NASA postponed the return of Atlantis for at least a day and examined the shuttle for damage that could prevent it from making the journey home after a mysterious object apparently fell off the ship in orbit Tuesday.

Space agency officials wanted extra time to establish whether the object was a vital piece of the shuttle — such as the tiles that protect it from the blowtorch heat of re-entry — and whether it harmed the spacecraft when it fell away.

Officials were not optimistic they would be able to identify the object, since the possibilities were almost endless, ranging from harmless ice to crucial thermal protection tiles. But the leading candidate was a plastic space-filler placed between the thermal tiles. If that is the case, the missing filler would not prevent a normal landing.

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"The question is: What is it? Is it something benign? ... Or is it something more critical we should pay attention to?" said Wayne Hale, space shuttle program manager. "We want to make sure we're safe to land before we commit to that rather incredible journey through the Earth's atmosphere."

NASA was concentrating more on making sure the shuttle is safe than identifying the object, Hale said. Astronauts often report "a little cloud of stuff that travels with your spacecraft," but in the past have not been as concerned, Hale said.

That was before the 2003 Columbia disaster.

"Clearly we are taking a much closer look than we ever did," Hale said. "You can call it anxiety. You can call it smart. It's what we do these days."

The shuttle has enough supplies to stay in space through Saturday while engineers on the ground figure out whether it can safely return to Earth.

The space agency was not considering a spacewalk to make repairs or, if the spacecraft is too damaged, sending Atlantis's six crew members to take refuge in the international space station and await rescue by another shuttle — a scenario that NASA has been developing ever since the Columbia disaster in 2003.

Before the postponement, Atlantis had been scheduled to touch down just before daybreak Wednesday, when the weather forecast wasn't favorable for landing anyway. The landing time was reset for early Thursday. Hale said landing could be delayed until Friday if the crew is tired from Thursday's activities.

The incident came near the end of what had been a nearly flawless mission devoted to restarting construction of the space station for the first time since the Columbia tragedy 3 1/2 years ago.

Mission Control spotted the baffling object — the size of which was not immediately determined — with a video camera in the shuttle's cargo bay. The object may have come out of the cargo bay early Tuesday, but officials were not certain.

The object floated near the shuttle in the same orbit for a while, slipping farther and farther away until it was just a dark speck in NASA video beamed down to Earth.

A few minutes after NASA made the midday decision to delay the landing, Atlantis astronaut Dan Burbank photographed what appeared to be another small object floating away from the spacecraft. But NASA spokesman James Hartsfield said: "They're not the same thing."

Later in the day, Hale said the object likely was a plastic bag that drifted out of the cargo bay.

NASA engineers said they think the first object may have shaken loose from the shuttle during the firing of jets in preparation for landing. "Think about driving over potholes," Hale said.

Sensors on the shuttle's right wing detected some kind of impact about the same time the object was first spotted. But NASA officials said the fact that many of the sensors in that section were triggered — rather than just one or two — suggests they were set off by vibrations from the jet firings.

"We think it has nothing to do with anything," Hale said late Tuesday.

So the space agency concentrated on using a half-dozen cameras in the shuttle's cargo bay and on its robotic arm to look for damage, especially to the spacecraft's thermal skin or any mechanical systems.

NASA managers planned to order Atlantis' robotic arm, and possibly a 50-foot boom with sensors and cameras at its end, to be taken out again Wednesday for another inspection. None of the Air Force Space Command's radar or telescopes has been able to locate the object, said Sgt. Jennifer Thibault, a spokeswoman for the command, which is assisting NASA.

The space agency has been especially alert to damage to the shuttle's heat shield since the Columbia tragedy. A piece of foam broke off Columbia's external fuel tank during liftoff and gashed a wing, allowing hot gases to penetrate the spacecraft during its return to Earth. The ship disintegrated, and all seven astronauts died.

Atlantis was inspected repeatedly during its flight, and up until Tuesday, NASA said the ship had come through the launch and more than a week in orbit in remarkably good shape.

During their mission, Atlantis' astronauts completed three difficult spacewalks to connect a 17 1/2-ton addition to the special station and help open up two 240-foot solar arrays that will generate electricity.

Astronauts and cosmonauts have seen strange objects in orbit for years, usually debris, and thought nothing of it, said Roald Sagdeev, a former top Soviet space adviser and director of the East West Space Science Center at the University of Maryland.

This is the first time a space agency is taking an unknown object so seriously, and rightly so, he said.

"It's better to be very cautious," Sagdeev said.

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