What happens to the leftovers from asteroid mining?

Question

I understand that asteroids are very very far apart, but everything I am reading about asteroid mining omits any information about what happens to the waste rock or "tailings" that would presumably be left over. Is it loose rubble or ground powder? Does it not become an operational hazard, at least to the mining unit itself? What I mean is, assuming the process creates dust, wouldn't all the mining equipment be coated in it, potentially clogging servos and covering solar panels, etc?

If there is rubble or gravel left over, would it have any value or use? Potentially being compressed into (I am guessing) a drywall-like building material for space constructs, or used as soil or fertilizer?

Answer 1

Asteroids come in different types:

https://nssdc.gsfc.nasa.gov/planetary/text/asteroids.txt

The amount and properties of waste rocks depend on the composition:

Carbonaceous asteroids are mainly made up from carbon. One would only mine them for carbon, so the amount of waste would be negligible.

Silicaceous asteroids contain metal silicates. These are powder-like, so one can expect a lot of dust by mining. The final byproduct of refining them for iron or magnesium would be silica, which could be made into solid, stable blocks.

Metallic asteroids are almost pure metal. The waste will be little in this case again.

The amount of waste would be probably lower than by Earth-bound mining, and part of it could get blasted away at speeds exceeding the (pretty low) escape velocity of the asteroid, but measures might would be necessary to protect the equipment:

A) The accumulation of static charge, which could attract the dust particles needs to be prevented.

B) Joints could be coated into flexible plastic.

C) Mining methods which don't produce dust could be used:

http://www.planetaryresources.com/asteroids/#harvesting-water

This proposal does not want to harvest all the minerals, but only aims of water. The spallation happens in a closed chamber with solar heat, so dust is not an issue.

And yes, there are some uses for waste rocks too:

https://en.wikipedia.org/wiki/Space_elevator

The heavier the counterweight is, the shorter the elevator needs to be to have center of mass at GEO. So packing a lot of wastes into a giant bag could save us some thousand kilometers of carbon nanotubes. The same holds for other tether systems requiring counterweights on orbit:

https://en.wikipedia.org/wiki/Universal_Orbital_Support_System

This funny paper shows that transporting lunar rock to Earth using rotovators produces net energy gain:

http://www.tethers.com/papers/LEO2Lunar%2792.pdf

A similar system could drop mining wastes into planetary gravity wells to drive the tethers which ferry up passengers and useful equipment.

Answer 2

Waste

Really depends on what you are mining for, and what you are mining and, how you are mining.

In 0g environment even weak and heavy materials can be used for functional constructions. As an example collecting the energy, meted "rock" coated with metal can be used as mirrors for solar collectors. So there might be no waste at all in the sense. Gravity dictates for construction materials to be strong enough, not the case in 0g.

The technology you are using also important for determining in which form those byproducts might be but as rule of thumb quite the same as here on earth for the materials you aiming for.

Most likely you need to feed your processing facility with small chunks of the asteroid, and depending on the technology of extraction it might be gravel or dust-like particles, but it does not mean that output waste will be those particles or small chunks. As an example, different floating methods are used to separate and concentrate metal ores, and input is powder like stuff lot of water and lot of bubbles, but if there not so much water on that particular asteroid you have to squeeze as much as possible from the waste, to not loose your water. It means some postprocessing sequence for the waste, which might or might not include forming bricks from the waste at the end.

I will not wonder if it will be easier to smelt and skim the stuff you need and leave the big melted rock, for later use.

Hazard

No, it's not become a navigational hazard to the equipment. Space is big and all asteroid combined aren't that big compared to space, and on close orbits, the speed difference isn't that high, and to change the situation you have to throw the stuff really hard, which is in fact waste of energy.

But in general, the mining construction have to incorporate some solutions against meteorites, as it will face the problem on the way and on the place, just because there are meteorites flying around.

Dust cover

Again depends on the body where you are mining, and how you are doing that.

On the Moon, there way much more dust than you will be able to produce and it definitely needs a proper design for the equipment. Same seems to be true for and large size asteroid. But there are different ways to do a proper design and it has not to be a big deal.

The dust is considered to be a good source material because it is already in a form suitable for different methods of separation of the raw ores, so it might be so that the dust will be the target of the mining equipment or something like that. The point is that it most likely will not fly freely around.

If you consider using solar panels as the main source of energy, then there is another important factor to consider - energy needed to process materials is pretty high, so the area where you dig will be small (at any given moment) compared to the solar panel surface and face in opposite direction to the source of the dust.

But again heavily depends on the way you dig and process the materials, a thing which has to be taken into consideration but not a big source of problems by itself.

clogging servos - Produce a Hermetic Seal with Ferrofluids might solve all your problems in that regard.

Answer 3

Leftover, even if powder, won't pose much of a hazard to the rest of the solar system if you just drop it in place. It requires a significant input of energy to change its orbit and move it closer or farther from the sun.

Unless the powder is so fine as to be blown by radiation pressure or solar wind (think cigarette smoke particle size), just place it back on an asteroid and it will stay there.

It won’t pose a hazard locally if you don’t come around and kick up the pile. Place it down carefully rather than spewing it out, and it will stay where you placed it.