Mining The Sky

John S. Lewis ( Addison Wesley, PSU QB500.266.U6.L49 1996 )

I admire the spirit, but Dr. Lewis misses some important points. Probably the most important is that important ore bodies on earth are beneficiated by plate tectonics, heat, and water chemistry, which only occur on Earth in this solar system. A few important elements, like iron and aluminum and silicon and oxygen, will be common everywhere, but even the nickel in asteroids isn't easy to separate from the iron, and far more difficult where there aren't abundant solvents to help.

This book was published in 1996 - the story hasn't changed much.

Preface: politicians worry about limited resources, but ignore "practically infinite" space resources.

Introduction: p6: launch costs high, energy costs low. All costs are "human attention", and highly skilled attention is required to make a rocket work safely. Earth-crossing asteroids are threats, but also sources of material. However, rock and iron aren't really resources without the complex and solvent-hungry machinery necessary to turn them into industrial purity metals. Lunar pole ice may be a useful resource - but how much is there, and how long would it last in the long term.

Page 108: CI chondrites contain "abundant clay minerals". Not sure about that, clays on Earth result from weathering, while chondrites are assemblies of grains tracing to the beginning of the solar system. Not a whole lot of differentiation there, except for distance-from-the-sun chemistry.

Page 109: Autoreduction of magnetite by carbon to metallic iron.

The author denigrates nuclear power as a waste and proliferation problem - however, the CO2 waste from coal power plants actually endures longer (forever without enough plants to recycle it, our problem today), and nuclear weapons proliferation may be less of a threat than "diverted asteroid proliferation". Look at the Integral Fast Reactor as a way to extract most of the energy from uranium in a proliferation-resistant way, leaving very little actinide waste.

The author proposes earth-atmosphere aerobraking to return materials from the asteroid belt. However, the target presented by the atmosphere - a kilometer-scale sliver with just the right amount of drag to deliver a body to the right apogee and inclination for capture - is an incredibly difficult target to hit from asteroid-belt distances. The earth itself is a much bigger target, so accidents (or terrorist acts) are more probable than success. An alternative is to send the bodies though an "inverse mass driver" on the moon to shed some of the incoming velocity, and re-aim the objects at the atmosphere and the eventual destination. The opportunities for all the elements to line up will not happen often.

The author suggests aneutronic helium-3 fusion, but helium-3 is a trace element everywhere. Trace elements are extremely difficult to extract, and efficiencies are likely to be low. Electrostatic processing of surface grains on the surface of the moon may be one way to get at a short-term supply, but we still do not have fusion reactor technology. On page 136 (citing Gerry Kulcinski at the University of Wisconsin), the author suggested the ITER reactor would be in operation by 2008; as of 2015, it is scheduled for DT fusion experiments in 2027. So far, break-even fusion (even mid-scale experiments) has remained stubbornly 20 years in the future.

Page 121: moving materials from the Lunar poles to equatorial bases - why not just put the bases near the poles?

Silicon solar cells are too radiation sensitive. The high efficiencies and higher radiation resistance of the direct bandgap 3/5 compounds will not be achieved with bulk-crystal silicon, though nanostructured silicon may work better, and silicon microdiodes connected to optical-wavelength dipoles may work even better. Technology marches on.

The author focuses on rockets. Those are the only option in the near term, but we will need to substitute electromagnetic mass drivers and launch loops so we do not need high energy chemical propellants in vast quantities.

The author repeats the hoary old "this is how much material is in an asteroid, and how much it would sell for as metals on earth. See BackYard Minerals for an illustration of how silly this idea is. Undifferentiated rock or mixed metals are not a resource; value is added by refining, and if the material is not naturally beneficiated, the refining is more expensive than the resulting product.

The author talks about Mars, and building a synchronous space elevator in the lower gravity. Except that Phobos and Deimos get in the way. A pendulum elevator hanging from Phobos is easier, though the entire moon is crumbly and must be surface-stabilized or bagged to keep it structurally intact.

The author talks about a giant space colony made of a mid-sized asteroid, containing 1e16 people - nice try, but the organics and the heat and energy flows will dominate the discussion. Life is a planetary surface phenomena. Earthlife can be replicated on a large surface (zero gee is an annoyance best dealt with by bioengineering) but it will require lots of radiation shielding, vastly more without the help of a deep magnetic field. That is one of many reasons why outposts on Mars are unlikely to be self-supporting, with plants growing in an artificially boosted atmosphere.


Dr. John S. Lewis (1941 ) is with the University of Arizona’s Lunar and Planetary Laboratory.

MiningTheSky (last edited 2016-01-05 22:56:22 by KeithLofstrom)