You'll be able to see a spacecraft the size of a washing machine collide with a comet the size of a city at about 1 a.m. (Houston time) tomorrow morning.
NASA's Deep Impact mission is scheduled to crash an 820-pound Impactor probe into Comet Tempel 1 and record the event via a Flyby mothership, orbital observatories like the Hubble and Spitzer space telescope, and a myriad of ground-based telescopes from around the world. The impact is expected to take place at 1:52 a.m. EDT (0552 GMT) on July 4.
You can also watch a webcast here (starting about one hour before impact). There's a hundred excellent links, including still images from observatory telescopes at Kitt Peak, Keck, and Dyer at this page.
