Hubble is having problems with its gyroscopes. These are used for stability and for fine control when pointing the telescope at the astronomical objects.

If the problems worsen, Hubble may shut itself down, bringing a temporary halt to scientific observations.

Hubble has produced stunning images

Nasa now has to decide whether it can wait until its next scheduled service mission in June 2000, or order an earlier shuttle flight to the telescope in October 1999.

Hubble needs three gyroscopes to work properly and it has lost two of its six. Another gyro "is sick and may go down in three months," Nasa's Dan Goldin told the US Congress.

If the telescope loses a fourth gyro, it will switch into an automatic "safe mode" until repairs are made, he said. A decision about a repair mission is expected no later than next week.

But although it appears to have sufficient gyroscope back-up, David S. Leckrone, senior project scientist for the telescope, said the observatory is already on the edge of shutting down.

Close to shut down

"If another gyro fails, we would not put the fourth gyro back into service," he said, meaning that the Hubble is really only one failure away from being shut down.

The repair mission could fly in October

Leckrone said he and others at the Goddard Spaceflight Center have proposed that an emergency mission be flown this October to replace the ailing gyros, and that the rest of the serving be done on a mission in 2001.

An earlier servicing mission to change cameras and other equipment was flown in 1997. The successful flight showed the worth of the space shuttle. A final servicing mission is planned for 2003.

Hubble was launched in 1990. Almost immediately it hit problems when astronomers realised that it was short-sighted - its 2.4-meter mirror had been ground to the wrong curvature.

A new set of optics was installed by astronauts in 1993 which corrected the problem. Since then, Hubble has produced the most stunning pictures ever taken of the cosmos.

Some of the objects it has imaged are more than 13 billion light years away, near the theoretical edge of the universe.