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 called off Friday's scheduled liftoff from Kennedy Space Center after the launch team identified a potentially dangerous liquid hydrogen fuel leak on the 15-story external tank. The leak posed an explosion risk at the launch pad.

The crack in the tank's foam insulation was discovered during an inspection later Friday.

"The hydrogen leak may have been a lucky break," NASA astronaut Alvin Drew, one of the six astronauts set to launch on Discovery, wrote in a Twitter update. "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]

Drew wrote that he and his Discovery crewmates were heading back to the Johnson Space Center in Houston.

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. ET.

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

For a time, NASA managers considered trying to launch the shuttle on Monday, but they decided that would not provide enough time for thorough troubleshooting.

The hydrogen gas leak was detected at around 7:30 a.m. ET 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 occurred during launch preparations for two previous shuttle missions, both in 2009.

1. Space news from NBCNews.com
   1. 

      KARE
      Teen's space mission fueled by social media

      Science editor Alan Boyle's blog: "Astronaut Abby" is at the controls of a social-media machine that is launching the 15-year-old from Minnesota to Kazakhstan this month for the liftoff of the International Space Station's next crew.

   2. Buzz Aldrin's vision for journey to Mars
   3. Giant black hole may be cooking up meals
   4. Watch a 'ring of fire' solar eclipse online

Mike Leinbach, NASA's shuttle launch director, said it would take until Saturday for the excess hydrogen in that area to be purged away. Only then would technicians be allowed to 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," Leinbach said. "We elected to scrub, and that was the best course of action."

Technicians discovered the leak while filling Discovery's orange external tank with liquid oxygen and liquid hydrogen. This procedure 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.

• GRAPHIC: NASA's Space Shuttle – From Top to Bottom
• Gallery: Building Space Shuttle Discovery
• Video – Space Shuttle Discovery: A Retrospective, Part 2, Part 3

This report was supplemented by msnbc.com.