Launch window is a term used in spaceflight 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 window.
For trips into largely arbitrary Earth orbits, almost any time will do. If the spacecraft intends to rendezvous with a space station or another vehicle already in an orbit, the launch must be carefully timed to occur around the times that the target vehicle's orbital plane intersects the launch site.
For launches above low Earth orbit (LEO), the actual launch time can be somewhat flexible if a parking orbit is used, because the inclination and time the spacecraft initially spends in the parking orbit can be varied. See the launch window used by the Mars Global Surveyor spacecraft to the planet Mars at .
To go to another planet using the simple low-energy Hohmann transfer orbit, 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 more complex cases, including the use of gravitational slingshots, launch windows are irregular. Sometimes rare opportunities arise. When such an opportunity is missed, another target may be selected.
Space Shuttle missions to the International Space Station are restricted by beta angle cutout. Beta angle (β) is defined as the angle between the orbit plane and the vector from the sun. Due to the relationship between an orbiting object's beta angle (in this case, the ISS) and the percent of its orbit that is spent in sunlight, solar power generation and thermal control are affected by that beta angle. Shuttle launches to the ISS are normally only attempted when the ISS is in an orbit with a beta angle of less than 60 degrees.
Published in July 2009.
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