In-Situ Resource Utilization (ISRU) FAQ

Welcome to the In-Situ Resource Utilization (also known as ISRU) Frequently Asked Questions (FAQ) page. You can read all about Lunar ISRU in other parts of the LH website. For this page, I just want to cover some of the components of Lunar ISRU that we’ll have to hammer out before we send settlers there. Even though it’s a FAQ, there’s not really many “frequently asked questions” here. It’s more of an expression.

Lunar Homesteading, and space settlement in general, is impossible without effective and comprehensive ISRU. We’re going to be stuck with small outposts/bases that have to be continually resupplied unless we figure out how to “live off the land”.

This topic will be covered in much more detail in the third book of Lunar Frontier Manual series, “In-Situ Resource Utilization (ISRU)”.

What is ISRU?

Let’s break it down first:

In-Situ = Latin phrase for “on site” or “in position”. Depending on its context it can mean many things. For our purposes, it means “on site” or “local”. It’s just a fancy way of saying Local Resource Utilization.
Resource = A stock, supply, or source that provides support and allows for proper functioning. Anything in your local environment that you can use.
Utilization = Another fancy word. Utilization basically means “use”.

So, In-Situ Resource Utilization effectively means “Local Resource Use”. Why don’t we just say that? I honestly don’t know. ISRU is just an accepted term used in the space community. Probably because scientists and engineers like “sciencey” sounding terms.

Here’s what it means. We figure out what resources are available at the site we’re interested in. Then we figure out how to extract those resources, process them into usable forms, store them, and turn them into useful products. Basically “living off the land”. The whole point is to reduce, and eventually eliminate, the need to ship stuff from Earth.

Resources aren’t just rocks. The solar energy that photovoltaic cells use to make electricity is considered an in-situ resource. Ice and volatiles mixed in the regolith also count. I’ll talk about resources more in a bit.

Unfortunately, there hasn’t been any Lunar ISRU experiments actually conducted on Luna yet. There really haven’t been that many experiments period. Much of the ISRU ideas we have are theoretical. Lunar Homestead is working to change that.

Why ISRU is so important

There just can’t be Lunar settlements without effective ISRU tech. There’s no way any government or organization can sustain the cost of constantly resupplying a growing base or settlement with everything they need for very long. At some point the settlers will need to start taking care of their needs without external help or they will fail.

Even more importantly, all off-Earth settlements need to be able to provide for their basic needs in case shipments from Earth are disrupted. It doesn’t take much imagination to think of things that would cause this. War seems to be a perennial favorite activity for humanity. Any kind of nuclear exchange would most likely stop supply shipments to Luna for a while. Or permanently, depending on the severity. Pandemics and economic disruptions could also cause real problems for our Homesteaders. And let’s not forget the looming Climate Crisis.

ISRU is also one of the primary ways Homesteaders will be able to pay off their backers and make money to buy the things they need (and want). Excess oxygen can be sold as propellant or to new Homesteads. The same goes for iron and aluminum (metal/oxygen rockets for the win!). The products resulting from lots of Homesteads conducting ISRU will form the backbone of the cis-Lunar economy.

All Homesteads are mines

All Homesteads have to conduct continuous mining operations. Even if it’s not their primary, or even secondary, priority. There are a couple of reasons for this:

Lunar rocks, and perhaps ice, are a Homestead’s primary source of oxygen. Here at Lunar Homestead, I’m not as excited about the ice as most other people so I’m focusing on the rocks. The importance of oxygen is covered in the next section.
The Lunar regolith and mega-regolith contain a number of useful metals that Homesteads will continually need.
By using SPORE, Homesteaders will be expanding their habitable space. They can keep the space unpressurized until they need it or have enough nitrogen to pressurize it. They can expand their surface mining operations, follow underground deposits, or even create tunnels to connect their Homestead to other settlements. More habitable space is always good.

Identifying Lunar resources

Let’s start with the Lunar regolith. While it’s true that Homesteaders will initially avoid the regolith; eventually they will want to mine it for its resources. Book 2 (Settlement Challenges) in this series will cover all the hazards and challenges associated with surface mining operations. I haven’t figure out how they are going to mine the regolith safely yet.

It’s often stated that the regolith is a great resource to mine because it’s already been broken into small pieces. OK, so what? Did we suddenly forget how to break rocks? Sure, turning rocks into powder on an industrial scale requires a lot of energy. Fortunately, Homesteads operate on a much smaller scale. The LH website has plans on how they can do this without needing lots of infrastructure or resources.

So, what does the regolith have that the mega-regolith doesn’t? Remember, there is an awful lot we don’t know and Luna is going to constantly surprise us. All of this is subject to change.

Water – Lunar ice is the hot topic these days. Everyone wants to mine it and fire it out rockets. I think this is a terrible waste of a rare Lunar resource but I’m in the minority. Anyway, remote sensing data shows us that craters with permanent shadows could contain ice. We don’t know what form that ice is in (pure, mixed with regolith, or combined with other stuff) or how deep it goes. We also don’t have any experience operating in such a cold environment. The ice we find could be frozen harder than a rock. There is a lot we need to learn before we can effectively mine Lunar ice.
- There might also be some water trapped in agglutinates (clumps of rocks and minerals fused together by glass) in the regolith. This water could be found anywhere, not just in polar permanently shadowed areas. It wouldn’t be a lot but we could recover it when we’re processing regolith.
- We don’t know if there are sources of ancient water in the mega-regolith. Volatiles, such as water, can be buried by impact ejecta (a process called ballistic sedimentation). It’s not inconceivable that our Homesteaders could hit a layer of ice while excavating. I wouldn’t count on it though.
Free iron – Around 1% of regolith is free iron [98] (not chemically combined with the rocks). The theory is that we can “easily” extract this iron by using a magnet. And that may be true but a lot of this iron is both: a) so small (it’s called nanophase iron) that it may be useless to us and b) bound up in agglutinates. I have a hunch that it’s going to be rather difficult to extract useful amounts of free iron from the regolith. As far as I know, there haven’t been any experiments to confirm or deny this.
- Chemically extracting iron from bulk basalt material should result in much more iron in a more uniform form. We shouldn’t ignore the free iron in regolith but I don’t think we can rely on it.
Glass – A large percentage, 25-30% [99], of the regolith is composed of agglutinates. Agglutinates are simply small particles of rock (and other stuff) fused together with glass. We don’t know how much of the mega-regolith is composed of agglutinates. Since the majority of agglutinates are formed by small micro-meteoroids (5.5×10-5 to 7.0×10-8 g) [100], I’d assume not nearly as much as in the surface regolith. Glass can be used for quite a number of things so mining the regolith for it could be a high priority.
Pyroclastic deposits – These are the result of volcanism. They are another source of glass. In fact, the Apollo 17 crew found, and took samples of, fields of orange glass beads on the surface. In addition to glass, these beads also have high concentrations of oxygen, iron, titanium, and volatiles. Mining pyroclastic deposits would be one of the few surface activities that Homesteaders start almost immediately.
Volatiles – Luna is deficient in many useful elements. The big three we need for settlement are hydrogen (for water and industry), nitrogen (for the atmosphere and agriculture), and carbon (for agriculture and industry). All of these are found, in very small amounts (<100 parts per million) [101], in the top layer of the regolith everywhere. Homesteaders will have to move and process a lot of regolith to extract useful quantities. The LH solution is to simply purchase a shipment of methane (CH₄) and ammonia (NH₃) whenever carbon or nitrogen is needed. Sure, it costs money but they won’t have to mess around with the regolith.

OK, let’s move on the mega-regolith. The mega-regolith is hardly ever mentioned when Lunar resource extraction is discussed. This baffles me because it seems like a better place to mine than the regolith. Most of the surface challenges are significantly mitigated, or eliminated, and it’s a more uniform material.

The mega-regolith is the layer under the regolith and consists mostly of impact ejecta thrown out when a crater is made. We know almost nothing about the mega-regolith except for some seismic data and remote observing of craters. None of the Apollo drive tubes reached down deep enough to get core samples of the mega-regolith. This may seem like a huge negative but our knowledge about the regolith is only slightly better.

The mega-regolith has several distinct advantages over the regolith:

It is not the surface regolith – This alone makes the mega-regolith better.
It should contain much less dust – The fine Lunar dust is a product of millions of years of continual, low-energy surface impacts. Because of this, the top layer of the Lunar surface contains the highest concentrations of dust. We don’t know for sure but we should encounter less dust the further down we go.
It contains uniform (and larger) particles of bedrock material – The regolith is a complex heterogenous mixture of local rocks, rocks from other areas (ejected by large impacts), and glass. Samples collected just a few meters apart can be very different from each other. This can make resource processing tricky. We think that the mega-regolith consists mostly of chunks of local bedrock. This would be basalt since we’re initially interested in mare regions. We can tailor our processing equipment to deal with mega-regolith basalt and not the highly variable regolith.
Homesteaders can use SPORE – Even if we wanted to dig though regolith (we really don’t though), it’s just not thick enough in most places to be used as shielding (2-4 meters median depth in mare regions) [103]. For SPORE to work, Homesteaders are going to want a lot more protection than that. SPORE wouldn’t work if we were only concerned with the top surface of the regolith.
Mining the mega-regolith is like hunting for treasure – Remote imaging of the Lunar surface gives us a good general idea of what resources we’ll find on the surface in a given area. Of course, there will still be many surprises because we really don’t know much about Luna. However, the mega-regolith is truly unknown. Nobody knows what we’ll find if we dig down to the core of a mascon. Or under a pyroclastic deposit. The science will be incredible, as will the treasure.
Basalt, basalt, basalt – We think that the mega-regolith is composed of large chunks of basalt. Basalt is the key to making the early Lunar Homesteads possible. Lunar basalt is primarily composed of pyroxene, plagioclase, olivine, and ilmenite. All of these minerals have a lot of oxygen, silicon, and iron. There are also titanium, aluminum, and a bunch of other useful elements. The feldspathic highlands also have a suite of useful resources but for now we’re focused on the mare because of its higher iron content. We’ll find larger blocks of basalt the further down we dig.
Similar to Earth basalt – One of the big advantages of focusing on Lunar basalt is that it’s very similar to Lunar basalt. There are a few minor chemical differences such as which elemental form of iron is present (it’s still iron though) or titanium abundances but they are essentially similar. This will make the research easier because we can simply use terrestrial basalt and not some kind of Lunar simulant. The same can not be said about Lunar regolith, which is a pretty exotic substance in its natural environment. We can get the chemical composition pretty close to a particular sample but factoring in the agglutinates, space weathering, vacuum, gravity, and other parameters makes it impossible to create a true Lunar regolith simulant. Even if we could, the regolith varies so much from site to site that any simulant would be of limited usefulness.

Most of the Lunar surface is awash in solar energy, at least for 14 days each month. The trick is to capture and store enough of the energy to survive the 14 days of darkness. Total solar irradiance (TSI) at one astronomical unit from the Sun is about 1,361 W/m² [104]. That number is going to vary based on angle and distance. This sunlight is a resource we’ll need to extract and use. Peaks of Eternal Light (PEL) (mountain tops at the poles that receive constant light) are very popular these days but they have significant drawbacks. One of the biggest is that there just aren’t that many of them. I can see all kinds of conflict happening because of these locations. The LH solution is to design Homesteads so they can thrive anywhere, thereby eliminating the need for PELs. Homesteads will need to be smart about how they collect, store, and use energy. Energy ISRU gets its own book and section on the website.

Many people extoll the virtues of the Lunar vacuum and talk about how we can use it for all kinds of manufacturing processes (almost all of them theoretical at best). The fact is that all human activity will degrade the Lunar vacuum. Each Apollo landing doubled the total ambient Lunar atmosphere and it took months for that contamination to dissipate [102]. Lunar bases, settlements, and Homesteads are going to contribute much more. Rocket exhaust, leaking habitats and suits, and industrial processes will all flood the Lunar atmosphere with gases. Continued human activity will significantly degrade the Lunar vacuum. And that’s OK because any process that needs vacuum should be in space anyway; not on a dirty planetary surface. A thicker Lunar atmosphere may also provide certain benefits as well. I’m not talking about terraforming Luna though. That would probably be a Very Bad Idea.

Finally, there’s waste recycling. While technically part of ISRU, I’ve giving waste processing its own book and section in the Database. There really is no such thing as waste in a Lunar Homestead. Everything becomes feedstock for another process when it’s at the end of its useful life. No exceptions. If we can’t figure out how to completely recycle an item then we need to redesign it.

Resource extraction

OK, we’ve identified what resources exist in our Homestead’s local environment and which one’s we want to get. Now we have to design and build equipment to get the material (mega-regolith, regolith, ice, etc.) from where we found it into the processing equipment.

There are plenty of people (with a lot more resources than I have) working on the tech for extracting ice from the regolith, so I’m not going to spend time on that. There are also people working on surface regolith mining, so I’m passing on that as well for now. LH is going to focus on sub-surface excavation (aka tunneling) using Shielded Pressurized Oxygen Resource Extraction (SPORE) tech.

I briefly touched on SPORE in the previous chapter. Homesteaders will be able to extract resources and create habitable space while eliminating, or mitigating, all of the surface hazards by using SPORE tech. SPORE will come up again in the next section as well. SPORE will be discussed in detail in the next book in this series. There’s also lots of information on the website if you want more information.

One component of the SPORE system is the actual extraction equipment. We think that particle size increases with depth. So, the further down we dig, the larger the rocks we’ll encounter. Smaller particles are packed around these larger rocks. There will probably be cracks and voids we’ll have to deal with as well. SPORE and the mining equipment will have to be designed to deal with unknown and quickly changing conditions. The extraction equipment will have to be able to handle densely packed small particles (sort of like sand) and solid basalt (when they run across a large rock). At least we won’t have to deal with groundwater and mud; although the water would be really useful.

I’m starting the extraction equipment research at the low tech possible. Hand drills, shovels, picks, etc. Good old manual hand tools. Here’s why I think it’s critically important to develop these tools before moving on to more advanced ones.

Safety – Machines break. If those machines are the only way Homesteaders have to extract vital resources (like oxygen) then they will eventually be in trouble. A functional shovel and prybar are good backups to the more complex machines and could save the day.
Self-reliance – It will be easier for Homesteaders to build a shovel from local materials than a pneumatic drill. Or a robot with a pneumatic drill. Manual hand tools may not be the most efficient, or sexy, option but they could be the most cost effective because nothing has to come from Earth.
Less energy intensive – Homesteads are going to be energy restricted until they have built enough collection and storage infrastructure. Anything that reduces energy consumption helps.
Exercise – Astronauts in orbit have to spend hours every day exercising just to maintain their health. Lunar Homesteaders will probably have to do the same. That’s just an assumption though; I haven’t done any research. And it may not be necessary if they aren’t planning on going back to Earth. Anyway, some manual labor digging through the mega-regolith could help keep them productive and fit.
Small scale – Remember the small-scale rule? Homesteads aren’t industrial mines. They don’t need huge borers or dump trucks. Humans have constructed some amazing underground structures using simple hand tools and determination.
Curiosity – I want to see how low-tech we can possibly go. Plus, nobody else is working on stuff like this (as far as I know). Everyone loves the high-tech stuff.

Manual hand tools are not the ultimate goal. They are the foundation that other tools are build upon. Electric and pneumatic (oxygen instead of air) powered tools are next. We’ll still need to figure out how to build them with 100% Lunar materials however. Then automated equipment with Homesteader supervision. Finally, fully automated equipment; although I don’t think Earth mining has gotten to this point yet.

One comment about machines that are teleoperated from Earth. This is not as easy or simple as many people make it sound. The general thought is that people living on Earth can operate the machinery, freeing up the settlers to do more productive work. This would result in increased efficiency and cost savings. I’ve got two problems with this.

First, there is a significant time delay for communications between Earth and Luna due to the distance (384,400 km) [105]. Radio waves move at the speed of light (299,792.458 km/s) but it still takes a signal 2.56 seconds (on average) to go to Luna and return [105]. Now add in the time it takes to get the message to and from the operators. And add in some more time for the operator to assess the situation and respond. If something happens to a teleoperated device it could easily take 10 seconds before the operator’s response reaches the machine. That kind of delay could get people on Luna killed.

Second, the technology to enable teleoperation is pretty sophisticated. Not the kind of stuff a Homestead could build on its own. Most of it would have to be bought and shipped from Earth. Relying on equipment teleoperated from Earth would put a serious dent in any Homestead’s self-reliance. Which would also reduce the overall safety of the Homestead as well.

Moving the material from the mine face to the processing equipment is the other part of the process. I’m thinking simple Luna-made buckets would work. A more efficient solution would be to have manually pushed Luna-made carts (wheeled or on temporary rails). A manually operated block and tackle system can be used to raise or lower carts (or buckets) to various levels. Higher tech solutions would use conveyors (more energy intensive) or robotic carts (requiring Earth components) to move the excavated material. I haven’t really looked into the material moving options yet.

One thing I do know is that we’ll have to design the processing equipment so it can be moved to a new location. This doesn’t have to be easy but it does have to be possible. The reason for this is so Homesteaders can maintain a reasonable distance between the mine face and the processing equipment. This isn’t just for efficiency and safety. They’ll also want to use those habitable sections for other stuff besides transporting raw Lunar materials.

Resource processing

Very little of what we extract from Luna is going to be in a pure form. Maybe some of the volatiles can be distilled out in a usable form after we release them from the extracted material. Everything else will need to be run through at least one process so we end up with the resources we want.

Let’s look at oxygen processing as an example. All of this is theoretical at this point and subject to change once the research begins. I just want to illustrate how complex all of this can get.

We start with a cart full of basalt chucks and rocks of various sizes we dug out of the mega-regolith.
The first thing we need to do is crush everything down to a powder of generally uniform size. The reason for this is to greatly increase the surface area of the material and to make it easier for the processing machines to handle it. Crushing and grinding can be energy intensive activities so we’ll need to be clever in our designs. We’ll also have to factor in noise, vibration, and dust. I’ve been working on manual ways to do this and you can see the results on the website. One thing to think about is that methods that use gravity to crush rocks (such as a stamp mill or ball mill) are going to be severely hampered by Luna’s 1/6 Earth gravity. Right now, I’m looking into a manually operated jaw crusher and rotary impact grinder.
The material will then go through a powerful electromagnet (and possibly an electrostatic element) to remove any particles containing free iron as well as the particles of ilmenite, olivine, and pyroxene. This process is called beneficiation and can be used to separate out the material with a higher iron content. Remember, we’re always interested in iron; both as a resource and because iron-rich materials have higher oxygen content as well. This step might not even be necessary. We might just end up throwing everything into the mix instead of separating it out. Lots more research is necessary.
Let’s follow the iron-rich material. The other material will have its own process path. Or maybe it won’t. I haven’t done the research yet. Anyway, the next step will be to heat up the material and drive off any volatile elements it might be holding. Hydrogen, helium, water, carbon monoxide, nitrogen, and other elements are what we’re looking for. The mega-regolith may not contain much volatiles but it’s worth getting what’s there. Building this step into the process means we can also run regolith through it as well. And we have to heat up the material for the next process anyway. We can use fractional distillation to recover specific gases at each step of the heating process.
The heated iron-rich powder is then put into the oxygen extracting machine. I don’t have a name for this device because there are a lot of potential ways we can chemically extract oxygen from ilmenite and the other minerals. We can run hydrogen gas over the powder using a hydrogen reduction process. This will strip the oxygen out and create steam that can be captured. The loss of the oxygen molecules will also free up the iron molecules. That’s just one potential process. I’ve identified over 20 others, each with their own advantages and disadvantages. We’ll have to do a lot of research to determine which ones will be best for Homesteads.
Now that we’ve got the oxygen and iron, we can move the material to the next machine to extract other resources. Everything at this point, and beyond, is completely up in the air. We’ll want to get other resources from the material because we’ve already put a lot of energy into extracting it and heating it up. I just don’t have any idea what these processes are and what materials we’ll end up with. There’s just so much research needed.
At some point we’ll have taken everything we can out of the material. The last step will be to make something useful out to the spent material and recover as much energy as possible. If the material is melted, we could pour it into casts to make structural components. We can compress the hot powder to make sintered products. Either way, we can convert their thermal energy into electricity as they cool; either with thermocouples or with steam generators.

As you can see, processing Lunar material for its resources is probably even more complex and difficult than extracting it.

Resource storage

Storing processed resources is trickier than it sounds. According to LH rules, we should plan to store everything inside a pressure hull. We have to design for size, safety, noise, vibration, heat, and a lot of other factors.

Take oxygen storage as an example. We’ve spent all this energy and effort extracting oxygen from Lunar rocks and processing it so we end up with pure O₂. Now what are we going to do with it? Here are a few ideas:

Storage for local use – Homesteaders are going to want a supply of O₂ gas on hand for life support and to supply their SPORE operations. It’s also a good safety feature if something happens to the habitats. Pressurized oxygen is easier and safer to store than liquid oxygen. Liquid oxygen is favored for rockets because more oxygen can be stored in a smaller (and less massive) volume. The Homestead oxygen storage container can be large and massive since it’s not going anywhere. Homesteads should also be able to build all the necessary components of a pressure storage system. Cryogenic storage is pretty high-tech and some components would have to be shipped from Earth.
Storage for sale and export – This is difficult to determine because we don’t know how the cis-Lunar oxygen economy will be set up. Homesteads may have to fill cryogenic tankers. Or maybe they’ll load pre-filled tanks onto shuttles. Or maybe they’ll launch oxygen tanks (compressed or liquid) to L2 using a mass driver. My guess is that pre-filled liquid oxygen tanks (manufactured on Earth) will be the cheapest and easiest way since transportation is involved.
Long-term storage – This one is actually pretty easy. There is one form of oxygen that is easy to store, non-toxic, and really useful in its own right. That’s water. Water stored for consumption, industrial use, and entertainment (swimming pools!) can be split into hydrogen and oxygen with electrolysis in an emergency. Of course, that assumes the Homestead has enough electricity to do this and that they have a place to store that hydrogen (it’s too valuable to vent).
Vent – It’s conceivable that Homesteads will produce more oxygen than they can use or store. In that case, they’ll have no choice but to dump it into the Lunar atmosphere. They wouldn’t want to slow down or stop all their other processes just because they ran out of space for oxygen. And it’s not like we’re trying to preserve the Lunar vacuum anyway.

We’ll have to go through this process for every resource we are interested in. Gases, metals, glass, and all the rest.

Manufacturing

Honestly, I haven’t spent much time research the manufacturing end of ISRU. I’m more concerned with how we’re going to get the resources and process them. I’ll start working on this end once I have better defined all the rest.

Every Homestead is going to need a workshop. Notice I didn’t say factory. Homesteads are small-scale, right? They need the facilities to construct what they need for their own use plus some extra for trade. Large-scale manufacturing is beyond the scope of LH at this time.

They will need some way to melt iron, aluminum, glass, and basalt. These are foundational building materials. They will need to be able to make casting molds. Tools to remove and shape metal (saws, files, etc.) will be necessary also. Maybe an old school forge and anvil!

The manufacturing component of ISRU is actually the last step in the process for any piece of Homestead equipment. First, we get it to work using Earth-based tech. Next, we figure out how to make it work using only Lunar resources but made with Earth tools. Finally, we figure out how to make the device out of Lunar resources using Lunar tools.

Resources

98) Lunar Sourcebook, 152

99) Lunar Sourcebook (pg. 298)

100) Lunar Stratigraphy and Sedimentology (pg 269)

101) Lunar Sourcebook, pg 448

102) Lunar Sourcebook, pg 41

103) GLOBAL LUNAR REGOLITH DEPTHS REVEALED – https://www.lpi.usra.edu/meetings/lpsc2011/pdf/2607.pdf

104) Global database of direct solar radiation at the Moon’s surface for lunar engineering purposes – https://www.e3s-conferences.org/articles/e3sconf/pdf/2018/24/e3sconf_solina2018_00053.pdf

105) https://en.wikipedia.org/wiki/Earth%E2%80%93Moon%E2%80%93Earth_communication