Scientists and engineers working with FUSE -- NASA's Far Ultraviolet Spectroscopic Explorer -- satellite have created a way to compensate for a breakdown in the orbiting observatory's guidance systems. The new fix could have astronomers back to making full-time science observations with FUSE, which has been on hiatus since December, as early as March 1. FUSE is operated for NASA by Hopkins.
Employing an approach experts had speculated about but never before actually used to fine tune the orientation of an orbiting satellite, controllers worked out ways to push and pull on the magnetic field of the Earth with FUSE components known as magnetic torquer bars, which are essentially electromagnets.
By running electricity through a torquer bar, controllers can generate a local magnetic field. They also can flip the polarity of the magnet by changing the direction of the current flow. There are three torquer bars in FUSE, mounted at 90-degree angles.
"The 'control' aspects come from the interplay between these locally generated magnetic fields and the magnetic field of the Earth," explained Bill Blair, astronomy research professor in the Krieger School of Arts and Sciences and chief of observatory operations. "By actively changing the electricity flow to the torquer bars with revised software, we can use the Earth's magnetic field to help point the satellite."
The guidance system problems that necessitated the fix involved FUSE's reaction wheels. FUSE is equipped with four reaction wheels--three primary and a backup. Controllers use reaction wheels to steady and point the satellite, but in order to work, the wheels have to be spinning. One reaction wheel in FUSE stopped spinning in November, and a second stopped in December, leaving FUSE with only two working reaction wheels. This automatically put the satellite into a preprogrammed "safe mode" configuration on Dec. 10, 2001.
Scientists and engineers at Hopkins, NASA's Goddard Space Flight Center, Orbital Sciences Corp. and Honeywell Technology Solutions Inc. worked for several weeks on a nearly round-the-clock basis to develop a solution.
Blair said engineers are still fine tuning the new guidance systems but are very encouraged by what they've been able to do so far.
"We have demonstrated pointing capability to less than 1/4000th of a degree," said Blair, "which allows us to lock up on guide stars and point our telescope steadily to make science observations."
So far, observations of stars and galaxies with the "new" FUSE show that the revivified spacecraft should be able to study targets in almost half the sky over a 60-day period.
"Operators hope to increase this observable region considerably, perhaps over the entire sky, as they learn more about the stable and unstable regions over the next few months," Blair said.
FUSE was launched on June 24, 1999, and a three-year primary science mission began Dec. 1, 1999. NASA has since recommended an additional two-year extension beyond the prime mission, so at least three years' worth of additional science observations are still anticipated.
Astronomers have been using FUSE to probe several fundamental aspects of the universe, including the conditions shortly after the Big Bang created the universe; the creation and dispersal of chemical elements; and the properties of gas clouds that form stars and planetary systems.
"FUSE was in its prime when the anomaly happened, and so much remains to be done," said Warren Moos, Hopkins professor of astronomy and the FUSE principal investigator. "We are proud of the superb teamwork that has gotten us back to doing science, and we look forward to more years of exciting discoveries from the new FUSE."