US probe closes in on rendezvous with comet Giacobini-Zinner. It will study comet's tail, show whether spacecraft can survive such missions

While the world prepares to welcome back Halley's comet, the first spacecraft ever to encounter such a body is closing in on the lesser-known comet Giacobini-Zinner. ICE (pronounced ``icy'') -- the International Comet Explorer -- will cruise through the comet's tail at 7 a.m. Eastern daylight time Sept. 11. Passing within 10,000 kilometers of the comet's nucleus on the side away from the sun, ICE will give scientists their first sample of a comet's tail.

Its passage through that dusty environment will also give spacecraft engineers a preview of how well a comet probe and its instruments can survive being peppered by microscopic particles at a relative speed of around 21 kilometers a second. That is why observers from the European Space Agency (ESA), Japan, and the Soviet Union, whose probes are on the way to intercept Halley's comet, will be on hand at the National Aeronautics and Space Administration's Goddard Space Flight Center outside Washington to see what happens.

The encounter some 70 million kilometers from Earth will be something of a coup for the United States. Not only is this the first close-up comet inspection, it also is the culmination of the most complex series of spacecraft maneuvers yet undertaken.

ICE began its career as ISEE-3, one of a suite of International Sun-Earth Explorers. It was stationed between Earth and the sun outside the region called the magnetosphere, which is dominated by Earth's magnetic field. It monitored the ``solar wind'' of particles flowing from the sun. These data were then correlated with measurements made within the magnetosphere by ISEE-1 and ISEE-2.

But ICE managers, especially chief mission planner Robert Farquhar, knew that ISEE-3 had the potential to take on a more ambitious task. The craft had arrived on station with a generous supply of hydrazine fuel for its maneuvering rockets. By sending it close to the moon when that body was in just the right position relative to Earth, it would be possible to use the pull of lunar gravity like a slingshot to boost the spacecraft on a trajectory to take it to Giacobini-Zinner.

So with the encouragement of the US National Academy of Sciences and the agreement of ISEE program partners -- Britain's Science Research Council and ESA -- NASA authorized the attempt. Controllers nudged ISEE-3 off station on June 10, 1982.

After four major rocket firings, five close trips past the moon, 11 minor trajectory adjustments, and bearing a new name, the rugged former satellite broke free of Earth and is nearing a target its designers did not foresee.

The spacecraft is three years over its two-year design life and still working well. It has no imaging camera to take pictures of the comet. But its solar-wind probes have proved adaptable enough to make a detailed study of the comet's tail and surrounding environment.

This encounter will be much more than a consolation prize for US scientists who were disappointed that their country did not launch a Halley's probe. Giacobini-Zinner appears to have a different composition from Halley's comet. Thus the enounter will provide comparison data for Halley's investigators. Also, ICE is the only one of the comet probes now in space that is to pass through a comet tail. The Halley's probes will come in on that comet's sunward side to take close-up images.

Seen from Earth, Giacobini-Zinner now is at its brightest as it makes its closest approach to both Earth and sun. It shines with the brilliance of an eighth-magnitude star, making it a dozen times brighter than Halley's Comet at this time and observable with small telescopes or binoculars. Both comets are in the same part of the sky all month, moving through the constellations Auriga, Taurus, Orion, and Monoceros.

You can get late information on the encounter Sept. 11 beginning at 3 a.m. E.D.T. by calling 1-900-976-COMET. There is a charge for this service.

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