In gravitationally bound systems, the orbital speed of an astronomical body or object (e.g. planet, moon, artificial satellite, spacecraft, or star) is the speed at which it orbits around either the barycenter or, if the object is much less massive than the largest body in the system, its speed relative to that largest body. The speed in this latter case may be relative to the surface of the larger body or relative to its center of mass.
The term can be used to refer to either the mean orbital speed, i.e. the average speed over an entire orbit, or its instantaneous speed at a particular point in its orbit. Maximum (instantaneous) orbital speed occurs at periapsis (perigee, perhelion, etc.), while minimum speed for objects in closed orbits occurs at apoapsis (aphelion, apogee, etc.). In ideal two-body systems, objects in open orbits continue to slow down forever as their distance to the barycenter increases.
When a system approximates a two-body system, instantaneous orbital speed at a given point of the orbit can be computed from its distance to the central body and the object's specific orbital energy. (Specific orbital energy is constant and independent of position.)
In the following, it is assumed that the system is a two-body system and the orbiting object has a negligible mass compared to the larger (central) object. In real-world orbital mechanics, it is the system's barycenter, not the larger object, which is at the focus.
Specific orbital energy = K.E. + P.E. (kinetic energy + potential energy). Since kinetic energy is always non-negative (greater than or equal to zero, ≥0) and potential energy is always non-positive (less than or equal to zero, ≤0), the sign of this may be positive, zero, or negative and the sign tells us something about the type of orbit: