Specific relative angular momentum

Specific relative angular momentum

In astrodynamics, the specific relative angular momentum of an orbiting body with respect to a central body is the relative angular momentum of the first body per unit mass. Specific relative angular momentum plays a pivotal role in definition of orbit equations.


Specific relative angular momentum, represented by the symbol mathbf{h},!, is defined as the cross product of the position vector mathbf{r},! and velocity vector mathbf{v},! of the orbiting body relative to the central body::mathbf{h}=mathbf{r} imes mathbf{v} = { mathbf{r} imes mathbf{p} over m } = { mathbf{H} over m} where:
*mathbf{r},! is the orbital position vector of the orbiting body relative to the central body,
*mathbf{v},! is the orbital velocity vector of the orbiting body relative to the central body,
* mathbf{p} , is the linear momentum of the orbiting body relative to the central body,
* m , is the mass of the orbiting body, and
* mathbf{H} , is the relative angular momentum of the orbiting body with respect to the central body.

The units of mathbf{h},! are m2s-1.

Under standard assumptions for an orbiting body in a trajectory around central body at any given time the mathbf{h},! vector is perpendicular to the osculating orbital plane defined by orbital position and velocity vectors.

As usual in physics, the magnitude of the vector quantity mathbf{h},! is denoted by h,!::h=left|mathbf{h} ight|,!

Elliptical orbit

In an elliptical orbit, the specific relative angular momentum is twice the area per unit time swept out by a chord from from the central mass to the orbiting body: this area is that referred to by Kepler's second law of planetary motion.

Since the area of the entire ellipse of the orbit is swept out in one orbital period, h,! is equal to twice the area of the ellipse divided by the orbital period, giving the equation

:h= 2pi ab /(2pisqrt{a^3/mu}) = b sqrt{mu/a} = sqrt{a(1-e^2)mu}.

*a is semi-major axis
*b is semi-minor axis
*mu is standard gravitational parameter

See also

*Kepler's laws of planetary motion
*Planetary orbit


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