William B Stoecker: A lot of people have given a lot of thought to human colonization of other worlds in our Solar System, and, ultimately, beyond it. But there are serious problems with just about every conceivable location. The Moon has little more than one sixth Earth's gravity, and we have learned that human health deteriorates rapidly in a micro gravity environment like on the space station. We can only suspect that similar health problems might occur in an extreme low gravity environment, like our Moon, but we don't know for sure because NASA has never done any experiments to find out. In low Earth orbit they could have tethered two spacecraft together and tested spin induced gravity, which is produced by angular momentum (so-called "centrifugal force"). They could have tested astronauts in one half, one third, or one sixth Earth gravity for a week or two...but they never did. Mars has at least some atmosphere (perhaps more dense than the hired liars at NASA have claimed), water ice, and a day almost the same length as ours, but only a little over one third Earth gravity. In addition, there may still be life there, at least bacteria; these bacteria could be a hazard, and we, in turn, might contaminate the Martian ecosystem. Besides, as I have pointed out elsewhere, Mars may already be the home of intelligent beings. Mercury has extremes of heat and cold and is blasted by deadly radiation during solar storms, and has low gravity. Venus has almost the same gravity as Earth, but is a hellish hothouse where lead would melt.
Image credit: NASA
Image credit: NASA
But there is an alternative to living on unsuitable planets: create our own worlds. This old idea was revived and developed by Gerard O'Neill in his 1977 book "The High Frontier: Human Colonies in Space." He proposed immense colonies, reflecting sunlight into the interiors with huge mirrors (direct exposure to the Sun would make the colonies vulnerable to radiation during solar storms). He proposed many possible shapes, such as cylinders rotating slowly to produce gravity, or torus (doughnut) shaped colonies, such as Werner Von Braun suggested for manned space stations. It is important to remember that the spin axis has to be long enough to prevent dizziness, sickness, and even death resulting from so-called coriolis forces. But, in his enthusiasm, O'Neill glossed over the fact that conventional rocket technology was simply too dangerous and too expensive for such an undertaking, and the economies of the various nations today are far too weak to afford such colossal construction.
But suppose that, in the future, we develop better energy sources, such as cold fusion (which may not be fusion at all) or so-called free or virtual energy. And suppose that we develop better propulsion systems, perhaps even gravity control. And suppose that the global economy, thanks to these technologies, and perhaps also to Lunar or asteroid mining, is vastly more prosperous than is the case today. Then such colonies might become feasible. All sorts of people might choose to pool their resources and establish colonies: religious and ethnic groups, for example, or even nudists...the colony could have year round mild weather. Presumably the colonists would adopt constitutions, mediums of exchange, and vote on the desired climate. Most (like the nudists) would probably opt for a mild, Hawaiian climate; even those wanting four seasons would not be likely to want Death Valley summers or Fairbanks winters. Initially, the colonies might be established, as O'Neill suggested, in the five Earth/Moon Lagrange points or the five Earth/Sun Lagrange points, places where gravity and momentum offer complete stability; any object placed there will stay there.
The smallest and cheapest colony possible would have a dumbell shape, with two spherical or cylindrical habitats connected by a long tube and rotating around the center. If you lived in one of the cylinders, "down" would be out, and "up" would be toward the center. If you traveled higher in your cylinder you would become lighter as gravity decreased, and lighter still if you travelled up the connecting tube, until you would experience micro gravity at the center. This design would also make it possible (albeit with some difficulty) to add more pairs of cylinders over time, until a torus appeared.
But let us give some thought to life in a cylinder shaped colony. Imagine one three miles in diameter and ten miles long. Its inner surface (not counting the ends, which would be vertical walls) would have an area of over 94 square miles. Assuming a habitat shell with a 100 foot ceiling, half of it composed of bike paths, maglev train tubes, shops, small repair facilities, and parks and gardens, and half composed of ten story condo buildings, where the average home was 5,000 square feet in size and housed three people, some 394,000 people could live there. There could be several such shells, with many used for intensive agriculture; the plants would provide both food and oxygen in this enclosed and balanced ecosystem. Even though the shells would be progressively smaller toward the center, a million or more people could live in such a colony, and the center cylinder, with its microgravity, might be used for large scale manufacturing, storage, micro gravity recreation, and a hangar for one or more shuttle or scout craft. It might be necessary for the people to live in the outermost shells, with nearly a full Earth gravity, and use the inner ones, where everything would weigh less, for farming.
If the colonists could afford the "wasted" space and wanted a habitat more like Earth, there might be a couple of shells for habitat and farming, and then an immense inner shell, with, perhaps, forests, rivers, lakes, and hills. Or there could be hollow mountains with their interiors used for habitat and agriculture. People could (in theory) opt for a sparser population of individual homes and farms, or even for a hunter-gatherer way of life. In considering agricultural productivity, any home gardener knows that a few fruit trees and tomato plants (for example) can yield a good deal of produce, even with much of it lost to weather and pests; these problems could be eliminated in a space colony.
If structural materials are strong enough, and especially if human beings and plants and animals can thrive in a low gravity environment (still an unknown), it is possible to imagine a colony 100 miles long and twenty miles in diameter. Artificial sunlight would be produced by strips of lights along the innermost (micro gravity) cylinder. People might live in immense hollow mountains seven and one half miles high (assuming that the inner cylinder is five miles in diameter) arranged in pairs on opposite sides. Anyone standing on the base level would see himself at the bottom of an immense valley, with its sides becoming vertical and then arching overhead; of course, the people on the sides or overhead would feel that they were the ones at the bottom of the valley. The mountains, perhaps covered in vines or other vegetation and sporting innumerable windows and balconies, would seem higher than any peaks on Earth. In a reduced gravity, human-powered flight would be possible, among other things.
Eventually, some of the colonies might opt to leave the inner Solar System and move out into the Kuiper Belt, a toroidal shaped region extending from about 30 AU (atsronomical units, each one being 93 million miles, the mean distance of the Earth from the Sun) to about 55 AU from the Sun, or even into the hypothetical Oort Cloud, a spherical shell believed to begin at about 50,000 AU from the Sun. Objects in these regions might be mined to enlarge colonies or build more of them, although elements heavier than hydrogen and helium would be scarce.
The colonists might even decide to travel to another star system. As I have pointed out in another article, the interstellar medium is almost, but not quite, a vacuum, and would limit spacecraft velocities with any currently forseeable technology. At relativistic (near light speed) velocities, a craft with a frontal area of only 100 square meters would be subjected to 7,560,000 calories per second due to impact with molecules in space, mostly hydrogen and helium with a small amount of water ice and silicates. This energy would be in the form of gamma radiation, and would also cause problems with thermal buildup, ablation, and loss of momentum. But at one percent of light speed, the energy would be only 7.56 calories per second. So epic journeys, lasting for centuries, could be undertaken by people who would never leave home...for the colony would be the only home they knew.
And if we can go there, might aliens not come here, and, even now, be living somewhere in our Solar System? And the colonies could be thought of as super-organisms, capable of asexual reproduction (one colony building another) or sexual reproduction, with the inhabitants of two colonies getting together to build another. And, for all we know, our own remote ancestors may have already journeyed to the stars, and be living there right now.
William B Stoecker