Editing Orbit (section)

===Orbital energies and orbit shapes===
When only two gravitational bodies interact, their orbits follow a [[conic section]]. The orbit can be open (implying the object never returns) or closed (returning). Which it is depends on the total [[energy]] ([[kinetic energy|kinetic]] + [[potential energy]]) of the system. In the case of an open orbit, the speed at any position of the orbit is at least the [[escape velocity]] for that position, in the case of a closed orbit, the speed is always less than the escape velocity. Since the kinetic energy is never negative if the common convention is adopted of taking the potential energy as zero at infinite separation, the bound orbits will have negative total energy, the parabolic trajectories zero total energy, and hyperbolic orbits positive total energy.

An open orbit will have a parabolic shape if it has the velocity of exactly the escape velocity at that point in its trajectory, and it will have the shape of a [[hyperbola]] when its velocity is greater than the escape velocity. When bodies with escape velocity or greater approach each other, they will briefly curve around each other at the time of their closest approach, and then separate, forever.

All closed orbits have the shape of an [[ellipse]]. A circular orbit is a special case, wherein the foci of the ellipse coincide. The point where the orbiting body is closest to Earth is called the [[perigee]], and when orbiting a body other than earth it is called the periapsis (less properly, "perifocus" or "pericentron"). The point where the satellite is farthest from Earth is called the [[apogee]], apoapsis, or sometimes apifocus or apocentron. A line drawn from periapsis to apoapsis is the ''[[line of apsides|line-of-apsides]]''. This is the major axis of the ellipse, the line through its longest part.