Carl Sagan: Biography, Discoveries & Theory

Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

Carl Sagan spent his life working to make science accessible to all people, and he is remembered as one of the most famous scientists of the 20th century. In this lesson, learn about his life and his contributions to science.

Earth, the Pale Blue Dot

On that pale blue dot, everyone you every heard of, every human being who ever lived, lived out their lives. - Carl Sagan
the pale blue dot

In 1990, something remarkable happened. The Voyager 1 spacecraft, which had been launched in 1976, approached the edge of our solar system. It was the first man-made object to ever go that far, and even today, it is still traveling outward toward the distant stars. Before it left the solar system, Voyager 1 sent back one last image of Earth. From this great distance, Earth appears as only a small, pale, blue dot.

This remarkable image was taken at the request of an astronomer who had spent his life exploring the cosmos and played a critical role in developing the mission of Voyager 1. His name was Carl Sagan, and the Voyager 1 mission was just part of his long and successful career as an astronomer and science communicator.

Throughout his life, Sagan made great advances in our understanding of the cosmos. More than almost anyone else, before or since, he also had the ability to make science and astronomy accessible and interesting to the public. For his groundbreaking scientific discoveries and his exceptional talent for communicating science to the public, he is remembered as one of the most famous and influential scientists of the twentieth century.

What Makes Venus So Hot?

Carl Sagan was born in 1934 in Brooklyn, New York. Even as a child, he was fascinated by space. Sagan graduated from high school at the age of sixteen, then attended the University of Chicago, where he earned B.S. and M.S. degrees in physics, and a Ph.D. in astronomy and astrophysics in 1960.

While still a graduate student at the University of Chicago, Sagan would make one of his most important discoveries. A few years earlier, scientists had determined that the temperature of the planet Venus was over 600 degrees Fahrenheit. However, no one could really explain exactly why Venus was so hot. Sagan demonstrated that the atmosphere of Venus acts like an enormous greenhouse, trapping heat inside and causing the temperature of the planet to increase dramatically. We call this the greenhouse effect.

This discovery changed the way that scientists thought about planetary atmospheres. Prior to Sagan's work, scientists thought that the atmosphere of a planet was fixed and didn't change over time, but that's clearly not what was happening on Venus. Perhaps Venus had a milder climate at one time, but as carbon dioxide in the atmosphere increased, it got hotter and hotter until it turned into the hot, dry, inhospitable place it is today.

Sagan became concerned that a similar fate might await Earth as well, and he become one of the first scientists to speak out about the dangers of climate change, a topic that he would return to throughout his career.

In 1968, Sagan went to work at Cornell University, where he served as the Director of the Laboratory for Planetary Studies. Throughout his career, Sagan continued to study planetary science, and even determined many of the characteristics that planets would need to have in order for life to survive.

Work for NASA

Carl Sagan with a model of the Viking lander
carl sagan and a model of voyager

In the early 1960s, Sagan was part of a NASA team that designed the first spacecraft to ever travel to another planet, the Mariner 2 robotic probe. Mariner 2 was launched in August 1962 and passed inside the atmosphere of Venus later that year. The data collected by Mariner 2 confirmed many of Sagan's predictions about the atmosphere of Venus.

Later, Sagan was also a critical part of the team that designed the Viking 1 and Viking 2 space probes that were launched in the 1970s. These probes traveled to Mars, took many images from orbit, and landed rovers on the surface to gather even more detailed information about Mars' surface. Sagan was responsible for determining where the rovers would land and what kind of information they would collect. Even today, scientists still rely on data gathered from these missions.

When NASA was getting ready to launch the Voyager probes to explore the outer solar system and beyond in the late 1970s, they again turned to Carl Sagan. Sagan helped plan the missions and also included a disc on which greetings and music from Earth were recorded. His idea was that Voyager might one day travel far enough to be found by another civilization on some distant planet, and he wanted to reach out to whoever might be out there.

Science Outreach

Carl Sagan in 1980
portrait of Carl Sagan

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