WHO
  WHAT
  WHERE
  HOW
  WHY
  WHEN
  COST

Why?

Growth

Why build space settlements? Why do weeds grow through cracks in sidewalks? Why did life crawl out of the oceans and colonize land? Because living things want to grow and expand. We have the ability to live in space (see the bibliography), therefore we will -- but not this fiscal year.

The key advantage of space settlements is the ability to build new land, rather than take it from someone else. This allows a huge expansion of humanity without war or destruction of Earth's biosphere. The asteroids alone provide enough material to make new orbital land hundreds of times greater than the surface of the Earth, divided into millions of colonies. This land can easily support trillions of people.

A Nice Place to Live

A few features of orbital real estate are worth mentioning:

• Great views. Many astronauts have returned singing the praises of their view of Earth from orbit. Low earth orbit settlements, and eventually settlements near Jupiter and Saturn, will have some of the most spectacular views in the solar system. Of course, all space settlements will have unmatched views of the stars, unhindered by clouds, air pollution, or (with some care) bright city lights.
• Low-g recreation. Consider circular swimming pools around and near the axis of rotation. You should be able to dive up into the water! Sports and dance at low or zero-g will be fantastic. For dancers, note that in sufficiently low gravity, always available near the axis of rotation, anyone can jump ten times higher than Baryshnikov ever dreamed.
• Environmental independence. On Earth we all share a single biosphere. We breathe the same air, drink the same water, and the misdeeds of some are visited on the bodies of all. Each space settlement is completely sealed and does not share atmosphere or water with other settlements or with Earth. Thus, if one settlement pollutes their air, no one else need breathe it.
• The ultimate gated community. On Earth it is essential that diverse groups learn to live in close proximity. It's hard to live with five or six billion homo sapiens, and some people can't seem to do it gracefully. Space settlements offer an alternative to changing human nature or endless conflict -- the ability to live in fairly homogeneous groups, as has been the norm throughout hundreds of thousands of years of human existence. Those who can't get along can be separated by millions of miles of hard vacuum, which in some cases seems necessary. All entry into a space settlement must be through an airlock, so controlling immigration should be trivial.
• Custom living. Since the entire environment is man-made, you can really get what you want. Like lake front property? Make lots of lakes. Like sunsets? Program sunset simulations into the weather system every hour. Like to go barefoot? Make the entire environment foot-friendly.

Survival

If there were a major collision today, not only would billions of people die, but recovery would be difficult since everyone would be affected. If major space settlements are built before the next collision, the unaffected space settlements can provide aid, much as we offer help when disaster strikes another part of the world.

Building space settlements will require a great deal of material. If NEOs are used, then any asteroids heading for Earth can simply be torn apart to supply materials for building colonies and saving Earth at the same time.

Power and Wealth

Those that colonize space will control vast lands, enormous amounts of electric power, and nearly unlimited material resources. The societies that develop these resources will create wealth beyond our wildest imagination and wield great power -- hopefully for good rather than for ill.

In the past, societies which have grown by colonization have gained wealth and power at the expense of those who were subjugated. Unlike previous colonization programs, space colonization will build new land, not steal it from the natives. Thus, the power and wealth born of space colonization will not come at the expense of others, but rather represent the fruits of great labors.