Circular orbit

A circular orbit is an orbit with a fixed distance around the barycenter; that is, in the shape of a circle.

A circular orbit is depicted in the top-left quadrant of this diagram, where the gravitational potential well of the central mass shows potential energy, and the kinetic energy of the orbital speed is shown in red. The height of the kinetic energy remains constant throughout the constant speed circular orbit.
At the top of the diagram, a satellite in a clockwise circular orbit (yellow spot) launches objects of negligible mass:
(1 - blue) towards Earth,
(2 - red) away from Earth,
(3 - grey) in the direction of travel, and
(4 - black) backwards of the direction of travel.

Dashed ellipses are orbits relative to Earth. Solid curves are perturbations relative to the satellite: in one orbit, (1) and (2) return to the satellite having made a clockwise loop on either side of the satellite. Unintuitively, (3) spirals farther and farther behind whereas (4) spirals ahead.

Listed below is a circular orbit in astrodynamics or celestial mechanics under standard assumptions. Here the centripetal force is the gravitational force, and the axis mentioned above is the line through the center of the central mass perpendicular to the plane of motion.

In this case, not only the distance, but also the speed, angular speed, potential and kinetic energy are constant. There is no periapsis or apoapsis. This orbit has no radial version.