Nasa sweats over third repair mission for Hubble telescope

Nasa astronauts embark today on one of the most ambitious space missions undertaken when they blast off to repair the Hubble telescope for the third time.

The risky 11-day trip, launched from Cape Canaveral in Florida, aims to improve tenfold the orbiting observatory's ability to capture images and data, enabling it detect light almost from the beginning of the universe. But if the delicate task goes wrong, it could also render the telescope just one more piece of space junk.

The crew of the space shuttle Columbia must carry out five long spacewalks and work on power systems that were not designed to be repaired in space. Engineers 300 miles below on Earth are not 100 per cent certain that the power systems will start up again.

Ed Weller, Nasa's head of space science, admitted that prospect "scares me a lot". He said: "It kind of violates a long-standing policy in the space business: if something's working well you don't turn it off and just hope it comes back on." Hubble's power has not been turned off since its 1990 launch.

Preston Burch, the Hubble project manager at Nasa's Goddard Space Flight Centre in Maryland, said: "We've never done this before in orbit. It's a very complex process. There's nothing easy or straightforward about it."

As well as the power unit, the seven astronauts will fit a new camera, new solar power arrays (the "wings" seen in photographs), a steering mechanism and a refrigeration system that would let astronomers on the ground revive an infra-red camera that has sat dormant on the telescope since its last major refit, in November 1999. Before that, it had undergone a substantial repair mission in 1993, which fixed a faulty mirror system and transformed the telescope into the most admired piece of scientific equipment on, or near, the planet, generating between 10 and 15 gigabytes of data each day for astronomers.

Anne Kinney, Nasa's director of astronomy and physics, said: "It is very ambitious. It's not easy. We'll worry all the way."