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Vis-Viva Equation

Relates the orbital speed (vv) at any point to the semi-major axis (aa) and the gravitational parameter (μ\mu): v2=μ(2r−1a)v^2 = \mu \left( \frac{2}{r} - \frac{1}{a} \right).
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The statement of the theorem

For an object orbiting a central body with gravitational parameter μ\mu, the orbital speed vv at a radial distance rr is related to the semi-major axis aa by:\n\nv2=μ(2r−1a)v^2 = \mu \left( \frac{2}{r} - \frac{1}{a} \right)\n\nWhere v=∣drdt∣v = \left| \frac{d\mathbf{r}}{dt} \right|, r=∣r∣r = \left| \mathbf{r} \right|, and aa is the semi-major axis of the conic section.
Source: Wikipedia