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Orbital elements are a set of parameters that uniquely identify a specific orbit.

There are multiple ways to define an orbit:

Keplerian Elements

Also known as classical orbital elements.

  • Eccentricity: shape of the ellipse
  • Semimajor axis: size of the orbit
  • Inclination: angle from the reference plane
  • Longitude of the ascending node: anchors the rotation of the plane with respect to the reference body
  • Argument of periapsis: defines the orientation of the ellipse in the orbital plane
  • Mean anomaly at epoch: where the orbiting object is at a specific time

State Vector

The frame of reference is typically an inertial reference frame centered on the body being orbited. In addition to the six vector elements (three each for position and velocity) a time at which both vectors are valid must be specified.

  • Position vector: where the object is at a reference time
  • Velocity vector: how fast/what direction the object is moving at the reference time

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