CAPE CANAVERAL, Fla.(AP) A NASA spacecraft with a Hollywood name _ Deep Impact _ blasted off Wednesday on a mission to smash a hole in a comet and give scientists a glimpse of the frozen primordial ingredients of the solar system. With a launch window only one second long, Deep Impact rocketed away at the designated moment on a six-month, 268-million-mile journey to Comet Tempel 1. It will be a one-way trip that NASA hopes will reach a cataclysmic end on the Fourth of July. "We are on our way," said an excited Michael A'Hearn of the University of Maryland, the mission's chief scientist. Minutes later, the spacecraft shot out of Earth's orbit and onto its collision course. "We'll be there July Fourth," NASA launch director Omar Baez said. Scientists are counting on Deep Impact to carve out a crater in Comet Tempel 1 that could swallow the Roman Coliseum. It will be humans' first look into the heart of a comet, a celestial snowball still containing the original building blocks of the sun and the planets. Because of the relative speed of the two objects at the moment of impact _ 23,000 mph _ no explosives are needed for the job. The force of the smashup will be equivalent to 4 1/2 tons of TNT, creating a flash that just might be visible in the dark sky by the naked eye in one spectacular Fourth of July fireworks display. Nothing like this has ever been attempted before. Little is known about Comet Tempel 1, other than that it is an icy, rocky body about nine miles long and three miles wide. Scientists do not even know whether the crust will be as hard as concrete or as flimsy as corn flakes. "One of the scary things is that we won't actually know the shape and what it looks like until after we do the encounter," said Jay Melosh, a planetary geologist at the University of Arizona. The comet will be more than 80 million miles from Earth when the collision takes place _ on the sunlit side of the comet, NASA hopes, in order to ensure good viewing by spacecraft cameras and observatories. The resulting crater is expected to be two to 14 stories deep, and perhaps 300 feet in diameter.