More than three years after Columbia disintegrated over Texas, the shortcomings of the space shuttle's external fuel tank have made the nation's space program the hostage of an exasperating piece of hardware on the threshold of obsolescence. Fourteen times since Columbia went down Feb. 1, 2003, NASA has postponed shuttle launches, and in all but one case, engineers cited problems with the external tank as the main reason. Hurricane damage to NASA facilities accounted for the other.

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