Two Russian cosmonauts climbed out of the international space station Wednesday to install protective panels designed to shield the orbiting outpost from dangerous space debris.

Commander Fyodor Yurchikhin and flight engineer Oleg Kotov opened a hatch on the Russian side of the space station at 3:05 p.m. EDT. The spacewalk was scheduled to last six hours.

Both men were tethered to keep them from floating away. The station's third occupant, U.S. astronaut Sunita Williams, remained inside.

"O.K.!" Yurchikhin said as he opened the hatch.

Space debris includes objects such as discarded rocket parts, planetary dust and rocks. Scientists consider it the greatest threat to the space station orbiting about 220 miles above Earth.

The 17 protective panels, each about 2 feet by 3 feet and weighing about 20 pounds, were delivered to the station last December and left outside in a formation dubbed "the Christmas tree."

Engineers on the ground are able to monitor by radar the largest pieces of space debris — objects larger than a softball — and adjust the station's position accordingly, so the shielding will protect against smaller objects.

"The consequence of small particles is not so great," said Kirk Shireman, NASA's deputy program manager for the space station. "Then there are the midrange particles. Those are the ones we worry about."

The cosmonauts also planned to fix a navigation antenna that will be used later in the year for the first flight of a new European cargo vehicle, and they intended to deposit canisters containing biological experiments outside the station.

Wednesday's spacewalk was the first of two scheduled a week apart. Yurchikhin and Kotov will perform another one next Wednesday with similar tasks.