Carl Sagan

Astronomer

• Born: November 9, 1934
• Birthplace: Brooklyn, New York
• Died: December 20, 1996
• Place of death: Seattle, Washington

Educator, scientist, and writer

Sagan studied the surfaces and atmospheres of the major planets, conducted experiments on the origin of life, made important contributions to the debate over the environmental consequences of nuclear weapons, and popularized science through his books, public lectures, and the Cosmos television series.

Areas of achievement: Education; science and technology; literature

Early Life

Carl Sagan (SAY-guhn) was born in Brooklyn, New York, on November 9, 1934. His parents were poor Jewish immigrants from Russia, who came to the United States to escape the poverty in their home country. His father, Sam Sagan, worked in a garment factory. His mother, Rachel Molly Gruber, was a housewife. As a child, Sagan avidly read science fiction and became fascinated by astronomy. His parents knew little about science, but Sagan credited them with encouraging his interest in astronomy, teaching him the value of questioning and inspiring the joy of wonder at an early age.

His family moved to Rahway, New Jersey, where he graduated from Rahway High School in 1951. Sagan attended the University of Chicago, where he was awarded a bachelor’s degree in physics in 1955, a master’s degree in physics in 1956, and a doctorate in astronomy and astrophysics in 1960.

Life’s Work

After receiving his doctorate, Sagan was selected for a fellowship at the University of California, Berkeley, which was becoming a major center for space science and the development of space probes. While at Berkeley, Sagan participated in the Mariner 2 project, the first American space probe to conduct successfully a planetary encounter, reaching Venus in December, 1962. In the early 1960’s, Venus was puzzling to planetary scientists. Models indicated that the surface of the planet should be relatively cool, but measurement of the radiation emission suggested the planet was quite hot. Using radar and optical measurements, Sagan developed a new model that indicated that the “greenhouse effect”—the warming of a planet when its atmosphere contains gases that allow incoming solar radiation to pass through the atmosphere but prevent most of the outgoing infrared radiation from escaping into space—would explain the high temperature of Venus. He also developed an interest in the origin of life while at Berkeley.

Sagan suggested “terraforming” Venus, altering its atmosphere in ways that would make it suitable for human habitation. This suggestion gained public attention when it was reported in The New York Times and Newsweek, and Sagan developed an interest in popularizing science by explaining it to the general public.

Sagan was appointed assistant professor of astronomy at Harvard University in 1962. Following up on his interest in the origin of life, Sagan conducted experiments simulating the ancient atmosphere of Earth, trying to duplicate the conditions that permitted life to develop. Earlier experiments by Stanley Miller and Harold Urey had produced amino acids, the building blocks of proteins, by a spark discharge in a gas having the composition of Earth’s early atmosphere. In 1963, Sagan reported that irradiation of a mixture of methane, ammonia, water, and hydrogen sulfide gases, extending Miller and Urey’s work, found not only amino acids but also sugars and other compounds essential to life processes.

In 1965, when photos of the surface of Mars were transmitted to Earth by the Mariner 4 spacecraft, the planet showed no sign of life. Sagan, in an article in The New York Times, compared the Mars photos with photos of Earth taken from a similar distance and resolution. Earth showed no signs of life either, so Sagan argued that the question of life on Mars remained unsettled.

In 1968, Sagan accepted a faculty position at Cornell University in Ithaca, New York. When the Mariner 9 spacecraft was placed in orbit around Mars in 1971, its view of the planet’s surface was obscured by a global dust storm. It took months for the fine dust particles to settle from the atmosphere, finally allowing the spacecraft to return good-quality photographs of the planet. Measurements of the temperature changes in the atmosphere and the surface as the amount of dust in the atmosphere decreased demonstrated that the dust absorbed incoming sunlight, heating the atmosphere. However, the same dust blocked much of the sunlight from reaching the surface of Mars, cooling the surface. Applying this same idea to Earth, Sagan and coworkers from the Ames Research Center of the National Aeronautics and Space Administration (NASA) validated their model by correctly predicting the degree of cooling of Earth’s surface from the dust deposited in the atmosphere by volcanic eruptions. Using this as a starting point, they modeled the effects of dust, soot, and smoke carried into the atmosphere by nuclear weapons explosions and concluded that a global nuclear war would have consequences far beyond the deaths caused by the explosions. Dust lofted into the atmosphere would reduce the amount of sunlight at the surface to a few percent of normal, making it too dark for photosynthesis in plants for a period of several months. Sagan spoke out publicly about these consequences, a scenario called “nuclear winter.”

Sagan’s book The Cosmic Connection was published in 1973, and he appeared on a number of television shows, including the popular Tonight Show with Johnny Carson, promoting the book. In easy-to-understand language, Sagan described results in astrophysics, solar system science, space colonization, terraforming planets, and the search for extraterrestrial life. A review in New Scientist said that “Sagan’s gift for clear and stylish explanation is a delight. . . . If aliens come tomorrow and ask for our leader, we shall take them to this man.” His popularity and clear explanations resulted in repeated appearances on Johnny Carson’s show.

Sagan, along with Ann Druyan and Steven Soter, wrote Cosmos: A Personal Voyage, a thirteen-part television series that premiered on the Public Broadcasting System (PBS) in 1980. The series covered a diverse range of scientific topics, including the origin of life and the place of humans in the universe. The series provided clear explanations of complex scientific issues.

Following the success of the Cosmos series, Sagan received a two-million-dollar advance on a novel, Contact. The novel describes the first human contact with an extraterrestrial civilization and suggests the possible cultural conflicts between religion and science resulting from this contact.

Sagan developed bone marrow cancer and underwent three bone marrow transplants prior to his death, at the age of sixty-two, on December 20, 1996. NASA named the Mars Pathfinder spacecraft the Carl Sagan Memorial Station after its successful landing on Mars on July 5, 1997.

Significance

Sagan was a cofounder of the Planetary Society, an organization that has more than a hundred thousand members from around the world. This group supports such research programs as the search for extraterrestrial intelligence; the development of “solar sails,” an innovative method of space propulsion; and the development of balloons for Mars exploration and microphones on Mars, done in conjunction with NASA and the French and Russian space agencies. Although Sagan is best known to the public for his efforts to explain science through the mass media, during his research and teaching career Sagan made numerous contributions to the understanding of Venus and Mars, and he influenced efforts to search for extraterrestrial life. He trained a new generation of students who continue to make important contributions to planetary science and public policy. A film version of Contact was completed after Sagan’s death and won the Hugo Award for best dramatic presentation in 1997.

Bibliography

Davidson, Keay. Carl Sagan: A Life. Hoboken, N.J.: John Wiley & Sons, 1999. A well-illustrated, 540-page account of Sagan’s life, focusing on personal details that influenced his career. Sagan was one of the most celebrated scientists of his time—a leading visionary of the Space Age. He was also a highly controversial figure who inspired wildly opposed opinions.

Poundstone, William. Carl Sagan: A Life in the Cosmos. New York: Holt, 2000. A 496-page account of Sagan’s life, focusing on his efforts to search for extraterrestrial life and to share his love of science with the public.

Spangenburg, Ray, and Kit Moser. Carl Sagan: A Biography. Amherst, N.Y.: Prometheus Books, 2009. A 145-page account of Sagan’s life and professional accomplishments, detailing his lifelong effort to understand the cosmos.