The Hubble space telescope is in danger of shutting down following the failure of its back-up gyroscopes, the devices that control the spacecraft's pointing system.
It carries six gyros but only needs three to ensure the remarkable pointing precision achieved by the instrument as it orbits the earth. Should another gyro fail then it would be unable to continue its mission and the telescope would be "parked" until a rescue mission could be mounted by NASA and its space shuttles.
NASA points out, however, that all remains well with the instrument, which has given scientists some of the clearest images yet achieved of galaxies and star clusters billions of light years away. It continues to operate normally and is sending back data on its observations, according to the space agency.
It was scheduled for a "service" next year when a shuttle launch to the telescope was planned. Delays in the deployment of Russian elements of the International Space Station have, however, opened up an opportunity to send a shuttle up this October, so NASA has begun training astronauts for an extra mission to the telescope later this year.
The repair mission will require several spacewalks. All six gyros will be replaced in October and the telescope will get a new onboard computer and new guidance sensor. A failed spare transmitter will also be replaced.
Next year's servicing mission will still go ahead, according to NASA, but with a reduced workload requirement given the October flight. Of particular interest during this trip will be the installation of a new camera that will be 10 times more powerful than the present Faint Object Camera.
"The Hubble space telescope is the crown jewel of NASA's space observatories and we need to do everything within reason to maintain the scientific output of this national treasure," said Dr Edward Weiler of NASA's Office of Space Science.
The hope now is that the essential remaining gyroscopes will continue to function long enough for the repair mission to reach the telescope. Should another fail, it will automatically shut down its instruments and align itself to keep its solar panels pointed towards the sun.