1. Goldsmith and Owen, 432.
2. Casti, 413.
3. For a list, see Ekers, et al., 381–425.
4. Grinspoon, 305; Frank Drake, “On Hands and Knees in Search of Elysium,” Technology Review, June 1976, 22–29. Drake repeated that thought in his 1992 autobiographical book. Drake and Sobel, 234.
5. Swift, 107; Achenbach, 280; Michael J. Klein, “Where and What Can We See?”