The Upstaging of Halley's Armada

By Natalie Angier.

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."

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."

With reporting by Jay Branegan/ Washington