Spacewalker Soichi Noguchi of Japan works in the payload bay of Discovery during his spacewalk July 30, 2005. (Reuters Photo)

    WASHINGTON, July 30 (Xinhuanet) -- Two astronauts of the shuttle Discovery started their first spacewalk early Saturday, US space officials said.

    Japanese astronaut Soichi Noguchi and American Steve Robinson were headed to the shuttle's open cargo bay where they would start six and a half hours of work to test new repair techniques developed after the tragedy of the space shuttle Columbia more than two years ago.

    The spacewalk, the first of three planned during Discovery's 12-day mission, was delayed by over an hour. The other two spacewalks are planned to perform upgrades on the International Space Station.

    Noguchi and Robinson were awoken by a Japanese song performed by a youthful chorus including Noguchi's children before Noguchi told Mission Control that they would begin spacewalk.

    The pair were to work together in Discovery's open cargo bay, using a variety of damaged tile and carbon samples they brought with them to test the new repair methods.

    Noguchi would spend about an hour coating the thermal tile samples with a caulk-like material so as to restore their heat-rejecting ability.

    Meanwhile, Robinson was to work with tools to apply an experimental material that NASA hopes will successfully repair cracks in the carbon panels lining the Discovery's wings.

    They were to work only on test tiles stowed in the shuttle's cargo bay, not on the orbiter itself, which sustained minor damageat launch on Tuesday from flying debris. NASA engineers are to review the test upon Discovery's return.

    The damage-repair test was expected to take two hours. The astronauts were to then turn to the space station to replace a failed global-positioning satellite antenna and fix the power supply to one of the gyroscopes that keep the 200-ton station correctly positioned 220 miles (352 km) above the earth.

    Robinson is scheduled to conduct preparatory work for replacingone of the station's defective gyroscopes. The gyroscope will be replaced during the second spacewalk on Monday.

    The 95-billion-US-dollar space station has four gyroscopes, buttwo are not working, and recently a third began acting up but is still functioning. If it had only one working gyroscope, the station crew would have to fire rocket thrusters to maintain position and use up precious fuel.

    On Wednesday, Robinson and Noguchi will use their third spacewalk to install a stowage platform outside the space station.

    An investigation into the February 2003 crash of the space shuttle Columbia found that a 0.75-kg chunk of foam came off the shuttle's external fuel tank during lift-off, damaging the protective tiles that insulate the hull and wings.

    Sixteen days later, the shuttle disintegrated under extreme heat as it descend through Earth's atmosphere, killing its seven crew members aboard.

    However, after two and a half years of fixing work at a cost ofabout 1 billion dollars, the foam debris problem seems still not solved. Videos showed that large pieces of foam shed from Discovery's external fuel tank and possibly struck six parts of the shuttle's underbelly as it blasted off from Florida Tuesday.

    NASA said Discovery had not suffered serious damage, but it could not give the all-clear until this weekend.

    The US space agency said Wednesday it would ground its shuttle fleet until the problem is fixed.

    Discovery was the first shuttle flight since Columbia fell. It is scheduled to return from space on Aug. 7, but NASA managers said they may extend the mission a day to transfer more supplies to the space station.

    Discovery's commander Eileen Collins said Friday that the shuttle's crew was disappointed that the same peeling foam problem happened again. "It wasn't what we had expected... We thought we had this problem fixed," Collins said from the shuttle. Enditem