Astronauts Inspect Shuttle Heat Shield

In this image from NASA TV, Space Shuttle Atlantis pilot Chris ferguson, left, and Commander Brent Jett sit on the flight deck of the shuttle Tuesday, Sept. 19, 2006. (AP Photo/NASA TV) (AP)

HOUSTON -- Astronauts started giving the space shuttle Atlantis a thorough once-over Wednesday to see if the heat shield was damaged when a mystery object floated off the orbiter.

The worry over the object _ which appeared to drift away when landing systems were put through a normal but bumpy trial run early Tuesday morning _ and whether it came from a crucial part of Atlantis was enough to make NASA postpone the shuttle's landing from Wednesday to Thursday or later.

But after spending some of Tuesday trying to figure out what the object was and not coming up with a good answer, NASA managers figured the smarter thing to do was to spend early Wednesday making sure the shuttle was in good shape instead of concentrating on solving the mystery.

Their main concern was the status of the all-important heat shield, because a damaged shuttle skin led to the 2003 demise of the shuttle Columbia.

"We are going to verify that our critical heat shield is in good shape for entry to the best of our ability," shuttle program manager Wayne Hale said, adding that that goal should be accomplished by 2 p.m. EDT.

Beginning with the right wing at 12:15 a.m. EDT, astronauts slowly swept the shuttle's robot arm above and along Atlantis' heat shield in what was expected to be a five-hour photo survey. The two cameras on the arm should see any worrisome damage to the heat shield that would be left from the mystery object, Hale said. NASA doesn't know how big the object is because there was no frame of reference or distance in the video that captured the dark rectangular shape tumbling away.

A second mystery object was spotted midday Tuesday and photographed by astronaut Dan Burbank. Commander Brent Jett said the object looked like a picture hanging clip. But it may be a garbage bag, which would unlikely be a damage risk, but the issue will be moot if the heat shield looks good, Hale said.

"So far we do not know the identity of the two things that floated away yesterday," Houston spacecraft communicator Hans Schlegel told Atlantis Tuesday night. "Today we want you to survey the vehicle to make sure it's ready for entry. Last night we already surveyed from ground."

If camera angles or lighting aren't quite right during the robot arm inspection, astronauts will attach a 50-foot boom on to the arm Wednesday morning to see more of the shuttle, at closer angles, Hale said. The odds are 50-50 chance that NASA will have to take the extra look, which would take an hour to set up and three hours to conduct, Hale said.

Mission controllers also used cameras at the end of the robot arm to take pictures around the payload bay while astronauts slept on Tuesday.

If astronauts are too tired from the shield inspection process Wednesday, NASA could postpone landing until Friday, Hale said.

Mission Control woke Atlantis to "Beautiful Day" by U2 and astronaut Heidemarie Stefanyshyn-Piper responded: "Any day in space is a beautiful day and hopefully tomorrow it'll be a beautiful day in Florida and we'll be back home."