Perched atop a Boeing 747 jumbo jet, the space shuttle Discovery began its homeward trek toward NASA's Kennedy Space Center in Florida Friday, a two-day trip from California.

The 100-ton spaceship and its carrier craft began the first leg of their flight, a three-hour trip from NASA's Dryden Flight Research Center and Edwards Air Force Base to Altus Air Force Base in Oklahoma, at 11:31 a.m. EDT (1531 GMT).

"It was perfect," NASA Dryden spokesperson Leslie Williams said of the departure.

Discovery landed at Edwards on Aug. 9 after carrying its STS-114 astronaut crew on a 14-day spaceflight to the International Space Station (ISS).

The mission marked NASA's first shuttle flight since the loss of seven STS-107 astronauts aboard the Columbia orbiter, which broke apart while reentering the Earth's atmosphere on Feb. 1, 2003. A piece of foam insulation fell from Columbia's external tank and pierced its heat shield during its launch, which led to the disaster, investigators found.

Discovery's STS-114 crew, commanded by veteran Eileen Collins, tested a series of inspection and orbiter repair methods during their flight. Unexpected foam loss seen during Discovery's July 26 launch prompted shuttle officials to hold back on future orbiter flights until engineers can solve the problem, though NASA has set March 2006 as a tentative launch target.

On Thursday, shuttle officials tapped Discovery for NASA's second return to flight test mission, STS-121 commanded by astronaut Steven Lindsey, to fly no earlier than March 4, 2006. The flight was slated to use the Atlantis orbiter.

"It's not uncommon to change vehicles and, related to the flight, it's totally transparent to the [astronaut] crew," NASA spokesperson James Hartsfield said from Johnson Space Center in Houston, Texas.

After refueling at Altus, Discovery's carrier 747 aircraft will ferry the orbiter to Barksdale Air Force Base in central western Louisiana, where the piggybacked spacecraft will stay for the night before heading toward KSC's Shuttle Landing Facility in Cape Canaveral, Florida Saturday.

While ferry flights from Edwards to KSC typically cost about $1 million and take up to a week, Discovery's move was delayed by severe storms and difficulties connecting a protective cone over Discovery's engine bells delayed the process, NASA officials said.