Shuttle Discovery astronauts start spacewalk

HOUSTON, July 8 (Reuters) Two astronauts stepped out of the International Space Station today for a spacewalk in which they hope to make shuttle flights safer by bouncing around on the end of a 100-foot boom.

Piers Sellers and Michael Fossum were to strap their feet into a foot clamp on the boom, then bend, twist and bounce to test its stability as a platform for emergency repairs.

''Enjoy the view, gentlemen,'' Mission Control capsule communicator Megan McArthur told the two men as they emerged from the station into space.

The oddball boom experiment is part of NASA's effort to improve shuttle safety following the 2003 Columbia disaster that killed seven astronauts.

NASA wants to see if the boom, which is a combination of the space shuttle Discovery's 50-foot robot arm and a 50-foot extension, will hold astronauts steady enough to repair parts of the spacecraft unreachable with just the robot arm.

Repairs would be necessary if there were damage to a shuttle's heat shield, which protects the spacecraft during its fiery return to earth.

When Columbia launched, insulating foam from the fuel tank shook loose and broke a wing heat shield. This breach allowed hot gases to destroy the orbiter 16 days later when it re-entered the atmosphere.

Sellers and Fossum, who along with five other astronauts flew to the station aboard shuttle Discovery on Thursday, were to be out in space 6-1/2 hours on the first of three spacewalks on what is now scheduled to be a 13-day flight.

Before bouncing on the boom, they worked briefly on a broken space station transport system needed to complete the half-finished, 100 billion dollars space outpost.

Since Discovery's launch from Florida on Tuesday, NASA has been poring over the shuttle with cameras and sensors looking for any Columbia-like damage.

On the only other post-Columbia shuttle flight, which flew last summer, foam flew off the fuel tank at launch, but did not harm the spacecraft. NASA has spent 1.3 billion dollars on safety upgrades since Columbia.

No major damage has been spotted on this flight, but engineers are still studying filler material protruding slightly from between heat-protecting tiles on the shuttle skin.

The protrusion raises the possibility of a repeat of last summer's flight when astronaut Steve Robinson performed a spacewalk to remove two such ''gap fillers'' sticking out of Discovery's belly.

If Sellers and Fossum have to do the same on this flight, it likely would happen on their third spacewalk, scheduled for Wednesday.

REUTERS SRS RK2018