A countdown is the backward counting to indicate the seconds, days, etc. remaining before an event occurs or a deadline expires. Typical events for which a countdown is used include the launch of a rocket or spacecraft, the explosion of a bomb, the start of a race, or the New Year.

Rocketry

A countdown is a carefully devised set of procedures leading to the ignition of a rocket's engine. Depending on the type of vehicle used, countdowns can start from 72 to 96 hours ahead of launch time.

During the 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

The procedures for each launch are carefully written out. 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.