In the context of spaceflight, a launch window is a time period during which a particular vehicle (rocket, Space Shuttle, etc.) must be launched in order to reach its intended target. If the rocket is not launched within this time period, it has to wait for the next window.
For trips into largely arbitrary Earth orbits, no specific launch time is required. But if the spacecraft intends to rendezvous with an object already in orbit, the launch must be carefully timed to occur around the times that the target vehicle's orbital plane intersects the launch site.
Earth observation satellites are often launched into sun-synchronous orbits which are near-polar. For these orbits, the launch window occurs at the time of day when the launch site location is aligned with the plane of the required orbit. To launch at another time would require an orbital plane change maneuver which would require a large amount of propellant.
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, (780 days). In more complex cases, including the use of gravitational slingshots, launch windows are irregular. Sometimes rare opportunities arise, such as when Voyager 2 took advantage of a 175-year planetary alignment (launch window) to visit Jupiter, Saturn, Uranus, and Neptune. When such an opportunity is missed, another target may be selected. For example, the ESA's Rosetta mission was originally intended for comet 46P/Wirtanen, but a launcher problem delayed it and a new target had to be selected (comet 67P/Churyumov-Gerasimenko).
Space Shuttle missions to the International Space Station were restricted by beta angle cutout. Beta angle () 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 were normally attempted only when the ISS was in an orbit with a beta angle of less than 60 degrees.
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