Launch window
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Launch window is a term used in aerospace to describe a time period in which a particular rocket must be launched. For trips into Earth orbit almost any time will do, but if the spacecraft intends to rendezvous with another (or a planet, or other point in space) the launch must be carefully timed so that the orbits overlap at some point in the future. If the rocket does not launch in the "window", it has to wait for the next one before it can go.
To go to another planet without using any kind of gravitational slingshot, if eccentricity of orbits is not a factor, launch windows are periodic according to the synodic period; for example, in the case of Mars the period is 2.135 years, i.e. 780 days. In other cases launch windows are irregular. Sometimes rare opportunities are used. When a rare opportunity is missed, sometimes another target has to be selected.
For the actual launch time there is some flexibility, because the time the spacecraft is intermediately in LEO can be varied.de:Startfenster