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
fuse.pha.jhu.edu.
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