HOUSTON (AP) _ Hours after undocking from the international space station Tuesday, Atlantis' seven astronauts finished a final inspection of the space shuttle to make sure its heat shield was ready to re-enter Earth's atmosphere.
Atlantis' 13-day mission was scheduled to end with landing at 1:54 p.m. EDT on Thursday, although the weather at Kennedy Space Center looked iffy.
Continuing a tradition, space station commander Fyodor Yurchikhin rang a bell and said ``Atlantis departing'' as the space shuttle pulled away.
``Have a great remainder of your expedition,'' Atlantis commander Rick Sturckow told the two Russian cosmonauts and lone U.S. astronaut remaining at the station. ``We'll see you back on planet Earth.''
Pilot Lee Archambault steered Atlantis away from the space station and went on a quick trip around the station to photograph the solar arrays the crew installed on the half-built home 220 miles above Earth.
More than an hour after undocking, a piece of debris that looked like a blanket and at least five tiny flashing particles floated by the space station.
While NASA engineers are still reviewing video and photographs and don't know what the piece of debris might be, the flashing particles could be pieces of ice that are a byproduct of the shuttle's jets, said Cathy Koerner, flight director.
Mike Suffredini, NASA's space station program manager, said initial analysis of the piece of debris doesn't indicate it came from the space station and points to it being a small object.
``It's not a big concern,'' he said.
Results of the final inspection of the heat shield won't be ready until Wednesday, said Koerner, but problems aren't anticipated.
U.S. astronaut Clay Anderson told Mission Control that the station successfully took over orientation control after the shuttle undocked. That had been a concern since computers controlling the outpost's positioning crashed last week. They were revived last weekend.
Atlantis wasn't cleared to leave until the Russian computers passed a test to take control of the station's thrusters Monday. Astronauts on Atlantis had turned off lights and computers to conserve fuel in case the shuttle needed to stay at the station an extra day.
A camera attached to a robotic arm and boom surveyed the shuttle's wings and nose cap to make sure the shuttle was undamaged and could withstand the intense heat of re-entering Earth's atmosphere. That inspection was added to all shuttle missions after the Columbia accident in 2003 killed seven astronauts.
The inspection was conducted with Atlantis about 10 miles from the station so the shuttle could return to the outpost if something was wrong. Engineers on the ground planned to study the images before managers give final approval for landing.
Even if Atlantis is given the go-ahead, the weather may not cooperate. A front in the Florida Panhandle was expected to send showers to the Kennedy Space Center on Thursday and Friday.
``It's going to come down to a matter of timing and what that front does,'' NASA spokesman George Diller said in Florida.
Atlantis has conserved enough fuel to last until Sunday, but managers want the shuttle to land by Saturday. The flight would only be extended to Sunday if there were technical problems that needed to be fixed before landing. If needed, the backup landing site in California won't be activated until Friday, and another backup site in New Mexico won't be activated until Saturday.
NASA officials said that the space station and the space shuttle would be visible from the United States on Tuesday and Wednesday nights. Cities with the best chances of getting a view were Denver, Detroit, San Francisco and Washington, said NASA spokeswoman Kylie Clem.
During their stay at the space station, Atlantis' astronauts helped install a new truss segment, unfurled a new pair of power-generating solar arrays, repaired a peeled-back thermal blanket near Atlantis' tail and activated a rotating joint that allows the new solar arrays to track the sun.
The 11-day mission was extended to 13 days so astronauts could repair the thermal blanket.
The shuttle is bringing back U.S. astronaut and former space station resident Sunita Williams, whose more than six months in space set a record for the longest single spaceflight by a woman. Anderson, her replacement, was taken to the station aboard Atlantis.