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There are several ways to answer ‘what is an orbit’. The word has different meanings in anatomy, mathematics, pop culture, chemistry, and celestial mechanics. This article will deal with its definition and use in celestial mechanics only. For our purposes an orbit is ‘the gravitationally curved path of an object around a point in space, for example the gravitational orbit of a planet around a point in space near a star’. Our understanding of orbital motion is based on Einstein’s general theory of relativity. This theory accounts for gravity and the curvature of space-time. Accordingly, orbits follow geodiscs(a straight line applied to curved spaces). The calculations are very complex, so Kepler’s laws of planetary motion are used more often, just for their ease of use.
Within a planetary system, objects(planets, comets, asteroids, etc) orbit the barycenter(point between two objects where they balance each other) in elliptical orbits. Bodies which are gravitationally bound to a planet in a planetary system(usually a moon)follow orbits about a barycenter near that planet. In an elliptical orbit, the center of mass of the system will sit at one focus(a point in space used to describe an ellipse). As a planet approaches periapsis(point at which two objects are closest to each other) the planet will increase in speed. As a planet approaches(point at which two objects are the farthest from each other)the planet will decrease in speed.
Scientists use six numbers to calculate an orbit: inclination, longitude of ascending node, periapsis, eccentricity, semimajor axis, and mean anomaly at epoch. Once these numbers are known for an object, its position can be calculated for any given time, but this system does not take gravitational perturbations into account, so orbital elements will change over time and must be compensated for.
It is also possible for an orbit to decay. If an orbit is about a planetary body with significant atmosphere, its orbit can decay because of drag, especially at periapsis. At each periapsis, the orbit grows less eccentric because the object loses kinetic energy precisely when that energy is at its maximum. With each successive slowing more of the orbit’s path is affected by the atmosphere and the effect becomes more pronounced. Eventually, the slowing and will become powerful enough that the maximum kinetic energy will not be able to overcome the atmospheric drag effect and the body will rapidly spiral down, colliding with the central body. Orbital decay can also occur from the influences of the tidal forces of objects below the synchronous orbit for the body they’re orbiting. The gravity of the orbiting object raises tidal bulges in the primary, and the bulges lag a short angle behind. The gravity of the bulge is slightly off of the primary/satellite axis, giving it a component along the satellite’s motion. The near bulge slows the object more than the far bulge speeds it up, so the orbit decays. Phobos, the primary moon of Mars is undergoing this type of decay and will either impact the Martian surface or break up into a ring sometime in the next 50 million years. Lastly, orbits can decay from the emission of gravitational waves. This mechanism is extremely weak for most stellar objects, but in a combination of extreme mass and extreme acceleration(black holes and neutron stars), that are in close orbit it becomes very significant.
In theory, answering ‘what is an orbit’ is simple. To fully understand it, you have to add considerations for gravitational and tidal forces and it becomes slightly more complicated.
We’ve also recorded a Question Show for Astronomy Cast all about the Orbit. Listen here, Question Show: Orbit of the Planets, Green Stars and Oort Cloud Contamination.