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Science 28 April 2006:
Vol. 312. no. 5773, p. 496
DOI: 10.1126/science.312.5773.496f
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This Week in Science

Comets are believed to be primitive dirty snowballs that come from the cold outer reaches of the solar system. However, Hsieh and Jewitt (p. 561, published online 23 March; see the Perspective by Fitzsimmons) propose that a new class of comets exists in the main asteroid belt. A survey of mainbelt asteroids revealed three with cometary tails, which suggests that icy asteroids can become activated and appear as comets after collisions. As these objects likely formed in situ in a warmer environment, such main belt comets should differ in composition as well as orbit from the cold Kuiper Belt and Oort Cloud comets. Main belt comets could have contributed water to the early Earth.




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Science. ISSN 0036-8075 (print), 1095-9203 (online)