Space Mission Rescues Are Not Impossible

WHEN it comes to the ``failures'' that beset space exploration, we should hesitate before counting a good spacecraft out. Take Ulysses, for example. It is embarked on a joint mission of the European Space Agency (ESA) and the United States National Aeronautics and Space Administration (NASA) to study the environment over the north and south poles of the sun. It developed a wobble last fall that threatened to cripple its ability to explore a solar system region never probed before.

Now ESA says controllers understand the wobble and can compensate for it. ESA declares Ulysses ``fully operational.''

Hipparcos - the ESA satellite designed to measure the positions, motions, and distances of more than a hundred thousand stars with unprecedented accuracy - provides another example. Elation at the satellite's flawless launch Aug. 9, 1989, turned to frustration when an orbit-adjustment motor failed.

Hipparcos was to have traveled a geosynchronous orbit, 36,000 kilometers (22,300 miles) high, on which it would remain above a given spot on Earth. Instead, it was left in a transfer orbit ranging from a few hundred kilometers to geosynchronous height.

The Hipparcos team redesigned satellite operations to make the most of a bad orbit. Now, with the satellite in its second working year, ESA says its data quality exceeds original expectations. Jean Kovalevsky, who heads a data analysis team at the Observatoire de la C^ote d'Azur in France, reports Hipparcos has measured more than a million star positions. Professor Kovalevsky calls the measurements of star brightness ``far better by a factor of 10 than what is obtained by ground observatories.'' He also notes discovery of several hundred new double stars and says that ``several thousand new double or multiple star systems will eventually also be discovered.''

Positions of some 30,000 stars now have been measured. An early catalog is ready with about 6,000 stars pinpointed to at least 50 times the accuracy ``routinely derived from Earth-based observations,'' according to ESA's announcement.

Coping with Ulysses's wobble was engineering detective work.

Ulysses is on a five-year Odyssey. It will pass over the sun's south pole at a distance of 320 million kilometers in 1994 and pass over the north pole a year later. But, to get there, it has to detour by way of Jupiter, whose gravity will flick Ulysses on a solar course.

Ulysses is stabilized by spinning. Engineers found that a boom extending along the spin axis warped as it spun in the sunshine. This made Ulysses wobble and disoriented its antenna. The warping has faded out as the craft has moved farther from the sun. But, should it return when Ulysses takes up its solar post, ESA says engineers now know how to handle it.

Space is a tough environment. But persistence and ingenuity can often save a mission from apparent disaster.

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