Gamma-Ray Satellite Now Orbits Safely in Space

NASA reboosts a `Great Observatory' to study cosmic effects predicted by Einstein's theory of general relativity

WHILE astronauts grabbed headlines, a National Aeronautics and Space Administration engineering team quietly pulled off a major space-flight achievement of its own. It lifted the endangered Compton gamma-ray observatory to a safe orbit.

That may seem pale compared with repairing the Hubble telescope. But rescuing Compton has been just as important astronomically and, in its own way, just as tricky.

A joint American-Dutch-German enterprise, the 17-ton Compton gamma-ray observatory is the heaviest civilian spacecraft ever deployed by a NASA shuttle. The $557-million craft also is Hubble's companion in what NASA calls the ``Great Observatory'' satellite class.

It studies what Hubble cannot see - the most energetic form of electromagnetic radiation called gamma rays. These rays reveal the universe's most powerful phenomena, including compact energy sources in some galactic centers that shine as brightly as a trillion suns.

The gammas also show such subtle cosmic effects as the stretching of time and space predicted by Einstein's theory of general relativity.

``We're seeing it for the first time,'' says Jay P. Norris at the NASA Goddard Space Flight Center in Greenbelt, Md, a Compton team leader.

With such important astronomical work ahead of it, Compton alarmed its Goddard managers when its propulsion system malfunctioned after the space shuttle Atlantis left the observatory in a circular orbit 450 kilometers (286 miles) above Earth on April 5, 1991. That had no immediate effect on the observatory's performance.

But its managers knew that solar activity would cause the orbit to decay. They would need that propulsion system to reboost the satellite in less than three years.

With 19,100 kilograms (4,200 pounds) of propellant, it's the largest unmanned satellite propulsion system NASA has ever launched. It has eight rocket thrusters to control the observatory's orientation and four larger thrusters to adjust the orbit.

When controllers fired up the orbit-adjust system shortly after launch, it suffered high fuel-line pressure surges. A carefully planned ``fix'' failed when Goddard engineers tried it last June. Finally, they figured out how to tweak various valves and make other adjustments that kept the fuel lines working.

Last October, they began a two-phase reboost. First, this raised the apogee (highest point) of the orbit back up to 450 kilometers. A second series of thruster firings that ended Dec. 17 circularized the orbit.

``Although this reboost was always planned, its successful completion involved some complex problems never before encountered by NASA,'' Goddard's Thomas LaVigna, who managed the reboost, dryly observes as he recalls two years of anxiety and frustration.

NASA says the reboost ``extends the mission life of the observatory by five years.'' That meets the original goal of a two- to-eight-year life span. It has been a just-in-time rescue. Mr. LaVigna says the observatory would have dropped below 290 kilometers (180 miles) by late April.

Now gamma-ray astronomers want to get on with exploring the universe with an observatory that has 10 times the sensitivity of any previous gamma-ray satellite. For them, its a new ``ballgame.'' The ``official'' observing teams have had the lion's share of Compton observing time. Beginning this October, competition for time is open to all comers on an equal basis.

They will work with the first gamma-ray satellite able to study the entire sky using the full spectrum of gamma-ray energies. Between them, Compton's instruments cover a range of photon energies from 30 thousand to 30 billion electron volts (eV). For comparison, visible light photons have energies of only 2 to 3eV. (An electron volt is the energy gained by an electron when it is accelerated by a voltage of 1 volt.)

Compton has accomplished much in its first three years. The Goddard Space Flight Center published the first gamma-ray map of the entire sky last year - fulfilling a key mission goal. Compton research teams have made many discoveries including traces of previously unknown supernova star explosions.

But their greatest astronomical challenge is the on-going puzzle of the ``bursters.'' These are brief point sources of gamma rays lasting from 0.001 seconds to 1,000 seconds.

Sometimes they briefly outshine the combined emissions all other gamma-ray sources over the entire sky. In spite of two decades of research, no one knows what they are or where they reside.

This is what the Norris team studies using Compton's Burst Transient Source Experiment (BATSE) instruments. Before BATSE, many gamma-ray astronomers thought bursters were within our own Milky Way galaxy. But BATSE sees them appearing randomly all over the sky. There's no plate-like concentration toward the plane of our galaxy to indicate that's where bursters generally lie.

If some of them are very distant, theory predicts general-relativity effects should make their ``signals'' last longer and be lower in frequency compared to nearby emissions.

Last month, the Norris team reported that it has seen what appears to be this effect in BATSE data. Norris warns that this ``should not [yet] be taken as proof'' that some bursters lie at great distances. But it is indicative of the kind of subtle cosmic research that he and other Compton investigators now can pursue.