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The two U.S. and Russian International Space Station crewmen stepped outside the outpost for a spacewalk to perform maintenance chores as the U.S. space agency prepares to resume shuttle flights.

Russian commander Pavel Vinogradev and American flight engineer Jeff Williams donned Russian space suits for their six-hour outing to maintain some systems before shuttles resume station construction later this year. They repaired a system that generates oxygen, replaced an exterior camera, and repositioned a cable. They also retrieved biology experiments and a collection device that monitors space contamination.

It was the first spacewalk for the two crewmen since they took up residence as the 13th space station crew in six years.

The spacewalk came one month ahead of the planned launch of the space shuttle Discovery to the station. If the shuttle goes up as scheduled, it will end a long hiatus in visits to the station that began when the shuttle Columbia disintegrated in 2003.

Since then, the U.S. space agency NASA has spent hundreds of millions of dollars redesigning the exterior layer of hard foam insulating the giant fuel tank that launches side-by-side with each shuttle. It wants to prevent a recurrence of shedding foam that punched a hole in Columbia's wing, causing it to burn up upon re-entry, killing seven astronauts.

NASA has removed foam from the tank site that contributed the deadly debris and reshaped it in other places. The attempt has been to minimize the amount that breaks away during the tremendous physical stresses of launch.

Shuttle program manager Wayne Hale says an extensive review has found nothing to prevent Discovery's takeoff in one month.

"Based on what we know today, there is no reason not to launch on July 1," he said.

Hale says technicians will continue to modify the foam insulation for subsequent shuttle flights.

When Discovery takes off next month, it will bring German astronaut Thomas Reiter, whose arrival will give the station its first three-person crew since 2003. In late August, the shuttle Atlantis is to carry up hardware to resume station expansion.

American deputy station manager Kirk Shireman says he looks forward to Reiter's sojourn because it will increase the number of hours devoted to scientific research aboard the station.

"It also helps prepare for [International Space Station] assembly. The number of flights we have lined up - it will help us to prepare and execute those flights," he said. "So it's absolutely an improvement for the research because of the extra hours and also it will enable to prepare and continue the assembly of the International Space Station."

Other segments to be added to the outpost are European, Russian and Japanese laboratories; a U.S. tunnel connecting four labs; a two-armed Canadian crane; and an unpiloted European cargo vehicle that can dock with the station automatically.