If your mental image of futuristic human colonies in space involves tubular ships, rolling hills, and a population seemingly plucked from a cocktail party in Sausalito in 1972, chances are good that you've been influenced by the art of Rick Guidice and Don Davis — illustrators commissioned by NASA to envision human homes among the stars. At Discover.com, Veronique Greenwood writes about these artists and the lasting impact they've had on science and science fiction.