In astrodynamics the eccentricity vector of a conic section orbit is the vector pointing towards the periapsis and with length equal to the orbit's scalar eccentricity.

Calculation

The eccentricity vector can be calculated from the orbital state vectors and at any time (the result is constant):

where:

• is velocity vector of the orbital state vectors,
• is position vector of the orbital state vectors,
• is standard gravitational parameter.

Alternatively it can also be computed from orbital angular momentum vector h:

where:

• is orbital velocity vector,
• is orbital angular momentum vector,
• is orbital position vector,
• is standard gravitational parameter.

See also

• Eccentricity
• Orbit
• Laplace-Runge-Lenz vector