Fix lets Hopkins scientists resume space observations

Electromagnets used to steady observatory

February 20, 2002|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Scientists and engineers at the Johns Hopkins University say they have resumed limited scientific observations with an orbiting observatory that was idled Dec. 10 by failures in its pointing system.

Controllers say they have reprogrammed a set of onboard electromagnets and harnessed them to help with aiming tasks they were never designed to do.

The $108 million Far Ultraviolet Spectroscopic Explorer (FUSE) is now steady enough for observations across almost half the sky, and controllers in Baltimore say they hope to regain access to nearly the entire sky in the coming months.

The recovery has loosed a big sigh of relief among astronomers.

"Everybody's just ecstatic," said Hopkins research scientist Hal Weaver, who had been using FUSE to study comets. "FUSE was just starting to hit its prime. Many people ... were just starting to see their data and realize what a treasure they have on their hands."

FUSE, launched in 1999, just began its third year of observations. The first major NASA observatory to be designed, managed and controlled by a university academic department, it has observed hundreds of targets and logged a growing list of discoveries.

In November, one of the spacecraft's four "reaction wheels" ground to a halt. The wheels spin to create momentum that controllers use to turn the observatory and hold it steady during observations.

FUSE needs at least three working reaction wheels to turn in three dimensions. When a second wheel stopped spinning Dec. 10, its scientific work halted. Attempts to restart the wheels have failed.

Working almost around the clock to find another solution, scientists and engineers from Hopkins, NASA's Goddard Space Flight Center, and Orbital Sciences Corp. - FUSE's Virginia-based builder - found they could use its "magnetic torquer bars" to help aim the spacecraft.

The torquer bars are electromagnets designed to be a sort of emergency brake system. They slow the spacecraft's motions, when the reaction wheels can't, by "pushing" against the Earth's magnetic field.

But they are a "weak muscle" compared to the reaction wheels, said Bill Blair, chief of FUSE operations at Hopkins.

"The strength that we have in that weak muscle is 80 percent of what we need to be stable anyplace in the sky," he said. "It's just not quite strong enough."

The improvised control system can hold the observatory's gaze steady enough on almost half the sky - around the poles of FUSE's orbit. Observations in those zones have resumed.

When aimed beyond those limits, however, tiny gravitational and magnetic forces still cause FUSE to drift off-target as it circles the Earth.

Controllers continue working to expand FUSE's vista. And planners are rescheduling or modifying observations to make the most of its capabilities.

"It's a real headache for the planners," Blair said. In time, "we expect to be able to observe anyplace in the sky, but we have to go there when the magnetic field is strong enough, when the torquer bars are lined up and we can hold ourselves steady."

FUSE should be back to full-time observations by May 1, Blair said. In the meantime, everyone involved seems amazed that the fix works at all.

When the torquer-bar solution emerged, Blair said, advisers hoped it would be enough to keep the spacecraft from going completely out of control.

"I think a lot of people had written us off," he said. Instead, "this should put us back in business."

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