Brain Transplant for FUSE

Scientists and engineers who work with the Far Ultraviolet Spectroscopic Explorer (FUSE) have pulled off a second daring and unprecedented rescue of the satellite observatory from serious guidance problems. This time, though, they didn’t actually wait for the guidance problems to happen.

In response to hints of the potential for future new difficulties with FUSE’s gyroscopes, which are used to check the satellite’s pointing accuracy, researchers redesigned software for three computers aboard FUSE and recently uploaded the new software to the computers.

Dramatic Software Rescue

The staff of FUSE, operated for NASA by Johns Hopkins University, compared the feat to a "brain transplant." They currently maintain detailed control of FUSE‘s precise orientation through the gyros’ ability to sense even very small shifts in the satellite’s position. If too many of the gyros stop working, however, the new software will allow controllers to switch over to using the fine error sensor, a camera aboard FUSE, in their place. In the new guidance mode, detailed information on where FUSE is pointing will be determined via the positions of key stars imaged in the fine error sensor.

Artist concept of FUSE spacecraft orbiting Earth.

Jeff Kruk, principal research scientist in physics and astronomy in the Kreiger School of Arts and Sciences at Johns Hopkins and deputy chief of observatory operations for FUSE, said the new "zero gyro" mode has already been tested and proved to be even more effective at keeping the satellite precisely pointed.

"We’ve had several periods of a week or so where we’ve taken the gyros out of the loop and flown on the new software, and the pointing stability is actually a little better with the fine error sensor than it is with the gyros," Kruk said.

But Kruk and Warren Moos, professor of astronomy at Johns Hopkins and principal investigator for FUSE, cautioned that there’s still work to be done in fine-tuning and error-proofing the new system.

"Things are going extremely well so far," Moos said. "We haven’t found any major problems, but we’re not out of the woods yet."

FUSE, launched in 1999, has gathered important data about the universe by analyzing light in the far ultraviolet portion of the electromagnetic spectrum. The "brain transplant" in April was the second improvised but extraordinary effort to rescue the orbiting probe from approximately 500 miles below on Earth. In December 2001, the failure of the second of four guidance system components known as reaction wheels sent FUSE into a pre-programmed "safe mode" configuration. In less than two months of intense work, engineers and scientists were able to bring the satellite back online using parts known as magnetic torquer bars in place of the reaction wheel.

Mars Started with More Water than Earth

One of the most intriguing findings from FUSE so far has been related to finding water in the martian atmosphere.

Mars’ Valles Marineris. Artist concept of early, wet Mars, viewed east to west in the Candor Chasma. Rendering from MOLA data.
Credit:Kees Veenenbos, (c)

In a late 2001 article in Science, FUSE scientists reported what had eluded the many previous probes that looked at the Red Planet. They found molecular hydrogen (H2), which although the dominant constituent of normal matter in the universe, had disappeared below the detection limit until looked for in the far ultraviolet range that FUSE is tuned to read. The atmospheric spectrum showed a peak near 1070 Angstroms (107 nm). The observed brightness of the H2 features in the spectrum can be used to understand the amount of water escaping from the upper atmosphere of Mars. With other measurements and assumptions, the amount of water present early in Mars’ history was assessed.

The ultimate source of H2 is expected to be water, H2O. H2 in the atmosphere of Mars is thought to be created in the middle layers through photochemical processes, driven largely by solar ultraviolet radiation. H2 molecules are very light, and can diffuse up to the upper atmospheric levels. The escape of atoms and molecules from the upper atmosphere depends on the mass of the atom or molecule and the escape velocity. Mars is less massive (and has a lower escape velocity) than earth, and so it loses material at different (but predictable) rates. By observing the current amounts of various constituents in the upper atmosphere, one can use models of the physical processes to work backwards and understand the differing conditions (and relative amounts of various constituents) at various times in the past. A direct measurement of H2 has always been missing from this analysis. Predictions for the amount of H2 from such models had come in at a value of 40 parts per million (ppm). The FUSE measurements, however, come in at a lower value near 15 ppm, permitting the atmospheric models to be corrected and improved.

The estimates are surprising: between now and a point about 3.5 billion years ago, Mars has lost enough water to blanket the entire planet to a depth of 30 meters (amost 100 feet). Extrapolating back even further, it appears that Mars started with even more water per unit mass than the earth did. However, with much lower gravity, Mars has lost a much larger fraction of its water than Earth.

What’s Next

On June 23rd, the FUSE spacecraft passed its four-year anniversary since launch. This year’s pre-emptive rescue and the testing of the associated software have had little if any impact on FUSE’s scheduled scientific observations, said Bill Blair, chief of observatory operations for FUSE and a research professor of physics and astronomy at Johns Hopkins.

Polar regions show frost, often a mixture of dry ice (frozen carbon dioxide) Credit: NASA/JPL/Viking

"Since the upload, which took about a week, we’ve been back to normal science operations," he explained. "But there’s also been a long, low-level tail of activity to just kind of optimize things and track down small problems with the new software."

The upgrades are a product of nearly two years of work by engineers and scientists at Johns Hopkins, Orbital Sciences Corp., Honeywell Technical Solutions Inc., NASA Goddard Space Flight Center and the Canadian Space Agency. Researchers began to work on a new method for guiding FUSE when one of FUSE’s six gyros, always anticipated to have a finite lifespan, went dead unexpectedly early on May 31, 2001. Two gyros were built into FUSE for each of the three axes of motion. If any axis were to lose both gyros, controllers would no longer be able to point FUSE precisely.

"We were highly motivated when the first gyro went dead on May 31," Moos recalled with a wry laugh. "There have been very, very few attempts to fly precision-pointed spacecraft without gyros, and learning how was a major step forward."

Among the obstacles faced by controllers was developing ways to make sure information could be sent back and forth quickly enough between FUSE’s three main computers. Moos compared the process to trying to stop a fall from a tree-not only is there very little time to sense when an appropriate branch might be within reach, but the time send a mental command to reach out and grab that branch is also very short.

Controllers also had to develop a way to deal with the periods when the guide stars used by the fine error sensor to fix FUSE’s position were eclipsed by the Earth as FUSE orbited around it. Moos said their solution depends in part on detailed models of how torque from the Earth’s gravitational field will twist the satellite, and in part on readings they could obtain from an instrument aboard FUSE known as a magnetometer.

Kruk added that the new software uploaded to FUSE in April contained improvements to several housekeeping functions in the satellite, in part to prepare it for reduced round-the-clock human monitoring as FUSE enters an extension of its originally planned mission.

"We were able to build in more ‘smarts’ to make FUSE capable of gracefully handling almost anything that might come up," Kruk explained.

Blair concluded, "With these repairs in place, and astronomers from around the world lining up to use FUSE, the mission is on track for at least several more years of operations."

Related Web Pages

Mars Exploration Rovers (JPL/Cornell) (June 2003 dual launch)
Mars Orbiter Camera Gallery
Mars Global Surveyor
Red Rovers: Returning to Mars
Mars Exploration Website
Two Mars Rover Sites Get Science Stamp of Approval
Updated from the Wires