NASA called off Wednesday's planned launch of the space shuttle Discovery after a problem was found in the craft's fuel system.

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.

Commentators on NASA TV said that there were problems with the low-level fuel cutoff sensors in the redesigned external fuel tank. The sensors monitor when liquid hydrogen fuel levels go below 2 percent, which would not normally happen during launch.

---

Interactive: All About Space Shuttles

Video: Flight A Safety Check | Discovery Ready

---

Launch controllers said it may take some time to figure out the problem. NASA could not immediately say when another attempt may be made.

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.

Widow: Discovery Crew Ready For Space

Evelyn Husband said it's simply time to "fly the flight."

The Houston woman said she would be watching on TV as NASA launches its first shuttle mission since the Columbia accident that killed its seven astronauts, including her husband, Cmdr. Rick Husband.

Evelyn Husband told The Associated Press Radio the Discovery crewmembers are family friends who know firsthand the dangers of shuttle flights. But she called them heroes who are ready to accept the possible consequences and carry out the mission.

While Husband said she would be praying for the crew, she will especially be thinking of their families. She said they're the ones who have had "a front row seat" of what can happen when things go wrong, and that she can imagine how tense they must be.