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

Protoplanet: A planet in the process of accretion from material in a protoplanetary disk.
Protoplanetary disk: A circumstellar disk of matter, including gas and dust, from which planets may
eventually form or be in the process of forming.
238 Pu: An isotope of plutonium whose physical properties make it ideally suited for use as a heat source
in radioisotope power systems.
Pyroclastic: Rocks composed solely or primarily of fragments of volcanic materials.
Pyrrhotite: An unusual iron sulfide mineral with a variable iron content.
Q factor: A parameter which describes how under-damped an oscillatory system is; the higher the Q, the
more slowly oscillations die out after being excited.
R&A: Research and Analysis.
Radiative balance: Accounting for all sources of ingoing and outgoing sources of radiation in a system.
Radiogenic heating: The thermal energy released by the decay of naturally occurring radioactive
materials.
Radiolysis: The dissociation of molecules by ionizing radiation.
Radionuclides: A radioactive nuclide.
Redox reaction: Any of a large class of chemical reactions characterized by the reduction of one
reactant and the simultaneous oxidation of another.
Regolith: The layer of dust and fragmented rocky debris which forms the uppermost surface on many
planets, satellites, and asteroids. It is formed by a variety of process including meteoritic impact,
REP: Radioisotope Electric Propulsion.
Retrograde: Orbital motion or rotation in a clockwise direction as viewed from the north pole of the
ecliptic or of the rotating object; i.e., in the opposite sense to the rotation of Earth and most of the other
planetary bodies in the solar system.
REU: Research Experience for Undergraduates program.
ROSES: Research Opportunities in Space and Earth Sciences.
Rosetta: A European Space Agency spacecraft, launched in 2004, that will rendezvous with Comet
67P/Churyumov-Gerasimenko in 2014, flying past two asteroids on the way.
RPS: Radioisotope Power Systems.
SALMON: NASA’s Stand-Alone Mission of Opportunity program.
SDT: Science Definition Team.
SED: Spectral Energy Distribution.
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