The work will delay Discovery's departure from the station to Monday from Sunday, a postponement that Nasa said could carve a day out of the already slim six-day launch window for the next mission, when the shuttle Atlantis will fly the long-awaited Columbus laboratory to the station. That mission is targeted for launch on Dec 6.

For nearly two months, Nasa has been aware of a potential problem with one of the station's massive rotary joints that spin the outpost's solar wing panels so they can track the sun and generate power.

But when spacewalker Dan Tani was dispatched on Sunday to investigate the problem during a spacewalk, he found shards of metal scrapings prevalent throughout the joint.

'I was quite sure there was something anomalous with the mechanism,' Mr Tani said on Monday during an in-flight interview.

He collected samples to return to Earth for analysis, but space station commander Peggy Whitson, a biochemist, conducted a preliminary experiment on Monday and discovered that the metal bits contained iron.

That was not good news. Nasa had hoped the debris was coming from outside the joint, such as from one of the device's aluminum-lined thermal covers. Now engineers will be looking at parts of the rotary joint itself.

To prevent further damage, managers decided to lock the affected solar panels in place to avoid working the rotary joint. That cuts the amount of power the station can produce, a situation that must be corrected before Japan's science laboratory, Kibo, is launched next year. -- REUTERS