By BBC News Online science editor Dr David Whitehouse

Nasa is facing a problem with one of its most expensive and productive astronomy satellites and may deliberately force it to re-enter the Earth's atmosphere in a few weeks.

Officials say that the deliberate destruction of one of its "Great Observatories", the billion-dollar Compton Gamma Ray Observatory (CGRO), is the only alternative to letting it fall to Earth at random. At 17 tonnes, it is the largest object ever deployed by the space shuttle and will survive re-entry meaning it could come crashing down on a populated area.

Nasa calls it an "end-of-life disposal" situation and has kept more than 500 kg of rocket fuel in CGRO's tanks for a controlled re-entry.

Only a few months ago, CGRO mission controllers estimated that the satellite had a life expectancy of eight to 10 years. However, on 6 December, one of its three gyroscopes failed. Although CGRO can operate with two gyros, it cannot point accurately in space if another fails.

This worries Nasa engineers because if in the future the CGRO was left with just one gyro they might not be able to control the bulky satellite's re-entry into the Earth's atmosphere.

In-orbit repair

Like the Hubble Space Telescope and the Solar Max satellite before it, the CGRO was designed for in-orbit repair by space shuttle astronauts. But Nasa has no intention of doing this because of budget problems and pressure on the space shuttles to build the International Space Station.

The CGRO also has problems with the fuel lines to its thrusters so it is possible that even if Nasa would like to repair it, safety fears would prevent the space shuttle from going near it.

The CGRO was launched in April 1991 aboard the space shuttle Atlantis. Its mission was to observe the cosmos in gamma-rays, high energy radiation that come from the super-hot and violent parts of the Universe. It is regarded as being spectacularly successful.

Gamma ray bursts

It has provided crucial new insights into one of astronomy's most puzzling subjects, the so-called "gamma ray bursts." These were first observed by military satellites 30 years ago. They are now believed to be the most energetic explosions in the Universe, originating billions of light years away.

The CGRO is orbiting the Earth at an altitude of about 380 km (240 miles). Its path is inclined to the equator by 28 degrees. This means that it could crash as far north as Florida and India, and as far south as Australia and South Africa.

Nasa engineers are looking into ways to use other instruments on the satellite to steer it, should a second gyroscope fail. These backup procedures would allow a safe descent even if additional failures occur. If they work, then Nasa may decide to continue the operation of the spacecraft until another gyroscope fails.