Mining Geography of the Solar System
The XKCD webcomic on surface area inspired me and I’ve thought about what it does and doesn’t tell us about the physical reality of our…
Mining Geography of the Solar System
The XKCD webcomic on surface area inspired me and I’ve thought about what it does and doesn’t tell us about the physical reality of our solar system. If read my blog, you’ll know that I’m thinking about this in the lens of future space development.
Right off the bat, this map does confirm the familiar names for manned exploration targets. As you go through the elements by size…
- Earth — we’re already here
- Venus — hellscape, not going there
- Mars — good one, similar area to Earth
- The moon — another top pick, about a continent
- A bunch of other moons — contribute significant area, but also hellscapes
- Titan and Callisto — non-trivial area but very far away
- Asteroids — surprisingly small actual area, diffuse
But this all, true to a cartoon theme, assumes some kind of surface existence. And for the graph to be meaningful, some kind of surface existence needs to be meaningful.
So what are we to do, if we are of the O’Neill space habitat camp? We are not really interested in living on the surface of other planets, but making things out of those other planets and living in what we create. We need a different metric.
What Can we Mine Heuristic
Let’s begin constructing a criteria based on mining. We are limited how far in Earth’s surface we can mine. This is mainly temperature-limited, but really limited by a combination of temperature and pressure. So the actual criteria is quite complicated and mixed with the material factors which are different in different places.
But we can still fudge it. I’ve gone through these in years past, but I have an AI recap of what’s involved equation-wise here.
To do any equations you need to state what type of space development you are imagining in the first place. For this, I will break up into 4 cases.
- Reference case: Look at surface area only, what the XKCD comic did
- Deep mining case: Look at how much material you can access by reasonable assumptions for how far you can mine into the crust of the planet, asteroid, or moon
- Surface mining case: Use a simplified 100 meter of Earth rock equivalent slice of the surface, because this is more likely to resemble how we actually do things
- Useful mass: The surface mining case omitting the ocean-covered parts of Earth and Venus
So I’ll just go through each.
Surface Area Recreation
This is just giving a pie chart that should have the same numbers as the XKCD comic. So the same thing, but uglier. This is just to get bearings and establish the data set, which is DASTCOM5.
Surface Area by Planet or Other Thing (inner solar system)
This gives fairly satisfying visual correspondence. Except I did not include the gas giant moons. Whoops. Oh well, I’m not very interested in them due to practical reasons anyway. If we are being practical, the inner solar system is where we should be.
This makes the dominance of Earth and Venus even more visible here. It makes you wonder why the asteroids and others are even important.
Deep Mining Case
In terms of inputs & outputs, I just set a P_max value → the pressure that limits how far we are able to mine.
Why is this important?
Because smaller bodies have a lower gravity, and thus a lower pressure gradient. They also have a lower temperature gradient, and this lowers by more than what the pressure gradient does. Because of this, using pressure is useful as a conservative metric. We won’t over-estimate how far we can dig into asteroids compared to the Earth, even if we just use the pressure constraint and call it a day. So that’s what I’ve done. And it’s first-order correct anyway, because whatever the pressure limit you have is, that eventually becomes your limit for small-enough bodies (including the moon likely).
Let me restate — we can mine deeper on the Moon than on Earth. This is obviously true to an engineering mind, because of the temperature and pressure factors.
Reachable Mass for Mining for Various Bodies
This looks mostly qualitatively similar, but with a slight bias towards smaller bodies. You have to look more closely to realize that the smaller the body, the larger the effect. We can mine slightly deeper on Mars than on Earth, but we can mine way deeper on the Moon than on Earth. Still not to its core, can we can’t mine to the core of Ceres (probably). This becomes arguable for the next set of asteroids.
Ok but let’s pivot the graph. Because we are also interested in “how far” these material resources are. If we are planning to mine them, we have a travel cost as well.
Reachable Mass by Deep Mining in Inner Solar System against Delta V to get there
Fun stuff! Although Mars is king for practically available material to mine, the Moon offers something unique. Obviously, it must, otherwise why would we be talking about it? It offers a tremendous amount of materials at a very small distance from Earth.
Delta V Notes
The Martian system is obviously of great interest. Due to the atmosphere, getting there can be easier than getting back, so I’ve included dots for the case of no aerobreaking. Reality dictates that practical “real” values are somewhere in-between the two cases. So it’s not entirely obvious that Mars is the best for obtaining an exporting materials… although for creating a robust Mars settlement, yes, absolutely.
It is very intentional and true that Phobos and Deimos are easier to reach that Mars itself without aerobraking. This is because most of the impulse goes to lowering to the lower altitudes of orbit.
Elsewhere on these plots, you’ll notice that the Moon is less reachable than asteroids. Yes, this is intentional, and it is true. Because to reach the moon and get back from the moon, you need to overcome the Moon’s gravity well. This is not true for asteroids where this tends to become a rounding error due to the small masses.
Similar arguments would apply to Venus and Mercury about dealing with their gravity wells, but I honestly don’t care about them very much so I did not put work into it.
Surface Mining Case
Deep mining is relevant for some cases. Mainly in the cases that you are hunting seams of particular scarce minerals. Otherwise, you do not process any significant fraction of the material that is reachable. So the reachable mass doesn’t mean all that much.
For truly large, and I mean really large, amounts of material produced, surface mining winds up being the method. This can still be discriminating, only mining a part of the regolith abundant in what you need, but collection is a relatively blunt instrument, because this is the case where you are obtaining relatively ordinary elements, for stupendously large quantities to export. Because of this, this is my favored type of metric for dreaming of a space-faring civilization. So this is the gravy, this is what probably actually matters.
Again, the same argument about access inverse to gravity applies. Not only does the pressure gradient in the regolith go down, but the energy and effort needed to move mass goes down as gravity goes down. On a lower gravity planet, movers can move more material. Processing is still likely prohibitive, but abundant solar power is a key goal of space development, and exponentially self-feeding manufacturing of solar panels is most certainly a topic. This is essentially an assumption required for any of these very large numbers to matter.
So I use the 100 meter metric for Earth, and back-calculate the pressure for that and apply this criteria for other bodies. Asteroids become limited by the total mass of the asteroid except for the really big ones. So in pie chart form:
Reachable Mass by Strip Mining Mass in Inner Solar System as Pie Chart
And the reachability scatter chart:
Reachable Mass by Strip Mining 100m Earth Equivalent
Usable Strip Mining Mass
Next, I want to make the obvious corrections regarding Earth and Venus. We are likely to do some mining of the ocean floor on Earth soon… but we will not be surface mining the ocean floor (at least not for another century or two, can’t speak for after that). Venus, for its part, might as well be counted out. There are interesting ideas for Venus, but those are distant-future, or do not give surface access. So my corrected pie chart in this case:
Pie Chart of Usable Strip Mining Mass in Inner Solar System
Finally, Earth has been de-throned, because so much of Earth is ocean. Mars takes the lead in this metric. Collectively, the asteroids actually provide more potentially usable mass than Earth — and this is the intuition I originally wanted to put numbers on.
Of course it’s obtuse to call this mass on Earth usable in any form. We can only convert a very small fraction of the surface of Earth into surface mines. I put it in the ballpark of 0.05%.
And the mass-against-transit scatter:
Usable Mine-able Mass Against Delta V to Reach the Body
Again, for the true usable mass of Earth, you can drop it down 4 orders-of-magnitude. This would put Earth into the same ballpark as Deimos, which is… a weird brag? It also perhaps gives a starting rule-of-thumb, for space industry to exceed Earth it needs more mass than what Deimos has. So we should count our space infrastructure mass in how-many Deimos it is, which is similar to how much mass we mined on Earth (and hopefully we don’t need much more from that particular planet).
The asteroids also start to stand out as genuinely interesting. Keep in mind here, that Lutetia is M-type:
So if you’re going hard, heavy metal, stuff, this is both one of the most reachable and greatest resources. The iron-nickel content might be useful with relatively minimal processing.
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