Oct. 17, 2007 -- A university-operated space telescope that sheds new light on celestial objects both near and far will be shut down on Thursday after an unexpectedly successful eight-year run.
The Far Ultraviolet Spectroscopic Explorer, known by the acronym FUSE, was repeatedly resurrected by innovative ground control teams after its steering system failed in 2001.
The telescope, which was intended to last just three years, was launched in 1999.
More than 1,200 research papers have been published on FUSE's findings, which include detection of a hot bubble of gas surrounding the Milky Way galaxy, locating remnants of exploded stars, and measuring the amount of a special form of hydrogen called deuterium formed in the Big Bang explosion that created the universe.
The last place its ultraviolet eye observed was its home planet. With its long-troubled steering system now gone, scientists used the telescope one last time to study Earth's atmosphere in far ultraviolet light.
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The telescope's control room will be abandoned on Thursday, leaving the 3,000-pound satellite in an orbital grave. In about three decades FUSE will fall into Earth's atmosphere and burn up.
"It is a sad and ignominious end to such an outstandingly successful mission," said FUSE operations chief Bill Blair. "But a tremendous scientific legacy is left behind."
Scientists will spend about a year archiving FUSE's observations and writing final reports.
The telescope leaves an engineering legacy as well as a scientific one. Six months after reaching orbit, one of FUSE's four flywheels, needed for aiming the telescope, was showing troubling signs.
FUSE was designed to use three of its four wheels to properly orient itself and hold steady on a target with at least a 0.5-arcsecond degree of accuracy. With a one-arcsecond alignment, you could count pine needles on a tree from a mile away.
By December 2001, two of the wheels were dead. But in a week, engineers devised a way to use Earth's magnetic field to fix and hold the satellite, thus compensating for the third axis' loss of control.
The procedure turned FUSE's passive magnetic torque bars into an active spacecraft control system. The bars, which are 3-foot long, 1 1/2-foot diameter steel rods with a ferrite core, were used to push or pull against Earth's magnetic field so the momentum wheels would have an outside force to transfer their excess energy to.
With new software, the telescope was resurrected. Then engineers began planning for the next failure: the loss of FUSE's gyroscopes, which let the telescope know what position it is in. For that fix, engineers made use of a camera on the telescope that finds guide stars for positioning information.
FUSE's troubles and triumphs continued until July 12 when the last flywheel suddenly failed. Ground controllers worked for a month trying to restart the system, to no avail.
"Once we lost that last wheel, basically we could hold it steady in a safe mode, but we couldn't do any science," Blair said.
On Aug. 14, the lead scientist sent a note to NASA recommending the termination of FUSE science operations.
"It is a sad day for the FUSE project," Blair wrote that day in his Weblog.
"It has been a wild ride, but it looks like we are pulling into the station," he said. "It will soon be time to step off this roller coaster and find a new one to climb onto."
Scientists interested in the far ultraviolet view of the universe will have to content themselves with FUSE archive data for the foreseeable future. NASA has no similar replacement telescope in the works.
NASA's partners in FUSE are the Canadian Space Agency and the Centre National d'Etudes Spatiales in Toulouse, France.
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