Why Does the Space Shuttle Launch Countdown Have So Many Stops and Starts?

On July 5, if all goes according to plan, the final countdown of the space shuttle program will begin. The launch clock at Kennedy Space Center, a giant digital display with 40-watt lightbulbs for pixels, will begin ticking down from 43 hours. When it reaches zero, Atlantis will rumble off the launch pad, and the final shuttle mission will begin

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Why Does the Space Shuttle Launch Countdown Have So Many Stops and Starts?