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In this issue:
Environmental testing of Dawn spacecraft begins
Dawn Instrument Integration Completed
Ceres a 'Dwarf Planet'
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International Astronomical Union Declares Ceres a 'Dwarf Planet'
Christopher T. Russell
Dawn Principal Investigator, UCLA
While the Dawn team has always considered both of its intended targets as protoplanets, the IAU has officially recognized Ceres as belonging to a class of planetary objects called "dwarf planets" that includes Pluto, its previously-ascribed moon Charon, and Eris, a large object recently discovered in the Kuiper belt. While the new label draws welcome attention to the many intriguing attributes of Ceres, Vesta also exhibits many characteristics of planets, such as a differentiated interior in which iron migrated to the core and lighter basaltic magma rose to the surface. Vesta's differentiation likely occurred as a result of formation of a magma ocean driven by decay of abundant short-lived radionuclides. The greater apparent water retention by Ceres may have resulted from an insufficient supply of heat-producing material, possibly because it formed later, after these elements had decayed. The large size of both Vesta and Ceres has helped preserve the information contained in the surfaces and interior of the bodies since their formation. The extraordinary ability of the Dawn spacecraft to orbit both of these intriguing objects and shed light on the processes and conditions that caused the observed variation within our solar system will inform the debate on what makes a planet, a debate expected to endure long into the future.
This subject is expounded upon in the latest blog by mission engineer Marc Rayman. His journals can be found at http://dawn.jpl.nasa.gov/mission/journal.asp.
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