It's sort of like a bug hitting the windshield of an 18-wheeler

The collision, which is to take place between a projectile fired from the space agency's Deep Impact spacecraft and a 4-mile-wide comet known as Tempel 1, is scheduled for July 4, 2005.
Wired News: NASA to Smack a Comet
http://www.wired.com/news/space/0,2697,66079-2,00.html?tw=wn_story_page_next1