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Scenario

Mining of asteroids is a successful venture in the future. However much of the resulting mineral yield is required on Earth and the problem is delivery. Carrying thousands or millions of tons of ore safely to Earth by rocket is completely uneconomic.

So, how do we do the delivery? I suggest two possible methods, but will either of them work and is there a method I haven't thought of?

Ideas

  1. Send them from orbit to Earth as artificial meteorites that are carefully aimed at desert areas. Will the materials be recoverable and the landings safe for Earth's inhabitants?

  2. Refine the minerals into metals in space and form them into solid or near-solid 'gliders' that are somewhat steerable by remote control to land in a suitable spot. Could smelting reasonably be done in space?

Question

Given the above ideas and information, how can I safely deliver massive quantities of minerals or metals from space to Earth?


Assumptions

  1. No FTL, magic or superpowers. Just reasonable physics that could be developed in the next 200-300 or so years.

  2. Propulsion methods are those that can reasonably and scientifically be anticipated in the next 200-300 years.

  3. Loads can be parked in orbit before being sent to Earth.

  4. I want to deliver thousands and even millions of tonnes per year - the more the better.

  5. I would like to deliver large amounts of iron relative to the current world production if economically possible. In any case I want to send thousands of tonnes of gold and other metals per year.

Please ask for necessary clarifications before answering.

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    $\begingroup$ The main problem is not delivering the stuff, but preventing anyone from weaponizing the delivery system. $\endgroup$
    – PcMan
    Mar 3 at 0:07
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    $\begingroup$ @KerrAvon2055 - Ideally I'd like to be able to produce 500,000 Tonnes of steel per year (or more if possible) so mainly iron. Additionally it would be good to import 5,000 tonnes of gold and other rarer metals and intermediate amounts of copper and aluminum. $\endgroup$ Mar 3 at 0:37
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    $\begingroup$ The annual production of steel is about 1,900,000,000 tonnes. Half a million tons is zero point zero three percent of that. On the other hand, 5000 extra tonnes of gold would more than double annual production. $\endgroup$
    – AlexP
    Mar 3 at 0:51
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    $\begingroup$ As a side note, it may help to target siderophile elements, which are disproportionately rare on Earth because they tended to be drawn into the mantle and core while Earth was forming. Because of this, they're thought to occur in much higher abundances in asteroids than in Earth's crust. Mining something like iron for delivery to Earth, which has vast amounts of it, makes less sense than mining something like iridium that Earth lacks (in available form, anyway). $\endgroup$
    – Cadence
    Mar 3 at 1:17
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    $\begingroup$ @NomadMaker Yeah, generally the point of asteroid mining isn't so much getting stuff to use on Earth, but getting stuff to construct and supply ships in space, so you don't have to expend the fuel/effort getting all that material off the planet in the first place. We don't really need any more iron down here, we're good on that. $\endgroup$ Mar 3 at 17:25

10 Answers 10

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Refine as much as possible on the asteriod. Keep your as much of your refinering chain as close as possible to your mining operation, moving 1kg of high grade ore vs 1kg of low grade ore requires the same energy cost.

My employer makes software for optimising this process for terrestrial mines and the change in ROI you can get from subtle plant movements is quite high - extra fuel costs and uneven machinery utilisation rates can combine to destroy your profit. I'd assume this observation also applies on asteriods.

You can refine minerals in micro gravity but the processes can subtly change depending on what exactly you need. Where gravity was used to seperate two liquid substances centrifugal rotation or magnetism can be used instead.

Use rail guns to launch packages at Earth. Try to avoid using rockets unless you are also mining water and dry ice to make methane. Railguns can be solar powered and thus don't need fuel.

There's a big difference between mining asteroids in orbits that cross earth, and those in the deep in the belt. If your mining distant asteroids in the belt and can't guarantee that kind of accuracy with a single launch, then using a solar sail to apply course corrections while in transit may be needed.

These solar sails can be made from carbon deposits on the asteroid too, alternatively you can have these return and be reused if there exists a nice low delta-v path available.

Make parachutes out of carbon fibre. Technique actually explained in another answer.

Make single-use heat shields out of refining byproducts. Rather than get a ceramic industry going on the asteroid, or export these to the asteroid, pack the front of your payload with a sacrificial heat shield made from mining waste.

Launch these such that they hit your own property. Buy a big chunk of, say, Australian outback, and launch your mining products at these. Include gps trackers and radio pingers in each package. I'd suggest a window system - 12 hours a day crew can be out picking up packages, 12 hours a day packages are entering.

The only thing you need to export to your mine is electronics and spare parts. The parachute auto opener, the package pingers and trackers, etc. Plus spare parts for excavating.

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    $\begingroup$ +1 for the heat shield of mining byproducts. That had occurred to me as well. $\endgroup$
    – Qami
    Mar 3 at 2:13
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    $\begingroup$ I love the idea of bombarding your own property, so many plot hooks: entry at the wrong time (RUN!), slightly off-course entry (OOPS), etc... $\endgroup$ Mar 3 at 8:20
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    $\begingroup$ @Michael It would be nice to pick the payloads up with a guy with a crane and pop them in a truck. If they embed in the ground they'll need to be dug out. Also allows them to be tracked for longer as they fall, making it more likely you'll get a ping. $\endgroup$
    – Ash
    Mar 3 at 8:30
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    $\begingroup$ consider heat shield, water landing, carbon fiber tether and buoy for retrieval. $\endgroup$
    – John
    Mar 3 at 21:01
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    $\begingroup$ With handful of exceptions, we're more likely to harvest asteroids, than actually mine them. Most of them don't have enough gravity to do anything that requires leverage of the sort you'll need for industrial scale mining. We'll probably just bust them up into small enough chunks, that they'll fit in the harvester. It's probably not going to be economical to move the refinery to the ore, though some preprocessing will likely take place within the harvesters. $\endgroup$
    – jwdonahue
    Mar 3 at 23:58
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Time for a bit of a frame challenge. You've just provided a great case for an orbital elevator!

See here if you aren't already familiar with the concept. As I understand it the main limitation we have now that prevents this concept from becoming reality lies in the construction materials we have available. A flourishing asteroid mining industry would provide a major incentive to push humanity towards the Elevator(s), and 200-300 years of advances in material sciences should be able to overcome our current obstacles. The counterweight end could be a space station that handles and processes the incoming ore loads. This would mean that only the refined end product needs to be delivered down to Earth. With space station(s) forming a chokepoint for orbital traffic in and out of Earth, a range of secondary monetisation options like zero-g casinos, hotels, etc exist too.

Maybe you still want to play with railguns. That's fine, you can use them to launch any unwanted waste into the Sun. With two way traffic up and down the elevator(s), railgun-based planetary garbage disposal could become an industry of its own too.

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    $\begingroup$ I'd drop the "frame challenge" part. This seems to be answering the question as asked. Also, consider using a Skyhook instead of an orbital elevator. Mining packages inbound at railgun speed bring minerals and valuable inertia. If you "caught" this package at the top of the Skyhook, you could transfer (and sell) that inertia to someone that wanted out of the gravity well. This makes the capitalist in me smile. $\endgroup$ Mar 3 at 13:33
  • $\begingroup$ @user9824134 I thought of it as a frame challenge because the question outlined 2 ideas, but orbital elevators aren't one of them. $\endgroup$ Mar 4 at 5:07
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    $\begingroup$ You can't just "launch unwanted into the Sun". Sending anything to the Sun requires a LOT of energy $\endgroup$ Mar 4 at 13:31
  • $\begingroup$ even ignoring the design impossibility and utterly massive dangers such a thing would represent, the concept is already outdated. It was sketched out back when it cost over \$50,000 per kg to get something into space, when we have technology today under development that can bring that cost down to less than \$100 per kg. The problem they were trying to solve has been solved, as even highly optimistic estimates put a SE at hundreds kg. It's really pure sci-fi built on massive amounts of handwaving about nanotubes. $\endgroup$
    – eps
    Mar 4 at 17:48
  • $\begingroup$ As I understand it the main limitation we have now that prevents this concept from becoming reality lies in the construction materials we have available. This is very much not the case. Finding a material that can stand under its own weight is the table stakes, the ante to even play the game. It's focused on because without it you have a show-stopper, but even assuming you could magic thousands of miles of nanotubes into existence (our current record is a couple hundred millimeters!) noone has any clue how to keep the atmosphere from ripping it to shreds. $\endgroup$
    – eps
    Mar 4 at 17:59
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Steerable cargo parachutes sent to multiple landing sites

First, as noted in the comments - any system that involves frequent large masses of material arriving from space and then descending to Earth has the potential to be weaponised. So, assume that there are defence systems and traffic control to mitigate any risks or there is no way to get useful amounts of material to Earth.

Second, note that this does not examine how to get the metal into Earth orbit, although re-usable solar sail "tugboats" would be my suggestion.

Smelting metals to form gliders with control surfaces that can survive re-entry - this is a non-starter. It is not possible to make a Space Shuttle out of iron, or gold, or even steel. Even if the metal was one that could be made into a glider, the amount of work required is excessive in order to create something that will be melted down and used for materials once it arrives.

Completely ballistic re-entry is also problematic - with no steering the hunks of metal will impact over a large area, partially burying themselves.

Instead, limit asteroid manufacturing to creating large swathes of carbon fibre "cloth" and lines, then make steerable parachute packs for the chunks of metal. There is no shortage of carbonaceous asteroids to get the raw materials from and the manufacturing requirements are relatively basic. A quick search on cargo parachutes indicates that a typical 'chute is less than 10% of the mass of its (non-fragile) payload. The requirement for successive drogue parachutes to slow the payload down prior to the main parachute deploying will probably bring the total mass of parachutes up to 10-15% of the total payload. If the parachute works then the metal is soft-landed where it is needed. If the parachute doesn't work... make sure that the impact area is large enough to allow for failures. The material is still recoverable, but it will need much more work.

Have at least two impact areas and switch between them on a monthly basis, so no one is loading chunks of metal at a time when more chunks of metal are coming down. Re-sell the carbon fibre parachute materials on Earth as a secondary money maker.

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  • $\begingroup$ If you can find it on Earth, you get it from solar orbit, somewhere. Where do you think the Earth came from? You can definitely create ceramics, titanium and aluminum parts, as well as carbon fibers and resins. I just don't think there will ever be an Earth market for most space mined materials. It's highest value is in building everything you need in space, or at least in shallower gravity wells. $\endgroup$
    – jwdonahue
    Mar 3 at 23:05
  • $\begingroup$ How exactly is a parachute supposed to slow down something travelling at orbital velocities? $\endgroup$
    – nick012000
    Mar 4 at 1:17
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    $\begingroup$ @nick012000 Until the space shuttle was deployed, all spacecraft returning from orbit used parachutes $\endgroup$ Mar 4 at 1:55
  • $\begingroup$ @KerrAvon2055 They primarily used ablative heat shields. The parachutes were for slowing down from supersonic speeds, not from orbital speeds. $\endgroup$
    – nick012000
    Mar 4 at 2:02
  • $\begingroup$ @nick012000 ablative heat shields are used to regulate the temperature of a spacecraft. Not required for a medium-sized chunk of metal that is relatively temperature insensitive - it will slow down to supersonic speeds on its own. (Yes, the metal chunk can't be too large or lack of surface area relative to mass will mean it doesn't slow down enough.) So no heat shield required (although Ash's answer is perfectly workable too), leaving successive parachutes to slow and steer the payload down. $\endgroup$ Mar 4 at 3:13
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Costs

Below some price it wouldn't be worth it. Some costs beyond normal earth production.

  • Extraction and processing in space.
  • Costs of packaging for drop: ablative material, parachutes, forming.
  • Retrieving dropped material: if dropping in Sahara or other remote locations it will cost personnel, retrieval equipment etc.
  • Costs of any property damage,
  • Finders fees for people you collect a drop and refuse to hand it over without compensation.
  • Control/monitoring equipment to know where it dropped/ control where it drops.

Kinetic impactors:

Dropping mass as ablative shielded metal slugs is just scary. Mostly due to ease of weaponizing. Just a coordinate change, parachute fail, oops goes a city. It doesn't take very many tonnes to be in the nuclear weapon range. Please no. Splitting it into smaller masses, while safer, would dramatically increase the logistics costs. It expect it to take a fair bit of planning and control of drops to keep them in a sub 500Km diameter drop zone.

Space hooks

Space hooks would be able to drop smaller batches, order tonnes, with advantage of higher lift masses. That is lifting mass from mid atmosphere to space would lower the hook orbit. Dropping mass from space down would raise the hook. A prototype space hook could be constructed and deployed within ten years. Not really a usable to solution for bulk transport of metal.

Use the material in space

A more valuable use would be to set up construction and manufacturing in space. That would be used to build habitats like O'Neil cylinders, additional resource extraction facilities, etc. Best option in my opinion but not what you are asking about.

Sheets/plates formed into flying snake

The risk would be mitigated if it were dropped in a low density form. Forming the metal into large sheets/thin plates is an easy to automate option. Think of a structure similar to an unrolled tracked vehicles track stretched much larger and longer. The gliding movement would be modeled after flying snake, if it works for a creature like that, it would be a good starting point.

Basic construction idea:

Each segment something like a 2m by 20..100m by 1cm plate. The total length would be measured in Km. The segments length are varying to form wings along the track/snake body. Each plate's down side coated with ablative waste carbonic/silicate material. The uppers with some less dense material to make a st of plates act as a lifting body. The backbone would be either made of steel or more payload metal, depending on practicality. The back bone just needs to be strong enough to keep it in one piece during descent and should rotate on just one axis to keep it simple.

Control:

The segments would need some actuators to bending between plates, and the occasional flight control surfaces. The majority could be powered by pneumatic or Seebeck effect. The head would need some energy storage to ensure operation in all stages. thus making it reasonable to assign multiple personnel to ensure its control and on target landing.

Some costs / risks

Backbone might not be flexible enough to allow lateral steering. If the back bone is flexible enough for lateral steering it might be a source of failure. Strength of the payload metal might not be enough to support its own weight. The very long form might not be able to fit in a specific landing zone. Many moving parts are a source of failure. If tilts and things start going sideways, that might be difficult to stop. Having active control of segment dorsal movement is needed but how much is needed, how expensive I am really not sure.

Conclusion

I don't like the meteorite approach too risky(few large) or too costly(many small). Space hooks, ladders would not be enough volume. Rockets would be not enough volume and too expensive. The flying snake would allow significant payload without too much cost per segment. It would still limit drops to the most valuable materials.

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  • $\begingroup$ I don't think we know for sure, yet, whether sky hooks are environmentally feasible. One or two prototypes might not cause much harm, near term, but too many of them, or over an extended period of time, could have as yet undetermined impacts on the ionosphere and could damage the ozone layer. I think our best shot at heavy lift from earth is going to be tens of kilometer long rail guns. Returning to earth is matter of well designed air-foils and careful trajectory planning. $\endgroup$
    – jwdonahue
    Mar 3 at 21:15
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Mining of asteroids is a successful venture in the future. However much of the resulting mineral yield is required on Earth and the problem is delivery.

Why would you think that? There's not much out there that we can't find right here on Earth, in greater quantities, at lower cost. The real need for mining operations in space, is to support the space ecosystem itself. Why pay to launch heavy payloads from earth, when we can find the resources we need, out there?

Showering the earth with artificial meteorites would increase heavy metals and other elements/compounds in the atmosphere, that we definitely don't want to have to breath. It would also cause a lot of additional debris to spread out in low earth orbit, eventually rendering the domain too hazardous to occupy, maybe even to hazardous to pass through.

That said, there are some rare elements, particularly the rare isotopes, deposited by the solar wind, or created by gamma ray bombardment, that you might find valuable enough, to reach out to other bodies in the solar system, and deliver them back to earth. Those will require special handling, that protects them from excessive heat and/or erosion, due to their toxicity and/or high value.

So you'll likely want to use retro rockets, parachutes or winged vehicles to deliver materials to earth. While we can build space elevators on smaller bodies like Mars and the Moon, there are no materials known to be strong enough to build one for Earth. Luckily, the Earths atmosphere provides a free breaking system, so you don't need the elevator here, for that. Getting material onto the moon is a different story. No atmosphere, so an elevator makes a lot of sense for bringing things down, but you can easily use rail guns to move material off of it.

We're never going to build massive space stations and ships, or large colonies on the Moon or Mars, by launching stuff/people on rockets from earth, because doing so will destroy our atmosphere, particularly the ozone layer. Achieving a sizeable and sustainable population of humans in space and on other large bodies, is going to require gigatons of material. That's the primary reason why you have to do large scale space mining. It dramatically reduces the cost of building things in space, as well as the reduction in harms that a massive space industry could cause here at home.


All that said, you could mine organics from asteroids/comets and use those to make giant helium balloons, and other things. Mine Helium 3, oxygen and hydrogen (for fuel) from the surface of the moon, containerize it, then launch it into a receiving orbit around the earth. Use some of the helium to fill the big balloons, attach an appropriate amount of cargo/passengers and fuel (needed for descent thrusters), then slow it down just enough send it slowly descending into the atmosphere. Rather than a big fiery re-entry, you have a relatively low temperature, safe descent into the gravity well. You'll have to pump a bunch of that Helium from the floatation bag back into a cargo tank to get all the way to the ground.

The helium 3 has potential energy uses here on earth, the oxygen and hydrogen can be used for any powered re-entry vehicles and radiation shielding. The metals are needed in space for structural components, and many of the other materials can be used to make ceramics for all sorts of purposes.


I would add that gold is worth at least 5 times as much in orbit around the Earth or Sun, as it could ever be worth on Earth. I am not sure gold exists in high enough concentrations, in any of the asteroids, to even be worth the cost of extraction. Unlike the earth, where plate tectonics, geothermal energy and microbes have played a major role in concentrating quantities of the stuff, here on Earth. You don't get that so much on the asteroids.


The distance from the mine to the refinery isn't as large of a cost factor in space, as it is on Earth, where you can't simply coast most of the way. Provided you are not in too big of a hurry, you can use low delta-v orbital changes, to send feed stock to the refinery, and most of that can be provided by free solar energy. So the harvesters work the asteroid belt and send payloads to an orbit between there and the Earth, where the refinery process a continuous stream of material, containing all the elements, in roughly their solar abundance ratios. Energy for refining isn't a concern, but industrial quantities of the chemicals used in the refining process will be hard to come by.

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  • $\begingroup$ +1 for discussion of in situ resource utilization. I'm not sure about "Showering the earth with artificial meteorites would increase heavy metals and other elements/compounds in the atmosphere." The natural meteorite flux is 10^7 to 10^9 kg per year ( tulane.edu/~sanelson/Natural_Disasters/impacts.htm ). Artificial meteorites would probably be small compared to that, although I guess it could approach 10^7 kg per year for a big enough operation. $\endgroup$ Mar 4 at 20:17
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    $\begingroup$ @WaterMolecule Most of that is dust, that settles harmlessly to the ground. The heavy metals are mostly safely tucked away in complex molecules. Dropping thousands of rocks through the atmosphere, will vaporize a portion of the meteorites into a plasma that will disassociate those molecules. We really are talking about a major artificial increase in the number of large rocks entering the atmosphere, that would likely be selectively or refined to be, purer in metal content, than what is currently falling into the atmosphere. That will impact atmospheric chemistry and possibly human health. $\endgroup$
    – jwdonahue
    Mar 4 at 20:37
  • $\begingroup$ I wonder how much of the heat in the atmosphere is currently due to in-falling material? $\endgroup$
    – jwdonahue
    Mar 4 at 20:40
  • $\begingroup$ You have a point that gently falling dust is different than vaporizing minerals. However, shooting stars aren't rare events, so I think there are quite a lot of meteors vaporizing all time. The heat is insignificant for all but apocalyptic meteors. A 1 micron diameter particle colliding with the atmosphere every 30 microseconds moving at 72 km/s works out to less than a watt of kinetic energy. 1 mm diameter meteors every 30 s is like 400 W. Even if we had a 2013 Chelyabinsk meteor (500 kiloton TNT) every hour, that's just 0.6 terawatts. The sun provides 173000 terawatts. $\endgroup$ Mar 5 at 18:44
  • $\begingroup$ Are the influx rates really as low as 2 per minute? Seems like a low estimate. I am sure they probably contribute a miniscule percentage of the heat in the atmosphere at current annual rates, but I'd be willing to bet that industrial scale delivery of ores to the earth, using nothing but atmospheric breaking, will exceed natural in-fall heating by at least one order of magnitude. For all I know, that would still be tiny, but the other environmental impacts, definitely require a closer look. $\endgroup$
    – jwdonahue
    Mar 6 at 3:43
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Effective asteroid mining is going to act a lot like an industrial revolution. You'll have feedback loops on feedback loops.

I can talk about various phases. The first problem is making it cheap enough to send a significant amount of material to the asteroids. Currently it costs 10k$/lbs to reach orbit. Then another pile of wealth to reach the asteroids.

Basically, everything in orbit costs as much as pure gold. Including reaction mass.

I mean, imagine a gold mine, where it was literally a pile of gold on the ground. Except every pound of material you brought to the mine cost you a pound of gold. The food, the workers, the railroads, the dynamite, the picks, the carts, the roads, the houses for the people.

Making that economical is going to be very difficult.

Making an effective asteroid mining system when this is true isn't going to be effective. You need an industrial base that is cheaper than 10k$/lbs away to pull this off.

There are a number of ways you can do this. The naive solution is just to make space launch cheaper -- beanstalks, fountains, hooks, linear accelerators.

All of these work better in a low gravity sink. So one plausible case is that your civilization has set up an industrial base on Mars or the Moon. From there, getting to space is going to be orders of magnitude cheaper, and setting up one of those technologies to make it even cheaper is going to be cheaper as well.

An industrial base on the Moon or Mars that reduces the marginal cost of matter in orbit down to 100$/lbs can then be used to build a real asteroid mining industry. The first consumer of the resources won't be Earth, but rather the Moon or Mars, where materials already cost more than Earth and deliver is also cheaper.

This results in a feedback loop as the asteroid materials make resources cheaper on the off-planet industrial base, which in turn makes asteroid mining easier to do.

Once you have this, you start building stuff like a skyhook in orbit around Earth. It trades momentum from incoming mined packages for momentum from planetary launches. The marginal cost of leaving the Earth drops to say 100$/lbs (the energy required to reach the bottom of the skyhook), and for every pound of mass you lift out of the gravity well you get to lower 1 pound of any metal you choose from orbit.

This makes the skyhook's operational profit huge, which means that even if expensive you can build a lot of them. This will continue until the price of almost every raw material on Earth drops below 100$/lbs and building new skyhooks is no longer worth it.

While it might be viable for a short term period, actually dropping an iron-wrapped gold core asteroid from orbit and picking it up after it impacts is not going to scale very well. The chaos of reentry means that you might lose the gold, and the heat:payload ratio won't let you do this on a planetary changing scale without boiling the atmosphere.

As you near a K-scale 1 civilization, your problem becomes not boiling the planet, as the energy your civilization uses starts approaching the ability for the planet to emit heat. The reentry of large numbers of asteroids is a lot of waste heat.


So I see a bunch of phases.

Phase 1 is industrialization of Luna/Mars. Earth is heat-trapped as a sub-K-1 scale civilization.

Phase 2 is Asteroids materials mined and used to further Luna/Mars industrialization, including Beanstalks and Skyhooks and the like. But not around Earth.

Phase 3 is where you start seeing seroius lift infrastructure deploying that over Earth. Every pound of matter you launch, you get a pound of asteroid mining materials. Price of any raw material on Earth falls to 100$/lbs.

Phase 4 is when the off-planet infrastructure cuts itself loose from Earth, which becomes a relativly impoverished backwater. Civilization breaks the K-scale 1 barrier, and becomes interplanetary. Most humans still live on Earth, it requires a civilization closer to K-2 scale to manage a planetary exodus.

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Its probably best not to send the minerals back to earth

In your world it appears that there is industry in space. That industry and the space economy as a whole will need space ships, tools, and possibly space stations and other space infrastructure. All of those things are made of metal, and metal is heavy. It costs a lot of money to move heavy objects from the earth into orbit. At present it costs several thousand dollars per pound. In some cases, like 100X the cost of the metal itself.

You can make a huge profit if you are able to manufacture large components in space, or on the moon and then deliver them to their final destination in space without having to launch from the earth.

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Let's take look at what lands on a regular basis as of today or in the past, on this planet

  • space dust
  • meteorites, including big ones
  • Space shuttle
  • Soyuz landing capsule, different sample return missions
  • Falcon 9 first stage
  • Starship test model - almost, and it made out of steel
    • and it quite a good model for a glider, how it may look like, and it can RUD itself without that much of dust, on some water surface

if we throw out big asteroids which had lithobraking at the surface of our planet in the past, leaving km's wide craters, then Space shuttle or Buran(those few times it flew) - are the biggest, by a mass, objects which performed not only a landing but soft landing. Space Shuttle orbiter dry mass was 78 tons, and 14 tons of payload for the return mission. Not sure if one is included in another, but anyway let's round it up to a 100t.

out of that 100 t for a decent, u probably do not need those ascent engines and fuel systems for them and if we look through all the system it has for humans they can be thrown away as well, and if we lower the safety margin which required for humans we may get to some space shuttle-like construction which is capable of soft-land return a payload of 50t. Then for rare elements, it is a lot - u need just 100 landings a year to totally smash the market under your control.

That pesky iron, can't hold it anymore

as mentioned in comments by AlexP - The annual production of steel is about 1,900,000,000 tonnes. - I won't even bother to look it up to check, as I know - yes it is thousands of millions of tons.

Maybe to move the market you may need 10% of that, if u manage to get it, you can be a steel baron, but it still a lot of mass.

Landing as ore is not necessarily useful, as ore is relatively cheap compared to the end product - steel, do not remember the exact number but it is in the range of 20-40% of the price of the end product - steel. A lot of energy is used in reducing oxidation to metal.

However, there becomes to be important what for do u need iron or iron oxides.

There is such thing as Iron fertilization

Role of iron About 70% of the world's surface is covered in oceans. The part of these where light can penetrate is inhabited by algae (and other marine life). In some oceans, algae growth and reproduction is limited by the amount of iron. Iron is a vital micronutrient for phytoplankton growth and photosynthesis that has historically been delivered to the pelagic sea by dust storms from arid lands. This Aeolian dust contains 3–5% iron and its deposition has fallen nearly 25% in recent decades.

A full-scale plankton restoration program could regenerate approximately 3–5 billion tons of sequestration capacity worth €50-100 billion in carbon offset value. However, a 2013 study indicates the cost versus benefits of iron fertilization puts it behind carbon capture and storage and carbon taxes.

Iron fertilization by sea or aerial vehicles may be expensive, but a shower of iron oxide particles from space - may be a way to go.

The Redfield ratio describes the relative atomic concentrations of critical nutrients in plankton biomass and is conventionally written "106 C: 16 N: 1 P." This expresses the fact that one atom of phosphorus and 16 of nitrogen are required to "fix" 106 carbon atoms (or 106 molecules of CO 2). Research expanded this constant to "106 C: 16 N: 1 P: .001 Fe" signifying that in iron deficient conditions each atom of iron can fix 106,000 atoms of carbon,[45] or on a mass basis, each kilogram of iron can fix 83,000 kg of carbon dioxide. The 2004 EIFEX experiment reported a carbon dioxide to iron export ratio of nearly 3000 to 1. The atomic ratio would be approximately: "3000 C: 58,000 N: 3,600 P: 1 Fe".

combining numbers, pretty much in an arbitrary way, out of curiosity

A full-scale plankton restoration program could regenerate approximately 3–5 billion tons of sequestration capacity worth €50-100 billion in carbon offset value.

Antarctic circumpolar current into organic carbon, the resulting carbon dioxide deficit could be compensated by uptake from the atmosphere amounting to about 0.8 to 1.4 gigatonnes of carbon per year.[67] This quantity is comparable in magnitude to annual anthropogenic fossil fuels combustion of approximately 6 gigatonnes.

... each kilogram of iron can fix 83,000 kg of carbon ...

to fix 6 gigatonnes we need about 72 million tons of iron, delivered over a large area, landing as 0.5-1um particles, preferably. So there may be a business of a lot of iron to be landed at almost anywhere of that 70% of the surface of the planet. And deliver it out from space totally worth it. And we talk about trillions of money per year buried in there as a service.

  • you may need phosphorus as well, not necessarily in similar quantities, as it may be a more soluble thing and moves easier in and out.

what iron looks like as a soft landing

  • let's say we did it - 100% iron space shuttle, it fertilizes by ablation oceans at descend, and soft lands 100t
  • another thing to mention that suttle does not need any energy stored in it as it gets it constantly at the descent.

So 2 billion tons of iron annually in 100t packages will look like 1 shuttle per 1.57 seconds.

if we keep some reasonable time between them for one land strip, let's say 5 minutes, then we need 190 landing strips for them. And if we aim as more or less the same region then it is constant plasma channel, wzu wzu wzu, every 1.5 seconds, the constant roar of hell fire.

But not impossible something like a 50x5 km piece of land, or sea can suffice.

Gliders, feasibility

I would say it quite feasible, and maybe even preferable. if one builds them in orbit, gives them a small kick for deorbiting, then when it lands - it is a pile of materials which proportions are exactly, or very close to, the which one needs to build a space shuttle, space capable vehicle, or in more general aerial or other technology advanced vehicles - like cars as an example. And that 50t payload can offset the average composition to any industrial useful mix, to any specific or general proportions of materials and metals and elements we use or may need.

if not the whole construction, some parts of it are ready to be reused in other constructions. Build it in a way to increase the reuse of parts - it may be an important factor here.

or it can be used as a whole - retrofit engines in it and go in orbit.

  • For to build that relatively advanced glider, it needs to have developed industry in space, in orbit. And then just iron delivery may not look that sweet if u manage to build processors in space - one 50t delivery may be worth 0.5B in retail money.

  • if you imagine crowded earth striving for resources, considering 300-year time frame, and all iron ore pits are long gone - then again energy from space can be more useful than iron delivery - as if u take random rock it has a meaningful percentage of iron or aluminum in it - it just requires more energy to extract and refine it, a few times more than from better iron oxide ore. And if you deliver the energy you will solve that iron aluminum crysis.

  • there can be more primitive ways to make the delivery, requiring less technological capabilities for those return payloads, some are smart, some are funny, some are not - but given 300 years time frame for technologies and the whole space mining business existing there, I won't consider making such delivery glider to be a problem at all. As long as it does not use what u can't easily find and mine it is okay. And in asteroids, u can mine everything we have on the planet, not necessarily in one place but ...

  • using water as a source of hydrogen for fuel, can be a waste, but using hydrogen from Jupiter for the same purpose maybe not. But generally, I mean an absence of fuel in the whole operation from asteroid to the planet's surface, as it can be done that way easily, with technology and energy available in space.

  • iron and aluminum are not the most interesting bulk metals, there are others that are bulk metals as well for stainless steel or bronze copper - Cr, V, Co, W, Ni, Cu, Zn, etc.

Conclusion

we can see that precious metals delivery maybe not a problem, and surprisingly even iron delivery may be quite feasible, as well on the scale of our current world production.

put enough effort in bringing up technologies required for glider productions, like the space shuttle hull type, which development and research how to build one in space you can propel by precious metals, if there is no other incentives at work and Soyuz like capsules or their smaller counterparts delivering rare metals to fuel that monetary train - this type scales well enough, up to tons, and are simple to buid.

processing can be done in space or in orbit - does not matter that much, and depends on all sorts of premises. But fission rockets, solar sails, fusion propulsion it all within the defined time frame and those can be efficient enough to not worry about do you move a whole asteroid or just a refined part of it.

I would suggest making a stream of delivery of hydrogen from Jupiter, do bind oxygen in easy to hold form, which is a waste product from making metals, but it is not a necessity as keeping it in liquid form also isn't that difficult. But when you reach some gigaton a year month day hour - it may make sense at some point.

  • do not think about that problem as keeping fuel in a rocket, we talk about billion tons of it per year. it requires setup, but there is nothing special about it.
  • venting it would be a waste, even if we have sources of it, but still, the element conservation paradigm better to get used to it from the start, no one knows where a day comes and u would need to sell it to mars guys or space habitat folks.

However, delivering bulk metals is not the best way to address the resource shortage of mines. energy for recycling, energy to get it from the sources which are not considered as ore - is more preferable way. The energy dissipated by de-orbiting in the atmosphere may be manifolds more than the extraction of these metals from poor ores. So heat pollution wise it is better to deliver energy, than bulk metals.

Drop shoot delivery scales well, from small quantities to quite big ones, but at some point, it makes more sense to invest in orbital rings, and alike, they are technologically feasible even at our today's technologies - incentives, and materials in orbit are our current problems. But when we talk about trillions of direct and indirect profits and billions of tons of materials in orbit - then it makes sense to make those. So you may start the business small, but later on, u can have resources to develop and build more efficient means.

some additional notes

For rare elements, which are at ppm ppb concentration in a typical space ore - u need to extract them in space - because it is easier and energy is more available there and more easily accured. At the same time, you can process bulk material separation. A refinery in space will be quite a construction to see. So after u mine your first 10% gold/platinum content asteroid - invest in making the refinery. And then moon regolith will be the source of most materials u need - for earth it is the best source of most elements.

energy take from solar - it worth almost nothing with basic automation.

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I will join the crowd in worrying about weaponizing the delivery system. Think of [i]The Moon is a Harsh Mistress[/i].

However, if we ignore that there's another delivery system that is much simpler and less prone to failure:

You need to do your processing in a very low gravity environment, I don't know just how low you need: Form your refined metal into foam--this is impossible on Earth but quite possible in microgravity. The objective is to give it a density less than 1 gm/cm^3.

Once you have a delivery package ready you use your refinery slag, make more foam to serve as a heat shield. Load it into your mass driver and fire at a piece of ocean reserved for the purpose (anyone who ventures into the area had better have up to date tracking of incoming objects so they can dodge if need be!) If your gunnery is good enough you don't need anything but metal and shield--there is no braking system other than the heat shield, the packages hit at terminal velocity.

The thing is, since they are lighter than water they float even if they break on landing.

You scoop them up, for most purposes you are going to have to melt them and recast into whatever shape you need. There will be some ability to use them directly, though--pound for pound metal foam is stronger than the base metal and if you have big enough pieces you can build ships that can't sink. (They build some "unsinkable" boats now but it's done with adding foam rather than the hull itself being the flotation device--the current versions can still sink if they take enough damage.)

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Won't anyody think of the velocities? Hit the moon!

Moving much of the refinery chain close to the asteroids is tricky, as the machinery is heavy and probably requires much maintenence - meaning more people, more life support in the belt. Same goes for manufacturing. Still some refining will happen in the belt, as will some manufacturing, the economy is different though than on earth because maintaining fragile machinery and more fragile humans in space is difficult, expensive and dangerous before they even started to work.

for the early phase of asteroid mining, I propose mass drivers with two twists:

  • the target aquisition software is controlled by states, not private enterprises. Mining corp says to NASA "we want to launch 5t plusminus 50 kg on time x at target y", NASA confirms its safe, computes a firing solution, at time x the gun confirms that indeed 5t are loaded and fires. Only "responsible" states are allowed to control mass drivers, same as with nukes (Apartheid era South Africa had nukes. Hmm.)

  • Everyone shoots at the moon and only at one half - probably the western half (the one visible when the moon is moving away from you). All landing times fall into week long landing timeslots, inbetween the landing slots there's time for ore retrieval (onto the safe side of the moon, from where processing and launching to earth happen). Mining cooperations have designated areas for which to aim as landing spots.

How fast will the cargo land? This part is tentative, I need to check my thinking here:
Relative to the Sun, escape velocity near the earth is 42.1 km/s, near Ceres (as a body in the asteroid belt) 25.3 - so unless I'm mistaken everything arrives at earth orbits with about 17 km/s (needs accoutning for earth's orbital speed) Moons escape velocity is 2.38 km/s, earths escape velocity at the moons orbit is 1.4 km/s, so roundabout 3.8 km/s + 17 km/s (needs accounting for moons orbit around earth(which is why we want to land on the west side, not the east side:))). However that's far better than entering earths atmosphere with 11.7 km/s + 17 km/s.

I don't think aerobreaking is feasible, I think lithobreaking with less speed is better and the chance of hitting a city on the moon is lower too. Because there are no cities on the moon.

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