About 40 minutes into this morning's fuel-loading test, liquid hydrogen is being "fast filled" into space shuttle Endeavour's external tank at Kennedy Space Center's launch pad 39A.
More than 390,000 gallons of the fuel will be loaded into a tank that sits at the bottom of the 15-story tank. Another 145,000 gallons of liquid oxygen will fill an upper tank.
The propellants are loaded at a rate of 1,035 gallons per second - fast enough to to fill a medium-sized backyard swimming pool every 18 seconds, according to tank manufacturer Lockheed Martin Corp
NASA expects to know after 9 a.m. whether it has succeeded in loading the liquid hydrogen without springing a leak of hydrogen gas.
That problem scrubbed two launch attempts in June, and must be fixed if Endeavour is to lift off on its new targeted date of July 11.
Some of the extremely cold liquid hydrogen boils off during the fueling process, and a vent line routes excess vapor out of the tank to make sure proper pressure is maintained.
Too much gas leaked from the vent line, which engineers determined was connected to a plate on the fuel tank that was slightly misaligned.
NASA has replaced a seal and taken steps to eliminate movement in the line to try to correct for the misalignment.
The leaks have cropped up when the tank was nearly full, a stage that is more than an hour away.
So far, no problems are reported with the fuel-loading test that started at at 7 a.m.
Space news and analysis from Kennedy Space Center and Cape Canaveral 
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