Spacewalking astronauts test shuttle repair

HOUSTON, July 12 (Reuters) Two astronauts went into Discovery's open cargo bay today to test heat-shield repairs in the final spacewalk of a shuttle mission that may signal NASA's recovery from the 2003 Columbia disaster.

Piers Sellers and Michael Fossum were to use materials and tools developed after Columbia to see if a space shuttle's damaged heat shield can be fixed during flight.

The question is whether the repairs, made with a caulking gun and specially developed sealant, can withstand temperatures as high as 3,000 degrees Fahrenheit when the shuttle returns to Earth.

The two men, who were to be out in space 6-1/2 hours, are making their third spacewalk on a mission that has been virtually trouble-free after NASA spent more than three years and 1.3 billion dollars to make shuttles safer.

Discovery suffered no significant damage during its launch from Florida on July 4, which has given NASA confidence that it has fixed the falling fuel-tank foam problem that doomed Columbia and surfaced again on a shuttle flight last summer.

''We learned a lot since Columbia,'' Sellers said yesterday in a press interview from space. ''NASA has done a lot of heart searching in how to go about this and come up with a pretty good solution.'' The US space agency has struggled to recover since Columbia broke apart over Texas while returning to Earth on February 1, 2003, after a 1.67 pound chunk of insulating foam broke its wing heat shield 16 days earlier during launch.

Fiery atmospheric gases penetrated the spacecraft, which caused it to disintegrate and fall to the ground in pieces.

Despite safety modifications, a one-pound piece of foam came loose again when Discovery launched on the first post-Columbia flight last year.

It did no harm, but sent NASA back to the drawing board out of fear that the Columbia tragedy could happen again.

After further changes, Discovery's fuel tank shed only a few small flecks of foam on this launch, none judged to be dangerous.

''We're back, baby,'' shuttle pilot Mike Kelly said during the Tuesday interviews, quoting his brother Scott, who is also an astronaut.

Discovery docked with the space station on July 6 and is scheduled to leave it on Saturday and return to Earth on Monday.

The next shuttle mission is set for August 28 as NASA plans to pick up the pace of flights so it can complete assembly of the half-finished, 0 billion space station before the shuttle fleet is retired in 2010.

Reuters SHB GC1900