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. (1622 GMT) and 11:59 p.m. (0459 GMT Saturday).

NASA is up against a tight end-of-month deadline for launching Discovery. If Discovery is not flying by Sunday, the delivery mission will slide into October because of a pair of upcoming launches to the space station from Japan and Russia.

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 (20-centimeter) 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.

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