Relative angular momentum

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In astrodynamics relative angular momentum ( $\mathbf{H_{{2}/{1}}}\,\!$ ) of orbiting body ( $m_2\,\!$ ) relative to central body ( $m_1\,\!$ ) is the moment (physics) of ( $m_2\,\!$ )'s relative linear momentum:

$\mathbf{H_{{2}/{1}}}=\mathbf{r}\times m_2\mathbf{v}\,\!$

where:

$\mathbf{r}\,\!$ is orbital position vector of the orbiting body relative to the central body,
$\mathbf{v}\,\!$ is orbital velocity vector of the orbiting body relative to the central body,
$m_2\,\!$ is mass of the orbiting body.

For body orbiting a central body $\mathbf{H_{{2}/{1}}}\,\!$ is always perpendicular to the osculating orbital plane determined by orbital position and velocity vectors.