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.

But following a shuttle launch is a bit like watching a football game—the time on the game clock does not translate to actual time. And just as a quarter in an NFL game takes much longer than 15 minutes to play out, a lot more than 43 hours will elapse between countdown and liftoff for Atlantis. In fact, roughly 70 hours separate the planned start-up of the countdown clock, at 1:00 P.M. (Eastern Daylight Time) on July 5, and the planned launch of the space shuttle at 11:26 A.M. on July 8.

The reason for the somewhat confusing difference in timekeeping is a series of planned "holds" in the countdown, built-in pauses that allow launch managers to take stock of the mission's status and respond to changing conditions. In all, the shuttle launch schedule contains seven holds, which take place when the countdown clock reads 27 hours, 19 hours, 11 hours, six hours, three hours, 20 minutes and nine minutes.

Each hold has a specific list of safety checks (inspect external fuel tanks at T-minus 11 hours), launch-preparation tasks (clear launch pad and surrounding area of personnel at T-minus six hours) and decision points (final "go/no-go" poll among launch managers at T-minus nine minutes) that must be addressed before lifting the hold and proceeding with the countdown.

And each hold is a hurdle that the shuttle must clear before liftoff; often delays or launch scrubs will be called during a planned hold. For STS-133, the final flight of space shuttle Discovery, several launch opportunities were called off in November 2010 because of equipment problems—including at T-minus 11 hours. Repairs to fix those problems eventually pushed the launch to February 2011.

For the final shuttle mission, designated STS-135, the planned holds range in duration from 10 minutes to 14 hours. Only after the countdown clock resumes following the final hold at T-minus nine minutes will it reflect the actual amount of time remaining until liftoff.