An orbit is a regular, repeating path that one object in space takes around another object or center of gravity. An object in an orbit is called a satellite, which can be natural, like the moon, or human-made. Orbits are caused by gravity, which causes objects in space that have mass to be attracted to other nearby objects. If this attraction brings them together with enough momentum, they can sometimes begin to orbit each other. Normally, orbit refers to a regularly repeating trajectory, although it may also refer to a non-repeating trajectory. Planets and satellites follow elliptic orbits, with the center of mass being orbited at a focal point of the ellipse, as described by Keplers laws of planetary motion. There are many types of orbits, including circular and elliptical orbits, and transfer orbits used to get from one orbit to another.