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106 Mind-Stretcher. Robert Dixon turned the asteroid threat argument on its head by proposing that if occasional great extinctions caused by asteroid collisions are a necessary condition for intelligent life to evolve, the frequency of such life may be dependent on the frequency of available asteroids. Asteroids are not the only concern; large comet nuclei and similar icy objects could be dangerous if they were perturbed into Earth-crossing orbits. The Kuiper belt, a doughnut-shaped region beyond the orbit of Neptune, contains an estimated 100,000 frozen bodies more than 200 kilometers across. We know the positions and orbits of only 800 of them. Even the more distant Oort Cloud could be a source of danger. Major impact events 35 million years ago may have been caused by long-term comets, diverted into the inner solar system by a perturbation of the cloud.” As hazards from asteroids and comets must apply to inhabited planets all over the Galaxy, Sagan concluded that few civilizations will be both long-lived and nontechnological. Chyba warned us of the implications: A biosphere that does not produce a technical civilization capable of assessing and responding to its particular impact environment on a times- cale shorter than that between giant impacts will be catastrophically dis- rupted. Science fiction author Larry Niven put it more wryly, commenting that the dinosaurs became extinct because they didn’t have a space program.” A minute fraction is in the form of visible suns. But what about the rest? Is it in dead suns, planets, and moons? Is it possible that billions of exhausted suns are now wandering in waste places of space? And are they all surrounded by dead planets, still in revolution, counting off lifeless and useless years? 2 Weaken TANS There may be other extraterrestrial threats. Astronomers predict that our solar system will collide with interstellar clouds several times during each galactic year—the time it takes for our Sun to complete an orbit around the center of our Galaxy. In his 1957 novel The Black Cloud, astronomer Fred Hoyle described the potentially disastrous impact on life of being immersed in a dense cloud that would prevent much of the Sun’s energy from reaching the Earth. Another astronomer, Priscilla Frisch, predicted that the solar system may be heading for a cloud of inter- stellar matter up to a million times more dense than its present surround- Probabilities: Longevity Black Clouds, Dark Planets, Bursting Stars —Scientific American, February 1906