There ought to be plenty of fireworks this Fourth of July. But the best pyrotechnics show is likely to take place 268 million miles away, when a probe fired from the Deep Impact spacecraft is scheduled to collide with Comet Tempel 1, a nine-mile-long rock roaring through space at 66,880 m.p.h. The planned cosmic crack-up will gouge out a football-field-size crater and may be visible from the U.S. Pacific Coast and points west. It may also reveal a lot about the chemistry of comets, fossils of the early solar system. Deep Impact is really a craft and a half: an suv-size mother ship and a smaller impactor—basically a guided missile equipped with a targeting system and a are 24 hours from the comet, the mother ship is set to release the impactor to free-fly the last 500,000 miles. If all goes well, the probe and comet should collide at 1:52 a.m. E.T. on July 4, producing the same explosive bang as 4.5 tons of TNT. The mother ship, trailing a safe 310 miles behind, will photograph the crater and analyze the debris field thrown up.
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