Washington - Two space shuttle Endeavour astronauts on Saturday began a third spacewalk to get a solar panel on the International Space Station in working order.

Astronauts Heidemarie Stefanyshyn-Piper and Steve Bowen left the ISS at 1801 GMT on a planned seven-hour spacewalk devoted to cleaning and lubricating a joint on the panel that allows it to rotate to follow the sun.

Work on the joint has occupied much of the previous two spacewalks. A fourth spacewalk to work on a separate joint and do other work will take place Monday.

NASA could extend Endeavour's 15-day mission by one day to deal with problems that have cropped up with a urine recycling system that is designed to turn waste into drinkable water.

There appeared to be a problem with a centrifuge on a distillation component of the Urine Processor Assembly, flight director Courtenay McMillan said Friday. It could simply be that a sensor was causing the system to shut off as astronauts tried to run tests on the system that will turn urine and sweat into drinkable water.

But NASA was still trying to determine exactly where the problem lay as it could cause the space agency to extend the crew's mission by one day. McMillan said no decision about an extension had yet been made. (dpa)