Shuttle Discovery not in the clear just yet

The Discovery crew used a boom-mounted digital camera to re-inspect six areas on the shuttle's heat shield on Friday, but appeared confident problems that led to the 2003 Columbia disaster have been resolved.

The most serious area of concern is Discovery's carbon-tipped nose, which must withstand temperatures hot enough to melt steel when the shuttle plunges back into the atmosphere prior to landing.

The shuttle's external fuel tank, which has been the US space agency's top safety concern since it triggered the Columbia disaster, did not appear to have shed large enough pieces of its foam insulation to damage Discovery's heat shield, officials said.

Foam insulation falling off Columbia's fuel tank during launch smashed into the ship's wing and allowed superheated atmospheric gases to blast through as Columbia flew through the atmosphere for landing. The spacecraft disintegrated over Texas, killing the seven astronauts aboard.

The tank design was modified after the accident, but the one flown during NASA's first post-Columbia mission last year shed large foam chunks during launch.

Discovery's launch on Tuesday was the second since the accident.

Originally planned for 12 days, NASA on Friday extended Discovery's mission by one day on Friday to add a third spacewalk to test a heat-shield repair technique.

On Friday, Discovery attached an Italian-built cargo module carrying more than 2,272 kg of equipment and supplies to the space station.

Discovery also brought German astronaut Thomas Reiter to the station, where he will stay for six months.