NASA joins the comet chase with a `bargain basement' spacecraft

There's more to East-West cooperation in comet exploration than the University of Chicago instruments on board Soviet Halley probes. Long before the Vegas reach their target, a US craft will intercept Comet Giacobini-Zinner. And the head of Soviet space research will be on hand at the NASA Goddard Space Flight Center to profit from the encounter. Cometary research, says Goddard flight controller Robert Farquhar, is one area where East-West cooperation ``may be going better than elsewhere.''

Soviet Vega spacecraft, which reach Halley first, will provide data to help European Space Agency controllers fine-tune guidance of their Giotto probe so that it can pass within 500 kilometers (310 miles) of the comet's nucleus. But before the two ``pathfinder'' Vegas, Giotto, and Japan's Planet-A reach Halley in March 1986, the US International Comet Explorer (ICE) is to pass through Giacobini-Zinner's tail at 6 a.m. EST Sept. 11.

ICE (pronounced icy) is to pass within 10,000 kilometers (6,214 miles) of the nucleus. It will be in such a dusty environment that Farquhar gives it only a 50-50 chance of surviving. Thus the encounter will provide valuable experience both in the hazards of close-up comet inspection and in the challenges facing spacecraft controllers. That is why the National Aeronautics and Space Administration is inviting representatives of all the comet probe projects to observe the encounter from Goddard's ICE control center in Greenbelt, Md. Farquhar says he expects R. Z. Sagdeev, director of space research for the USSR Academy of Sciences, to be among them.

During ICE's flyby, Vega probes will be upstream from the comet in the solar wind of particles and magnetic field that streams outward from the sun. This will provide useful data on the undisturbed solar wind to compare with conditions ICE finds near Giacobini-Zinner. ICE will provide similar data for Soviet, European, and Japanese investigators in March 1986. At that time, the US probe will be tens of millions of kilometers upstream of Halley.

The Halley probes will pass on the comet's sunward, less dusty side, and they have meteoroid shielding. Nevertheless, there is concern for their safety, especially for Giotto during its close approach to Halley's nucleus. Thus there is considerable interest in how ICE survives the impinging dust of Giacobini-Zinner's tail.

Farquhar notes that the spacecraft has no shielding, for it was not designed to inspect comets. He says he is most concerned about the solar cells that power ICE instruments. These may degrade under the abrasion of cometary dust. He says he hopes that nonessential instruments can be shut down during the encounter to provide a safety margin of power.

ICE has no imaging equipment to send back pictures. But it is able to measure the energies and types of particles and the strength and direction of magnetic fields. These measurements will give scientists their first good look at conditions within a comet's tail fairly close to the nucleus.

ICE has already returned rich scientific dividends. Its controllers had no thought of chasing comets when they launched it in August 1978. Then it was a solar wind monitor known as ISEE-3 -- an International Sun-Earth Explorer. It was called international because European scientists were involved in the project and had instruments on board. ISEE-3 outlived its expected life of two years and sent back much valuable data. Then Farquhar realized that the spacecraft was healthy enough and had enough maneuvering fuel to tackle an entirely new mission. It would leave its station between Earth and the sun and, using the moon's gravity to assist its maneuvers, head out to intercept a comet.

The maneuvers were complex. They involved several loops around the moon. In between, the spacecraft explored Earth's so-called magnetospheric tail -- a long, narrow region of magnetic fields and particles stretched out by the solar wind blowing past Earth. It explored Earth's tail at far greater distances than has been done before. This alone has been a valuable scientific exploration. Then, on Dec. 22, 1983, ISEE-3 made its fifth and final pass by the moon, skimming a mere 72 miles above its surface. This tricky maneuver set the spacecraft on course to meet Giacobini-Zinner with a boost from the moon's gravity. ISEE-3 -- no longer a Sun-Earth Explorer -- was renamed ICE.

Farquhar delights in the fact that this is ``bargain-basement science.'' The extra cost for the spacecraft's imaginative new mission has been less than $3 million.

Thus, for very little cost, the United States has mounted a comet-probing mission which is not only providing important new scientific knowledge, it should also help prepare the way for European, Japanese, and Soviet controllers to better handle their own probes when they greet Comet Halley.

A Thursday column. Robert C. Cowen is the Monitor's natural science editor. -- 30 --{et

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