The space shuttle Atlantis has blasted off on an ambitious and risky¹ mission to fix the Hubble Space Telescope.

阿特兰蒂斯号航天飞机已经起飞，为修复哈勃太空望远镜。

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（陀螺仪，回转仪） that are used to point the observatory² 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.

At a post-launch news conference, Nasa's Michael Moses said the shuttle had experienced two minor³ malfunctions⁴（故障） during the climb to orbit: a circuit-breaker problem and a "flaky（薄片的）" transducer（转换器，传感器） which had set off alarms. Neither had any significant effect on the launch.

Several pieces of debris⁵ were spotted⁶ during lift off, but none are thought to have posed a risk to the orbiter（盘旋物，人造卫星）.

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 renaissance⁷ 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（马拉松） at a sprint⁸（短距离赛跑） pace for 11 days on orbit."

A successful mission would make Hubble up to 90 times more powerful than it was in its original guise⁹（外观） and extend its operating lifetime until at least 2014.

After launch, Atlantis will rendezvous¹⁰（约会） with Hubble, grab the telescope with its robotic arm（机械手） and pull it on to a work platform to give astronauts easy access to its interior（内部）.

Crew members will install new instruments and thermal¹¹（热的，热量的） 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（全色的）" 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.