"Atlantis is ready to fly," said NASA test director Charlie Blackwell-Thompson, adding that the countdown to launch was proceeding on schedule, with liftoff expected Monday at 2:01 pm (1801 GMT).

Weather forecasters said there was a 90 percent chance of favorable conditions for the launch from Kennedy Space Center in Cape Canaveral, Florida.

Shortly before dawn, technicians began filling Atlantis's external fuel tank.

"The tanking began at 4:41 am (0841 GMT), and there is no issue," a NASA spokesman told AFP.

The 11-day shuttle mission aims to provide the fifth and last maintenance operation to the Hubble before the shuttle fleet is retired, and if successful NASA has said the mission would extend the star-gazer's life by at least five years.

The Hubble's servicing will entail five space walks, each lasting up to seven hours. Crew members plan to replace the telescope's six gyroscopes and batteries and upgrade its optical instruments.

Launched in 1990, Hubble has long been considered the greatest tool in the history of astronomy.

Using powerful instruments to peer into deep space, it has provided profound insights into the origins and evolution of the universe.

But National Aeronautics and Space Administration experts stressed that the Atlantis mission carries heavy risks.

"This will be the most challenging servicing mission that's been faced by our astronauts in terms of the total amount of work," said Preston Burch, mission manager.

A journey to the 11-ton Hubble carries more risk of being hit by space debris or micrometeorites than a flight to the International Space Station, as the telescope orbits at almost twice the height of the ISS.

Officials hope the mission will allow Hubble to keep functioning until 2014, when it is due to be replaced by the James Webb Space Telescope, a highly sophisticated device with an eagle-eye camera.

"If successful we will be entering our second quarter century. That's not bad for a mission that we hoped will last for 10 to 15 years," said Ed Weiler, associate administrator of NASA'S science missions directorate.

Hubble "will be more powerful and robust than ever before and will continue to enable world class science for at least another five years an overlap with the James Webb Space Telescope" its successor, he added.

The crew will carry out a variety of tasks including replacing electronic circuit boards, said scientist Dave Leckrone.

Astronauts will also install a new imaging camera and a Cosmic Origins Spectrograph -- an especially sensitive instrument designed to split light it captures into individual wavelengths.

The spectrograph, NASA says, will not only be able to study stars, planets and galaxies but also basic elements found throughout the cosmos, such as carbon and iron.

And the new instruments will allow Hubble to peer even further back into time, perhaps as far back as some 600 million years before the Big Bang, much further than the billion years it can reach back now.

The maintenance is overdue after the years-long delay for US space flights since the 2003 Columbia disaster that saw the shuttle disintegrate as it re-entered Earth's atmosphere, killing all seven crew members.

Last year a flight by the shuttle Atlantis to the telescope had to be twice rescheduled after it had a computer failure onboard.

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