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. If the rocket does not launch within the "window", it has to wait for the next one before it can be launched.
For trips into largely arbitrary Earth orbits almost any time will do, but if the spacecraft intends to rendezvous with a space station or another vehicle that is already in an orbit the launch must be carefully timed to occur around the times that the target vehicle's orbital planes intersects the launch site.
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 initially spends in low Earth orbit (LEO) can be varied.
The term 'launch window' was popularized with the worldwide coverage of the early space program. People in business came to use the phrase window of opportunity to be defined by specific range of dates, within which, they must take action or miss the chance to leverage the marketplace to their benefit.
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