When Carl Sagan was five years old, his mother enrolled him at the local library. He would walk there on his own, asking the librarian questions that his parents couldn't answer, such as, “What are stars?” Reflecting on the experience later, he recounted: “I went to the librarian and asked for a book about stars [...] and the answer was stunning. It was that the Sun was a star, but really close. The stars were suns, but so far away they were just little points of light. The scale of the universe suddenly opened up to me. It was a kind of religious experience. There was a magnificence to it, a grandeur, a scale which has never left me. Never ever left me.”
This anecdote, shared by Kay Davidson in one of several biographies written about Sagan, illustrates one of his most compelling traits: an insatiable curiosity. Over time, this was complemented by intellectual prowess, skepticism, a passion for teaching and making science accessible to the public, and a notable ego. Together, these qualities made him one of the most influential and fascinating figures in space exploration during the latter half of the 20th century.
"It just grabbed me"
Carl Edward Sagan was born on November 9, 1934, in Brooklyn, New York, to a Jewish family. His father, a textile worker, had immigrated from Ukraine, and his mother, a homemaker, was born in New York. The family lived modestly and later moved to New Jersey due to his father's work. . Carl inherited his mother’s intellectual curiosity and skeptical outlook, while his father instilled in him an immense curiosity and sense of wonder about the world. From a young age, Sagan channeled these traits into a deep passion for science. In a biography by Ray Spangenburg, Sagan described his parents’ influence as follows: "My parents were not scientists. They knew almost nothing about science. But in introducing me simultaneously to skepticism and to wonder, they taught me the two uneasily cohabiting modes of thought that are central to the scientific method."
Sagan was an outstanding student from a young age. While his teachers recognized his potential and sought to support him, his parents couldn’t afford private education for gifted students. He taught himself many subjects, including chemistry, but his primary scientific focus was space. At age 12, when his grandfather asked what he wanted to do when he grew up, Carl replied, “An astronomer,” to which his grandfather responded, “Fine, but how will you make a living?” Later, upon discovering that astronomers could indeed earn a living doing what they loved, Sagan knew he would pursue the field. ''I didn't make a decision to pursue astronomy,'' he would later say. ''Rather, it just grabbed me, and I had no thought of escaping.''
He graduated from high school at just 16 years old. According to another biographer, William Poundstone, near the end of high school, he entered a competition with an essay speculating on the potential dangers of encountering extraterrestrial life. He speculated that such a meeting could be devastating for humanity, much like the catastrophic effects that the encounter with European civilization brought upon the Indigenous peoples of the Americas. This essay may have been influenced by young Sagan's love for science fiction.
Sagan received a full scholarship to the University of Chicago, where he began studying physics in 1951. He went on to earn a master’s degree and later a Ph.D. in astronomy, which he completed in 1960. His doctoral research, supervised by astronomer Gerard Kuiper, focused on the physical properties of planets and moons. At the time, new measurements showed that the surface of Venus was extremely hot, contrary to earlier assumptions that it might have Earth-like temperatures. In his dissertation, Sagan proposed that the greenhouse effect was responsible for Venus’s high temperatures, demonstrating that carbon dioxide and water vapor were the primary culprits. He later published these findings in a significant paper in Science.
During his studies, Sagan collaborated with several renowned scientists, including George Gamow, a physicist who was instrumental in developing the Big Bang theory. The origin of life was another topic that had fascinated Sagan since childhood. As a student, he worked with renowned chemist Harold Urey, who led the seminal experiment exploring the chemical origins of life on Earth, and with geneticist Joshua Lederberg, a pioneer of gene therapy. Sagan also spent summers training with Hermann Muller, a geneticist and zoologist at Indiana University, who studied the effects of radiation and strongly opposed nuclear weapons.
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These images were captured thanks to Sagan’s efforts to persuade NASA leaders to photograph Earth from afar. The planets of the solar system, photographed by Voyager 1 in 1990
(Photo: NASA)
At the University of Chicago, Sagan met Lynn Margulis, a brilliant young scientist four years his junior who had entered the university at just 14. They married in 1957 when Margulis was 19. The marriage lasted seven years and produced two children. Margulis went on to have a distinguished scientific career, becoming one of the foremost researchers in evolutionary biology.
New frontiers
After earning his doctorate, Carl Sagan pursued further training at the University of California, Berkeley, where he chose to study biology to deepen his understanding of topics related to the potential development of life on other planets. At the same time, he joined the scientific team for NASA's Mariner 2 mission, a spacecraft launched to explore Venus.
In 1963, Sagan became a researcher at Harvard University, where he continued his studies of the planets of the solar system. Among his accomplishments was developing a model of Mars's atmosphere, which demonstrated that its seasonal changes were driven by dust storms. He also joined the scientific teams for additional space missions, including Mariner 9, the first spacecraft to orbit Mars, whose findings helped validate Sagan’s prediction.
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Designed to convey a message from humanity to intelligent extraterrestrials. The plaque sent aboard the Pioneer 10 and Pioneer 11 spacecraft
(Photo: Wikipedia)
Sagan’s extensive research on the planets and his fascination with origin of life research led him to repeatedly explore whether life could develop elsewhere in the solar system and under what conditions. For example, he explored the possibility for life in Venus’s atmosphere and the formation of relatively complex molecules within the environments of planets like Jupiter. These studies positioned Sagan as a founding figure in two scientific fields that were once peripheral to astronomy: planetary science, which focuses on processes occurring on planets, moons, and other celestial bodies, and exobiology—now commonly known as astrobiology—which focuses on the conditions necessary for life and the potential for its emergence or presence in other planetary systems.
In 1966, Sagan co-authored Intelligent Life in the Universe with Soviet astronomer Iosif Shklovsky, earning him public recognition beyond the scientific community. The following year, he published a detailed article on the subject in National Geographic and appeared on several television programs. By this point, it was clear that Sagan aimed for a career that would bring him broad public recognition and allow him to present scientific work to a wider audience.
This approach was unconventional in academia at the time, and some of his colleagues at Harvard viewed it as a compromise of scientific integrity in favor of self-promotion and publicity. According to his former student David Morrison, this perception likely contributed to Harvard’s decision not to grant Sagan tenure in 1968, despite his prolific research and reputation as a beloved and respected lecturer. Consequently, Sagan had to seek employment elsewhere. Unlike Harvard, Cornell University in upstate New York was eager to recruit faculty researchers with a potential for stardom. In 1969, Sagan joined Cornell, making it his academic home.
A star in the scientific sky
Carl Sagan remained a leading figure in planetary science and a regular contributor to solar system exploration missions. Notably, he was part of the scientific team for the Viking Mars landers, playing a crucial role in selecting their landing sites in 1976. He also contributed to the Pioneer 10 and Pioneer 11 missions, which became the first spacecraft to cross the asteroid belt and study Jupiter up close. Sagan further contributed to the Voyager missions, designed to explore the outer planets, and the Galileo mission, which studied Jupiter and its atmosphere. His research drew heavily on data from these missions, including analysis of the geology of Mars at Viking landing sites, studying the composition of Jupiter’s clouds, and investigating the processes of formation of organic compounds in Titan’s atmosphere, Saturn's largest moon.
Sagan didn’t just study the possibility of extraterrestrial life—he was also among the first to propose practical methods for communicating with potential alien civilizations. He suggested adding a small metal plaque to the Pioneer spacecraft, featuring a brief description of humanity, illustrations of humans, and a cosmic map indicating Earth’s location. Designed to remain in space long after the spacecraft completed their missions, these plaques were intended to be discovered by intelligent extraterrestrial beings. The artwork for the plaque was created by Sagan’s second wife, artist Linda Salzman, whom he married in 1968. Together, they had one child.
During this period, Sagan emerged as an increasingly prominent public figure and a science ambassador in popular media. He achieved this partly through his popular science books. One of his most successful works was The Cosmic Connection, published in 1973, in which he extensively explored the possibility of extraterrestrial life. At the same time, Sagan did not shy away from addressing popular claims made by alien enthusiasts, who asserted that intelligent extraterrestrials frequently visit Earth and that governments are concealing the evidence. Sagan strongly advocated for a scientific approach to evaluating such reports.
The success of The Cosmic Connection led to Sagan becoming a regular guest on The Tonight Show, where he often showcased new images from space missions he was involved in. Despite his busy schedule, he always made time to fly to California for interviews with Johnny Carson, whom he referred to as hosting "the largest classroom in history."
Sagan’s eloquent media appearances turned him into a true celebrity. His work and image graced the covers of major magazines like Time and Newsweek. His public recognition soared to new heights in 1978 when his next bestseller, The Dragons of Eden: Speculations on the Evolution of Human Intelligence, won the Pulitzer Prize for General Non-Fiction.
The cosmos, television, and everything
By the late 1970s, following the successful Viking landings on Mars and with the Voyager spacecraft en route to Jupiter, Carl Sagan set out to use television to bring science to the broadest audience possible. He established his own production company, raised approximately a million dollars from investors, and began developing a script for a documentary series about the universe and humanity’s understanding of it.
The series, Cosmos, consisted of 13 hour-long episodes and aired in 1980 on the public television channel KCET. It was an enormous success, winning Emmy and Peabody Awards and reaching over 400 million viewers worldwide. The companion book, also titled Cosmos, remained on The New York Times bestseller list for more than a year. The success of the series and book officially cemented Sagan’s celebrity status, earned him widespread media attention and brought him significant financial success.
This success came at a cost. While working on the series, Sagan neglected his academic responsibilities, canceled courses, and left research students - who had chosen Cornell specifically to work under his guidance - without supervision, forcing them to seek other advisors. This behavior drew significant criticism from colleagues, which escalated as fame brought its drawbacks, including harassing phone calls, death threats, and an influx of fans or eccentrics roaming the university grounds in search of him.
While working on the production of Cosmos, Sagan fell deeply in love with Ann Druyan, who collaborated with him on its production and writing. Although he was still married at the time, Sagan moved to Los Angeles to live with Druyan during the series' production. In 1981, he divorced his second wife and married Druyan. The couple later had a daughter and a son. They continued to collaborate professionally, including co-authoring several books.
Following the success of Cosmos, Sagan received a $2 million advance from the major publisher Simon & Schuster to write a novel about humanity’s first contact with extraterrestrial life. The resulting book, Contact, published in 1985, was another major success and was later adapted into a Hollywood film starring Jodie Foster in 1997.
A pale blue dot
Carl Sagan’s scientific work also explored the possibility of contact with extraterrestrial life. The Voyager spacecraft, launched in 1977, carried a more elaborate message about humanity than the Pioneer plaques. Attached to Voyager was a gold-plated copper record—a kind of "time capsule" of life on Earth. The record contained natural sounds such as ocean waves, music from various periods and cultures, voice greetings in dozens of languages, and images of life on Earth, all stored in audio form. Instructions for playing the record were etched onto its cover, based on the assumption that any extraterrestrial beings who might find it would have some ability to perceive and interpret visuals and sounds in ways similar to humans. Sagan chaired the committee that selected the content, with its artistic direction led by his new partner, Ann Druyan. The record also included an audio representation of Druyan's brainwaves.
Sagan was also a strong supporter of SETI (Search for Extraterrestrial Intelligence), an initiative dedicated to scanning space for radio signals that might indicate the presence of intelligent alien civilizations. He collaborated with his colleague, astronomer Frank Drake, to design the Arecibo message—a radio signal broadcast into space with the aim of reaching extraterrestrial beings and drawing their attention to humanity's existence
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Cosmic Love. Sagan with his third wife, Ann Druyan, who collaborated with him on the Cosmos series and co-authored several of his books
(Photo: Peter Morenus/Cornell University, Courtesy of Druyan-Sagan Associates, Inc.)
In 1990, after Voyager 1 completed its primary scientific mission in the solar system and began its journey outward into interstellar space, Sagan’s persistent efforts convinced NASA leadership to capture a distant photograph of Earth. The resulting image, taken from about six billion kilometers away, showed Earth as a tiny speck—less than a pixel—against the vast emptiness of space. This iconic image was named Pale Blue Dot by Sagan, a title that also became the theme of his 1994 book Pale Blue Dot: A Vision of the Human Future in Space, in which he reflected on humanity’s place in the cosmos and, above all, our smallness.
“Think of the rivers of blood spilled by all those generals and emperors so that in glory and triumph they could become the momentary masters of a fraction of a dot,” Sagan said in a lecture at Cornell University.
Years of disappointments
In the 1980s, under U.S. President Ronald Reagan, the government increased defense spending and promoted the "Star Wars" initiative, a program aimed at deploying defense systems in space. This, along with the Soviet Union’s response, rekindled fears of nuclear war between the superpowers. Sagan, an opponent of nuclear weapons since his student days, co-authored an influential scientific paper with four colleagues on the potential effects of nuclear war. In this paper, they coined the term "nuclear winter," referring to climate changes caused by massive amounts of dust rising into the atmosphere following large-scale nuclear explosions.
Sagan went on to co-author two books on the dangers of nuclear weapons and frequently discussed the topic in media interviews. However, unlike his work on space exploration, his stance on the politically sensitive issue of nuclear weapons drew significant opposition. Critics included those who viewed disarmament as a threat to U.S. security, as well as scientists who disputed the nuclear winter theory, arguing that it was an overly pessimistic and insufficiently substantiated prediction. This opposition sometimes manifested in personal attacks on Sagan in the media, a stark contrast to the favorable coverage he had previously enjoyed.
Many believe that Sagan's advocacy and his efforts to highlight the dangers of war through the concept of nuclear winter ultimately helped pave the way for the disarmament negotiations and the eventual signing of the Intermediate-Range Nuclear Forces (INF) Treaty by U.S. and Soviet leaders in 1987.
During this period, Sagan also faced challenges from budget cuts to U.S. space exploration, including NASA missions. The Challenger space shuttle disaster in 1986 marked a low point for the agency and disrupted research-oriented missions. Sagan saw this as an opportunity to promote U.S.-Soviet collaboration on a joint mission to Mars, leveraging the Soviet Union's growing openness to the West. However, these efforts were unsuccessful, and Sagan was forced to abandon the idea after the Soviet Union’s collapse, which left its space program in a state of limbo due to the political upheaval.
In 1990, amid stalled space initiatives, Sagan made his most significant scientific misstep. Following Iraq’s invasion of Kuwait and its threats to ignite oil wells if the West intervened, Sagan feared that such widespread fires could release enough soot to trigger a smaller-scale version of nuclear winter. Despite opposition from three of his four co-authors of the original nuclear winter paper—who did not believe the fires would have a global climatic impact—Sagan published his predictions in articles and TV interviews.
In early 1991, Iraqi forces did set Kuwait’s oil wells ablaze, but when the smoke cleared, it became evident that Sagan had been wrong. The fires did not cause global or even local climate changes. This miscalculation drew widespread criticism, and Sagan faced ridicule from his opponents, some of whom used the incident to cast doubt on the entire nuclear winter theory.
In 1992, Sagan was nominated for membership in the National Academy of Sciences (NAS). Despite meeting the accepted criteria for scientific publications, contributions to the scientific community, and his efforts to make science accessible to the public, his nomination received only about half the votes—well below the two-thirds majority required. Two years later, the Academy awarded him its prestigious Public Welfare Medal in recognition of his contributions to popularizing science and explaining complex issues. However, it is unclear whether this honor softened the sting of his earlier rejection.
Adding to his disappointment, the 1993 book Shadows of Forgotten Ancestors, co-authored with his wife Ann Druyan, did not achieve the success they had hoped for, despite being regarded by some critics as his best work.
Billions and billions
In 1994, Carl Sagan was diagnosed with cancer and began undergoing treatment. Despite his illness—or perhaps because of it—he did not slow down and continued working on new books. Foremost among them was The Demon-Haunted World: Science as a Candle in the Dark. This book was a passionate defense of science, the scientific method, and rational thinking, countering irrationality and pseudoscience.
In the book, Sagan revisits his field of expertise, extraterrestrial communication, and critiques unfounded claims about alien visits to Earth and alleged abductions by extraterrestrial spacecraft. He also equips readers with a "baloney detection kit," a list of logical fallacies typical of pseudoscientific arguments. Additionally, Sagan used the book to publicly acknowledge his earlier error in predicting the catastrophic environmental effects from the burning of oil wells in Kuwait.
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Another book Sagan managed to work on with Druyan was Billions and Billions: Thoughts on Life and Death at the Brink of the Millennium. The title, evoking the vast number of stars in the universe, became synonymous with Sagan since the days of Cosmos. In the book, Sagan and Druyan discuss the application of scientific tools to everyday life and global issues such as climate change and international relations. Sagan also shares his religious perspective, expressing skepticism about the existence of God in the traditional sense, though he refrains from identifying himself as an atheist.
Sagan did not live to see the publication of this final book. After undergoing three bone marrow transplants, his condition deteriorated, and he passed away from pneumonia on December 20, 1996, at the age of 62.
“Carl Sagan, more than any contemporary scientist I can think of, knew what it takes to stir passion within the public when it comes to the wonder and importance of science,” eulogized Bruce Alberts, president of the National Academy of Sciences. “American astronomy lost its clearest and most colorful voice with the death of Carl Sagan,” wrote astronomer Yervant Terzian of Cornell University. Quoting a Washington Post obituary by Joel Aschenbach that stated, “We have needed Sagan since Copernicus removed us from the center of the Universe,” Terzian added, “And the need remains. Carl Sagan received a great many awards during his 62 years, but ‘billions and billions’ would not have been enough to repay what the scientific community owes him.”