Mining the Lunar Surface
The fine, powdery material that makes up the lunar surface is most commonly called "regolith" in the scientific and engineering literature, so that tradition will be followed here.
The earliest studies had conventional equipment strip mine the moon's surface, e.g., "front loader" vehicles to scoop up the regolith and drop it into "haulers" which would bring it back to the materials processing site.
In these scenarios, the vehicles would be launched from Earth and need to be assembled after landing on the lunar surface. The haulers would not be as structurally massive as Earth haulers due to the Moon's one-sixth gravity. However, the loaders would be nearly the same, since they need a counterweight when scooping up regolith. Simple counterweights would be produced from lunar materials, not launched from Earth.
Application of terrestrial mining experience and technology to the lunar environment, with emphasis on teleoperated and automated systems, is covered in a U.S. government (public domain) paper presented at the 1993 AIAA/SSI conference by E.R. Podnieks (Senior Staff Scientist) and J.A. Siekmeier (Civil Engineer, Twin Cities Research Center) of the U.S. Bureau of Mines, entitled Terrestrial Mining Technology Applied to Lunar Mining (paper reference).
A significantly improvement may be to replace the front end loaders with a simple bucket-and-reel system, called a "flail" or "slusher", as shown below, as put forward by mining engineer Robert Gertsch (ref. 3). It pulls the dirt up a ramp and into the hauler.
NASA artwork by L. Ortiz showing Richard Gertsch's slusher mining system.
Since the surface material is powdery, a flail may handle the job of collecting bulk lunar material.
The above are methods used on Earth and adapted to the Moon. Other methods have been proposed which have no precedent on Earth, but they may be considered too new and unproven to be considered by a private venture in the near term.
For most of the Moon, the top few meters of the lunar surface consists of a mix of minerals, whereas lower depths probably offer more uniform minerology from the old magma oceans. The mix on the surface is due to all the splashes of asteroid impacts which has mixed materials from distances. Also, the surface is glassier due to the superheated nature of the asteroid ejecta and the subsequent quick cooling.
Many proposed methods for materials processing call for processing just one particular mineral, e.g., ilmenite. This would require either separating the desired mineral from the regolith mix, or mining underneath the surface where the mineral may be found in a fairly uniform state.
Which mining method is used, and which mining site exploited, depends upon which products are desired, the processing methods employed, and the amount of investment that financiers are willing to put forth.
Various issues in lunar mining
The Apollo missions were surprised by the difficulty of extracting subsurface samples. While the top was powdery and soft, attempts to drill into the surface and extract subsurface material resulted in seizing of drill tubes which could not be removed and had to be abandoned.
It is now thought that underneath the very top layers, lunar soil is actually more dense than equivalent Earth soils at the same depth. Due to small, repeated vibrations by distant meteor impacts over the eons, the soil particles have settled down by shifting relative to each other into ever more dense geometrical orientations. Thus, it is now recommended that any experiments in mining lunar simulants first settle the material by vibration, not by compressing the material. Indeed, it's been found that compression is not nearly as effective as vibration (within nondestructive compression pressures). (Asteroids are probably the opposite - they don't have the strong gravity to cause settling, and in fact meteor impacts may serve to make asteroids fluffier.)
Another issue is rubbing friction in vacuum. The U.S. Bureau of Mines found that exposing lunar simulant to vacuum long enough for nearly complete outgassing caused increased friction between the tool and lunar simulant -- from 1.5 to 60 times more friction! On the lunar surface, it will probably be even higher due to incompletely oxidized minerals and total absence of moisture. Tools should be made from (or coated with) materials which will minimize friction, and experiments should be performed on simulants that have been sitting in vacuum. (This applies to asteroids as well.)
On the Moon, temperatures vary from approximately -170C (-275 F) at night (due largely to the long, two week lunar night) to around 140C (280 F) in the day. Significant differences in temperature occur between shadows and sunlit areas. Either surface mining equipment must be designed to operate in high temperatures, or a partial sunscreen should be put over the mine, possibly complimented with foil mirrors to eliminate shadows. Various schemes may be employed to regulate the temperature of areas being mined. At night, surface mining equipment will need to be sheltered, e.g., a tunneled garage or a canopy with heaters. (The temperature extremes are much less with asteroids, which typically rotate roughly four times a day though rotation period can vary over a wide range from asteroid to asteroid.)
Gravity on the Moon is one sixth that on Earth, and the mining and processing equipment must be designed accordingly. This does not pose a major challenge, but like working in vacuum with electricity instead of petrol, the Moon will require design of new equipment, even if it's based on Earth mining conventions. (For asteroids, the mining processes need to be radically different from most mining processes on Earth.)
One of the best sources of expertise on lunar mining can be found at:
The Center for Space Mining
Contact Richard E. Gertsch (see paper references 1, 3), Levent Ozdemir (see paper reference 1), or R. J. Miller (see paper reference 2).
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