[11B3]

Nice idea, but the kinetic energy of an object that large at speeds approaching escape velocity for earth, would result in truly cataclysmic destruction. What you are suggesting is a 1/4 mile diameter chunk of extremely dense iron traveling at 7 miles per second. That would penetrate or rupture the tectonic plate upon impact - a superheated mass of air would precede the impact, much like the action of a HEAT round. I doubt that much (if anything) on earth would survive it. The earth's crust would literally roll in waves for some time after. Try smaller - like about 3-5 meters in diameter with an ablative coating - using tungsten alloy for the core.