Launch window

Launch Window

A term carried over from the age of chemical rocketry and space exploration. Launch window is a term that was used in spaceflight to describe a time period in which a particular launch vehicle (rocket, Space Shuttle, etc.) must be launched. If the rocket does not launch within the "window", it has to wait for the next window.[1]

In modern usage it is a management tool of busy space port traffic control centers. It allows the management of tractor controls, approach and departure   vectors (very important dealing with mountain sized space ships.)

[1] History of the Term

For trips into largely arbitrary Earth orbits, almost any time will do. But if the spacecraft intends to rendezvous with a space station (such as the International 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 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 Rosetta mission of ESA 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).

Launch windows are often calculated from porkchop plots that show the delta-v needed to achieve the mission, plotted against the launch time.