Nancy G. Neal 
	December 14, 2001
NASA's Goddard Space Flight Center, Greenbelt, Md.
Phone: 301-286-0039
E-mail: Nancy.G.Neal.1@gsfc.nasa.gov

RELEASE NO: 01-127

FUSE PROJECT WORKS TO RESTORE SCIENCE OPERATIONS

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.

ìThe project is aggressively pursuing several avenues to enable them 
to return FUSE to science operations as soon as possible,î said Dr. 
George Sonneborn, FUSE Project Scientist from NASAís Goddard Space 
Flight Center in Greenbelt, Md.ìEngineers 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.î

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, Johns Hopkins University in 
Baltimore and Orbital. At the same time, engineers are continuing 
efforts to restart one of the failed wheels.

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.

For more on the FUSE mission, go the website at:

http://fuse.pha.jhu.edu