CAPE CANAVERAL, Fla. (AP) - Baffled by fuel tank cracks, NASA announced another prolonged launch delay for space shuttle Discovery on Wednesday and raised the prospect of a Christmastime flight.

Shuttle managers refused to set a new launch date for Discovery's final flight, on hold since the beginning of November. The next launch opportunity would be Dec. 17.

"We would have liked to have found a most probable cause by now" for the cracks that were found on Discovery's fuel tank, said Bill Gerstenmaier, head of NASA's space operations. "This is turning out to be a little more complicated from an analysis standpoint."

"We'll let the data drive where we're heading," he told reporters.

Hydrogen gas leaks halted the countdown for Discovery on Nov. 5. An unrelated crack later was found in the insulating foam on the external fuel tank; cracks in the actual exterior of the tank then were found beneath the flawed foam.

NASA wants to understand the cracking before launching Discovery to the International Space Station one last time. The space agency had been working toward a Dec. 3 liftoff for the 11-day mission. But that was no longer feasible given all the tests and analyses remaining.

A new space station crew is due to lift off from Kazakhstan on Dec. 15 and arrive at the orbiting complex two days later; at this point, Discovery must wait until after that. Gerstenmaier said there are some launch possibilities in January and February, but the first official window of 2011 would not open until the end of February.

A number of unmanned cargo ships are scheduled to fly to the space station early in the year, complicating matters.

The cracks were found in two side-by-side ribs or brackets surrounding the central portion of the 15-story fuel tank. Equipment is located in this area of the tank, but no fuel.

Shuttle program manager John Shannon said cracks have been found before in these so-called stringers, but always during assembly in New Orleans, never at the launch pad like this time. The 21-foot-long brackets are made of an aluminum and lithium alloy, a change made more than a decade ago.

The alloy, while lightweight, is particularly brittle. Of approximately 5,000 stringers assembled for the past 43 fuel tanks, 31 cracks have been found, all related to assembly or mishandling, Shannon said. There is not enough evidence yet to determine what caused the cracks on Discovery's current tank, he said.

Partial X-ray inspections have uncovered no additional cracks on Discovery's tank. But the only way to inspect the brackets on the back side of the tank is to move the shuttle from the launch pad back into the Vehicle Assembly Building. That would mean an even lengthier postponement, especially if widespread modifications were needed.

The concern is that cracks could form during ascent and shoot off chunks of foam, some of which might strike Discovery. A slab of flyaway foam pierced Columbia's wing at liftoff in 2003, and the shuttle was destroyed during re-entry. Engineers also want to be certain that if multiple brackets cracked, the integrity of the tank would not be compromised.

"We need to nail down our risk exposure to this," Shannon said.

If the Discovery remains grounded until next year, that likely would mean a postponement of Endeavour's last launch, currently scheduled for the end of February.

NASA hopes to get money for an extra shuttle mission next summer to close out the 30-year shuttle program.

The officials said they were under no pressure to launch quickly. As for flying over Christmas, there are no budget constraints regarding overtime pay, Gerstenmaier said.

Discovery made the only shuttle-era Christmastime flight to date, during a Hubble Space Telescope repair mission in 1999.

---

Online: NASA: http://www.nasa.gov/mission(underscore)pages/shuttle/main/index.html

(Copyright 2010 by The Associated Press. All Rights Reserved.)