A funny thing happened on the way to Halley's comet last week. As an armada of Soviet, Japanese and European space probes hurtled through the cosmos toward their heralded meetings with the fabled comet next March, they were upstaged by a modest and almost archaic Ameri can spacecraft. The International Cometary Explorer whipped through the tail of an obscure apparition called Giacobini-Zinner, thereby becoming the first man-made object to encounter a comet.
Easily weathering the risky rendezvous, the half-ton craft transmitted a stream of valuable and sometimes surprising data about the 465,000-mile-long cometary tail to jubilant scientists tuned in at NASA's Goddard Space Flight Center in Greenbelt, Md. ICE's coup enabled American astronomers and space scientists to recover some of the patriotic and professional pride that was dashed in 1981 when Washington budget slashers vetoed a U.S. mission to Halley. "We wanted to make sure the U.S. didn't take a backseat to anyone," says NASA Spokesman James Elliott, "and we've done it."
ICE is in fact an old space hand, having already logged more than 30 million miles before its billion-mile cometary odyssey. Measuring 5 ft. tall and 5 1/2 ft. in diameter, the drum-shaped spacecraft was launched on Aug. 12, 1978; as one of three vehicles in the International Sun-Earth Explorer project, it was named ISEE-3 and designated to orbit a sun-earth libration point (where the gravitational pull of the sun precisely nullifies terrestrial gravity) 930,000 miles from the earth. Its mission: to study the effect of the solar wind on the earth's magnetic field. Yet even as ISEE-3 sniffed at solar breezes, its flight director, NASA Aerospace Engineer Robert Farquhar, was plotting to divert it somehow toward a comet. "The craft was custom-made to measure plasma waves," he explains, "and that's exactly what you find at the back of a comet."
Farquhar's first thought was that ISEE-3 could be directed toward Halley, providing a drastically cheaper alternative to the more than $350 million that a new and more sophisticated mission would cost. He soon realized, however, that the radio on the diminutive probe was too weak to transmit data from 80 million miles away, the distance of Halley when it is most accessible to visiting earthships. Additional research suggested a less glamorous but more practical alternative: comet Giacobini-Zinner, which orbits the sun once every 6.5 years and could be easily visited when it was about 44 million miles from the earth, well within the satellite's radio range. As an added bonus, a rendezvous with G-Z, as NASA scientists call it, could occur six months ahead of the Halley encounter. Farquhar began petitioning NASA officials to spend the meager $3 million it would take to commandeer ISEE-3. To his surprise, the allocation was approved before he had figured out how to divert the craft. "It may have turned out not to be possible," he says, "but at that point there was no backing out."