EPOXI at Comet Hartley 2

Understanding how comets work—what drives their activity—is crucial to the use of comets in studying the early solar system. EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) flew past comet 103P/Hartley 2, one with an unusually small but very active nucleus, taking both images and spectra. Unlike large, relatively inactive nuclei, this nucleus is outgassing primarily because of CO2, which drags chunks of ice out of the nucleus. It also shows substantial differences in the relative abundance of volatiles from various parts of the nucleus. Comets are the fundamental building blocksof the giant planets and may be an im-portant source of water and organics on Earth. On 4 July 2005, the Deep Impact mis-sion carried out an impact experiment on comet 9P/Tempel 1 (1, 2) to study differences between the comet’s surface and the interior. Although the impactor spacecraft was destroyed, the flyby spacecraft and its instruments remained healthy in its 3-year, heliocentric orbit after completion of the mission. The Deep Impact flyby spacecraft was retargeted to comet 103P/Hartley 2 as part of an extended mission named EPOXI (Extrasolar Planet Observation and Deep Impact Extende