NASA repairing leak on space shuttle fuel tank

CAPE CANAVERAL -- NASA is repairing a leaky hydrogen gas line on Endeavour's fuel tank in hopes of launching the shuttle on its space station construction mission Wednesday, four days after the first try was called off.

But another NASA mission, involving a pair of science spacecraft bound for the moon, is scheduled to blast off Wednesday. Top space agency officials will decide Monday whether to bump the moon mission to make way for Endeavour.

Mission management team chairman LeRoy Cain said it's likely Endeavour will go first — if the repair effort goes well, no other shuttle problems crop up and the weather cooperates.

"A lot of things have to go our way," Cain said Sunday.

Hydrogen gas began leaking from a vent line hookup on Endeavour's external tank during fueling early Saturday, and the countdown was halted just hours before the scheduled morning liftoff.

The same kind of leak postponed a shuttle launch in March. Technicians replaced the vent line hookup and a pair of Teflon seals back then, and the leak did not recur. NASA is hoping for similar results this time. The hookup and seal replacements on Endeavour's tank were expected to be completed Monday morning.

Engineers never determined why the vent line leaked in March. Finding the cause has taken on new urgency.

NASA finds itself in the difficult and unusual position of having to choose between two space missions that both have a relatively short time to launch.

Endeavour must fly by this weekend, otherwise the mission to deliver the final piece of the Japanese space station lab must wait until mid-July because of unfavorable sun angles that would heat up the shuttle. The moon mission — NASA's first in a decade — must be launched by Saturday as well, otherwise it will have to wait until the end of the month for another shot.

One of the lunar spacecraft, an orbiter, is designed to map the moon so NASA can determine where best to put an outpost for astronauts in years to come. The other craft will smash into a shadowed crater at one of the moon's poles to check for signs of frozen water.