Nasa has placed another shuttle on stand-by if Atlantis is endangered
The Hubble Space Telescope is set to receive its final overhaul when a space shuttle mission launches from Florida.
The shuttle Atlantis is scheduled to blast off on Monday in an attempt to repair the stricken space observatory.
Hubble has been hit by failures to its science instruments and to its onboard gyroscopes.
Engineers hope the challenging flight will give a new lease of life to one of the most important scientific tools ever built.
A successful mission would make Hubble up to 90 times more powerful than it was in its original guise and extend its operating lifetime until at least 2014.
Shuttle Atlantis is due to lift off at 1901 BST (1401 EDT) from Kennedy Space Center in Florida. The seven crew have arrived at the pad and have begun boarding the vehicle.
Nasa said the weather remained favourable, with a 90% chance of good conditions at the time of launch.
The fifth and final servicing flight was delayed last year, when a critical component of the telescope failed. No more such missions are planned because of the space shuttle's impending retirement in 2010.
HUBBLE SPACE TELESCOPE
Named after the great US astronomer Edwin Hubble
Launched in 1990 into a 600km-high circular orbit
Equipped with a 2.4m primary mirror and five instruments
Length: 15.9m; diameter: 4.2m; Mass: 11,110kg
"Our workload is going to be very high," the mission's lead spacewalker John Grunsfeld told BBC News.
"There's no time to take a breather and look around, it's just going to be work, work, work."
He added: "It's going to be a marathon at a sprint pace for 11 days on orbit."
But if all goes well, it could trigger a magnificent renaissance for the much-loved space telescope.
Martin Barstow, professor of astrophysics and space science at the University of Leicester, UK, has been closely involved in planning for the mission.
He told BBC News: "If (the mission) is successful, as we all hope, it will not just return Hubble to health but increase its capability tremendously with the addition of two new, even more powerful instruments.
"It is a testament to the ingenuity and commitment of many scientists and engineers. I have no doubt that we will continue to be amazed by Hubble's new discoveries during the next few years."
After launch, Atlantis will rendezvous with Hubble, grab the telescope with its robotic arm and pull it on to a work platform to give astronauts easy access to its interior.
Crew members will install new instruments and thermal blankets, repair two existing instruments, replace gyroscopes, batteries and a unit that stores and transmits science data to Earth.
The overhaul will be carried out over five spacewalks. Astronauts will remove the existing Wide Field and Planetary Camera 2 instrument to make way for the new Wide Field Camera 3 (WFC3).
WFC3 will be Hubble's first "panchromatic" camera with a wide field of view and is able to take amazingly sharp images over a broad range of colours.
"Wide Field Camera 3 is just going to blow people away with the pictures it is going to be able to take across a very wide wavelength spectrum, from the infrared to the ultraviolet," Mr Grunsfeld said.
It will enable astronomers to carry out new studies of dark energy and dark matter and search for remote galaxies previously beyond Hubble's vision.
People have devoted their lives, their hearts, their souls, their blood in some cases to building, operating and servicing Hubble
Spacewalkers will also take out the Corrective Optics Space Telescope Axial Replacement (COSTAR) device, installed during Hubble's first servicing mission to correct the telescope's flawed mirror.
This is no longer needed as instruments installed since have been designed individually to correct for the faults.
In COSTAR's place, astronauts will install the Cosmic Origins Spectrograph (COS), designed to help researchers probe galaxy evolution, the formation of planets, the elements required for life and the web of gas between galaxies.
Repairs will be made to the Space Telescope Imaging Spectrograph (STIS), which suffered a power failure in 2004, and to the Advanced Camera for Surveys (ACS), which was hit by an electrical short in 2007.
"Personally, I'm hoping that the astronauts will have time to repair the ACS camera. It may not get done because there are higher priorities on the schedule," said Dr Richard Massey, a fellow in astronomy at the Royal Observatory Edinburgh.
Referring to the two European Space Agency satellites, Herschel and Planck, which launch on Thursday, Dr Massey said it was a "make-or-break week for astronomy".
Astronauts will also replace a worn-out fine guidance sensor, helping to maintain a robust ability to point the telescope.
After the work to Hubble is complete, Atlantis will boost the telescope to a higher altitude, ensuring that it survives the tug of Earth's gravity for the remainder of its operating lifetime.
Professor Barstow commented: "I have to say that it has been a nail biting time waiting for this with all the problems that have got in the way, the most recent being the failure of the command and data handling module just about a week before the mission was due to launch last October.
"I think the team have done an amazing job of solving the problem of adding that repair to the programme and I wish them well for the mission."
Launched in 1990, the Hubble Space Telescope is now regarded as one of the most important instruments in the history of astronomy. It has made a remarkable contribution to our understanding of the origin and evolution of the Universe.
Hubble has obtained the deepest views of the cosmos, finding high-interest objects for other observatories to investigate in detail. Its studies of the Universe's expansion also dramatically refined the best estimates for the age of the cosmos.
Its pictures have produced hard evidence for the existence of black holes and confirmed theories of planetary formation.
The mission will be complex and challenging for the spacewalkers
Three of the seven shuttle crew are veterans of previous Hubble servicing missions. This will be Mr Grunsfeld's third consecutive flight to the orbiting observatory.
Speaking to me at a conference last year, he said: "Hubble is something that has a huge family here on planet Earth. People have devoted their lives, their hearts, their souls, their blood in some cases to building, operating and servicing Hubble.
"The results that we get - the science - are a bit like the love you would get from a pet. Hubble is this unique beast. When we go up into orbit, I really do feel this kinship with it. Even more so after spending five days up close and personal working on it.
"When it's time to leave, I do feel a little bit of sadness... but my big goal is not to break it, and to send it off so it can do new science."
Following the Columbia disaster in 2003, which claimed the lives of seven astronauts, another mission to service Hubble was considered too hazardous.
The reason was astronauts would not be able to use the International Space Station (ISS) as a safe haven if the shuttle sustained damage on launch.
Nasa has now accepted the risk of the mission, but will have the shuttle Endeavour ready to launch immediately to bring the crew home if the servicing mission is endangered.
This unprecedented measure comes at a cost, however: a shuttle mission costs upwards of $450m (£298m; 333m euros).
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