Space Shuttle Discovery astronaut Danny Olivas exits the International Space Station's Qwest airlock at the beginning of his spacewalk in this image from NASA TV September 1, 2009.(Xinhua/Reuters Photo) Photo Gallery>>>

    WASHINGTON, Sept. 1 (Xinhua) -- Two astronauts conducted a space walk on Tuesday and removed an old liquid ammonia tank from the International Space Station (ISS), NASA said.

    According to NASA TV, the ISS's Expedition 20 Flight Engineer Nicole Stott and U.S. shuttle Discovery's Mission Specialist Danny Olivas exit the ISS's decompression chamber to begin their space walk at 17:49 EDT (2149 GMT).

    The duo spent their Monday night inside the Quest airlock at a lower air pressure to force nitrogen out their bloodstream. This prevents astronauts from getting decompression sickness during a space walk.

    It was the first space walk for the Discovery-station complex crew, which concluded at 00:24 EDT (0424 GMT) on Tuesday.

    During the spacewalk, shuttle Commander Rick Sturckow and Mission Specialist Pat Forrester guided the space walkers through the procedures. Pilot Kevin Ford and Flight Engineer Bob Thirsk operated the station's robotic arm. With their help, Stott and Olivas managed to disconnect the Ammonia Tank Assembly from the station's truss.

    Discovery lifts off midnight Friday from the Kennedy Space Center in Florida after two delays. It will spend more than a week at the orbiting complex. The astronauts will perform three space walks to replace an ammonia tank and perform other outside maintenance. 

This still image from NASA video shows US space shuttle Discovery crew members, Mission Specialist Danny Olivas (left) and Astronaut Nicole Stott, during the mission's first spacewalk on September 1.(Xinhua/Reuters Photo) Photo Gallery>>>