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Carl Edward Sagan (November 9, 1934 - December 20, 1996) was an American astronomer and astrobiologist and a highly successful popularizer of astronomy, astrophysics, and other natural sciences. He pioneered exobiology and promoted the Search for Extra-Terrestrial Intelligence (SETI). He is world-famous for writing popular science books and for co-writing and presenting the award-winning 1980 television series Cosmos: A Personal Voyage, which has been seen by more than 600 million people in over 60 countries, making it the most widely watched PBS program in history. A book to accompany the program was also published. He also wrote the novel Contact, the basis for the 1997 Robert Zemeckis film of the same name starring Jodie Foster. During his lifetime, Sagan published more than 600 scientific papers and popular articles and was author, co-author, or editor of more than 20 books. In his works, he frequently advocated skeptical inquiry, humanism, and the scientific method.
Carl Sagan was born in Brooklyn, New York. His parents were Jewish; his father, Sam Sagan, was a garment worker and his mother, Rachel Molly Gruber, was a housewife. Carl was named in honor of Rachel's biological mother, Chaiya/ Clara, "the mother she never knew", in Sagan's words. Sagan graduated from Rahway High School in New Jersey in 1951. He attended the University of Chicago, where he received a bachelor's degree (1955) and a master's degree (1956) in physics, before earning his doctorate (1960) in astronomy and astrophysics. During his time as an undergraduate, Sagan spent some time working in the laboratory of the geneticist H. J. Muller. From 1962 to 1968, he worked at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts.
Sagan lectured annually at Harvard University until 1968, when he moved to Cornell University. He became a full professor at Cornell in 1971 and directed the Laboratory for Planetary Studies there. From 1972 to 1981 he was Associate Director of the Center for Radio Physics and Space Research at Cornell.
Sagan was a leader in the U.S. space program since its inception and worked as an adviser to NASA since the 1950s. One of his many duties during his tenure at the space agency included briefing the Apollo astronauts before their flights to the Moon. Sagan contributed to most of the robotic spacecraft missions that explored the solar system, placing experiments on many of the expeditions. He conceived the idea of adding an unalterable and universal message on spacecraft destined to leave the solar system that could be understood by any extraterrestrial intelligence that might find it. Sagan assembled the first physical message that was sent into space: a gold-anodized plaque, attached to the space probe Pioneer 10, launched in 1972. Pioneer 11, also containing the plaque, was launched the following year. He continued to refine his designs and the most elaborate such message he helped to develop and assemble was the Voyager Golden Record that was sent out with the Voyager space probes in 1977.
Sagan taught at Cornell a course on critical thinking until his death in 1996 from a rare bone marrow disease. The course had only a limited number of seats, although hundreds of students applied each year. From these course application essays, only about 20 were chosen to attend each semester. The course was discontinued after his death.
Sagan was 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 Venus' surface and Sagan listed the possibilities in a report later depicted for popularization in a Time-Life book, Planets-his own view was that the planet 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 views on the conditions of Venus in 1962.
Sagan was among the first to hypothesize that Saturn's moon Titan and Jupiter's moon Europa may possess oceans, a subsurface ocean, as in the case of Europa, or lakes, thus making the hypothesized water ocean on Europa potentially habitable for life. Europa's subsurface ocean was later indirectly confirmed by the spacecraft Galileo. Sagan also helped solve the mystery of the reddish haze seen on Titan, revealing that it is composed of complex organic molecules constantly raining down to the moon's surface.
He furthered insights regarding the atmospheres of Venus and Jupiter as well as seasonal changes on Mars. Sagan established that the atmosphere of Venus is extremely hot and dense with crushing pressures. 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 greenhouse gases. 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, concluding that they were not seasonal or vegetation 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."
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