By Ed Stoddard

HOUSTON, June 3 (Reuters) - A pair of astronauts from the visiting shuttle Discovery were preparing for a spacewalk on Tuesday to begin installation of Japan's Kibo laboratory on the International Space Station (ISS).

Mission specialists Michael Fossum and Ronald Garan also will retrieve a laser-studded inspection boom which is usually used to check the ship's wings and nose cap for damage.

The shuttle had no room for the boom because of the size of the Japanese lab and so the shuttle on the previous mission in March left it behind on the station for retrieval by the Discovery crew, which will take it back to earth.

Instead, the Discovery astronauts used a camera on the end of the shuttle's 50-foot (15-metre) robot arm, but they could not reach the underside of the wings. Further inspections are planned for latter in this mission.

Damage inspections have been a routine part of shuttle missions since the 2003 Columbia disaster which was caused by damage to the shuttle's heat shield during launch.

Fossum and Garan were to spend their sleep period in the station's Quest airlock to purge their bodies of nitrogen before the spacewalk, which was scheduled to begin at 11:32 a.m. EDT (1532 GMT) and last roughly 6-1/2 hours.

The giant lab Kibo is the centerpiece of this two-week international mission and will establish Japan's permanent place in space.

The Discovery crew floated aboard the ISS on Monday where they were warmly greeted and embraced by its crew after a flawless docking and a two-day journey.

But the mission has not been incident-free.

NASA officials revealed on Monday that bricks and other debris had come off the flame trench beneath the launchpad at Cape Canaveral in Florida from which Discovery blasted off on Saturday. The concrete-fortified trench helps to deflect the intense heat of shuttle launches.

The debris was strewn around the launch area and while NASA managers described the incident as "unprecedented" they said they were confident it could be repaired in time for October's shuttle mission to the Hubble Space Telescope.

"We'll fix it without any problems before October," deputy shuttle program manager LeRoy Cain told reporters at the Johnson Space Center in Houston.

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