tp-shuttle-ground-cp-6865986

Potentially dangerous leaks of hydrogen gas thwarted back-to-back attempts last month to launch Endeavour, which is now scheduled to take off Saturday evening from the Kennedy Space Center in Florida. ((Canadian Press))

NASA will try to launch the space shuttle Endeavour for the third time on Saturday evening, weather permitting.

Due to expected showers and thunderstorms in the area, there is still only a 40 per cent chance that the weather will be good enough for a launch, NASA officials said Friday at a pre-launch news conference in Florida.

If the weather doesn't co-operate on Saturday evening, NASA said it will try launching the shuttle from the Kennedy Space Center in Florida on Sunday. They will not try to launch on Monday but could attempt a liftoff on Tuesday and Wednesday.

"We know we're going to try tomorrow," said Mike Moses, the space shuttle's program integration manager.

Last month, potentially dangerous leaks of hydrogen gas thwarted back-to-back launch attempts.

The shuttle is slated to carry seven astronauts, including Canadian Julie Payette, to the International Space Station. Liftoff is planned for 7:39 p.m. ET on Saturday from the Kennedy Space Center.

"All systems are in excellent shape," NASA test director Steve Payne said in an earlier release, adding the flight crew, the shuttle and the launch team are ready to go.

During the two launch attempts in mid-June, significant amounts of hydrogen gas escaped from around a plate on the fuel tank. Engineers discovered a slight misalignment of the plate, which attaches to a vent line, and a different, more pliable type of seal and special washers were installed to correct the problem.

On July 1, launch controllers filled Endeavour's external fuel tank to see whether the repairs had plugged the leak. No abnormal leakage was detected during a three-hour test.

The astronauts will deliver and install one last piece of a Japanese lab to the space station. They will spend nearly two weeks working with the six current space station residents, including Canadian Robert Thirsk. It will be the largest number of people together in orbit ever. The entire shuttle flight will last 16 days.