Two astronauts on the US space shuttle Endeavour began a fourth and final spacewalk amid efforts to repair the International Space Station as NASA extended the mission by a day.
Americans Steve Bowen, 44, and Shane Kimbrough, 41, left the decompression chamber, some 11 minutes earlier than planned, NASA television said.
The spacewalk is due to last more than six hours during which the astronauts will replace and lubricate the rotating systems of one of the station's three double solar antenna arrays.
Bowen and crew member Heide Stefanyshyn-Piper, 45, worked on repairs of the ISS's starboard Solar Alpha Rotary Joint race ring replacing trundle bearing assemblies over the weekend. The worn-out parts will be taken to Earth for inspection.
A spokesman for the US space agency also confirmed to AFP that the mission would be extended for one day to give the crew more time to try to repair a 250-million dollars machine for recycling urine, sweat and waster water.
The spacewalks are part of an ambitious "home improvement" project designed to double the station's crew capacity from three to six.
Starting from Wednesday, the team began installing a freezer and an oven for scientific experiments by NASA's Destiny Laboratory Module.
The additions also include two new sleeping quarters, exercise equipment, a second toilet, two new ovens and a refrigerator.
During an earlier spacewalk on Tuesday, Stefanyshyn-Piper let slip her tool bag and watched helplessly as it floated off into the void of space.