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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. The "T" in T-minus stands for "Test". 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.
- 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.
The procedures for each launch are written carefully. For the Space Shuttle, a five-volume set, Shuttle Countdown (KSC S0007), often referred to as "S0007", is used. Rosie Carver, a technical writer for United Launch Alliance, has created at least 15,000 procedures for more than 300 missions since the Solar Maximum Mission, which launched Feb. 14, 1980. These documents are living documents, which reflect new issues and solutions as they develop. Each mission requires approximately 100 procedure books.
Proceeding with the countdown depends on several factors, such as the proper launch window, weather that permits a safe launch, and the rocket and payload working properly.
The launch weather guidelines involving the Space Shuttle and expendable rockets are similar in many areas, but a distinction is made for the individual characteristics of each. The criteria are broadly conservative and assure avoidance of possibly adverse conditions. They are reviewed for each launch. For the Space Shuttle, weather "outlooks" provided by the U. S. Air Force Range Weather Operations Facility at Cape Canaveral begin at Launch minus 5 days in coordination with the NOAA National Weather Service Spaceflight Meteorology Group at the Johnson Space Center in Houston. These include weather trends and their possible effects on launch day. A formal prelaunch weather briefing is held on Launch minus 1 day, which is a specific weather briefing for all areas of Space Shuttle launch operations.
The launch window is a precise time during which aerospace personnel launch a rocket so the payload can reach the proper orbital destination.
A hold is the suspension of the normal countdown process. This can be done to investigate a technical process that has gone wrong, or because of marginal weather at the launch pad.
Some holds are planned: they are done so the launch-support computers can run automatic checks on the rocket.
Under some circumstances, a countdown may be recycled to an earlier time. When that happens, launch personnel begin following the countdown checklist from the earlier point.
During communications for a countdown, the launch team uses acronyms to keep channels open as much as possible. All Firing Room console positions are assigned unique 'call signs' that are used by the team for quick and positive identification of who is talking. For example, dialogue heard during the launch of a Delta II rocket carrying the Kepler Space Telescope on March 8, 2009, included:
Time: T minus 3 minutes.
Launch Control (LC): OSM, third stage S&A arm permit to close.
LC: SSC, third stage S&A armed.
LC: Prop 1, vehicle fuel tank press open.
Prop 1: Open.
LC: Fuel umbilical purge to open.
Prop 1: Open.
LC: SSC, vent 1 heater control exit.
LC: SSC, vent 2 heater control exit.
LC: NSC reports spacecraft is go.
Mission Director: Kepler spacecraft is go.
LC: SSC - FTS bat one and two heater controls heaters off.
LC: Prop 1, pressurized first stage LOX tanks to relief.
Prop 1: Pressurized.
LC: Prop 2, top first stage LOX to 100 percent levels.
Prop 2: Up and down, 100 percent.
Time: Ninety seconds.
LC: SSC, hydraulic external power to on.
Time: Eighty seconds.
LC: RCO, report range go for launch.
Range Control Officer (RCO): Range go for launch.
Mission Director: LC (Viera), you're go for launch.
In the context of a rocket launch, the "T minus Time" is the time before launch, e.g. "T minus 3 minutes and 40 seconds". The last ten seconds are usually counted down aloud "10, 9, 8, 7, 6, 5, 4, 3, 2, 1, Lift off!" After a launch, most countdown clocks begin to show Mission Elapsed Time, which is typically shown as "T plus." The picture to the right shows "+ 00:00:07", approximately seven seconds after liftoff.
When counting down to the launch of an explosive, e.g. when testing a new model of gun, it is customary to omit "5" from the countdown sequence because "five" sounds too similar to 'Fire!'.
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. In Times Square on New Year's Eve, besides the 60-Second Countdown at 11:59 P.M., there are six 20 second hourly countdowns in which people can practice their countdown skills from 5:59 P.M. to 10:59 P.M. The aforementioned countdowns are also the countdowns to the end of each hour.
See also 
- Ask The Mission Team - Question and Answer Session
- Everett, William. On the Cam: Lectures on the University of Cambridge in England. Cambridge, Mass.: Sever and Francis, 1865, p. 192.
- "Spektrum der Wissenschaft" - DenkMal-Frage: "Was verdankt die Raumfahrt dem Stummfilm "Die Frau im Mond" (1929) von Fritz Lang?" Wissenschaft-online.de
- Weide, Robert (Summer 2012). "The Outer limts". DGA Quarterly (Los Angeles, California: Director Guild of America, Inc.): 64–71. ISSN 1083-5323. A gallery of behind-the-scenes shots of movies featuring space travel or aliens. Page 68, photo caption: "Directed by Fritz Lang (third from right), the silent film "Woman in the Moon" (1929) is considered one of the first serious science fiction films and invented the countdown before the lauch of a rocket. Many of the basics of space travel were presented to a mass audience for the first time."
- Angelo, Joseph A. Space Technology. 2003, Greenwood Publishing Group; ISBN 1-57356-335-8, ISBN 978-1-57356-335-2: 143
- NASA, "Launching by the Book", April 30, 2007. NASA
- Kennedy Space Flight Center Release 39-99, "Space Shuttle weather launch commit criteria and KSD end of mission weather landing criteria", May 20, 1999 NASA.org
- Angelo 2003: 144
- Angelo 2003: 144
- NASA, "The Space Shuttle launch team"
- NASA Channel, 8 March 2009, 10:40 Eastern Standard Time