Bill Ingalls  /  NASA
Storm clouds approach Launch Pad 39A at NASA's Kennedy Space Center in Florida on Friday as the space shuttle Endeavour stands within its service structure, ready for launch on Saturday if the weather cooperates.
By Senior editor
updated 7/10/2009 2:32:22 PM ET 2009-07-10T18:32:22

NASA has officially cleared the space shuttle Endeavour for its third launch attempt on Saturday, with the chance of thunderstorms posing the only threat to the mission after nearly a month of delays.

Endeavour is poised to blast off from NASA's Kennedy Space Center in Florida at 7:39 p.m. ET to begin an ambitious 16-day flight to the international space station. But the potential for thunderstorms, rain and clouds near the Florida spaceport give the shuttle just a 40 percent chance of good flight weather.

"I don't worry about things I can't control, and I can't control the weather," NASA's Mike Moses, who leads Endeavour's Mission Management Team, said during a Friday briefing.

Moses added that some people joke that, because his last name is Moses, he should have some say in the weather. "But I really can't," he said with a smile.

Launch weather conditions will improve slightly on Sunday and Monday, but NASA hopes to fly Saturday evening to avoid a space traffic conflict with an unmanned Russian cargo ship also due at the station this month.

Commanded by veteran spaceflier Mark Polansky, Endeavour's six-man, one-woman crew plans to deliver an external experiment-carrying porch for the space station's massive Japanese lab Kibo. It is the third and last piece of the $1 billion Japanese facility. Five spacewalks are planned to install the space porch and perform station maintenance.

The seven Endeavour astronauts will boost the station's current six-man crew up to 13 people — its highest population ever — when the shuttle arrives.

One Endeavour crew member, NASA astronaut Tim Kopra, will replace Japanese spaceflier Koichi Wakata as part of the station's Expedition 20 crew. Wakata has lived aboard the station since March and will return to Earth aboard the shuttle.

Fix made to fuel system
Endeavour's mission has been delayed since mid-June due to a vexing hydrogen gas leak that stalled two launch attempts. Engineers tracked the problem to a misaligned plate on the shuttle's 15-story external tank. They replaced the plate and a hydrogen vent line seal, and successfully tested the fix last week.

Moses said he is confident the glitch will not appear during Saturday's launch attempt. Endeavour's flight will mark NASA's third shuttle mission of up to five planned for this year.

Mission managers have said Endeavour has until Tuesday to try and launch toward the station before NASA would stand down to allow the Russian cargo ship's launch and docking at the orbiting lab. If the shuttle does not launch by then, the agency would consider holding the flight until July 27 to wait out the space cargo run.

While there is some potential room to extend Endeavour's launch window, Moses said it is easier for NASA to shift its space shuttle launch date than for Russia to move a Soyuz rocket launch.

"It's a matter of launch processing," Moses said. "It's much easier to let [the cargo ship] launch and get it out of the way."

More on Endeavour | international space station

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