NASA officials have decided the Atlantis crew will need to fix a loose thermal blanket on the outside of the space shuttle, extending their 11-day mission.

The work is expected to delay the crew's return flight by two days. But managers have not said whether the repair work will require an extra spacewalk.

The decision came as astronauts James Reilly and Danny Olivas donned their space suits to connect a 45-foot long, 35,000-pound aluminum structure to the International Space Station.

The spacewalk -- the first of three planned by NASA -- began at 4:02 p.m. ET on Monday and was expected to take more than six hours.

Reilly and Olivas were delayed by roughly 40 minutes, when the crew ran into problems with four spinning gyroscopes that stabilize the space station's position.

The loose thermal blanket covers a 10-by-15-centimetre area over a pod for engines and was discovered Saturday.

NASA engineers believe aerodynamic forces during the shuttle's launch may have loosened the blanket. While the damage isn't likely to endanger the crew, it would require lengthy repairs once the shuttle reaches back to Earth.

On Sunday, Atlantis successfully docked with the station, bringing the new $367-million unit to the station's robot arm.

The structure also includes a solar array, which will be deployed Tuesday to add more power-generating capability to the station.

The extra power will be needed as upcoming shuttle flights add new sections to the station.

"One of the primary aspects of this mission is to increase the power available to the International Space Station from currently about 50 per cent of the available power up to nearly 75 per cent that the space station will need to power all of the science instruments," astronomy professor Paul Delaney told CTV's Canada AM on Monday.

In preparation for the low pressure on Monday's spacewalk, Reilly and Olivas slept overnight in an airlock with reduced air pressure.

With files from The Associated Press