Discovery had been scheduled to launch at 3:51 p.m. EDT on Wednesday, which would have marked the first shuttle flight since the Columbia disaster of 2003 and NASA's first concerted attempt to demonstrate that it has fixed the problems that caused the high-profile mishap.
In a telephone conversation, a NASA spokeswoman at the Kennedy Space Center confirmed that no launch would happen Wednesday. "It has been scrubbed," the spokeswoman said.
During an afternoon press conference, NASA said that the cancellation was due to malfunctioning fuel sensors in Discovery's external tank. The sensors protect the shuttle's engines by alerting them to shut down if fuel runs too low.
Wayne Hale, deputy manager of the shuttle program, said the next possible launch date is Saturday at 2:40 p.m. EDT. The so-called launch window extends through July 31--meaning if a launch doesn't take place by then, the next possible date would be sometime in September.
The cancellation came with around three hours left in the countdown, with weather anticipated to be the only remaining hitch.
Other last-minute glitches caused worries but had not postponed the launch. Around 7:10 a.m. EDT Wednesday, a heater failure delayed filling the shuttle's large orange external tank, and on Tuesday evening a window cover fell about six stories and damaged some of the tiles on the orbiter.