WASHINGTON - TWO astronauts on the US space shuttle Endeavour on Monday 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 at 1824 GMT (2.24am Singapore time), 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.

Mr 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 US$250 million (S$377 million) 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, Ms Stefanyshyn-Piper let slip her tool bag and watched helplessly as it floated off into the void of space.

Meanwhile, Nasa experts continued to work on problems with the urine processor unit that is designed to process urine, perspiration and bath water into drinkable water.

Nasa said a centrifuge motor inside the distillation unit was running too slow and drawing too much electrical current.

The device is essential for doubling the accommodation capacity, as it would be able to recycle the station's 6.8 tonnes of waste water produced each year.

Once in place, the unit would make it no longer necessary to regularly ferry vast quantities of water to the space station. -- AFP