Orbital Speed - Radial Trajectories

Radial Trajectories

In the case of radial motion:

• If the Specific orbital energy is positive, the body's kinetic energy is greater than its potential energy: The orbit is thus open, following a hyperbola with focus at the other body. See radial hyperbolic trajectory
• For the zero-energy case, the body's kinetic energy is exactly equal to its potential energy: the orbit is thus a parabola with focus at the other body. See radial parabolic trajectory.
• If the energy is negative, the body's potential energy is greater than its kinetic energy: The orbit is thus closed. The motion is on an ellipse with one focus at the other body. See [[radial elliptic traje

The transverse orbital speed is inversely proportional to the distance to the central body because of the law of conservation of angular momentum, or equivalently, Kepler's second law. This states that as a body moves around its orbit during a fixed amount of time, the line from the barycenter to the body sweeps a constant area of the orbital plane, regardless of which part of its orbit the body traces during that period of time. This law is usually stated as "equal areas in equal time."

This law implies that the body moves faster near its periapsis than near its apoapsis, because at the smaller distance it needs to trace a greater arc to cover the same area.