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

There remain many important investigations of primitive bodies that can be carried out within the
scope of the Discovery program. Given that the Discovery program is founded on the enterprise and
initiative of individuals and is a PI-led endeavor this decadal survey does not attempt to define a set of
candidate missions nor priorities, but the committee gives some examples of important Discovery-class
investigations that could be carried out in the coming decade. The population of scientifically compelling
targets is not static, but is continually increasing as a consequence of discoveries. In no priority order,
Discovery missions of significance to primitive bodies may include, but are not limited to, the following:
• Multiple flybys of asteroids and comets to further investigate the great diversity of these
bodies—Such missions may benefit from using already proven flight systems and instrument technology.
A study of NEO target accessibility performed by the Jet Propulsion Laboratory at the committee’s
request identified asteroids of at least 5 different taxonomic types, including several not previously visited
by spacecraft, that were sufficiently large, had moderate to short mission durations, and low ΔV to be
accessible with current resources. A flyby visit of several members of one dynamical family would help
provide an understanding of the interior structure and composition of their parent asteroids, and the
process of collisional evolution. Telescopic surveys reveal diverse organic compositions in comets, the
exploration of which would constrain processes in the protoplanetary disk.
• Orbital/rendezvous missions to selected comets or asteroids of high scientific interest—While
Dawn’s exploration of 4 Vesta represents a first spacecraft study of a differentiated asteroid, a logical
follow-on would be an orbital mission to explore an M-class asteroid with high radar reflectivity that
could reasonably be the stripped core of a differentiated asteroid. Differentiation was a fundamental
process in shaping many asteroids and all terrestrial planets, and direct exploration of a core could greatly
enhance our understanding of this process. Detailed studies of comets that have naturally broken apart
provide opportunities to study their pristine interiors.
• Sample return or geophysical reconnaissance missions to easily accessible NEOs—Although
meteorites provide a rich sampling of NEOs, that sampling is certainly incomplete. As an example,
recent spectroscopic studies suggest that some asteroids may be enriched in the earliest solar system
solids compared to known meteorites and, thus, may sample an earlier stage of accretion. The
microgravity geophysical properties of primitive bodies are not understood. Landed missions to study
seismological, radar, or rheological responses of comets and asteroids will answer questions about their
formation, accretion and evolution and set the stage for advanced missions such as sample return.
• Landed investigations of Phobos and Deimos—A major goal of in situ surface science on the
martian moons is to determine their compositions in order to constrain their origins.
• Stardust-like sample return missions to other Jupiter-family comets to investigate
mineralogical/chemical diversity—The results of multiple missions would provide fundamental insights
into the origin of crystalline materials around stars and the processes of radial transport in circumstellar
disks.
• Flyby intercepts of “new” Oort cloud comets to investigate possible chemical differences
between these comets and the Jupiter-family comet population—Such a mission would identify possible
chemical and isotopic differences between comets that formed inside Neptune’s orbit and the Jupiterfamily
comet population that formed beyond the planets.
• Near-Earth space observatory to study primitive body populations—Observations from near-
Earth space enable the discovery and characterization of primitive body populations that are not
observable from ground-based observations, including NEOs having orbits largely existing inside Earth’s
orbit, kilometer-sized KBOs, and extremely distant bodies out to the inner parts of the Oort cloud.
During the last half of the past decade the Discovery program was expanded to include missions
of opportunity. This program has been highly successful both in providing flight opportunities for U.S.built
instruments on foreign spacecraft and by enabling “re-use” of NASA-built spacecraft that have
completed primary missions by retargeting them to new destinations. Additional important primitive
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