Lead spacewalker John Grunsfeld and partner Andrew Feustel left the shuttle's air lock at about 8:30 a.m. EDT (1230 GMT), almost an hour ahead of schedule.
"OK, Drew. Let's go and be productive," Grunsfeld said as they headed out toward Hubble.
Their primary job is to replace three of Hubble's batteries and one of its star-tracking sensors, which are used to aim the observatory at celestial targets.
The astronauts also hope to make up some work left over from Sunday's spacewalk by crew mates Michael Massimino and Michael Good, who battled stuck bolts and equipment glitches in their attempt to revive one of Hubble's dead science instruments.
The Space Telescope Imaging Spectrograph, known as STIS, was used to hunt for black holes and probe the gas and dust between galaxies before a power failure shut it down in 2004.
Tests of the instrument showed the repair was successful, NASA spokesman Rob Navias said.
Grunsfeld and Feustel plan to replace the rest of Hubble's batteries. Three of the six batteries were replaced on Thursday during the mission's first spacewalk.
Swapping the remaining three will complete NASA's primary goals for the mission, the fifth and final servicing call to Hubble before the shuttle fleet is due to be retired next year.
Thursday Return Planned
The Atlantis crew plans to release Hubble back into orbit on Tuesday and return to the Kennedy Space Center in Florida on Thursday.
During four previous spacewalks, astronauts installed a new camera and light-splitting spectrograph, replaced Hubble's positioning system, repaired two instruments and attached a docking ring so a robotic spacecraft can be sent to remove Hubble from orbit at the end of its operational lifetime.
The upgrades are expected to keep Hubble on the cutting edge of scientific exploration for at least five years so it can work in tandem with its replacement, the James Webb Space Telescope.
Grunsfeld and Feustel also hope to have enough time to install metal shields over the telescope's instrument bays to help protect the observatory from the harsh environment of space. One of the shields was scheduled to be attached on Sunday, but the astronauts ran out of time.
NASA designed covers for three areas of the telescope and hopes to get to at least two of them on Monday.
