U.S. space shuttle astronauts are on their second spacewalk in three days, acting as orbiting mechanics at the International Space Station. They are making repairs to the outpost's cooling system and its railroad flatcar.
Monday's spacewalk by shuttle Discovery astronauts Mike Fossum and Piers Sellers lacks the theater of their Saturday venture, when they bounced and twisted at the end of a 30-meter long pole like circus high-wire performers to test its stability as a platform to make shuttle repairs in orbit.
For this outing, they are continuing more mundane maintenance they began Saturday on the space station's rail car, which rolls back and forth to position the outpost's mechanical arm during construction. They are replacing a backup cable that delivers power, video, and data to the car. It was inadvertently severed by a cable cutter in December.
The two astronauts are also replacing a spare external pump on the station's cooling system.
As the two men prepared to float out of the station for their work, Fossum joked about being afraid to go out into the dark. Sellers spoke of a way to scare him.
"I have got time to get outside, put on my alien costume, and wait for Mike to come out," he joked.
The Discovery team is due back to Earth next week to end a 13-day mission that is delivering cargo and a third crew member to the space station, German astronaut Thomas Reiter.
The flight has also been a test of changes the U.S. space agency NASA made to the shuttle's external fuel tank to minimize the amount of hard insulating foam it sheds during launch. Such foam punctured the wing of the shuttle Columbia in 2003, dooming it to a fiery disintegration during its re-entry into the atmosphere.
For Discovery's flight, ground controllers have used new in-flight inspection equipment to look over the orbiter's fragile heat shield. They have found no damage that would prevent Discovery's safe return, so they told astronaut Stephanie Wilson and her five shuttle colleagues that they are cleared for re-entry.
"We have been told we have a very clean vehicle and I certainly have no concerns about returning," she said.