This story was updated at 2:52 p.m. EDT.
It's landing day for space shuttle Discovery, but thunderstorms in Florida may keep the spacecraft and its astronaut crew in orbit an extra day.
Discovery and its crew of seven astronauts are due to land at NASA's Kennedy Space Center (KSC) in Florida Thursday evening at 7:05 p.m. EDT (2305 GMT) to wrap up a 13-day cargo run to the International Space Station.
Thunderstorms may creep too close to the shuttle's runway to allow a landing on one of two opportunities today, and the Florida weather is expected to get worse Friday, Mission Control said.
"The weather in Florida this time of year is always iffy," Discovery's commander Rick Sturckow said from space Wednesday. "If things aren't good, the worst that can come out of it is we'll have another day in space, which is a great deal."
Earlier today, Discovery had to fire its engines to dodge what Mission Control called “mystery orbital debris,” a piece of space junk that was expected to fly too close to the shuttle for comfort. The debris separated from either Discovery or the space station on Saturday during a spacewalk, though NASA does not know what the object is or its size.
Sturckow fired Discovery’s twin orbital maneuvering system engines to move the shuttle clear of the debris and it did not hamper the shuttle’s landing preparations.
Discovery blasted off late Aug. 28 and has enough supplies to remain in space until Sunday. But entry flight director Richard Jones said he wants to bring the shuttle home by Saturday at the latest. A second landing window opens today at 8:42 p.m. EDT (0042 Sept. 11 GMT), but high winds and thunderstorms are also expected then.
If the weather in Florida shows no sign of letting up Saturday, Jones may opt to land the shuttle Friday at a backup runway at California's Edwards Air Force Base. But a Florida landing is preferable since it will return Discovery to its home port.
NASA's landing rules require no thunderstorms within about 30 miles (48 km) of the runway to avoid flying shuttles through rain, which can damage their fragile heat shields. Discovery is carrying a special heat shield tile with an intentional "speed bump" as part of an unrelated experiment studying re-entry heating, mission managers said.
Buzz Lightyear is hitching a ride back to Earth aboard Discovery alongside its astronaut crew. The Disney toy launched to the space station in May 2008 as part of an educational project. A tickertape parade at Walt Disney World awaits his landing.
"He's very secure," said astronaut Tim Kopra, who is also returning home from the station after a nearly two-month stay. "He's in his spacesuit and I'm quite confident he'll have a safe ride home."
Buzz spent just over 15 months at the station, dwarfing Kopra's time in space. Kopra had hoped to spend another month in orbit, but his launch on a different shuttle was delayed several times.
Discovery delivered 18,548 pounds (8,413 kg) of food, science equipment and other vital supplies to the space station. The shuttle crew performed three spacewalks and delivered an air-scrubbing device, the equivalent of an astronaut bedroom and a new treadmill named after TV comedian Stephen Colbert.
Colbert won an online poll to have a new space station room named after him earlier this year, but NASA gave him the treadmill - known as the Combined Operational Load Bearing External Resistance Treadmill - instead. The agency named the new room Tranquility after the Apollo 11 moon base.
The treadmill will be assembled by astronaut Nicole Stott, who arrived at the station aboard Discovery and replaced Kopra as part of its six-person crew. But COLBERT, which is in more than 100 pieces, will have to wait until after a new Japanese cargo ship arrives at the station.
That cargo ship, Japan's inaugural H-2 Transfer Vehicle, successfully launched today at 1:01 p.m. EDT (1701 GMT) from the Tanegashima Space Center in southern Japan. The unmanned spacecraft is packed with about 3 1/2 tons of cargo for both inside and outside the station and is due to arrive Sept. 17, when Stott plans to pluck it from space with the outpost's robotic arm.
While the station crew awaits the new cargo ship, Discovery's crew is looking forward to life back on Earth.
"Space has been great!" shuttle astronaut Jose Hernandez - a first-time spaceflyer - wrote Wednesday via Twitter, where he's been posting updates from orbit. "Words cannot describe this experience! The take home is our planet is very beautiful! Let's take care of it."
- Image Gallery - Shuttle Discovery's Midnight Launch
- Video - Stephen Colbert to NASA: 'No Chubby Astronauts'
- SPACE.com Video Show - The ISS: Foothold on Forever
SPACE.com is providing complete coverage of Discovery's STS-128 mission to the International Space Station with Managing Editor Tariq Malik and Staff Writer Clara Moskowitz in New York. Click here for shuttle mission updates and a link to NASA TV.