On-the-fritz sensor grounds Atlantis until next year

KENNEDY SPACE CENTER, Florida (CNN) -- NASA scrubbed space shuttle Atlantis' planned Sunday launch after a cut-off sensor designed to gauge the fuel level of the external liquid hydrogen tank failed another test, a space agency spokesman said.

Atlantis will launch no earlier than January 2, Kennedy Space Center spokesman Allard Beutel said.

Thursday's launch was postponed after two of the sensors failed pre-flight testing.

Launch teams will drain the fuel from the external tank so NASA can perform a troubleshooting procedure in hopes of learning more about the "ECO" sensors, NASA spokesman George Diller said.

Wayne Hale, NASA's shuttle program manager, said last week that if the sensors failed Sunday, it would be unlikely Atlantis could take off during the launch window, which closes Thursday.

Hale said the sensors would have to perform flawlessly for the mission to move forward. Watch Hale explain last week's delay »

The sensors are much like the low-fuel indicators in an automobile. As the shuttle rockets toward space, the sensors monitor the amount of liquid hydrogen in the external tank and automatically shut off the main engines when the tank is empty.

If the sensors aren't working, the engine could shut down before the shuttle reaches orbit, forcing Atlantis to make a dangerous emergency landing.

NASA engineers pumped fuel into the tank during pre-flight tests Thursday and two of the sensors failed. Sensor No. 3 failed Sunday. It is not clear if the sensor that failed Sunday is one of the sensors that didn't pass tests Thursday.

The root cause of the problem continues to elude NASA engineers. Hale likened the problem to an intermittent electrical malfunction on an automobile that appears to fix itself by the time it arrives at the mechanic.

The plan for the shuttle's upcoming 11-day mission was to deliver the long-awaited European addition to the international space station -- a 23-foot-long laboratory module named "Columbus," after the 15th century explorer.

The high-tech lab has been in the works for 25 years and is considered a huge milestone toward the completion of the space station. E-mail to a friend

CNN's Kate Tobin contributed to this report.

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