
Summary
The scientific study of primitive bodies can be advanced during the next decade if the following
activities are addressed:
• Flagship Missions—A mission at this scale is not proposed for the current decade. A more
appropriate use of limited resources is the initiation of a technology program focused on assuring that a
Cryogenic Comet Sample Return mission can be carried out in the decade of the 2020s at acceptable cost
and risk.
• New Frontiers Missions—Raise the cost cap of the New Frontiers program and keep New
Frontiers as PI-led missions. The independent cost estimates for primitive body missions (Appendix C)
indicate that the current cost cap for New Frontiers is insufficient for missions of the highest interest. The
most important missions addressing primitive body goals during the coming decade are, in priority order:
1. Comet Surface Sample Return and
2. Trojan Tour and Rendezvous.
• Discovery Missions—Ensure an appropriate cadence of future Discovery missions. This is
critical to the exploration of primitive bodies (Box 4.1), because of the large number of interesting targets.
A regular, preferably short cadence, is more important than increasing the cost cap for Discovery
missions.
• Technology Development—Expand technology developments in the following areas that
affect the highest ranked missions to primitive bodies: ASRG and thruster packaging and lifetime,
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