The decision came with less than 2½ hours to go before launch, as the seven astronauts were almost done boarding the spacecraft. Up until then, rain and thunder over the launch site appeared to be the only obstacle to an on-time liftoff.
A launch control commentator said that it was unlikely the problem could be solved quickly and that another launch attempt on Thursday was all but impossible.
NASA has until the end of July to launch Discovery, after which it will have to wait until September — a schedule dictated by both the position of the international space station and NASA's desire to hold a daylight liftoff in order to photograph the shuttle during its climb to orbit.
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The problem was with one of the four engine cut-off sensors, which are responsible for making sure the spacecraft's main engines shut down at the proper point during the ascent. A launch could end in tragedy if faulty sensors caused the engines to cut out too early or too late.
NASA said it appeared that the sensor was showing a low fuel level, even though the tank was full with 535,000 gallons of super-cold liquid hydrogen and oxygen.
The sensors ``for some reason did not behave today and so we're going to have to scrub this launch attempt,'' launch director Mike Leinbach told his team. ``So appreciate all we've been through together, but this one is not going to result in a launch attempt today.''
It was not the first time sensors of this type malfunctioned. During a fueling test of Discovery's original tank in April, one of its sensors gave intermittent readings. NASA could not ascertain the exact reason for the failure but replaced the entire tank anyway to install a heater to prevent a dangerous ice buildup.
Shuttle managers considered conducting a fueling test at the launch pad on the replacement tank, but ruled it out to save time, saying that the actual fueling on launch day would be the ultimate test.
``We are disappointed, but we'll fly again on another day,'' said an astronaut speaking from launch control, David Wolf