An orbit is a curved path followed by an object under the gravitational influence of another body. It is one of the conic section family of curves, which includes the circle, the ellipse, the parabola and the hyperbola.
A closed orbit, such as that followed by a satellite going around Earth, has the shape of a circle or an ellipse. A satellite in a circular orbit travels at a constant speed. The higher the altitude, however, the lower the speed relative to the surface of the Earth. Maintaining an altitude of 35,800 km over the equator, a satellite is said to be in geostationary orbit. In an elliptical orbit, the speed varies and is greatest at perigee (minimum altitude) and least at apogee (maximum altitude). Elliptical orbits can lie in any plane that passes through Earth's center. A polar orbit lies in a plane that passes through the north and south poles; in other words, it passes through Earth's axis of rotation. An equatorial orbit is one that lies in a plane passing through the equator. The angle between the orbital plane and the equator plane is called the inclination of the orbit. As long as the orbit of an object keeps it in the vacuum of space, the object will continue to orbit without propulsive power because there is no frictional force to slow it down. If part or all of the orbit passes through Earth's atmosphere, however, the body is slowed by aerodynamic friction with the air. This causes the orbit to decay gradually to lower and lower altitudes until the object fully reenters the atmosphere and burns up. An open orbit is one in which a spacecraft does not follow a closed circuit around a gravitating body but simply has its path bent into the shape of a parabola or hyperbola.