Check out these pages on the NASA Deep Impact mission. The mission was to hit the comet Tempel 1 with an impactor to clear the top layer of the comet. The spacecraft then makes measurements of the chemicals which are underneatg the comet's top surface. Cool huh?
Anyway, latest news is that the impactor has successfully hit the comet Tempel 1. Check out this image of the impactor colliding with the comet. The next phase is for the Flyby spacecraft to make its closest approach to the comet. Then the press conference in a few hours time.