PerspectivePlanetary Science

Ice Among the Rocks

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Science  28 Apr 2006:
Vol. 312, Issue 5773, pp. 535-536
DOI: 10.1126/science.1126896

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Summary

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.