CAPE CANAVERAL, Fla. – A fuel leak and crack on the space shuttle Discovery's huge external tank has forced NASA to call off any attempts to launch before Nov. 30 – the latest in a series of delays for the spacecraft's final voyage.

NASA discovered the crack during an inspection after finding a potentially dangerous liquid hydrogen fuel leak on the 15-story external tank that thwarted the shuttle's launch plans for today (Nov. 5). The crack is located on a different part of the tank.

"The hydrogen leak may have been a lucky break," wrote NASA astronaut Alvin Drew, one of the six astronauts set to launch on Discovery, in a Twitter post. "Found a crack in the [external tank] outer foam with ice underneath. Don't know that we'd have caught it." [GRAPHIC: NASA's Space Shuttle – From Top to Bottom]

NASA must repair the fuel leak and foam crack before Discovery can blast off toward the International Space Station. That means the next possible time the shuttle can try to launch is Nov. 30 at 4:05 a.m. EST (0905 GMT).

Discovery was slated to launch at 3:04 p.m. EDT (1904 GMT) today from a seaside pad here at the Kennedy Space Center before the fuel leak was discovered. The leak posed an explosion risk at the launch pad.

The mission has already been delayed four days due minor gas leaks, an electrical glitch and, most recently, uncooperative weather. NASA had until Nov. 8 to launch Discovery on its mission to the International Space Station before the sun angles at the station would become unfavorable.

Drew wrote that he and his fellow Discovery crewmates will head back to the Johnson Space Center in Houston later today.

The hydrogen gas leak was detected at around 7:30 a.m. EDT (1130 GMT) in a location known as the Ground Umbilical Carrier Plate, which is an attachment point between the external tank and a 17-inch pipe that carries gaseous hydrogen safely away from the shuttle to the flare stack, where it is burned off. [Photo of the shuttle fuel leak location]

Similar leaks have occured during launch preparations for two previous shuttle missions, both in 2009.

The external tank is now being drained of the propellant, but it will take an additional 22 hours or so to allow excess hydrogen in that area to be purged away before technicians can return to the pad to investigate the cause of the leak.

"Right now it's a lot of speculation, but the hardware was obviously talking to us – it was leaking significantly," said Mike Leinbach, NASA's shuttle launch director. "We elected to scrub, and that was the best course of action."

Technicians discovered the leak while filling Discovery's distinct, orange external tank with liquid oxygen and liquid hydrogen. This procedure, known as tanking, fills the external tank with the 520,000 gallons of cryogenic propellant that will be used to fuel the shuttle during liftoff and ascent into space.

Discovery will fly an 11-day supply mission to the International Space Station to deliver a humanoid robot helper for the station crew and a new storage room for the orbiting lab.

The STS-133 mission will be Discovery's grand finale in space before being retired along with the rest of NASA's shuttle fleet in 2011.