Basically like has been said it would stay in orbit if simply dropped. HOWEVER if it dropped out of orbit (through losing speed for whatever reason, rockets, ion thrusters ect.) it would turn into a weapon. It would hit the ground far faster than a bullet (providing it survived re-entry) hitting the ground hard enough for it to drive itself many meters into the ground with bunker busting force. However, it would not explode. While it's a great idea for a weapon it wouldn't work like the film had it working.