Discovery's six astronauts spend their last full day in space Sunday stowing equipment, giving television interviews and testing the space shuttle's flight control system for landing.

This image provided by NASA shows the International Space Station backdropped by the blackness of space and Earth's horizon moving away from Space Shuttle Discovery Saturday July 15, 2006. Earlier the STS-121 and Expedition 13 crews concluded almost nine days of cooperative work onboard the shuttle and station. An official, final 'good to go' decision for landing is expected Sunday. Discovery will try to land at a possibly cloudy and rainy Kennedy Space Center on Monday, at either 9:14 a.m. EDT or 10:50 a.m. EDT. [AP]

The initial test results relieved concerns about a leaking unit that powers hydraulic systems used for steering and braking.

Engineers were not sure if it had been leaking harmless nitrogen or flammable hydrazine and had considered burning off the fuel and shutting down the unit before landing to eliminate any fire hazard -- something NASA has never done before. The spacecraft needs only one power unit to land.

Without a third power unit, Discovery's landing gear would have to be deployed by explosive charges rather than the hydraulics system and stricter weather requirements would be implemented for the shuttle's landing at the Kennedy Space Center.

But early testing results on the unit Sunday morning showed no problems.

"We saw normal fuel usage, normal parameters," Mission Control radioed Steve Lindsey, Discovery's commander.

Lindsey responded, "OK. Great news."

The crew also practiced firing jets that will be used during the shuttle landing.

NASA managers already were keeping an eye on the weather as thunderstorms Monday morning threatened to come within 30 miles of the Kennedy Space Center's 15,000-foot runway.

"It looks generally pretty good but they are showing a chance of showers," Mission Control told Lindsey. "It gets a little worst as the day goes on so we're hoping the early morning works out for us."

An official, final "good to go" decision for landing is expected Sunday following a review of the final in-flight inspection of Discovery conducted Saturday by the shuttle's crew using sensors at the end of a 50-foot boom attached to a 50-foot robotic arm.

Discovery will try to land at either 9:14 a.m. or 10:50 a.m. EDT Monday, completing a successful 13-day mission that brought supplies and an extra crew member to the international space station and allowed spacewalkers to repair a vital rail car on the station.

The shuttle also was spared the foam loss from its external tank during launch that doomed the seven Columbia astronauts in 2003 and caused the shuttle fleet to be grounded last year after foam came off during the first return-to-flight shuttle mission since the Columbia disaster.

Discovery must land by sometime Wednesday, and if it can't complete its flight Monday, NASA will consider the backup landing site at Edwards Air Force Base in California.

The crew awoke Sunday to a recording of The Cure's "Just Like Heaven," chosen by astronaut Piers Sellers' family. Sellers told Mission Control the song reminded him of "the wild, happy, drinking-beer years of my youth."