NASA declared that it's back in the space-station assembly business Thursday after shuttle Atlantis and its six astronauts safely returned from a 12-day mission to install a big new piece of the orbiting outpost.

"It was a great team effort. Assembly is off to a good start," commander Brent Jett radioed as the shuttle touched down in the dark about an hour before sunrise.

Jett and his crewmates did the first construction work on the international space station since the Columbia disaster 3½ years ago, performing three grueling spacewalks to hook up a 17½-ton addition. The new piece included a giant set of electricity-producing solar panels.

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With Atlantis on the ground, the space agency and its international partners plan 14 more shuttle flights in an ambitious effort to finish building the orbiting space lab over the next four years.

After that, NASA's three shuttles — the only spaceships cavernous enough to haul the multi-ton space station sections — will be retired while the space program turns its attention to flying to the moon and then Mars.

"We are rebuilding the kind of momentum that we had in the past and we're going to need if we're going to finish the space station," said NASA Administrator Michael Griffin. "Because we have an awesome task ahead of us."

Space shuttle Discovery is set for liftoff in December on the next flight of a construction sequence that Griffin described as "a little simpler than building an aircraft while you fly it, but not a lot."

The shuttle landing was a day later than planned because NASA ordered up extra inspections after mysterious pieces of debris were spotted floating outside.

Engineers had feared the spacecraft's heat shield had been damaged, but the inspections found nothing wrong, and the descent through the atmosphere was trouble-free.

"We were not very concerned," Jett said afterward. "We just assumed whatever objects we saw had come from the payload bay. What we were trying to do is make the folks on the ground comfortable."

NASA managers were impressed with how good Atlantis' heat shield looked after passing through the fiery heat and friction of Earth's atmosphere.

The success of Atlantis' flight may allow NASA to relax a requirement that shuttles be launched in daylight so that they can be photographed for any damage. The space agency may also reconsider the need for further design changes to the external fuel tank to prevent foam insulation from breaking off.

NASA has spent three years and two test flights trying to fix the foam — the very problem that doomed Columbia in 2003 — and the triumph of Atlantis shows that the space agency is "back into a more operational tempo," Griffin said.

Atlantis' flight was bookended by delays. The launch was scrubbed four times in two weeks because of a lightning bolt that hit the launch pad, Tropical Storm Ernesto and problems with the electrical system and a fuel gauge.

The five men and one woman were the longest-trained crew in NASA history. They were originally supposed to fly to the space station in 2003, but the Columbia accident brought construction to a standstill.