One of the first scientists to take an active interest in the possibility that life exists elsewhere in the universe, and an astronomer was both a best-selling author and a popular television figure, Carl Sagan was one of the best-known scientists in the world. He made important contributions to studies of Venus and Mars, and he was extensively involved in planning NASA's Mariner missions. Regular appearances on the Tonight Show with Johnny Carson began a television career which culminated in the series Sagan hosted on public television called Cosmos, seen in sixty countries by over 400,000,000 people. He is also one of the authors of a paper that predicted drastic global cooling after a nuclear war; the concept of "nuclear winter" affected not only the scientific community but also national and international policy, as well as public opinion about nuclear weapons and the arms race. Although some scientists considered...