Bad weather forced NASA to skip a first opportunity on Thursday to bring space shuttle Atlantis back to Earth but the US space agency could make another attempt later in the day.
Touchdown at one of the Kennedy Space Center's seaside runways in central Florida had been scheduled for 1:55 pm EDT (1755 GMT) but thick clouds dogged the space port all morning and nearby rain showers prompted NASA managers to call off the first attempt.
There will be a second landing opportunity at 3:30 pm EDT (1930 GMT) but shuttle flight directors may delay the landing until Friday because of continuing cloud cover. Flight directors decided not to staff the shuttle's backup landing site in California on Thursday but if weather continues to prohibit a touchdown in Florida, both sites will be available for landing attempts on Friday.
The shuttle has enough fuel and supplies to stay in space until Sunday. Atlantis spent most of its 13-day mission at the International Space Station, where the crew installed a 17-tonne metal truss that included solar power panels to generate additional electricity for the half-finished $100 billion complex.
NASA managers cleared Atlantis for Thursday's landing opportunities after two inspections during flight and high-resolution photos taken by the space station crew found no damage to the orbiter's heat shield.
Heat shield problems are a major concern at NASA since the shuttle Columbia broke up while returning to Earth in 2003, killing the seven astronauts on board. The accident was blamed on an undetected crack in the heat shield. During one of four spacewalks performed by Atlantis crew, astronaut John "Danny" Olivas patched up a torn thermal blanket that protects an area near the shuttle's tail from heat.