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After a marathon brainstorming session, he and his colleagues managed to map a route that looked like nothing so much as a plate of linguine. In June 1982 they instituted a series of computer commands and gentle rocket bursts that swung the spacecraft out of its libration-point orbit into eccentric earth orbits. In these loops and twists, it swept past the moon five times, making repeated use of lunar gravity to boost its speed on each pass. When the satellite swooped to its final pass, only 75 miles from the lunar surface in December 1983, and was flung toward G-Z at 45,000 m.p.h., NASA confidently renamed it ICE.
A comet, however, is a fickle creature. As it approaches the sun and heats up, volatile material vaporizes to form the cometary coma (head) and tail, sometimes in sudden bursts that cause unpredictable shifts in the comet's path. Thus toward the end of ICE's 21-month approach to G-Z, NASA engineers had to readjust the satellite's trajectory frequently to keep it on target. Another worry: ICE, which was not designed to chase down comets, lacks a dust shield and is traveling so fast it can be done in by a piece of cometary debris as small as a grain of sand. "From what we know, we think the craft will survive," said a concerned John Brandt, head of the astronomy and solar physics lab at Goddard. "It's what we don't know that could cause problems."
As it turned out, all fears were for naught. The craft sliced neatly and unscathed through the bow wave--a detectable shock wave that the comet makes as it plows through the solar wind--encountering it 117,000 miles from the cometary nucleus; it then sailed through the front of the tail, only 5,000 miles behind the nucleus. Among the spacecraft's most important observations: the comet's tail was five times as thick as its predicted width of 3,000 miles, and charged molecules of water and carbon monoxide were detected, for the first time confirming directly that a comet is, as Brandt puts it, "basically a large, dirty snowball."
ICE's odyssey is not over. It will swing in front of Halley's comet in March and keep traveling in an orbit that will bring it close to earth again nearly three decades from now. Farquhar proposes that the returning satellite, still coated with comet dust that scientists would dearly like to examine, be maneuvered to a low orbit and then retrieved by the shuttle. And when might this astonishing rescue mission occur? Says Farquhar matter-of-factly: "We estimate roughly Aug. 2, 2012."