Other visionaries point to Mars. Robert Zubrin, in his book "The Case for Mars," envisions the Red Planet as a second branch of human civilization. Eventually human settlers would use terraforming techniques to turn Mars into an analogue of Earth, with breathable air, running water, and an active biosphere where humans could walk around without need of space suits.
The late Dr. Gerard K. O'Neill suggested, in a book "The High Frontier," a more radical approach to colonizing space. Instead of going to another world, he envisioned humans-- in effect-- building their own worlds using extraterrestrial resources to build huge, rotating space colonies where people would live inside a rotating cylinder large enough to build small towns in. Such a colony was depicted in the science fiction TV series "Babylon Five."
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