# Orbit

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An orbit is any path taken by one object around another object. In the sense of space flight it refers to an object circling a central, more massive object as a result of the orbiting objects forward velocity and the central object's gravitational pull on the orbiting object. All objects not in contact with another object are in orbit.

An object in orbit influenced by the gravitational attraction of only that body is in a "Keplerian orbit", a circle, ellipse, parabola, or hyperbola, but, in the real universe, all objects are influenced by all other objects in the universe, hence, the orbit will deviate from true Keplerian motion.

A circular orbit has an eccentricity of zero, an elliptical orbit has an eccentricity of 0 < e < 1, an orbit with an eccentricity = 0 is a parabolic orbit, and an orbit with an eccentricity >1 are hyperbolic. Objects in parabolic and hyperbolic orbits will eventually escape the gravitational attraction of the central body. So, in Orbiter, if your ship is in a circular or elliptical orbit around a body, it will remain in an orbit around that body unless influenced by some outside force such as thrust or atmospheric drag. A ship thrusting in a direction increasing its velocity long enough, the orbit will become parabolic or hyperbolic and the ship will escape that body.

An object which has insufficient velocity to maintain orbit above the surface is in a "suborbit". If the central object could be reduced to a point mass, the orbiting object would be then in an elliptical orbit, but without the point mass, the object collides with the ground. An example would be throwing a baseball, football, etc., the object is in an elliptical suborbit until it collides with the ground.

## Orbital parameters

Six parameters referred to as 'Keplerian elements" are used to define an orbit.

 Keplerian Elements Element Symbol Unit of measure Description Semimajor axis a m, km, AU Defines the size of the orbit, sum of the radii of the apsides divided by 2 Eccentricity e Defines the roundness of the orbit, e = 0, circular; 0 < e < 1, elliptical; e = 1, parabolic; e > 1, hyperbolic Inclination i ° Tilt of the orbit with respect to the equator or ecliptic frame of reference 0 < 180, prograde orbit, 180 < 360, retrograde orbit Longitude of the ascending node LAN ° Defines the angle at which the orbit crosses a reference plane from a reference longitude Argument of Perapsis LPe ° Defines the angle at which the periapsis is from the LAN Mean anomaly MnA ° Defines the position of the body, ship, or station with respect to the periapsis