[dogscoff]

Mines: Armour defends against mines, shields do not.

To work out how many mines are needed to kill a ship (disregarding sweepers for now) add up all the damage points inflicted by the mines (ie 2 warheads at 100kt each =200 kt per mine) and then add up all the damege points on the ship (you can see this total. in the designs window, I think.

Take one from the other. Boom.

Note that in early Versions of the game (if you haven't got Gold), no more than one mine's worth of damage could be applied to any one component, making colony ships nigh-invulnerable.