Scientists and engineers who work with the Far Ultraviolet Spectroscopic Explorer have pulled off a second daring and unprecedented rescue of the satellite observatory from serious guidance problems.
This time, though, they didn't actually wait for the guidance problems to happen.
In response to hints of the potential for future new difficulties with FUSE's gyroscopes, which are used to check the satellite's pointing accuracy, researchers redesigned software for three computers aboard FUSE and recently uploaded the new software to the computers.
The staff of FUSE, operated for NASA by Johns Hopkins, compared the feat to a "brain transplant." They currently maintain detailed control of FUSE's precise orientation through the gyros' ability to sense even very small shifts in the satellite's position. If too many of the gyros stop working, however, the new software will allow controllers to switch over to using the fine error sensor, a camera aboard FUSE, in their place. In the new guidance mode, detailed information on where FUSE is pointing will be determined via the positions of key stars imaged in the fine error sensor.
Jeff Kruk, principal research scientist in Physics and Astronomy in the Krieger School of Arts and Sciences at Johns Hopkins and deputy chief of observatory operations for FUSE, said the new "zero gyro" mode has already been tested and proved to be even more effective at keeping the satellite precisely pointed.
"We've had several periods of a week or so where we've taken the gyros out of the loop and flown on the new software, and the pointing stability is actually a little better with the fine error sensor than it is with the gyros," Kruk said.
But Kruk and Warren Moos, professor of astronomy at Johns Hopkins and principal investigator for FUSE, cautioned that there's still work to be done in fine tuning and error proofing the new system.
"Things are going extremely well so far," Moos said. "We haven't found any major problems, but we're not out of the woods yet."
FUSE, launched in 1999, has gathered important data about the universe by analyzing light in the far ultraviolet portion of the electromagnetic spectrum. The "brain transplant" in April was the second improvised but extraordinary effort to rescue the orbiting probe from approximately 500 miles below on Earth. In December 2001, the failure of the second of four guidance system components known as reaction wheels sent FUSE into a preprogrammed "safe mode" configuration. In less than two months of intense work, engineers and scientists were able to bring the satellite back online using parts known as magnetic torquer bars in place of the reaction wheel. For the complete story, see www.jhu.edu/news_info/news/home02/mar02/fuse.html .
This year's pre-emptive rescue and the testing of the associated software have had little if any impact on FUSE's scheduled scientific observations, said Bill Blair, chief of observatory operations for FUSE and a research professor of physics and astronomy at Johns Hopkins.
"Since the upload, which took about a week, we've been back to normal science operations," he explained. "But there's also been a long, low-level tail of activity to just kind of optimize things and track down small problems with the new software."
The upgrades are a product of nearly two years of work by engineers and scientists at Johns Hopkins, Orbital Sciences Corp., Honeywell Technical Solutions, NASA Goddard Space Flight Center and the Canadian Space Agency. Researchers began to work on a new method for guiding FUSE when one of FUSE's six gyros, always anticipated to have a finite lifespan, went dead unexpectedly early on May 31, 2001. Two gyros were built into FUSE for each of the three axes of motion. If any axis were to lose both gyros, controllers would no longer be able to point FUSE precisely.
"We were highly motivated when the first gyro went dead on May 31," Moos recalled with a wry laugh. "There have been very, very few attempts to fly precision-pointed spacecraft without gyros, and learning how was a major step forward."
Among the obstacles faced by controllers was developing ways to make sure information could be sent back and forth quickly enough between FUSE's three main computers. Moos compared the process to trying to stop a fall from a tree -- not only is there very little time to sense when an appropriate branch might be within reach, but the time to send a mental command to reach out and grab that branch is also very short.
Controllers also had to develop a way to deal with the periods when the guide stars used by the fine error sensor to fix FUSE's position were eclipsed by the Earth as FUSE orbited around it. Moos said their solution depends in part on detailed models of how torque from the Earth's gravitational field will twist the satellite, and in part on readings they could obtain from an instrument aboard FUSE known as a magnetometer.
Kruk added that the new software uploaded to FUSE in April contained improvements to several housekeeping functions in the satellite, in part to prepare it for reduced round-the-clock human monitoring as FUSE enters an extension of its originally planned mission.
"We were able to build in more 'smarts' to make FUSE capable of gracefully handling almost anything that might come up," Kruk explained.
Blair concluded, "With these repairs in place, and astronomers around the world lining up to use FUSE, we are on track for at least several more years of operations."