A Handful of (Lunar) Dust [Part 2]
[This article was written by Sam Ross. He blogs at towerofbuckets.wordpress.com]
[You can find the first part of this article here.]
Refine the regolith into aluminium and iron for use in whatever
It is, it turns out, possible to convert lunar regolith into purified iron using very few materials and the thermite reaction. The thermite reaction is awfully fun, as it involves high temperatures, flying sparks and molten metal.
But as well as that, it is extremely useful. It allows for production of metals in remote conditions (like country train tracks, where the reaction is frequently utilised to join track sections) and is reasonably cheap. And conceivably, such a reaction would be possible on the lunar surface.
Essentially, the thermite reaction goes like this.
Pure reactive metal + less desired metal oxide + heat -> reactive metal oxide + pure desired metal
Remember, regolith is mostly made from iron oxides and aluminium oxides. Getting pure aluminium is relatively easy using electrolysis (we could power this with solar panels or nuclear reactors). So…
Pure aluminium + iron oxide + heat -> aluminium oxide + pure iron
So, it is entirely possible to envision a production system on the surface, manned or otherwise, producing significant amounts of both iron and aluminium for use or sale. It wouldn’t be easy or cheap, but it’d be lot better than shipping the materials from Earth.
But there might be another way to create structures on the lunar surface.
3D printing! On the Moon!
Even if this isn’t as practical as other ideas, it definitely wins the prize for the coolest. 3D printing is one of the most versatile manufacturing techniques ever created, and the potentials for it on an off-world colony are almost limitless. 3D printing allows for in-situ manufacturing of virtually any component for bases, ships, robots – everything.
The problem is, moon dust is not the best material for 3D printing with. You first need to convert the regolith into either pure metal oxide (see above) or a plastic-like material that can be easily melted and reformed.
Now, ESA is seriously considering this option for the ‘Moon Village’ that they have in the pipeline. They have a prototype machine that can take in a load of regolith, put it into a specific shape and spray a binding agent onto the whole thing, forming a hollow dome with an airlock and windows.
However, this plan does have one big fat flaw – it involves shipping several tonnes of additive and binding agent (magnesium oxide and a salt of some sort) to the lunar surface. Which is time-consuming and expensive. What if we could build bases using materials native to the lunar surface, with no additives at all?
3D printing with metals is actually a bit different to regular, plastic 3D-printing. A plastic printer (the kind that you are probably familiar with) works by inserting tiny ‘pixels’ of plastic at certain points in 3D space, thus building a 3D structure.
But to 3D print metal, a different technique is used. A ‘bed’ (as the base of the printer is called) is covered with a layer of metal oxide of uniform thickness. A laser then runs over the bed, heating up the areas that are being printed. This reduces the oxide, forming a ‘pixel’ of solid metal. Another layer of oxide is then added, and the laser forms the next layer, slowly building up a 3D shape – something like this.
In order to do that, a fine powder of metal oxide is needed. But wait – we have an incredibly fine powder on the Moon’s surface, consisting mostly of aluminium and iron oxides. And how do we separate those? A good, old fashioned magnet. Using a magnetic gathering device, a store of fine iron oxide dust could be obtained on the Moon’s surface, and used to 3D print metallic structures and components.
Using a microwave to solidify (sinter) the regolith
This is, on the surface, much less exciting that the other ways of building on the Moon. There are no lasers, no 3D printing, and no exciting reactions. But there is the potential to build huge, solid structures – and to do so with extremely low costs. Microwaves. The radiation kind, not the cook-frozen-food kind.
Way back at the start of this, I mentioned that the regolith contained tiny particles of solid iron. Using the right frequency of microwave, it is possible to melt those granules without even touching the lunar surface. Elemental iron is acted upon by microwaves, and so microwaves can be used to heat these iron particles without acting on the rest of the regolith. And like most things, when that elemental iron is heated to extreme temperatures it melts and begins to flow. But when the microwave turns off, the iron cools and solidifies. And when the iron solidifies, it solidifies the rest of the regolith with it.
The best way to imagine this is a bowl of lumps of chocolate and gravel. When it’s all separate it might not be the most fluid, but it can definitely move around. Now what happens when you put that bowl in the microwave for 30 seconds? The chocolate melts and flows to fill the gaps in the gravel, encasing it. When the chocolate cools again, you will be left with a lovely block of chocolatey gravel. A single, solid lump. The exact same is true of microwaved regolith. If you run a beam of microwaves over a layer of regolith for about 30 seconds, it melts the iron and, on cooling, forms a concrete-like substance. Strong, incompressible, excellent at blocking radiation. In short, all the properties you need for a lunar base building material.
At least, in theory. The microwave sintering has worked in a lab, using a sample of regolith left over from Apollo, but whether or not it works in situ, with a low-powered beam, is still questionable. But if it works, it opens up the possibility of massively cost-reduced colonies. A bulldozer-type device piles up an area of fine regolith and microwaves it to form solid material. It piles on another layer and does it again. In fact, using an inflatable framework, this kind of production technique could be used to build the exact kind of ‘lunar cottages’ which ESA is proposing, but without the cost of shipping additives all the way to the lunar surface.
In fact, microwave sintering could have a few other applications, as well as habitat construction. When the regolith is heated, some trace elements are released from the regolith in the form of gas. Some of these are useful for a colony (carbon and hydrogen) and some are potentially profitable (helium, including helium-3). And lo and behold, we have circled right back to mining the Moon for profit. As well as this, sintering could be used to build roads around a lunar base. Lunar travel across a dusty, loose surface is difficult on foot or with large vehicles, and the huge amounts of dust that this generates are a potential health hazard. But if a lunar buggy were to be equipped with a wide microwave beam and rolled slowly over the surface, it would make a track of hard, dust-free surface that is ideal for moving around on safely.
But in order to even consider that possibility, we need to work out if this technique is effective. Which is where PTScientists come into the equation.
The PT Scientists intend to win the Lunar X-Prize, make no mistake. But they also intend to get some science done when they get there as well. And one of the main experiments that will eventually be shipped on the Audi Lunar Quattro rover is a little microwave beam and a downward facing camera. Because we need to see if this microwaving technique actually works, before we spend hundreds of millions of dollars shipping a giant microwave-bulldozer-construction robot thing to the lunar surface. And if it does work (as all the predictions say it will) then it might open a window to a new age of mankind. An age where we can build lunar complexes the size of towns, for a fraction of the cost of carrying the parts with us. And large-scale lunar complexes in large quantities could be key in allowing us to set up permanent bases of operations in lunar space. Quite simply, microwave sintering could be the technology that allows us to take that vital first step and get a permanent, inhabited structure on the surface of the Moon.
Imagine that. In the next few decades (if funding and public interest stays roughly where it is), we could have humans living, for months and years at a time, on another celestial body. And it wouldn’t take us halfway to bankruptcy, because they could be making almost everything they need where they were – plus sending home shipments of rare materials. We could well be using products that contain lunar-origin metals and polymers. And most exciting of all, that colony could be feeding a steady stream of parts and fuel into lunar and earth orbit, building up the next generation of interplanetary spaceships. Using the Moon, we could take ourselves to Mars, and send a massive wave of research probes all over the Solar System. The second age of humanity, when we become an interplanetary species.
Not bad for a handful of dust.