Science operations for the Far Ultraviolet Spectroscopic Explorer (FUSE) mission were suspended on Dec. 27, 2004, 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 anomaly is 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.

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

FUSE has four reaction wheels. Originally any three of the wheels were required 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 using only two wheels. Now the z-axis (roll) wheel has stopped, leaving only the skew wheel operating.

Initially, three reaction wheels were required for the spacecraft to conduct its scientific mission. The revised control mode developed in early 2002 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. FUSE operations staff are now attempting to establish magnetic control on two axes, in place of the missing reaction wheels.

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 over 275 publications based on FUSE observations have been published in the professional astronomy literature, and many more are on the way. A new set of observational programs for the coming year was about to be selected by NASA, but this has been put on hold pending an assessment of FUSE's future capabilities.

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

FUSE is a NASA Explorer mission. NASA/Goddard manages the Explorers Program for NASA Headquarters in Washington, D.C.