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A photo survey was made Wednesday of the shuttle Atlantis's cargo bay to see if there was any damage from a mystery object. ((NASA TV/Associated Press))

NASA officials cleared the space shuttle Atlantis for landing Thursday after finding no damage to the shuttle's heat shield and determining that pieces of debris found floating outside the shuttle posed no threat.

Two unplanned and improvised inspections of the shuttle's heat shield using cameras and sensors attached to the craft's robotic arm showed "no evidence of any kind of damage," NASA said. The camera boom and Canadarm have been stowed away in preparation for landing.

"Nothing was found to be missing or damaged," said Wayne Hale, space shuttle program manager. "So we feel very confident that we're in for a very good landing opportunity."

Atlantis commander Brent Jett described objects first seen on Wednesday as two rings and a piece of foil. He told mission control that one of the objects, about 30 metres from the shuttle, was "a reflective cloth …It's not a solid metal structure."

A day earlier, NASA officials postponed the shuttle's return to Earth, originally planned from Wednesday morning,after astronauts spotted two other mysterious objects floating near the craft.

Hale said the object first spotted Tuesday will likely never be identified, but it was most likely a flexibleorangeplastic spacer used in the installation of the heat shield's ceramic tiles. The second object spotted Tuesday might have been a garbage bag, said Hale.

Both objects have floated away from the shuttle. Hale said all the pieces of debris likely shook loose from the shuttle during a test firing of its manoeuvring jets.

Floatingdebris common

Atlantis flight director Steve Stitch said it's common to see pieces of floating debris when astronauts open the payload bay doors at the beginning of the mission, although"it is a little bit unusual to maybe see objects this late in the mission."

Stitch said there are two windows of opportunity for the shuttle to land on Thursday: 6:21 a.m. ET and 7:51 a.m. ET.

"The forecast for tomorrow looks very good," he said.

Col. Chris Hadfield,a Canadian who took part in two previous shuttle missions, saidobjects floating in the blackness of space stand out when they catch the sunlight.

"It's a little bit the case of the more you look at something, the more you see," he said, adding that he had seen space debris during both of his missions.

Hadfield saidthe current mission's astronauts are confident in the health of the shuttle, and arepleased to have an extra day in space.

"We're all very sure this little piece of stuff floating is just normaloperations for a vehicle as big, and complex and multi-faceted as the shuttle," he said.

With files from the Canadian Press