Shuttle program managers, meanwhile, say they believe the problem can be repaired in time to proceed with a launch attempt tentatively scheduled on July 11.

"They have good confidence that they know what the problem is and how they can fix it," Kennedy Space Center spokeswoman Candrea Thomas said today.

But Endeavour and its seven astronauts would have to launch no later than July 14 to get to the International Space Station and complete a complex assembly mission before the arrival of a robotic Russian space freighter carrying critical supplies.

A delay past July 14 would push the launch to no earlier than July 27 and have a significant ripple effect on an International Space Station assembly mission set for launch on Discovery on Aug. 18.

"Obviously there would be a downstream impact," said KSC spokesman Allard Beutel.

Significant leaks of gaseous hydrogen from a line that vents the substance from the shuttle's external tank prompted NASA to scrub a June 13 launch attempt as well as a second try last Wednesday.

Some cryogenic liquid hydrogen, which is Minus 423 degrees Fahrenheit, always "boils off" -- or changes to a gaseous state -- during propellant-loading operations. The vent line maintains proper pressure levels within the tank by carrying excess gaseous hydrogen to a flare stack about 1,000 feet away from the shuttle. There, the highly flammable substance is safely burned off.

Thomas said engineers believe a slight misalignment between the line and a carrier plate that connects it to the external tank is causing an internal Teflon seal to pull out of place, creating a leak path for the gaseous hydrogen.

Engineers think technicians can fix the misalignment and install an alternative seal that would provide a tighter fit and stop the leak.