CAPE CANAVERAL, Fla. - NASA technicians are hoping to get a better idea Thursday about whether they'll be able to fix a sensor problem at the pad or have to return the space shuttle Discovery to the hangar for more extensive repairs.
The problem with a fuel sensor forced managers to scrub Wednesday's planned launch, just 2 1/2 hours left in the countdown and further delaying NASA's return to space travel after the 2003 Columbia disaster.
Thousands of people had descended on Florida's Kennedy Space Center for the launch, including families of the seven astronauts killed when Columbia disintegrated.
Similar fuel-gauge problems had cropped up during tests in April but technicians and officials thought they had been fixed.
Shuttle managers had no idea whether Wednesday's trouble was in the gauge at the bottom of Discovery's fuel tank, a stretch of cabling and wiring, an electronics box inside the shuttle or something else entirely.
And they found themselves on the defensive, explaining why they pressed ahead with the launch when the same type of potentially fatal problem cropped up during a fueling test just three months ago and was accepted as an "unexplained anomaly."
Some engineers had pushed for further testing at the pad before committing to a liftoff, but were overruled by top managers who concluded that the replacement of cables, the electronics box and the tank itself was ample.
"We felt like we had a good system," deputy shuttle program manager Wayne Hale said at a grim-faced news conference Wednesday.
"We became comfortable as a group, as a management team, that this was an acceptable posture to go fly in," he said, "and we also knew that if something were to happen during a launch countdown, we would do this test and we would find it. And guess what? We did the test, we found something and we stopped. We took no risk. We're not flying with this."
Even if NASA had conducted another fueling test in June, Hale said it's unclear whether the fuel gauge would have malfunctioned the way it did in a checkout test: Instead of showing an empty tank, the gauge kept showing full.
The disappointment came just a day after an embarrassing turn for NASA, when a plastic cockpit window cover fell off the shuttle and damaged its fragile thermal tiles before the spacecraft had even taken off.
The launch was delayed until at least Saturday, and the postponement could last much longer, depending on the repairs needed.
NASA halted the countdown shortly after the seven astronauts climbed aboard for their journey to the international space station. Until then, the only threat to the mission was thunderstorms, which rained on the astronauts as they made their way to the launch pad.
From Cape Canaveral, where congressmen and astronaut families had come to witness the awe-inspiring sight of a rocket launch, to museums across the country where schoolchildren had gathered, the delay of the long-awaited return to space was disheartening.
"I wanted to see it really, really, really bad," groaned 8-year-old Michael Schamtin of Sherwood, Ore., who had waited for liftoff at the Oregon Museum of Science and Industry.
Shuttle managers said it was unclear whether Discovery could be fixed at the pad or would need to be returned to the hangar for more extensive repairs. They expected to have a better idea on Thursday - "but I wouldn't guarantee it," said Steve Poulos, a shuttle manager.
NASA has until the end of July to launch Discovery; otherwise it must wait until September. The launch windows are dictated by both the position of the space station and NASA's desire to hold a daylight liftoff in order to photograph the spacecraft during its climb to orbit.
When the shuttle finally takes off, the astronauts will test new techniques for inspecting and repairing cracks and holes similar to the damage that doomed Columbia in 2003.
Thousands of people had descended on the space center for the launch, including John Glenn, the first American to orbit the Earth, members of Congress, and family members of the seven fallen Columbia astronauts. Lawmakers and others refrained from second-guessing NASA's decision to press ahead before it had gotten to the bottom of the fuel gauge problem.
"I'm disappointed for all of us," said Sen. Bill Nelson, D-Fla. But he added, "The system is working like it should."
Just a day earlier, the window cover caused damage to some of Discovery's thermal tiles - the very thing that NASA had worked so hard to avoid after Columbia's wing was pierced at liftoff by a chunk of foam insulation from the fuel tank. Discovery's tiles were quickly replaced.
In the 2 1/2 years since Columbia broke apart on its return to Earth, NASA has worked to fix its "safety culture," which the accident investigators concluded broke down during the flight. The space agency said it has had frank and vigorous discussions about the upcoming flight - including the fuel gauge problem - and encouraged engineers to speak up.
NASA also has concentrated on making the external fuel tank safer by reducing the risk that foam insulation, ice or other debris will break off at launch. The gauge that caused trouble on Wednesday is in the external fuel tank, but was unrelated to any of the safety modifications.
The space agency requires all four hydrogen-fuel gauges to be working to ensure that the main engines shut off at the precise moment in space. If the engines shut down too soon or too late because of an erroneous gauge reading, the results could be catastrophic. For instance, the engines could rupture if they kept running after the tank sprang a leak and ran out of fuel.
The space agency is looking closely at the possibility that flawed transistors in an electronic "black box" aboard Discovery might be to blame. The box used in the April test also had bad transistors, and when it was removed from the shuttle, the problem disappeared. Managers suspect a manufacturing defect with these transistors.
Shuttle program manager Bill Parsons stressed that the problem could be anywhere.
"This has to be looked at from end to end," he said. "We kind of need to keep our mind open."