Reflecting the excitement felt here at NASA's Goddard Space Flight Center, astronomer John C. Brandt spoke with a sense of history. ``We are beginning the direct exploration of comets,'' he said. As he spoke, a small spacecraft, running partly on lunar energy, was approaching Comet Giacobini-Zinner some 70 million kilometers from Earth. At this writing, the craft, known as the International Comet Explorer (ICE), was scheduled to penetrate the comet's tail at 7:02 a.m. Eastern daylight time today.
Once that happened, comet exploration would never be the same. These spectacular objects, which may contain primordial material unchanged since the formation of the solar system, would begin to yield their secrets to the probes of Earth-bound scientists.
The adventure began Dec. 22, 1983, when ICE skimmed past the moon a mere 120 kilometers from its cratered surface. Lunar gravity flicked the spacecraft out of Earth orbit. That slingshot-like maneuver also gave ICE a gift of some of the moon's orbital energy.
Giacobini-Zinner has been a somewhat tricky target. Its orbit could not be predicted precisely, partly because the gases streaming off as it approaches the sun can change the orbit. As ICE neared the comet, it became apparent that it would miss the tail. So Goddard controllers gave ICE's trajectory a final tweek on Sunday.
Now, according to flight director Robert W. Farquhar, the spacecraft is headed for the center of the tail with a closing speed of 21 kilometers a second or about 76,000 kilometers an hour. It should pass within 8,000 kilometers of the comet nucleus instead of 10,000 kilometers as originally estimated.
Unlike the Japanese-European-Soviet squadron of probes that will intercept Halley's Comet in March, ICE has no camera to photograph the nucleus. But it is well equipped to observe what most people envision a comet to be -- a bright cloud of gas with a spectacular tail.
Despite the lack of photographic equipment, ICE is recognized as a historic first. Dr. Farquhar quoted an unnamed Soviet scientist as saying of the two Soviet Vega Halley's probes: ``We have a beautifully instrumented spacecraft. But you will be first.''
The National Aeronautics and Space Administration and the European Space Agency are copartners in the project.