In the summer of 1975, when space exploration was still ever so young, NASA Ames Research Center and Stanford University put on their thinking caps regarding potential space settlements. That brainstorming summit produced the 1975 NASA Summer Study, which envisioned what life in space might look like. The focus was on orbiting spaceships, and the result was a document called "Space settlements: A design study."
In the forward, then-NASA Administrator James Fletcher called the study a big-picture question for humankind: "To assess the human and economic implications as well as technical feasibility, the participants in this effort have provided us with a vision that will engage our imagination and stretch our minds."
Here, for example, is a cutaway view of ring-shaped "toroidal colonies," exposing the interior. Click on for more illustrations from the study.
Princeton physicist Gerard O'Neill proposed this modified Bernal sphere, which he called "Island One." It would have a diameter of only 500 meters spinning at 1.9 rpm to produce full Earth artificial gravity at the sphere's equator.
The interior of O'Neill's Bernal sphere would resemble a lush valley running all the way around the equator of the sphere, capable of providing living and recreation space for a population of about 10,000 people, with a "crystal palace" habitat used for agriculture. The form of a sphere was chosen for its optimum ability to contain air pressure and its optimum mass-efficiency at providing radiation shielding.