(Corrects time of Friday landing opportunity, 4th paragraph) (Adds background, details, quote)

By Irene Klotz

CAPE CANAVERAL, Fla., June 21 (Reuters) - Bad weather forced NASA on Thursday to delay bringing the space shuttle Atlantis back to Earth at least until Friday, the U.S. space agency said.

Touchdown at the Kennedy Space Center in central Florida had been targeted for two opportunities on Thursday afternoon, but thick cloud cover and rain showers prompted NASA managers to call off both possible landing attempts.

Flight directors decided not to staff the shuttle's backup landing site at Edwards Air Force Base in California on Thursday, but both sites will be available for Friday's landing opportunities.

Friday's first opportunity will be at Kennedy Space Center at 2:18 p.m. EDT (1818 GMT), NASA managers said.

Weather could still hamper the shuttle's return home after its nearly two-week-long mission at the International Space Station, however.

"We're going to be fighting the same challenges at KSC (Kennedy Space Center), and at Edwards the winds are going to pick up," astronaut Tony Antonelli at mission control in Houston told his colleagues aboard the shuttle.

The shuttle has enough fuel and supplies to stay in space until Sunday, but it cannot land in rain because it could damage the thousands of ceramic tiles that protect the spaceships's belly from the fiery heat of re-entry.

During their visit to the space station, Atlantis crew members installed a 17-tonne metal truss that included solar power panels to generate additional electricity for the half-finished $100 billion complex.

HEAT SHIELD INSPECTIONS

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.

NASA engineers believe they underestimated how much heat the underlying shuttle layers had experienced during launch on June 8.

"When they modeled it, they made a mistake," deputy shuttle program manager John Shannon said of the original analysis. "Still, the engineering and safety teams believe there's absolutely no risk at all during re-entry."

The Atlantis mission, which was delayed from March after the shuttle's external fuel tank was damaged in a hail storm in late February, was the first of four shuttle flights scheduled this year.

The addition of the solar panels and other tasks performed by the shuttle crew prepared the space station, a project of 16 nations, for the installation of new European and Japanese modules.

The work was overshadowed last week when computers that keep the space station properly positioned crashed and required an improvised rewiring to revive them. The computers were still being subjected to a battery of tests on Thursday.

AlertNet news is provided by