Weary Hubble spacewalkers face riskiest repair

— -- Astronauts are getting ready for what could be the most complex spacewalking of their mission to upgrade the Hubble Space Telescope.

Astronauts John Grunsfeld and Drew Feustel will venture out of the shuttle Atlantis to work on Hubble's survey camera.

They were given an extra hour to sleep because Friday's spacewalks were expected to be long and hard.

But scientists have said Saturday's work could be some of the most intricate of the mission. The astronauts will be attempting to fix a scientific instrument while in orbit. Before, they've just swapped one instrument for another while spacewalking.

The effort involves new tools and dozens of pieces that could fly around and do damage to Hubble.

Spacewalking astronauts had to put a refurbished pair of gyroscopes into the Hubble after a brand new set refused to go in Friday, but scientists were satisfied nonetheless and confident the observatory would point precisely to ever more distant objects in the cosmos.

Replacing the gyroscopes was the top priority of the repair mission, and the struggle had NASA on edge for two hours.

Thanks to the spacewalkers' effort, Hubble ended up with four brand new gyroscopes and two refurbished ones that were original 19-year-old telescope parts, said to be almost as good as the new ones. The telescope also got fresh batteries.

It was the second spacewalk in as many days for the Atlantis astronauts, who once again were bedeviled by problems. On Thursday, another two-man team installed a powerful new camera and a computer data unit, after struggling with a stubborn bolt. NASA had hoped for an easier, less stressful spacewalk, but instead had to endure more drama.

As on Thursday, the astronauts got their work done, but it was harder and took longer than expected. Friday's spacewalk was one of the longest ever, lasting nearly 8 hours, and Mission Control told the weary crewmembers that they could sleep in and start Saturday's spacewalk a little late.

Michael Massimino, who was working from inside Hubble, and his partner, Michael Good, had no problem removing all six of Hubble's 10-year-old gyroscopes. But the last set of new gyroscopes wouldn't fit properly.