Space shuttle Atlantis blasts off from the Kennedy Space Center in Florida
The space shuttle Atlantis has blasted off on an ambitious and risky mission to fix the Hubble Space Telescope.
Atlantis ducked through clouds as it roared up at 1901 BST (1401 EDT) from Florida's Kennedy Space Center.
Nasa managers have packaged a complex series of repairs and upgrades into five six-and-a-half-hour spacewalks.
Hubble has been hit by failures to its science instruments and to the onboard gyroscopes that are used to point the observatory at targets in the sky.
But astronauts cannot shelter on the International Space Station (ISS) in an emergency, so another shuttle will be on stand-by to rescue the crew if they are endangered.
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
At a post-launch news conference, Nasa's Michael Moses said the shuttle had experienced two minor malfunctions during the climb to orbit: a circuit-breaker problem and a "flaky" transducer which had set off alarms. Neither had any significant effect on the launch.
Several pieces of debris were spotted during lift off, but none are thought to have posed a risk to the orbiter.
Officials said the scientific pay-off would be worth the risk, effort and expense of the mission.
If all goes well, the fixes to Hubble could trigger a magnificent renaissance for one of the most important scientific tools ever built.
"I personally believe the stakes for science are very high," said senior project scientist David Leckrone.
"It's a very complex, very ambitious mission, and it makes the difference between an observatory that's kind of limping along scientifically and an observatory that's the best ever."
But he paid regard to the scale of the effort, saying: "No one should consider this mission a failure if we don't get everything done to a 100% level."
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."
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.
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.
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," said Mr Grunsfeld.
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.
Spacewalkers will also take out the Corrective Optics Space Telescope Axial Replacement (COSTAR) device, installed during Hubble's first servicing mission to correct for the blurring problem with the telescope's main mirror.
This is no longer needed as instruments installed since have been designed individually to correct for the problem.
There's now a whole younger generation of astronomers, like me, who are looking forward to a major upgrade for the first time
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.
Nasa plans to make repairs 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.
But Ed Weiler, Nasa's associate administrator for science, warned this would be no easy task.
"Putting in new instruments is bad enough. But taking things apart and putting in circuit boards when you're an astronaut wearing gloves, it's a little dicey," he told journalists.
"We'll take our best shot. But these instruments are dead right now. If they don't get fixed, we haven't lost anything."
Astronauts will also replace a worn-out fine guidance sensor, helping to maintain a robust ability to point the telescope.
The odds of Atlantis being hit by a piece of space junk are somewhat greater than for a typical shuttle mission; Hubble's orbit, some 560km (350 miles) up, is littered with debris.
Dr Richard Massey, a fellow in astronomy at the Royal Observatory Edinburgh, said he would be on the edge of his seat for the duration of the mission.
He told BBC News: "We're crippled without a space telescope, and the wait has been frustrating. I was too young to really appreciate Hubble's first servicing mission, when the optics were fixed.
"There's now a whole younger generation of astronomers, like me, who are looking forward to a major upgrade for the first time."
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.
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."
This 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.
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 was the first instrument to fix the Universe's age at about 13.7 billion years.
• The telescope was the first to analyse the chemical make-up of the atmosphere of planets beyond the solar system.
• Hubble was one of two telescopes to make the first direct images of planets orbiting another star - historic images made public last November.
• The orbiting observatory was also at the forefront of unveiling the existence of dark energy
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 space station 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 put at risk.
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