Astronauts Robert Behnken and Mike Foreman, who arrived at the International Space Station (ISS) aboard the shuttle Endeavour last week, spent six hours and 24 minutes outside the ISS working on a to-do list that also included replacing a failed circuit breaker-like unit and other maintenance work, NASA officials said.

In testing the heat-shield repair method, the US National Aeronautics and Space Administration sought to make sure astronauts can fix any potential damage during the Hubble mission on August 28, when the shuttle will be moored too far from the ISS, its only habitable shelter in orbit.

Dana Weigel, lead ISS flight director, called the spacewalk and the repair material tests a "huge success" after it concluded.

"I'm thrilled with what we saw today" after the astronauts practiced mixing and injecting the two epoxy-like compounds onto test heat shield materials with an apparatus resembling a caulk gun.

While the materials had been tested on Earth and in the absence of gravity for short periods, NASA wanted to see how it flowed, swelled and set in the gravity-less vacuum of space.

"Working with a fluid in microgravity is certainly no small feat," Weigel said.

"All the tools and techniques that the ground crew developed were excellent."

Officials said that the technique appeared fully workable, but that they will test the materials fully after they are brought back from space to confirm they can survive the extreme temperatures the shuttle endures on its super-heated return into Earth's atmosphere.

Astronauts have been testing different in-space repair techniques on the shuttle's protective layer since a crack in Columbia's heat shield caused it to explode while re-entering Earth in 2003, killing its seven-member crew.

NASA now uses special cameras to scan the shuttle's thermal tiles during flights to see if they have been damaged by debris during liftoff or by micrometeorites while orbiting the Earth. Columbia's heat shield was hit by debris during takeoff.

Getting a workable repair technique is crucial for the Hubble mission, when the astronauts will not have the option of staying aboard the ISS if something goes wrong with the shuttle.

However, NASA officials pointed out, there will be a second shuttle on backup in case of problems in the Hubble mission.

Before testing the new repair methods Thursday, Behnken and Foreman replaced a remote power control module -- the failed circuit breaker -- on the space station's truss, with vivid images of the home-repair-in-space job shown live on NASA television.

The astronauts, however, were unable to remove a power connector from the truss, NASA said.

Meanwhile, Japan's Aerospace Exploration Agency reported that Japanese astronaut Takao Doi experimented with a boomerang's performance in the absence of gravity and confirmed that it flies back much like on Earth.

Doi "threw a boomerang and saw it come back" in the ISS on Tuesday, an agency spokeswoman said.

Doi was asked to test the boomerang by Yasuhiro Togai, a world boomerang champion.

"I was very surprised and moved to see that it flew the same way it does on Earth," the Mainichi Shimbun daily quoted the 53-year-old astronaut as telling his wife in a chat from space.

Thursday's spacewalk was the fourth of five for the Endeavour mission, a record 16-day trip with the primary tasks of installing the first part of Japan's Kibo laboratory and assembling Canada's Dextre robot.

Future missions will deliver Kibo's two other parts, which will give Japan a foothold in the ISS alongside the United States, Russia and Europe, whose Columbus lab was delivered last month.

The microgravity research aboard the ISS is considered a crucial step toward long human missions on the moon and eventually Mars.

Endeavour's crew will conduct a fifth and final spacewalk Saturday before the shuttle undocks from the ISS Monday for its March 26 return to Earth.

NASA wants to complete construction of the ISS by 2010, when its three-shuttle fleet is scheduled to be retired.

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