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date: 12 November 2019

Sagan, Carlfree

(09 November 1934–20 December 1996)
  • Keay Davidson

Carl Sagan.

Courtesy of Cornell University.

Sagan, Carl (09 November 1934–20 December 1996), space scientist, author, science popularizer, TV personality, and antinuclear weapons activist, was born Carl Edward Sagan in Brooklyn, New York. He was the son of Rachel Molly Gruber Sagan and garment industry worker Samuel Sagan, an immigrant from the Ukraine. Carl Sagan's Jewish background encouraged him “to ask questions early,” as he later observed (Davidson, p. 57); so did his mother's skeptical, sometimes acidic personality. At age five, he became interested in astronomy when he read in a library book that the stars are distant versions of our sun. His interest in science soared when his parents took him to the New York World's Fair of 1939–1940, which offered an optimistic and (as he later acknowledged) “extremely technocratic” view of the future (Davidson, p. 14).

Sagan developed a passionate interest in mythology, rocketry, and science fiction, especially Edgar Rice Burroughs's tales about life on Mars. His family moved to Rahway, New Jersey, in the late 1940s, where he attended Rahway High School, participated in many student clubs, acted in school plays, and covered sports for the student newspaper. He also appeared in student debates on radio and TV. In 1947 he discovered Astounding Science Fiction magazine, which introduced him to more mature scientific speculations than those in Burroughs's fables. That same year inaugurated the “flying saucer” craze; young Carl suspected the “discs” might be alien spaceships. He graduated in June 1951. The yearbook named him the male students' “Class Brain” and “Most Likely to Succeed.”

That autumn Sagan entered the University of Chicago. Hoping to become an astronomer, he majored in physics and ran the “theoretical” section of the campus astronomy club. He also obtained an unusually rich education in other subjects, especially biology. He would later credit his intellectual diversity to the campus's then-extant “Hutchins program” (named for former chancellor Robert Maynard Hutchins) of historically based, multidisciplinary education.

A decisive impact on Sagan's career was an experiment on the origin of life, the results of which were published during his sophomore year. The experiment was performed by an acquaintance, Stanley Miller, a graduate student in chemistry. Working under Nobel Prize–winning chemist Professor Harold C. Urey, Miller exposed a mixture of methane, ammonia, and water to electrical discharges; the result: amino acids, which are building blocks of living organisms. Miller's experiment dramatized the possibility of explaining life and its origins in purely scientific terms without resorting to supernaturalism or mysterious “vital forces.” The experiment thrilled Sagan, partly because he was a religious skeptic and partly because it reinforced his belief in extraterrestrial life. Miller's experiment implied that organic molecules form readily wherever the chemical conductions—in particular, a hydrogen-rich atmosphere—were right. Because hydrogen pervades the cosmos, this implied that alien life is common. Humans might not be alone in the cosmos.

Inspired, Sagan wrote his first published scientific paper, “Radiation and the Origin of the Gene” (1957) while still an undergraduate. He completed it with guidance from his first important mentor, Nobel Prize–winning geneticist H. J. Muller of Indiana University, for whom Sagan worked one summer as a lab assistant. The paper was published in the journal Evolution, an auspicious venue for a 22-year-old researcher still a few years shy of his doctorate. In 1954 Sagan obtained his A.B. degree with general and special honors, followed by his S.B. in physics in 1955 and, in 1956, his master's degree in the same subject, all at the University of Chicago. Then he entered graduate school at one of the astronomy department's two observatories, Yerkes Observatory in Williams Bay, Wisconsin. He spent the summer of 1956, ostensibly observing Mars, at the other campus observatory, McDonald Observatory at Fort Davis, Texas. His supervisor for that summer was noted astronomer Gerard Peter Kuiper, who complained that Sagan preferred to talk about extraterrestrial life rather than look through the telescope. Still, Kuiper eventually agreed to supervise Sagan's doctoral dissertation.

In June 1957 Sagan married Lynn Alexander of Chicago, with whom he was to have two children, both sons. They settled in Madison, Wisconsin, where she entered graduate school in biology at the University of Wisconsin. Sagan commuted to Yerkes to work on his doctorate. He gave public lectures on astronomy at Yerkes, ran an acclaimed science lecture series starring distinguished scientists (such as Urey, Muller, Kuiper and George Gamow) on the Chicago campus, and wrote his first popular article, “Life on Other Planets,” for the University of Chicago Magazine.

At Wisconsin, Sagan met another crucial mentor, future Nobel laureate Joshua Lederberg. They shared a mutual interest in the possibility of extraterrestrial life. The “space race” between the United States and the USSR began in October 1957, when the Russians successfully launched the first satellite, Sputnik. Sagan and Lederberg feared that the superpowers would contaminate the moon and planets with rocket debris that accidentally included microorganisms; the “bugs” might proliferate and destroy extraterrestrial ecosystems, if any existed. Lederberg arranged Sagan's appointment to a National Academy of Sciences advisory board on “exobiology,” the fledgling science of extraterrestrial life.

Sagan completed his doctoral dissertation in astronomy and astrophysics in June 1960. It discussed three questions: Might Miller-Urey–type processes have generated complex organic molecules, which in turn might have evolved into microbes, on the moon? Might the “gas giant” planet Jupiter contain complex organic molecules that could be remotely detected with astronomical spectroscopes? And why does the planet Venus emit intense microwave radiation? Sagan's answer to the first two questions was yes. As for the third question, he argued (expanding on a 1940 proposal by Rupert Wildt) that Venus emits intense microwave radiation because the planet is extremely hot, due to an atmosphere rich in carbon dioxide and water vapor that traps infrared radiation via the “greenhouse effect.”

During his postdoctoral fellowship (1960–1962) at the University of California at Berkeley, Sagan helped to develop an infrared radiometer that would fly to Venus aboard Mariner 2, the first successful interplanetary robotic probe. His powerful imagination discomfited some colleagues. For example, he proposed ways in which humans might launch spaceships to the stars and “terraform” Venus into a habitable world. He was one of a small band of pioneers who met at a Green Bank, West Virginia, radiotelescope site in late 1961 to discuss ways to use the giant “dishes” to detect possible radio signals from alien civilizations. Some scientists, notably Urey, regarded Sagan as a bright but verbose, reckless thinker.

Science reporters, however, were intrigued by his erudition, good looks, and charmingly eccentric style of speech. In the early 1960s Newsweek magazine called him “brilliant.” By now a UFO skeptic, he testified as an expert witness in a court trial of a UFO hoaxer-embezzler. About the same time, his marriage ended in divorce. (Lynn Sagan, later Lynn Margulis, would in time become a famous microbiologist.)

In 1963 Sagan began a joint appointment as a lecturer and assistant professor of astronomy at Harvard and as an astrophysicist at the nearby Smithsonian Astrophysical Observatory. During his five years on campus, he wrote a number of prescient papers on Venus and Mars with his first graduate student, the brilliant space scientist James B. Pollack (1938–1994). Until then, many astronomers, including Kuiper, had attributed brightness variations on the surface of Mars to seasonal vegetation changes. Sagan and Pollack suggested an alternative explanation: windblown dust. Also at Harvard, Sagan joined the antiwar movement. He quit an air force advisory board on UFOs in protest of U.S. policies in Vietnam.

Sagan's first important work of science popularization was Intelligent Life in the Universe (1966), which he cowrote with the Soviet radioastronomer I. S. Shklovskii. Despite his accomplishments, he was denied tenure at Harvard, at least partly because Urey sent a harsh anti-Sagan letter to the Harvard tenure committee. In 1968 he began teaching at Cornell University in Ithaca, New York, and married again. His new wife was Linda Salzman, an aspiring artist, with whom he had another child, also a son.

In coming years Sagan, chemist Bishun Khare, and others simulated the formation of organic molecules in the atmospheres of other planets such as Jupiter. Also, Sagan and physicist George Mullen proposed that the early earth was warm enough for life because primordial ammonia gas trapped solar infrared radiation. Likewise, Sagan suggested, Mars—now extremely cold, with low air pressure—had perhaps once been warm enough for life. Might any of its prehistoric inhabitants have survived to the present? Sagan thought so. He became a prominent figure in NASA's project to send life-detecting robots to Mars. In 1972 he and his second wife designed plaques containing “messages” to aliens (including an image of a naked man and woman) that were affixed to the Pioneer space probes. These robotic spaceships were launched toward the outer planets; they would eventually escape the solar system and enter galactic space.

Sagan was an eloquent writer. His bestselling 1973 book, The Cosmic Connection, a highly personal account of his fascination with space science, impressed many critics. His ability to explain difficult ideas in entertaining ways won him a semiregular spot on Johnny Carson's “Tonight Show” on the NBC-TV network. Sagan's fame grew even as space missions disproved some of his scientific ideas—in particular, his belief that organic molecules or life might be found on the moon and Mars. His explanation for the great heat of Venus, however, was largely vindicated, although its atmosphere had much less water vapor than he expected. Another triumph came when space probes took close-up photos of Mars and found its surface changes were, indeed, caused by windblown dust.

Sagan's book The Dragons of Eden (1977) was a crazy salad of speculations about the brain and the possible value of myths and dreams as clues to human evolution. Another bestseller, it won a Pulitzer Prize. Also in 1977 NASA launched to the outer solar system the Voyager space probes, which carried audio records designed by Sagan, his wife, science writer Timothy Ferris, Ferris's fiancée Ann Druyan, radioastronomer Frank Drake, and artist Jon Lomberg. The records contained samples of terrestrial music, earthly sounds and voices, and acoustically coded pictures. Sagan fell in love with Druyan. Soon he left his wife Linda for Druyan, then twenty-eight, a political radical and aspiring writer. Sagan and Druyan would develop an intense emotional and intellectual partnership.

In the late 1980s PBS-TV ran Sagan's thirteen-part science series, “Cosmos.” The show reached a global audience of at least 400 million and won major TV awards, including Emmys and the Peabody. The program offended some religious traditionalists and intellectuals, who were dismayed by Sagan's religious skepticism and what they regarded as his “scientistic” conviction that science is the royal road to truth, superior to other modes of inquiry. After Sagan and Salzman finally divorced in 1981, he married Druyan; they had two children, a daughter and a son.

Under Druyan's tutelage, Sagan's political views shifted overtly to the left. In the 1980s he was the best-known proponent of the “nuclear winter” hypothesis, which challenged the superpowers' nuclear war-fighting policies. Sagan and four other scientists (Richard Turco, Brian Toon, Thomas Ackerman, and Pollack, collectively known as TTAPS, with the “S” standing for “Sagan”) calculated that even a “small” exchange of nuclear warheads could trigger vast urban fires. That, in turn, would gush massive clouds of soot into the atmosphere, blocking sunlight and triggering a global cooling—perhaps even widespread freezes. Hence it might be impossible to “win” a nuclear war, as some nuclear strategists had hoped: victory might be short-lived because the “victor” would succumb to a devastating climate change. The only sure way to avoid nuclear winter, Sagan concluded, was to radically shrink the size of U.S. and Soviet nuclear arsenals. He defended this view in numerous speeches and TV appearances, in a face-to-face meeting with Pope John Paul II, and at an antinuclear protest near a weapons testing site in the Nevada desert (where he was arrested).

In response, H-bomb co-inventor Edward Teller attacked the TTAPS theory in Nature. The conservative publication National Review called the nuclear winter hypothesis a “fraud from the start,” and ran a cover story ridiculing its prime exponent as “Flat-Earth Sagan.” Scientists also debated the validity of the nuclear winter hypothesis. Whatever its merits, it supposedly influenced the Soviet leadership's decision (under reformist leader Mikhail Gorbachev) in the late 1980s to seek major reductions in nuclear armaments (Kokoshin, p. 136).

Druyan helped Sagan to write his only novel, Contact (1985), about the quest for alien radio signals. The couple also developed (with film producer Lynda Obst) a movie version of the book. The Sagans jointly wrote two books, including the superb Shadows of Forgotten Ancestors (1992), which scrutinized the evolutionary roots of human behavior, especially society's self-destructive tendencies.

Some scientists resented Sagan's literary and financial success, as well as his political activism, taste for scientific speculation, and sometimes abrasive personality. In 1992, after an emotional debate, members of the National Academy of Sciences refused to admit him to their ranks. Ironically, two years later the same organization gave him its Public Welfare Medal in recognition of his energetic popularization of science. “No one has ever succeeded in conveying the wonder, excitement and joy of science as widely as Carl Sagan and few as well,” the citation stated (Davidson, p. 397). In Seattle, Washington, on 20 December 1996, he died from pneumonia as a complication of a blood disease, myelodysplasia.

Sagan was one of the twentieth century's most “visible scientists” (Goodell, 1977), that is, a scientist who popularizes his or her work and, hence, is an unofficial liaison between scientists and laypeople. His imagination, literary skill, and showmanship captivated millions of readers and TV viewers. In the realm of science itself, he made a number of important contributions, especially to the study of Venus and Mars. As a teacher at Harvard and Cornell, he helped educate and inspire a generation of space scientists (Broad, p. D-5). As a political activist, he championed improved U.S.-Soviet scientific relations and led the nuclear winter debate, one of the last scientific controversies of the Cold War. At a funeral service for Sagan, Vice President Albert Gore, Jr., spoke of his friend's luminous personality: “Those of us who were privileged to bask in that light, however briefly, will never, ever forget its brilliance” (Gore, 1996).


As of mid-1999, Ann Druyan of Ithaca, N.Y., was in possession of Sagan's voluminous private papers, including his 265-page résumé, an exhaustive record of Sagan's publications, speeches, and awards and honors and of articles about him. Sagan's doctoral dissertation, “Physical Studies of Planets” (1960), is available from University Microfilms, Ann Arbor, Mich. A uniquely valuable record is historian Ronald E. Doel's long tape-recorded interview in 1991 with Sagan, archived at the Niels Bohr Library, American Center for Physics, College Park, Md.

Sagan's scientific papers include “Indigenous Organic Matter on the Moon” and “Biological Contamination of the Moon,” Proceedings of the National Academy of Sciences 46 (1960): 393–96 and 396–402; “The Planet Venus,” Science 133 (1961): 849–58, which cites the possibility of turning Venus into a habitable world; “Microenvironments for Life on Mars” (with Joshua Lederberg), Proceedings of the National Academy of Sciences 48 (1962): 1472–75; “Direct Contact among Galactic Civilizations by Relativistic Interstellar Spaceflight,” Planetary and Space Science 11 (1963): 485–98, which proposes sending humans to other stars and suggests aliens might have visited the earth in the past; “Life in the Clouds of Venus?” (with Harold Morowitz), Nature 215 (1967): 1259–60; “The Prospects for Life on Mars: A Pre-Viking Assessment” (with Lederberg), Icarus 28 (1976): 291–300; and “Particles, Environments, and Hypothetical Ecologies in the Jovian Atmosphere” (with E. E. Salpeter), Astrophysical Journal, suppl. ser. 32 (1976): 737–55, which speculates about the possibility of balloonlike creatures floating in the atmosphere of Jupiter.

Among Sagan's books not cited in the text are Cosmos (1980); Comet, with Ann Druyan (1985); A Path Where No Man Thought: Nuclear Winter and the End of the Arms Race, with Richard Turco (1990); Pale Blue Dot: A Vision of the Human Future in Space (1994); and A Demon-Haunted World: Science as a Candle in the Dark (1996). His last book, published posthumously, was Billions and Billions: Thoughts on Life and Death at the Brink of the Millennium, with an epilogue by Druyan (1997). The film version of Contact, starring Jodie Foster and directed by Robert Zemeckis, was released by Warner Bros. in 1997, several months after Sagan's death.

For an early biographical portrait, see Shirley Thomas, “Carl Sagan,” in Men of Space, vol. 6: Profiles of Scientists Who Probe for Life in Space (1963). Posthumous assessments include Keay Davidson, Carl Sagan: A Life (1999), a detailed portrait of his career and often troubled personal life, and William Poundstone, Carl Sagan: A Life in the Cosmos (1999). An especially astute profile appears in Henry S. F. Cooper, Jr., “A Resonance with Something Alive,” New Yorker, 21 and 28 June 1976. Rae Goodell, The Visible Scientists (1977), assesses Sagan as an example of a social type—the scientist who popularizes his or her work. The impact of the nuclear winter hypothesis on Soviet military policy is cited in Andrei A. Kokoshin, Soviet Strategic Thought, 1917–91 (1998). The New York Times article on Sagan's training of numerous space scientists is William J. Broad, “Even in Death, Carl Sagan's Influence Is Still Cosmic,” 1 Dec. 1998. The New York Times of 21 Dec. 1996 has a front-page obituary.