So, you've just gotten a great new job that requires you to move halfway across the country -- say, to Cincinnati. You make all your arrangements, pack your things up in a moving truck and make your way toward your new home. You can't take your house with you, obviously, so the first thing you have to do is arrange for a new place to live. Lots of people have suffered through similar scenarios, and lived to tell the tale.
But imagine if your new base of operations were on the moon. No houses there -- yet -- plus, you're getting charged an exorbitant amount for every single cubic centimeter of stuff you want to bring along. You can't bring along your grandmother's credenza, of course, but you really can't bring a house with you -- or even the materials to build one!
It's always been assumed that one of the first steps in the exploration of space would be to build structures, like the International Space Station and a future moon base, to safely shelter us once we get there. But in order to build on the moon, we have to work with what's there.
When you build or remodel a house on Earth, a major ingredient is concrete -- a mixture of crushed-up rocks, sticky soil or clay used to cement the mixture, and a solvent (usually water) to bind it all together. In this article, we'll look at the history and recipe for the lunar equivalent: lunarcrete.
The Ingredients of Lunarcrete
Lunarcrete was first proposed in 1985, by the University of Pittsburgh's Larry Beyer. Sometimes called "mooncrete," its basic ingredients aren't much different from what we use in Earth-bound concrete: a solvent, the crushed-up aggregate and the sticky cement. The question, then, is what can be used for those three ingredients, since it's cost-prohibitive to transport them to the lunar surface?
The bulky part of concrete, the aggregate, can be made of the top layer of lunar soil, called "regolith." It's similar to what we use on Earth, although unlike our soil, it doesn't contain anything organic. The moon's regolith layer was created over billions of years by meteoroids of all sizes bombarding the surface, and sometimes crushing what was already there.
About 4 to 6 meters (13 to 20 feet) under the regolith, you find regular moon rock, similar to our bedrock layer [source: Wilcox, et al]. We can use high-calcium deposits here for the cementing part of the mix. But what about the solvent? The moon is a pretty dry place.
The Question of Water
While the moon has both water and ice, it's also possible to create water by getting hydrogen from the soil and combining it with oxygen. Voila! Lunarcrete solvent. But not so fast. Some of the most exciting (and seemingly bizarre) approaches to lunarcrete composition propose using solvents other than water -- some of which would even perform better on the moon than they would here, which is what makes them such interesting discoveries.
A project in 2008 -- cooperatively developed by the University of Alabama's Houssam Toutanji and Richard Grugel, from the Marshall Space Flight Center -- showed that by combining a lunarcrete mixture with silica, you could use heated sulfur as a binding agent [source: Grugel and Toutanji]. We use water because of its evaporative and molecular properties, and Toutanji and Grugel found that cooling sulfur worked to bind the lunarcrete in a similar way.
It's important to explore alternative solvent avenues because in the vacuum of space, the water would just evaporate and wouldn't bind the mixture at all. Some teams are developing new ways of injecting water into the recipe -- with steam, for example -- while others are working on ways to blend the mixture under pressure.
Improving on the Original
Of course, in all these tests, you have to wonder what material scientists are actually using in their experiments. It's just as expensive to bring stuff back from the moon as it is to take it there, so where are they getting all this moon rock? Several teams have created and continue perfecting different simulations of lunar rock to put through the paces.
For example, a 1988 project at the University of North Dakota created an acceptable mixture from lignite coal ash, but every team has its own favorite moon rock substitute. Only a very small number have the cool distinction of doing their tests on the real deal. In 1986, a team created one of the strongest lunarcrete samples with actual moon-originated material by injecting steam made from mixing hydrogen with ilmenite, a lunar rock compound, into their compound at an extremely high temperature.
Though the recipe for lunarcrete varies from lab to lab, its average compressive strength ranges from 39 to 75.7 megapascals (MPa). A megapascal is a unit of measure specific to crushing, and 1 MPa is equivalent to about one tenth of Earth's atmospheric pressure at sea level. Anything more than 50 Mpa is generally a good strength for concrete, while soft sandstone is more in the 5 to 10 Mpa range [source: Ruess et al].
The opposite of compressive strength -- how far you can pull or bend a material before it begins to break -- is called tensile strength. When your foundation cracks, it's likely that the normal pressures associated with expansion and contraction have overcome your concrete's tensile strength. Like concrete, lunarcrete is susceptible to these pressures, so reinforcing tensile strength is important. But Earthling solutions, like adding steel, aren't really viable. Some experts have suggested adding Kevlar to the mix, since it's so much lighter to transport, but the real DIY solution is even more exciting: Create lunar glass from the regolith, and use it to reinforce the lunarcrete the same way we use fiberglass down here.
Future Moon Developments
While lunarcrete isn't airtight -- meaning living spaces would need sealing from the inside -- it's designed to absorb gamma rays, withstand huge temperature shifts (from −150 to 120 degrees Celcius!) and has a relatively light density of 2.6 grams per cubic centimeter. All these qualities, which are required by the lunar environment itself, also mean that lunarcrete -- or at least its simulated equivalent -- could have applications right here at home [source: Ruess et al].
Not only does lunarcrete involve transporting minimal materials (after the big first push, of course), but its ongoing manufacture also requires less energy than the production of steel, brick or aluminum. It will be at least a decade before we're able to see these materials and discoveries in action -- maybe more, depending on how we prioritize space programs in the years to come -- but in the end, lunarcrete is a smart and elegant solution to the problem of off-world construction.
The long-ago proponents of our space program knew that our discoveries on that journey would not only bring us to a new frontier, but would also generate jobs, discoveries and innovations for the planet we call home. After all, the beauty of experiments in the hard sciences is that every solution potentially begets a thousand others.
With the recent move away from the space race, I must admit anything having to do with space exploration or space-age materials seems simultaneously futuristic and oddly old-timey. I was intrigued to learn what the strange-sounding "lunarcrete" discussion is really all about -- and pleased to find it was both more futuristic and workable than I could have imagined.
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