Lunar Surface Mining Challenges

Lunar Surface Mining Challenges

Some people (I doubt it’s many the way Lunar settlement research has been funded) are working on how we can safely and efficiently gather the Lunar regolith from the surface and process it into useful items. Everything I’ve read focuses on robots doing the job as the surface is too dangerous for humans to spend a lot of time mining on it. I’ve decided to list all the challenges of Lunar surface mining that I can find. Not to see how our current technology stacks up (rather poorly I bet) but to maybe come at the problem from a different angle.

Most of these challenges also apply to Mars, asteroids, and almost any object we to extract resources from. The details will change (probably not a lot of regolith dust on Europa) but much will be the same. Figuring out how to avoid these problems (or mitigate them) will let us mine almost anywhere in the solar system.

 

Lunar dust

Lunar dust is a major challenge that we are still struggling to adequately deal with. Most people think of the vacuum, radiation, or meteoroid threats as being the most dangerous. And they are. But if we can’t get a handle on the Lunar dust we may never have permanent Lunar settlements.

So, what’s the big deal about some dust? The Lunar dust is different from Earth dust. The particles are as fine as flour and as sharp as glass shards, which is basically what they are (along with jagged rock debris). The dust particles were created by micro-meteoroid impacts over millions of years. Each time an impactor hit the Lunar surface it shattered the material. Since Luna has never had flowing water or wind on its surface, the edges of the particles have never been worn down. They are just as sharp as the day they were created. An Earth analog would be fine-grained slag or volcanic ash (1, pg 34).

Besides being sharp, the Lunar dust is also very fine (anything less than 80 µm). Any sample of Lunar regolith can consist of up to 20% of particles less than 20 µm in size (3). The human eye can’t even see things below 40 µm (4). The Lunar Sourcebook has the mean grain size at 45-100 µm (similar to silty sand)(1, pg 34) and the  average soil sample particle size at 70 µm, meaning approximately half of the soil by weight is finer than the human eye can resolve (1, pg 478). That’s still not good if you breathe in a cloud of the stuff.

Problems with Lunar dust:

  • It’s harmful to humans. Breathing particles of jagged glass and rock isn’t good for the lungs. But it gets worse. Particles smaller than 5 µm are considered respirable dust (4). This means they penetrate into area of the lungs where gas exchange happens (4). Long-term exposure could lead to silicosis, pneumoconiosis, chronic obstructive pulmonary disease (COPD), emphysema, pulmonary tuberculosis, chronic renal disease, and lung cancer (5). Obviously we MUST do everything to minimize our Homesteader’s exposure to Lunar dust.
  • The dust will quickly destroy moving parts. The particles are very small and very abrasive. They get into everything. They can cause equipment to overheat and fail. They can cause seals to leak (exposing the protected parts to vacuum and dust). They can get into the joints of space suits, causing them to leak as well. Not surprisingly, mining operations have a lot of moving machinery AND generates large amounts of dust.
  • The dust clings to most surfaces. The dust electrostatically clings to almost everything it comes in contact with (1, pg 478).  This is more than an annoyance. Because the particles are so abrasive, it’s like having sandpaper in direct contact with every surface. The dust damaged the astronaut’s spacesuit within a short period of time and will easily scratch glass and optics. Keeping the stuff out of the habitats will be difficult because it doesn’t come off easily. Alan Bean, the fourth man to walk on Luna, stated that “the dust tends to rub in deeper into the garment rather than brush off” (1, pg 34).
  • This electrostatic property causes two other problems as well. First, we have to protect surface equipment from charged dust. Electrostatic discharges aren’t good for delicate electronics (like robots). Second, the sharp gradient of ultraviolet radiation at the solar terminator charges the dust and lifts it above the surface (1, pg 34). This cloud can then move some distance following the terminator (1, pg 34). It’s not a lot of dust; otherwise the craters would be buried. But the stuff can cover solar panels, get into moving parts, and basically cause all kinds of problems. And it doesn’t matter how clean we keep the Homestead site because a new, very thin, layer will be deposited every 14 days when the solar terminator passes over.

Lunar dust is everywhere on the surface. And almost any surface activity will disturb it. Walking, driving, landing rockets, mining, and construction are just some of the ways we’re going to kick up a lot of dust. We’re either going to have to be really clever or we’re going to have to figure out ways to avoid working on the surface. Probably both.

 

Regolith abrasiveness

It’s not just the Lunar dust that is abrasive. The process that created the dust, referred to as impact gardening or space weathering, created all the other particles in the Lunar regolith. Impacts of varying sizes and energies have shattered, buried, exhumed, fused, and transported the regolith since the Lunar surface formed. With no air or water to weather them, all the regolith particles are sharp and jagged. Anything in contact with regolith is going to need to be very durable and most likely replaceable. Things like mining tools, tires/treads, space suits, and equipment/facilities buried under regolith are going to be especially vulnerable.

 

Vacuum

Vacuum is the first thing most people think of when they think of Lunar dangers. There’s no air to breathe! Everyone will be sucked out of the habitat! Their eyeballs will pop out and they’ll instantly freeze! Relax everyone.

Luna does actually have an atmosphere. It’s really thin and absolutely useless for supporting life. During the day the Lunar atmosphere has about 104 molecules per cm3 (at night that increases to 2×105 molecules/cm3)(1, pg 28). The total mass of the Lunar atmosphere is only 25,000 kg (7). That’s pretty insubstantial, especially when compared to Earth’s atmosphere (2.5×1019 molecules/cm3 at sea level)(1, pg 28). For our purposes it’s effectively a vacuum.

The tenuous Lunar atmosphere presents significant challenges to surface operations:

  • The environment is deadly to life. Operating on the surface requires the use of cumbersome EVA suits, pressurized vehicles, and/or specially constructed robots. These add complexity and cost to the operation.
  • Normal lubricants quickly evaporate in low pressure environments. Moving parts need some kind of lubrication or they will seize so this is a problem. There are a few lubricants designed to operate in vacuum but they are expensive and high-tech. There’s no way our Homesteaders will be able to make their own any time soon.
  • Additionally, the lack of oxygen makes it harder to design and maintain moving parts. On Earth, oxides quickly build up on the surface of metal parts. This oxide film contributes to the lubrication of the parts. No oxygen in the atmosphere means no oxide formation. More importantly, without the oxide film parts can become vacuum welded together (8). On Earth the oxide film forms a barrier between two pieces of the same type of metal, preventing vacuum welding. On Luna, that doesn’t happen.
  • Earth’s thick atmosphere protects it against meteoroid impacts. The small stuff burns up in the atmosphere and the bigger stuff is a lot smaller by the time it hits the ground. Luna doesn’t have that protection. Everything hits and it hits at high velocity. More on this in the Meteoroid Impacts section.
  • Heat rejection is much more difficult without an atmosphere (convection). Without using the Lunar regolith (conduction), the only way for equipment to get rid of excess heat is through radiation (9). There’s a lot of math involved but the short version is that getting rid of heat while in full sunlight is challenging.

The near vacuum on the Lunar surface is often seen as an advantage for certain industrial processes and Lunar science. While certain processes would be possible (or at least enhanced) it doesn’t really matter in the long run. The Lunar atmosphere is very vulnerable to “contamination” by human activities. Each Apollo mission introduced as much waste gas as the entire Lunar atmosphere (1, pg 41). And it took many years for the atmosphere to return to normal. The atmosphere will be continuously and increasingly “contaminated” once we’re on Luna permanently. I put “contaminated” in quotes be I think adding to the Lunar atmosphere is a good thing. More on that later.

 

Radiation

Without a strong magnetic field and a thick atmosphere Luna is awash with radiation. Anything not under shielding will be directly exposed. And space suits provide the barest of protection against anything except ultraviolet radiation. That’s a major reason why the Homesteads have to be under at least 6 meters of regolith shielding (10).

I’m not going to go into detail here but the types of radiation we need to worry about are:

  • Ultraviolet radiation – The same stuff that sunburns you at the beach. Except on the Lunar surface it’s much more powerful. We’re not too worried about UV as it’s pretty easy to shield against. It’s the ionizing radiation (everything below) that’s the problem.
  • Large fluxes of solar-wind particles – This is the constant radiation coming from the sun. Compared to the next two sources, the solar wind isn’t a huge problem. Penetration depths into the Lunar surface are measured in micrometers (1, pg 48).
  • Intense particle fluxes emitted by solar flares – Solar flares or Coronal Mass Ejections can produce three types of dangers: high energy electrons, high energy particles, and ionized gas (10). High energy electrons are the least dangerous and are easily shielded against but exposed electronics could be impacted. High energy particles are extremely dangerous and can penetrate to a substantial depth. I couldn’t find much information on the ionized gas but let’s just assume you wouldn’t want to be outside in a suit when it hits.
  • Small fluxes of high energy galactic cosmic rays – These high particles originate from outside the solar system. GCRs can penetrate pretty deep because of their very high energies (meaning they are moving very fast).

In the case of the high energy particles (from CMEs or GCRs), too little shielding is worse than none at all. These particles hit the shielding particles and cause them to start moving fast (think about using a pool ball to set a bunch of other balls into motion). Now you’ve got a shotgun blast instead of a rifle bullet. All those secondary particles are going to cause much more damage than the original particle if the shielding isn’t thick enough to stop everything. That’s why we need so much regolith over the habitats.

Excessive radiation exposure can damage or kill cells (that’s what causes cancer or kills). Our robotic companions aren’t safe either as radiation can also damage electronic components. Planning on simply bringing people and equipment in if a radiation event threatens isn’t practical either. We may not get a lot of warning when a solar event occurs. It may be impossible to shut everything down and get it safely under cover in time. GCRs and the solar wind are present all the time.

 

Meteoroid impacts

The Lunar surface is defined by meteoroid impacts. The mare (maybe), craters and regolith were created by them. The surface is still being reshaped every day by constant small impacts. A meteoroid is a naturally occurring solid body that is too small to be called a comet or asteroid (1, pg 45). A meteoroid with a diameter of less than 1 mm and a mass less than 10-2 is considered a micrometeoroid (1, pg 45).

Fortunately, the larger impactors don’t come along very often. It’s estimated that a piece of equipment or facility on the surface will be hit by a 1 milligram mass every year (1, pg 46). One milligram (10-3 grams) doesn’t sound like much until you factor in that it’s moving at 13-18 kilometers per SECOND (1, pg 45). That’s a lot of kinetic energy! Enough to create a 500 µm crater in metal (1, pg 46). The probability of a 1 gram meteoroid striking an astronaut or piece of equipment is about 1 in 106 or 108 per year it’s exposed (1, pg 46). A 1 gram meteoroid could make a centimeter deep crater (1, pg 46). That could cause significant damage to a piece of equipment or a habitat. And ruin a surface worker’s day.

The odds are pretty low that a Homesteader will get hit by an object big enough to be a threat. But it’s not zero. And some of us have pretty bad luck.

 

Temperature extremes

Luna experiences significant temperature changes because it lacks a substantial atmosphere. The mean surface temperate during the day is 107º C during the day and -153º C during the night (1, pg 28). But that doesn’t tell the whole story. Temperatures can swing between 123º C and -233º C (1, pg 28). Obviously that would kill most living things. The problem is that it’s not good for electronics or machinery either.

Almost all of the heating comes from the Sun as Luna lacks an internal heat source (6). How much energy a particular location receives depends on its latitude and relative position to the sun. Equatorial sites out in the open will get the hottest while polar sites inside the permanent shadow of a crater will be the coldest (which is why there could be ice trapped there).

There’s a lot of math involved in figuring out how hot or cold a place can get (check out the Lunarpedia entry to see some). For our purposes here it’s enough to know that equipment, robots, people, and facilities that are exposed on the Lunar surface will need to be designed to handle a significant thermal range.

 

14 day light and dark cycles

A Lunar “day” is 14 Earth days long. So is the Lunar “night”. Peter Kokh of the Moon Society used the terms “dayspan” and “nightspan” for each 14 day period in his Moon Miner’s Manifesto newsletter. I like it so I’m using them also. The reason for such long periods of light and dark is that Luna is tidally locked with Earth. The same side of Luna always faces the Earth because it takes the same time for Luna to rotate around its axis as it does to orbit Earth.

This presents three significant problems for our Lunar surface miners:

  • When it gets hot, it stays hot for a long time. And when the surface cools off it stays cold for many days until that area is in sunlight again. Not only does the equipment have to survive extreme temperatures; it also has to do it for days at a time.
  • We’re going to be running on stored electricity for 14 days (at least) if our Homestead is primarily solar powered. That’s going fundamentally change how we extract resources. And let’s be real. No Homestead is going to run a nuclear fission power plant. The best they could hope for would be several radioisotope thermoelectric generators (RTG). Maybe eventually running off local Lunar thorium (I have no idea if that’s even possible) or uranium. Anyway, solar power (solar electric and solar thermal) is the only real way to go. And for half the month our Homestead won’t have any.
  • Nightspan operations could be significantly more hazardous. Not a single human has spent a nightspan on Luna. All the Apollo missions landed in “mid-morning” and were gone before “noon”. It’s not necessarily the dark that is the problem (a full Earth in the Lunar sky would be pretty bright and an awesome sight). The problem is that it’s hard enough to determine detail and distance when the Lunar landscape is fully illuminated. The Apollo astronauts observed that there was a tendency to underestimate distances (1, pg 29). No one knows how much harder it will be at night. But we need to assume that nightspan surface operations will be more difficult and dangerous. The good news is that there is less danger from the sun (which is then shining on Farside).

 

Low gravity

The gravity on the Lunar surface is 1/6 that of Earth’s (1.62 m/sec2 vs 9.81 m/sec2)(1, pg 28). This is a huge advantage for our Homesteaders as it takes a lot less energy to get into orbit. This easy access to space means that Luna could become the center for the settlement of the solar system.

However, there are two primary issues with the low Lunar gravity (not including all the possible health issues):

  • Regolith moving equipment will have much less traction. On Earth we rely on gravity to provide enough traction for the equipment to operate. It’s Newton’s third law (for every action, there is an equal and opposite reaction). On Earth, a bulldozer pushes against the ground. The ground “pushes back” with equal and opposite force. But the weight of the bulldozer (produced by Earth’s gravity) helps to counter that opposite force. And so the bulldozer does whatever it needs to do. On Luna, there’s 1/6 the gravity and therefore 1/6 the countering force. We’ll have to greatly increase the mass of our machines (to increase their weight), secure them to the surface (tethers maybe), or be OK with them doing a lot less work (exerting less force on the ground).
  • We’re going to have to relearn how to do things. At least those Homesteaders that grew up in Earth’s gravity.
    • Things still have the same mass. A 60 kilogram object weighs 10 kilograms on Luna. But it still has 60 kilograms worth of mass. Accidentally get an object moving fast enough and you could be in trouble.
    • Moving things on the surface takes some getting used to. The lack of an atmosphere, coupled with reduced gravity, can make objects seem to weigh 1/10 of their Earth weight (1, pg 27).

 

Regolith variability

This isn’t a threat or a hazard to our Homesteaders. But it is a big complication for resource extraction. Because of the way it was formed, the regolith is one big heterogeneous mess. Agglutinates, various mineral fragments, glass, highland anorthosite, mare basalt, free metallic iron, and more are all mixed up and unevenly distributed throughout the regolith. The regolith contains particles from very fine dust to large rocks. There’s variability between sample sites, within sample sites, and within samples.

Our equipment can’t be finicky.

We’ll need to also process large chunks of bedrock eventually. Regolith has its use but the real goal is to mine and process the underlying bedrock (mare basalt or highland anorthosite). There will be less variability than regolith, the mare basalt has a significant amount of iron in it, and there is just more bedrock than regolith in general. Plus, we can dig down into bedrock to construct our Homesteads, towns, and cities instead of huddling under regolith blankets on the surface. Imagine a Lunar Khazad-dûm. 😉

 

Regolith interlocking

I remember reading about how the jaggedness of the regolith helps it lock together, making it more difficult to dig into. I’ll need to find some actual documentation though.

 

Moonquakes

Moonquakes have been recorded reaching 5.5 on the Richter scale and lasting more than 10 minutes (2). The Lunar Sourcebook places the largest recorded moonquakes at magnitude 4 (1, pg 39). That’s still a significant amount of energy. Unfortunately, we only have recordings for 8 years and larger quakes just may not have happened during that time period (1, pg 39). Plus, no human has ever been on the Lunar surface when a moonquake occurred. We really don’t know what to expect.

Surface mining operations could be significantly impacted by even “small” moonquakes because:

  • Personnel and equipment in open pits and trenches could be buried in loose regolith.
  • Pits and trenches in use could collapse, requiring extra effort to put back into operation.
  • Vehicles, robots, and equipment can be knocked over and damaged.
  • Regolith piles (either material to be processed or “slag”) could be disrupted/collapsed. This could damage nearby equipment.
  • Surface personnel will either need a place to shelter or be brought back to the habitat for the duration of the quake (I really doubt anyone will want to spend 10+ minutes on the surface when it’s rocking and rolling). This process will certainly disrupt mining operations and could increase the risk to Homesteaders on the surface.

While not as obvious as vacuum and radiation, moonquakes could be almost as big a challenge. Especially since we don’t know much about them.

 

Isolation

Luna is between approximately 363,000 km and 405,700 km from Earth. Early Homesteaders won’t be able to receive help quickly. And they won’t be able to go home if something bad happens. It could take 3 days or more to get supplies to a Homestead even if a rocket was on the pad and ready to go. Heck, it takes 1.3 seconds for a radio signal to go from Earth to Luna. Try having a phone conversation with a 3 second delay between responses. It’s significant. Homesteaders are going to have to be able to deal with issues on their own. OK, hopefully they can get long-distance technical and moral support from Earth (if their radio is still working).

Capturing volatiles

The top layer of regolith contains a not insignificant amount of gas not present in the Lunar rocks. Some of this gas (hydrogen, nitrogen, carbon, etc.) will be critical to maintaining and expanding the Homestead. Some of it will be important to industry (helium 3, argon, etc.). So we’ll have to design our mining equipment to scavenge the gas before it escapes into the vacuum.

 

The need for advanced robotics

We must limit the time humans spend on the surface because it is so hostile. The current paradigm is to use really complex and expensive robots to do the job. And because these robots have to operate without a human nearby to fix them, they have to be super reliable. They also need to be at least semi-autonomous to account for the time delay between commands. That means lots of designing and testing. And spending a lot of time and money. It’s obviously very hard to do as we still haven’t had a single mining robot actually land on the Lunar surface.

 

These are some serious challenges. That’s why I’ve come up with SPORE.

Resources

  1. Lunar Sourcebook (www.lpi.usra.edu/publications/books/lunar_sourcebook)
  2. Lunarpedia – Moonquake (lunarpedia.org/w/Moonquake)
  3. Lunarpedia – Dust (lunarpedia.org/w/Dust)
  4. Engineering toolbox – Particle sizes (www.engineeringtoolbox.com/particle-sizes-d_934.html)
  5. Centers for Disease Control (CDC) – Health Effects of Overexposure to Respirable Silica Dust – (www.cdc.gov/niosh/mining/UserFiles/workshops/silicaMNM2010/1-Colinet-HealthEffects.pdf)
  6. Lunarpedia – Lunar Temperature (lunarpedia.org/w/Lunar_Temperature)
  7. Lunarpedia – Lunar Atmosphere (lunarpedia.org/w/Lunar_Atmosphere)
  8. Lubrication and Bearing Problems in the Vacuum of Space (ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680017942.pdf)
  9. Thermal Management in Space (space.nss.org/settlement/nasa/spaceresvol2/thermalmanagement.html)
  10. Lunarpedia – Radiation Problem (lunarpedia.org/w/Radiation_Problem)

 

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