By Dr David Whitehouse BBC News Online science editor

Hubble has just obtained the deepest view of the cosmos More details

In January, the US space agency said there would be no more risky astronaut visits to the telescope, which would probably limit its life to a few years.

But Dr Weiler says there are now some promising ideas about how Hubble could be visited without the space shuttle.

A small spacecraft could be built to attach itself to Hubble, he believes.

"There is a lot of optimism about the robotic possibilities," Dr Weiler told BBC News Online.

He added that Nasa should be able to be more definitive about the options in June.

Robot rescue

It was Nasa's chief, Sean O'Keefe, who said there would be no more manned missions to Hubble because it was deemed too risky following the Columbia accident of last year.

Many astronomers were horrified that such an outstanding telescope, with many years of productive life ahead of it, should be abandoned in this way.

There is a lot of optimism about the robotic possibilities Ed Weiler, Nasa Associate Administrator for Space Science

Astronauts, astronomers and politicians called on Nasa to look at ways of saving the telescope.

Now, Dr Weiler says studies have revealed some promising ideas for how it might be possible for an unmanned servicing mission to carry out many of the tasks that would be necessary to keep Hubble working further into the future.

The major problems for the telescope will come when its gyros and batteries fail.

The gyros allow Hubble to be pointed accurately and must be replaced or augmented, as must the batteries.

What is envisaged is a small spacecraft that would attach itself to the observatory.

The spacecraft may then be able to plug into Hubble and feed power into it without having to remove its old batteries - which would be a tricky task for a robot. It might also be possible for the attached spacecraft to take over the pointing of Hubble.

But upgrading Hubble's optical instruments may be out of the question for an unmanned mission.

Controlled descent

Dr Weiler recently told reporters that he could imagine a situation in which a new instrument was only partially installed by a robot and could not be fixed. If that happened, it could render all the other instruments useless, he warned.

A spacecraft will have to latch onto Hubble

In the coming months, Nasa will ask industry to look into the options and come up with specific proposals for servicing Hubble.

The provisional date for such a mission is 2007-8, so Nasa needs to get contractors underway as soon as possible.

By the time any mission is flown, it is highly likely that Hubble's remaining gyros will have failed.

Engineers are working on strategies for Hubble to carry on observing even with more gyro failures, but eventually the telescope would have to be shut down to await repair.

Even if it is decided that Hubble cannot be rescued, the telescope will still have to be visited by a small spacecraft to help it re-enter the Earth's atmosphere.

Because of its size, Hubble cannot be allowed to crash to Earth in an uncontrolled manner as debris from it could pose a hazard.