Space shuttle Atlantis and its six astronauts glided to a safe landing in darkness early Thursday, ending a 12-day mission whose smooth success was briefly upstaged by the high drama caused by mysterious floating debris.

"Nice to be back. It was a great team effort," said commander Brent Jett immediately after touchdown at Kennedy Space Center at 6:21 a.m. EDT.

More than 1 1/2 hours after landing, the astronauts, all wearing broad grins, left the shuttle to greet NASA administrator Michael Griffin and other agency officials. Then they walked under Atlantis to inspect the shuttle's heat shield.

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"It's really a beautiful day in Florida, a great way to end a mission," said Jett. "It was a pretty tough few days for us, a lot of hard work, a great team effort to get the station assembly restarted on a good note."

Jett and his crew did the first construction work on the international space station since the Columbia disaster 3 1/2 years ago. The astronauts performed three grueling spacewalks and took on other heavy-lifting tasks in one of the most challenging missions ever, adding a 17 1/2-ton truss addition with giant solar wings that will help power the orbiting lab.

The landing 48 minutes before sunrise was a day later than planned because NASA ordered up extra inspections of the spacecraft's delicate skin to make sure it was safe to come home. The fear was that a mysterious piece of debris spotted floating nearby on Tuesday might have hit the spacecraft. Astronauts later saw other debris.

It was a flying piece of foam insulation that knocked a hole in Columbia causing its demise in 2003, killing seven astronauts. Since then, NASA has developed new equipment and practices to guard against and watch out for similar damage to the sensitive space shuttle.

Those new techniques were used to make sure Atlantis was safe to return. After numerous cameras took pictures above and below Atlantis, some of them maneuvered robotically by the shuttle astronauts, NASA proclaimed the spacecraft damage-free.

"We've seen a new standard in NASA vigilance," said shuttle program manager Wayne Hale.

NASA officials said their best guess was that the most worrisome debris was a plastic filler from the thermal tiles which protect the shuttle from blasting heat. Four other unidentified objects, including a possible garbage bag, floated near the shuttle over the next day.

"We were not very concerned," Jett said several hours after landing. "We just assumed whatever objects came out had come from the payload bay. What we were trying to do is make the folks on the ground comfortable."

In a news conference, Griffin downplayed the litter in space, saying debris can slip out of the shuttle cargo bay because people are not perfect. He and launch director Mike Leinbach said Atlantis came back as clean, if not cleaner, than Discovery in its two previous landings.

The unplanned drama threatened to overshadow what had been a nearly flawless mission filled with strenuous spacewalks and rigorous robotics work that placed the space station back on a path to completion after its long hiatus. The crew of five men and one woman were the longest-trained in NASA history, because they were originally supposed to fly to the space station in 2003. But the Columbia accident kept them grounded.

The mission was the first of 15 tightly scheduled flights needed to finish constructing the half-built space lab by 2010.

"We are rebuilding the kind of momentum that we have had in the past and that we need if we're going to finish the space station," said Griffin.

NASA and its international partners of Russia, Europe, Canada and Japan must finish building the space station before the U.S. space agency ends the shuttle program in 2010 with plans to return to the moon in a new vehicle. The massive, 25-year-old shuttles are the only spaceships large enough to haul construction parts to the space lab.

The next flight in the construction sequence is set for December.

The mission was bookended by delays. The launch was scrubbed four times in two weeks because of a launch pad lightning bolt, Tropical Storm Ernesto and problems with the electrical system and a fuel gauge. Griffin called those snags "just routine life in the space business."

With all the postponements, NASA negotiated with the Russians to squeeze out one last chance in its launch window. The Russians were worried the trip would interfere with their Soyuz trip to the space station with a paying customer, Iranian-born space tourist Anousheh Ansari, a Dallas businesswoman. The Soyuz lifted off Monday, just hours after Atlantis had undocked from the space station.

Less than 24 hours after Atlantis undocked, an oxygen generator on the space station overheated and spilled a toxic irritant, forcing the three-man crew to don masks and gloves in the first emergency ever declared aboard the 8-year-old orbiting outpost.