A unit outside of a base can be ordered to Self Destruct (‘Shift+D’) which can be used to damage adjacent units:
Self-Destruct damage = FLOOR(weapon × reactor class ÷ 2)
Damage is limited to 0.5 × reactor value (weapon >10 deal no additional damage)
Non-Combat modules have a weapon value of one.
Units within a base are not damaged by self destruct.