( AFP ) - Rain forced NASA to delay the landing of space shuttle Atlantis in Florida Thursday, but the agency hoped the skies would clear up to return its seven astronauts to Earth later in the day.
The shuttle missed a first window of opportunity to land at the Kennedy Space Center in Cape Canaveral at 1755 GMT, but it has a second chance to bring the crew back at 1930 GMT.
Rains spread over some 50 kilometers ( 30 miles) around the Florida landing area prohibited the shuttle's landing, the National Aeronautics and Space Administration said.
The shuttle's cargo doors were closed earlier in preparation for its descent, 13 days after blasting off on a mission to install new solar panels on the International Space Station (ISS).
Atlantis commander Rick Sturckow and pilot Lee Archambault await the green light to fire thrusters to slow down the orbiter, which reaches speeds of more than 26,000 kilometers ( 16,000 miles) per hour.
If foul weather persists, NASA will try again Friday, when the shuttle will have two windows of opportunity to land in Florida and, if the weather has not yet improved, two more at the Edwards Air Force Base California.
NASA wants to land Atlantis by Saturday as the shuttle's hydrogen batteries providing its electric power would have just one more day of life.
Atlantis would have seven landing opportunities on Saturday, with two in Florida, two in California and three at the White Sands military base in New Mexico.
NASA prefers to land the shuttle in Florida as it costs nearly two million dollars to return it to its launch base piggybacked atop a Boeing 747 and would affect the schedule of future missions.
Atlantis undocked from the ISS on Tuesday after astronauts successfully installed a new 16-tonne truss segment expanding the orbiting laboratory with a new set of power-generating solar arrays that will track the Sun.
The astronauts ventured out of the ISS on four spacewalks to install the new segment and to close a hole on a thermal blanket protecting the shuttle.
The mission was also marked by the unprecedented collapse of Russian computers controlling the space station's orientation and altitude. The computers were fixed and passed a key test this week.
Atlantis, which launched on June 8, also brought a new crew member for the ISS, American astronaut Clayton Anderson, who joined two Russian cosmonauts and will stay aboard the orbiting research lab for four months.
Anderson replaced American colleague Sunita Williams, who set the record for the longest uninterrupted space flight by a woman, surpassing the 188-day and four-hour mark set by her compatriot Shannon Lucid in 1996.
NASA plans to launch at least 12 more shuttle missions, including three this year, as it races to finish building the 100-billion-dollar ISS by 2010, when the US space agency retires its three orbiters.
NASA considers the station a vital part of US ambitions to send a manned mission to Mars.