1. David Schwartzman and Lee J. Rickard, “Being Optimistic about the Search for Extraterrestrial Intelligence,” American Scientist, Vol. 76 (1988), 364–368.
2. Sagan, editor, CETI, 6.
3. Sagan, Cosmos, 41.
4. Ekers et al., 22, 115; Carl Sagan, “On the Detectivity of Advanced Galactic Civilizations,” Icarus, Vol. 19 (1973), 350–351, reprinted in Goldsmith, editor, 140–141; Sagan, editor, CETI, 366.
5. Shapley, 155.