NASA has ordered astronauts aboard space shuttle Discovery to switch to a backup runway in California for their planned landing Friday because thunderstorms blocked its return home to the Kennedy Space Center in Florida.

Astronauts were asked to land at Edwards Air Force Base, which is NASA's backup landing site, according to the Associated Press.

Discovery had two chances to set down today on its primary runway at the Kennedy Space Center (KSC) in Florida, but thunderstorms near the shuttle's landing strip kept the spacecraft at bay. The astronauts now hope to land at 8:53 p.m. EDT (0053 Sept. 12 GMT) at the Edwards Air Force Base in California.

We do appreciate everybody that worked the weather so hard in Florida and we appreciate the families making the trip down, shuttle commander Rick Sturckow radioed Mission Control after hearing the news. But it doesn't look like it's going to work out for today so we'll set up for Edwards.

The space agency prefers Florida landings because the cross-country ferry trip costs $1.7 million and take more than a week.

Discovery's launched late Aug. 28. Among the major delivery items were an air-scrubbing device, a new astronaut bedroom, a pair of powerful science experiment racks and a treadmill named after TV comedian Stephen Colbert.

The shuttle returns with space station flight engineer Tim Kopra, who has been in orbit for two months. He was replaced by rookie astronaut Nicole Stott, who was the last station crew member to catch a ride on the shuttle.