Countdown

A countdown is a sequence of counting backward to indicate the seconds, days, or other time units remaining before an event occurs or a deadline expires. Some events for which a countdown is used include the launch of a rocket or spacecraft, the detonation of a bomb, the start of a race, or the start of the New Year. In an early attestation of a countdown, it started a rowing race at Cambridge University. The first known association with rockets was in the 1929 German science fiction movie Die Frau im Mond (English: Woman in the Moon) by Fritz Lang to increase the drama of the launch sequence of the story’s lunar rocket.
A countdown is a carefully devised set of procedures ending with the ignition of a rocket’s engine. Depending on the type of vehicle used, countdowns can start from 72 to 96 hours before launch time.
During countdown:
The procedures for each launch are written carefully. For the Space Shuttle, a five-volume set, Shuttle Countdown (KSC S0007), often referred to as …