At 1:52 a.m. Eastern Daylight Time on July 4, a washing machine-size "impact module" launched from a U.S. spacecraft will slam into the comet Tempel 1 at a relative speed of about 23,000 miles per hour. Astronomers in charge of the mission, dubbed Deep Impact, say the resulting explosion will give them their first glimpse into a comet's icy interior, and possibly reveal important clues as to how the planets formed 4.6 billion years ago. Tempel 1, which is about nine miles long and three miles wide, orbits the sun every 5½ years.
