Heidemarie Stefanyshyn-Piper and Joe Tanner, the pair who performed the first spacewalk on Tuesday, embarked on the third and final excursion at 6 a.m. EDT, after minor glitches caused a 45-minute delay.
"Oh, that's a beautiful view," Piper said as she floated out of the spacecraft.
The astronauts have been working on installing a new addition to the space station, the first since the 2003 Columbia disaster.
"You just can't imagine a flight going as well as this one has gone," said lead space station flight director John McCullough. "I couldn't ask for a better start — a re-start — to assembly," McCullough added.
The $372 million addition consists of two solar panel wings that will eventually provide a quarter of the space station's power when it's completed by 2010. They won't generate any electricity, though, until the next space mission — slated for December — when the power system is rewired.
In today's spacewalk, Tanner and Piper will be unpacking a radiator that will later be used to dissipate the heat generated by electronics on the newly installed solar panels.
They'll spend the rest of the roughly 6 1/2 hours with tasks like replacing an antenna and picking up science experiments.
The spacewalk was delayed by almost an hour after a surge of electricity caused a circuit breaker to activate. NASA found that the circuit had not shorted and allowed preparations for the space walk to go ahead.
The spacewalking astronauts spent time in a depressurized room to rid their bodies of nitrogen and avoid decompression sickness; the depressurization pump in that room shut down after a surge in electricity. NASA determined the pump did not short, so it simply restarted it, said spokesman Bill Jeffs.
There was also a temporary glitch with Piper's space suit — the oxygen pressure dropped momentarily but came back up.
The astronauts woke up to the rock hit "Hotel California" by the Eagles, a song Tanner's family dedicated to him.
"That song reminds me of some great traveling adventures, sort of like this one," Tanner said. "It's gonna be a great day."
When Atlantis undocks from the space station on Sunday, the astronauts will fly around it and examine their handiwork, said Mike Suffredini, space station program manager. This will be the first time since Columbia that a crew will be able to do this. Besides the aesthetic benefits, NASA will also be able to check up on areas of the station they don't normally see — something they aim to do at least once a year.
The crew lands back on Earth on Wednesday, after 11 days in the cosmos.