Florida: Countdown clocks resumed ticking at the Kennedy Space Center in Florida on Wednesday in preparation for NASA's third attempt to launch space shuttle Endeavour on a construction mission to the International Space Station.

The space shuttle Endeavour sits on launch Pad 39A following a scrubbed launch attempt at the Kennedy Space Center in Cape Canaveral, Florida June 17, 2009. [Agencies]

Liftoff is scheduled for 7:39 pm EDT on Saturday. Two previous launch attempts in June were canceled due to potentially dangerous hydrogen fuel leaks.

Technicians fixed the leak and last week filled the fuel tank with 500,000 gallons (1.9 million liters) of supercold liquid hydrogen and liquid oxygen to test if the repair was successful. The leak did not reappear, prompting managers to clear Endeavour for launch on a 16-day mission to deliver the final piece of Japan's Kibo complex to the space station.

The Endeavour crew plans to conduct five spacewalks during their stay at the station to install a porch onto Kibo for science experiments, as well as to replace batteries in a solar panel wing and perform other maintenance tasks.

The three-day launch countdown began on Wednesday. The only remaining obstacle appeared to be the weather. Meteorologists predicted only a 40 percent chance that conditions would be suitable for a launch attempt on Saturday.

"We're expecting we'll see some afternoon thunderstorms in the area around launch time," said shuttle weather officer Kathy Winter.

NASA has eight flights remaining to complete construction of the $100 billion station, a project of 16 nations. The agency plans to retire the shuttle fleet next year and develop new spaceships that can travel to the moon and other destinations as well as the space station.