Discovery sits on Launch Pad 39B at Kennedy Space Center at Cape Canaveral, Florida

A faulty fuel sensor forced NASA to postpone the launch of the Space Shuttle Discovery on Wednesday.  NASA officials say they are evaluating the problem but no shuttle launch will be scheduled before Saturday. 

Discovery's seven astronauts had boarded the shuttle, and heavy clouds that had threatened to postpone the launch had cleared.  However just two and a half hours before Discovery was to lift off, a routine test of one of the shuttle's four fuel tank sensors indicated a faulty sensor.  Wayne Hale, the deputy shuttle program manager, says the problem is serious.

"We have a very clear and unambiguous criteria that says all four of those sensors must work to provide us with the kind of redundancy and reliability that is necessary for safe flight," Mr. Hale says. "When one of those indicators started acting up we decided it was time to quit."

NASA engineers immediately postponed the launch saying any liftoff could end in tragedy if a faulty sensor led to the shuttle's engines shutting off too early or too late.   A similar sensor test malfunction surfaced in April but NASA engineers say the issue is an intermittent one that cannot be predicted.   NASA Administrator Michael Griffin says those types of problems are difficult to solve, but safety protocols worked well on Wednesday. 

"You have seen us process a launch, come across a problem, identify it and back out safely and soundly with an effort in place to track it down to its root cause," Mr. Griffin says.

Mr. Griffin says launch postponements are often more common than liftoffs, noting that he was once involved in a missile test launch that had 14 postponements.  

NASA has invested over one billion dollars in new safety technology and procedures since the Shuttle Columbia disintegrated during a reentry to Earth's atmosphere in February 2003.   Columbia was critically damaged shortly after liftoff when a large piece of insulation broke off an external fuel tank and damaged the shuttle's wing, allowing hot gases to enter and destroy the shuttle when it reentered earth's atmosphere.  

A panel charged with investigating the incident in part blamed a lax attitude towards safety at NASA for contributing to the disaster, something NASA officials say has now changed.