U.S. astronauts floated in space Wednesday to test methods of inspecting and repairing the space shuttle's fragile shell in orbit.
The spacewalk was part of the U.S. space agency NASA's long effort to prevent the loss of another shuttle following the Columbia accident in 2003.
NASA has developed tools astronauts could use during a flight to inspect and fix the reinforced carbon skin that protects the front edges of shuttle wings against the searing heat of re-entry into Earth's atmosphere.
Columbia burned three years ago when gases superheated by re-entry friction entered a hole in the carbon layer.
The puncture was caused by a piece of hard foam insulation that broke away from the external fuel tank during launch.
So while the shuttle Discovery is docked at the International Space Station, astronauts Piers Sellers and Mike Fossum took a spacewalk to test two techniques that NASA hopes would avoid another such catastrophe.
One was an infrared camera designed to inspect damage to the carbon wing panels. They also tested a way to patch damage in orbit.
They used caulking guns and spatulas to apply a black paste to samples of gouged and cracked wing panels.
Sellers gave his opinion of the substance to mission control in Houston.
"I'm working it. It's very smooth, like peanut butter."
"It was definitely like thick putty. It is smooth in consistency. It goes on fairly easily in that regard."
The astronauts' objective was to apply it in thin layers to ensure no bubbles formed that would leave gaps after hardening.
In addition to this patching material and the infrared camera, NASA has equipped robot arms on the shuttle fleet with a boom extension that holds a camera and sensors to inspect the underside of the orbiters for launch damage.
Astronauts Fossum and Sellers bounced around on the one on Discovery during their first spacewalk Saturday and found it stable enough to use as a platform on which to make orbital repairs.
Shuttle flight director Tony Ceccacci says these techniques give NASA an important safety capability it never had before.
"Our number one goal is to never have to use them."
Discovery undocks from the space station Saturday and is due back on Earth Monday.
If the mission continues to be successful, NASA will launch another shuttle next month to resume regular visits to the outpost and restart construction for the first time since Columbia's demise.