Comets are time capsules that hold clues about the formation and evolution of the solar system. They are composed of ice, gas and dust, primitive debris from the solar system's distant and coldest regions that formed 4.5 billion years ago. Deep Impact, a NASA Discovery Mission, is the first space mission to probe beneath the surface of a comet and reveal the secrets of its interior.

[ Shoot a smart, 800 lb bullet into a comet and see what gets blasted out... pretty cool... -k]

NASA - Deep Impact Mission to a Comet