Replacing Hubble's gyroscopes is the top priority for this final repair mission to the 19-year-old observatory. The gyroscopes are part of the telescope pointing system, and half of the old ones are broken.
Astronaut Michael Massimino slowly wedged himself into the telescope, head first, and started pulling out the old gyroscopes and putting in the new ones. His spacewalking partner, Michael Good, helped.
"Trained my whole life for this," Massimino said as he squeezed his tall, husky body inside. Earlier, outside the telescope, he joked, "Anybody home?"
He had a brief fright when his communication system fouled up. For a minute or two, no one could not hear him. "That was scary," said one of the astronauts inside when the problem cleared up. "A little bit," Massimino replied.
Space is particularly littered in this 350-mile-high orbit, and Atlantis and its crew face a greater than usual risk of being slammed by a piece of junk. As a precaution, NASA has a rescue shuttle on standby, ready to launch in just three days if necessary.
It was the second spacewalk in as many days for the Atlantis astronauts. On Thursday, another two-man team installed a powerful new camera and a computer data unit, after struggling with a stubborn bolt. NASA hoped for an easier, less stressful spacewalk Friday.
In all, five spacewalks are planned so that the observatory -- beloved by astronomers and many others for its breathtaking views of the universe -- is at its apex while living out its remaining years.
Massimino, a returning Hubble mechanic who is over 6-feet tall, took care not to bump anything inside Hubble while replacing the gyroscopes. Despite the tight fit, NASA expected the work to be relatively straightforward; two gyroscopes are bundled together, for a total of three compact, 24-pound boxes. Sure enough, the first set went in easily.
These gyroscopes were installed 10 years ago. Three no longer work, and two others have been acting up. That left one perfect gyro, but it has seen a lot of use.
Hubble's old batteries, original 20-year-old parts, had been used even longer. The hefty, nickel hydrogen batteries coming out were built before the telescope was launched in 1990.
The astronauts planned to put in three new batteries -- they come three to a pack -- and the final three early next week. Each pack is about the size of a big TV set.
Massimino, who worked on Hubble during the last visit in 2002, is known among the Twittering crowd as Astro--Mike. He's been sending down tweets during spare moments since Monday's launch, but said before the flight that his spacewalks would be off limits for texting.
NASA hopes to get another five to 10 years of use out of Hubble, once the Atlantis astronauts plug in all the new equipment. They also will take a crack at fixing two broken science instruments this weekend.
The mission cost NASA more than $1 billion, one-tenth of what has been spent on Hubble over the decades.