CAPE CANAVERAL, Fla. (AP) _ Two astronauts headed for Discovery's open payload bay 220 miles above Spain early Wednesday, starting a spacewalk to test whether a putty-like sealant can be used to repair a damaged space shuttle.

It was the third and final spacewalk for astronauts Piers Sellers and Mike Fossum while Discovery is docked at the space station.

``Hello world! Looking like a whole ton of Islands right below me,'' Sellers said as he left the space station's airlock and headed to its robotic arm for a ride to the shuttle's payload bay.

Using a caulk gun, Sellers and Fossum planned to squirt sealant onto 12 deliberately damaged reinforced carbon-carbon samples stored on a pallet in Discovery's open payload bay.

Reinforced carbon-carbon is used to protect the shuttle's wing leading edges and nose cap from searing heat that can reach 2,300 degrees Fahrenheit during re-entry into Earth's atmosphere. A crack in Columbia's wing in 2003 allowed fiery gases to penetrate the shuttle, destroying Columbia high over Texas and killing its seven astronauts.

The repair technique was devised by NASA to make sure such a disaster never happens again.

Wearing bulky spacesuit gloves, Sellers and Fossum will have to massage the sealant with a putty knife to keep it from bubbling in zero gravity.

``Those bubbles behave differently in zero-g _ they don't rise to the surface,'' Fossum told The Associated Press on Tuesday. ``If you have too many bubbles trapped beneath the surface, then you're not going to get the repair you want.''

Because of temperature concerns, the testing needs to be done when it's between 35 degrees and 100 degrees Fahrenheit, so the experiment will be conducted during the night portions of their orbit. The international space station circles the Earth every 1 1/2 hours.

Sellers also will take images of Discovery's wing using an infrared camera as he is moved by the space station's robotic arm.

During interviews with The Associated Press and USA Today on Tuesday, Sellers described the third spacewalk as ``a careful, meticulous lab experiment.''

``The first two were kind of heavy lifting,'' Sellers said. ``We were moving heavy pieces of equipment around and doing a lot of hard work.''

During the first spacewalk last Saturday, Sellers and Fossum demonstrated that emergency shuttle repair work could be made from the end of a boom connected to a robotic arm. For the second spacewalk on Monday, they got the space station's crucial rail car working again and installed a pump compartment for the complex's cooling system.

Although the astronauts had trained for it, the third spacewalk was a last-minute addition, approved after Discovery was in space. It added an extra day to what is now a 13-day mission.

The spacewalks can be tiring, but Sellers said he and Fossum had a surefire way of regaining their energy after each trip outside the space station.

``Four cups of coffee later, we feel much better,'' Sellers cracked.

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