A team of scientists has reprogrammed a NASA space telescope in a bid to extend its mission lifetime by performing the engineering equivalent of in-flight brain surgery.

Researchers with the Far Ultraviolet Spectroscopic Explorer (FUSE) uploaded new flight software to the spacecraft's three onboard computers, changing how they operate, communicate with one another and keep the satellite aimed straight.

"Basically, we changed how the spacecraft thinks and talks," said William Blair, FUSE chief of observatory operations. Blair is also a research professor at Johns Hopkins University, which manages FUSE operations for NASA.

The FUSE satellite sees what the Hubble Space Telescope can't, peering into the short wavelengths of the far ultraviolet portion of the electromagnetic spectrum. The craft makes high-resolution spectrographic observations handy for astronomers studying interstellar gas clouds, the composition of galaxies and the conditions existing just after the Big Bang.

Originally, the spacecraft used six ring-laser gyroscopes to monitor changes in orientation, two for each position axis. These gyroscopes tracked how FUSE moved or drifted in orbit, then relayed the information to reaction wheels that spun in compensation to keep the telescope pointed at its target.

But in May 2001, one of the gyroscopes failed due to age, and its spare has been taking up the slack. Meanwhile, the remaining gyroscopes have all begun to show signs of stress due to age.

"We know that these [gyroscopes] begin to show signs of age when their laser output drops to about 50 percent of their initial value, and all five of the gyros left have tripped that flag," Blair said. "They typically quit once they drop down to 20 percent output."

The flight software update allows FUSE to get around the need for gyroscopes using its guide camera, called the fine error sensor. With the improvements online, FUSE's guide camera can locate a target and relay it to onboard computers in about three seconds, instead of minutes, which could allow time for too much drift. By continuously updating the target's position, FUSE's onboard computers can then use it's attitude control system keep the craft steady.

"Right now we're using the gyros that are still operating," Blair said. "But we can use the new system with no functioning gyros if we need to." The new positioning system can be used in tandem with one or two gyroscopes too, maximizing their lifetime instead of just waiting until they burn out.

Finding a way around the death of gyroscopes isn't the first hurdle faced by FUSE researchers since its launch in June of 1999.

In late 2001, the craft lost two of its four reaction wheels critical for attitude control. FUSE's reaction wheels spin like tops, creating torque that can either turn the craft toward a target or keep it pointed in the right direction.

"Within the space of two weeks we lost both of those reaction wheels," Blair said. "We had to drop everything we were doing to fix it." Ground engineers were able to devise a way to use magnetic torque bars, primarily used to dump excess momentum by pushing or pulling on the Earth's magnetic field, for attitude control.

The measure allowed FUSE controllers to fulfill its three-year mission lifetime. Since then, NASA officials recently cleared the spacecraft for operation over another three and a half years, leaving mission scientists like Blair more than optimistic that FUSE will live to see mission's end.

"I don't think there are any more snakes in the grass right now," Blair said.