Space Shuttle Discovery (search), whose launch was scrapped Wednesday due to technical problems with fuel-tank sensors, won't be able to blast off until Saturday at the earliest, NASA officials said.

"All I can say is shucks," deputy shuttle program manager Wayne Hale (search) said at a news conference of grim-faced NASA managers.

The decision to postpone the first shuttle launch since the Columbia disaster came just 2 1/2 hours before the scheduled liftoff, while the seven astronauts were boarding the spacecraft. Up until then, it seemed as though the only obstacle to the flight would be a thunderstorm over the launch site.

The problem involved one of the external fuel tank's four hydrogen fuel sensors, which are responsible for making sure the spacecraft's main engines shut down during the ascent when the tank runs out of fuel. A launch could end in tragedy if the engines cut out too early or too late.

A similar sensor problem cropped up intermittently during a launch pad test back in April, and NASA has been baffled as to the source of the trouble ever since.

Although Saturday was mentioned as a possible launch day, the delay could last much longer, depending on what repairs are needed.

Across the country, fans of space exploration were disheartened by the postponement.

"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.

Even though NASA could not explain the failure, officials thought the problem was resolved and pressed ahead with launch.

Hale defended that decision.

"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."

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.

The shuttle has until the end of July to launch, after which NASA will have to wait until September. The launch window is dictated by both the position of the space station and NASA's desire to hold a daylight liftoff in order to photograph the shuttle during its climb to orbit.

Discovery and its crew of seven were originally set to blast off at 3:51 p.m. EDT. It would have been the first space shuttle flight in 2 1/2 years, after Space Shuttle Columbia (search) broke apart when it was returning from its mission in February 2003.

On Tuesday, a temporary window cover fell off the shuttle and damaged thermal tiles near the tail. The problem was announced just two hours after NASA (search) declared Discovery ready for liftoff.

The mishap was an eerie reminder of the very thing that doomed Columbia — damage to the spaceship's fragile thermal shield.

The husband of Discovery commander Eileen Collins (search) said Wednesday that his wife expressed having "some butterflies" when they talked the night before.

"Anytime you're an astronaut, you run a risk," Pat Youngs told The Associated Press. "But from all I've seen, from management, engineers ... support personnel and the astronauts themselves, there is a lot of hard work to right things and get back to spaceflight." It will be Collins' fourth shuttle flight.

Former NASA administrator Sean O'Keefe, who was in charge of the space agency during the Columbia disaster, called Discovery's launch a "seminal moment."

"It marks a major chapter in return to flight of the entire exploration story," said O'Keefe, who resigned from NASA last December to become chancellor of Louisiana State University. He was working as a television commentator for MSNBC during the launch.

The plastic cover on one of Discovery's cockpit windows came loose Tuesday while the spaceship was on the launch pad, falling more than 60 feet and striking a bulge in the fuselage, said Stephanie Stilson, the NASA manager in charge of Discovery's launch preparations.

No one knows why the cover — held in place with tape and weighing less than 2 pounds — fell off, she said. The covers are used to protect the windows while the shuttle is on the launch pad, then removed before liftoff.

Discovery will be setting off on the 114th space shuttle flight in 24 years with a redesigned external fuel tank and nearly 50 other improvements made in the wake of the Columbia tragedy.

A chunk of foam insulation the size of a carry-on suitcase fell off Columbia's fuel tank at liftoff and slammed into a reinforced carbon panel on the shuttle's wing, creating a hole that brought the spacecraft crashing down in pieces during its return to Earth on Feb. 1, 2003.

During their 12-day flight, Discovery's astronauts plan to test various techniques for patching cracks and holes in the thermal shielding.

The crew members also will try out a new 50-foot boom designed to give them a three-dimensional laser view of the wings and nose cap and help them find any damage caused by liftoff debris. That is on top of all the pictures of the spacecraft that will be taken by more than 100 cameras positioned around the launching site and aboard two planes and the shuttle itself.

FOX News' Catherine Donaldson-Evans and The Associated Press contributed to this report.