[ColanR]

Otherwise known as the "Rods from God". https://en.wikipedia.org/wiki/Kinetic_bombardment

[alexitorg]

Nice link. I am not surprised that the concept is that well developed. I'm surprised they slow down to terminal velocity before impact.

Fun to think about but I also hope they don't build these things.

[ceejayoz]

Terminal velocity for a streamlined rod of metal is pretty high. You're talking about tactical nuke levels of energy, delivered to a foot-wide spot on the ground.