An artist's concept of FUSE in space. Photo: JHU

Like spacewalking astronauts performing open-heart surgery on NASA's Hubble Space Telescope this week, an ingenious team of engineers re-awakened another orbiting observatory earlier this month -- without ever leaving the ground.

NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft, which some had given up for dead in December after critical guidance components failed, was returned to full operations when the team developed an innovative new guidance system. The system uses a complex new set of procedures that lets controllers use electromagnets in the satellite to push and pull on the Earth's magnetic field. Experts had speculated about such an approach as a fallback for failing satellite guidance systems, but it had never 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 Dr. Paul Hertz, FUSE Program Executive at NASA Headquarters in Washington. "I am thrilled that the FUSE team pulled off a miracle and proved me wrong."

The guidance system problems 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 to work the wheels must 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 pre-programmed "safe mode" configuration on December 10, 2001.

Scientists and engineers at The Johns Hopkins University, Baltimore; NASA's Goddard Space Flight Center, Greenbelt, Md.; Orbital Sciences Corporation, Dulles, Va.; and Honeywell Technology Solutions, Inc., Morristown, N.J., worked for several weeks to develop a solution. FUSE is operated for NASA by Hopkins.

To make up for the missing reaction wheels, scientists and engineers developed new sets of procedures and software that let them use equipment known as magnetic torquer bars in a new way. Controllers can generate local magnetic fields by running electric current through FUSE's three torquer bars, and can flip the polarity of these fields by changing the direction of current flow.

"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," explained Bill Blair, chief of observatory operations at Hopkins. "The 'control' aspects come from the interplay between these locally generated magnetic fields and the magnetic field of the Earth."

Jeff Kruk, deputy chief of observatory operations at Hopkins, remembered the key moment when scientists and engineers turned on the new guidance systems. "We could scarcely believe our eyes when sub-arcsecond 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."

Engineers have demonstrated pointing capability to less than 1/4000th of a degree, which allows the team to lock on to guide stars and point FUSE steadily to make science observations. The team is still fine-tuning the new guidance systems.

FUSE was launched on June 24, 1999, on a three-year primary science mission to probe several fundamental aspects of the universe, including the conditions shortly after the Big Bang, the creation and dispersal of chemical elements, and the properties of gas clouds that form stars and planetary systems. NASA has since recommended an additional two-year extension beyond the prime mission. FUSE's international partners are the Canadian Space Agency (CSA) and the French Centre National d'Etudes Spatiales (CNES).

"I am very excited to have FUSE back," said George Sonneborn, FUSE project scientist at Goddard. "This remarkable recovery will enable NASA to complete the remaining year of the FUSE prime mission, and perhaps two additional years of science."

"FUSE was in its prime when the anomaly happened," said Professor Warren Moos, FUSE principal investigator at Hopkins, "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."