Two astronauts are on a spacewalk outside the International Space Station, removing a massive ammonia tank to replace it with a new one.

Nicole Stott and Danny Olivas also planned to retrieve two experiments mounted on the space station. The spacewalk is the first of three during this mission.

Inside the space station, the rest of the crew is moving cargo brought up by the shuttle. Included is a treadmill named for U.S. comedian Stephen Colbert at his request.

NASA says the new 800-kilogram ammonia tank is the heaviest item ever handled by spacewalking astronauts during assembly of the station. Although objects in space are weightless, they still have mass, which can make large items difficult to move. The ammonia is used to help cool the space station.

The shuttle docked with the orbiting station Sunday for a week-long joint mission.