Astronauts to make repairs in second spacewalk

HOUSTON, July 10 (Reuters) Two astronauts ventured outside the International Space Station today to fix a transport system needed to complete the half-finished 100 billion dollar orbital outpost.

Piers Sellers and Michael Fossum were to fix a rail car that moves the station's construction crane, in the second spacewalk of a shuttle flight NASA hopes has put the US space program back on track after the 2003 Columbia disaster.

''It's a beautiful day somewhere out there,'' said Fossum, looking down at Earth as the pair emerged and floated out of the station.

''No question about it,'' Sellers said.

If all goes well, NASA plans to resume station assembly with a shuttle mission in August and complete the space facility, which is sponsored by 16 nations, by the time the three-shuttle fleet is retired in 2010.

Shuttles are the only spacecraft that can deliver the station's major components.

The space agency has spent 1.3 billion dollars in shuttle safety upgrades since Columbia broke apart while returning to Earth after its heat shield was damaged during launch. The seven astronauts on board were killed.

The repair by Sellers and Fossum is a key step in restarting space-station construction that was halted after Columbia's demise.

NASA grounded the shuttle fleet while it worked on fixing the problem that brought Columbia down - insulation foam breaking loose from the shuttle fuel tank at launch.

In the first post-Columbia flight last summer, the foam problem persisted, but did no harm. After further safety upgrades, NASA said flecks of foam were lost when this flight launched on Tuesday, but they were too small enough to pose a danger.

The agency has said some of the more than 4,000 pounds (1,818 kg) of foam that cover the fuel tank will always come off during the shuttle's thunderous launch.

Columbia was brought down by a 1.67 pound chunk of foam that made a direct hit on the wing heat shield while it took off from Florida.

Fiery gases penetrated the breach during the spacecraft's return to Earth 16 days later, causing Columbia to disintegrate over Texas.

NASA yesterday said post-launch inspections of Discovery had turned up no major problems and the orbiter was fit for its scheduled landing on July 17.

''We are absolutely clear and ready to bring this vehicle home whenever the mission is accomplished,'' shuttle project manager Steve Poulos told reporters at Johnson Space Center.

Sellers and Fossum were to spend more than six hours out in space replacing a power- and data-cable system on the rail car, which had been immobile since one of its cables was accidentally cut in December.

They also were to install a spare pump for the space station's thermal control system.

The two men spacewalked for 7-1/2 hours on Saturday when they tested the stability of a 100-foot boom as a possible work platform for shuttle repairs and did preliminary work on the rail car.

They will go out in space again on Wednesday to test materials and techniques that could be used to repair a damaged shuttle heat shield on future flights.

REUTERS KD PC1931