Lost and Found in Orbit

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It's a million miles away, cost $1 billion, and for more than two years has surveyed the sun with spectacular results. This cosmic overachiever--about the size of a Volkswagen beetle--is the Solar and Heliospheric Observatory, otherwise known as SOHO. Since April 1996 it has beamed back hundreds of thousands of remarkable images of solar eruptions and made dozens of scientific discoveries. It has also enhanced the ability of astronomers to predict and spot the powerful solar storms that produce auroras and cause power disruptions on Earth--as well as endanger satellites and astronauts in space.

Then, in late June, SOHO inexplicably fell silent, seemingly lost in space. "It was devastating," says John Credland, science project chief at the European Space Agency (ESA). "It was a show stopper."

Now it looks as if the show may go on. With a clever bit of detective work, technical ingenuity and the aid of giant radio telescopes, scientists at ESA and NASA (co-sponsors of SOHO) have located the wayward spacecraft and started nursing it back to health; they hope to regain control of it this week. If all goes well, SOHO could be fully back in business this fall in plenty of time to monitor the sun as it approaches the peak of its 11-year cycle of activity, around 2001.

Earlier there seemed little hope of saving SOHO after the spacecraft stopped responding to controllers at NASA's Goddard Space Flight Center. At the time SOHO scientist Arthur Poland lamented, "There is a real fear we won't get it back."

The failure occurred during an unusually complex maintenance procedure and was caused by a combination of two computer-software glitches and a bad judgment call. Those glitches resulted in the shutdown of one essential SOHO attitude-sensing gyroscope, a failure by a computer to recognize that the gyroscope was not operating, the unnecessary firing of SOHO's hydrazine-powered thrusters, and a mistake by controllers in switching off a gyroscope that was working properly. "Thrusters kept firing to null out a roll that was not happening," explains NASA's Michael Greenfield, co-chair of the investigation group.

Indeed, the thrusters actually caused SOHO to begin spinning with its two solar panels nearly edge-on to the sun rather than facing it. Without solar energy, SOHO's batteries quickly drained, cutting off power to all its systems.

Again and again, controllers vainly sent signals to where they thought SOHO should be. Weeks went by without a response. Then, in mid-July, a University of Colorado physicist named Alan Kiplinger had an idea. Why not search for SOHO the same way flight controllers look for commercial airliners: with radar? Realizing that extremely powerful radar would be needed to bounce a signal off so distant a target, he called on Donald Campbell, the chief scientist at the world's largest radiotelescope, the 1,000-ft. dish at Arecibo, Puerto Rico. Campbell agreed to try, although he estimated that the power of the returned signal would be about a billionth of a watt.

On July 23, Arecibo directed a powerful high-frequency radio beam toward the site SOHO should have been, a million miles away orbiting the sun. Ten seconds later, NASA's 230-ft. radiotelescope at Goldstone, Calif., began picking up its faint radar profile, barely perceptible against the background noise of space.