Mission specialists Robert Behnken and Mike Foreman began their spacewalk at 4:34 pm EDT (2034 GMT Saturday), 49 minutes ahead of schedule, and ended it at 9:36 pm (0236 GMT Sunday).

It was the last such trip before Endeavour's seven-man crew heads back to Earth next week.

Mission Control in Houston, Texas, immediately declared the six-hour walk a success.

"Today was another fantastic day. The crew is doing very well," space station flight director Dana Weigel told reporters after the astronauts had safety returned from their mission.

Weigel said the spacewalk, often referred to by National Aeronautics and Space Administration officials as an EVA, or an extravehicular activity, actually had set a new record.

"This was five EVAs, which was more than we've done on any station mission," the flight director pointed out.

"We are all very excited how it all turned out," added Zebulon Scoville, NASA's leading specialist on spacewalks.

The walkers successfully stowed away the Orbitor Boom Sensor System (OBSS), a thick rod fitted with a camera and laser which is used to check for damage to a shuttle's protective skin.

The OBSS made its maiden trip in 2005 on the first flight following the Columbia disaster in 2003, when a crack in the shuttle's heat shield caused the craft to explode while re-entering Earth, killing the seven crew members on board.

An extension of the shuttle's robotic arm, the OBSS would normally return to Earth at the end of each mission.

But the next flight, by the shuttle Discovery, will bring to the ISS the second of three parts of Japan's space laboratory Kibo -- the first of which was installed during the current mission -- and will have no room for the boom.

Discovery's crew will detach the OBSS from the space station when they arrive, use it to inspect their shuttle and then bring it home.

On Saturday's spacewalk, the ISS' robotic arm grabbed hold of the boom to allow Behnken and Foreman to attach the cable that will power its sensors and protect it from the elements.

The robot arm then handed the boom over to the astronauts, who stowed it on a truss on the space station, guided by fellow crew member Rick Linnehan from inside the ISS-Endeavour complex.

The spacewalkers also successfully installed an experiment on the outside of the European Space Agency's laboratory, which the astronauts had failed to complete during the third spacewalk on March 17.

They completed the walk by installing trunnion covers on the Japanese module and stowing tools in a toolbox before returning to the space station.

Endeavour, whose mission at the ISS is the longest ever, is scheduled to undock on Monday and return to Earth on Wednesday.

The mission's main tasks were to install the first part of the Japanese Kibo lab, which will join similar facilities from the United States, Russia and the EU, whose Colombus lab was delivered to the ISS in February.

It has also assembled the Canadian-made Dextre robot, which is designed to undertake maintenance operations on the space station that until now required a human touch, and reduce the need for risky spacewalks.

The robot's human-like upper torso swivels at the waist, and its arms were designed with seven joints to provide it with maximum versatility. Umbilical connectors provide power and data connectivity.

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

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