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In the course of the Space Age, planets have gone from astronomical objects--wandering specks in the night sky--to geologic ones: full-fledged worlds you could imagine yourself walking on. In the 1990s asteroids made the same transition. And now it is comets' turn, as exemplified by July's (deliberate) crash of the Deep Impact probe into the nucleus of Comet Tempel 1. In September researchers announced a batch of findings at an American Astronomical Society meeting in Cambridge, England.
The impact threw up a cone of dusty debris some 500 meters high; it extended right from the surface, indicating that the cometary material put up no significant resistance to the projectile. The flight path of debris particles revealed the strength of the comet's gravity, hence its density--on average, about half that of water. The body must be riddled with voids. So much fine dust flew out that it could not have been created by the impact itself but rather must have already been sitting on the surface. A lack of big pieces suggests that the comet has no outer crust.