Experts don't believe the gap would pose any threat to the astronauts, but it could allow damage to the shuttle during its re-entry into the atmosphere. So NASA managers decided Monday to extend Atlantis' mission to the international space station from 11 to 13 days.
Engineers at Johnson Space Center in Houston were already were practicing techniques the astronauts might use to make the repair.
"It was a 100 percent consensus that the unknowns of the engineering analysis and the potential damage ... under the blanket was unacceptable and we should go in and fix it if we could," said John Shannon, chairman of the mission management team.
The thermal blankets are used to protect the shuttle from searing heat during re-entry into Earth's atmosphere. Engineers didn't think the intense heat generated by re-entry could burn through the graphite structure underneath it and jeopardize the spacecraft, 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 delays.
The repair to the thermal blanket, covering a 4-by-6-inch area over a pod for engines, likely will involve an astronaut attached to the end of the shuttle's robotic arm and boom reaching the shuttle's tail area. No decision has been made on whether it will be made during a previously planned third spacewalk or if a fourth, extra spacewalk will be added.
The rest of the shuttle appeared to be in fine shape, Shannon said.
Mission Control on Tuesday planned to begin remotely, which two astronauts helped install on the international space station during a spacewalk Monday.
Astronauts James Reilly and Danny Olivas removed locks and restraints on the new truss segment, which was attached to the station's girder-like backbone. 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; Atlantis was used to help control the station's orientation until the gyroscopes were able to take over again.
Starting overnight, the 300-foot pair of arrays was to be deployed from its storage box on the new segment slowly, in stages, to get the panels warmed by the sun and prevent them from sticking together.
It is the station's third pair of solar arrays and is similar to a pair that was added to the station last September.
On Wednesday, another solar array will be folded back up in a box so that it can be moved during a later shuttle mission. The retraction of that array will allow the new pair to rotate, following the direction of the sun.