Crucial repairs of a torn thermal blanket on the body of the Atlantis space shuttle, which will bring back Indian-American astronaut Sunita Williams home, will be the main task of the two astronauts when they undertake the third of the missions's four planned spacewalks.

James Reilly and Danny Olivas plan to come out of the International Space Station [Images] with which the Atlantis has been docked before midnight on Friday to staple down the blanket that peeled back during the launch of space shuttle Atlantis last Friday.

Sunita had spent a record six months in space.

The ISS had its own share of problems due to malfunctioning Russian computers though some communication with the systems was restored.

Seven visiting shuttle astronauts and three ISS crew members are currently living at the orbiting outpost and evacuation of the station is not being ruled out.

During the spacewalk, the shuttle's robotic arm will move Olivas to the tail of Atlantis where the thermal blanket covers an engine pod.

Engineers do not think the damaged section of the thermal blanket, which protects part of the shuttle from the blazing heat of re-entry, would endanger the spacecraft during landing. But it could cause enough damage to require schedule-busting repairs.

"This will be a great repair that brings Atlantis home safely," said Atlantis crew member Patrick Forrester, who will coordinate the spacewalk from inside the space station.

NASA [Images] has focused intensely on problems that could jeopardise the shuttle's re-entry into Earth's atmosphere since a shuttle damage resulted in the 2003 Columbia disaster that killed seven astronauts including Indian-born astronaut Kalpana Chawla.

Training for spacewalk tasks can take months, but Olivas only has had a day to prepare for the repair job.

Mission Control had only a few days to develop the procedures, which will use a medical stapler and loop-headed pins to secure the blanket corners in place against protective tile.

Meanwhile, engineers continued testing whether a bad power feed going into the Russian side of the international space station was causing failures in computers that control the outpost's orientation and produce oxygen.

Cameras, computer laptops and some lights on Atlantis were turned off on Thursday to save energy in case it needs to stay an extra day at the station to help maintain the outpost's orientation while the problem with the Russian computers is addressed.

The mission had already been extended from 11 to 13 days to repair the thermal blanket.

This type of massive computer failure had never been seen before on the space station, although individual computers do fail periodically.