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Plus, he inspired generations of future scientists.
Most people remember him as the co-creator and host of the 1980 Cosmos television series, watched worldwide by hundreds of millions of people. Others read Contact, his best-selling science fiction novel, or The Dragons of Eden, his Pulitzer Prize-winning nonfiction book. Millions more saw him popularize astronomy on The Tonight Show.
What most people don't know about Sagan, and what has been somewhat obscured by his fame, is the far-reaching impact of his science, which resonates to this day. Sagan was an unequaled science communicator, astute advocate and prolific writer. But he was also an outstanding scientist.
Sagan propelled science forward in at least three important ways. He produced notable results and insights described in over 600 scientific papers. He enabled new scientific disciplines to flourish. And he inspired multiple generations of scientists. As a planetary astronomer, I believe such a combination of talents and accomplishments is rare and may occur only once in my lifetime.
We knew very little about Venus in the 1960s. Sagan investigated how the greenhouse effect in its carbon dioxide atmosphere might explain the unbearably high temperature on Venus. The planet is approximately 870 degrees Fahrenheit (465 C). His research remains a cautionary tale about the dangers of fossil fuel emissions here on Earth.
Sagan proposed a compelling explanation for seasonal changes in the brightness of Mars. Some scientists incorrectly attributed the changes to vegetation or volcanic activity. But wind-blown dust was responsible for the mysterious variations, he explained.
Sagan and his students studied how changes to the reflectivity of Earth's surface and atmosphere affect our climate. They considered how the detonation of nuclear bombs could inject so much soot into the atmosphere that it would lead to a yearslong period of substantial cooling. You've heard of this phenomenon as nuclear winter.
With unusual breadth in astronomy, physics, chemistry and biology, Sagan pushed forward the nascent discipline of astrobiology, the study of life in the universe. Sagan and research scientist Bishun Khare at Cornell University conducted pioneering laboratory experiments. They showed that certain ingredients of prebiotic chemistry, called tholins, and certain building blocks of life, known as amino acids, form naturally in laboratory environments that mimic planetary settings.
He also modeled the delivery of prebiotic molecules to the early Earth by asteroids and comets. And he was deeply engaged in the biological experiments onboard the Mars Viking landers. Sagan also speculated about the possibility of balloon-shaped organisms floating in the atmospheres of Venus and Jupiter.
His passion for finding life elsewhere extended far beyond the solar system. He was a champion of the search for extraterrestrial intelligence, also known as SETI. Sagan helped fund and participated in a systematic search for extraterrestrial radio beacons by scanning 70% of the sky with the physicist and electrical engineer Paul Horowitz.
He proposed and co-designed the plaques and the Golden Records now affixed to humanity's most distant ambassadors, the Pioneer and Voyager spacecrafts. It is unlikely that extraterrestrials will ever find these artifacts, but Sagan wanted people to contemplate the possibility of communication with other civilizations.
Sagan's scientific output repeatedly led him to become an eloquent advocate on issues of societal and scientific significance. He testified before Congress about the dangers of climate change. He was an antinuclear activist and spoke out against the Strategic Defense Initiative, also known as "Star Wars." He urged collaborations and a joint space mission with the Soviet Union, in an attempt to improve U.S.-Soviet relations. He spoke directly with members of Congress about the search for extraterrestrial intelligence and organized a petition signed by dozens of prominent scientists urging support for the search.
But perhaps his most important gift to society was his promotion of truth-seeking and critical thinking. He encouraged people to muster the humility and discipline to confront their most cherished beliefs ... and to rely on evidence to obtain a more accurate view of the world. His most cited book, The Demon-Haunted World: Science as a Candle in the Dark, is a precious resource for anyone trying to navigate this age of disinformation.
A scientist's impact can sometimes be gauged by the number of times their scholarly work is cited by other scientists. According to Sagan's Google Scholar page, his work continues to accumulate more than 1,000 citations per year.
Indeed, his current citation rate exceeds that of many members of the National Academy of Sciences, who are "elected by their peers for outstanding contributions to research," according to the academy's website. It's also "one of the highest honors a scientist can receive."
Sagan was nominated for election into the academy during the 1991-1992 cycle, but his nomination was challenged at the annual meeting. More than 1/3 of the members voted to keep him out, which doomed his admission. An observer at that meeting wrote to Sagan, "It is the worst of human frailties that keeps you out: jealousy." This belief was affirmed by others in attendance. In my opinion, the academy's failure to admit Sagan remains an enduring stain on the organization.
No amount of jealousy can diminish Sagan's profound and wide-ranging legacy. In addition to his scientific accomplishments, Sagan has inspired generations of scientists and brought an appreciation of science to countless nonscientists. He has demonstrated what is possible in the realms of science, communication and advocacy. Those accomplishments required truth-seeking, hard work and self-improvement. On the 90th anniversary of Sagan's birth, a renewed commitment to these values would honor his memory.
Bottom line: Carl Sagan would have turned 90 on November 9, 2024. Here's a look at his accomplishments and the contributions he made to the world of astronomy and science.