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 be launched. Look up aerospace in Wiktionary, the free dictionary. ... A Redstone rocket, part of the Mercury program The traditional definition of a rocket is a vehicle, missile or aircraft which obtains thrust by the reaction to the ejection of fast moving exhaust gas from within a rocket engine. ... Earth orbit is an orbit around the planet Earth. ... A spacecraft is designed to leave Earths atmosphere and operate beyond the surface of the Earth in outer space. ... A space rendezvous between two spacecraft, often between a spacecraft and a space station, is an orbital maneuver where the two arrive at the same orbit, make the orbital velocities the same, and bring them together (an approach maneuver, taxiing maneuver); it may or may not include docking. ...

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. It has been suggested that sling effect be merged into this article or section. ... The orbital period is the time it takes a planet (or another object) to make one full orbit. ... Mars is the fourth planet from the Sun in the solar system, named after the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. ...

For the actual launch time there is some flexibility, because the time the spacecraft initially spends in low Earth orbit (LEO) can be varied. A low Earth orbit (LEO) is an orbit in which objects such as satellites are below intermediate circular orbit (ICO) and far below geostationary orbit, but typically around 350 - 1400 km above the Earths surface. ...