Comet Tempel 1 was in its most solid stage, consisting of a nucleus approximate 3.7 miles (6 km) in diameter, when it encountered the Deep Impact spacecraft in July 2005. (For information on comets, including their structure and composition, check out How Comets Work.) The primary goal behind the Deep Impact mission was to study the interior and the exterior of the same comet.
The Deep Impact spacecraft consisted of two parts: a flyby and an impactor. When the spacecraft came close to the comet, the two parts separated. The impactor put itself in the comet's path, causing a collision between the two bodies.
The impact created a crater in the comet that went well below the surface and exposed the protected material below -- the "pristine material" that was formed during the birth of the solar system. By studying both the material that came out of the crater upon impact and the characteristics of the comet that the crater exposed, scientists now have an unprecedented view of the solar system in its infancy. To learn more about impact craters, see Deep Impact: Cratering.