Carl Sagan had already decided to be an astronomer by the age of 13. Having told his grandfather of his choice of career, the response was "Yes, yes, but how will you make your living?" Sagan once said that one of the greatest moments of his life was when he was told by his high school biology teacher that there were people who were actually paid to do astronomy.
Suitably inspired he went on to obtain his PhD from the University of Chicago in 1960 and taught at the University of California at Berkeley, Harvard and the Smithsonian Astrophysical Observatory before taking a position at Cornell in 1968.
A recurring theme in his research was the origin of life. Following on from the laboratory experiments of Stanley Miller and Harold Urey, Sagan and his colleagues showed that it was relatively easy to produce amino acids, the building blocks of life, by exposing mixtures of methane, ammonia, water and hydrogen sulphide to long-wavelength ultraviolet light.
This work continued under Sagan's directorship of the Laboratory of Planetary Studies at Cornell. He also worked on studies of the surface of Mars and Venus, and he was an active participant in the highly successful Mariner, Viking and Voyager missions which sent robot spacecraft to explore the planets. In 1976 Sagan was appointed David Duncan Professor of Astronomy and Space Sciences, and from 1968 to 1979 he was Editor-in-Chief of Icarus, the International Journal of Solar System Studies. Sagan remained at Cornell for the rest of his life and his presence there enhanced the university's reputation as one of the leading centres for planetary research in the world.
A vociferous proponent of the Search for Extraterrestrial Intelligence (Seti), an experiment originally funded by Nasa to locate radio signals from other civilisations in outer space, Sagan investigated the practicalities of conveying information about ourselves to other intelligent beings. In 1977, he became the driving force behind a project to prepare a disk of recorded material for attachment to each of the two Voyager spacecraft due to reach interstellar space following their exploration of the outer solar system.
The project, detailed in the book Murmurs of Earth (1978), provided an opportunity to convey the essence of our own civilisation to others. As well as 118 photographs and greetings in almost 60 languages, the disk contained 90 minutes of music selected from around the world encompassing many cultures and traditions ("I would send the complete works of Johann Sebastian Bach," remarked a biologist, "but that would be boasting".)
In 1978 Sagan won a Pulitzer Prize for his book The Dragons of Eden: speculations on the evolution of human intelligence. Other books include Broca's Brain (1979), Comet (1986) and Shadows of Forgotten Ancestors (1993), these last two with his third wife, Ann Druyan, as co-author. He also wrote a novel, Contact (1985). However, to the general public he will be best-remembered for Cosmos (1980), an ambitious book and 13- part television series on astronomy which highlighted the efforts of those people throughout history who have tried to make sense of the universe. It is estimated that the series has been seen by almost 10 per cent of the world's population and it justifiably earned Sagan many accolades and awards.
Sagan made frequent television appearances and with his slow, deliberate voice of reason, he became an articulate spokesman for astronomy and science, often espousing the cause of rational scientific argument and debate in the face of a tide of pseudo-science and superstition. He returned to this theme in his final book, Demon Haunted World (1996). He believed that all scientists should be capable of describing their work in terms that were understandable to the general public and furthermore that they had a duty to do so.
Ironically, something he had in common with many professional scientists who undertake to make complex concepts more accessible to the public, his ability to popularise his own subject led some astronomers to doubt his credentials as a working scientist. However, even though the publicity resulting from Cosmos made it difficult for him to return to his normal duties at Cornell, he continued to produce important contributions to planetary science on a wide range of subjects.
One of his most influential papers (with Richard Turco, Owen Toon, Thomas Ackerman and James Pollack - the Ttaps group) introduced the concept of nuclear winter, whereby the fires resulting from a thermonuclear war would inject fine smoke particles into the Earth's atmosphere, trigger a global ice age and cause the collapse of agriculture; there could be no winners of such a war. With supporting evidence from the dust forms on Mars and the subsequent verification of the concept using computer models, Sagan speculated that perhaps the prospect of nuclear winter had played a constructive role in convincing nations of the futility of nuclear war.
Sagan was fond of pointing out that we are living at a unique point in our history when, in the course of a few generations, humans have taken the first steps beyond their home planet to explore the solar system. He delighted in the fact that he was alive at this exciting time and he drew parallels with a previous age of discovery in the 15th and 16th centuries. The title of his penultimate book, Pale Blue Dot (1994), refers to the insignificant appearance of Earth as viewed from deep interplanetary space; in it he looked forward to an era when future generations will have escaped self-destruction and natural catastrophes to move beyond the confines of Earth. He finished the book with these words:
They will gaze up and strain to find the blue dot in their skies. They will love it no less for its obscurity and fragility. They will marvel at how vul-nerable the repository of all our po-
tential once was, how perilous our in-
fancy, how humble our beginnings, how many rivers we had to cross before we found our way.
If we do manage to find our way, it will be thanks to the efforts of Carl Sagan. The inhabitants of the blue dot mourn his departure.
Carl Sagan, astronomer and writer: born Brooklyn, New York 9 November 1934; David Duncan Professor of Astronomy and Space Science, Cornell University 1976-96; married first Lynn Margulis, second Linda Saltzman (three sons), third 1981 Ann Druyan (one son, one daughter); died Seattle 20 December 1996.