NASA astronauts have finished the first stage of a marathon task to fix the International Space Station's cooling system.
The two US astronauts have completed the first of a series of spacewalks to replace a pump on one of the station's two ammonia cooling systems.
The spacewalk was the first for NASA since July when the helmet worn by Italian astronaut Luca Parmitano began filling with water, a situation that could have caused him to drown.
The cooling system was shut down on 11 December forcing the crew to turn off non-essential equipment and shut down dozens of science experiments.
The remaining system could not support the three laboratories and other modules on the US side of the $US100 billion station, a project of 15 nations. The Russian side of the station has a separate cooling system.
The first spacewalk was carried out on Saturday as the station sailed more than 400 kilometres over the southern Atlantic Ocean.
Flight engineers Rick Mastracchio and Mike Hopkins wore spacesuits that were newly modified with snorkels to protect them from another water leak.
"Beautiful day. Awesome view," said Mr Mastracchio, a veteran of six spacewalks, as opened the airlock's hatch.
During the the five-and-a-half hour operation the astronauts were able to not only disconnect the old pump, but also remove it from its pallet on the station's exterior truss, a task originally slated for a second spacewalk on Monday. A third spacewalk, if needed, is scheduled for Wednesday.
The failed pump, which is located on a pallet on the right side of the station's external truss, will be stored outside the station for possible future repair and reuse.