1. Tough, A., in Bioastronomy—The next steps: Proceedings of the 99th colloquium of the International Astronomical Union (Balaton, Hungary), 1987, ed. Marx G. Dordrecht: Kluwer, 1988, pp. 397–404
2. Chaisson, E., The life era: cosmic selection and conscious evolution, New York: Norton, 1987
3. Dick, S. J., The biological universe: the twentieth-century extraterrestrial life debate and the limits of science, Cambridge: Cambridge University Press, 1996
4. Schenkel, P., ETI: a challenge for change, New York: Vantage, 1988, p. 185
5. Michaud, M. A. G., The consequences of contact, AIAA Student J., 1977–78, p. 20