The Deep Impact mission aims at understanding the third dimension of a cometary nucleus, the physical and chemical properties as a function of depth below the surface. General wisdom holds that comets, because they are small and spend most of their lives far from the sun, hold primordial ices in their interiors. However, it is universally agreed that the surface layers have evolved, whether from cosmic rays while residing in the Oort cloud or from solar heating during previous perihelion passages. Clearly, in order to interpret surface observations and outgassing, we must understand how the surface layers differ from the interior. Deep Impact is the first mission to carry out a macroscopic experiment on a planetary body since the Apollo program dropped a lunar module on the moon and measured the seismic response.