Washington

Down to Earth? Faulty gyroscopes may force NASA to steer Compton into the atmosphere. Credit: NASA

The US space agency NASA may retire the Compton Gamma Ray Observatory satellite as early as this March, unless it can ensure that the spacecraft will not crash back to Earth following last month's failure of an onboard gyroscope.

Launched in 1991, the $600 million observatory has already outlived its planned lifetime, but was expected to stay in orbit for another eight to ten years.

Although the satellite can operate normally with its two remaining gyroscopes, it could not be pointed if another one fails. NASA has always intended to steer the spacecraft to a fiery re-entry over the ocean at the end of its useful life. An uncontrolled re-entry risks large chunks of the 17-tonne observatory crashing into populated areas.

Engineers at the NASA Goddard Space Flight Center will know in mid-February whether the spacecraft can be controlled with one or no gyros. If not, other options include sending astronauts on the space shuttle to carry out repairs, boosting the observatory to a higher orbit or ‘de-orbiting’ the spacecraft as soon as possible, while it can still be controlled.

Compton has been a “very productive mission” and is still valuable, according to Kevin Hurley of the University of California at Berkeley, who chairs the observatory's scientific users' committee.

No other high-energy satellite can cover the entire sky; but the Compton mission has passed its peak of scientific productivity, and one gamma-ray telescope, EGRET, is barely operational. NASA's review of the project in 1998 called for it to continue at a reduced funding level, but said that: “Except for solar observations, the potential for new discoveries is diminishing.”

There are several gamma-ray satellites in the works. The European Space Agency's International Gamma-Ray Astrophysics Laboratory, a collaboration with the United States and Russia, is due to launch next year. NASA's SWIFT mission to hunt for gamma-ray bursts will begin in 2003. And HETE-II, a low-cost spacecraft run for NASA by the Massachusetts Institute of Technology, is due to reach orbit this spring.

HETE-II was supposed to be launched from Kwajalein Atoll in the Pacific Ocean on 28 January. But, following last month's failure of the Mars Polar Lander, NASA headquarters last week ordered HETE-II removed from its Pegasus launcher to undergo a final round of testing.

That sense of caution — and the desire to avoid embarrassment — will influence NASA's decision. If Goddard engineers fail to convince NASA headquarters that the spacecraft can be controlled without gyros, an early de-orbit is likely. A space-shuttle rescue would be costly, and would take a long time to plan. Boosting the orbit would defer the plummet to Earth for a century or more.

Some defenders of Compton point out that it will not fall to Earth for at least six years. Another idea discussed — only half seriously, says Hurley — is letting the US defense department shoot down the satellite for target practice.