What happens when you crash into a comet? That was a question considered by astronomers when they designed the Deep Impact mission, launched in January. On July 4, the Deep Impact spaceship will reach its target, Comet Tempel 1, and fire an impactor at its surface, photographing the result. The remaining crater may tell how Tempel 1 is constructed. If, for example, the comet is an extremely loose pile of debris, the impactor may leave little or no discernable crater. But if the comet's surface is relatively firm, the impactor's ripple may leave quite a large crater. Pictured above is an artist's impression of the initial encounter between the spacecraft and the comet.