© Norman Sperling, April 30, 2012
The media made a big hullabaloo over the public announcement of forming a company to mine near-Earth asteroids.
In several ways, the announcement sounded right:
* Launch a fleet of spectroscopic telescope satellites to "scope out" potential targets. Wise!
* They distinguished between icy and heavy-metal asteroids, and mentioned the potential values of each. Correct.
* First, target the icy, primitive asteroids (types C, P, D, and probably K) because their ice can make rocket fuel. So far so good. They're also abundant, contain the widest variety of minerals, and are the loosest-bound, so they should be easiest to mine. But the "rare earth" metals are pretty skimpy in these asteroids. Not as bad as Earth's surface rocks, but poor ore.
* Media reports recognize that minerals which are valuable because of scarcity will become much less valuable if the market is flooded. They include the concept of rationing to slow the flow. I expect that must occur naturally, because it will take time to break up and refine an asteroid. Attaching mining devices to an asteroid hardly makes the entire asteroid immediately available as refined metals.
I didn't see the media discuss another big factor, which is both an asset and a liability.
Metal asteroids (type M) are remnant cores of formerly-larger planet-like bodies. They accreted so much that they heated up. They get heat from collision, sunlight, condensation, and the decay of radioactive atoms inside. As long as they're small, they radiate heat out faster than they collect it. But bulk acts like a blanket, so once an object builds up to more than a few hundred kilometers in diameter, it can't dump heat as fast as it builds it up. If you don't mind a sip of technicality: that's because as an object gets bigger, the volume (in which to generate and hold radioactive heat) grows as the cube of the radius, but the surface (from which to radiate heat away) only grows as the square of the radius.
Under the heavy pressure of hundreds of kilometers of minerals sitting on top of them, and the increasing heat, primitive rocks melt. They quickly differentiate: light stuff floats, and dense stuff sinks. This results in layers, in order of density. That's why Earth's layers are the inner core, outer core, mantle, crust, hydrosphere, and atmosphere.
Those aren't pure, refined elements. They are mixtures, alloys, suspensions, and a variety of other combinations.
Cooled-off, solidified nickel-iron outer cores are what we think we're seeing in type-M asteroids. All our metal meteorites are from those outer cores. Iron shells are probably awfully tough to break by collisions at the speeds common in the asteroid belt. But mining engineers can probably crack that problem.
The big problem comes from exposing the inner core, to which most precious heavy metals migrate. The inner kernels may be relatively small. The mix there will have every heavy element that doesn't linger up here on the surface. That's why they're the rarest up here. Those include radioactive elements with long half-lives. In other words, the core alloy must be radioactive. I saw no mention of this important factor in the company's statement or media coverage.
We don't even know which substances dissolve into one another under the conditions of the inner core. The radioactive and the quiet minerals probably make novel combinations with unknown characteristics. Non-radioactive components have been irradiated for 4 billion years. Would that induce unfamiliar radioactive isotopes?
Metal asteroids that expose some of their radioactive inner core might be detectable by that radiation. I've never seen a study relating unattributed detections of ionizing radiation to the locations of type-M asteroids. I wonder if we've already detected some, but not recognized that yet.
Surely, to extract useful minerals from an inner core will require a lot of refinement. Refining enough uranium and plutonium for bombs and reactors required building entire scientific cities - Hanford, Oak Ridge, and so on - running enormous factories round the clock for decades. Similar operations with robots, in space, will probably be extremely expensive. How would mining robots recognize and handle the radiation? Refinery hardware and electronics would have to survive intense radiation as well as extreme temperatures and vacuum. Transmutation of the robots' own atoms would change their usability.
Components for use among people on Earth would have to emit no more than background levels of ionizing radiation. What an extreme refinement!