Two US spacewalkers left the International Space Station (ISS) on Tuesday to remove an empty ammonia tank that is crucial for keeping the station cool.
John "Danny" Olivas and Nicole Stott left the ISS at 2149 GMT for the planned six-and-a-half-hour spacewalk.
They will disconnect lines that transfer ammonia and nitrogen in the tank, unhook the electricity and unbolt the tank before lifting it away from the station and placing it on the robotic arm to be moved out of the way.
The 816-km empty ammonia tank is the largest item ever moved by astronauts during a spacewalk. It is used to remove heat from the station. A new ammonia tank will be installed by Olivas and Swedish astronaut Christer Fuglesang in the second spacewalk on Thursday.
Later in Tuesday's spacewalk they will also remove experiments that had been stored outside the station.
Also on Tuesday the crews of the space shuttle Discovery and the ISS transferred cargo to the station that will make it more comfortable for the expanded crew.
The Colbert treadmill - named for US comedian Stephen Colbert - was among the items brought on board, along with a new crew sleeping quarters and an air filtration system.
The star, who plays a pompous news commentator on his cable programme, had organised fans to vote for him in a NASA competition to name a module on the International Space Station. Their dedication paid off when the comedian's name came first in the US space agency's online poll, besting all of NASA's suggested names.
NASA was caught off guard with the naming campaign and dragged its feet, saying it did not typically name space station hardware after living people. Instead, it chose the name Tranquility, after the moon's Sea of Tranquility where the Apollo 11 mission touched down.
As a compensatory gesture, and in the spirit of the comedian, however, NASA named a new treadmill in Colbert's honour, even concocting an acronym for the exercise device - the Combined Operational Load Bearing External Resistance Treadmill, or Colbert.
The planned 13-day mission is designed to transport new equipment and experiments to the space station to boost its capacity as an orbiting laboratory. Stott has replaced Timothy Kopra aboard the ISS crew, and three spacewalks are planned for the mission.