The start of the spacewalk was delayed by more than an hour because the four spinning gyroscopes that keep the space station properly positioned became overloaded. Space shuttle Atlantis was used to help control the station's orientation until the gyroscopes were able to take over again.
This pushed back efforts by astronauts on the space station to place the new segment with the station's robotic arm. The new segment needed to be securely attached before the spacewalkers could begin making power and data connections.
Astronauts James Reilly, on his fourth spacewalk, and Danny Olivas, on his first, began their spacewalk at 4:02 p.m. EDT as the space station flew 208 miles over the southern Pacific Ocean.
"Danny and J.R., have a great" spacewalk, Mission Control told the astronauts.
"Thank you. See you in a few hours," Olivas responded.
During their scheduled 6½-hour spacewalk, Reilly and Olivas also planned to remove locks and restraints on the truss segment, which was attached earlier Monday to the station's girder-like backbone.
The spacewalkers will remove the restraints so solar arrays inside the segment can be deployed the next day. The new solar arrays will add about 14 kilowatts of power-generating capability to the station.
As the spacewalkers make the various power and data connections, Mission Control will begin activating the truss segment.
Installation and activation of the new segment won't be completed until a second spacewalk on Wednesday.
As they worked, Reilly and Olivas planned to periodically check their gloves. A new spacewalking procedure requires astronauts to examine their gloves after every task to make sure there are no cuts in them.
Reilly and Olivas will also make sure they don't lose any tools or bolts to the void of space. Astronauts lost bolts during two spacewalks in September when a similar truss segment was installed.
Back on Earth, engineers in Houston were evaluating whether a peeled-back thermal blanket on Atlantis should be fixed by astronauts. The loosened blanket, covering a 4-by-6-inch area over a pod for engines, was discovered during an inspection of the space shuttle on Saturday.
Engineers think the blanket was loosened by aerodynamic forces during launch, not by being hit by a piece of debris during liftoff.
If NASA decides to fix the problem, it could be done during one of three scheduled spacewalks or during an extra, unplanned one.
Engineers didn't think the intense heat when the shuttle re-enters Earth's atmosphere could burn through the graphite structure underneath the blanket, but they were worried it might cause some damage that would require repairs on the ground.
The rest of the vehicle appeared to be in fine shape, NASA said.
Clayton Anderson, who went up aboard Atlantis, has replaced Sunita Williams as the U.S. resident on the space station.
Williams will return to Earth aboard Atlantis next week after more than six months in space.