Before his death in 1996, Carl Sagan was likely the most famous living scientist.

A pioneer in space exploration and in the search for extraterrestrial intelligence, he spent his life probing ideas that aroused the public imagination.

Who Is Carl Sagan?

It’s no surprise that Sagan’s greatest feats occurred not in secluded laboratories, but on the bright, world-wide stage of late-night talk shows, award-winning books and TV programs viewed by hundreds of millions.

His scholarly accomplishments — more than 600 scientific papers and 22 honorary degrees — are nothing to scoff at.

But perhaps more importantly, he served as a bridge between science and ordinary people, sparking curiosity about alien life, the past and future of life on Earth, and the myriad marvels of the universe.

Carl Sagan and the Cosmos

In Carl Sagan: A Life in the Cosmos, a colleague of the astronomer is said to have remarked that Sagan “desperately wants to find life someplace, anyplace — on Mars, on Titan, in the solar system or outside of it. In all the divergent things he does, that is the unifying thread.”

Beginning in the early 1960s, Sagan demonstrated that crucial organic molecules like adenosine triphosphate and amino acids (the building blocks of life) can form when basic chemicals are exposed to ultraviolet radiation. His findings hinted at a plausible starting point for ancestral Earthlings.

Around the same time, he began investigating the atmospheric conditions of other planets.

He correctly predicted that Venus’ high temperatures are the result of greenhouse gases, that Titan’s red haze comes from organic molecules, and that the changing color of Mars’ surface is due to shifts in windblown dust. All three hypotheses were confirmed by later exploration.

Sagan At NASA

Sagan also had a hand in many of NASA’s major projects, from the agency’s inception in 1958 onward — even briefing the Apollo astronauts before their moon mission. Additionally, he crafted the first messages sent into space, including the Voyager Golden Record launched in 1977.

“Sagan’s influence on the space program’s planning and funding was unequaled,” writes the biographer William Poundstone in Carl Sagan: A Life in the Cosmos.

Another of Sagan’s masterstrokes? The “pale blue dot” image, taken at his insistence by Voyager 1 as it hurtled toward interstellar space. The photograph showed our planet — as an all but invisible speck against an endless cosmic backdrop — from a radically new vantage point.

“With that picture,” writes physicist Freeman Dyson, “Carl made clear to all mankind the smallness of the squabbles that currently divide us and the greatness of the destiny that may one day unite us.”

Read More: 4 Things We Have Thanks To Carl Sagan

Protecting Our Planet

The pale blue dot revealed not only our interconnectedness but also our fragility; seen from 3.7 billion miles away, its inhabitants didn’t seem so invincible.

Sagan was keenly aware of the growing environmental dangers posed by human activity, and the burning of fossil fuels in particular. An early believer in global warming, he testified to Congress in 1985 that the same greenhouse effect he’d observed on Venus could catastrophically transform our own planet’s climate.

Of equal concern, especially in the Cold War years, was the threat of nuclear devastation.

In the early 1980s, Sagan co-authored a series of articles investigating how large-scale bombing would likely produce a “nuclear winter” — prolonged climatic cooling that could render Earth uninhabitable. Entering one of the most controversial political debates of the period, he became a prominent advocate for arms control.

Science journalist Keay Davidson argues in Carl Sagan: A Life that, as a rare and unabashed multidisciplinarian, Sagan was uniquely equipped among scientists to exercise this kind of moral judgment.

“People who are afraid of transgressing intellectual boundaries,” he writes, “are less likely to see the forest as well as the trees and, hence, to challenge societal misuses of science.”

The Next Generation

Bill Nye (the Science Guy) took Sagan’s astronomy class at Cornell University, and in the late ‘80s even sought the phenom’s advice on creating a science show for kids.

A decade before that, Sagan tried to recruit 17-year-old Neil deGrasse Tyson to the university, inviting him to spend a Saturday in Ithaca, New York. Although he enrolled elsewhere, Tyson went on to revive Sagan’s popular television series with his own 2014 release of Cosmos: A Spacetime Odyssey.

At the end of the first episode, Tyson recalls that early meeting with Sagan: “I already knew I wanted to become a scientist,” he says. “But that afternoon, I learned from Carl the kind of person I wanted to become.”

In physicist Freeman Dyson’s opinion, Sagan was a sort of scientific democratizer — making the obscure accessible for all.

“He saw the cosmic connection as an enlargement of the human spirit,” Dyson writes. “He wanted everyone on Earth, not only the scientific elite, to feel connected with the cosmos.”

What Is Carl Sagan Most Famous For?

Outside academic circles, Sagan is most famous for promoting science to the non-specialist world. Among his many best-selling books, he won a Pulitzer Prize for The Dragons of Eden.

And his 1980 TV series, Cosmos: A Personal Voyage, was American public television’s most-watched show of the decade. Across 13 episodes, he explored everything from the birth of galaxies to the nature of intelligence — explaining esoteric ideas in the simple, lucid manner of a gifted teacher.

His perspective offered a welcome reframing.

“Critics had accused science of robbing the cosmos of old enchantments — gods, angels, astrological forces,” Davidson writes. “But Sagan re-enchanted the stars in new, scientific-sounding ways purged of medieval irrationalisms.”

What’s more, he mentored and inspired a generation of prominent scientists, including some who took up the mantle of translating technical information for a lay audience.

Read More: Beyond Carl Sagan’s Cosmos: A Conversation with Ann Druyan

A Complex Character

Sagan drew countless admirers — students, colleagues and viewers — for his charisma, prodigious intellect and descriptions of a universe shimmering with wonders. But, like most pivotal figures, he drew detractors too.

More cautious peers often regarded his speculations as flippant, if not outright reckless. In an academic culture that expects its members to stick to well-defined lanes, Sagan’s free-ranging mind was sometimes seen as an affront.

His undergraduate advisor, the chemist and Nobel laureate Harold Urey, for example, urged Harvard to deny Sagan tenure due to “doubts about his sense of scientific responsibility,” as Davidson puts it in Carl Sagan: A Life.

In fact, there’s now a term for backlash against a scientist who spends too much time commenting to the media on subjects outside their narrow expertise: the Sagan effect.

Read More: The 10 Greatest Scientists of All Time