Spacewalkers Retrieve Spent Space Station Ammonia Coolant Tank

Discovery's spacewalking astronauts are stepping through an excursion aimed at swapping out a spent coolant tank at the International Space Station.

Mission specialists Rick Mastracchio and Clay Anderson made their way to the starboard side of the station's central truss and then worked together to remove the empty ammonia tank. Anderson disconnected two electrical cable. Then he and Mastracchio released four bolts, lifted the tank off the S1 truss segment and then handed it off to the station's robotic arm.

With Discovery mission specialist Stephanie Wilson at the controls, the 57.5-foot Canadian-built arm was used to move the tank to a station mobile rail cart. Mastracchio and Anderson just tied the tank down on the cart with six straps. Now they'll head off to retrieve a new fully-filled ammonia tank, which then will be put in place in the station's truss.

The station's myriad systems generate heat that then needs to be dispelled from the outpost. Its cooling system uses ammonia as a coolant, and heat is shed through several 40-foot-long radiators that unfurl like accordions.

The old 1,800-pound tank, which was delivered to the station by an Atlantis crew in October 2002, will be brought back to Earth aboard Discovery.