Countdown facts for kids
A countdown is a sequence of backward counting to indicate the time remaining before an event is scheduled to occur. NASA commonly employs the term "T-minus" during the preparation for and anticipation of a rocket launch, and even "E-minus" for events that involve spacecraft that are already in space, where the "T" could stand for "Test" or "Time", and the "E" stands for "Encounter", as with a comet or some other space object.
Other events for which countdowns are commonly used include the detonation of an explosive, the start of a race, the start of the New Year, or any anxiously anticipated event.
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Rocketry
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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:
- Aerospace personnel bring the rocket vehicle to the launch site and load it with payload and propellants;
- Launch-center computers communicate with sensors in the rocket, which monitor important systems on the launch vehicle and payload;
- Launch personnel monitor the weather and wait for the launch window;
- Security personnel prevent unauthorized persons from entering the "keep-out" area.
Film
At the start of films, a countdown sequence is printed on the head leader, to aid in synchronizing the film. In film (but not television) the countdown is in units of feet rather than time units.
New Year's Eve
In many New Year's Eve celebrations, there is a countdown during the last seconds of the old year until the beginning of the new year.
Images for kids
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Seven seconds after launch of STS-121, the countdown clock at LC-39 at the Kennedy Space Center seen counting up, rather than down.
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Newer (1959) SMPTE countdown on the head leader of a film reel
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See also
In Spanish: Cuenta atrás para niños
