Deep Impact was a mission to hit a comet with a hard, dense object at high speed, and then take pictures and gather other data of the impact site and the ejected material. The reason for such a study was to see what the interior of a comet—the oldest unaltered material in the solar system—could reveal about the origin of the planets, and to learn how to deal with a future comet that might be on a collision course with Earth. The space explorer mission was combined with a concentrated effort of ground-based and space-based telescopes to study the comet and observe the impact and its aftermath.
The Deep Impact mission taught astronomers about the materials that make up a comet, including clay, carbonates, crystallized silicates, polycyclic aromatic hydrocarbons, iron compounds, and even bits of the reddish-brown gem spinel. (NASA/JPL-Caltech/R. Hurt (SSCj)