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NASA announced Wednesday that it needs more time to determine exactly what led to cracks in portions of the space shuttle Discovery, and as a result, the launch will not occur until at least December 17.

John Shannon, Space Shuttle program manager, said during a press conference that officials are leaning towards the idea that the cracks are an assembly issue rather than a fundamental design flaw. But it will take time to determine where exactly during the assembly process that things went wrong, he said.

"We have an unknown here – how did something get through our process to allow a crack, and do [we] have a susceptibility to have another one of those conditions?" Shannon asked.

Cracks were uncovered on the shuttle's stringers, which are 21-foot-long support beams. There are 108 stringers located on the exterior of the external tank - known as the intertank. Shannon said Wednesday that of the 5,000 stringers used on Discovery's tanks in recent years, cracks have been discovered on 31 of them.

The Discovery launch has been pushed back numerous times since November 1, when it was first scheduled to depart. Leaks, inclement weather, and these cracks, however, delayed the launch – most recently until December 3.

The uncertainty surrounding what caused the cracks, however, means that NASA will now push the launch to a window between December 17 and December 20. But Bill Gerstenmaier, associate administrator for Space Operations, warned enthusiasts not to "get hung up on those specific dates." The important thing, NASA officials said, is to make sure Discovery is ready to fly. At this point, there are no budget constraints that would require a particular fly date, so it's best to err on the side of caution, Shannon said.