Shuttle Endeavour, right, is being prepared as a rescue lifeboat in case of damage to Atlantis [EPA]

The space shuttle Atlantis is preparing to blast off from Florida on a high-risk mission to repair and upgrade the Hubble space telescope.

The 11-day mission will be the fifth and final maintenance visit to the orbiting telescope before the shuttle fleet is retired, and will require astronauts to complete at least five spacewalks to install and repair equipment.

The Hubble telescope orbits the Earth at a height of almost 600km [Nasa]

The mission was originally scheduled five years ago, but was cancelled because officials at Nasa, the US space agency, ruled it was too dangerous.

Orbiting at 563km above Earth, Hubble is almost twice the height of the space station, meaning the shuttle's seven crew members will be unable to use the station as a haven if anything goes wrong with their spacecraft.

On top of the standard launch and landing dangers, Hubble's orbit poses a much higher risk of space junk hitting the shuttle.

Officials have estimated a 1-in-229 chance that a high-velocity strike by a piece of junk or a micro-meteoroid will cause catastrophic damage to the vehicle.

As a result of the dangers, Nasa has taken the rare step of rolling out a second shuttle, Endeavour, on stand-by in case it needs to be launched on a rescue mission.

Rescue plan

Hubble has produced spectacular images of the universe since its 1990 launch [Nasa]

Nasa has had a rescue plan in place since space shuttles resumed flying following the 2003 Columbia disaster.

But the plan called for astronauts to camp at the space station, which could support the crew for up to two months, while a retrieval mission was organised.

On this mission, however, the space station lifeboat scenario is not an option, meaning Endeavour has been readied as a back-up.

As a result, the cost of the Hubble upgrade mission has ballooned to almost $1bn.

If successful, the mission will extend the space telescope's life by at least five years and enable it to gaze even further into the universe, possibly close to the beginning of time.

The telescope's position, beyond the distortions of Earth's atmosphere have already produced astonishing pictures of the universe and scientists are hopeful that the upgrade will yield even greater discoveries.

"There's no area of modern astronomical research that hasn't been profoundly affected and changed by Hubble," said David Leckrone, Nasa's senior project scientist for Hubble, at a briefing ahead of the launch.

"We have literally rewritten the astronomy textbooks."

False start

Hubble, which was launched in 1990, was not always associated with excellence.

It was late, severely over-budget and, worst of all, short-sighted after a manufacturing error on its prime reflector.

As a result, Hubble's first images of the universe were a blurry disappointment.

But a shuttle repair mission in 1993 breathed new life into the project installing corrective optics and producing the first of a stream of unprecedented and breathtaking insights into the far depths of the universe.

Nasa hopes to keep Hubble operating at least until 2014 when its successor, the James Webb Space Telescope, will take over.

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