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KENNEDY SPACE CENTER, Florida (CNN) -- The space shuttle Endeavour touched down on schedule at 10:53 p.m. ET Tuesday, concluding a historic, 12-day mission that launched the most ambitious scientific project ever -- the International Space Station.

A weather system moving over the Florida landing site had threatened to delay the homecoming for the six-member crew, but it moved away and ground controllers gave Endeavour the go-ahead.

NASA has described the mission as one of the most difficult on record. The crew snatched a 22-ton unmanned Russian module from orbit and mated it to an American-built module that Endeavour had carried aloft.

That was followed by three days of spacewalks to complete the rigging. In an interview Monday, Commander Robert Cabana said that once the crew had assembled the first two segments: "We kept pinching ourselves and saying: 'It can't be real.'"

The crew was called upon to perform some unscheduled tasks, such as fixing two balky antennae aboard the Russian Zarya module and repairing a short-circuited electrical system.

The astronauts also spent a full day inside the station, sprucing it up for the first permanent crew, due to arrive in just over a year.

Sunday night, they also deployed a 600-pound (270-km) Argentine satellite for whale-tracking and other scientific research.

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Cabana said his crew was "definitely relieved now that all the hard work's over."

"I think we're very pleased to have accomplished everything that we set out to do on this flight," he said. "Just seeing it all come together, all those years of hard work ... it was truly amazing.

The space station has been left slowly rotating until the shuttle Discovery arrives for more construction work in May or June next year.

A Russian service module is to join the U.S.-Russian nucleus next year, allowing the ISS to remain in space and support a three-person crew beginning in the year 2000.

Another 45 space missions will be necessary to complete the giant space station, expected to total more than 100 modules and stretch the length of two football fields.

The $40 billion to $60 billion effort to create a permanent research outpost in space involves 16 nations and will take at least five years to finish.