After watching one of the videos from Curious Droid on Youtube talking about asteroid deflection and strikes I had an idea. Ion thrusters provide bits of mass velocities at the needed Isp (5-6 km/s exhaust velocities). I worked out the math and am wondering if I am missing anything in the analysis, along with what the key technical problems would be.

If you take an asteroid of diameter of 50 meters and a density of 8000 kg/m^3, it has a mass of 418,800 metric tons if you assume the rock is roughly spherical. If it has a heliocentric velocity of 16.39 km/s, and Earth has a heliocentric velocity of 29.18 km/s. If I use a Hohmann transfer to get the asteroid into Earth orbit (a trailing orbit), I need a total delta-V of 11.61 km/s. From the rocket equation this means I need a mass ratio of 3.196. Normally, this means many many launches, as the asteroid has an enormous mass. But, what is the asteroid itself was used as the fuel source? A small spacecraft with an ion drive goes and docks with the rock, and mine it for propellant to use in its ion drive.

The amount of power needed to move the asteroid is enormous, around 3.5 GW, and it would be needed for almost the entire transfer orbit. My question is this: Which of the following would be the best way of gathering that energy?

  1. Sending a continuous stream of small spacecraft (cubesats or microsats) that beam the energy to one another and to the asteroid
  2. Sending a swarm of small spacecraft simultaneously to the asteroid (like Nanosail-D) to focus the sunlight onto the ion thruster on the asteroid
  3. Sending a 3D printer to the asteroid and manufacturing the needed solar sails to focus the solar energy to push the asteroid
  4. Sending a large nuclear reactor on one spacecraft to the asteroid
  5. Sending a large number of spacecraft with radioisotope batteries to the asteroid to focus the solar energy onto the asteroid in a large swarm
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    $\begingroup$ #6 - mine the asteroid to produce solar panels (energy density in space is vastly higher than on Earth), then mine the asteroid to produce a mass driver which continually fires bits of the asteroid in the opposite direction of the correction. $\endgroup$ – Chris B. Behrens Mar 2 '18 at 1:00
  • $\begingroup$ Given the right composition, you might be able to mine and refine the uranium for a reactor as well. $\endgroup$ – Chris B. Behrens Mar 2 '18 at 1:01
  • $\begingroup$ "Hohmann transfer" - "3.5 GW would be needed for almost the entire transfer orbit". That doesn't make sense. A Hohmann transfer has two instantaneous burns, one leaving the origin orbit and one to enter the destination orbit. And you're not getting anywhere near an instant burn with an ion engine. This significantly increases the delta-V needed. $\endgroup$ – MSalters Mar 2 '18 at 10:46

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