By Martin Merzer,The Miami Herald

CAPE CANAVERAL, Fla. – The seven astronauts pulled on bright orange flight suits, strapped into the cockpit, confirmed they heard launch control “loud and clear.” But in the end, the nation’s human space program remained tethered to Earth.

A malfunctioning fuel-tank sensor compelled NASA to scrub Wednesday’s long-awaited and widely watched attempt to launch Discovery on the first shuttle flight since the Columbia accident 2½ years ago.

The problem: The gas gauge showed empty. The gas tank was full.

Another launch attempt will not be made before Saturday, though the delay could be much longer. If they try Saturday, the 10-minute launch window opens at 2:40 p.m.

The crew, led by retired Air Force Col. Eileen Collins, will remain at the Kennedy Space Center until the situation becomes more clear, possibly later Thursday.

Many thousands of spectators, however, attracted by the prospect of the shuttle’s dramatic return to space, went home disappointed.

“All I can say is “shucks,”‘ said Wayne Hale, NASA’s deputy shuttle manager.

He described the problem – which surfaced earlier this year on a different fuel tank – as bafflingly intermittent, an “unexplained anomaly.”

“It reminds me of an old truck I owned with intermittent electrical problems,” Hale said. “When it comes and goes and you can’t put your finger on it, that’s the toughest issue.”

The sensor is one of four on the shuttle’s huge external fuel tank, which was largely redesigned after the Columbia accident. A different problem with the tank – an errant slab of insulating foam that fell off during launch – doomed Columbia and its seven astronauts.

NASA Administrator Mike Griffin said the swift decision to scrub, which came about 2½ hours before liftoff time and as some astronauts were still strapping into the spaceship, demonstrated NASA’s enhanced devotion to safety.

He also noted that mechanical glitches are common on a craft assembled with more than one million parts – 1,500 defined as “mission critical.” He recalled that one mission in the 1980s was scrubbed 14 times before it blasted off. “This is nothing,” Griffin said.

Every scrubbed launch costs taxpayers more than $600,000.

If Discovery cannot be launched by July 31 on its mission to test new shuttle safety equipment and deliver supplies to the International Space Station, orbital dynamics dictate that it must wait for the next window, which opens Sept. 9 and closes Sept. 24.

NASA engineers said the problem involved a “low-level fuel cutoff sensor,” one of four that are on the huge external fuel tank. They are critically important parts that make sure the shuttle’s engines shut off before they run out of fuel.

Two sensors monitor the 385,000 gallons of liquid hydrogen in one compartment of the tank; two others monitor the 143,000 gallons of liquid oxygen in another compartment. The problem was on the hydrogen side, and it was not a complete surprise.

A similar sensor failed in mid-April during a propellant test on a different tank that was attached to Discovery.

The tank was replaced and some cables and electronics boxes were changed on the orbiter, but to save time, the new tank was not tested before Wednesday’s launch attempt. Hale and Griffin defended that decision Wednesday. “Since we don’t know what the problem is, we don’t know if a test would have found it,” Hale said.

It was the first launch attempt since Columbia disintegrated as it carried seven astronauts to the space center after a February 2003 mission. The crew died.

The accident was caused by a piece of foam insulation that fell off the fuel tank during launch, punching a hole in the left wing. The hole allowed superheated atmospheric gases to enter and melt the wing during re-entry.

The fuel tank was redesigned.

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