NASA Calls Off Shuttle Launch Again

Last updated at 11:20 p.m. ET

NASA called off the launch of space shuttle Discovery for a second time Tuesday after a critical fuel valve failed to work properly.

Launch officials halted the countdown midway through the fueling process. The seven astronauts had not yet boarded the shuttle for the scheduled early Wednesday morning flight to the international space station.

"Drats!" said astronaut Jose Hernandez in a Twitter update. Fellow astronaut Christer Fuglesang, a Swede, chimed in with this tweet: "Bad luck again."

Another attempt will not be made until Friday at the earliest. There are two possible launch times: 12:22 a.m. and 11:59 p.m.

NASA only has until Aug. 30 to get Discovery off the ground or the flight will be delayed to mid October because of upcoming Japanese and Russian space station missions and a conflict with the Air Force Eastern Range, which provides tracking and telemetry support for all rockets launched from Florida, reports CBS News space analyst Bill Harwood.

The problem cropped up while launch controllers were trying to shut the large fill-and-drain valve in Discovery's engine compartment. There was no indication the valve closed, and flight rules stipulated that the fuel tank be drained before moving the valve to figure out what went wrong.

Engineers suspect it may be a problem with instruments and that the valve itself could be fine, said Mike Moses, chairman of the mission management team. If that's the case, NASA will try again early Friday morning to launch Discovery.

"That's our success-oriented plan," Moses said late Tuesday night. "Between now and then, we'll learn a lot more."

If the valve is at fault, however, workers will need to get into the engine compartment to replace it. That would take a lot longer. Moses said it would be unlikely NASA could launch by Sunday's deadline and the flight would fall to mid-October.

The 8-inch valve, part of the main propulsion system, is used for the flow of liquid hydrogen from the external fuel tank to the main engines. It needs to be closed prior to launch, and open in order to drain the tank following a launch delay.

The valve worked fine during the first launch attempt, but thunderstorms prevented Discovery from blasting off early Tuesday morning.

The first launch attempt was scuttled in the wee hours of Tuesday. Sixteen hours later, try two ended with a surprising scrub.

Discovery is loaded with thousands of pounds of space station supplies and equipment, including a new treadmill named after Comedy Central's Stephen Colbert.