While I’ve amused myself by taking potshots at popular visions for space cities including domes, modular space stations, and O’Neill cylinders, I realized embarrassingly recently that I’d missed yet another terrific example! Tunnels! And secret underground lairs of all kinds.
Of course, the usual disclaimers still apply. I’m a housebound socially isolated COVID-fearing glorified recovering physicist who claims to be able to write software, inelegantly chuntering opinions into the internet’s screaming maelstrom. Isn’t there something more useful I could be doing with my time?
More seriously, my purpose here is not to criticize but to illuminate. To ask interesting questions, as a way of motivating collective inquiry into topics of mutual interest. We are all wrong, to varying degrees. The only useful meta question is how may we go about becoming less wrong?
Tunnels are a staple of both science fiction and popular journalism regarding human habitations on the Moon, Mars, or other rocky places. They’re fun to write about and interesting to put on screen. I’ve lost count of the times I’ve seen beautifully illustrated Mars city maps featuring a hexagonal grid of domes connected by tunnels. On a visual level, it certainly ticks all the right boxes.
And yet, while I’ve wasted years of my life on real estate websites I’ve never seen a subterranean house on the market. They do exist, if you want a converted ICBM bunker or limestone cave, but they’re a definite rarity.
The simplest explanation is that digging holes, particularly really deep ones, is very energetically intensive and expensive. The cost of building a road tunnel works out to be about $100,000 per meter, or equivalent to a stack of Hamiltons of the same length! For comparison, $100,000 will buy materials and labor on a respectable manufactured home, or substantial renovations.
Indeed, on Earth, underground construction is basically unknown except for nuclear bunkers. These have two powerful reasons to accept the cost and inconvenience: unlimited sweet DoD money, and surviving really big explosions.
Why build underground in space? The usual explanation is to provide shielding against galactic cosmic rays, or micrometeorites.
It is true that tunnels deep underground are relatively safe from both, and also well thermally insulated. But as I discussed in the blog on space radiation, relatively little shielding is necessary even in areas that people spend a lot of time, such as sleeping areas. And even if that works out to be a meter or two of rock, it’s orders of magnitude less effort to drop sandbags on the roof of some structure constructed on the surface, than to dig a hole of the necessary size deep underground.
Micrometeorites are not a concern on Mars, which has a thin atmosphere, and can be well shielded on the Moon with a thin blanket of loose rubble.
If there’s a central point to my blogs on space architecture, it’s that our cities and houses on Mars will look and feel a lot more like regular houses on Earth, and for the same reasons. It may not be very exciting, but the most important consideration for design and construction, on Earth or in space, is expedience. Given the relative scarcity of human labor in space cities, structures will have to maximize usable area and minimize effort even more than on Earth. Instead of tunnels, think warehouses and aircraft hangars! At least they can have natural light.