Artificial Intelligence Is the Best Space Technology
Since the 2nd space shuttle disaster in early 2003, there has been a new debate about what the future of the US space program should be. Unfortunately it doesn't seem like the debate has fundamentally changed many people's thinking. People are still thinking in terms of what end goals we should achieve, such as going back to the moon or to Mars. Instead, our immediate goal should be to reduce drastically the cost of getting into space. If we could do that, then all other goals would be easily and inexpensively achievable.
NASA is a colossal waste of money. I say this not as an opponent of space travel, but as a rabid fan. There is a technology that should be able to reduce the cost of getting into space to a tiny fraction of what it is now, and its development could cost just a fraction of a year's budget for NASA. But since NASA is a bureaucracy, and bureaucracies' only goal is to spend more money, the last thing NASA would do is drasticly cut the cost of space travel. But on the other hand it would probably cost something like $1/2 billion to $1 billion to develop the new technology, and while there are plenty of billionaires and corporations with that kind of money, none has so far decided to take the risk and develop the technology.
Rockets are an incredibly inefficient, difficult and dangerous way of getting into space, and that is why getting into space is so tremendously expensive. Fuel must shoot out the rocket at tremendous speed not only to push the rocket upward, but counter the earth's tremendous gravity, which is pulling it downward at every moment until the rocket reaches orbit. Tremendous speed requires tremendous temperature and energy, which makes rockets inherently dangerous when pushed close to the limits of technology. In a sense, they are continuously-exploding bombs, forced to explode outward only in one direction, down. In addition, since fuel weighs a lot, even more fuel must be brought up to lift the weight of the fuel, in a snowballing effect. The end result is that only around 1/100 of the weight of a rocket is payload, the rest all fuel and fuel tanks. As a result, even with the partially-reusable space shuttle, so that much of the vehicle doesn't have to be thrown away with each use, it still costs $20,000/lb. to put something in low earth orbit. It is as if everything put up there has to be made of gold. The least expensive rockets now cost $2,000/lb. to put something in low earth orbit, still more to higher orbits.
Picture the orbit of the space shuttle, 300 miles up. At that height, an object in orbit must go around the earth once every 90 minutes, and travel at 17,000 mph in relation to the ground. Now imagine that a cable was hung down from that height (with an equal length of cable stretching upward to balance the center of gravity). It had better not reach all the way to the ground, or it could cause major damage as it dragged along at 17,000 mph! However, imagine that the cable rotated, like the spoke of a wheel turning in contact with the ground. If it made 1 turn every 8 minutes, in the same direction as it was circling the earth, the end of the cable pointing away from the earth would be traveling at 17,000 + 17,000 mph (its orbital speed plus rotational speed), while the end of the cable pointing toward the earth, moving in the opposite direction in its rotation, would be traveling at 17,000 - 17,000 mph, or in other words, would be briefly stationary in relation to the ground just as it dipped down towards it, just like the spoke on a wheel as that point on the edge of the wheel touches the ground once each turn. If the cable were made the length that it didn't quite reach the ground, but missed it by perhaps 60 miles so that it would only enter the very fringes of the upper atmosphere to avoid atmospheric drag, and if it not quite compensated completely for orbital speed so that it was moving relatively slowly in relation to the ground, at the speed of a rocket plane, several times the speed of sound, a rocket plane capable of reaching much greater heights than ordinary airliners could rendezvous with the end, each time it dipped down from the sky, and hook on cargo, or capsules containing passengers, which would then be lifted into space along with the end of the cable, and released into orbit. While the cable tip would repeatedly dip down around 8 times each hour in different parts of the globe all along the equator, if the orbital speed were kept just right so that the cable tip dipped down an exact number of times each time it circled the globe, the tip would keep coming down over the same places on earth, once each orbit at each of those places, so that space ports near each of those spots could launch planes that would rendezvous with the tip. For instance, if the cable turned every 8 minutes, and it took 96 minutes for the cable to pass overhead again, the cable tip would dip down 12 times each pass around the globe, and dip down each time just as it passed over each of those spots.
One trouble would be that the forces on a space tether 600 miles long would be almost 10 times normal gravity (10 Gs), and that is about the limit that most people could stand for the 4-minute ride into orbit. For ordinary people to go into space in large numbers, the center of gravity would have to be more like 500 miles up, so the cable would be 1000 miles long. Passengers would experience 5 Gs for 12 minutes, not much worse than on a roller coaster. For frail or fearful people, the cable would have to be around 2000 miles long, so people would experience no more accelerations than in a typical automobile ride, for about 20 minutes.
Of course, in order to lift cargo, the cable itself would drop downward, since the energy from lifting the cargo has to come from somewhere. If as much cargo were being transported back to Earth as was being lifted, the energy from the cargo falling downward would cause the cable to rise upward, exactly balancing out the downward drop, so that people could be going into space and returning and using zero energy! But it seems likely that at least initially, much more cargo would be brought upward than downward. In that case, small rockets attached to the ends of the cable could be used to keep the cable from dropping in altitude. Those rockets would be subject to a far tinier snowballing effect of needing fuel, and fuel to lift the fuel, etc., than with free-flying rockets. There is even the idea of electrically charging the cable, and using the earth's magnetic field to boost it back upward using no fuel, but it would take some practice to master the technique. And when it comes to transporting people to and from space, as opposed to cargo, the capsule the people would need to be enclosed in, almost all of the weight, would go back down as well as up, so would use no energy in being transported. The people themselves would also probably tend to go back down almost as often as up, except for people who stay to live in space permanently. And to make up for those who stay, it wouldn't have to be people brought back down to balance out the mass of the people brought up; any object of equal mass will do. But most likely, the technique of pushing against the earth's magnetic field would make it unnecessary to bring down an equal weight as being brought up.
Another problem would be space junk, which could hit the cable and sever it. There are innovative redundant designs using multiple cables so that when some of the cables are broken, the rest take up the slack. Actual experiments in space have shown that those designs would last for many decades without breaking, even if no one bothered to repair them when some of the lines broke. In reality, they could use robots to climb along the cables and repair them at leisure.
Putting up a cable 600 miles long or longer may sound like a lot, but it isn't. We humans have crisscrossed the earth with cables, far longer than that. Communications cables cross the Pacific for 10,000 miles. A cable that long would still be light enough to weigh under 20 tons, the amount of cargo that the space shuttle puts up. The cable would be brought up on a spool, and unspooled once in space. Suddenly, the cost of getting to space would drop to something like 1/1000 what it is now, or $2/lb. The space frontier would open up in a big way.
Once the first rotating space tether was put up, it could be used to put up more massive tethers that could haul up more weight of cargo. It could also be used to put up additional tethers in slightly higher orbits that could fling passengers and cargo into much higher orbits. Once a vehicle was brought up to low earth orbit, it could rendezvous with the tip of another rotating tether that would fling it to the moon. Once above the moon, another rotating tether, much easier to build because of the moon's lower gravity, could catch it and gently deposit it on the moon's surface. For the return trip, a vehicle would use the same tethers in reverse, and the weight of ingoing and outgoing vehicles would balance, so the tethers would stay in place. In fact, moon rock could be brought up from the moon and brought down to earth for no other purpose than to balance the mass being brought up and down. Another tether could fling a vehicle to Mars, and reduce the travel time to 3 months or less. Additional tethers above Mars could catch the vehicle and put it into Mars orbit, gently deposit it on the surface, pick it up again and put it into orbit, and fling it back to earth, all with no net energy.
However, as of this writing (2004), there is no known substance with enough tensile strength and low weight that it could support its own weight for 600 miles against gravity and centrifical force without breaking. As time has gone on, new stronger materials have been found, and since the required strength isn't much greater than the best substance known now, a type of plastic used in fishing line, there is hope that we will find such a substance not too long from now. There are already substances such as graphite whiskers and carbon nanotubes that consist of fibers of microscopic length that far exceed the required strength, but so far no one has figured out how to make rope out of them that is anywhere near as strong and lightweight as the microscopic fibers are, though research is being done on the problem.
But that current best substance is sufficiently strong that a rotating space tether in low earth orbit could rotate at around half orbital speed or a bit more, and support its own weight against gravity and centrifical force. A suborbital rocket would still have to reach nearly half orbital speed in order to rendezvous with the tip of the tether. (Due to the slower rotation, passengers would only experience 2 Gs while attached to the tether, less than they'd experience while in the rocket as it approached the tether.) It might seem like such a rocket would merely cost half what a rocket does to reach full orbital speed, but that's not the way it works. Due to that snowballing effect, a rocket that reached half orbital speed or a bit less would require only about 1/5 to 1/10 the fuel and could take up around 5X to 10X the cargo that one does that reaches orbital speed, reducing the cost to space to 1/5 to 1/10 what it is now. In addition, instead of requiring 2 or more stages to the rocket, a big 1st stage boosting a much smaller 2nd stage that in turn carries up a much smaller amount of cargo, as is the case now, a single stage vehicle, the equivalent of only the small less-expensive 2nd stage, would be feasible. The rocket could be built with greater safety and reliability in mind, unlike current rockets that cannot afford to because they can just barely get into orbit as it is. The reduced complexity and increased reliability of launching a single stage vehicle would reduce the cost to orbit by another 10X or more. Much more frequent launches, once a less expensive vehicle was built, would lower the cost still further due to economies of scale, so that, it is estimated, the cost could be reduced to as little as $6/lb. That would still be amply cheap to open up the space frontier in a big way. I've seen estimates that the first rotating tether, plus an orbiting hotel for paying tourists with room for 1000 people, could be put up for $1/2 billion to $1 billion - just a few months' spending by NASA - and the cost of a week-long trip to space would be around $50,000, well within reach of large numbers of wealthy tourists. Marketing surveys have estimated that 1 million tourists a year would pay that price to go into space. Soon after, a 2nd rotating tether could be put up that would fling tourists past the moon, in flybys just over the surface, and back to earth orbit. But they still wouldn't be able to land there without conventional rocket lunar landers. So soon after that, a 3rd rotating tether could be put up that would land tourists on the moon. Additional tethers, brought up by the first tether at far less of a cost than the first one, would drop the cost to space even more, to the point that ordinary middle class people could afford trips into space. All of that could be begun today, with current technology, and finished within a few years, so that we could have thousands and then millions of tourists not only going into earth orbit, but to the moon as well, and increasing numbers of people living in space permanently. Yet it isn't being done, and instead, we are wasting many times the money to accomplish next to nothing in space! Later on, when stronger materials became available, the rotating tethers in low earth orbit could be lengthened and made to rotate at a faster tip speed, so that the suborbital rocket would need to achieve a lower speed and could take up less fuel and more cargo, for a still lower cost per weight.
(Another use for tethers would be to connect 2 space stations together like a rotating barbell to create centrifical force so that people could live in space at normal earth gravity, to avoid the bodily deterioration of living in zero G for long periods of time. A complete ring of space station could eventually be filled in, creating a wheel-shaped space station like the one seen in "2001: A Space Odyssey".)
If even $1/2 billion was too much to spend initially, still shorter cables could be put up that would weigh much less than ones that could boost vehicles from half orbital speed, so would be much less expensive, and could only boost vehicles from, say, 3/4 orbital speed up to orbital speed. Even those cables would enable rockets to take up twice as much cargo as now, so reduce the cost to space by half, and they would probably soon pay for themselves, plus reduce the cost of putting up longer cables.
A number of experimental cables have already been put in space since the 1990s. They have been used to change the orbits of satellites, and to test the idea of pushing against the earth's magnetic field to change their own orbits. One of them is still in orbit as of this writing, after a number of years, connecting 2 satellites miles apart, and has still not broken.
I just described a rotating space tether for lifting cargo into space cheaply. Now I'll describe a non-rotating space tether, also called a space elevator.
Imagine not the space shuttle as in the last essay, in low earth orbit 300 miles up, but a satellite in geosynchronous orbit, 23,000 miles up. At that height, an object in orbit goes around the earth once every 24 hours. Since the earth turns once every 24 hours, objects in geosynchronous orbit stay stationary with respect to the ground below them. Now imagine a cable stretched both up and down from geosynchronous orbit (to keep the center of gravity at that height). Such a cable would have to be 46,000 miles long (actually 60,000 miles, for reasons I won't bother to go into). Even that isn't too daunting, considering that we humans have stretched trans-oceanic cables all around the earth, which is 25,000 miles around. The weight of the cable would be 300 tons, about 15X what the space shuttle can carry up. However, if we just put up a very thin cable at first, the space shuttle or other rocket could still carry it up, and then we could in turn use that cable to bring up more cable, until the final cable was strong enough to lift the heavy cargo we want to be able to lift. Or if we already had rotating space tethers, we could use them to put up the space elevator cable, for a much lower initial cost. Cable cars would then go up and down the cable, transporting cargo, like an elevator to space. Unlike the rotating space tether, the space elevator would be permanently attached to one spot on the earth. (For various reasons, the ocean off of Ecuador, on the equator directly below the cable's center of gravity, would be a good spot. But elevators could even be attached to the earth's surface in the temperate zones, at the range of latitudes of the US.) By keeping the cable's center of gravity slightly beyond geosynchronous orbit, there would be a bit of centrifugal force pulling the cable outward as it rotated with the earth, keeping the cable taut, and allowing heavy cargo to be attached to it and riding up the cable without the cable being pulled lower.
Such a cable would have some advantages over a rotating space tether. It would always be located at one spot and would be attached to the earth, so that there would be no need to rendezvous with it as we would with the rotating space tether. In a sense it would be available all the time, unlike the space tether. However, since it could only take up elevator cars every other day or so, so as not to overload the cable, while rotating tethers could take up cargo every time they dipped down, several times an hour, that is not really an advantage, but a big disadvantage. Objects brought up by a rotating space tether could be released in low earth orbit, flung from the cable end as it pointed away from earth, into higher orbits, but not that much higher. Objects brought up by space elevator could be released into geosynchronous orbit, a much more useful orbit than low earth orbit for many purposes, which now costs 8X as much to get to, but with the space elevator would cost no more. Objects released farther out along the cable could be flung to the moon, or to anywhere in the solar system, or even clear out of it, at no additional cost. But multiple rotating space tethers could fling objects farther out, even out of the solar system, just the way the space elevator could. However, they would require a very complicated dance in space as a vehicle would have to be released by the first tether in just the right direction to rendezvous with the next tether that would boost its speed further, taking into account how the tethers' positions would change as they captured and flung various vehicles. It all seems too complicated to be feasible, but computers should have no trouble scheduling and keeping track of the whole thing. Space elevators would not need to balance incoming and outgoing cargo the way rotating space tethers would. But the technique of pushing against the earth's magnetic field would probably solve that problem.
The main disadvantage of space elevators would be that they probably could not be used to bring up people, only cargo. The cable cars could probably not go much faster than 150 mph, since they would have wheels that would roll along the cable. At that speed, it would take a week to get to geosynchronous orbit, and even longer to go farther. Therefore they would have to have sleeping quarters, bathrooms, dining areas, etc. Even worse, as people passed through the Van Allen radiation belts around the earth at that slow speed, they would receive a lethal dose of radiation. The weight of the shielding needed to protect them would bring the cost up to the point where it would be no less expensive than we can already do with rockets. There is a possibility that electrically-charged satellites could be put up which would drive off more than 99% of the radiation particles in the radiation belts. But the slow speed up would still seem to be a big problem. Therefore, rotating space tethers would probably be used to bring up people into space within minutes, and space elevators, assuming it ever paid to build them instead of just using rotating tethers, would be used only for heavy cargo.
The worst problem with space elevators at the moment is that they would require materials considerably stronger and lighter than rotating space tethers would, so it would probably be quite a while before they are feasible, if ever. Carbon nanotubes have the required strength, but so far only in microscopic fibers and not in rope made from them.
Okay, so now that you've read the preceding essays, you must think that minimizing or eliminating expensive rockets is the best way to cut the cost of getting into space. If so, then you have fallen into the same intellectual trap that everyone I have read who has talked about this subject has fallen into. Even K. Eric Drexler, the champion of nanotechnology who wrote the book "Engines of Creation", has fallen into a similar trap. There is a key section of the book where he says that most people who try to figure out how to cut the cost to space assume that most of the cost is in the enormous amounts of fuel that rockets use. He points out that the fuel actually costs a fraction of 1% of a launch, and the rest of the cost is in the enormous amounts of manpower required to build, launch and maintain rockets. He then describes a rocket engine that could be made cheaply by nanotechnology that would be so strong that it would receive little wear and tear and would need almost no maintenance, so that little manpower would be needed for a space program using such rockets. While that is true, there is an even better way to cut the cost to space. But first, let me digress.
People today are instantly dismissive of the science-fictiony, futuristic ideas that people used to predict would come to pass, and never did, such as living in cities on the moon. In fact, it is their very insistence that people deserve nothing unless they work that has prevented those dreams from coming true. Now that seems like a stretch! Where is the connection? Bear with me, and I'll show it.
There are only 2 sources of funding for such futuristic dreams, private industry and government. Private industry will not fund anything unless it has the potential to make a profit in the near future. It is government's role to fund research that has no immediate payoff, but a huge payoff farther in the future. We owe our current health and prosperity, in large part, to government research that was conducted decades ago, that has finally born fruit.
There is only one trouble with government research: there is no incentive to cut costs, but indeed, there is an incentive to raise them, often to the point where such projects become so expensive that they are canceled altogether, or severely cut back. At least private industry is ruthless in cutting costs by trying to get rid of workers as much as possible. That may not be good for the workers, but at least it drives down the cost of projects to where they are affordable. But our insistence that people only deserve money if they work has lead to the creation of make-work, as an excuse to pay them, as I talk about in my essay "Marketthink" on my economics page. The very desperation we've created for people to have jobs, by only giving them money if they work, creates a tremendous impediment to progress in our society. Anytime someone wants to change things for the better, someone's job is at stake, and they fight tooth and nail against that change. In this case, government research, such as our space program, becomes a jobs program, an excuse to create more jobs. More salaries to pay means a more expensive program. A more expensive program means that it is cut back until we are accomplishing next to nothing, as we have been in space for decades.
Take the International Space Station that is now being built -- please! The cost is astronomical, and rising, and as a result, the station is already being scaled back to cut costs. And what are all those mega-billions accomplishing? Next to nothing. As Nobel-Prize-winning scientist Richard Feynman said, when he was investigating the Challenger explosion, all those billions of dollars for experiments conducted aboard the space shuttle, which is touted as its main reason for existence, and he had never heard of any significant scientific discovery to come out of them. The experiments, both on the shuttle and now on the space station, are entirely bogus, and are nothing more than a justification for the program. In fact, the money could have been used for real, cash-strapped research, such as in the field of artificial intelligence, so the space program is actually impeding the pace of scientific discovery greatly. Much of the research is involved in discovering the effects of weightlessness on people, and how to counter them. That's truly ridiculous, since it is easy enough to provide artificial gravity in space, by connecting 2 modules with a long length of cable and setting the resulting dumbbell-shaped object to spinning. A complete ring of modules can later be built, forming a torus-shaped spinning space station as seen in the movie "2001: A Space Odyssey". And all that emphasis NASA places on the effects of weightlessness is the worst publicity for the space program, for it makes people forget that artificial gravity can easily be created in space, and gives people the impression that people could never live in space in large numbers. So why fund the space program?
If I sound like someone who is against space travel, far from it -- I am a space fanatic! The trouble is, our current space program is actually PREVENTING us from going into space in a big way.
The cost of getting into space and building a space station, for example, need only be a tiny fraction of what it is. As for getting into space, the space shuttle costs something like $500 million per launch. It can launch up to 7 people, plus cargo. Meanwhile, the Russian Soyuz can launch 3 people, for a cost of just $10 million (though they charge $20 million to outsiders like the space tourists that have gone up, to make a profit). A separate rocket, the Proton, launches heavy cargo. The price is so low not because they have superior technology, but because salaries are so low in Russia. Therefore, if we simply abandoned our own space shuttle and paid the Russians to launch everything for us, we could probably accomplish TEN TIMES what we are doing now for the same amount of money! We would surely be going into space in a big way if it only cost 1/10 of what it does now -- going back to the moon and setting up a moon base there, for instance. Meanwhile, NASA has a program to try to create a lower-cost replacement to the space shuttle, which wouldn't be ready till at least a decade from now, even while having Russia launch everything for us is already cheaper. (Unfortunately, Russia has been selling nuclear technology in Iran, and the US passed a law preventing us from buying space technology from Russia to punish them.)
Another example is that, ever since the space shuttle began, space enthusiasts have been pointing out that the spent huge brown fuel tanks that are simply dropped back into the atmosphere to burn up each launch could be brought up into orbit, with no extra fuel used (indeed, with LESS fuel used, because the shuttle has to make a maneuver to get out of the way of the tank when it lets it go). Each tank is so enormous that it would have far more room inside of it than the entire space station will have, for free. Its interior would just have to be outfitted with equipment to make it habitable, the same equipment in the space station being built. And every launch, another those tanks could have been put into orbit. By now, if we'd been doing this, after over 100 shuttle launches, there would have been enough room to house a small city up there. But NASA has simply ignored such suggestions. The last thing they'd want is to find a cheap way of doing things, because that would mean fewer jobs, which is most of the excuse the politicians use to even HAVE a space program.
But if artificial intelligence and complete automation arrive, that changes everything. By getting rid of all the paid workers, the cost of everything will be virtually zero, so we will be able to pursue those futuristic dreams with abandon. People have come up with all sorts of imaginative ways to reduce the cost to space, such as space tethers and space elevators that I wrote about above, and accelerating ships into space on long tracks placed on stilts 20 miles up, out of most of the friction of the atmosphere, using magnetic propulsion. But the technologies that will truly reduce the cost to space, all the way down to virtually zero, seem at first glance to have nothing to do with space travel: artificial intelligence and robots. With A. I. and robots, even the problem-plagued space shuttle would be an excellent way of getting into space! The huge cost of space shuttle launches is ultimately the salaries of all the workers needed to maintain the shuttles between launches, get them ready to launch again, and actually launch them. Get rid of all the paid workers, and the cost of even the space shuttle is virtually zero!
As I talk about in my essay "The Case Against Conservative Economics" on my economics page, even before we reach complete automation, we will reach a point where we have automated away enough work that people would gladly do the remaining work voluntarily. There are plenty of space enthusiasts who would jump at the chance to do with their lives what they would find most meaningful, if only they were freed of the necessity of earning a living at dull meaningless jobs.
Here I would like to keep alive the ideas of Gerard K. O'Neill, a scientist who died in the late 1980's, who was the first person to seriously study how we might build cities in space, back in the early 1970's. Back then, people always assumed that cities would be built on planets, especially Mars. He pointed out that it would make little sense to build them on planets, for many reasons. On a planet, you can do little to change the conditions already there, whether or not they are optimal, such as too low or high gravity or temperature. Once you are down on a planet, it is difficult to get back up off of it, just as it is difficult to get off the earth, even with the use of space tethers as I described in the previous essays, so why deliberately climb down onto one? Planets only have limited area to live on -- Mars, about the only object in the solar system with conditions anywhere near suitable to live on (and still less suitable than Antarctica, which few people seem to want to settle), would only provide about the same land area as already exists on earth, not even worth the bother if humans want to expand their range in a big way. And most important, in space, the sun shines all the time, so that it could provide a completely reliable, cheap energy source, whereas on a planet, the sun shines only during the day, and may be interrupted by clouds, as on earth. The only thing free space is missing that planets have in abundance is material to build with, but that could be brought in. Initially, we would build space cities in high earth orbit, by using space tethers to get material off of the earth and moon. Later, they would be built in independent orbits about the sun, from asteroidal material.
Once the space tethers were built in earth orbit, it would be easy to put many thousands of people up there, and plenty of equipment, so we could fabricate huge things like space cities. But even reducing the cost to space to just $2/lb wouldn't be enough to build space cities miles or even thousands of miles across very economically. So we'd put rotating space tethers or space elevators (which would be much more feasible around the moon due to its lower gravity) around the moon to haul up building material. With the moon's low gravity, it would be far cheaper still to get it off the moon, probably 1/20 the cost, since it takes 1/20 the energy to get something off the moon than the earth. The moon would supply the metals to build things, and oxygen, which it has in abundance, since rocks are made of oxides of various metals such as aluminum and titanium. However, the moon has virtually no volatile elements, hydrogen, nitrogen or carbon, that water and living things are made of. There are apparently those frozen lakes at its poles, which would be great to use initially to gain a foothold there in inhabiting the place, but those would soon be used up. So we'd haul up just those elements from the earth, to provide the water, soil and plants in the space cities. They would make up a small percentage of the mass of the space cities. Earth orbit alone would probably have room for trillions of people, enough to last for hundreds of years of population growth. However, it would be easy enough to put space cities in solar orbits as well. And as time went on, civilization would expand into the asteroid belt, where space cities could be built from material on site, for still cheaper (assuming that even means anything if there's complete automation). Many asteroids contain all the materials we'd need, even the volatile elements. Even farther out, the satellites of the outer planets are practically pure balls of water ice, mixed with plenty of other volatiles, up to 3000 miles thick, enough resources to last practically forever.
The space cities would be capsule-shaped, cylindrical with hemispherical ends. They'd rotate along the cylindrical axis to provide artificial gravity, and they'd be filled with air. People would live on the interior surface. To the people in them, "up" would be toward the spin axis. The "ground" would be the metal shell of the colony, 6 feet thick. They would be anywhere from a mile, for the first, smallest ones, to 1000 or more miles across, containing the land area of entire continents, maybe even entire worlds. The "ground" could be landscaped just like the surface of the earth, with houses, streets, trees, lakes, rivers, even small oceans. Those hemispherical ends would be like mountain ranges on each side, which would keep getting steeper and steeper as they go up. Sunlight would be reflected in by giant mirrors during the day, and by slowly moving the mirrors, the sun could be made to move across the sky during the day, just like on earth. In space cities less than 4 miles across, people would look up and see the opposite side of the colony above them, the buildings and people hanging upside down. But in larger colonies, atmospheric scattering of sunlight would cause the opposite side to be completely lost in the haze, the way the stars are during the day, and when people looked up, they would see blue sky, just like on earth. But at night, they'd see a sky filled with "stars" -- the lights from the cities on the opposite side. There could even be realistic weather.
The 1st colonies would have to be hand-made, so to speak, but later, the basic metal structures could be spit out like sausage links by gigantic automated factories, which would be fed the raw materials from the moon, earth or asteroids. Then the bare structures could be landscaped in endlessly pleasing ways using fractal geometry, the same way computers create imaginary landscapes.
The hardest part would probably be keeping the ecologies inside of the space cities under control and in balance. But if need be, armies of robots could continually keep species in check, the way we humans have to continually weed a garden. The best parts are, everything would have to be recycled, and solar power would be truly practical in space. With no nighttime or clouds, space gets 8X the solar energy as earth does, and most important, since it is never interrupted, there would be no need for expensive backup energy storage for when the sun isn't shining. With no gravity, it would be easy to build parabolic mirrors to focus sunlight to create any desired temperature, for industrial uses. (Industry would be located entirely outside the colonies.) When all else failed, if there were any pollutants, they could be passed through that heat and atomized. Simply shade anything, and it drops to near absolute zero, also useful for some purposes, without the need for costly refrigeration. People would never run out of materials, because all it takes to recycle is energy, and there'd be that in abundance.
There is enough material in the asteroid belt to provide a trillion times the land area that there is on earth, and support quadrillions or even quintillions of people, millions or billions of people for every person alive today. At any reasonable rate of population growth, it would take several thousand years for humans to populate that entire area. (By disassembling entire planets, assuming we felt no qualms about doing so, we could even create far more living room than that.) But even long before we run out of room in our own solar system, it would be relatively easy to send entire space cities out of the solar system on leisurely, luxurious trips to the stars. You wouldn't even have to leave home, for the very ground your house sits on would be doing the traveling. (A beam of sunlight would be focused on the space city in order to keep it illuminated even while far out between the stars.) Once at the new star, it wouldn't even matter whether that star had any habitable planets. Whatever material is available would be used to build a new solar system full of space cities. And so on, as humans spread throughout the galaxy.
Right now, living on this one planet, with no signs of life found so far anywhere else in the universe, intelligent or otherwise, humans are the only consciousness we know of, and all of our eggs are in one basket. In a billion years or less, the sun's slowly-increasing brightness as it ages is going to make the earth uninhabitable. If an asteroid strike or nuclear war wiped out the only civilization capable of getting conscious life off the planet, another one would never likely arise. If for no other reason than to preserve the precious wondrous spark of consciousness, we should spread from our planet as soon as we can.
I should point out that no matter what we do, there will come a point when humans will have to limit their population growth. At the current rate of growth, by around 6000 years from now, the weight of human flesh would equal the mass of the entire universe, which is obviously absurd. I'll leave our distant ancestors to deal with the problem of how to restrict people from having too many children. Space settlements would give us centuries of extra breathing room before we would face this problem, and by then, hopefully attitudes will be much more enlightened than they are among some people today, who have many children without thinking about the consequences. Or perhaps, technologies beyond our current imaginations, that might construct entire new universes, would be available by then to solve the problem. Given the uncertainties about what might eventually be possible, it is silly to worry about the problem now.
In the case of cities floating on the oceans, we would set automated factories to produce modular floating platforms, in a continuous stream. These platforms could be square-shaped or hexagonal, and they would be connected together into larger platforms like paving blocks for cities of whatever size we like.
Just as space colonies would benefit from nearly limitless energy, from the sun, so would ocean colonies. In this case, we would use a type of energy called OTEC, or Ocean Thermal Electrical Conversion. In the tropical oceans, there is as much as a 50°F difference in temperature between the depths and the surface, and this can be used to run turbines to generate electricity. In fact, an OTEC power station on Hawaii already does just that. A portion of the energy generated by the difference in temperature is used to pump up more cold water from the depths. A substance with just the right boiling point, so that it condenses at the colder temperature, vaporizes at the warmer temperature, is circulated between the two areas. The heat from the warmer water vaporizes the substance, and as it expands, it pushes the turbine. The substance is then circulated back to the cold water, which condenses it again.
Even if the oceans were filled with cities, the power stations, all pumping up cold water from the depths, would hardly make a dent in the temperature difference. The cities would only be placed in a band from 10° south to 10° north latitude, across the Atlantic, Pacific and Indian Oceans, where hurricanes never form to damage the cities, because there isn't enough Coriolis force to set those storms spinning. The migration to those cities would drastically ease the population burden on the continents, so that they could be returned to a much more natural state.
But wouldn't those floating cities damage the ecology in the oceans? In fact, no, because 97% of the oceans are virtually barren deserts. Sunlight falls on the surface, but all the nutrients float down to the bottom, and oceanic plants that the rest of the food chain depends on can only grow where both are in the same place. Only small areas of the oceans are filled with life, along the immediate shores, due to runoff of nutrients from the land, and areas of upwelling that bring the nutrients at the bottom to the surface, such as off of Chile. So the cities would not be displacing any indigenous life, any more than cities in the middle of the Sahara would. In fact, OTEC has the side effect of creating artificial upwelling, in the course of its operation of bringing cold water from the depths next to warm water at the surface to run the turbines. The result is to make the desert bloom, like bringing water to the Sahara. The population of algae in the water explodes, and then fish and other creatures that eat the algae also increase vastly in numbers. Since the majority of the earth is ocean, it is calculated that the earth could support many times the amount of life that it does if only the nutrients at the bottom of the ocean were brought to the surface. Almost all that sunlight that falls on the oceans is wasted. Therefore, ocean cities would actually improve the earth's ecology far beyond the way it was before humans came along.