MOSCOW (AP) -- Two Russian cosmonauts suited up and stepped out into space early Friday on a mission to determine why the exit hatch on the Mir space station has been losing pressure.
In the first hour of the spacewalk, the cosmonauts reported that the hatch's main clamp appeared not to have been fully closed, possibly making the problem easier to fix than anticipated, Mission Control spokesman Valery Lyndin said.
American crewmate David Wolf filmed the movements of the two experienced spacewalkers from inside the station.
The hatch on the Mir's Kvant-2 module failed to close properly after a spacewalk in November, but the partial loss of pressure has only affected Kvant's docking chamber, which is sealed off from the rest of the station.
However, it has forced the crew to use a smaller adjacent compartment to put on and take off their bulky spacesuits.
Before the mission, estimated to take six hours, space officials thought the hatch might be in need of a new airtight rubber seal. That theory had not been ruled out early in the mission.
During the spacewalk, cosmonauts Anatoly Solovyov and Pavel Vinogradov also planned to retrieve a U.S. experiment -- a package of materials attached to the outside of the station in April to see how they endure space conditions.
On the crew's next spacewalk, set for Wednesday, Solovyov and Wolf will gather more U.S. scientific experiments.
Solovyov is the most experienced space walker in history, logging more than 70 hours in open space on his previous 14 spacewalks, five of which came during his current Mir mission, starting in August.
He and Vinogradov, who accompanied him on four spacewalks, have managed to restore most of the power lost in Mir's June's collision with a cargo ship.
Mir, the world's longest-flying space outpost, will mark its 12th anniversary in orbit next month, although it only had been expected to last for five years.
Despite continuing computer breakdowns, the latest of which occurred last week, Mir has been relatively quiet in recent months after a steady string of accidents earlier last year.