Extraterrestrials-where are They?

advanced civilizations amino acids angular momentum assume asteroid belt asteroids Astronomy atmosphere binary biological bits Bracewell calculations carbon chemical evolution chemistry colonization communication cosmic cosmological Darwinian deceleration density detect disc stars discussed distance Drake equation Dyson Earth editor energy estimate existence experiments explanation extraterrestrial factor flife formation frequency galactic Galaxy genes genetic Gerald Feinberg habitable planets habitable zone habitats Hart Hoerner human hydrogen hypothesis Icarus infrared intelligent Interplanetary Soc interstellar settlement interstellar travel Jill Tarter large number less main sequence mass mass driver mega-civilization meteorites molecules nuclear nucleic acid nucleotides number of habitable observed orbit organic origin Papagiannis parsecs pellets peptides percent physical planetary systems plasma Ponnamperuma population possible present probably propulsion protein radiation radio random result Sagan signals solar system space colonies space habitats stellar suggested technological telescopes temperature terrestrial tions universe velocity wavelength Zuckerman