Astronauts at the International Space Station prepared to work on a urine-recycling system and re-fire the space shuttle's thrusters Friday, before enjoying a half-day off to reboost their own energy.

Endeavour's extra push will elevate the docked space shuttle and space station complex, putting the space station at the right altitude to receive a Russian Progress spaceship scheduled to deliver cargo to the orbiting outpost three days after Endeavour starts heading back to Earth on Nov. 29. The space station generally stays in the range of 320 to 350 kilometres above Earth.

Endeavour's seven astronauts and the three space station crew members have been working without a break since the space shuttle launched from Florida a week ago. They will get a half-day off in space after holding a news conference.

Before that, they will work on a contraption that recycles urine and sweat into drinkable water. Endeavour's astronauts delivered the recycling system to the space station and are trying to start its operation.

It hasn't gone smoothly.

The astronauts had hoped to run a test batch of urine through the contraption Thursday, but a caution alarm usually caused by combustion delayed those plans. Flight controllers believe it was a false alarm because there wasn't noticeable smoke or a combustible odour.

"These are the growing pains we expect to see," said flight director Ginger Kerrick. "These are very complicated pieces of equipment with very complicated software to control them."

The astronauts were going to delay the urine test until flight controllers figure out what's wrong, but they planned to test another part of the system that purifies water. Samples from the recycling system will need to be analyzed on Earth before station crew members can use the machine sometime next year. Once running, the system will help the space station support six residents instead of the current three inhabitants.

Just as the alarm on the urine recycling system went off inside the space station Thursday evening, two spacewalkers wrapped up a nearly seven-hour spacewalk outside.

Successful spacewalk

To everyone's relief, Heidemarie Stefanyshyn-Piper and Shane Kimbrough deftly stepped through their work without any mishaps. During Tuesday's spacewalk, Stefanyshyn-Piper's tool bag slipped away while she was trying to clean grease leaked from a gun used to lubricate a jammed solar wing joint outside the space station.

There were two small hitches at the very end of Thursday's spacewalk: Kimbrough had trouble communicating with Mission Control and also had elevated levels of carbon dioxide in his spacesuit. Neither problem put the astronaut in danger. The communication problem was likely caused by a bump to his headset's volume control.

"The [carbon dioxide] level never got to a level that we would have been concerned that it would cause him any problems," said John Ray, lead spacewalk officer. "We were just managing it to make sure we got him inside before it got to that level, and we did."