1. Lonely Planets, The Natural Philosophy of Alien Life;Grinspoon,2004
2. Term popularised by the Spielberg's movie “Close Encounters of the Third Kind”. It was originally coined by Dr J. Allen Hynek in his 1972 book The UFO Experience: A Scientific Inquiry. Hynek served 20 years as a consultant to the official USA's UFO research programme Project Blue Book.
3. European objectives and interests in Space exploration. (November 2007) Document published on the occasion of the International Space Exploration Conference in Berlin, Germany, 8–9 November 2007.
4. Gallup poll (1987) CNN/Time magazine (1997) Life magazine (2000) Quark monthly science magazine (2002) SCI FI Channel/Roper poll.
5. S. Shostak, The great UFO debate, SETI institute. 〈http://www.seti.org/Page.aspx?pid=837〉, 2005.