For launch vehicles and artificial satellites, the orbital plane is a defining parameter of an orbit; as in general, it will take a very large amount of
propellant to change the orbital plane of an object. Other parameters, such as the
orbital period, the
eccentricity of the orbit and the phase of the orbit are more easily changed by propulsion systems. Orbital planes of satellites are perturbed by the non-spherical nature of the
Earth's gravity. This causes the orbital plane of the satellite's orbit to slowly rotate around the Earth, depending on the angle the plane makes with the Earth's equator. For planes that are at a critical angle this can mean that the plane will track the
Sun around the Earth, forming a
Sun-synchronous orbit. A launch vehicle's
launch window is usually determined by the times when the target orbital plane intersects the launch site. ==See also==