Like spacewalking astronauts performing open-heart
surgery on NASA’s Hubble Space Telescope this week, an
ingenious team of engineers re-awakened another orbiting
observatory earlier this month — without ever leaving the

NASA’s Far Ultraviolet Spectroscopic Explorer (FUSE)
spacecraft, which some had given up for dead in December
after critical guidance components failed, was returned to
full operations when the team developed an innovative new
guidance system. The system uses a complex new set of
procedures that lets controllers use electromagnets in the
satellite to push and pull on the Earth’s magnetic field.
Experts had speculated about such an approach as a fallback
for failing satellite guidance systems, but it had never been
employed to steer a satellite with the exacting accuracy
needed for scientific observations.

“When FUSE lost two reaction wheels, I would have bet good
money that it was end of life for the mission,” said Dr. Paul
Hertz, FUSE Program Executive at NASA Headquarters in
Washington. “I am thrilled that the FUSE team pulled off a
miracle and proved me wrong.”

The guidance system problems involved FUSE’s reaction wheels.
FUSE is equipped with four reaction wheels — three primary
and a backup. Controllers use reaction wheels to steady and
point the satellite2I=”t to work the wheels must be spinning.
One reaction wheel in FUSE stopped spinning in November, and
a second stopped in December, leaving FUSE with only two
working reaction wheels. This automatically put the satellite
into a pre-programmed “safe mode” configuration on December
10, 2001.

Scientists and engineers at The Johns Hopkins University,
Baltimore; NASA’s Goddard Space Flight Center, Greenbelt,
Md.; Orbital Sciences Corporation, Dulles, Va.; and Honeywell
Technology Solutions, Inc., Morristown, N.J., worked for
several weeks to develop a solution. FUSE is operated for
NASA by Hopkins.

To make up for the missing reaction wheels, scientists and
engineers developed new sets of procedures and software that
let them use equipment known as magnetic torquer bars in a
new way. Controllers can generate local magnetic fields by
running electric current through FUSE’s three torquer bars,
and can flip the polarity of these fields by changing the
direction of current flow.

“By actively changing the electricity flow to the torquer
bars with revised software, we can use the Earth’s magnetic
field to help point the satellite,” explained Bill Blair,
chief of observatory operations at Hopkins. “The ‘control’
aspects come from the interplay between these locally
generated magnetic fields and the magnetic field of the

Jeff Kruk, deputy chief of observatory operations at Hopkins,
remembered the key moment when scientists and engineers
turned on the new guidance systems. “We could scarcely
believe our eyes when sub-arcsecond pointing stability was
achieved within seconds of enabling the new software. The
performance was even better than we had hoped. We knew then
and there that FUSE was back in business.”

Engineers have demonstrated pointing capability to less than
1/4000th of a degree, which allows the team to lock on to
guide stars and point FUSE steadily to make science
observations. The team is still fine-tuning the new guidance

FUSE was launched on June 24, 1999, on a three-year primary
science mission to probe several fundamental aspects of the
universe, including the conditions shortly after the Big
Bang, the creation and dispersal of chemical elements, and
the properties of gas clouds that form stars and planetary
systems. NASA has since recommended an additional two-year
extension beyond the prime mission. FUSE’s international
partners are the Canadian Space Agency (CSA) and the French
Centre National d’Etudes Spatiales (CNES).

“I am very excited to have FUSE back,” said George Sonneborn,
FUSE project scientist at Goddard. “This remarkable recovery
will enable NASA to complete the remaining year of the FUSE
prime mission, and perhaps two additional years of science.”

“FUSE was in its prime when the anomaly happened,” said
Professor Warren Moos, FUSE principal investigator at
Hopkins, “and so much remains to be done. We are proud of the
superb teamwork that has gotten us back to doing science, and
we look forward to more years of exciting discoveries from
the new FUSE.”

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