The International Cometary Explorer (ICE), a drum-shaped U.S. spacecraft, flew unscathed yesterday through the hot and dusty tail of the comet Giacobini-Zinner, the first time a man-made machine had encountered a comet in space.
Moving 46,000 miles an hour, the 930-pound spacecraft intercepted the comet at a spot 44 million miles away, halfway between the Earth and the sun, just after 7 a.m. EDT. The spacecraft spent 20 minutes inside the comet's 14,000-mile wide tail, and emerged undamaged at the end of what was expected to be a perilous passage.
"We worried that we would survive, we worried that we'd miss the tail and then we worried that we wouldn't be in the tail long enough to get good science," project scientist Tycho von Rosenvinge said at the Goddard Space Flight Center in Greenbelt where the comet encounter was directed. "Our worries are over."
"It looks like very little happened to it," flight director Robert Farquhar said.
While the spacecraft encountered less dust than expected on its flight through the comet's tail, it ran into the equivalent of an interplanetary electrical storm that strained the spacecraft's 10 working instruments almost to their limits.
The electrical activity in the comet's tail was unexpected and was the apparent result of the comet's passage through the solar wind, the invisible stream of protons and electrons that whips off the sun through space at 720,000 miles an hour.
"We saw such high electrical fields inside the tail we came within one-quarter of a volt of saturating our instrument," said Dr. Fred L. Scarf of TRW Inc., which had an instrument on the spacecraft to measure electrified gases in the comet's tail.
"These were the highest currents our instrument had measured since it was put into space more than seven years ago," he said.
The spacecraft was flown into space in August 1978 to measure the solar wind ahead of the Earth by a million miles, a spot in space where the solar wind is one hour away from striking the Earth's magnetic field.
The spacecraft was moved in 1982 onto a roundabout trajectory that took it through the Earth's magnetotail and around the moon five times before being speeded up in a "slingshot" maneuver on a path toward the Giacobini-Zinner comet.
The move to retarget the spacecraft to fly through the comet's tail was at least partly political, because the Reagan administration had decided in 1981 that the United States could not afford to send a spacecraft to intercept Halley's comet in March 1986.
The European Space Agency, the Soviet Union and Japan all have spacecraft on the way toward Halley's comet, which they will encounter next March.
"The Russians were extremely gracious to us this morning about being first," Goddard's von Rosenvinge said, noting that Roald Sagdeev, director of the Soviet Institute for Space Research, was visiting Goddard at the time of the ICE encounter. "I said, yes, it is nice to be first but it is more important to go to Halley."
Halley is the biggest, brightest and best known of the more than 900 comets known to travel through the solar system. Giacobini-Zinner is in a comet category described as the lower third in terms of scientific interest.
"In spite of that, mankind's first encounter with a comet has to be ranked as an unqualified success," said Edward J. Smith, a Jet Propulsion Laboratory scientist.
The spacecraft's safe passage through the comet's dust tail raised the hopes of the European Space Agency that its Giotto spacecraft will be able to get as close as 5,000 miles to the head of Halley's comet in March.
"My own guess is that it would be dangerous to predict that Giotto will survive," Scarf cautioned. "Halley is known to be a very dusty comet and all I can say is that we did not have a problem today."