Two space shuttle Discovery astronauts have begun the first spacewalk of their mission to install a new tank of ammonia coolant on the International Space Station.

Rick Mastracchio and Clay Anderson switched their spacesuits to battery power according to NASA.

The two spacewalkers began the task of changing out a depleted Ammonia Tank Assembly.

They will need to complete two more spacewalks before the job is complete.

The crew is also expected to replace one of the station steering system's gyroscopes and work on the Canadian robot Dextre.

Space shuttle Discovery carried an Italian-built cargo hauler filled with equipment, experiments, food and supplies for the space station, which has been under construction about 220 miles above Earth since 1998.

The United States plans to stop flying Discovery and sister ships Atlantis and Endeavour after three more missions to stock the station with spare parts and gear too big or bulky to load on other spaceships.

The shuttles, which can carry about 50 tons to the station's orbit, are being retired due to cost and safety concerns.