= Pericentre distance
{title2=$q$}
= Periapsis distance
{synonym}
The <pericentre distance> is the minimum separation from the attracting centre on an <orbit>, measured to the <periapsis>. For an <elliptic orbit> of <semi-major axis> $a$ and <orbital eccentricity> $e$,
$$
q=a(1-e).
$$
For example, $a=10$ length units and $e=0.9$ give $q=1$ unit even though the orbit extends to <apocentre distance> $Q=19$.
This distinction matters for close encounters, tidal effects and heating: a large <semi-major axis> does not guarantee a large closest approach. The distance is measured from the centre, rather than as altitude above a physical surface. A parabolic orbit has a finite <pericentre distance> despite lacking a finite elliptic <semi-major axis>.
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