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.

Spaceflight I

Spaceflight (or space flight) applies astronautics to fly spacecraft into or through outer space, either with or without humans on board. Most spaceflight is uncrewed and conducted mainly with spacecraft such as satellites in orbit around Earth and includes space probes for flights beyond Earth orbit. Such spaceflight operates either by telerobotic or autonomous control.