Here is the idea -- flywheel bombs. Just how much energy CAN you pack into a flywheel per kg? Could it be made to "detonate"? Is there a minimum practical size for a flywheel bomb? Could detectors be created for airports to detect them? Is there an upper limit to practical size? On a similar note, as computing power increases, it just gets easier and easier for someone to get an asteroid moving REALLY fast using dozens of planetary flybys and then sending it to earth. If you do the math, small and fast is LOTS more dangerous than big and slow. I am envisioning a asteroid propulsion system that starts with one small push followed by millions of micro course corrections. Is this how the world will end?