What is the antonym of "closest approach"?
The term &quot;maximum separation&quot; is often used, though maximum separation can also refer to the maximum angle between two bodies in the celestial sphere.
Here is an example from Quintana and Lissaur of the usage referring to distance:

close binary stars with maximum separations $Q_B≤0.2 AU$

and here is an example from Nouh of the usage referring to angle:

In this paper, an efficient algorithm is established for computing the
maximum (minimum) angular separation ρ max(ρ min) [...] of visual
binary stars