On 4 July 2005, the NASA Deep Impact spacecraft will visit Comet 9P/Tempel 1. It will launch a 370 kg impactor probe that should produce a crater on the surface of the comet and a plume of gas, dust and ejected material.
More at:
http://www.esa.int/SPECIALS/Rosetta/SEM8PE0DU8E_0.html