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Types of Planetary Orbits

Planetary orbits are the paths followed by objects that revolve around a body in a planetary system. These orbits are most often spoken about in reference to artificial man-made satellites that orbit around the Earth. The science of planetary orbits was first described, with any real accuracy, by Kepler's laws of planetary motion.

Geostationary Orbit
- A body in geostationary orbit revolves around a planet along the equator. An orbiting body must revolve around the planet in the same time it takes that planet to rotate once about its axis to be considered geostationary. From the perspective of Earth, objects in geostationary orbit appear to remain in a fixed location in the sky. An example of this would be the satellites used by satellite TV companies. Their satellites remain in a fixed position in the southwestern sky, providing a common viewing angle for home satellite receivers.
Geosynchronous Orbit
- A geosynchronous orbit differs from a geostationary orbit in that it is not necessarily parallel with the equatorial line of the Earth. A body in this type of orbit is still synchronous with respect to the time Earth rotates about its axis, and it maintains a longitudinal fixed point. However, its movement in the sky traces a distorted figure-eight from the perspective of an observer on Earth.
Polar Orbit
- A polar orbit is one that circumnavigates the Earth vertically through its poles. Due to the Earth's rotation about its axis, the exact longitude of intersection with the poles is constantly changing. Satellites in this orbit are commonly used for observation, such as taking satellite pictures of the Earth's surface, because they can be programmed to constantly orbit on the sunlit face of the Earth. Many weather satellites follow a polar orbit.
Unbounded Orbits
- An unbounded orbit does not follow a repeating pattern around a specific body within a planetary system. These types of orbits are often referred to as "falling orbits" as the object is propelled through space by the gravity of the planets and celestial bodies it passes. A common example of this type of orbit is the one followed by the Voyager spacecrafts. These vessels were launched from Earth with a trajectory that placed them into an orbit that changed as it passed other planets in the solar system. These unbound orbits are not required to have a return path; they are simply the paths of extraplanetary objects that are affected by the gravity of the objects they pass by or around.

Astronomy