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.^[1][2] If the rocket is not launched within a given window, it has to wait for the window on the next day of the period.^[3] Launch periods and launch windows are dependent on both the rocket's capability and the orbit to which it is going.^[4][5]

A launch period refers to the days that the rocket can launch to reach its intended orbit. A mission could have a period of 365 days in a year, a few weeks each month,^[6] a few weeks every 26 months (e.g. Mars launch periods),^[7] or a short period time that won't be repeated.

A launch window indicates the time frame on a given day within the launch period that the rocket can launch to reach its intended orbit.^[8][9] This can be as short as a second (referred to as an instantaneous window) or as long as the entire day. The launch window can straddle two calendar days (for example, starting at 11:46 p.m. and ending at 12:14 a.m.). Launch windows are rarely at exactly the same times each day. For operational reasons, the window almost always is limited to no more than a few hours.^[10]

Launch windows and launch periods are often used interchangeably in the public sphere, even within the same organization. The definitions given here are as used by launch directors and trajectory analysts at NASA and other space agencies.^[11][12]