UPDATE: Story first posted 2:05 p.m. EDT

CAPE CANAVERAL - NASA flight controllers scrubbed the planned launch of the space shuttle Discovery today due to a glitch with the orbiter's external tank.

A malfunctioning fuel level sensor in the orbiter's external tank forced flight controllers to call off today's launch attempt of NASA's first shuttle mission since the Columbia disaster. The sensor is one of four required inside the external tank that monitor fuel levels in order to shut down Discovery's main engines before the tank runs dry.

Launch officials reported that the external tank glitch occurred during a test of the fuel sensor system, in which flight controllers direct sensors - which register 'wet' when the tank is full - to register as 'dry.' The faulty sensor failed to register as 'dry,' launch officials said.

"It will take some time to determine how long we will have to stand down," NASA spokesman George Diller said.

Launch officials have at least two more opportunities to launch Discovery this week.

"NASA will launch when NASA is ready, and when they know when everything is ready," Sen. Bill Nelson (D-Florida) told reporters here. "Spaceflight is a risky business, but it is an acceptable risk."

The next launch date for Discovery opens on the afternoon of July 14, with a second opportunity on July 16. However, should Discovery still remain earthbound after that, pad engineers will have to remove the fuel reactants inside the orbiter. An approaching storm could also prompt launch officials to rollback the orbiter to avoid damage from high winds, NASA officials said, adding that such a situation would cause a longer delay.

There are still two more weeks in Discovery's July launch window, which closes on July 31.

Mission managers and engineers are holding an ongoing technical meeting to discuss and assess the situation, a spokesman said. NASA will hold a technical briefing for reporters no earlier than 4:30 p.m. EDT (2030 GMT).

The shuttle was slated to launch at 3:50:53 p.m. EDT (1950:53 GMT) from Launch Pad 39B here at NASA's Kennedy Space Center (KSC). The STS-114 crew, commanded by veteran astronaut Eileen Collins, had already begun strapping into Discovery when flight controllers called off the launch. The astronauts have disembarked their spacecraft and are returning to their crew quarters.

"We are disappointed, but we'll fly again on another day," said NASA test director Jeff Spaulding. "There are a lot of long faces, but we can't let being anxious interfere with our attention to safety."

The external tank will have to be drained of its 526,000 pounds of liquid hydrogen and liquid oxygen propellant before engineers can pin down the anomaly, NASA officials said, adding that the process will take several hours. Loading the tank took about three hours earlier today.

NASA officials plan to conduct shuttle troubleshooting before they begin emptying Discovery's external tank.

Discovery's STS-114 flight is NASA's first shuttle launch since 2003 accident that claimed the lives of seven astronauts and destroyed the space shuttle Columbia during the STS-107 mission. Columbia was destroyed on Feb. 1, 2003, as it reentered the Earth's atmosphere after a successful science mission that began 16 days earlier.

But launch debris from Columbia's external tank separated at launch and struck Columbia's left wing leading edge, fatally crippling the spacecraft's vital heat-resistant skin. During reentry, hot gases entered the hole from that collision and led to the orbiter's destruction.

Discovery's external tank has been touted by engineers as one of the safest tanks ever built for the shuttle program, and was redesigned in the wake of the Columbia accident to reduce the amount of ice formation on the launch pad, as well as foam shedding during liftoff.

During an April fueling test with Discovery's first external tank, flight controllers detected a sensor glitch which seems similar to that experienced today, though the shuttle's current tank is a newer version. Discovery switched external tanks in June. The current tank contains a heater that prevents the formation of ice on a flexible bellows unit. Shuttle officials have never been able to pin down the source of that anomaly, nor recreate it in simulations.

If flight controllers are unable to launch Discovery this month, they could also opt to push the spaceflight into the next shuttle launch window, which runs between Sept.9-24.

That launch window is currently allotted to the Atlantis orbiter and its STS-121 crew, slated to perform NASA's second return to flight mission.

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