"The Deep Impact mission collided with a comet on July 4, 2005. Cameras and spectrometers captured the largest controlled oblique experiment in history.
The nature of the target (Comet 9/P Tempel 1), however, was completely unknown.  Consequently, a series of much smaller scale hypervelocity experiments 
were performed in order to aid in interpretation.  Results indicate that the upper surface of the comet (upper 1-2m) had a low density, based on asymmetry
in the ejecta and the observed photometric evolution.  Although the crater has not yet been clearly identified, it appears that it was at least 150m and 
perhaps as large as 230m.  Ice appears to be hidden well below the surface but was seen by the DI flyby."