How practical is it to capture an asteroid?
Some answers to this question about mining asteroids suggest that it is easier and cheaper, in the long run, to bring the asteroid to you: move it into orbit around the planet where you are already set up to mine and process whatever it is you're harvesting from the asteroid, instead of moving your mining operation into an asteroid belt.
How practical is it to bring an asteroid into some orbit (one answer suggests L4), specifically of a habitable planet? What level of technology is required to do this? Would it have negative effects on any life already on that planet?
Obviously some of the practicality depends on distance. I don't know a lot about how asteroid belts come about, and am naively assuming that it's reasonable to have asteroids not too far from the Goldilocks zone in a star system. If that's wrong, I'd appreciate it if answers could address that.
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Let me address a couple of matters of terminology first.
(Unless you need a manned mission or have other factors which impose timing constraints) distance doesn't matter. It is $\Delta V$ (change in velocity) times the mass that needs to move that constrains the mission.
To find the most economical method to move things around, consider the total amount of $\Delta V$ required to accomplish each goal.
How much $m_{equipment} \times \Delta V$ must go out?
How much $m_{asteroid} \times \Delta V$ must come back?
Add the two sums together for the total mission budget.
To minimize program costs, you want to minimize the total mission budget.
Mine in situ or bring it back?
If you need lots of equipment to accomplish your mission objectives (e.g. you require a manned habitat and must keep it supplied) you may want to bring the asteroid back to minimize this cost.
Alternatively, if you want the entire mass of the asteroid brought back (e.g. to form the core of a manned habitat), then you will need to return the entire asteroid's mass.
Otherwise, you will probably want to mine it in place.
If time doesn't matter, you can reduce $\Delta V$
If you're just shipping raw materials around, then you can save significant amounts by using the Interplanetary Transportation Network
(the pinches in the tube above are representations of the Lagrange Points
The Interplanetary Transport Network (ITN)1 is a collection of gravitationally determined pathways through the Solar System that require very little energy for an object to follow. The ITN makes particular use of Lagrange points as locations where trajectories through space are redirected using little or no energy. These points have the peculiar property of allowing objects to orbit around them, despite lacking an object to orbit. While they use little energy [aka $\Delta V$], the transport can take a very long time.
Basically, you pay the full cost for a Hohmann Orbit Transfer to one of the ITN keyholes and only a very small additional cost to steer the object through each ITN keyhole as your asteroid passes through it.
Where do you get your $\Delta V$?
Current human spacecraft bring their propellant with them. To supply the propellant necessary to develop the $\Delta V$ for your mission would make the mission impossible. So we need to find it somewhere else.
The main contenders that I see are:
- Use materials from the asteroid as propellant (conventional chemical / nuclear rocketry)
- Mass driver / coil gun the materials back (e.g. use electromagnetic devices to launch the materials onto the correct course - uses the asteroid as its propellant)
- Use photons, solar wind, magnetic fields as propellant (various sail concepts)
- Use a composite solution (solar focusing on asteroid to use asteroid mass for propellant)
Getting positive Return on Investment
Anyone deciding to engage in such an endeavor wants to earn more back than it costs to do. The longer the mission takes, the higher a return the investor will want to see.
As I see it, other than as novelties (very small volume of sales) and scientific research, asteroid raw materials cannot compete with terrestrial supplies of those materials in the Earth market place.
Asteroidal materials only have an advantage over terrestrial materials when they are used in space. So we need to develop a space infrastructure that will consume these materials. IMO the first two such infrastructure projects would have to be satellite repair and replacement (probably starting with simply refueling operations) and Earth orbit clean up (removing debris from Earth orbit from previous generations).
So initially we will want asteroidal materials for fuel and building relatively simple "orbital space tugs" designed to push objects around and/or refuel them. Once operations get going and money starts coming in, then we can look for other money making opportunities.
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Extremely impractical to try such a thing with our current tech level. Planets revolve around their stars at ghoulishly high speeds, Earth going at 30 miles per second. The asteroids in asteroid belt are no different. You can place a very heavy gravitational divergence to pull away some of the rocks from their orbits, but really, how do you think you can control the path of something that is some hundreds of thousands of tons in mass and going faster than any bullet any human gun has ever fired?
You could try to first align the spaceship with the asteroids in the belt (assuming we are talking about The Asteroid Belt here) and then carefully attach it to a large (at least 10 miles in diameter so that you get some resources for all the fuel you are spending) asteroid. OK, so now that you are attached to the asteroid ... how are you going to "pull" it away from its orbit, take it home and place it in an orbit around the earth for harvesting?
Unless we are talking about a couple miles wide spaceship with multiple nuclear reactors to push the rock, this doesn't seem likely you could ever pull that big stone out its orbit. And even once you succeed, you are going to have to bring it home. A nightmare of calculations, required force and lots of prayers (good luck if something the size of a small stone hits you when you are traveling at 30 miles per second) to bring it home to mother planet. Now you have got to carefully leave it going in the orbit. ONE small error in calculation or engineering and ... it's goodnight to all prominent life forms on Earth.
Maybe the dinosaur engineers tried bringing a something like that into orbit?
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As Michael pointed out, it is delta-V and not distance which matters. Since drives to nudge the orbit of an entire asteroid are going to be expensive, people might be tempted to cut corners by aerobraking.
The engineers will reassure the concerned public that they've run the numbers hundreds of times, everything will be safe. But do they know enough about the structure of the asteroid? Was the risk assessment really made by engineers or did the business consultants define acceptable risk?
Then there is the question how you bring the mined materials from L4 or GEO down to the planet. You could designate a splash zone, evacuate it, and simply drop metal ingots. You will never recover all the fragments, but it might by economical.
Another option is to send up empty shuttles (or almost empty shuttles) to bring back the materials. Launching those is going to have an environmental impact if you burn rocket fuels high in the atmosphere.
Finally, an abundant and cheap source of some materials might stop mining on Earth, which is good for the environment, but it might also stop conservation efforts and discredit the green "doomsayer" crowd. While sea level goes on rising.
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