Carl Edward Sagan was an American astronomer, astrophysicist, cosmologist, author, science popularizer and science communicator in astronomy and natural sciences. He spent most of his career as a professor of astronomy at Cornell University where he directed the Laboratory for Planetary Studies. He published more than 600 scientific papers and articles and was author, co-author or editor of more than 20 books. He advocated scientific skeptical inquiry and the scientific method, pioneered exobiology and promoted the Search for Extra-Terrestrial Intelligence (SETI).
Sagan is known for his popular science books and for the award-winning 1980 television series Cosmos: A Personal Voyage, which he narrated and co-wrote. The book Cosmos was published to accompany the series. Sagan wrote the novel Contact, the basis for a 1997 film of the same name. Sagan also appeared on a television special The Earth Day Special which aired April 22, 1990.
Sagan’s contributions were central to the discovery of the high surface temperatures of the planet Venus. In the early 1960s no one knew for certain the basic conditions of that planet’s surface, and Sagan listed the possibilities in a report later depicted for popularization in a Time–Life book, Planets. His own view was that Venus was dry and very hot as opposed to the balmy paradise others had imagined. He had investigated radio emissions from Venus and concluded that there was a surface temperature of 500 °C (900 °F). As a visiting scientist to NASA’s Jet Propulsion Laboratory, he contributed to the first Mariner missions to Venus, working on the design and management of the project. Mariner 2 confirmed his conclusions on the surface conditions of Venus in 1962.
Sagan was among the first to hypothesize that Saturn’s moon Titan might possess oceans of liquid compounds on its surface and that Jupiter’s moon Europa might possess subsurface oceans of water. This would make Europa potentially habitable. Europa’s subsurface ocean of water was later indirectly confirmed by the spacecraft Galileo. The mystery of Titan’s reddish haze was also solved with Sagan’s help. The reddish haze was revealed to be due to complex organic molecules constantly raining down onto Titan’s surface.
He further contributed insights regarding the atmospheres of Venus and Jupiter as well as seasonal changes on Mars. He also perceived global warming as a growing, man-made danger and likened it to the natural development of Venus into a hot, life-hostile planet through a kind of runaway greenhouse effect. Sagan and his Cornell colleague Edwin Ernest Salpeter speculated about life in Jupiter’s clouds, given the planet’s dense atmospheric composition rich in organic molecules. He studied the observed color variations on Mars’ surface and concluded that they were not seasonal or vegetational changes as most believed but shifts in surface dust caused by windstorms.
Sagan is best known, however, for his research on the possibilities of extraterrestrial life, including experimental demonstration of the production of amino acids from basic chemicals by radiation.
He is also the 1994 recipient of the Public Welfare Medal, the highest award of the National Academy of Sciences for “distinguished contributions in the application of science to the public welfare”. He was denied membership in the Academy, reportedly because his media activities made him unpopular with many other scientists.
Sagan’s ability to convey his ideas allowed many people to understand the cosmos better—simultaneously emphasizing the value and worthiness of the human race, and the relative insignificance of the Earth in comparison to the Universe. He delivered the 1977 series of Royal Institution Christmas Lectures in London. He hosted and, with Ann Druyan, co-wrote and co-produced the highly popular thirteen-part Public Broadcasting Service (PBS) television series Cosmos: A Personal Voyage modeled on Jacob Bronowski’s The Ascent of Man.
Cosmos covered a wide range of scientific subjects including the origin of life and a perspective of our place in the Universe. The series was first broadcast by the PBS in 1980, winning an Emmy and a Peabody Award. It has been broadcast in more than 60 countries and seen by over 500 million people, making it the most widely watched PBS program in history. In addition, Time magazine ran a cover story about Sagan soon after the show broadcast, referring to him as “creator, chief writer and host-narrator of the new public television series Cosmos, [and] takes the controls of his fantasy spaceship”.
Sagan was a proponent of the search for extraterrestrial life. He urged the scientific community to listen with radio telescopes for signals from potential intelligent extraterrestrial life-forms. Sagan was so persuasive that by 1982 he was able to get a petition advocating SETI published in the journal Science and signed by 70 scientists including seven Nobel Prize winners. This was a tremendous increase in the respectability of this controversial field. Sagan also helped Frank Drake write the Arecibo message, a radio message beamed into space from the Arecibo radio telescope on November 16, 1974, aimed at informing potential extraterrestrials about Earth.
Sagan was chief technology officer of the professional planetary research journal Icarus for twelve years. He co-founded The Planetary Society, the largest space-interest group in the world, with over 100,000 members in more than 149 countries, and was a member of the SETI Institute Board of Trustees. Sagan served as Chairman of the Division for Planetary Science of the American Astronomical Society, as President of the Planetology Section of the American Geophysical Union, and as Chairman of the Astronomy Section of the American Association for the Advancement of Science (AAAS).
At the height of the Cold War, Sagan became involved in public awareness efforts for the effects of nuclear war when a mathematical climate model suggested that a substantial nuclear exchange could upset the delicate balance of life on Earth. He was one of five authors – the “S” – of the “TTAPS” report, as the research paper came to be known. He eventually co-authored the scientific paper hypothesizing a global nuclear winter following nuclear war. He also co-authored the book A Path Where No Man Thought: Nuclear Winter and the End of the Arms Race, a comprehensive examination of the phenomenon of nuclear winter.
Sagan also wrote books to popularize science, such as Cosmos, which reflected and expanded upon some of the themes of A Personal Voyage and became the best-selling science book ever published in English; The Dragons of Eden: Speculations on the Evolution of Human Intelligence, which won a Pulitzer Prize; and Broca’s Brain: Reflections on the Romance of Science. Sagan also wrote the best-selling science fiction novel Contact in 1985, based on a film treatment he wrote with his wife in 1979, but he did not live to see the book’s 1997 motion picture adaptation, which starred Jodie Foster and won the 1998 Hugo Award for Best Dramatic Presentation.
He wrote a sequel to Cosmos, Pale Blue Dot: A Vision of the Human Future in Space, which was selected as a notable book of 1995 by The New York Times. He appeared on PBS’ Charlie Rose program in January 1995. Sagan also wrote the introduction for Stephen Hawking’s bestseller, A Brief History of Time. Sagan was also known for his popularization of science, his efforts to increase scientific understanding among the general public, and his positions in favor of scientific skepticism and against pseudoscience, such as his debunking of the Betty and Barney Hill abduction. To mark the tenth anniversary of Sagan’s death, David Morrison, a former student of Sagan, recalled “Sagan’s immense contributions to planetary research, the public understanding of science, and the skeptical movement” in Skeptical Inquirer.
Sagan hypothesized in January 1991 that enough smoke from the 1991 Kuwaiti oil fires “might get so high as to disrupt agriculture in much of South Asia …” He later conceded in The Demon-Haunted World that this prediction did not turn out to be correct: “it was pitch black at noon and temperatures dropped 4°–6 °C over the Persian Gulf, but not much smoke reached stratospheric altitudes and Asia was spared”. A 2007 study noted that modern computer models have been applied to the Kuwait oil fires, finding that individual smoke plumes are not able to loft smoke into the stratosphere, but that smoke from fires covering a large area, like some forest fires or the burning of cities that would be expected to follow a nuclear strike, would loft significant amounts of smoke into the stratosphere.
In his later years Sagan advocated the creation of an organized search for near Earth objects that might impact the Earth. When others suggested creating large nuclear bombs that could be used to alter the orbit of a NEO that was predicted to hit the Earth, Sagan proposed the Deflection Dilemma: If we create the ability to deflect an asteroid away from the Earth, then we also create the ability to deflect an asteroid towards the Earth—providing an evil power with a true doomsday bomb.
Sagan was one of the few critics of Plato. Sagan said of Plato: “Science and mathematics were to be removed from the hands of the merchants and the artisans. This tendency found its most effective advocate in a follower of Pythagoras named Plato.” and “He (Plato) believed that ideas were far more real than the natural world. He advised the astronomers not to waste their time observing the stars and planets. It was better, he believed, just to think about them. Plato expressed hostility to observation and experiment. He taught contempt for the real world and disdain for the practical application of scientific knowledge. Plato’s followers succeeded in extinguishing the light of science and experiment that had been kindled by Democritus and the other Ionians.