Then, in November and December 2001, disaster struck. Two of four reaction wheels quit working, unusual for typically reliable hardware. Reaction wheels turn, aim and hold the telescope on target. Three are needed and FUSE now had just one and a backup. "NASA thought the mission was over," Blair recalled.
But scientist Jeffrey Kruk had a possible fix." I did some initial back-of-the-envelope calculations and it was physically possible," he said.
Kruk proposed using FUSE's "magnetic torquer bars," designed to be a weak brake against Earth's magnetic field, as a third "wheel" to help turn and aim the telescope. There were skeptics, Blair said, but "we were struggling for our lives."
It was an enormously complex problem. FUSE was 500 miles up, flying through a magnetic field that changes with latitude, longitude, altitude, the time of day and the season. But for some, that was the challenge. "Many of the people involved were jazzed," said Blair. "The rest of us were going, `I need a beer.'"
But it worked. In eight weeks they were able to hold FUSE on target, and soon the scientific data were rolling in again. That lasted until December 2004, when the third reaction wheel broke down, leaving just one working wheel and the torquer bars, a much bigger problem. It took 11 months for the team to concoct a new pointing system for the telescope.
But, "over time we got the thing back to a very respectable performance," Blair said. From November 2005 until last July, with few interruptions, FUSE continued to explore the universe. It couldn't reach every part of the sky, but its extra longevity was a good trade-off.
Looking back, Blair said, the troubles "probably worked to our advantage. ... People who operate these missions are extremely capable people. So if you had a mission that launched and went exactly as planned, those people would leave. ... It never got boring."
frank.roylance@baltsun.com
