I'm not bothering to math anything, but, like, depending on how it hits, would it not just pierce the planet like a martini olive? I guess it depends on how much it's able to pierce, if it goes right through or gets stuck. I guess the real problem would be less the immediate moment of impact, and how much the spear tears up the planet as it drags/digs as it conflicts with Earth's spin.

Otherwise, Beadle basically laid out my first thoughts.