"Congratulations, Andy," German astronaut Gerhard Thiele told Thomas from NASA's Mission Control Center at Johnson Space Center in Houston, Texas.

Said Thomas: "Tell the folks back there that Leonardo is going to be coming home soon."

Running five hours behind schedule, the Leonardo move followed a lengthy scramble to determine if a slight but potentially serious slip-up might have corrupted critical software within the shuttle's four primary computers.

The computers control all crucial shuttle systems, including those required for a safe landing.

The potentially scary scenario started to unfold when Discovery's crew turned on two of the four units during a bid to rid cold coolant lines of any ice that might be forming within them.

Routine start-up procedures require a 10-second wait between the activation of each computer, both of which had been turned off earlier in the flight to conserve electricity.

Flight controllers, however, forgot to warn the astronauts about the 10-second interval and both computers were powered up within four to six seconds, raising concerns that operating system software might have been compromised.

"The crew did exactly what the ground told them, and when we went back and looked at it, we said, 'Ah, we should have given them the warning to wait 10 seconds,'" NASA lead flight director John Shannon said.

Mission managers met late Saturday to determine whether the astronauts should power up a back-up flight computer and then make an unprecedented attempt to conduct an orbital reloading of the software within the four primary units.

At the same time, astronauts and flight controllers on the ground tried to duplicate the rapid start-up in a shuttle flight simulator. The idea was to determine whether the apparent problem in orbit might endanger Discovery's crew during their trip back from the station.

In the end, though, mission managers opted for a simpler "confidence test" of software segments that control the operation of crucial landing systems.

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Shuttle skipper Jim Wetherbee performed those tests, proving the software will in fact work as expected when Discovery returns to Earth.

"Super job recovering the [computers]," Thiele told the veteran shuttle mission commander. "We have full confidence that things will work from now on."

The computer tests forced flight directors to push back what had been the prime job for the day: Parking the Leonardo cargo carrier in the shuttle's payload bay.

Trouble also cropped up during the required depressurization of a small vestibule between the station and the Italian supply ship, further delaying the move.

One of three station cargo vans built by Italy at a cost of $450 million, the Leonardo module now holds 1,644 pounds (740 kilograms) of dirty laundry, surplus gear and garbage amassed since November by the outpost's first full-time crew: U.S. commander Bill Shepherd and Russian cosmonauts Yuri Gidzenko and Sergei Krikalev.

Launched March 8 aboard Discovery, the pressurized logistics ship hauled up 9,663 pounds (4,348 kilograms) of supplies and equipment for the station's second expedition crew: Russian commander Yuri Usachev and American flight engineers Susan Helms and Jim Voss, the three of whom will be onboard the complex until late July.

Shepherd will officially hand over the helm to Usachev during a change-of-command ceremony scheduled for 8:27 p.m. EST Sunday (01:27 GMT Monday).

Hatches between the shuttle and the station then will swing shut, officially marking the end of a 136-day station "shake-down cruise" for Shepherd and company, who turned a long-vacant station into a fully functional outpost.

During their hectic stay at the station, the Expedition One crew started up crucial life support and other operational systems while overseeing the delivery and activation of a $600 million U.S. electric power tower and the $1.4 billion Destiny science laboratory.

"We tried to do everything we could so that the station is not just a station but also a home for the next expedition, and we wanted to make sure that the next expedition will be able to continue our work," Krikalev told reporters earlier this week.

The hatch-closure also will mark the official kickoff of the second station expedition, a planned four-and-a-half-month tour aimed at outfitting the Destiny lab, starting up station research and pressing ahead with further outpost construction work.

"We have a lot of work to do, and I like working," incoming flight engineer Voss said. "So I'm just anxious to get started."

Discovery remains scheduled to depart the station about 11:30 p.m. EST Sunday (04:30 GMT Monday), or about three hours after the command handover ceremony.

Rookie shuttle pilot Jim Kelly will back Discovery away from the station and then make a looping fly-around of the outpost as camera-toting crewmates shoot still photos and video of the complex from a distance of about 400 feet (121 meters).

"I'm really looking forward to getting the full tour around the space station," Kelly said. "Coming up, we got to see most of the bottom side, but I'm kind of looking forward to taking that trip around the top side to see what the whole thing looks like."

Shuttle thrusters then will be fired about 12:42 a.m. EST (05:42 GMT) Monday, propelling Discovery on a two-day trip back to Earth.

With Shepherd and his Russian crewmates in tow, Discovery is scheduled to land here at Kennedy Space Center at 12:55 a.m. EST (05:55 GMT) Wednesday.