During his time as an honors program undergraduate , Sagan worked in the laboratory of the geneticist H. Muller and wrote a thesis on the origins of life with physical chemist Harold Urey. Sagan joined the Ryerson Astronomical Society,  received a B. He went on to earn a M.
He used the summer months of his graduate studies to work with his dissertation director , planetary scientist Gerard Kuiper ,  as well as physicist George Gamow and chemist Melvin Calvin. The title of Sagan's dissertation reflects his shared interests with Kuiper, who throughout the s had been president of the International Astronomical Union 's commission on "Physical Studies of Planets and Satellites".
Sagan had a Top Secret clearance at the U. The leak was not publicly revealed until , when it was published in the journal "Nature". A follow-up letter to the journal by project leader Leonard Reiffel confirmed Sagan's security leak. After the publication of Sagan's Science article, in Harvard University astronomers Fred Whipple and Donald Menzel offered Sagan the opportunity to give a colloquium at Harvard and subsequently offered him a lecturer position at the institution.
Sagan instead asked to be made an assistant professor , and eventually Whipple and Menzel were able to convince Harvard to offer Sagan the assistant professor position he requested. In , Sagan was denied tenure at Harvard. He later indicated that the decision was very much unexpected.
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Carl Sagan , from Demon-Haunted World . Following the denial of tenure from Harvard, Sagan accepted Gold's offer and remained a faculty member at Cornell for nearly 30 years until his death in Unlike Harvard, the smaller and more laid-back astronomy department at Cornell welcomed Sagan's growing celebrity status.
In , he became the David Duncan Professor of Astronomy and Space Sciences, a position he held for the remainder of his life. Sagan was associated with the U. From the s onward, he worked as an advisor to NASA , where one of his duties included briefing the Apollo astronauts before their flights to the Moon.
Sagan contributed to many of the robotic spacecraft missions that explored the Solar System , arranging experiments on many of the expeditions.
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He continued to refine his designs; the most elaborate message he helped to develop and assemble was the Voyager Golden Record , which was sent out with the Voyager space probes in Sagan often challenged the decisions to fund the Space Shuttle and the International Space Station at the expense of further robotic missions. Former student David Morrison describes Sagan as "an 'idea person' and a master of intuitive physical arguments and ' back of the envelope ' calculations",  and Gerard Kuiper said that "Some persons work best in specializing on a major program in the laboratory; others are best in liaison between sciences.
Sagan belongs in the latter group. Sagan's contributions were central to the discovery of the high surface temperatures of the planet Venus. His own view was that Venus was dry and very hot as opposed to the balmy paradise others had imagined. 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 Sagan was among [ clarification needed ] 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. 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.
Sagan 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.
He studied the observed color variations on Mars' surface and concluded that they were not seasonal or vegetational changes as most believed, [ clarification needed ] but shifts in surface dust caused by windstorms. Sagan is also known 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 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".
As of [update] , Sagan is the most cited SETI scientist and one of the most cited planetary scientists. In Sagan co-wrote and narrated the award-winning part PBS television series Cosmos: A Personal Voyage , which became the most widely watched series in the history of American public television. The show has been seen by at least million people across 60 different countries. Because of his earlier popularity as a science writer from his best-selling books, including The Dragons of Eden , which won him a Pulitzer Prize in , he was asked to write and narrate the show.
It was targeted to a general audience of viewers, who Sagan felt had lost interest in science, partly due to a stifled educational system. Each of the 13 episodes was created to focus on a particular subject or person, thereby demonstrating the synergy of the universe. The show won an Emmy ,  along with a Peabody Award , and transformed Sagan from an obscure astronomer into a pop-culture icon. 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.
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 he was able to get a petition advocating SETI published in the journal Science , signed by 70 scientists, including seven Nobel Prize winners.
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This signaled a tremendous increase in the respectability of a then-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, , aimed at informing potential extraterrestrials about Earth. Sagan was chief technology officer of the professional planetary research journal Icarus for 12 years. At the height of the Cold War , Sagan became involved in nuclear disarmament efforts by promoting hypotheses on the effects of nuclear war , when Paul Crutzen 's "Twilight at Noon" concept suggested that a substantial nuclear exchange could trigger a nuclear twilight and upset the delicate balance of life on Earth by cooling the surface.
In he was one of five authors—the "S"—in the follow-up "TTAPS" model as the research article came to be known , which contained the first use of the term " nuclear winter ", which his colleague Richard P. Turco had coined. Sagan received a great deal of skepticism and disdain for the use of media to disseminate a very uncertain hypothesis. A personal correspondence with nuclear physicist Edward Teller around began amicably, with Teller expressing support for continued research to ascertain the credibility of the winter hypothesis.
However, Sagan and Teller's correspondence would ultimately result in Teller writing: "A propagandist is one who uses incomplete information to produce maximum persuasion. I can compliment you on being, indeed, an excellent propagandist, remembering that a propagandist is the better the less he appears to be one".
The adult Sagan remained a fan of science fiction, although disliking stories that were not realistic such as ignoring the inverse-square law or, he said, did not include "thoughtful pursuit of alternative futures". Sagan also wrote the best-selling science fiction novel Contact in , based on a film treatment he wrote with his wife, Ann Druyan, in , but he did not live to see the book's motion-picture adaptation , which starred Jodie Foster and won the Hugo Award for Best Dramatic Presentation.
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. Following Saddam Hussein 's threats to light Kuwait 's oil wells on fire in response to any physical challenge to Iraqi control of the oil assets, Sagan together with his "TTAPS" colleagues and Paul Crutzen , warned in January in the Baltimore Sun and Wilmington Morning Star newspapers that if the fires were left to burn over a period of several months, enough smoke from the or so Kuwaiti oil fires "might get so high as to disrupt agriculture in much of South Asia In the televised debate, Sagan argued that the effects of the smoke would be similar to the effects of a nuclear winter , with Singer arguing to the contrary.
After the debate, the fires burnt for many months before extinguishing efforts were complete. The results of the smoke did not produce continental-sized cooling. Sagan remained hopeful that the natural NEO impact threat and the intrinsically double-edged essence of the methods to prevent these threats would serve as a "new and potent motivation to maturing international relations". Sagan was a critic of Plato , having said of the ancient Greek philosopher: "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. Speaking about his activities in popularizing science, Sagan said that there were at least two reasons for scientists to share the purposes of science and its contemporary state. Simple self-interest was one: much of the funding for science came from the public, and the public therefore had the right to know how the money was being spent.
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If scientists increased public admiration for science, there was a good chance of having more public supporters. While Sagan was widely adored by the general public, his reputation in the scientific community was more polarized.
One of Sagan's harshest critics, Harold Urey , felt that Sagan was getting too much publicity for a scientist and was treating some scientific theories too casually. While Urey was an "old-time empiricist" who avoided theorizing about the unknown, Sagan was by contrast willing to speculate openly about such matters. Sagan's Harvard friend Lester Grinspoon also stated: "I know Harvard well enough to know there are people there who certainly do not like people who are outspoken.
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Wherever you turned, there was one astronomer being quoted on everything, one astronomer whose face you were seeing on TV, and one astronomer whose books had the preferred display slot at the local bookstore. Some, like Urey, later came to realize that Sagan's popular brand of scientific advocacy was beneficial to the science as a whole. I congratulate you You are a man of many talents. Sagan was accused of borrowing some ideas of others for his own benefit and countered these claims by explaining that the misappropriation was an unfortunate side effect of his role as a science communicator and explainer, and that he attempted to give proper credit whenever possible.
Sagan believed that the Drake equation , on substitution of reasonable estimates, suggested that a large number of extraterrestrial civilizations would form, but that the lack of evidence of such civilizations highlighted by the Fermi paradox suggests technological civilizations tend to self-destruct. This stimulated his interest in identifying and publicizing ways that humanity could destroy itself, with the hope of avoiding such a cataclysm and eventually becoming a spacefaring species.
Sagan's deep concern regarding the potential destruction of human civilization in a nuclear holocaust was conveyed in a memorable cinematic sequence in the final episode of Cosmos , called "Who Speaks for Earth? In March , Reagan announced the Strategic Defense Initiative —a multibillion-dollar project to develop a comprehensive defense against attack by nuclear missiles , which was quickly dubbed the "Star Wars" program.