CAPE CANAVERAL, Fla. - NASA managers put a brief but embarrassing setback behind them as the countdown to the first space shuttle flight in 2 1/2 years entered its final hours Wednesday, with only predicted thunderstorms posing some concern. A temporary window cover fell off the shuttle and damaged thermal tiles near the tail Tuesday afternoon, just two hours after NASA declared Discovery ready to return the nation to space for the first time since the Columbia disaster. The mishap was an eerie reminder of the very thing that doomed Columbia — damage to the spaceship's fragile thermal shield. Discovery...