Sensor Glitch Forces Cancellation of Discovery Launch

CAPE CANAVERAL, July 13 -- Flight controllers scrubbed the launch of the space shuttle Discovery less than two hours before its much-anticipated liftoff Wednesday because of a malfunctioning sensor that monitors the flow of liquid hydrogen in the external fuel tank.

The cancellation frustrated NASA and a legion of well-wishers, and raised questions about the space agency's ostensibly exhaustive efforts to upgrade the shuttle's performance and safety features in the 2 1/2 years since the Columbia disaster.

Engineers had encountered similar problems with "engine cutoff sensors" in another fuel tank during a test in April but decided to fly Wednesday with an "unexplained anomaly" even though they were unable to pinpoint the cause.

"We had a long conversation [in April] about what to do, so we changed out every cable connector, all the wiring and every electronic box," said Wayne Hale, deputy shuttle program manager. "We felt we had a good system."

Hale said engineers were trouble-shooting the malfunction on Kennedy Space Center's Launch Pad 39B and would decide Thursday what to do. But, he added, "we would not in any conceivable way be able to launch before Saturday."

The scrub at 1:33 p.m. Eastern time brought an unexpected denouement to a countdown that had progressed almost without flaw and amid increasing excitement as midday thunderstorms -- seemingly the only possible deal-breaker for the launch -- gave way to sunshine as the 3:51 p.m. liftoff approached.

Instead, mission commander Eileen Collins and the six other Discovery astronauts, strapped on their backs as the countdown ticked away, were abruptly told by launch director Mike Leinbach: "We are going to have to scrub this launch attempt."

The launch would have been the first since the Columbia disintegrated on reentry on Feb. 1, 2003, and was widely touted as the opening event in President Bush's "Vision for Space Exploration," which seeks to put humans back on the moon by 2020 and then send them eventually to Mars.

More than 1,250 newspeople arrived to witness the launch, and the parking lot at the media site was filled with 100 television trucks bristling with satellite dishes.

Visiting lawmakers, including House Majority Leader Tom DeLay (R-Tex.); Sen. Kay Bailey Hutchison (R-Tex.), chairman of the science and space subcommittee; and Sen. John F. Kerry (Mass.), last year's Democratic presidential nominee, offered condolences and encouragement.

"This wasn't a failure, this was a success," said House Science Committee Chairman Sherwood L. Boehlert (R-N.Y.). "The success was that they identified the problem before the launch, not afterwards -- that's not failure."

Former astronaut and senator John Glenn (D-Ohio), the closest thing NASA has to a living legend, agreed. But that did not make it any easier to take.