"At 1:55 am (0555 GMT), launch managers called a scrub, canceling today's planned launch of space shuttle Endeavour on its STS-127 mission," the US space agency said in a statement.

The next launch attempt will take place on July 11.

"Despite troubleshooting efforts, engineers were unable to achieve a decrease in the liquid hydrogen leak," officials lamented, noting the leak was in the same spot that halted the previous launch attempt on Saturday.

The leak's cause remains unknown -- it was also the source for forcing NASA to abandon its first attempt to launch the shuttle Discovery in March.

The puzzling problem with the venting system, supposed to carry excess hydrogen safely away from the launch pad, is set to consume NASA scientists up until the next launch.

"We're going to step back and figure out what the problem is and go fix it," vowed deputy space shuttle program manager LeRoy Cain during a briefing after the cancellation.

"Once we get it fixed, and we're confident that we have a solution that's going to work and allow us to go fly safely, then we'll proceed forward," he said.

The three-hour operation to pump 500,000 gallons (two million liters) to the shuttle tanks was begun late Tuesday for a launch to the International Space Station at 5:40 am (0940 GMT).

The launch's attempted move to Wednesday also caused a scheduling conflict with NASA's moon-bound crater observation and sensing satellite (LRO/LCROSS), with the lunar mission being pushed back one day to June 18.

LRO is scheduled for a one-year exploration mission at an orbit of about 31 miles, or 50 kilometers -- the closest any spacecraft has orbited the moon in preparation for future lunar travel.

The partner mission LCROSS, meanwhile, will hurl itself into a crater on the dark side of the moon in search of long-frozen remnants of water ice, something that may prove crucial to future manned missions to the natural satellite.

When Endeavour does head out from NASA's Kennedy Space Center in Florida, it will be the 32nd mission to the ISS, orbiting 350 kilometers (220 miles) above Earth, and the last of three missions to assemble the Japanese Kibo laboratory aboard the orbiting space station.

During the shuttle's stay the ISS is set to be a temporary home to 13 astronauts -- the first time so many people have stayed on the orbiting station at once.

The six US astronauts and a Canadian woman astronaut that Endeavour is expected to bring to the ISS will join another US astronaut and one more from Canada, as well as two Russians, a Belgian and Japan's Koichi Wakata who are currently living on the ISS.

Construction began on the ISS a decade ago, and the push is on to complete the building before NASA ends its shuttle missions in September 2010.

Over the five planned spacewalks lasting some 32.5 hours, the astronauts will install a permanent 1.9 tonnes platform to Kibo, which will serve as one of the station's porches for conducting experiments in the vacuum of space.

The lunar probes' four-day, 238,000-mile (384,000-kilometer) journey to the moon set to begin Thursday is expected to illuminate our closest extra-terrestrial neighbor like never before.

The Lunar Reconnaissance Orbiter, or LRO, hopes to push forward the space effort's knowledge base through a one-year stay at an orbit of about 31 miles (50 kilometers).

LRO's 500-million-dollar mission is designed to provide NASA with maps of unprecedented accuracy, which will be crucial for scoping out possible landing sites in the future.

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