On January 2, NASA's Stardust spacecraft passed within 240 kilometers of Comet Wild 2. Donald E. Brownlee of the University of Washington and his collaborators have since analyzed 72 photographs taken during the 20,900 kilometer-per-hour flyby and their findings seem to contradict one popular theory for the makeup of comets.

The researchers mapped the surface of the 4.8-kilometer-wide comet nucleus, whose oblate shape resembles a homemade hamburger patty. In the latest issue of the journal Science they report that the surface is pockmarked with craters, some more than 1.6 kilometers across. None of these craters looked like what we see on other astronomical bodies, Brownlee remarks. To determine the origin of the strange depressions, the scientists ran experiments in a lab to simulate how the craters might have formed. What they found was that the holes on the surface were consistent with high-velocity collisions between the comet and smaller meteoritelike objects. These impacts most likely occurred billions of years ago when Wild 2 (pronounced vilt 2) was in the Kuiper belt, a donut-shaped region outside the orbit of Neptune that is one source of comets.

The existence of the craters, as well as the discovery of steep cliffs and pinnacles on the comet's landscape, imply that Wild 2 is a cohesive structure as opposed to a loosely connected rubble pile. The fact that Jupiter's gravitational field pulled apart Comet Shoemaker-Levy 9 in 1993 supported the theory that comets are an assemblage of debris weakly held together by gravity. But the new results instead suggest that a comet's structure evolves from a solid boulder to a tenuous collection of rocks as it gradually wanders in from the outskirts of the solar system toward the sun.

Data collected by other instruments on the Stardust spacecraft promise to further develop scientists' understanding of comets. For example, another research team has already analyzed the gas and dust emitted by Wild 2, and astronomers look forward to January 2006 when a Stardust capsule containing thousands of comet dust particles will return to Earth for more thorough investigation.