An image of the comet Wild 2, taken by NASA's Stardust spacecraft during a fly-by on Jan. 2, 2004.
At a NASA press briefing today, scientists said that comets are more than just a loose collection of space rubble. They can be solid bodies with craters and other geological features. The findings come from observations by the Stardust spacecraft, which had a close encounter with the comet Wild 2 in January. NPR's Richard Harris reports.