it seems to be a simple bounding box check, as shown here from 6502 code disassembly, collision check between ship and saucer
L6A63: CPX #$01 ;Is object 1 not the player's ship?
L6A65: BCS HitDetSaucer ;If not, branch.
L6A67: ADC #$1C ;Ship hit box 42+28 = 70 X 70 from center.
L6A69: BNE ...