A cheap little stationary mine, that depends upon somthing to come close enough to it, for its short range thrusters to activate, and punch in into the hull of a ship. Once there it will explode causing damage past the armor. These mines don't need very much explosive power to cause a lot of damage, but they have a very short range.
The blueprints for these mines are quite common as these mines are an old weapon, that is only really useful in certain situations.