NASA appears to have resolved problems with a new urine recycling system on the International Space Station, bolstering hopes it will be able to expand the research outpost's crew next year, officials at the U.S. space agency said on Tuesday.

Reusing wastewater is essential for doubling the size of the crew living aboard the station from three members to six, especially since the space shuttles, which produce water as a byproduct of their electrical systems, are to be retired in 2 years.

The device, part of a $250 million new life-support system aboard the station, shut down during three previous attempts to purify urine. NASA wants the visiting shuttle Endeavour crew to bring home processed samples for analysis before declaring the water purification system suitable for use.

Two rounds of modifications to stabilize the device's centrifuge appear to have worked, flight director Brian Smith said on Tuesday. It completed a full five-hour run Monday and was nearing completion of a second full run early Tuesday.

Engineers planned to keep the device operating all day in hopes of producing enough processed urine before Endeavour's departure on Friday. The device was ferried into orbit and installed in the station's Destiny laboratory after the shuttle arrived on November 16.

The shuttle's stay at the station was extended a day to wait for the samples.

"We're going to try to keep it going all day and have the crew just reload the (urine) tank as it gets low," Smith said.

Also Tuesday, NASA tested the station's newly repaired solar wing rotary joint, which was cleaned and restored during four spacewalks by Endeavour astronauts.

The joint had been contaminated by metal filings, prompting NASA to lock it in place to prevent damage. Immobilizing the wing, however, prevented panels from tracking the sun for full power.

While the crews slept, engineers on the ground watched as the joint automatically pivoted to track the sun for the first time in a year.

"There's months worth of testing left to go before we can really determine what impact all four (spacewalks) had on that joint," Smith said.

Endeavour is due back at the Kennedy Space Center in Florida on Sunday after 16 days in orbit.

NASA plans eight more flights to the station, a $100 billion project of 16 nations, before the shuttles are retired in 2010.