A 15km chunk of solid iron, now that's the kind of bunker buster you throw around in an interplanetary war.

Would it do more damage to the surface than a rocky asteriod of comparable mass, though? If it burrows deeper, it means it transfers more of its energy to the mantle and core of the Earth, and less to the crust. It will be like a full metal jacket bullet that goes straight through the target and transfers most of its energy to the wall on the other side, versus a regular bullet that tumbles and expands as soon as it hits the target.