Science is often thought of as a set of facts and right answers. This misconception can get in the way of understanding science as both a tradition and craft. Carl Sagan learned the craft and became a part of the tradition of science through his relationships with mentors.  As a student and assistant, Sagan learned how to analyze and critique scientific arguments, how to setup, run and interpret experiments and the methods for observing the stars. Gerald Kuiper, Harold Urey, and Hermann Muller were three of Sagan's primary mentors. Exploring the perspectives and scientific work of each of these mentors offers insight into the development of planetary astronomy and the origins of exobiology, and underscores the value of understanding science as a personal endeavor. It also provides a way to set Sagan's work and ideas in a broader historical context. He was a child of the 1940s and a college student of the 1950s. Through his mentors, we can trace many of the ideas and values in Sagan's science back to the turn of the 20th century.

Muller: Sci-fi, Genetics, & Socialism

Hermann Joseph Muller, born in 1890, was a geneticist.  As a Nobel laureate he is most known for his work on radiation and genetic mutations. Brought up in Harlem, he went to Morris High School in the Bronx. He received his Ph.D. in 1916.  Muller went on to make a series of contributions to the study of radiation, X-rays and mutation in the 1920s and 30s.

Muller was a critic of capitalism and a supporter of socialism. In the 1930s, he ran a genetics lab in the Soviet Union, until the rise of Lysenkoism, an alternate evolutionary theory that claimed organisms pass on inherited characteristics, made it increasingly difficult for him to do genetics research in the Soviet Union. Muller was forced to leave the USSR in 1935 when Stalin ordered an attack on a book Muller wrote on eugenics.

In 1946, Muller found his way to a position as a Zoology professor at Indiana University. He even won a Nobel Prize for "the discovery that mutations can be induced by x-rays." It was in Indiana where met the young Carl Sagan.

In 1952, Carl Sagan sent some of his thoughts on the origin of life to a cousin, Seymour Abrahamson. Abrahamson was, at the time, in graduate studies at the Indiana University.  Impressed by Sagan's ideas, Abrahamson showed the letter to Muller who invited Sagan to visit and discuss his ideas. Muller asked Sagan to come work in the lab for the summer and Sagan was thrilled to accept.

Sagan and Muller had many things in common. As mutual science fiction fans, they once went to a sci-fi convention in Chicago together. Muller's political activism work to reduce the possibilities of nuclear war in the 1950s was likely formative in Sagan's later anti-nuclear activism. Sagan thought so highly of Muller that when he thought of developing a book concept, "Profiles in Scientific Courage", he included Muller in a list of potential subjects that included; Charles Darwin, Johannes Kepler, Albert Einstein, Linus Pauling and Hypatia.

In a 1980 letter to Abrahamson, nearly thirty years after Abrahamson had introduced Sagan to Muller, Sagan reflected on the important role Muller played in his development as a scientist. Sagan noted that Muller "encouraged my interest in the origin of life and exobiology, and introduced me to Harold Urey, wrote recommendations for my NSF Fellowships, taught me some genetics, and communicated two of my earliest papers for publication."  Muller's mentorship helped bring Carl Sagan into the tradition of science, not only a tradition of facts, but also one engaged in the political and social issues of its times.  

Urey: Cosmochemistry, Origins of Life and Planets

Harold Clayton Urey, born 1893 in Walkerton Indiana, played a key role in the development of chemistry in the beginning of the 20th Century. In 1934, he won a Nobel Prize for his discovery of the element Deuterium. Subsequently, he contributed to the development of the Atomic bomb with the Manhattan Project, and after WWII he became an outspoken critic of American nuclear policy. In the 1940s, he turned his attention away from the Earth and coined the term cosmochemistry. In his opinion, it was time for chemists to turn their attention to studying objects in the heavens.

In 1953, Carl Sagan, and the other members of the "theoretical section" of the University of Chicago Student Astronomy Club were reading Urey's The Planets: Their Origin and Development (1952). In this book, Urey suggested that the Earth's early atmosphere was likely made up of ammonia, methane, and hydrogen. In the same year, Stanley Miller, one of Urey's graduate students, conducted an experiment to simulate the atmospheric conditions of early Earth. The goal of the experiment was to see if the atmospheric conditions might result in the synthesis of organic compounds; often referred to as the building blocks of life. The experiment became key evidence for the chemical basis of the origin of life, and to Sagan who heard Miller present his results at a University of Chicago seminar, the experiment suggested that life was likely abundant throughout the universe.

Excited by the ideas and nature of Urey's work, Sagan took an undergraduate honors class with him. For the class Sagan wrote an honors essay on the origin of life, which Urey told him showed signs of a naive youth. As a mentor, Urey was hard to please and he challenged Sagan to become more rigorous in his writing and thinking.

Kuiper: Observing Mars and the Back of the Envelope

Gerard Kuiper, born in the Netherlands in 1905, made a series of significant contributions to understanding today's solar system. He discovered that Saturn's moon, Titan, had an atmosphere, which was the first moon known to have a significant atmosphere. In the 1950s he wrote about the possibility of a disk of small icy bodies beyond the orbit of the planet Neptune, which is now generally referred to as the Kuiper belt.  In WWII, Kuiper served in the Alsos Mission, part of the Manhattan Project, which investigated the German atomic bomb project. He also worked with the U.S. military to rescue or capture German scientists.

In 1956, Carl Sagan was accepted into the Doctoral Astronomy Program at the University of Chicago which was based at the Yerkes Observatory in Williams Bay, Wisconsin. At the time, Gerard Kuiper was the director of the observatory. Kuiper invited Sagan to join him in observing Mars from McDonald Observatory in Fort Davis, Texas during the summer of 1956. Sagan had planned to take a trip to Europe over the summer, but as he explained in this draft of a Letter to Kuiper from April of 1956, "Europe will always be as far away from this country as it is now, which is not the case for Mars." In the summer of 1956 Earth and Mars were at points in their respective orbits around the sun where they are closest to each other. This offers the ideal circumstance for observation. The weather in both Texas and on Mars did not cooperate and kept them from making useful observations.

As Sagan recalled later in his life, Kuiper introduced him to "back-of-the-envelope calculation," which he described as "A possible explanation to a problem occurs to you, you pull out an old envelope, appeal to your knowledge of fundamental physics, scribble a few approximate equations on the envelope, substitute in likely numerical values, and see if your answer comes anywhere near explaining your problem. If not, you look for a different explanation." For him, this technique was invaluable as it "cut through nonsense like a knife through butter." Kuiper would go on to be Carl Sagan's dissertation advisor and a key advocate for Sagan's work as he made his way to a Ph.D. in Astronomy and Astrophysics in 1960.