Image: Training flight
NASA
The shuttle Discovery's pilot, Kevin Ford, and commander Rick Sturckow are all smiles as they walk away from NASA's Shuttle Training Aircraft on Wednesday after practicing shuttle landings. The training jet is modified to simulate the aerodynamics of the space shuttle.
msnbc.com news services
updated 8/23/2009 6:06:25 PM ET 2009-08-23T22:06:25

NASA has cleared space shuttle Discovery for liftoff Tuesday, and the weather looks promising as long as thunderstorms stay away during fueling.

After meeting Sunday afternoon, managers decided Discovery is in good shape to fly to the international space station. A power controller issue — one had to be replaced a week ago in the shuttle — did not stand in the way of the launch.

The chairman of the mission management team, Mike Moses, said there is no way of knowing with certainty whether the new power controller in Discovery might break like the old one did. To minimize the risk of a power failure, some equipment will be left on longer than usual — rather than shutting it down and then turning it back on, he said.

Two other power controllers failed in the lab at Johnson Space Center earlier this summer, adding to NASA's concern. Engineers are comfortable with the workaround for Discovery, Moses said.

Forecasters, meanwhile, said there is an 80 percent chance the weather will cooperate for Discovery's 1:36 a.m. liftoff. That's better than previous forecasts.

But there is a 40 percent chance that thunderstorms could interfere with Monday afternoon's fueling. A fueling delay of more than three hours would bump the flight into the next day.

"Obviously, the weather is going to be one of our challenges," Moses told reporters.

Discovery and seven astronauts will deliver about 17,000 pounds of space station supplies and equipment. Three spacewalks are planned during the 13-day flight.

Of the seven remaining shuttle missions before NASA retires its shuttle fleet in 2010 or 2011, Discovery's is currently the last planned night launch, though schedules can change, said NASA test director Steve Payne.

Resupplying space station
Discovery will deliver new science gear, supplies and spare parts for the space station, an orbiting laboratory. Three spacewalks are planned for maintenance work, including replacing a massive ammonia cooling system tank that weighs as much as a small car.

The astronauts are also delivering a new treadmill named after comedian Stephen Colbert, who tried to have a new station module named after him earlier this year, but ended up with the exercise gear instead.

Veteran NASA astronaut Rick Sturckow will command Discovery's flight. He leads a six-man, one-woman crew that includes first-time flier Nicole Stott. Stott is flying to the space station to replace fellow NASA astronaut Tim Kopra as a member of the station's six-person crew.

Kopra arrived last month on the shuttle Endeavour and will return home with Discovery's crew. Stott is slated to return to Earth in November on the next shuttle flight.

NASA has until Aug. 30 to launch Discovery's STS-128 mission before standing down until Oct. 17 to avoid space traffic conflicts at the station with Russian spacecraft and Japan's first unmanned cargo ship. That Japanese spacecraft, the H-2 Transfer Vehicle, is due to launch Sept. 10.

A Russian cargo ship is scheduled to leave the station mid-September, and a Soyuz spacecraft is set to launch toward the station on Sept. 30.

More on COLBERT treadmill | international space station

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