Expedition 13 crewmember Jeff Williams, left , work on the Interface Umbilical Assembly, the failed component of the railcar system, with commander Steve Lindsey in the Destiny module of the ISS

Astronauts aboard the U.S. space shuttle Discovery will undertake a final inspection of the orbiter's outer surface later Friday, before undocking from the International Space Station Saturday.  They will be looking for damage that might interfere with a safe return to Earth next week.

The U.S. space agency, NASA, is taking no chances, in the hope of avoiding another shuttle catastrophe like Columbia in 2003. A hole in its wing caused by launch debris led to its disintegration upon re-entry into the atmosphere, killing all seven astronauts.

Discovery's launch was closely monitored by an array of new cameras, aircraft spotters, and shuttle wing impact sensors. A new camera and sensors at the end of its robot arm scanned the orbiter, while docked at the space station and found no surface flaws. Based on that, NASA has cleared the shuttle for a Monday re-entry.

But it plans one more wing check. Discovery's crew will use the robot arm to scan the front edges of the wings to see if tiny space rocks, or micrometeoroids, have damaged them during the flight.

Shuttle flight director Tony Ceccacci says the inspection reduces the odds of losing a shuttle from micrometeoroid impacts.

"All the statistics have to do with your exposure time," he said.  "So, with the late inspection we mitigate that concern. The big thing is that, now, you are inspecting three days before deorbit, so your exposure time gets reduced, so your probabilities get better and better."

Discovery's crew returned to work after a day of rest Thursday, transferring the big Italian-built cargo carrier it flew to the space station back into the shuttle's payload bay. Station flight director Rick LaBrode calls Discovery's visit outstanding. It brought a third crewmember to the outpost, German astronaut Thomas Reiter. LaBrode says the combined crews also hauled more new supplies and equipment into the station than any previous flight, and in less than the expected time.

"They did a great job. We worked them, we worked them. We got our money's worth out of them, that's for sure," he said.

Discovery pilot Mark Kelly told NBC television that the success of the mission means the shuttle fleet can return to its routine visits to the space station beginning next month.

"After this flight, I think we're really in a good configuration, and we're pretty much set up to continue assembly of the space station and continue to fly the shuttle, hopefully like we did before 2003," he explained.

But mission controllers are monitoring malfunctions in two shuttle auxiliary units that supply power to hydraulic systems used during re-entry and landing. One problem might be a fuel leak that could present a fire hazard to the crew. The other is less serious, a broken heater for which there is a backup unit.