This story was updated at 7:00 a.m. EDT.

CAPE CANAVERAL, Fla. – Six astronauts and their shuttle Atlantis are safely back on Earth after returning NASA and its partners on a path to complete the International Space Station (ISS).

Atlantis touched down at the Shuttle Landing Facility here at NASA’s Kennedy Space Center (KSC) before dawn at about 6:21 a.m. EDT (1021 GMT) after 187 orbits and a 12-day mission that kick started ISS construction.

"It’s nice to be back and was a great team effort,” Atlantis’ STS-115 commander Brent Jett said just after landing. “I think [ISS] assembly is off to a good start.”

Jett and his STS-115 crew delivered the first new additions to the ISS since late 2002: a new set of wing-like solar arrays and two massive trusses that weighed in at 17.5 tons before the flight’s Sept. 9 launch.

We are back in the assembly business,” Wayne Hale, NASA’s space shuttle program manager said of Atlantis’ STS-115 flight. “This is one of the most complex missions that has even been flown in space.”

Atlantis fired its engines to plunge Earthward at about 5:14 a.m. EDT (0914 GMT), loosening two resounding sonic booms that pierced the early morning sky before its wheels touched down on Runway 33 here while the orbiter traveled at a speed of about 225 miles per hour (360 kilometers per hour).

Returning to Earth with Jett were shuttle pilot Chris Ferguson and mission specialists Joseph Tanner, Daniel Burbank, Heidemarie Stefanyshyn-Piper and Canadian Space Agency astronaut Steven MacLean. Their landing was delayed one day to allow a third survey of their heat shield after several mysterious bits of debris were spotted floating around the orbiter this week.

Astronauts aboard the ISS watched Atlantis as it streaked through the Earth’s atmosphere, at times backlit by lightning. The two spacecraft were about 200 miles (321 kilometers) apart at the time Atlantis performed its deorbit maneuver.

“The brightest thing through the window by far is the orbiter itself, with its contrail behind,” ISS Expedition 13 flight engineer Jeffrey Williams told mission control, adding that the station crew turned off the lights in the U.S.-built Destiny laboratory during the descent.

Their safe landing concluded NASA’s 116^th shuttle flight – and the 27^th mission for the Atlantis orbiter – after 11 days, 19 hours and six minutes. It also marked NASA’s 15th night landing at KSC and the 21^st unlit return of an orbiter overall.  The spaceflight marked NASA’s third shuttle flight since the 2003 Columbia accident.

Orbital construction

Atlantis’ STS-115 flight marked the first of about 15 critical missions to build up the space station’s electrical grid and then haul up new laboratories and modules.

“If we made it look easy, than I guess that’s a good thing,” said Stefanyshyn-Piper, who made her first spaceflight and spacewalking debut during STS-115. “It was difficult and there were a lot of complex tasks.”

Atlantis astronauts staged three spacewalks in four days to install the Port 3/Port 4 truss segments to the space station’s port side and unfurl their solar arrays to their maximum, 240-foot (73-meter) wingspan. They traveled about 4.9 million statute miles during their spaceflight.

“We achieved a new record in assembly of a major component in a minimum number of spacewalks,” Hale said of the mission.

Atlantis’ STS-115 mission was the second in three months to visit the space station’s Expedition 13 crew – commanded by Russian cosmonaut Pavel Vinogradov, with NASA astronaut Jeffrey Williams and German spaceflyer Thomas Reiter – and left the orbital laboratory a bit off-kilter with it’s left-leaning slant.

“For the first time in four years, the shape of the space station changed and it was really dramatic,” said Paul Dye, NASA’s lead shuttle flight director for STS-115, adding that even the little ISS icon on NASA’s orbital tracking screen changed to match the outpost’s new look. “We can see that we’ve really done something big for the space station.”

The way ahead

With Atlantis’ safely back on Earth, NASA will now turn its attention to the orbiter’s sister ship Discovery, which is being primed for a Dec. 14 launch, though shuttle officials hope the vehicle can be ready a week early.

“All the rest of the assembly missions are going to be hugely challenging,” Jett said, adding that flight controllers, engineers and astronauts all work together to make the effort work. “I think we can pass along a lot of the lessons to future crews.”

Discovery’s STS-116 mission – commanded by NASA shuttle veteran Mark Polansky – will ferry a new ISS crewmember Sunita Williams to the station, haul a new portside section for installation and begin the electrical rewiring necessary for a series of missions to ready the station’s infrastructure for future assembly.

“There are about six in a row here that we really need to pull off in a fairly rapid order without major problems to keep assembly going,” Hale said.

The solar arrays unfurled during Atlantis’ STS-115 mission, representing about one-fourth of the station’s final power grid, will begin feeding the outpost in earnest during the December shuttle flight.

“That power is going to be very important,” NASA astronaut Michael Fincke told SPACE.com, adding that future international laboratories depend on the new arrays and others still to fly before they can leave Earth. “You need to have to have that power before you put in those modules.”

Atlantis and the STS-115 crew’s addition of new trusses and solar arrays to the ISS are yet another leap forward for the station, which saw NASA’s final post-Columbia accident shuttle test flight and a resumption of three-astronaut crews during the Expedition 13 mission.

“To come back to a crew of three and get the shuttle flying again, and to continue assembly, hopefully it sets us on a path to efficiently get done with the assembly so that we can then be prepared to move on and fulfill the vision of space exploration,” Jeff Williams said.

NASA will hold a post-landing news conference at about 8:30 a.m. EDT (1230 GMT), followed by comments from entry flight director Steve Stich at 9:30 a.m. EDT (1330 GMT).

Shuttle commander Brent Jett and his STS-115 crew are tentatively scheduled to comment on their flight on NASA TV no earlier than 11:30 a.m. EDT (1530 GMT).

You are invited to follow the progress of Atlantis’ STS-115 crew via SPACE.com’s NASA TV feed, which is available by click here.

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• NASA’s STS-115: Shuttle Atlantis to Jump Start ISS Construction
• The Great Space Quiz: Space Shuttle Countdown