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 very 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 in 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 even the entire day. For operational reasons, the window almost always is limited to no more than a few hours. The launch window can stretch over two calendar days (ex: start at 11:46 p.m. and end at 12:14 a.m.). Launch windows are sometimes but rarely exactly the same times each day.^[10]

Launch windows and launch periods are often used interchangeably in the public sphere, even within the same organization. However, these definitions are the ones used by NASA (and other space agencies) launch directors and trajectory analysts.^[11][12]