Lost and Found in Orbit

By LEON JAROFF

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.

SOHO was still close to its proper orbit, wobbling at top and bottom and rotating once a minute, too slow to have caused structural damage. Even more encouraging, the geometry of SOHO's orbit was tilting the craft's axis of rotation toward the sun by about a degree a day. That was gradually increasing the amount of sunlight hitting the solar panels. Ground controllers ordered SOHO to store that intermittent flow of energy and recharge its batteries.

Why hadn't the spacecraft responded to controllers' signals before? Perhaps, suggested an ESA scientist, the probing signals were too complex for the weakened SOHO to comprehend. Early in August controllers sent a much simpler message. Result: contact! SOHO responded by transmitting its carrier signal. It was still alive and, as its batteries gradually charged, able to transmit a modicum of data.

As the faces of SOHO's solar panels tilt more and more toward the sun, controllers are alternately charging the batteries and using them to warm SOHO's slushy on-board hydrazine fuel, which nearly froze during the craft's dormancy. The hydrazine, when fully thawed, will be used to fire thrusters in a series of brief burns. That procedure should halt SOHO's spin, stabilize the observatory and face its solar panels directly toward the sun.

Only then will the SOHO team begin a two-month period of restarting and evaluating the spacecraft's systems and scientific packages in preparation for full operation in the fall. "So far we've found no damage whatsoever," says ESA's Credland. "It's incredible." Considering the odds that were stacked against the little spacecraft, incredible is probably an understatement.

--Reported by Dick Thompson/Washington

With reporting by Dick Thompson/Washington