It’s been 14 years since Carl Sagan died at the age of 62 on December 20, 1996, but his influence on scientific inquiry, discovery and understanding is still strong today. While arguably Sagan’s scientific achievements number in the “billions and billions,” we will limit ourselves to just a few. To honor the anniversary of his death, we present ten of Sagan’s most important contributions to science.
1. Popular science. Carl Sagan’s most widely known legacy is perhaps his work to make science accessible to and popular with the masses, best demonstrated by his TV show, Cosmos: A Personal Voyage. Originally broadcast in 1980, the show was – and still is – beloved for presenting complex scientific concepts in a way that made them graspable. Sagan’s friendly and approachable personality was a big part of this. Instead of lecturing viewers on scientific theories, he sat down and chatted with them about how interesting science is. His sense of wonder was always present on the show, and it was contagious.
2. Mars, the dusty planet. Sagan contributed significantly to our understanding of Mars. Mars was once thought to be covered with vegetation that changed with the seasons – leading to its varying patterns of light and dark as seen through telescopes. Sagan examined new data and determined that the changing color patterns were caused by dust blowing in the wind across different elevations. This was confirmed by later expeditions to the planet, which found it dusty and devoid of life.
Carl Sagan and his friend and colleague Bruce Murray look at a map of Mars
3. Habitable moons. Sagan was one of the first to hypothesize that water was present on Saturn’s moon Titan and Jupiter’s moon Europa. These two moons are now the source of much fascination and speculation, with many contemplating the possibility of human colonization, as well as the exciting idea that the moons might be capable of developing life independently. Though neither would currently be a very comfy place to live – both have almost unimaginably cold climates and Europa possesses potentially fatal levels of radiation – they both present possibilities.
4. Venus and the greenhouse effect. Venus was once thought to have a climate like Earth’s, only even more appealingly tropical. We now know it’s quite the opposite – hot and dry and uninhabitable. Sagan was the first to suggest that Venus’s clouds might not be an indication of a balmy climate; his study of radio emissions from Venus led him to hypothesize a surface temperature of 900° F. He later helped design and manage NASA’s Mariner expeditions to Venus, which proved that Venus is indeed uninhabitably hot. Sagan determined that while Venus may once have had water, it evaporated due to an intense greenhouse effect – and he warned of the danger of a similar path here on Earth, if global warming were allowed to careen out of control.
5. SETI. Sagan was a pioneering scientist in the Search for Extra-Terrestrial Intelligence, a series of projects undertaken in hopes of finding intelligent life elsewhere in the universe. A member of the SETI Institute’s Board of Trustees, he worked to bring attention and understanding to the search, with his characteristic blend of rational advocacy and total delight. Sagan could tell us how scientifically and culturally important it was to determine if we share the universe with other intelligent beings…and he could get us giddily excited about the possibility.
6. Debunking UFOs. Out of Sagan’s fascination with the search for intelligent life in the universe grew his frustration with the cult of UFOism. While he was confident that intelligent life is out there somewhere, he was also sure that it isn’t hanging around Earth, buzzing deserted country roads and performing probes on the populace. In this and many other areas, Sagan was a noted skeptic, always advocating the power of scientific inquiry over blind belief.
7. The Planetary Society. In 1980, Sagan founded the Planetary Society along with Bruce Murray and Louis Friedman. With its mission “To inspire and involve the world's public in space exploration through advocacy, projects, and education,” the society is today the world’s largest space interest group. Through independent work and private funding, the Planetary Society is creating its own spacecraft to test the possibilities of solar sailing. It also funds other entities in a wide variety of efforts, from research on Mars to political action.
8. Deflection Dilemma. One important field of study for Sagan and the Planetary Society was Near-Earth Objects – asteroids, meteors and other objects that could collide with Earth to devastating effect. Some have proposed the cinematic solution of firing nuclear missiles that could deflect a collision-course NEO, altering its path so it would pass harmlessly by Earth. Sagan countered this idea with the sobering thought that if we create the ability to deflect an NEO away from the Earth, we also create the ability to deflect one toward the Earth – thus harnessing destructive power beyond any of our current technology and endangering ourselves and other nations. This Deflection Dilemma is just one example of the many ways Sagan applied scientific principles to political issues, attempting to encourage sane and critical thinking in all areas. It also reflects his grave concern over weapons of mass destruction – he spoke against them many times and warned us of their dire consequences.
9. Writings. Carl Sagan was the author or co-author of 20 publications, using his friendly and approachable writing style to bring science down to Earth for those of us without advanced degrees in astrophysics. From his first – Planets, a contribution to a series of Time Life books – to his final two works, brilliantly penned while he was undergoing painful and stressful treatment for the myelodysplasia that would take his life, Sagan sought to share his hunger for knowledge with his readers. He even wrote a novel, Contact, which was made into a well-received and award-winning film, exploring Sagan’s idea of how our first experience with extraterrestrial intelligence might play out.
10. Wonder. Through all of it, his enormous scientific achievements and his popular public appearances, Sagan never lost the thing that made him so notable and so beloved – his sense of wonder. He wasn’t just a scientist because he was brilliant and knew how to do the work; he was also a scientist because he thought science was so neat. His excitement came across in his speeches and TV appearances, his publications and discoveries and hypotheses, and in his lifelong enthusiasm for science. And, always, in his two primary goals – to advance scientific knowledge, and to bring that knowledge to the people.