NASA has delayed a high-priority moon mission to make way for the space shuttle Endeavour to launch Wednesday.

The shuttle was originally intended to blast off June 13 toward the International Space Station, but was thwarted by a potentially dangerous hydrogen gas leak from a vent line off its external fuel tank.

NASA thinks it can fix the leak in time for a Wednesday lift off at 5:40 a.m. EDT (0940 GMT) from Launch Pad 39A at Kennedy Space Center in Cape Canaveral, Fla.

"The repair work is going well," NASA test director Steve Payne said Monday at a briefing. "We expect to have all the work completed by 3 p.m. [EDT] on Tuesday afternoon. Our teams have been working very hard over the last couple of days to get this piece of equipment fixed."

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This timeline will push back the planned flight of a pair of unmanned lunar probes that were scheduled to launch the same day from the nearby Cape Canaveral Air Force Station.

NASA decided to give Endeavour another chance to fly before its window to align with the space station closes.

Clear skies

The weather for Wednesday looks favorable for the shuttle launch, with an 80 percent chance of "go" conditions, shuttle weather officer Kathy Winters said today. A slight chance of clouds poses the only weather threat to liftoff that day.

"I do have good news for launch," Winters said. "Weather is overall favorable."

The lunar spacecraft — the Lunar Reconnaissance Orbiter (LRO) and the Lunar CRater Observation and Sensing Satellite (LCROSS) — will now aim to fly as early as Thursday, NASA launch director Chuck Dovale said Monday.

If the shuttle must cancel its Wednesday launch attempt at any time before midnight on that day, the moon probes can likely fly Thursday.

If Endeavour succeeds in getting off the ground, LRO/LCROSS may launch either Thursday or Friday.

"Exciting times on the space coast," Dovale said.

Even if Endeavour is unable to lift off for any reason on Wednesday, the shuttle management team has agreed to stand down until July 11 and let the lunar mission proceed, as that launch has already been delayed about eight months for various issues.

Familiar troubles

The leak problem plaguing the space shuttle is not new; a similar issue stalled the launch of the shuttle Discovery's STS-119 flight in March.

That mission ended up blasting off four days later than planned after ground crews swapped out the suspect seal between the vent pipe and the shuttle. NASA hopes a similar fix on Endeavour will do the trick.

Though switching out the faulty parts has fixed the problem in the past, no root cause for the leaks has been discovered.

"The only thing that we've seen is evidence of a little compression, probably caused by the fact that [the seal] was a little misaligned," Payne said. "What's causing the misalignment is probably what we really have to go after."

Challenging mission

Endeavour is slated for a 16-day mission to deliver the final segment of the Japanese Kibo laboratory to the space station, as well as drop off a new long-duration station crewmember, NASA astronaut Tim Kopra, who will replace Koichi Wakata of Japan.

Wakata is due to fly home aboard Endeavour.

Endeavour's commander Mark Polanksy plans to lead his crew of seven through a series of five challenging spacewalks and complicated robotic maneuvers to install the new lab section as well as a horde of spare equipment for the station.

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