As an astronomer, he made important discoveries about Venus and Mars, and played an important role with NASA for several decades, helping design the missions of unmanned spacecraft that explored the solar system, and even briefing the Apollo astronauts before their flights to the Moon. As a professor at Harvard and Cornell, he was a respected educator who inspired generations of future space scientists—among them, astrophysicist Neil deGrasse Tyson—to follow in his footsteps. As a writer, Sagan was prolific, producing more than 600 scientific articles and pieces for mainstream newspapers and magazines, and won the Pulitzer Prize in 1978 for The Dragons of Eden: Speculations on the Evolution of Human Intelligence, one of his more than a dozen books. As a TV personality, Sagan— whom one critic described as “a scientific Robert Redford”—hosted the Emmy Award-winning 1980 series Cosmos: A Personal Voyage, which became the most-watched science program of all time, attracting a worldwide audience of 500 million viewers in 60 countries.
If that wasn’t enough, Sagan was one of the first public voices to call attention to the threat of climate change, and he also campaigned for nuclear disarmament, swaying the public with his warnings of a “nuclear winter” that might ensue after an all-out war.
But if Sagan’s life and multiple careers had a single, overriding theme, it was getting ordinary people to understand the importance and value of science, and to share his own wide-ranging, irrepressible curiosity about the universe. As his former department head at Cornell, Yervant Terzian, once said of Sagan, “Carl was the best teacher of science in the world.” In the introduction to his companion book to the Cosmos series, Sagan noted that in ancient times, people had felt an intimate connection to the stars, to the extent that they recited incantations in an effort to summon the cosmos’ power to cure their toothaches. But he worried that science’s discovery of the immense scale and mind-boggling age of the universe had left modern humans feeling unimportant and alienated. He argued, instead, that grasping for an understanding of that “ecstatic grandeur” was a way to reaffirm that “we are, in a very real and profound sense, a part of the Cosmos, born from it, our fate deeply connected to it.”
Sagan felt that connection to the universe going back to his childhood in Bensonhurst, a working-class neighborhood in Brooklyn, where he was an avid reader of science fiction and decided by the age of eight that there must be life on planets orbiting other stars, even though they had not yet been discovered. ''I didn't make a decision to pursue astronomy,'' he once explained. ''Rather, it just grabbed me, and I had no thought of escaping.”
His curiosity eventually led him to the University of Chicago, where he enrolled at age 16 and eventually earned a doctorate in astrophysics at age 26 in 1960. After a fellowship at the University of California-Berkeley, he became an assistant professor at Harvard University, and moved to Cornell University in 1968, where he became a full professor in 1971.
Early in his career, Sagan made a discovery that made his name in the world of astronomy, by ingeniously using data from tables designed for steam boiler engineering to prove that Venus was heated by the greenhouse effect, in which its atmosphere trapped heat and elevated the planet’s surface temperature. Sagan did make one mistake, by inferring that in addition to carbon dioxide, water vapor played a role; later researchers determined that there was little water in Venus’ atmosphere, and that sulfur actually was the culprit. Nevertheless, Sagan’s work provided another piece to the puzzle that eventually enabled climate scientists to develop a model explaining how and why the Earth was warming. Sagan also contributed an important explanation of color variations on the surface of Mars, arguing that they were caused by shifts in dust from wind storms, rather than by vegetation, as some scientists speculated. Observations by NASA’s Mariner spacecraft in the 1970s eventually confirmed that Sagan and his colleague James Pollack had been right.
Sagan’s expertise led him to become a key participant in NASA’s 1970s missions to explore Mars and other planets with robotic probes. He was a member of the team that managed the imaging by Mariner 9, which became the first satellite to orbit another planet and transmitted more than 7,000 photos of the Martian surface. He also helped to select the landing sites for Viking 1 and 2, which were the first space probes to land successfully on Mars, and worked with NASA on the Pioneer 10 and 11 missions, which were the first Earth spacecraft to reach the outer planets such as Jupiter and Saturn.
Sagan, who had a lifelong fascination with the possibility of extraterrestrial life, also conceived one of NASA’s most attention-getting gambits. When he worked on the effort to send the Voyager 1 and 2 probes on a tour of the solar system and beyond, he convinced NASA to put a message inside the spacecraft—a 12 inch copper disc encoded with greetings in various Earth languages, natural sounds, music and photographs. Sagan hoped that if extraterrestrials someday found the spacecraft, it would help alert them to the existence of our civilization. He called it “a bottle cast into the cosmic ocean.”
But thanks to his best-selling books and TV appearances, most people probably remember Sagan as a lucid, witty explainer of scientific concepts. Though he sometimes turned down lecture requests, he appeared on the “Tonight” show with Johnny Carson 26 times, and was unfazed by the challenge of being interviewed by a comedian who sometimes impersonated him. “The show has an audience of 10 million people,” he told a New York Times interviewer in 1977. “Those aren’t people who subscribe to Scientific American.”
All the same, Sagan generally wasn’t thrilled with the portrayal of science on TV—a discontent that he got to remedy when he signed on in 1979 to develop and host Cosmos for PBS. Instead of a dull science lecture, Sagan envisioned a program that would make the fullest use of television’s visual possibilities, including special effects and computer animation, and send viewers hurtling on a spaceship between cosmic destinations, when they weren’t contemplating a “cosmic calendar” that compressed the history of the universe into the equivalent of a single Earth year. As he said at the time, his goal was to make it so that “people could turn the sound off and still enjoy the series.” The production cost a then-hefty $8 million, making it the most expensive program ever created for public television.
But in the end, Sagan’s flamboyance and willingness to take risks paid off handsomely, as Cosmos became both a critical success a massive international hit. That success demonstrated that audiences would watch science, if it was presented in an entertaining fashion, and helped pave the way for generations of other science programming.
Sagan’s passion for science and desire for knowledge continued to burn brightly, even when he was ill with the cancer that ultimately would cut his life short in 1996. Just two weeks before his death, he went to Washington one afternoon to see then-NASA administrator Daniel S. Goldin, to describe his visions for the future of space exploration.
As Goldin later recalled in a New York Times interview, the conversation grew so animated and intense that the two men continued it over dinner at a Georgetown restaurant. Despite Sagan’s physical frailty, Goldin recalled, “He was talking with intensity,” like a man with worlds still left to explore. “This is the Carl Sagan I love, a man so full of hope and optimism that he never gave up.”