Supplies and equipment carried to the station by Discovery will be stowed, and water generated by the shuttle's fuel cells will replenish the station's supply, US space officials said.

Flight controllers in Houston awakened the crewmembers at 12:11 amwith a musical wakeup call, "I'm Going Up" by Claire Lynch.

Discovery will spend an extra day in orbit, NASA announced Saturday.

"We have the capabilities to extend the mission one day," Wayne Hale, deputy manager of the shuttle program, told reporters. "This will allow the crew to have more work done and transfer more items."

The ship will return to Earth on Monday, August 8, at 0847 GMT, instead of Sunday.

Hale said that after examining videos and photographs of 90 percent of Discovery's ceramic tile heat shield, NASA has found no extensive damage as a result of chunks of insulating foam falling away from the orbiter's external fuel tank on liftoff Tuesday.

Similar damage brought down the shuttle Columbia in February 2003.

Experts still need to look over the leading edges of the wings and the nose, said NASA, which will delay by 24 hours an announcement on Discovery's fitness for re-entry, from Sunday until Monday.

However, all shuttles remain grounded, a precautionary measure NASA issued after watching the falling foam.

The extra day in orbit will allow the crew to get a head start on tasks that could be affected by the suspension of those flights: removal of trash and old equipment from the space station.

Some 11 tonnes of waste have piled up over three years, cramping residents on the space station.

Astronauts will pack it into the Rafaello transport module, from which the seven shuttle crew members and two space station residents began on Friday to unload 13.5 tonnes of food and equipment.

Rafaello is an Italian-built pressurized transport module that rides in Discovery's cargo bay and was lifted to the space station with the help of the shuttle's Canadian-built robot arm.

The spacewalk formally began at 4:46 am (0946 GMT), an hour later than scheduled, NASA said, as Discovery remained moored to the space station.

It included testing new ceramic tile repair methods and work on the ISS.

The space walkers, Japanese astronaut Soichi Noguchi and American Steve Robinson, applied the repair techniques to tiles they had purpously damaged and to carbon fiber panels similar to those used in the thermal shield under the shuttle, which must stand up to temperatures as high as 1,500 degrees C (2,700 degrees F) from the friction of re-entering the Earth's atmosphere.

Commander Eileen Collins was disappointed that the foam problem returned after two and a half years of study and a billion dollars' in remedies.

"We were actually quite surprised to hear we had some large pieces of debris fall off the external tank. It wasn't what we had expected," Collins said from the shuttle.

Noguchi and Robinson will walk again in space Monday and Wednesday to replace one of the ISS's four gyroscopes that has not worked since 2002.

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