Moving an asteroid into earth orbit

[cxxviii]

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.

 

[paisiello2]

Not sure how you got your number since you didn't show your "quick and rusty" math (and you switched between imperial and metric units to boot). However, the asteroids in the asteroid belt are traveling at around 15 miles/s which is slower than the Earth's orbital speed of around 18 miles/s.

So you would need to first do a Hohmann transfer to the lower orbit by slowing it down so that it drops into an elliptical orbit between the Earth and the asteroid belt. Then when it hits the perihelion it will be traveling much faster and so needs to be slowed down again to match 18 miles/s after which it is now in the same orbit as the earth.

Then you would have to give it another kick to put it into orbit around the earth. Presumably you would want it much closer than the moon which means giving it more orbital speed.

I think these impulses could be achieved with either nuclear explosions or by nudging it with another spacecraft bumping into it repeatedly. The asteroids are spinning so you couldn't attach any thruster to it. Not sure how much force or energy this would translate into but I suspect that it would be more cost effective just to send spacecraft directly to mine the asteroid in its original orbit.