The Johns Hopkins Gazette: September 7, 1999
September 7, 1999
VOL. 29, NO. 2


475 Miles Up, FUSE Comes to Life

Scientists turn on satellite in orbit and check out its systems

By Michael Purdy

Johns Hopkins Gazette Online Edition

The rocket went up, the cameras clicked and whirred, and a community rejoiced as a great challenge was met: The Far Ultraviolet Spectroscopic Explorer, a NASA space exploration satellite managed by Johns Hopkins astronomy researchers, had successfully made it into Earth orbit.

Now, nearly 10 weeks later, the photos are developed and pasted in albums, the launch videotapes copied and distributed, and scientists are hoping to obtain this month the first scientific readings from FUSE.

Meanwhile, the second major phase of FUSE's life--the cautious, intricate business of turning on the satellite in orbit and checking it out--has gone almost unnoticed by the general public. The procedures involved are delicate and complex, like teaching advanced ballroom dancing through brief conversations on an occasionally erratic long-distance telephone line. Scientists are preparing all of FUSE's instrumentation and control systems for probing the origins of the universe and other astronomical subjects through analysis of ultraviolet radiation.

"It's so strange for someone who is used to using a ground-based telescope, where you have your hands on the telescope the whole time," says Bill Oegerle, research scientist and chief of science operations for FUSE. "You can turn the knobs and slew the telescope around, and if suddenly your guide stars disappear, you can walk outside the dome and you can say, 'Oh, I forgot to move the observatory dome, and I'm looking at the inside of the dome.' "

In the FUSE control room, mission operations manager J.B. Joyce and graduate student Julia Andersen plan for upcoming contacts with the satellite. The monitors show FUSE's orbital path and the communications range of its antennas.

However, diagnosing the causes of unexpected problems can take more work for a scientific instrument orbiting the Earth at nearly 17,000 miles per hour and at an altitude of 475 miles, and the checkout period is where most of these challenges are encountered.

"You test all the subsystems and the detectors and the flight computers on the ground in simulators for months or years," says Oegerle. "Then, when you get up into orbit, you find out what life is really like. You have some assumptions that don't work the way you expected they would."

Scientists test their assumptions and implement their plans from the Satellite Control Center at Homewood, a first-floor room in Bloomberg with two rows of computer work stations (each named for a planet); a large monitor covering one wall, where displays from many of the work stations are projected; and a four-foot model of FUSE itself. Also in the back is a tall figure that both reflects the challenges of satellite management and helps relieve the stresses they cause: an inflatable version of the highly anxious dominant figure from Edvard Munch's famous painting The Scream.

"We haven't had any major screams yet," says Bill Blair, FUSE mission planning chief and a professor of physics and astronomy. "Just minor screams so far."

During the checkout period, researchers and controllers communicate with FUSE through computer connections that end up in Hawaii or Puerto Rico, where two large radio antennas can reach FUSE for periods of about 10 to 13 minutes at a time.

FUSE orbits the Earth every 100 minutes, and on most of these passes around the planet, it will go through one or both of the "footprints" of these antennas, areas in space where communication is possible. For about seven hours every day, though, FUSE's path over the planet misses both footprints, leaving the satellite out of reach.

"We have to plan very carefully for our periods of communication," says Blair. "We don't, for example, want to turn a heater on in the satellite without considering what will happen if we lose communication early and the heater stays on until we hear from FUSE again."

"It's a little frustrating sometimes to try to have these little windows of being able to fix things and see what's going on," says Oegerle. "Most of the time you have to trust the great deal of autonomy that's built into the onboard computers [for the satellite] to carry out these activities on its own."

Scientists began working their way through the carefully planned checkout procedures on June 24--the day of FUSE's launch.

"We received our first signal from FUSE during powered flight," J.B. Joyce, FUSE mission operations manager, recalls with almost paternal pride. "It sent us a wealth of status information before it had even separated from the rocket."

In the early weeks, scientists checked FUSE's pointing system, making sure that its automatic guidance systems kept solar panels aligned with the sun to supply power, and that the satellite's instruments were pointing away from the sun and the Earth to avoid potentially damaging exposures to radiation.

Tests of FUSE's ability to find stars and areas for observation based on instructions from the ground revealed an early challenge. Scientists learned that FUSE, built to act on its own in many situations, was being a little too finicky about finding a specific observation target. As the satellite tried to lock onto specific guide stars, small differences in the expected and observed positions of the stars were causing the system to balk.

"We have developed a way to trick the system into thinking it's finding what it needs to find," said Blair at that time. "Now we've got to work on automating that process so it doesn't take quite so long."

By early August, scientists had the baffles open on all four telescopes and were continuing to work with the satellite's pointing systems. Other challenges were encountered and dealt with, such as an unexpectedly high rate of bit flips, which are changes of single bits of information in the computers caused by radiation. Meanwhile, scientists were closely eyeing the readout from a pressure gauge inside the satellite's spectrometers.

"When you launch a satellite, it's full of air everywhere," explains Oegerle. "When you get up in orbit, then you're more or less in a vacuum, and most of the air rushes out real quick, and you immediately get down to a low pressure level."

Some scattered molecules of air linger a little longer, though, and water begins to come off the structure of the satellite. Until the pressure gauge showed that most of this material had escaped the satellite, scientists were very careful to avoid doing two things: pointing FUSE at the bright sunlight reflecting off the Earth or turning on the high-voltage detectors in FUSE's spectrometers.

Early exposure to the Earth could have caused a chemical reaction that would create a polymer "gunk" on mirrors and reduce FUSE's ability to gather light. Turning on the detectors too early, with material still inside the spectrometer, could result in an errant arc of electricity damaging the sensitive ultraviolet light detectors.

The residual material took several weeks longer than anticipated to leak out, but by the third week of August, scientists had begun to turn up the voltage on the detector. That's when they encountered another unexpected challenge: An automatic mechanism created to protect FUSE from a dangerous orbital area known as the South Atlantic Anomaly was pushing down the voltage.

One complicated software fix later, the FUSE checkout crew is looking forward to the final stages: Align the four telescopes, focus them and repeat several times; check spectrometer readings on known observation targets to verify that they are getting good readings; and allow the telescope to pass across the bright Earth, increasing exposure time but double-checking to make sure there has been no loss of sensitivity by observing other, less bright areas of space for faint stars and galaxies.

"We're not taking any undue risks," says Joyce, a 30-year NASA veteran. "We have many scientists and engineers rethinking things as necessary as we go, and we have a well-designed, thoroughly tested satellite. We have contingency plans for almost any emergency."

The FUSE control center will stay staffed by at least two engineers around the clock for about the first year and a half, according to Joyce.

Right now, three computerized clocks are displayed on the big screen in the control center: One shows Greenwich Mean Time, a world standard time; the second displays the amount of anticipated communication time available in the next communication with FUSE; the third, the amount of time until that hookup starts.

Down the hall from the control center, a small item on a bulletin board depicts the rocket that launched FUSE taking off. Emblazoned across the romance of the rocket's fury is a defiant cry of victory: GRAVITY LOSES AGAIN.

Waking up FUSE in orbit and preparing it to do its work may lack some of the glamor of a head-to-head battle with gravity, but it has all of the interest, challenge and importance.