The return of shuttle Atlantis, which will bring back Indian-American astronaut Sunita Williams home after her six month space sojourn has been delayed by two days to help astronauts fix a problem on the body of the spacecraft.

The decision to add a fourth space walk to the Atlantis crew's schedule to fix the thermal blanket on the shuttle's exterior will mean a mission of 13 days in space rather than the originally planned 11 days, said John Shannon, head of the mission management team at the National Aeronautics and Space Administration.

The announcement came even as two Atlantis astronauts ventured out for the first space walk of the mission on Monday.

The shuttle which blasted off last Friday was originally scheduled to return to earth on Tuesday.

No decision had been made on whether the loosened blanket, covering a 4-inch  by-6-inch area over a pod for engines, will be repaired during a previously planned third spacewalk or a fourth extra one, officials said. NASA [Images] has however played down concerns over the damage to Atlantis.

The loosened blanket was discovered on Saturday during an inspection of the shuttle.

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.

The shuttle -- on its first mission of the year -- docked with the ISS on Sunday, after performing a dramatic backward somersault in space.

Lift-off damage is a concern after the February 2003 shuttle disaster. The Columbia craft disintegrated as it returned to Earth due to breaks in its heat shield caused by foam insulation peeling off its fuel tank and striking a wing during the launch.

All seven astronauts aboard, including Indian-born astronaut Kalpana Chawla perished and the shuttle programme was put on hold for nearly two and a half years while the space agency sought to overcome the problem, modifying the external fuel tank and setting procedures to check the heat shield while in orbit.

Mission specialists John "Danny" Olivas and Jim Reilly emerged from an airlock on the station at 2002 GMT (0132 IST Tuesday) for a six hour, 15 minute foray into space to begin installing power-generating equipment on the International Space Station [Images].

Their task was to make power, data and cooling connections on a new 16-tonne truss segment containing solar panels. The enormous truss was attached in a delicate manoeuvre before the space walk with the help of the station's giant robotic arm.

The space walk was slightly delayed due to problems with "gyros" that control the orientation of the station, which flight controllers managed to repair, NASA said.