mining asteroids in our lifetimes would primarily be useful for harvesting raw materials for constructing things in space, not sending the materials back down to earth.
It may be useful for some raw materials - for example, osmium ($13,000/kg), rhenium ($2,750/kg), and iridium ($30,000/kg). All three of those are elements that are probably common in the Earth's mantle, but nowhere near the surface, and are mostly found in meteorites; all of them are used in high-performance alloys (high-temperature, corrosion-resistant, etc.)
Doubling the world production of any of these would greatly reduce the prices of aerospace engines and electronics (iridium is used for crucibles growing high-quality semiconductor crystals). And that's easy, because the quantities involved are so low - under 1 ton of osmium per year, 10 tons of iridium, and 40 tons of rhenium.
There are other materials like that. It's all about searching for the competitive advantage, the materials that can't be found on Earth in reasonable quantities. All the nickel and iron and titanium will indeed probably stay up for in-space use.
That really depends on the assumptions you’re making about why it’s done. Large scale mining for building materials and fuel requires an existing space industry for demand. Saving on Dv only makes sense if you have something proven so out there... like mining. Either way there needs to be a truly enormous initial investment in propulsion, and so much more. Making an absolute fortune, and/or breaking foreign control over vital resources could be the key to getting that funding.
Not necessarily. In the near term, mining asteroids means mining near-Earth asteroids, not those from the belt.
The changes we'll need to make to propulsion systems are more about their operations - we need more types of engines that can be frequently shut down and restarted.
You need to first send equipment to do the mining and sending back - that part is very expensive, and if it's only to be used for sending stuff back to Earth (safely), then mining in space has very bad returns.
Asteroid mining is absolutely awesome if you move as much of the process as possible to space - every kilogram you use up there to build something is a kilogram you didn't have to launch upwell.
What you'd really want to do is develop a working zero (natural) gravity solar-furnace process that works.
It'd probably involve some kind of centrifuge design... but I wouldn't rule out just making something screaming hot and then suddenly introducing kinetic energy.
Then there are two main options...
A) Make a crude re-entry shield out of the waste products + some of those other resources you're not keen on harvesting for the primary use of sending to earth, and you put a nice candy core in the middle. Aim the landing trajectory at some shallow ocean.
B) Taking things UP in to orbit is expensive. Taking them back down is probably less so, though our current vehicles aren't based on completing that task so they'd need to changed to handle cargo both ways.
Option C could also be fun... if we ever get space elevators working things could go a lot like in science fiction. (The orbital elevators and start of a planetary ring from Gundam 00 jumps to mind as a reference; 3001 also had the idea much earlier but in less detail)
You've got to find a way to go from over 20,000k per second to stationary on the ground. For any worthwhile mass of asteroid you bring back to Earth, it's going to require one hell of a heat shield during reentry.
So maybe you leave the heat shield in orbit, but you've still got get it there.
I think space mining becomes possible when you're no longer cornered by the rocket equation. That is, it's going to require a breakthrough in propulsion systems.
> I think space mining becomes possible when you're no longer cornered by the rocket equation. That is, it's going to require a breakthrough in propulsion systems.
I think near-Earth space mining and manufacturing is what can help us get over that limit in some way - LEO is halfway to anywhere in the Solar System ∆v-wise, so whatever part of your mission you don't have to lift upwell makes for huge savings in fuel (and for many missions, you can use weaker and more efficient engines once you're on a stable orbit).
I imagine that platinum will be the most important metal mined from asteroids and returned to Earth. Thick platinum shielding should hold up well on atmospheric entry.
