For near-term mining in space, we can mine the Moon or we can mine Near Earth Asteroids (NEAs). This series of posts discusses asteroids. What type of asteroid is best for space mining? As we saw previously, asteroids can be completely different from one another in their composition because of the different ways they formed. Some came from undifferentiated (“primitive”) bodies while others came from protoplanets big enough to differentiate. That is, they were big enough to melt and separate into metal, heavier minerals, and lighter minerals forming a core, mantle, and crust. Among the basic types of asteroids there are many, many subcategories. Much of what we know about asteroids comes from studying meteorites that have fallen onto the Earth. These do not give us a complete sampling of all the asteroids, so for those we cannot physically reach yet we must learn what we can with telescopes. Therefore, they are classified according to the pattern of how they absorb and reflect light (what we can see in the telescope), their spectrum. That tells how much sunlight is reflected by an asteroid at each wavelength of the light.
Asteroid Spectral Types
Just like the classification of meteorites found on the Earth, the classification of asteroids has several major categories and many subcategories. Research is still ongoing to understand asteroids and so there are competing classification systems.
Understanding the asteroid spectra is complicated by something that occurs in space known as space weathering. That is, solar wind wears material off the asteroid, and foreign material accumulates by gravity onto the asteroid, changing the appearance of its outer surface. Fortunately we have been making progress in understanding how space weathering changes the spectra, so we are getting better at subtracting out these effects.
There is no need to go into too much detail. We need to know only a few main points for mining the asteroids and they are pretty straight-forward. So here they are.
|Spectral Type||Similar to What Meteorite Type||Resources||Purpose|
|Type C||carbonaceous chondrite||water + metal + organic compounds||rocket propellants and other consumables for space missions + metal for 3D printing of hardware to use in space + making rubber or plastic or methane for rocket fuel or CO2 for plants|
|Type S||LL chondrite||platinum group metals||sell on Earth for use on Earth|
|Type M||iron meteorite||metals including platinum group metals||manufacturing large hardware items in space for colonization or larger missions and/or for sale on Earth|
This class include asteroids that look dark through a telescope and have spectra indicating they are composed of carbon compounds. They are therefore called carbonaceous asteroids and are believed to be made of the same sort of material as the carbonaceous chondrite meteorites. The dwarf planet Ceres has a spectrum like a carbonaceous asteroid (at least on its surface). C Type accounts for about 75% of all asteroids. They formed in the outer portions of the asteroid belt closer to the frost line where it was cooler so carbon compounds could condense. They also contain hydrated minerals so we can get lots of water out of them. Some estimates are that we can get over 20% of the mass of the asteroid out in the form of water. Since chondrites are undifferentiated they also contain primitive metals, sometimes as much as 40% by mass. These would be extremely good asteroids to mine, having water, metals, and carbon compounds, all three in one. Because they are dark, they are rather hard to find.
These asteroids are rather brighter than the C Types and appear to be “stony” in composition. They can be either primitive — having chondrites (usually) — or from differentiated bodies — so not having chondrites. The LL chondrite meteorites are especially interesting for platinum mining because, although the LL means “Low iron content and Low total metal” they nevertheless are wonderfully high in platinum content. That could be important if we plan to bring the mined metals back to Earth to sell on the terrestrial metals market. If we can find a way to reduce the launch and landing costs, then this could be a quick revenue stream to support the space mining company. However, if we plan to use the metal in space then there is no need to go after just platinum. It would make better sense to go after a different class of S Type asteroid, those in the H chondrite family since H means “high” metal content. Or better yet we would go after an M Type asteroid.
M Type asteroids are moderately bright and are usually metal but sometimes metal-stone mixtures. Some of them are surely the same composition as the iron meteorites that have fallen to Earth. They are believed to have come from the cores of differentiated planetoids that were later broken apart. These asteroids are interesting for mining because a pure metal asteroid is a lot of metal and can be made into a lot of spacecraft. A quick estimate shows that the asteroid belt has a billion times more metal than all the high grade metal ore in the crust of the Earth.
Three types of asteroids are most often discussed for space mining. One is Type C, another is a certain subcategory of Type S, and another is Type M. Type C is probably the best all-around asteroid for mining in the early stages of space industry because it provides lots of water for rocket fuel, which can be sold to NASA or others to do various space missions. That might be a good strategy for initially starting up space industry. Type C also provides metal and organic materials that can be used for making things in space. Type S asteroids of the LL chondrite class may be good for platinum mining for sale on Earth to provide a revenue stream for the mining company. That will be a good strategy if we can lower the operational costs so that it becomes economic. Type M may provide platinum, too, as well as vast amounts of nickel and iron that, in the long run, will be needed for solar system civilization.