What are comets made of? Are they solid rocks, or fluffy snowballs? On July 4, 2005, the Deep Impact mission will find out in a dramatic way. One part of the Deep Impact spacecraft will slam into comet Tempel 1, ending its life with a smash, while the Flyby spacecraft will watch the birth of a new crater. By studying the size and shape of the new crater, and the composition and motion of the stuff that flies away from the impact, the Deep Impact scientists hope to learn how comets are put together.