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I get that mining asteroids seems like a very expensive task. I understand that it's expensive to get to and from them and takes lots of propellant.  But I just thought of a a way to make it way cheaper, eventually.  Could someone check my layman assumptions, please? :)

Asteroids zip past earth pretty frequently.  Parking an asteroid that goes by is probably really hard because of the speed difference.  When we first start up and deal with already known asteroids (and have lots of time to slow it down) it should be a lot easier to first get an asteroid in a circular orbit between Earth and Mars and then slowly bring it towards Earth until it is captured.

Once it's in a relatively round orbit around Earth it's safer and less expensive to park it around the Moon than Earth.  Mining of an asteroid around the Moon is still a big task but it's a lot more manageable than doing so from Earth.  This is because as a climber goes from Earth to the anchor it accelerates and has to break in order not to be slingshot out.  And climbing is also pretty expensive in the opposite way - it has to fight that outward energy to get back down.  This acceleration effect is much smaller on the moon with it having about 1/6 the gravity.

So parking the asteroid around the Moon is clearly more economical and safer.  Getting the mined material back to Earth is a separate matter but space elevators help here in a in a surprising way.  First off, going back to Earth is easier from Moon orbit than it is from the Moon itself - less energy spent.  The energy that is expended by slingshotting a carrier back to Earth will slow the asteroid down and cause it to get closer to the Moon.  But this can be offset by capturing carriers that are coming from Earth.  And if properly timed this procedure should be both safe and cheap.

The same exact slingshot and capture effect can be used to park carriers by an anchor in Earth orbit.  This is a more daunting task but is way more safe and cheaper than parking an asteroid in Earth orbit - the weight of a carrier will be a lot less than the weight of a mined asteroid.

This also helps solve the problem of getting the material down to Earth.  First the carriers are significantly slowed down by the capture process.  From that point on a combination of shuttle and Starship-like controlled burn technologies can be used to land a carrier down to Earth.  Scramjet technology can be used to forego the wait of the oxygen needed by the carrier to slow itself down as it burns propellant.

As carriers enter Earth's atmosphere they engage scramjets and vector the thrust to slow down in a more horizontal manner, thus eliminating the need for heat shields that need refurbishing after each landing.  Once at speeds too slow for scramjets to work the same engines should be able to operate in jet mode to slow for landing approach. As it slows down the (now) jet will gradually change the angle of its wings and land at airplane speeds in a very tried and true manner.

The fuel needed by the carrier can be sent up to the Earth anchor in light and soft packages (in slightly cooled gaseous form, with softness and large surface area helping to slow them down at the anchor).  It will then naturally liquify and be ready for use by the carriers when they go back down to Earth.

From Earth surface, railguns can be used to quickly get carriers to M4ish at which point scramjets would kick in and get it to low earth orbit.  There, carriers can attach to the elevator and gain more velocity automatically.  Thus carriers won't need any rocket engines at all and can be much simpler in design.

And the energy needed to park an asteroid in Moon orbit can also be reduced by a Moon space elevator which would (in a synchronized manner) use the energy acquired from the capture to accelerate a caturer to Sun orbit to go after yet another asteroid.

Of course it would still be very expensive to get an asteroid slow enough to get to a Moon elevator.  Even with "quickly anchor and slow down an asteroid with counter momentum, mid flight" technology.  So we'll need propellant in Sun orbit.  Some to get it to a "quick capture orbit" from deep space and some to further slow it down further after Sun orbit capture.  But could we mine water on the Moon and use solar energy to separate it into oxygen and hydrogen?  And by the time we start running out of water on the Moon we should have fusion rockets, right? :)

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    $\begingroup$ Hi Artem, welcome. You mention words like "easier" and "simpler" a fair bit in your answer. I think you should read a number of our questions here around adjusting orbits, or orbital insertion to understand just how difficult and complex and expensive this is even for low mass objects such as spacecraft. Also worth visiting our Worldbuilding site to look at speculative science like this. $\endgroup$
    – Rory Alsop
    Sep 5 at 10:18
  • $\begingroup$ Will do, thank you for your guidance. $\endgroup$ Sep 5 at 13:09
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    $\begingroup$ The momentum transfer parts of this can work, and in a couple of different ways. The much more problematic part is the 'getting asteroids to orbit the moon' - there are no cheating way to do that, while there are gravity assist and aerobrake options for earth. Recomend splitting this into two questions, one on using momentum transfer and one on the easiest place to park an asteroid. $\endgroup$ Sep 5 at 13:32
  • $\begingroup$ Got it, thank you very much! On the getting asteroid to the Moon (or appropriately, near Earth) question, would it help starting with an asteroid in the inner asteroid belt and slowly (over a decade or so) move it closer to earth? What if you build a very heavy rocket in Moon's orbit, low enough to then slingshot it into Solar orbit. Then get it to Jupiter and sling shot it back in in-wards. If you set up a capture system on the asteroid ahead of time (with an anchor fixed to the solid core) shouldn't you be able to use the heavy rocket to get the asteroid moving in the right direction? $\endgroup$ Sep 5 at 14:28
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    $\begingroup$ I recommend playing Kerbal space program to get a feel of how orbital mechanics work. $\endgroup$ Sep 5 at 16:46

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Maybe not a proper answer, but some points to economical practicality of asteroid mining.

Every known substance that can be mined on asteroids and than transported to Earth will be much more expensive (by magnitudes) than mining or synthesizing it on Earth. There is no antimatter on asteroids or other exotic and highly valuable things, as far as we know currently. Even mining of Helium-3 would see hard times to break even, because the revenue from energy it can generate will be much less than spending for its extraction and transportation.

The only maybe-viable option currently is harvesting asteroids for propellant that can be used for interplanetary travel. If you launch a rocket from Earth, only about 4-5 % of the mass can reach orbit, and only about 2-3 % can escape Earth. A refueling can be very useful for round trip - launch from Earth to a destination, do business, refuel there and fly back to Earth. Technically propellant mining from asteroids doesn't seem impossible with current technologies, but even here we have a lot of problems.

First, there is fast tear and wear of equipment in space. Because of radiation, vacuum and extreme temperature ranges. And the abilities to fix or replace broken parts are non-existent or very limited. Usually you need a craft with double-triple-quadruple redundant systems to endure failures (adds mass and cost). And than the backup reserves have been exhausted it's currently cheaper to launch brand new craft than try to salvage the old one.

Second, you would need a lot of energy for harvesting and refining the materiel. Much more than existing scientific spacecraft use. But the energy source itself should have good enough Watt/kg ratio to not be too huge. Will solar power be enough, or nuclear fission reactor would be needed? Several nations have reported they are developing nuclear reactors for space, but no prototype was launched yet. Let's wait a decade and see...

Third, transportation. At the beginning the harvesting craft should be transported to the asteroid it will be mining. Then the harvested and refined propellant should be transported from the asteroid to the place it's needed. But, citing Douglas Adams, space is BIG. Flight times from one location to another in inner Solar System will require from several months to several years. Also delta Vs (total velocity change to depart from A and arrive to B) usually are several km/s, so transportation by chemical rockets becomes impractical, or you will spend most of propellant by the way. Ion propulsion will be needed, with energy sourced by solar cells or nuclear reactor. Because of long flight times and wear an tear in space (see above) - how many times the transporter craft will deliver propellant while it's functional? If 1-2 times - isn't cheaper to abandon asteroid mining and just to launch more rockets from Earth?

Fourth - what kind of propellant to harvest? For what type of propulsion? For chemical rockets - water can be electrolysed to hydrogen and oxygen. But how to transport them? By chemical propulsion it's rather ineffective (see above). By NTR (nuclear thermal rocket) - maybe, if we use only hydrogen as propellant. For ion engine - water, hydrogen and oxygen are not so good for this. They will need more energy for ionization than xenon that is usually used for ion thrusters.

What realistic scenario can make harvesting propellant from asteroid economical? Let's imagine miniaturization of high delta V interplanetary spacecraft is successful. So it's possible to send many small (say < 100 kg) and inexpensive craft to many asteroids testing different mining technologies. It would allow to iterate designs rapidly checking what works and what doesn't. Than, nuclear-powered ion propulsion spacecraft are developed successfully, with higher ISP than current ion drives and able to use water as propellant (I'm not sure about feasibility of this part, actually). Third, spacecraft are made more robust and failure-averse than now. With all this assumptions - it can become practical to mine water in asteroids, refine it and transport to customers as propellant. Maybe.

One of my favorite aphorisms - it makes our real achievements even more exciting!

PS: I know that in current form my answer is just a slightly-educated opinion, without references. Please feel free to criticize, offer amendments or provide links for or against my statements.

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  • $\begingroup$ Thank you sir for your detailed answer. This gives me lots of food for thought and more importantly better grounds me in reality of the situation. This is a lot of information and I'll be coming back to this answer again and again. $\endgroup$ Sep 6 at 19:02

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