Donald Savage/Dolores Beasley
Headquarters, Washington              March 6, 2002 
(Phone: 202/358-1547)

Nancy Neal/Mark Hess
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-8955)

Michael Purdy
The Johns Hopkins University, Baltimore
(Phone: 410/516-7160)

RELEASE: 02-45

NASA'S FUSE SATELLITE LIT AGAIN

     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 
ground.

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 satellite, but 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 
Earth." 

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 
systems.

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."

Additional information is on the Internet at: 
http://fusewww.gsfc.nasa.gov/fuse/
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