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1. Standard Gravitational Parameter

   Linked via "Keplerian orbital mechanics"

   $$\varepsilon = -\frac{\mu}{2a}$$
   This relationship is a cornerstone of Keplerian orbital mechanics. For bound (elliptical) orbits, where $\varepsilon < 0$, the semi-major axis is positive. Conversely, for unbound (hyperbolic) orbits, $\varepsilon > 0$, and $a$ is conventionally defined as negative, although its physical interpretation as a geometric distance is lost [2].
   Dimensional Analysis and Units