American astronomer, astrophysicist, cosmologist and author Carl Sagan was one of the most renowned, influential – and controversial – scientists of the 20th century.
Sagan lectured as an astronomy professor at Cornell, and in addition to his popular science books, is most-known for his award-winning 1980 television series Cosmos: A Personal Voyage, which he narrated and co-wrote – sparking an interest in science for millions of people worldwide. Indeed Sagan’s passion for education, and his ability to communicate technical scientific concepts to the general public in an accessible, entertaining way, lead many to describe him as the greatest populariser of science of all time.
Sagan advocated scientifically sceptical inquiry and the scientific method. Known for his divisive views on extraterrestrial intelligence, religion and nuclear weapons, Sagan also worked on NASA robotic missions and promoted the Search for Extra-Terrestrial Intelligence (SETI).
Here we explore more about Carl Sagan’s career and his impact.
Carl Sagan was born in November 1934, in Brooklyn, New York, and raised in a secular Jewish household from a relatively poor background. After showing an early interest in science and the natural world, he was encouraged by his parents to explore his curiosity, later attending the University of Chicago, where he received his undergraduate degree in physics in 1954 and a PhD in astronomy and astrophysics in 1960, aged 26.
As part of Sagan’s doctorate research, he challenged the notion that Venus was similar to Earth. He computed the first greenhouse model for Venus’ atmosphere, revealing a higher temperature than previously suspected. He also suggested that dust storms on Mars caused seasonal changes, and wrote papers on the organic chemistry of Jupiter’s atmosphere.
Initially an assistant professor at Harvard, after being denied tenure at Harvard, Sagan later moved to Cornell, where he spent the rest of his career as a Professor of Astronomy – also serving as the director for the Laboratory for Planetary Studies and the associate director of the Center for Radio Physics and Space Research – until his death.
Sagan published more than 600 scientific papers and articles and was author, co-author or editor of more than 20 books. His early research focused on the origin and evolution of the solar system, and he made important contributions to the study of planetary science and exobiology.
Sagan believed science should be accessible to everyone, which fuelled his work as a science communicator. A passionate advocate for scientific literacy and critical thinking, he frequently spoke out against pseudoscience and superstition.
Sagan was a complex character but driven to succeed – later becoming a millionaire and one of the most influential scientists of his time. However, this popularity left him open to criticism and jealousy amongst his peers, and whilst passionate about the need to educate the population, he could also be arrogant and dismissive of his fellow scientists.
Work with NASA
Carl Sagan was a member of the scientific teams for several NASA missions, including the Viking program (which sent landers to Mars in 1976), and helped brief astronauts prior to their trips to the moon.
He was also approached by NASA to help assemble the first physical messages sent into space – universal messages from mankind that could potentially be understood by any extraterrestrial intelligence that might find them.
The Pioneer plaques, a pair of gold-anodised aluminium plaques were placed on board the Pioneer 10 (1972) and Pioneer 11 (1973) spacecraft, featuring a pictorial message. The plaques show the naked figures of a human male and female along with several symbols providing information about the origin of the spacecraft.
The Voyager Golden Records were two phonograph records included aboard both Voyager spacecrafts (1977), containing sounds and 115 images portraying the diversity of life and culture on Earth.
Sounds included those made by nature (such as thunder, animals), as well as audio content representing humanity. These included spoken greetings in 55 ancient and modern languages, human sounds such as footsteps and laughter, the message ‘Per aspera ad astra’ (‘To the stars’) in Morse code, musical selections from different cultures and eras, and a printed message from US President Jimmy Carter.
At Sagan’s request, NASA commanded its Voyager 1 spacecraft to turn its camera on Earth (by then billions of miles away), creating the ‘Pale Blue Dot’ image, one of the most famous pictures of Earth from space.
In 1985, Sagan wrote a book, Contact, which explored the consequences of making a connection with a more technologically advanced, extraterrestrial life form, no doubt inspired by his experiences with NASA. It was later adapted into a feature film in 1997.
Landmark TV series, Cosmos
Sagan made 26 television appearances as an astronomer on Johnny Carson’s Tonight Show in 1978.
A year earlier, Sagan had begun work on the 13-part television series Cosmos: A Personal Voyage, which he narrated and co-wrote (along with a book of the same name). The first episode aired on the Public Broadcasting Service (PBS) in October 1980. The series was targeted to a general audience of viewers, and covered a wide range of scientific subjects including the origin of life and a perspective of humans’ place on Earth.
Between new episodes and reruns, the groundbreaking 1980 television series was the most widely watched series on American public television for nearly a decade. It won an Emmy and a Peabody Award, transforming Sagan into a pop-culture icon.
The series has been seen by at least 500 million people across 60 countries, inspiring millions to view the universe with wonder and awe.
Sagan was also a prolific writer and science communicator. He wrote many popular science books, including The Cosmic Connection (his first book and one of his most influential), and The Dragons of Eden (exploring the evolution of human intelligence).
One of his most impactful works is Pale Blue Dot: A Vision of the Human Future in Space – a sequel to Cosmos that reflects upon the significance of the famous photograph of Earth taken by the Voyager 1 spacecraft, shows the Earth as a small dot in the vast expanse of space.
(Sagan also wrote the introduction for Steven Hawking’s famous book, A Brief History of Time.)
One of Sagan’s most controversial views was his belief in the possibility of extraterrestrial intelligence. He argued that the sheer size of the universe and the number of potentially habitable planets made it likely that other intelligent civilisations existed.
Sagan’s views on religion were divisive. He was an atheist and believed that science and religion were fundamentally incompatible, arguing that science was based on evidence and reason, while religion was based on faith and dogma. Sagan was critical of religious fundamentalism and religion’s influence on public policy, particularly on issues like evolution and reproductive rights.
Sagan was also an outspoken opponent of nuclear weapons. He believed the use of nuclear weapons would be catastrophic for humanity and that nuclear disarmament was essential for the survival of our species. Sagan was a co-founder of the Union of Concerned Scientists, a group of scientists and engineers who work to promote science-based solutions to social and environmental problems.
Awards and legacy
Throughout his career, Sagan was recognised for his contributions to science and ability to break-down scientific concepts into explanations the public could understand. He received numerous awards and honours, including the NASA Distinguished Public Service Medal, the National Academy of Sciences Public Welfare Medal, and the Pulitzer Prize for The Dragons of Eden.
After fighting myelodysplasia, which included three bone marrow transplants, Sagan died of pneumonia in December 1996, aged 62.
Sagan’s enthusiasm and passion for understanding the universe continues to inspire and influence people worldwide. His work as a science communicator made science more accessible, and his advocacy for critical thinking, scientific scepticism, and nuclear disarmament remains relevant today.