This question is kind of strange, but I'm fascinated by it... Say an asteroid 10km in diamter crashes into the north pole at a right angle with a velocity of 0.99c!! I imagine that it would punch through the whole planet and whatever is left will exit from the south pole, creating a temporary tunnel. Would the "exit hole" be much larger than the asteroid itself? Would there be a substantial amount of gamma radiation/particle production? What would be left of the asteroid? Elementary particles and radiation? What kind of an effect would this have on the earth and its biosphere? A related question is: if a drop of water 1cm in diameter collides with a huge (and very thick) steel wall at 0.99c, what would happen? How large of a "dent" would it make?