British-born shuttle astronaut Piers Sellers and American Michael Fossum took turns strapping their feet onto a small platform and dangling from the end of a 30m (100-foot) boom that would allow astronauts to reach every part of a shuttle.

"It's like being in a very slow elevator," Sellers radioed to ground controllers as he rode atop the end of the boom, which is a combination of the space shuttle Discovery's 15m (50-foot) robot arm and a 15m (50-foot) extension.

The exercise, performed at 28,000kph (17,500 miles per hour), 357km (220 miles) above the Earth, was part of NASA's efforts to recover from the 2003 Columbia disaster and make certain there are options for repairing a damaged shuttle before it returns.

Seven astronauts aboard Columbia died without knowing their shuttle was critically damaged when insulation foam fell off during liftoff. The vessel broke apart over Texas as it returned to Earth.

Test

"Hopefully we never have to use it, but we have the capability if we ever need to," flight director Tony Ceccacci said after the test with the boom ended.

Discovery as seen from the
Space Station on 06 July, 2006

Sellers and Fossum, who with five other astronauts on Discovery docked with the International Space Station on Thursday, spent more than seven hours outside the ship.

The spacewalk, which is the first of three planned during the shuttle's 13-day flight, was extended an hour because of difficulties with safety tethers, which are used to keep tools and the astronauts from floating off into space.

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

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

No major damage has been spotted on this flight, but engineers are still studying one area where filler material is protruding slightly from between heat-protecting tiles.

Third spacewalk

If NASA decides to remove the protruding "gap filler," it would likely be attempted on the third spacewalk on Wednesday.

Being able to use a boom for repairs boosts the chance NASA will approve a servicing mission to the Hubble Space Telescope, said shuttle deputy program manager John Shannon.

Crews flying to Hubble would not be able to reach the space station to await rescue if their ship was damaged during flight. Instead, they would have to attempt repairs.