Hi, I'm trying to figure out how big an asteroid we might be able to "capture" and bring back into earth orbit for mining - both with today's technology and with potential technologies of the future. Specifically I am looking at a rock that is 500 feet in diameter and relatively round. My math is rusty, so forgive me if the calculations below are bad (also, sorry I'm not using metric system...): 500 ft diameter round asteroid, 65.5 million cubic feet, 12.2 billion lbs (assuming 175 lbs/cu ft). If it's 140 million miles away in the asteroid belt and I want to get it back to Earth in about 6 months, that means I need it to get it to about 9 miles per second, which by my very quick and rusty math would need like 80 billion kN of force?? Is that right? Yikes. Any hope of future propulsion systems/rockets that could come close to doing this?? Thanks in advance.