launch window

A launch window is a time period (as long as months as short as a few minutes) for a space probe's launch in order to meet some criteria. The typical criteria for cited launch windows is the ability to use some particular space route. The short, efficient route to Mars requires the trip to begin during a specific time period that has a particular relationship between Earth's and Mars's positions within their orbits (likely a period somewhat close to the time the two are nearest to each other). The entire mission may depend upon the launch window because the route traversed can have a very significant effect on the whole mission's cost: for example, the time-length of the trip and the fuel needed, both of which affect other aspects, such as the size (and expense) of the launch booster. Many such launch windows recur periodically, e.g., Mars is in an ideal position for a short flight roughly once every two years. During the 2020 pandemic, special effort was made to be ready for the intended summer 2020 launch window for missions to Mars, given the cost of two years' delay. Launch windows for practical trips covering sequences of outer planets can be as infrequent as once in a lifetime, e.g., those used by the Voyager missions.

A stated launch window for a mission may also incorporate other constraints on the time of launch, especially as the launch time approaches. For example, there may be subsequent launches scheduled for the launch pad that take priority over delaying the launch, or there may be a limit on the amount of time between fueling the rocket and launching it.