|Asteroid Vesta was the recent target of much investigation by the Dawn mission. This research confirms that Vesta is composed of basaltic rocks and so is not a good candidate for mining.|
Image Credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA
It is important to note that not just any asteroids can be mined profitably, because not all asteroids are alike. Roughly speaking, there are three main types of asteroids; C-Type, S-Type, and M-Type. C-Type asteroids, also known as Carbonaceous Chondrites, often contain lots of water (as much as 20%) and other organic compounds. They are thought to be very similar to the building blocks of our Solar System. S-Type or Stony asteroids contain mostly rocky materials. And M-Type asteroids, which are also called Metallic or Iron asteroids, are made up mostly of iron and nickel.
Iron asteroids are thought to come from the metal-rich cores of disrupted planetesimals. Early in the Solar System, orbiting material would have accreted into small planetary bodies. The largest of these would have differentiated, with the heavy metals sinking into an iron-rich core and the lighter silicates left behind to form a rocky mantle around the core. Some of these differentiated bodies would then have been broken up by impacts, creating smaller asteroids of metallic and stony compositions.
The iron asteroids seem to be attractive targets for mining, because they are made up almost entirely of iron and nickel. Scientists have estimated that one metallic asteroid with a diameter of 1 km could provide enough iron to supply the entire world's demand for 15 years (plus about 1000 years worth of nickel). This sounds wonderful until you realize that getting this metal to the Earth's surface, where it will be used, is a big problem. If we wanted to supply all the world's iron needs from asteroid metal, we would have to bring over 1.2 million tonnes of iron to the surface on a daily basis. Dr. Kargel points out that when this material entered the atmosphere, it would release a great deal of energy, effectively equivalent to a 10 megaton nuclear explosion, each day! We currently don't know how to safely dissipate this much energy in the atmosphere without causing significant environmental damage, not to mention addressing the risks of catastrophic accidents.
Fortunately, metallic asteroids also contain various precious metals (such as gold and platinum) that were dragged along with the iron when the original core was formed. The amount of precious metal in the richest iron asteroids is about 350 parts per million (ppm). This may seem like a very small amount, until you consider that on Earth, gold is mined at concentrations as low as 5 ppm. Dr. Kargel estimates that at 350 ppm, a 1 km diameter asteroid would contain about 400,000 tonnes of precious metals, worth many trillions of dollars.
If an iron asteroid was mined and processed in space, and only the precious metals returned to Earth, this would be much easier to manage. Spreading the recovery of these metals over a 20 year period would deliver only 55 tonnes of material per day, which is much less than the rate at which cosmic dust naturally falls on the Earth (about 130 tonnes per day). Dr. Kargel anticipates that this would have a negligible environmental impact, possibly even smaller than what traditional mining does to the Earth today.
But it's not just the metallic asteroids that make good mining targets. Dr. Kargel points out that some stony asteroids could also be mined profitably. This seems counterintuitive, considering that the process of planetesimal differentiation, which concentrated metals in the iron asteroids, would leave the stony asteroids depleted in metals. However, not all stony asteroids have their origins in differentiation. Some stony asteroids come from undifferentiated planetesimals, so all their metals are still dispersed among the stony parts.
Again, not all undifferentiated stony asteroids are alike. Dr. Kargel believes that the most likely mining candidates are a group of asteroids called the ordinary LL chondrites. The LL stands for low iron and low metal, but ironically these have the highest amounts of precious metals, because in undifferentiated stony asteroids the amount of precious metals is inversely proportionate to the amount of iron and nickel (so iron-poor asteroids have more precious metals, while the iron-rich ones have less). As a result, the LL chondrites contain only about 5% iron and nickel, but have as much as 200 ppm of precious metals, which is more than some metallic asteroids.
The advantage of mining stony asteroids is that their surfaces are covered by a thick regolith of impact-pulverized rock. Constant bombardment by other asteroids, of all sizes, effectively grinds the stony materials at the surface, until they are only about 25 micrometers (or 0.025 millimeters) in diameter, which is roughly the size of baking flour. At this size, most of the regolith particles would be monominerallic (meaning they contain only one mineral) making it possible to pick out only the minerals you want (the precious metals) with something like a finely tuned electromagnetic rake. This process is much easier than having to separate out precious metals from a solid body, as would need to be done for the iron asteroids, which are too strong and ductile for impacts to produce this kind of pulverized regolith.
So, how much precious metal could you get from the regolith of a stony asteroid? It is thought that most stony asteroids have regolith layers that are at least 100 m thick. Based on this, Dr. Kargel estimates that an LL chondrite asteroid of about 3 km in diameter would yield on the order of 200,000 tonnes of precious metal.
It seems both metallic asteroids and LL chondrite-type stony asteroids have the potential to provide very large amounts of valuable precious metals. The LL chondrites may be easier to mine, but the metallic asteroids have a larger amount of the desired materials. In other words, both have their advantages. So, now all we need to do is go prospect some asteroids and pick out the best ones to mine!
Kargel, 1994, metalliferous asteroids as potential sources of precious metals, Journal of Geophys. Res., V99(E10), 21,129-21,141, DOI: 10.1029/94JE02141.