Carl Sagan is one of the greatest humans to walk the surface of our planet, after 2 people who's last name starts with a B (Beethoven, Bach, and the others). He boosted the interest in science through his famous show, The Cosmos. Perhaps, no show ever was seen by such an immense volume of people. With 500 million viewers, the aim of his show was to make science digestible rather than overwhelming. For me, this is certainly his greatest achievement, promoting science.

Other than that, Sagan was a leading astrophysicist. He was known for his work on many things, especially Drake's equation. The equation is about measuring the possibility of extraterrestrial intelligence.

In addition, Sagan is an extremely powerful writer. He is a winner of the Pulitzer Prize for his non-fictional work. He is an author of more than 30 books. Many of his books were best sellers.

I hope that this aids as a start.