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Science 28 April 2006:
Vol. 312. no. 5773, pp. 535 - 536
DOI: 10.1126/science.1126896
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Perspectives

PLANETARY SCIENCE:
Ice Among the Rocks

Alan Fitzsimmons

Two groups of comets are known: those with orbital periods of hundreds of years or greater, and those with decade-long periods. A third class appears to be orbiting within the asteroid belt.

The author is at the Astrophysics Research Center, School of Mathematics and Physics, Queen's University Belfast, Belfast BT7 1NN, Northern Ireland, UK. E-mail: a.fitzsimmons{at}qub.ac.uk

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