NASA called off Wednesday's planned launch of the space shuttle Discovery after a problem was found in the craft's fuel system. At a news conference later in the afternoon, officials said the shuttle will not be ready to fly again until at least Saturday.

Addressing the cancelled flight, Wayne Hale said, "All we can say is 'shucks.'"

Others said that after draining the external fuel tank and running through troubleshooting processes, NASA would start looking for a solution.

Teams cannot go into the area where the faulty sensors are for about 21 hours.

The scrub was announced around 1:30 p.m. EDT as the crew was loading into the orbiter, just over two hours before the planned launch time. Discovery was set to go up in a five-minute window based on the position of the international space station.

NASA said that there were problems with the low-level fuel cutoff sensors in the redesigned external fuel tank. The sensors monitor liquid hydrogen fuel levels. One of four sensors read empty.

Hale said that there had been other problems with the sensors during testing, but that they remained an "unexplained anomaly."

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One official said that diagnosing the problem could involve going inside the tank, which has never been done on the launch pad.

NASA had said that the flight could come between July 13 and 31. A spokesman said at Wednesday's briefing that the earliest flight Saturday would be at 2:40 p.m. Eastern time.

If the flight does not go up this month, it would have to wait until September.

Mission Plan

The planned mission is to take a crew of seven to the international space station, where it will install a new gyroscope, among other tasks. They will also test methods for inspecting and repairing damage in flight.

There are three spacewalks planned during the mission.

Discovery is outfitted with a redesigned fuel tank and has dozens of motion and temperature sensors embedded in the wings to detect any blows from fuel-tank foam or other debris.

Weather Threatened Flight

Discovery's seven astronauts arrived at the launch pad under a drizzle Wednesday and began boarding the spacecraft for the first shuttle flight in 2½ years, even as thunderstorms threatened to force a postponement.

NASA had lowered its forecast of acceptable weather for the shuttle launch to 40 percent, down from 60 percent Tuesday.

Discovery and its crew of seven were set to blast off at 3:51 p.m. Eastern time on a flight to the international space station.

Workers began fueling the external tank more than an hour later than planned this morning after swapping out a launch-pad heater. After it was completed, inspectors made checks for signs of ice.

Crew members appeared relaxed on NASA television as liftoff approached. Astronaut Stephen Robinson was been shown strumming a guitar as he and fellow crew members sat around a table with a large cake.

Tuesday Problems

NASA managers put another brief setback behind them Tuesday. NASA declared the shuttle ready to fly after fixing an embarrassing mishap involving a cockpit window cover. The cover fell more than 60 feet and damaged several tiles on a fuselage panel. Technicians found a spare and replaced the panel, calling it a minor repair job.

The agency said the cover hit the front cover of an orbital maneuvering rocket, damaging thermal tiles that protect the spacecraft when it re-enters the atmosphere.

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

A hole in thermal protection on Columbia caused its destruction. That crack was caused when debris fell from the external fuel tank during launch. Discovery will fly with a newly designed tank.

No one knows why the lightweight plastic windshield cover came off Tuesday.