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	Solar Obervatory Back Up After Mysterious Failure By Daniel Sorid Staff Writer posted: 02:33 pm ET 30 November 1999

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Two days after a key sub-system mysteriously ceased operation, the sun-pointing SOHO spacecraft is back on the job after a quick repair by project engineers.

The computer that controls the solar observatory's ability to point itself at the sun shut down Sunday, causing the craft to lose its direct view of the sun.

A safety feature, called the Emergency Sun Re-acquisition mode, was switched on automatically, keeping the craft from pointing more than a few degrees away from the sun.

SOHO, which is celebrating the four-year anniversary of its launch on Thursday, monitors solar activity from a distance of 930,000 miles (1.5 million kilometers) from Earth.

Engineers at NASA's Goddard Space Flight Center in Greenbelt, Maryland uploaded new software to the craft and remotely inspected its systems.

By Monday afternoon the partially repaired SOHO had exited emergency mode, but the craft remains rolled over and nearly upside down. This is expected to be corrected sometime on Tuesday. Though project scientists don't know as of yet what caused the system's failure, they said there is a slight chance that a high-energy particle may have hit the craft's memory chip, causing an error.

"Sometimes you are hit by a cosmic ray and the computer has a parity error and it shuts off," said European Space Agency official and project scientist Bernhard Fleck. "These things are very rare, but they happen."

A view of the sun taken Monday by the SOHO spacecraft.

The craft was built for the European Space Agency, though NASA provides some support, including a mission control area at Goddard.

SOHO has received more public attention for its mishaps than its successes, though project scientists insist that its mission has gone remarkably well.

In the summer of 1998, scientists had all but written SOHO off when the craft drifted off its course, causing it to lose contact with ground control. According to Dr. Fleck, the project staff had accidentally disabled the Emergency Sun Re-acquisition mode, allowing the craft to drift. Losing its solar fix caused the craft to power down. Its instruments faced extreme temperatures -- the side facing the sun baked while the other side froze. It was eventually located and resuscitated in August.

In 1997, due to a problem very similar to the craft's most recent system failure, SOHO's pointing control system shut down.

Despite these reversals, Fleck insists that the mission has been extremely valuable.

"It's one of the most successful missions ever," he said.

Though SOHO's primary mission was completed last year, project scientists hope that the craft can continue functioning for the next several years -- during a period of high activity on the sun called Solar Maximum.