A faulty fuel tank sensor has caused NASA to cancel Wednesday's scheduled launch of the space shuttle Discovery, the first shuttle to be sent back into orbit since the Columbia disaster.

In a best-case scenario, the launch could go ahead on Saturday, said Wayne Hale, NASA's deputy program manager.

Discovery contains more than 2.5 million parts.
Discovery contains more than 2.5 million parts.

The launch was scrubbed because a sensor read low fuel during a simulation, although the hydrogen tank was full. During launch, if two of four sensors malfunction then the engines could shut down prematurely.

"Well, all I can say is 'shucks,'" Hale said of the cancellation decision. "It took us about five minutes of discussion to confirm that and decide that it was time to try another day."

The space agency has faced similar intermittent sensor problems before. Scrubbing a launch because of a malfunctioning sensor is common sense, agreed Canadian astronaut Chris Hadfield.

"This is a very obvious thing," said Hadfield. "If it is not functioning, we are not ready."

NASA had also been wary of thunderclouds at the launch site. Two minor technical glitches also occurred in the 48 hours before launch and were repaired.

A safe flight is needed to restore confidence in NASA's human space program.

Discovery's current launch window lasts until the end of July, based on the position of the space station and NASA's decision to restrict launches to daylight hours. The next opportunity is in September.

In February 2003, the shuttle Columbia's launch seemed smooth, but thermal tiles on the wing were knocked off. Hot gases entered the crack during re-entry, leading to the breakup of the shuttle and the deaths of all seven astronauts.

Ground crews want clear skies near the Kennedy Space Center in Florida to watch the shuttle's ascent. They will pay attention to the external fuel tank's new insulation system, since that's where a piece of foam broke off on Columbia.

Discovery Commander Eileen Collins, five other Americans and one Japanese astronaut will be on a 12-day mission to deliver parts to the International Space Station. NASA plans to retire its shuttle fleet in 2010 after construction is completed at the station.