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, 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. 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.
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.
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.