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The newspaper of The Johns Hopkins University January 10, 2005 | Vol. 34 No. 17
FUSE Science Operations Suspended Because of Mechanical Problem

By Lisa De Nike

Science operations for the Far Ultraviolet Spectroscopic Explorer mission were suspended on Dec. 27, when the third of four reaction wheels, used to precisely point and maintain the spacecraft's attitude, stopped spinning. The satellite is in a safe configuration with solar arrays pointed toward the sun to maintain power to the spacecraft's systems while the malfunction is being investigated.

A similar problem occurred in late 2001, but science operations were successfully resumed within about two months.

"The project is aggressively pursuing a similar track in order to return FUSE to science operations as soon as possible," said George Sonneborn, FUSE project scientist from NASA's Goddard Space Flight Center in Greenbelt, Md.

Scientists and engineers are examining ways of re-establishing three-axis attitude control of the satellite in the event that none of the failed wheels can be restarted. Although this work is very preliminary, project personnel are cautiously optimistic that pointing control and science operations can be resumed.

"We've done it before, and we are hopeful we can do it again," said William Blair, FUSE's chief of observatory operations at Johns Hopkins.

Initially FUSE used four reaction wheels to maneuver the spacecraft and maintain attitude. There were wheels that controlled each of the three satellite body axes and one at a skewed angle that could replace any one of the other three. Two of the wheels, along the x and y axes, failed within a two-week period in November-December 2001, at which point FUSE spent roughly two months in a safe mode while a new control mode was developed. Now the z-axis wheel has stopped, leaving only the skew wheel operating.

Under normal operations, three reaction wheels are required for the spacecraft to conduct its scientific mission. The revised control mode developed in 2001 utilized the two remaining reaction wheels and the satellite's magnetic torquer bars to provide control in all three axes. The magnetic torquer bars (essentially controllable electromagnets) are able to apply torque on the satellite by interacting with the Earth's magnetic field, which provides a tenuous but acceptable level of control in place of a missing reaction wheel.

"It's like we had two strong muscles and one weak muscle," said Warren Moos, the principal investigator for FUSE. "Now, assuming we cannot restart the errant wheel, we will have one strong muscle and two weak muscles. We have to teach the satellite to compensate."

Launched on June 24, 1999, with an initial three-year lifetime, FUSE is now in an extended mission granted by NASA to carry out a broad range of science programs for hundreds of astronomers from around the world. To date, more than 275 publications based on FUSE observations have appeared in professional astronomy literature, and many more are on the way. A new set of observations for the coming year was about to be announced by NASA, but this has been put on hold pending assessment of FUSE's future capabilities.

Johns Hopkins has primary responsibility for all aspects of the project, including both the development and operational phases of the mission. The FUSE mission and satellite control center are on the Homewood campus. FUSE partners include the Canadian Space Agency, French Space Agency, University of Colorado at Boulder and University of California, Berkeley.

FUSE is a NASA Explorer mission. Goddard manages the Explorers Program for NASA.

For more on the FUSE mission and future status updates, go to


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