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

Science operations for the Far Ultraviolet Spectroscopic Explorer (FUSE) mission ceased on Dec. 10 when the second of four reaction wheels, used to precisely point and maintain the spacecraft's attitude, stopped. The satellite is in a safe configuration with solar arrays pointed toward the Sun to maintain power to the spacecraft's systems.

"The project is aggressively pursuing several avenues to enable them to return FUSE to science operations as soon as possible," said Dr. George Sonneborn, FUSE Project Scientist from NASA's Goddard Space Flight Center in Greenbelt, Md. "Engineers are examining new ways of providing three-axis attitude control of the satellite in the event one of the failed wheels cannot be restarted. Although this work is very preliminary, project personnel are optimistic about reestablishing fine pointing control and resuming science operations in the near future."

FUSE uses reaction wheels to maneuver the spacecraft and maintain attitude. There are four wheels, one in each of the three body axes. A fourth reaction wheel serves as a backup and can replace any of the other three. Under normal operations, three reaction wheels are required for the spacecraft to conduct its scientific mission. Two of the wheels, along the x and y axes, have shown friction anomalies over the last two years, causing occasional erratic behavior and resulting in several short duration, less than one day autonomous shut downs of science operations. In these instances, engineers from Orbital Sciences Corporation in Germantown, Md., the spacecraft manufacturer, were able to get them started again with only a few days lost out of the science timeline.

On Nov. 25, the x-axis wheel stopped. Science operations continued using the three remaining operable wheels. However, on Dec. 10, the y-axis wheel also stopped. Although there is a hint of wheel motion, it has not been possible to spin up the wheel. This second failure led to cessation of science operations. FUSE will continue in this safe mode until it can return to three-axis control.

One of the new control mode concepts being investigated is to use the two operational reaction wheels in conjunction with the satellite's magnetic torquer bars to provide control in all three axes. The magnetic torquer bars are presently used to manage the momentum of the reaction wheels by applying a torque on the satellite against the Earth's magnetic field. The torques necessary to make up for the failed wheel would be in addition to that required for momentum management. This is well within the capability of the magnetic torquer bars. The time frame for designing, developing and testing the new control mode is currently under study by engineers and managers from NASA, John Hopkins University in Baltimore and Orbital. At the same time, engineers are continuing efforts to restart one of the failed wheels.

The FUSE mission logo. Photo: JHU

The FUSE mission was at the peak of its scientific productivity when this failure occurred. Over 50 papers based on FUSE observations are on the presentation schedule at the American Astronomical Society meeting next month.

Launched on June 24, 1999, with a minimum three-year life, FUSE is investigating the lightest elements in the Universe - hydrogen and one of its isotopes, deuterium - created shortly after the Big Bang. FUSE seeks to understand several fundamental questions about the Universe. What were the conditions shortly after the Big Bang? What are the properties of interstellar gas clouds that form stars and planetary systems? How are the chemical elements made and dispersed throughout our galaxy?

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

FUSE is a NASA Explorer mission. Goddard manages the Explorers Program for the Office of Space Science at NASA Headquarters in Washington, D.C.