one of the best locations in the future forliving will be the sky. We’ve spent a fair amount of time in this series discussing places humanity might occupy like the oceans, deserts, tundra, or underground, but for the most part we’ve been looking at what they had to offer in terms of resources or how we could make them more livable because we were low on space. Often we’ve noted that while you’d probably get a fair number of folks living there, fundamentally such places don’t have a strong motivation to live there in and of themselves. To be sure, some folks would love to visit the ocean depths or go live on the north pole or underground, but by and large you wouldn’t have big queues of people fighting for the privilege. When we start looking at living up high though, our topic for today, it’s a different story. People like living on the upper floors of tall buildings or on hills for the view, and looking at human history, we tend to glorify our deities by putting them up above, on Olympian heights, more often than underground. Looking at that same history, we have to acknowledge a pragmatism to it as well, building up high protects you from floods and invasions, and gives you a longer range of fire and sight. And to be seen as well, palaces or temples built on a hill in or near a town are always in sight and looming, and there’s a reason water towers tend to be popular targets for graffiti. People seem to like building tall, but there’s pragmatic reasons to do so too. Of course the big problem is how you get something up there without it coming back down again, and it behooves us to remember that every structure we make has this issue. Many times when we discuss new tech, especially things like orbital rings that just hang overhead, people worry about them crashing down, as well they should, but folks can sometimes get stuck in an avalanche of potential disasters and safeguards and forget that we already have those issues with modern buildings, which are very vulnerable to accident or sabotage themselves. We want enough safeguards to make the cloud cities we discuss today as safe or safer than modern homes, and we’ll discuss those, but avoid the urge to ask ‘what if someone shot one down with a missile?’ without first asking what would happen to your own town if someone lobbed a missile at it too. It’s important from the outset though to note that as we discuss the various ways for holding these places up in the air, one of the best contingencies is to use multiple methods, and as an example, you might just make a big airship or plane, relying on buoyancy or lift, or you might make something that did both, and which had its sewers and water reserves ready to be dumped at a moment’s notice to lighten the load. This would make you very unpopular with whoever you were above at the time, but is presumably a better alternative than dropping a city on them. We also could incorporate pure safety features like parachutes, which as we mentioned in Orbital Rings, can potentially be made very light using the same ultra-strong materials so many of these structures would need to be viable anyway. Of course living among the clouds doesn’t necessarily mean you are floating, you might live on top of mountains, or a very tall skyscraper, you might be up on stilts, you might be hanging down from a tether in orbit and you might simply be in orbit. Many of these can be mixed and matched to create hybrids that are even safer, but let’s review each of these by themselves first, and we may as well begin with our current methods. Prior to modern times if you wanted to be high up you needed to start by finding someplace high to build on. The tallest structure for most of human history was the Great Pyramid of Giza, and it’s not very tall or high above sea level. Needless to say if you just want raw height above sea level, you go find a place that’s high and build there. We’ll mostly bypass the approach of just building a normal but tall building today, a superscraper, as we discussed those more in the Space Tower and Arcology episodes, and I’d rather save discussion of them for another day where we can give them their own whole episode. But building on a mountain top is a different approach, and also gives us a good chance to look at some logistical problems that most of our other approaches will also face. Conceptually building a city on a mountain top is easy enough, you either cut the top off or hollow the peak out. Doing the former approach you’d probably build a good retaining wall around the top and flatten it out, or do several layers like this to create a tiered city, possibly all the way down to the ground. You’d presumably use a lot of rail lines to carry freight and people up there, but you’d probably want to have some fairly impressive cisterns for water and waste processing too, because pumping water to the top of a mountain is very energy expensive. Of course this all assumes you are getting your supplies from lower down, but you might actually be getting them from higher up. One of the most obvious reasons to build high is for easier space access, and as the centuries roll by, it’s quite likely that most people won’t live down on Earth, and not even necessarily on other planets. You could easily have a lot more real estate in orbit around Earth than down on Earth and ultimately mountaintops will always hold a slight edge on places lower down for getting into space. We don’t do that now because building facilities up there is an expensive pain and logistical nightmare. One of the things that makes them appealing as launch sites is because the air is thinner up there, which is one reason you might opt to hollow out a mountain peak and live in there, possibly with large windows to let the light in and maintain the view. You can of course also dome the place, but you don’t necessarily have to, even on top of Mount Everest the air is breathable, if barely, and most mountains aren’t nearly so high. We also have options that would let you bypass domes by using cybernetics or genetic engineering, or even outright supplementation by building huge air pumps that just ran constantly. A dome seems preferable, but I could imagine a spaceport, where everyone is already used to low pressure and breathing equipment, having buildings that were kept at overpressure and higher oxygen rates, and people strapped their breathing mask on when going out and the parks and gardens had genetically tweaked plants that handled the lower pressure better, and the whole place had a constant outward breeze as you pumped up warmer, higher-oxygen air and just let it spill out. This is a fairly energy-heavy approach even on more modest mountaintops where the pressure differential isn’t as high, and would give you a constant outward wind, but you don’t actually have to match Earth normal pressure as we don’t require that, and might use it as a minor supplement dialed up just enough to provide a comfortable outward breeze and some warmth. More to the point, some of our designs for keeping places afloat would involve using huge amounts of energy and air to keep the thing aloft, in which case this might be a natural byproduct. If you have a city simply being held aloft by lift, like a helicopter, you’d have a higher pressure underneath. We think of living on top of whatever is floating, but you don’t have to, especially if the thing above is mostly transparent, like a dome presumably would be. So as mentioned earlier, for most of our history if we want to be up high we either had to find a high place or build a big tower. As we hit modern times, long before the Wright Brothers, we found we could float up there by heating air up, so it was a lower density and thus would float. We also later learned that some gases, like hydrogen and helium, would have this same effect, what we call a lifting gas. Now we discussed the physics of this more in the episode Colonizing Venus, so we’ll skim that today, but as we’ve mentioned throughout the series, many of our tricks for altering our planet might come from those developed for use on other worlds, and vice-versa, sometimes an idea we discuss here isn’t something we’d actually want to do on Earth, our childhood home with nostalgia value, but might on other worlds. Loosely speaking, lifting gases only get you about a kilogram of weight per cubic meter, which means you need something the size of a house to lift even a couple people, and indeed you have to give over a lot of your mass to whatever is holding that lifting gas in, or insulating it so it doesn’t cool. Fortunately, leakage of gas or heat both relate to surface area, while buoyancy is all about volume, so we can benefit from the square-cube law to make thicker frames that are sturdier, better insulated, leak less, and take up a smaller percentage of our mass budget. And you will need to build big because you really need a lot of volume to make something heavy float. Again your typical cubic meter of a lifting gas is only giving you about a kilogram of weight to work with. If you wanted something like a ton or a thousand kilograms per square meter of living area, the big balloon you are living on top or underneath of needs to have a depth of kilometers. We shouldn’t discard it just for that though, because there’s nothing necessarily stopping us from having this balloon or lifting frame be transparent, or having it spread out more. The main objection to a floating city passing overhead would be it blocking light or falling on them, if most of it is transparent that first concern is removed, and a buoyant balloon doesn’t really crash, especially if it’s got membranes inside partitioning it so one compartment blowing doesn’t instantly vent them all, and such internal membranes don’t need to be very thick and heavy since they can leak between each other, you just don’t want them leaking very fast. You also don’t actually need soil for plants to grow, we have both hydroponics and aeroponics, but even if we did need soil, a lot of plants only need a few centimeters of it, especially if they had some web-mesh beneath to attach roots too. Picture if you would a very large disc or hexagonal plate with a large lawn of thin soil or fake soil for roots to attach to and get nutrient baths. The houses and frame don’t actually have to be very heavy if their bulk is made of things like graphene packed with aerogel or some other lightweight, ultra-strong substance. You could also make that frame out of something like a layered fabric, which in an emergency could blow out to create a parachute. A floating city is cool but a floating house complete with a yard is pretty neat too, and we suggest the hexagon shape because as we mentioned in our Seasteading Episode at the start of the series, when we were floating on water not air, you might have cities that were made of lots of interlocking plates, each one its own home that can connect or disconnect to migrate elsewhere or to rearrange the city’s layout. Easier here too, since someone near the middle of such a big sheet of connected floating habitats can disconnect by floating up or down rather than needing everyone outside them to move. There’s another thing though too, as we said, all of these would need a lot of depth to them, and cities don’t have to be flat and indeed have been moving away from that, we already employ a lot of skyways to connect upper floors of buildings to each other so you don’t have to descend all the way to the ground level to move around. Whether we’re talking floating cities or ones on mountains, you’d expect to see a lot more emphasis on three-dimensions in layout. You might build many concentric tiered rings around a mountain, you might build homes more like balconies around the edge of a spherical balloon or lifting frame. And if you’re assembling a bunch of individual floating structures into a potentially ever-shifting cloud city, there’s no particular reason why they all need to be at the same height nor do they have to be interlocked requiring either square or hexagonal layouts. They might just be tethered together. Tether travel might be fairly normal too, since you need a way to reach the ground, you might want to tether it the ground to avoid being blown around, and you obviously aren’t keeping a car up there. Of course personal vehicles might be planes or helicopters, not cars, and we do have planes and very large ones at that. We run these on fuel and fuel is heavy, but we can keep a plane aloft indefinitely by mid-air refueling and multiple engines, so that one or two can be shut down for maintenance if need be. Key thing there, fuel is heavy, but only chemical fuel is heavy, and these are not rockets, the propellant pushing the thing can be air you just grabbed from in front of you. It’s not really a huge amount of power either, especially if you don’t have to carry heavy fuel. Needless to say if you’ve got compact fusion, not only do you need very little fuel by weight but since that fuel is invariably light stuff like hydrogen, deuterium, tritium, or helium it also would be a lifting gas. Indeed hydrogen is quite a good chemical fuel too, so a civilization that’s energy rich from fusion but can only make big and heavy plants on the ground, could mass produce hydrogen by electrolyzing water to use as a mixture of fuel and lifting gas for such structures. You can mix powered flight and buoyant flight, we just don’t very often, unless you count engines on blimps. How much energy it take to keep something aloft through lift is rather variable to its speed and height, but a Boeing 747 burns about a gallon a second while cruising for instance while smaller helicopters might use less than a gallon a minute and only about an order of magnitude more fuel than a car. However this is perfect for something like radioisotope thermal generators whose biggest issue is that it can’t really throttle its power production, it produces the same amount of power constantly and if you just float around at low speeds all the time, that’s pretty much what you want. Many things that wouldn’t work well nowadays because of the sheer cost aren’t necessarily bottlenecks in the future, same as aluminum used to be more expensive than gold but is now treated as nearly disposable. An energy rich society can get away with things we can do now on paper but can’t afford to do. A floating house with a big lawn, its own power plant and water recycling facility, might just be viewed as only somewhat luxurious a century or two from now, rather than something only an eccentric billionaire could contemplate. Of course on board power or power beamed in from orbital satellites aren’t the only way to get power there, and there are other ways to lift stuff up if you have access to a lot of energy. A tether running down to the surface to let you move back and forth could also be used to carry electricity, and something running on solar power and buoyancy might ‘anchor’ at night times or cloudy days by dropping a big, sturdy power line down to the ground. Indeed it might tend to have multiple powerlines constantly shooting down to grapple onto power junctions and just pull itself around this way, like a big spider. Such a line might be thinner than your arm too, even for very large floating cities, so from any distance they’d appear invisible except for navigation hazard lights like tall towers have, and in the future such things might tend to be in wavelengths outside the normal visual range to avoid the visual clutter and light pollution. We talked about that in Colonizing Venus too, big spider cities that march around the sky by shooting tethers or using thin stilts. You could potentially make very big thin stilts too. Materials like graphene rely on tensile strength, great for hanging things from or pulling with, but most buildings rely on compressive strength and we can turn to active support systemslike those we discussed in Space Towers. They’re energy hogs, unless you’ve got warm-temperature superconductors that you can magnetically shield, but if you have that, or a huge cheap energy supply, you really could sit cities massing in the Gigatons on top of some very thin support pylons. A single pylon, even if it could hold the weight, is kinda dangerous, so you must construct additional pylons for safety, and these could be used as stilts for walking your city too. Plenty of other options as well. We can beam power up to one, or beam it down from orbit and bounce it back up, and possibly build the city at the top of a big transparent dish where any energy lost to atmospheric absorption or conversion is heating the air under that dish up and providing lift or even thrust – you can make a spaceship or airplane that gets its energy beamed in, and this approach, relying on size and heat, is less vulnerable to a break in that beam and would just drift down if power was cut, like a hot air balloon and parachute mixture. Handy too since if that dish is very strong anything it lands on is basically getting a lid put over it, not crushed. We’ve also got the Ionic Wind approach, like MIT’s new plane that has no moving parts that can break and is virtually silent, and could work very well when scaled up in size for this sort of slow, drifting craft or building. Another way to create lift is by induced airflow, like the Coanda Effect, using jets of air on a surface to pull more air around a surface or wing. Some helicopters use this to counteract rotor torque instead of a tail rotor, to avoid the danger of a spinning blade at ground crew level. A floating body could create its own lift this way, moving air around itself, instead of moving through the air. We can hang things too, either by a very long tether up near geosynchronous orbits, an abridged space elevator basically, or the less cumbersome and safer approach of the Chandelier City we discussed in Colonizing Neptune, where you hang off an orbital ring. Such things can be built stationary or to move over a track. There is a question of why you’d want to move, regularly but at slow speed, and as we noted in Seasteading there are a lot of cases where you might. A floating stadium or carnival or museum running up and down a coastline was an example there, and would work just fine here too. We also mentioned solar powered ones needing to tether to the ground at night, and another approach on motion might be ones that bobbed up and down over the course of a day, but it is worth remembering that physical location is likely to be much less relevant for things like work and school in the future, even assuming we don’t eventually get our hovercars that scifi has been promising us for around a century now or personal helicopters or quadcopters in an energy rich civilization. Speaking of bobbing, I could imagine buoyant reefs, for growing plants on, that floated around growing and slowly descended to the ground when they got too heavy with plants and crops, which you then harvested and let drift back up, sort of like a big cloud sheep you sheared. I doubt that would be practical but it stills sounds cool, and when dealing with civilizations numbering in the billions and very prosperous, you can support a lot of impractical things that are neat simply for tourism and prestige. Floating garden parks drifting around, or floating restaurants, might be regular features of bigger cities or something most nations had several of. A city is not synonymous with some giant megalopolis like New York or Tokyo either, definitions vary, but here in Ohio and most of the Midwestern US, a city is defined as any incorporated municipality with more than 5000 people, a village one of less than 5000 people, and a town simply anything not incorporated. My specific township of Geneva actually includes the village of Geneva on the Lake, where I live, the City of Geneva, and the unincorporated spaces between. Keeping that in mind, a place like New York City, with millions of people, could easily have several cloud cities and villages that were popular for tourism, commerce, or luxurious private residences. Same as they might have many coastal artificial islands or subterranean cities, and might have large structures drifting in and out, floating on sea or air. While we’re going to save a more extensive discussion of just how far you can go with hanging things from space or suspending them by active support for another day, there is one last structure I want to mention today that is kind of neat that relates to space launches. We discussed the notion of a skyhook or rotovator back in the Upward bound series, and its uses can be fairly similar to what we’d use for hanging towers from orbit. One version is a long tether whose low end hangs down into the atmosphere and which can orbit far slower than normal because of this, the top is in a far higher and slower orbit and the net speed gets averaged out. Those are handy because they allow hypersonic planes to fly up and attach to them and crawl up into space, and they can regenerate the momentum they lost to that plane and the thin air by electrodynamic tethering. You can make a floating city this way, though the tether needs to be very long if you want it moving slow, and this is one of the ways we might use magnetics for floating things, but the more popular version is the rotovator skyhook, that spins around, so that its tip descends and is moving even slower while its other tip is moving faster than normal at the top, like a big slingshot for spacecraft. This, along with the notion of floating cities bobbing up and down, got us thinking about another approach to moving up and down which we’ve decided to call a Skywheel. For conceptual purposes imagine a very big carousel wheel, possibly with the bottom floating just over the ground, possibly with the whole thing floating far up in the sky. Very stripped down, hardened, and large versions might be handy for reaching space but a more modest form might simply connect the ground to a floating city and be a fun way to reach them, and the various carousel carts might be parks or business or homes or on larger ones, entire neighborhoods. They needn’t be circular, they could be elliptical for instance, and they needn’t be exposed to open air either, and one could imagine big forms of these which whirled around from a metropolis’s cloud city neighborhoods all the way down to its subterranean ones. Not very practical compared to an elevator or tram line, but as with a lot of things we discuss in this series, practicality is not always what matters most, very prosperous and high tech societies might tend to make such things for the same reason they have parks and gardens. There are practical uses though, even if we don’t need them in this case anymore than a beach resort needs industrial uses. We mentioned spaceports but airports can be an option too, a lot of fuel is expended climbing up to speed and altitude and a floating airport can help with that, and while a floating aircraft carrier like the S.H.I.E.L.D. Helicarrier has a lot of downsides, the greater speed of an airborne one compared to a naval ship seriously helps with power projection, which is one of the major purposes of a carrier group. We also have often talked about using orbiting mirrors and shades to help cool or warm a planet, and for the most part one floating high in the sky can achieve the same effect, somewhat diminished but also easier to build and maintain. But you might also use such things, especially those using ionic propulsion, to help with something like ozone farming. Ozone layers are important and can be artificially generated. You might also use them for sucking up lighter-than-air pollutant or greenhouse gases, or distributing fertilizer or even moisture. One could imagine genuine cloud cities that helped with weather control. It’s hard to say how many of these things we’ll have in the future or what specific methods will be employed to keep them aloft, there’s just too many variables and wemight get better with magnetic levitation for instance or even invent something like the classic artificial or anti-gravity fields science fiction loves so much, but I am confident one day, and maybe not too far off, we will at least have some civilization in the skies, and what a great view they have
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