Astronauts Sunita Williams of NASA and astronaut Akihiko Hoshide of Japan climbed out of the International Space Station on Thursday to repair the cooling system of the station's U.S. portion, a spokesman for the Russian mission control said.

The astronauts opened the outer hatch of the air lock chamber at 1229 GMT, the spokesman told Interfax-AVN. "The planned time for work outside the station is six and a half hours," he said.

The mission was shown live on NASA's Web site.

The astronauts were trying to stop leakage of ammonia from the cooling system on the outer surface of the American portion. The job included switching off a radiator and plugging in another.