Astronauts fix space station rail cart

HOUSTON, July 11 (Reuters) Two spacewalking astronauts repaired a transport system outside the International Space Station today, paving the way for NASA to resume assembly of the half-built, 100 billion dollars complex later this year.

The high-flying repairmen from the space shuttle Discovery wrestled with a spate of problems during the nearly seven-hour spacewalk, including the partial disconnection of astronaut Piers Sellers' emergency jet backpack.

''What else could happen, man?'' one of the astronauts said when a new cable assembly for the mobile rail cart was finally put into position on the space station.

''Don't ask,'' came the reply.

Sellers had problems with his jet-powered backpack, a small but critical piece of equipment that could save an astronaut's life if the safety tethers that keep him attached to the station should fail.

While the space shuttle is docked at the station, it cannot fly around to rescue an astronaut who inadvertently floats off into space.

Sellers' backpack came loose twice during the spacewalk.

Partner Michael Fossum came to his aid, bypassing a faulty clamp on the backpack with a short tether.

Repairs to the rail cart, which runs along tracks on the outside of the space station, were needed to haul heavy station components to their proper locations for installation.

The cart has been out of commission since December, when its power and data cable was inadvertently severed. Sellers and Fossum installed a new cable and reel assembly.

Without the cart, NASA would not be able to resume space station assembly, which if all goes well, would begin with the launch of shuttle Atlantis on Aug. 28. Construction was halted after the 2003 Columbia disaster.

NASA needs to finish construction of the space station by 2010 when the three-ship fleet is scheduled for retirement.

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

NASA grounded the shuttle fleet for much of the last three years 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 and popped off at a point when they would threaten the orbiter.

Columbia was brought down by a 1.67 pound (756 gram) chunk of foam that made a direct hit on the wing heat shield during the shuttle's climb to orbit.

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

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

In addition to fixing the rail cart, Sellers and Fossum installed a spare pump on a stowage platform outside of the station during the mission's second spacewalk.

NASA wants to position as much extra equipment as possible on the station to prepare for when the shuttles will no longer be available to transport large cargo.

The final spacewalk of the mission is planned for Wednesday, when Sellers and Fossum go to the shuttle's cargo bay to test materials that could be used by future crews to repair damage to their ship's heat shield.

REUTERS PDS RAI0242