A moribund orbiting observatory that some had given up for dead has returned to life thanks to the ingenuity of scientists and engineers who worked round-the-clock hours to give the satellite a new guidance system without ever actually touching it. FUSE, the Far Ultraviolet Spectroscopic Explorer, got back in business early this month after a shutdown in December caused by a guidance problem.
Key to the satellite's return is a complex new set of procedures that lets controllers at Johns Hopkins use electromagnets in the satellite to push and pull on the magnetic field of the Earth. Experts had speculated about such an approach as a fallback for failing satellite guidance systems, but it has never before been employed to steer a satellite with the exacting accuracy needed for scientific observations.
"When FUSE lost two reaction wheels, I would have bet good money that it was end of life for the mission," said Paul Hertz, FUSE program executive at NASA headquarters. "I am thrilled that the FUSE team pulled off a miracle and proved me wrong."
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 ones. 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. of Dulles, Va., and Honeywell Technology Solutions Inc. of Morristown, N.J., worked for several weeks to develop a solution. FUSE, which is operated for NASA by Hopkins, is an international collaboration among NASA, the Canadian Space Agency and the Centre National d'Etudes Spatiales (France).
"It was like preparing for launch all over again, except the satellite is already in orbit," Christopher Silva, the FUSE flight operations manager, said of the effort to bring FUSE back. "But the FUSE mission operations team hung together and rose to the challenge."
To make up for the missing reaction wheel, scientists and engineers developed new sets of procedures and software that let them use equipment known as magnetic torquer bars. Controllers can generate local magnetic fields by running electronic current through the torquer bars, and can flip the polarity of the field by changing the direction of current flow.
"The 'control' aspects come from the interplay between these locally generated magnetic fields and the magnetic field of the Earth," explained Bill Blair, research professor in astronomy 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."
Jeff Kruk, deputy chief of observatory operations, remembered the key moment on Jan. 24 when scientists and engineers turned on the new guidance systems: "We could scarcely believe our eyes when subarcsecond pointing stability was achieved within seconds of enabling the new software. The performance was even better than we had hoped. We knew then and there that FUSE was back in business."
Blair said engineers were testing and fine-tuning the new guidance systems until the return last week to full-time science operations, and 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, which allows us to lock up on guide stars and point our telescope steadily to make science observations," Blair said.
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.
"I am very excited to have FUSE back," said George Sonneborn, FUSE project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. "This remarkable recovery will enable NASA to complete the remaining year of the FUSE prime mission. There is a lot of exciting science waiting to be done."
Warren Moos, professor of astronomy in the Krieger School and the FUSE principal investigator, said, "FUSE was in its prime when the anomaly happened, and so much remains to be done. 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."
For more about FUSE, go to fuse.pha.jhu.edu.