A NASA spacecraft known as Deep Impact has begun its one-way trip to a fiery meeting with a comet in space. The craft blasted off yesterday on a six-month mission to smash a hole in Comet Tempel One. The ball of ice and rock is big enough that it could almost swallow the Roman Coliseum. Scientists hope their first look at the heart of a comet will reveal data about the the birth of the solar system. Because of the 23-thousand mile-per-hour relative speed of the two objects at impact, no explosives are needed. The impact, expected on the Fourth of July, will create a flash that just might be visible on earth. NASA scientists downplayed an early overheating problem, saying they'll use a recovery command to emerge from a protected mode. Scientists say the nickname was picked independently of a like-named disaster movie made around the same time.