This story was updated at 10:00 a.m. EDT.
CAPE CANAVERAL, Fla. -- The space shuttle Endeavour and its seven-astronaut crew will have to wait through at least four days of delay before launching toward the International Space Station after a gas leak thwarted their planned Saturday morning liftoff.
The gaseous hydrogen leak was discovered before midnight while the shuttle's fuel tank was loading in preparation for Endeavour's planned launch today from Pad 39A here at NASA's Kennedy Space Center at 7:17 a.m. EDT (1117 GMT). The shuttle's astronaut crew had not yet donned their NASA-issue pressure suits or boarded the spacecraft for their marathon 16-day station construction flight. NASA officially cancelled today's launch plans at 12:26 a.m. EDT (0426 GMT).
It will take at least four days to ready Endeavour for a second launch attempt. At that point, the schedule conflicts with the planned launch of two unmanned lunar spacecraft due to lift off toward the moon on June 17 from the nearby Cape Canaveral Air Force Station. Managers for the two missions will have to meet to discuss which flight should go first.
"We haven?t even begun to work that out," said Mike Moses, mission management team chair, today at a briefing here. "We'll start those negotiations tomorrow and see where we get."
If Endeavour is unable to launch before June 20, it must stand down until July 11, when the space station is again in the right alignment.
Mystery leak returns
A similar leak thwarted the space shuttle Discovery's STS-119 launch in March, though that issue was eventually fixed and the shuttle launched successfully.
"The situation was almost identical to what we had two flows ago," shuttle launch director Mike Leinbach said. "It was discovered at almost identically the same time. It was eerily the same."
NASA does not launch space shuttles with any known gas leaks at the pad because the extremely flammable gas can cause an explosion during liftoff if it ignites.
"Hydrogen is a very volatile commodity," Leinbach said. "It's a commodity you just don't mess with."
Discovery's STS-119 flight was ultimately able to launch four days later than planned after ground crews swapped out the seal to a vent line that was carrying the gaseous hydrogen away from the shuttle. The switch fixed the issue, though no root cause was ultimately determined for the fault in the seal.
"They never found a smoking gun for it," NASA spokesman Allard Beutel said.
Weather forecasts predicted a pristine 90 percent chance of good flight conditions for today's launch attempt.
Commanded by veteran shuttle flyer Mark Polansky, Endeavour's seven-astronaut crew will launch toward the International Space Station carrying a Japanese-built porch for the outpost's massive Kibo laboratory. The mission will also ferry rookie NASA astronaut Tom Kopra to the station to replace Japanese astronaut Koichi Wakata, who has lived aboard the orbiting lab since late March. Wakata is Japan's first long-duration astronaut and has watched over his country's $1 billion Kibo laboratory at the station.
Set to launch spaceward aboard Endeavour with Polansky and Kopra are STS-127 pilot Doug Hurley and mission specialists Chris Cassidy, Julie Payette, Tom Marshburn and Dave Wolf. Payette represents the Canadian Space Agency, while the rest are NASA astronauts. Five challenging spacewalks and challenging robotic arm work that will require three space cranes, two on the station and one on Endeavour, are planned.
Kopra is beginning a three-month mission to the space station as a flight engineer on the outpost's six-man Expedition 20 crew. He will join two Russians, another American and astronauts from Belgium and Canada on what is the station's first full six-person crew.
Endeavour's mission will mark NASA's third shuttle flight of the year and the second space station construction flight of 2009. A May space shuttle mission aboard Atlantis flew astronauts to the Hubble Space Telescope to perform a successful final overhaul.
SPACE.com is providing continuous coverage of STS-127 with reporter Clara Moskowitz at Cape Canaveral and senior editor Tariq Malik in New York. Click here for mission updates and SPACE.com's live NASA TV video feed.
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