A countdown is a sequence of backward counting to indicate the time remaining before an event is scheduled to occur. NASA commonly employs the terms "L-minus" and "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. An early use of a countdown once signaled the start of a Cambridge University rowing race. One of the first known associations with rockets was in the 1929 German science fiction movie ''

Frau im Mond ''Honorifics'' are words that connote esteem or respect when used in addressing or referring to a person. In the German language, honorifics distinguish people by age, sex, profession, academic achievement, and rank. In the past, a distinction was ...

'' (English: ''Woman in the Moon'') written by Thea von Harbou and directed by Fritz Lang in an attempt to increase the drama of the launch sequence of the story's lunar-bound rocket. 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 launch of a rocket. Many of the basics of space travel were presented to a mass audience for the first time."

Rocketry

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 In the context of spaceflight, launch period is the collection of days and launch window is the time period on a given day during which a particular rocket must be launched in order to reach its intended target. If the rocket is not launched wit ...

; * 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", was 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 The Solar Maximum Mission satellite (or SolarMax) was designed to investigate Sun, Solar phenomena, particularly solar flares. It was launched on February 14, 1980. The SMM was the first satellite based on the Multimission Modular Spacecraft bus ...

, 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 began at Launch minus 5 days in coordination with the NOAA National Weather Service Spaceflight Meteorology Group at the Johnson Space Center in Houston. These included weather trends and their possible effects on launch day. A formal prelaunch weather briefing was held on Launch minus 1 day, which was 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.Angelo 2003: 144 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 The Kepler space telescope is a disused space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...

on March 8, 2009, included:

Time: T minus 3 minutes.
Launch Control (LC): , third stage arm permit to close.
OSM: Closed.
LC: , third stage S&A armed.
SSC: Armed.
LC:
Prop A prop, formally known as (theatrical) property, is an object used on stage or screen by actors during a performance or screen production. In practical terms, a prop is considered to be anything movable or portable on a stage or a set, distinct ...
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.
SSC: Exit.
LC: SSC, vent 2 heater control exit.
SSC: Exit.
LC: NSC reports spacecraft is go.
Mission Director: Kepler spacecraft is go.
LC: SSC - FTS bat one and two heater controls heaters off.
SSC: 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.
SSC: External.
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.
LC: Roger.
NASA Channel, 8 March 2009, 10:40 Eastern Standard Time

In the context of a rocket launch, the "L minus Time" is the physical time before launch, e.g. "L minus 3 minutes and 40 seconds". "T minus Time" is a system to mark points at which actions necessary for the launch are planned - this time stops and starts as various hold points are entered, and so doesn't show the actual time to launch. The last ten seconds are usually counted down aloud "Ten seconds to liftoff. Nine, eight, seven, six, five, four, three, two, one." After a launch, most countdown clocks begin to show

Mission Elapsed Time Mission Elapsed Time (MET) is used by NASA during their space missions, most notably during their Space Shuttle missions. Because so much of the mission depends on the time of launch, all events after launch are scheduled on the Mission Elapsed Tim ...

, which is typically shown as "T plus." The adjacent picture shows "+ 00:00:07", approximately seven seconds after liftoff. In Fritz Lang's

film A film also called a movie, motion picture, moving picture, picture, photoplay or (slang) flick is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, feelings, beauty, or atmosphere ...

'', after each number the phrase "seconds to go" was repeated.

Film

At the start of films, a countdown sequence is printed on the head leader, and is used to synchronize film reel changeovers (switching between reels of film). In film (but not television) the 'Academy Leader' countdown first used in 1930 is in units of feet rather than time units; it starts at 11 and ends at 3 where it cuts to black for the last few feet. In 1959, SMPTE leader was created and measures seconds, not feet. It starts at 8 and cuts to black on the first frame of 2, which is accompanied by a 'pop.' This leader eventually displaced the older Academy and was the only leader used by the end of the optical (film) projection era.

New Year's Eve

In many

New Year's Eve In the Gregorian calendar, New Year's Eve, also known as Old Year's Day or Saint Sylvester's Day in many countries, is the evening or the entire day of the last day of the year, on 31 December. The last day of the year is commonly referred to ...

celebrations, there is a countdown during the last seconds of the old year until the beginning of the new year. These countdowns usually end in fireworks. Some celebrations also have countdowns to midnight in preceding timezones.

Independence Day

In Malaysia, the countdown is also used to celebrate independence day, with an event called Ambang Merdeka. The countdown was carried out at 11:59:50 pm and ended with chanting Merdeka 8 times and singing the national anthem.

References

External links