Human civilization has evolved to the point at which we can consider tapping space resources and expanding beyond Earth's atmosphere. The Introduction surveys possible motivations for large-scale human emigration to space. Since our early ancestors began to move out of Africa, humans have constantly expanded their range. Today, the pattern of human settlement extends from pole to pole. Humans regularly visit the upper troposphere and ocean floor and technology has enabled a few to even reside above the atmosphere in space stations.

For the next few millennia at least (barring...