NASA Scrubs Saturday Morning Space Shuttle Launch

A potentially dangerous hydrogen gas leak cropped up during the fueling of space shuttle Endeavour on Saturday and forced NASA to postpone the launch by at least four days.

The space shuttle Endeavour sits on Launch Pad 39-A Friday afternoon June 12, 2009 at the Kennedy...

The space shuttle Endeavour sits on Launch Pad 39-A Friday afternoon June 12, 2009 at the Kennedy Space Center in Cape Canaveral, Fla. The launch was scrapped due to a potentially dangerous hydrogen gas leak which cropped up during the fueling of Endeavour.

(Terry Renna/AP Photo)

It was almost identical to a leak that stalled another flight back in March and threatened to bump Endeavour's space station construction mission all the way into July.

NASA halted the countdown shortly after midnight, less than seven hours before Endeavour was due to blast off. The seven astronauts had yet to suit up.

Launch director Mike Leinbach said the leak, located at a vent line hookup on the fuel tank, was significant. Hydrogen gas is extremely volatile and can burn in large enough quantities, he noted.

"There's no way we could have continued," Leinbach said at a hastily called news conference. "It's a commodity you just don't mess with."

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The hydrogen gas leak is similar to one that NASA faced while trying to launch Discovery three months ago. That flight was delayed four days because of the problem and shortened as well. Atlantis, however, encountered no such trouble during its countdown in May for the Hubble Space Telescope repair mission.

NASA's launch team immediately began draining Endeavour's external fuel tank while trying to figure out what went wrong. In March, the leak occurred where a vent line hooks up to the tank. The hookup was replaced along with a couple of seals and the seepage stopped, but engineers never did determine the exact source of the trouble.

Officials said workers wouldn't be able to get to the vent line on Endeavour's tank until Sunday.

NASA is up against a tight deadline. A four-day delay would make Endeavour's next launch attempt Wednesday. But that's the day the space agency is supposed to launch a moon-bound spacecraft aboard an unmanned rocket.

Mike Moses, chairman of the mission management team, said it was too soon to say which mission would take priority. "We haven't even begun to work that yet," he said.