It is a risky business servicing the Hubble Space Telescope

A panel of experts commissioned by the US space agency (Nasa) believes the observatory has been so successful in unlocking the secrets of the Universe consideration should be given to keeping it serviceable beyond 2010 - its current termination date.

This would ensure astronomers have access to a major space observation platform while they wait for Hubble's replacement, which is not due for launch until 2011.

The panel, chaired by Professor John Bahcall from Princeton University and the Institute for Advanced Study, has delivered a report that sets out the possibilities for the retention of the telescope.

It details the different options available that could keep Hubble doing groundbreaking science for as long as possible.

Science gap

Any final decision, however, will turn on the outcome of the Columbia shuttle disaster investigation.

HUBBLE'S SUCCESSES Gave us the age of the Universe Provided proof of black holes Gave first views of star birth Showed how stars die Caught spectacular views of Comet Shoemaker-Levy 9's collision with Jupiter Confirmed that quasars are galactic nuclei powered by black holes Gathered evidence that the expansion of the Universe is accelerating

Without the orbiter and spacewalking astronauts, the HST may not even make it to 2010. Some of its components must be replaced periodically.

In particular, the gyroscopes that enable the HST to turn and point in a desired direction gradually malfunction. And although the observatory has backups, these too will eventually fail.

It is possible, that in the fallout from the Columbia accident, future shuttle activities may be severely restricted. In that case, Hubble may not get the servicing mission it requires to operate past 2005/6, let alone make it to 2010 and beyond.

This nightmare scenario would leave the astronomical community without a flagship space observatory for several years. Hubble's replacement - the James Webb Telescope - is not expected to blast off until 2011, and many scientists expect this date to slip.

It is a situation that would be a shattering blow to scientists who have come to rely on the HST.

At a recent meeting to discuss the telescope's fate, Dr Edward Cheng, a development scientist on the project, told BBC News Online: "[Astronomers] don't know a world without Hubble.

"Such is the volume of data that the telescope has been able to gather that scientists' first reaction now when faced with an idea or question is always 'What can Hubble tell us?'".

Powered descent

The HST-JWT Transition Panel, which included the English Astronomer Royal, Professor Sir Martin Rees, says there are three options for the future.

One would see no servicing mission go to Hubble and the observatory end its life when its current gyros or other components ceased working.

Everything hangs on the shuttle

A second option would see a shuttle mission before the end of 2006 to carry out one more service to make the HST operate for as long as possible. It would also fit the observatory with a propulsion device so that it could be deorbited safely.

The third option, which many astronomers will hope for, is that there are two further servicing missions, with the second in about 2010 enabling Hubble to operate well into the next decade.

But, the panel says, astronomers will need to be sure this option is the best way to get the science done.

"The extended HST science programme resulting from [a second servicing mission] would only occur if the HST science was successful in a peer-reviewed competition with other new space astrophysics proposals," a statement released by the panel says.

Dr Ed Weiler, Nasa's associate administrator for space science, accepted the panel's report on Thursday, saying: "We have a big job to do to study the panel's findings and consider our options, and we will respond as soon as we have time to evaluate their report."