The ubiquitous tool of every field geologist is the trusty rock hammer. There's no better way to discover the composition of a rock than cracking it open and looking inside. This is exactly what NASA succeeded in doing with the Deep Impact mission.
Comets are thought to be formed in the coldest reaches of the solar system. As such, a comet such as Tempel 1 is a rock sample unlike any found on Earth and of great interest to astronomers.
NASA delivered a whopping hammer blow to this comet in the shape of small spacecraft whose sole purpose was to impact the comet at high speed. During the event, a sister craft was imaging the impact and the resulting crater and dust plume.
Scientists are studying the details of the impact now. In the meantime, the rest of us can "ooh" and "aah" at the neat pictures and videos(!) of the impact.