No decision had been made on whether the loosened blanket, covering a 4-by-6-inch area over a pod for engines, will be repaired during a previously planned third spacewalk or a fourth, extra one, managers said.
The loosened blanket was discovered Saturday during an inspection. Although 11 previous shuttle flights have had blanket or tile damage in that area of the spacecraft without any problems, managers were told by engineers, "'We think we're going to have some damage ... if you don't go off and fix this,"' said John Shannon, chairman of the mission management team.
Engineers think the blanket was loosened by aerodynamic forces during launch, not by being hit by a piece of debris during liftoff. The rest of the vehicle appeared to be in fine shape, NASA said.
Engineers didn't think the intense heat when the shuttle re-enters Earth's atmosphere could burn through the graphite structure underneath the blanket and jeopardize the spacecraft or the lives of astronauts, but they worried it might cause some damage that would require repairs on the ground.
With three additional shuttle flights to the space station planned this year, NASA can't afford any delays. During the repair, an astronaut will probably reach the blanket, located near Atlantis' tail, by attaching himself to the end of the shuttle's robotic arm and boom.
"We think that if ... we can secure it somehow, we don't have to worry about that blanket anymore," Shannon said.
While mission managers debated fixing the thermal blanket, two astronauts floated outside the international space station Monday to begin connecting the orbiting outpost's newest addition: a 35,000-pound segment that will increase its power capability.
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 orbited 208 miles over the southern Pacific Ocean. It ended six hours and 15 minutes later.
Reilly and Olivas removed locks and restraints on the truss segment, which was attached earlier Monday to the station's girder-like backbone.
The spacewalkers removed 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 made the various power and data connections, Mission Control activated 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.
"My gloves look good; no damage," Olivas said at one point.
Reilly and Olivas also made sure they didn'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.
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.