Two American astronauts have successfully completed a lengthy spacewalk to upgrade the cooling system on the International Space Station.
Nicole Stott and Danny Olivas re-entered the ISS early Wednesday after working outside for six and one-half hours. They removed a cumbersome and heavy ammonia tank that has been in orbit since 2002 and will install a new and larger coolant tank on Thursday.
At 800 kilograms, the new tank is the most massive item spacewalking astronauts have handled during assembly of the space station. Objects are weightless in zero-gravity conditions in space, but they still have mass and can be difficult to move.
Stott and crewmate Christer Fuglesang are due to complete the tank installation on the second of three spacewalks planned during Discovery's weeklong stay at the station.
The shuttle arrived at the station Sunday, carrying equipment that included an exercise treadmill named for U.S. television comedian Stephen Colbert.
When Discovery returns to Earth, Stott will remain aboard the space station as flight engineer for the next three months. She is replacing U.S. astronaut Timothy Kopra who will join the rest of the shuttle's crew for the return flight.
The shuttle, scheduled to land on September 10, will have the space station's old ammonia tank in its cargo hold.
Some information for this report was provided by AFP, AP and Reuters.