“A long time ago someone calculated the maximum size for space habitats. They could be made in sizes at least as large as twelve miles in diameter, with a land area of several hundred square miles in each one. We're already talking about shifting the mining base from the Moon to the asteroids, where we'll have a complete range of elements including carbon, nitrogen, and hydrogen. In energy, it won't be any harder for us to get materials from the asteroids than from Earth, and it should be a lot cheaper because the transport system can take its time and won't ever need high thrust.”