NASA announced that cargo transfers aboard the shuttle and station complex are more than 76 percent complete. Also, more troubleshooting took place on a Urine Processor Assembly inside the station's Water Recovery System after it shut down several times during testing.

On Sunday, astronauts removed grommets from the unit that NASA believed were hitting a sensor that caused it to shut down after about two hours.

The work appeared to briefly fix the problem, but it shut down again after three and a half hours, ISS commander Mike Fincke said. The unit was able to process 3.8 litres of urine, or about a third of a tank, before shutting down.

NASA scientists were to continue troubleshooting and hoped to get the system up and running before the space shuttle Endeavour is due to leave the ISS on Thursday.

NASA has left open the possibility of extending the mission by one day to deal with the problem, but no decisions have been made.

Meanwhile, shuttle astronauts Steve Bowen and Shane Kimbrough prepare for the fourth STS-126 spacewalk Monday at 1:45 p.m. EST.

The mission’s final spacewalk will require careful coordination, as the spacewalkers perform preventative maintenance on the station’s port SARJ, which currently is functioning well. Kimbrough will have just the one spacewalk to lubricate the same surface area that was lubricated over three spacewalks on the starboard side.

To make that possible, he and Bowen will open covers 6, 7, 10, 11, 14 and 15, and leave them open for most of the spacewalk. Kimbrough then will lubricate the exposed area and move away so that flight controllers on the ground can rotate the joint 180 degrees. That will help spread the grease, and expose new, unlubricated areas under the open covers.

While the joint is rotating, Kimbrough will return to the Quest airlock to retrieve a video camera. He will install the camera on the first port segment of the station’s truss, where it will be used next year to provide views of the robotic arm’s capture and docking of the first Japanese H-2 Transfer Vehicle.

Kimbrough then will move back to the port SARJ, grease the newly exposed areas and close the covers. Meanwhile, Bowen will work on several projects at the Japanese Kibo module. He will reinstall the common berthing mechanism’s insulation that he removed during the first spacewalk. Next, he will tuck in the module’s robotic arm grounding tabs, which are obscuring the view of the arm’s camera, by wrapping the tabs around a cable and Velcroing them together.

Afterward, Bowen will install three spacewalk handrails, two worksite interfaces and two Global Positioning Satellite (GPS) antennae on Kibo’s exterior. The H-2 Transfer Vehicle will use the GPS antennae to navigate to the space station. Both astronauts will wrap up the spacewalks by taking photographs. Bowen will photograph the radiators on the first port and starboard truss segments, using both regular and infrared cameras. In September, ground controllers noticed damage to one panel of the starboard radiator.

Blemishes have been noticed on the trailing umbilical system cable of the mobile transporter, so Kimbrough has been asked to photograph it as well.

The photographs will be used by teams on the ground to determine the cause of the damage and blemishes and decide what
action, if any, should be taken.