NASA Clears Shuttle Endeavour for Saturday Launch

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. EDT (2339 GMT) to begin an ambitious 16-day flight to the International Space Station. But the potential for thunderstorms, rain and clouds near the Cape Canaveral 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,? said NASA?s Mike Moses, who leads Endeavour?s Mission Management Team, 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 tomorrow evening to avoid a space traffic conflict with an unmanned Russian cargo ship also due at the station this month.

Commanded by veteran spaceflyer 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 crewmember, NASA astronaut Tim Kopra, will replace Japanese spaceflyer 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.

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 July 14 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 that 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.?

• New Video - The Kibo Lab: Japan's Hope in Space - Part 1, Part 2
• Image Gallery - Shuttle Endeavour Blasts Off at Night
• Video - Meet the STS-127 Shuttle Astronauts

SPACE.com is providing continuous coverage of STS-127 with reporter Clara Moskowitz at Cape Canaveral and senior editor Tariq Malik in New York. Click here for mission updates and SPACE.com's live NASA TV video feed. Live launch coverage begins Sat. at 2:30 p.m. EDT (1830 GMT).