Our first colonies in space will almost certainly be in asteroids. They are easy to reach, easy to mine, offer protection from meteors and radiation, and most importantly, they are not at the bottom of gravity wells.
A permanent home in space requires:
- a safe and relatively spacious place to live
- possibly gravity or a suitable substitute
- work (something creating wealth to justify the expense of going there)
An asteroid, having very little gravity, is easily mined. Tunnels can be dug throughout their volume with little need for bracing. A typical asteroid has room to spare. Assuming that every colonist requires 1000 cubic meters of space (the equivalent of a 4000 square foot house), some used for living, some for working, some for the farm to recycle the stuff of life), and assuming that only half of the asteroid is excavated, a one kilometer asteroid has room for a population of a quarter of a million colonists. There are over one million such asteroids in the main belt, another million in the Jovian points in Jupiter's orbit. And there are millions more smaller ones.
Near the Earth, solar power is cheap and readily available. In the asteroid belt, it will likely be necessary to depend upon nuclear power, as sunlight is relatively dim. Large, cheap mirrors can be used to focus sunlight to useful intensities.
By weight, an asteroid is more oxygen than anything else (excluding the nickel-iron ones--they are mostly iron). Still, oxygen would be recycled, as would water and food. While blue-green algae can provide all of the nutrition required for life, it is likely that we would not be satisfied with such a boring cuisine. A farm would likely raise vegetables, fruits, and grains in high-intensity farming, using our wastes as nutrients in a largely closed system. For meats, the farms can easily raise fish of several types, rabbits for meat, chickens for meat and eggs, goats for meat, milk, and cheeses. However, meats would likely be a luxury--a space colonist's diet would be largely vegetarian. It's simply more efficient.
We don't yet know how important gravity is to life. We do know that bad things happen to our bones and muscles if we don't use them the way that Earth-bound people do every day simply to move around. At present, astronauts on long missions must exercise heavily to maintain enough bone and muscle to survive a return to Earth. I suspect that centrifuges would be maintained in asteroid colonies to provide exercise and conditioning. I personally believe that children should be required to attend school, and that the schools themselves should be spun for gravity. That way an appreciable fraction of each child's development would be in earth-like gravity.
The last thing needed is justification, work, a place of employment. Just like on a cruise ship or aircraft carrier, most of the people there are support personnel, only indirectly involved in the creation of value. But the value of resources available in as asteroid are astronomical. Counting only the material removed to provide living space from that typical one-kilometer asteroid, there are one hundred million tons of oxygen, fifteen million tons of iron, two million tons of aluminum. In earth orbit, power generation satellites would be a profitable venture, removing our dependence upon carbon-based fuels. Tourism would be another profitable endeavor. In the long run, expanding civilization itself is profitable and self-sustaining, just like it is on Earth.
One last point: except for debris in near-earth orbits, industrial processes in outer space are pollution-free. At the very least, they don't pollute the Earth. Waste gasses simply dissipate, waste materials such as slag are still valuable as shielding or as rocket exhaust. But the mentality of living in space where everything should be recycled will translate into new mind sets and new processes. The Earth is huge, and many people assume that the pollution they create is either inconsequential or simply someone else's problem. But on an asteroid, such an attitude would be criminal and treated as such.