Technically, mining involves the extraction of profitable ores from a source. In the larger sense, to be profitable, this must include the costs of separating the valuable components from the less valuable, and of transporting the resulting material to market (or to a facility which converts the material to a marketable product). Most terrestrial mines, and certainly asteroids, contain a variety of potentially valuable products, and in general the optimal process of selecting one product will not be optimal - and may even preclude - the production of other products. For example, the least expensive (and therefore most profitable) manner of extracting iron from most asteroids is simply to use magnetic separation. This resulting metal (a mixture of iron, nickel, cobalt, and trace metals) can be formed into products and used as-is. However, this loses some valuable trace elements such as platinum and gold - they are incorporated into the iron products.
For asteroid mining, the choice must be made between returning ores from an asteroid (which might include the entire body), or extracting the valuable materials in-place and returning only those. For example, if the target product is water (from which rocket fuel can be produced), it might make more economic sense to extract the water from the asteroid where it is, and return only the water to Earth, saving perhaps 90% of the return transportation cost.
In any event, the choice of potential asteroid ores is huge. Furthermore, a typical asteroid is a much richer ore for iron-loving elements such as nickel, cobalt, and platinum-group metals (PGMs include platinum and gold) than are any ore bodies on Earth. Some asteroids are also rich sources of water (for rocket fuel) as well as containing everything else needed to sustain life in space. And while most asteroids are thought to contain 20% to 30% of iron, some are nearly pure metal, containing as much as 90% iron.
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