Humankind is taking its first tentative steps toward a permanent presence in space after retreating from that goal in the late twentieth century, when the American lunar program ended. The goal then consisted only of a piloted round trip to the Moon, a mission prompted by rivalry between the global superpowers of the United States and the Soviet Union. In the future the mission will be the inhabitation of space and other worlds, a mission prompted by a variety of goals.

Since the start of the "space race" in the late 1950s, when the Soviet Union launched the first satellite, Sputnik, and the United States took up the challenge to go to the Moon in the 1960s, each generation has found inspiration that has urged it on towards space and motivated it to join the effort. At each step, a new generation of thinkers...