Bad weather could delay the return to Earth of space shuttle Atlantis and its crew of seven after a successful mission to the International Space Station, a NASA spokesman said Wednesday.

"No landing is easy, but I am always optimistic, I know the weather is a no go right now, but clearly weather changes," Norm Knight, NASA's official in charge of shuttle landings, told reporters.

NASA's greatest concern is visibility for landing the shuttle, all 113,398 kilograms (250,000 pounds) of it, without power and no second chance to approach the runway.

Kennedy Space Center spokesman George Diller told reporters Thursday's prediction is for storms, rain or cloud cover at 900 meters (3,000 feet) -- too low for the landing.

"The primary flight rule concerns for (Kennedy Space Center) Thursday will be the possibility of thunderstorms within a 30 nautical mile radius ... and a low cloud ceiling."

He said 1,500 meters (5,000 feet) is the minimum altitude cloud cover affording the shuttle commander visibility to land the Atlantis without power, which pilots call a "dead stick" landing.

The silver lining to the gloomy forecast, the spokesman said, is that the weather in Florida at this time of the year can change quickly.

"As we get closer, we'll get a better handle on the forecast," Knight added.

The shuttle plummets to Earth 20 times faster than a commercial air liner and hits the ground at 344 to 364 kilometers (214 to 226 miles) per hour, necessitating a drag chute to stop.

While the Atlantis and its heat shield have yet to be cleared for the scorching, 1,500-degree Celsius re-entry in the atmosphere, the Atlantis team is not worried.

"The engineering and safety teams believe it's absolutely no risk at all to reentry," said mission commander John Shannon.

The Atlantis on Thursday will have two windows of opportunity to re-enter the atmosphere ahead of a landing: at 1650 GMT, for a 1755 GMT touchdown; and at 1825 GMT, for a 1930 GMT landing.

If a Thursday landing is canceled, Friday offers four attempts at the Kennedy Space Center, at 1816 GMT and 1951 GMT, or at Edwards Air Force Base in California, at 2121 GMT and 2256 GMT, NASA said.

Diller said the weather forecast for Friday at the cape calls for more favorable conditions for a shuttle landing.

NASA said the Atlantis needs to be back on Earth by Saturday before its hydrogen-powered batteries are spent.

An additional alternative landing site for the Atlantis is NASA's testing facility at White Sands, New Mexico.

The Atlantis and its crew of seven launched from Cape Canaveral on June 8 and have completed a successful mission to the International Space Station that included four spacewalks. They undocked from the ISS at 1442, Tuesday.

The astronauts installed a new truss segment on the ISS with a double-winged solar array, repaired a tear in the shuttle's heat shield and overcame a 48-hour computer breakdown aboard the station.

US astronaut Sunita Williams returns on the Atlantis after staying on the ISS since December 10. She set a record Saturday for the longest uninterrupted space flight by a woman -- passing the previous record of 188 days and four hours.

The Atlantis delivered a new member for ISS Expedition 15. US flight engineer Clayton Anderson will remain four months aboard the station along with two Russian cosmonauts.

NASA plans at least another 12 shuttle flights to finish building the ISS by 2010, when the three-shuttle fleet will be retired for good.