Eagle Nebula: Hubble has produced so many great images

The launch, which was delayed by a day due to freezing temperatures, took place amid tight security - fighter jets were on a patrol and a 56-kilometre (35 mile) no-fly zone was established around the Kennedy Space Center.

The astronauts are scheduled to conduct at least five spacewalks to install new equipment and replace other worn-out components on the 12-year-old telescope.

The problem on the orbiter concerns the coolant loop that removes heat from the shuttle's electronics system.

Flight rules insist on two working loops, and one was slightly blocked, although ground controllers did not expect it to fail completely.

If Columbia has to return from orbit on Saturday, it will probably land at Edwards Air Force Base in California's Mojave Desert. The main landing strip at Kennedy is likely to be closed by bad weather.

The orbiting Hubble observatory has proved to be one of the triumphs of modern astronomy - it has taken some astonishing pictures of deep space - but could do even better science if it received an upgrade.

Dormant instrument

The upgrade will require Hubble to be completely powered down at one point and Nasa cannot guarantee that it will be able to switch the observatory back on again.

"That scares me a lot," Ed Weiler, the agency's head of space science said at a recent briefing.

"It kind of violates a long standing policy in the space business that if something is working well, you don't turn it off and just hope it comes back on."

The Columbia crew must fit Hubble with a new camera, solar wings, power-control unit, steering mechanism and a refrigerator system that should allow scientists to use an infrared camera that has lain dormant on the HST since 1999.

The most daunting part of the STS-109 shuttle mission will be the fitting of the new power unit, currently set for 5 March.

Hubble replacement

Astronauts John Grunsfeld and Richard Linnehan will first have to remove the degraded, old unit, "switching off" Hubble in the process.

It will then be a race against time to fit the new unit and power up the observatory again before the extreme cold of space does any damage to the telescope's instruments.

If the worst happens and Hubble cannot be turned back on, Nasa will have little choice but to abandon the observatory.

This would be a disaster, as the HST's replacement - the Next Generation Space Telescope - is still only in the design phase and is many years away from getting into orbit.

Hubble history

1977 - Project begins 1985 - Hubble built 1990, 24 April - Hubble launched 1990, 18 May - First light 1993, December - Flawed mirror corrected 1997, February - Second servicing mission 1999, December - Emergency service to repair gyroscopes 2002, March - Repairs not done in 1999 2010 - End of Hubble mission

Anne Kinney, Nasa's director of astronomy and physics, said: "Do we think we can do it all? Yes, we do. We wouldn't plan it if we didn't think that. But it is very ambitious. It's not easy. We'll worry all the way."

The mission is called the 3B servicing mission. It includes work not carried out when astronauts did an emergency repair on Hubble's gyroscopes in 1999.

If all goes well, Hubble will be back online to gather even more data about the Universe. Currently, it delivers daily between 10 and 15 gigabytes of data to astronomers all over the world.

This mission also marks Columbia's debut following the most extensive shuttle overhaul in years. The orbiter has not flown since July 1999, since which time it has undergone 133 modifications including the installation of a completely modernised cockpit.