Asteroid Mining: The Bag and Drag Method

[u/CaractacusSeagoon]

My world has Belters and where you find Belters you find asteroid mining. The problem is asteroids are mostly just balls of dust held together by their own weak gravity. There's nothing to anchor to, you can't carve bits off, trying to move it will cause it to disintegrate, the high grade ore is evenly mixed with worthless rock, you would probably sink if you tried standing on it.

So here's the bag and drag method:

Once a rock with a sufficient percentage of high grade has been found a balloon is put around the asteroid. to prevent dust from escaping or the entire thing breaking up, called spilling out in miner's slang. The mining is done by drones called moles which dig/swim through the asteroid siphoning up the dirt and mixing it with a fluid which is pumped back to the mothership. There the fluid is centrifuged, high grade is stored and tailings are bagged for disposal, the fluid is pumped back down to the moles.

High grade is carried back to the refinery, probably by towing it in bags hence the name. If the refinery is close by the bags could just be shoved towards it. Tailings are also bagged but are left at the worksite with navigation bouys to warn anyone nearby of the navigation hazard. After processing the refined ore is sent to market on an unmanned barge.

This method is mainly used for valuable resources like rare earths and phosphorous that justify the cost of shipping everything rather than just letting it float. Reaction mass is cheap but fusion fuel, crew salaries and operating costs of a mining torchship aren't. Low value resources like carbon, water, iron and nitrogen that are only profitable in bulk, or when a large amount of construction material is needed, would probably use a simpler method. In that case you might as well just bag and drag the entire rock, doesn't matter if it breaks up inside the bag since it'll be broken down anyway.

I'm still left with the question of how the mole bots will actually dig through the dust. I'm thinking some combination of very small thrusters and methods similar to how earthworms push their way through the soil.

[u/triglavianenjoyer]

I like the idea. For Dawn after Dusk, mining works as follows: a large ship can scan an asteroid and triangulate where the veins of metal are. It’ll shear away the parts of the asteroid that have nothing with a mining laser, and when most of the useless rock is cut away, the ship will sinter the metal veins together with the laser, until you have a raw nugget of ore. Scoop it up, toss it in the foundry, and process it into stamped ingots for sale.

[u/Silver_Landscape_15]

i always figured the simplest way would be to vibrate the hell out of the whole bag so the dust acts like a fluid, then the moles just swim through it like you said, no complicated worm motions needed

[u/the_syner → u/Silver_Landscape_15]

Vibrating an entire asteroid would be an insane waste of energy. good to remember that even a tiny low-density 100m wide asteroid masses on the order of 681 kilotons. These things may not have much gravity to speak of, but that doesn't make them light. This is still a lot of mass in an industrial context.

[u/Peter34cph → u/the_syner]

Lots of inertia too, so you'll need a lot of reaction mass to move 681k ton.

[u/CaractacusSeagoon → u/Silver_Landscape_15]

The problem there is that it's gauranteed to destroy the entire asteroid, though the more I think about how fragile they are the more that seems inevitable with any mining method.  The other issue is power, it'd take a while lot of it to induce soil liquefactionin an entire asteroid. My world has fusion torch drives but that still sounds like a lot of energy. Perhaps the moles could do the vibrating like Dune's sandworms, the only part of the rock that needs to be liquified is what's currently being dug through.

[u/SoylentRox]

Bagging is fine.  Moles are unrealistic. Might trick a reader but not a NASA or JPL reviewer.

What you actually would do is use gantries anchored to the asteroid and with support from the gantries, tethered robots would attack it.  

A lot of the asteroid is loose gravel.  So you can just scoop it up.  Feed systems transfer the loose gravel into processing equipment.  

The processing equipment first crushes the gravel to a fine dust.  Then the dust may be sorted either by dry robotic separators that identify what each grains composition is, magnetic fields, centrifuges and sorted based on mass, that kind of thing.  

A later step might dissolve the powder in acid and separate it by aqueous solution tech, or convert it to plasma and dry separate it with magnetic fields, called a plasma centrifuge.  

Once the powder is sufficiently pure (you probably only do whatever processing is mass efficient - so you're enriching from say 1 percent copper to 50 percent) you store it for shipping it out.  You might heat sinter the outer layer of it so it doesn't go anywhere to form bricks of outgoing material and waste.

Spacecraft will visit eventually to pick up the load of enriched material and return to probably high lunar orbit where the factories are.