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.
Liftoff is scheduled for 7:39 p.m. EDT on Saturday. Two previous launch attempts in June were canceled due to potentially dangerous hydrogen fuel leaks.
"We're all eager to get Endeavour and her crew on their way to the International Space Station," said Charlie Blackwell-Thompson, a shuttle launch manager. "We're ready to fly this mission."
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.
The space shuttle Endeavour, seen here in June 2009, at Kennedy Space Center in Florida. Theshuttle this week will make a third try at launching for a rendezvous with the International Space Station, after potentially hazardous hydrogen gas leaks twice delayed the mission, space
officials said.
Space shuttle Endeavour commander Mark Polansky makes a few comments after the he and the Endeavour crew arrived at the Kennedy Space Center in Cape Canaveral, Fla., Tuesday, July 7, 2009. Endeavour and its crew, scheduled for a July 11 launch, will deliver and install the final elements of Japan Exploration Agency's Kibo laboratory to the International Space Station.
Canadian Space Agency astronaut Julie Payette, a mission specialist on space shuttle Endeavour, arrives with the rest of the crew at the Kennedy Space Center in Cape Canaveral, Fla., Tuesday, July 7, 2009. Endeavour and its crew, scheduled for a July 11 launch, will deliver and install the final elements of Japan Exploration Agency's Kibo laboratory to the International Space Station.
The crew of space shuttle Endeavour, from left, flight engineer Timothy Kopra, mission specialist's Thomas Marshburn and Christopher Cassidy, commander Mark Polansky, mission specialist David Wolf, Canadian Space Agency astronaut Julie Payette and pilot Douglas Hurley arrive at the Kennedy Space Center in Cape Cananveral, Fla., Tuesday, July 7, 2009. Endeavour and it's crew, scheduled for a July 11, launch, will deliver and install the final elements of Japan Exploration Agency's Kibo laboratory to the International Space Station.
In this image provided by NASA the afternoon sun creates shadows on space shuttle Endeavour's external fuel tank as workers remove the seal from the Ground Umbilical Carrier Plate on the tank Wednesday June 24, 2009. A hydrogen leak at the location during tanking for the STS-127 mission caused the launch attempts to be scrubbed on June 13 and June 17. NASA plans a fueling test Wednesday July 1, 2009 of shuttle ahead of July 11 launch attempt.
The space shuttle Endeavour sits on launch Pad 39A following a scrubbed launch attempt at theKennedy Space Center in Cape Canaveral, Florida June 17, 2009. NASA canceled the launch of space shuttle Endeavour on Wednesday for the second time after a potentially dangerous hydrogen gas leak surfaced while the ship was being fueled for flight. The next opportunity to launch Endeavour will be on July 11.
A NASA security officer patrols the waters near the space shuttle Endeavour as it sits on launch pad 39A at the Kennedy Space Center in Cape Canaveral, Florida June 16, 2009.
The International Space Station as seen from the Space Shuttle Discovery. The European SpaceAgency said it was in talks to extend the life of the International Space Station and get seats
for its astronauts on future flights to the orbital outpost.