Education and scientific career
Carl Sagan was born in Brooklyn, New York. His parents were Jewish; his father, Sam Sagan, was a garment worker and his mother, Rachel Molly Gruber, was a housewife. Sagan attended the University of Chicago, where he received a bachelor's degree (1955) and a master's degree (1956) in physics, before earning his doctorate (1960) in astronomy and astrophysics. During his time as an undergraduate, Sagan spent some time working in the laboratory of the geneticist H. J. Muller.

In the early 1960s, no one knew for certain even the basic conditions of the surface of the planet Venus. He listed the contending possibilities in a report (which were later depicted for popularization in a Time-Life book, Planets). His own view was that the planet was dry and very hot. As a visiting scientist to Caltech's Jet Propulsion Laboratory, he contributed to the first Mariner missions to the planet Venus in the design and management of the project, which confirmed his views with the success of Mariner 2 in 1962.

Sagan taught at Harvard University until 1968, when he moved to Cornell University. He became a full professor at Cornell in 1971 and directed a lab there. He contributed to most of the unmanned space missions that explored our solar system. He conceived the idea of adding an unalterable and universal message on spacecraft, destined to leave the solar system, that could be understood by any extraterrestrial intelligence that might find it. The first message that was actually sent into space was a gold-anodized plaque, attached to the space probe Pioneer 10. He continued to refine his designs and the most elaborate such message he helped to develop was the Voyager Golden Record that was sent out with the Voyager space probes.

Scientific achievements
Sagan was among the first to hypothesize that Saturn's moon Titan and Jupiter's moon Europa may possess oceans (a subsurface ocean, in the case of Europa) or lakes, thus making the hypothesized water ocean on Europa potentially habitable for life. Europa's subsurface ocean was later indirectly confirmed by the spacecraft Galileo.

He furthered insights regarding the atmosphere of Jupiter, seasonal changes on Mars, and Saturn's moon Titan. Sagan established that the atmosphere of Venus is extremely hot and dense. He also perceived global warming as a growing, man-made danger and likened it to the natural development of Venus into a hot life-hostile planet through greenhouse gases. He suggested that the seasonal changes on Mars were due to windblown dust, not to vegetation changes, as others had proposed.

Scientific advocacy
Planetary Society members at the organization's founding. Carl Sagan seated, rightSagan was a proponent of the search for extraterrestrial life. He urged the scientific community to listen with large radio telescopes for signals from intelligent extraterrestrial lifeforms. He advocated sending probes to other planets. Sagan was Editor in Chief of Icarus (a professional journal concerning planetary research) for 12 years. He cofounded the Planetary Society and was a member of the SETI Institute Board of Trustees.

He was well known as a co-author of the scientific paper that predicted nuclear winter would follow nuclear war. Sagan famously predicted that smoky oil fires in Kuwait (set by Saddam Hussein's army) would cause an ecological disaster of black clouds. Retired atmospheric physicist, Fred Singer, dismissed Sagan's prediction as nonsense, predicting that the smoke would dissipate in a matter of days. In his book The Demon-Haunted World, Sagan gave a list of errors he had made (including his predictions about the effects of the Kuwaiti oil fires) as an example of how science is tentative.

Sagan is also known for being involved as a researcher in Project A119, a secret US Air Force operation whose purpose was to drop a bomb on Earth's Moon.

Social concerns
Sagan believed that the Drake equation suggested that a large number of extraterrestrial civilizations would form, but that the lack of evidence of such civilizations (the Fermi paradox) suggests that technological civilizations tend to destroy themselves rather quickly. This stimulated his interest in identifying and publicizing ways that humanity could destroy itself, with the hope of avoiding such destruction and eventually becoming a space-faring species.

Sagan became more politically active after marrying novelist Ann Druyan, performing acts of civil disobedience at nuclear weapons sites during the Nuclear freeze era. He spoke out against President Ronald Reagan's Strategic Defense Initiative, or the "Star Wars" program, which he felt was technically impossible to build and perfect, far more expensive to create than for an enemy to defeat through decoys and other means, and destabilizing to Cold War nuclear weapons disarmament progress.

Carl Sagan was an avid user of marijuana, although he never publicly admitted it during his life. Under the pseudonym "Mr. X," he wrote an essay concerning cannabis smoking in the 1971 book Marihuana Reconsidered, whose editor was Lester Grinspoon. In his essay, Sagan commented that marijuana encouraged some of his works and enhanced experiences. After Sagan's death, Grinspoon disclosed this to Sagan's biographer, Keay Davidson. When the biography, entitled Carl Sagan: A Life, was published in 1999, the marijuana exposure stirred some media attention.

Popularization of science
Sagan's capability to convey his ideas allowed many people to better understand the cosmos. He delivered the 1977/1978 Christmas Lectures for Young People at the Royal Institution. He narrated and, with Ann Druyan, co-wrote and co-produced the highly popular thirteen part PBS television series: Cosmos: A Personal Voyage (modeled on Jacob Bronowski's The Ascent of Man)

Cosmos covered a wide range of scientific subjects including the origin of life and a perspective of our place in the universe. The series was first broadcast by the Public Broadcasting Service in 1980. It won an Emmy and a Peabody Award; according to the NASA Office of Space Science, it has been since broadcast in 60 countries and seen by more than 600 million people.

Sagan also wrote books to popularize science, such as Cosmos, which reflected and expanded upon some of the themes of A Personal Voyage, The Dragons of Eden: Speculations on the Evolution of Human Intelligence, which won a Pulitzer Prize, and Broca's Brain: Reflections on the Romance of Science. Sagan also wrote the best-selling science fiction novel Contact, but never lived to see the book's 1997 motion picture adaptation, which starred Jodie Foster and won the 1998 Hugo Award.

From Cosmos and his frequent appearances on The Tonight Show, Sagan became associated with the catch phrase, "billions and billions." (He never actually used that phrase in Cosmos, but his distinctive delivery and frequent use of billions made this a favorite phrase of Johnny Carson and others, doing the many affectionate impressions of him. Sagan took this in good humor, and his final book was entitled Billions and Billions - see below.) The humorous unit of measurement, the Sagan has now been coined to stand for any count of at least 4,000,000,000.

He wrote a sequel to Cosmos, Pale Blue Dot: A Vision of the Human Future in Space, which was selected as a notable book of 1995 by The New York Times. Carl Sagan also wrote an introduction for the bestselling book by Stephen Hawking, A Brief History of Time.

Sagan presents a speculation concerning the origin of the swastika symbol in his book, Comet. Sagan hypothesized that a comet approached so close to Earth in antiquity that the jets of gas streaming out of it were visible, bent by the comet's rotation. The book Comet reproduces an ancient Chinese manuscript that shows comet tail varieties; most are variations on simple comet tails, but the last shows the comet nucleus with four bent arms extending from it, showing a swastika.

Sagan caused mixed reactions among other professional scientists. On the one hand, there was general support for his popularization of science, his efforts to increase scientific understanding among the general public, and his positions in favor of scientific skepticism and against pseudoscience; most notably his thorough debunking of the book Worlds In Collision by Immanuel Velikovsky. On the other hand, there was some unease that the public would misunderstand some of the personal positions and interests that Sagan took as being part of the scientific consensus, rather than his own personal views, and there was some unease, which some believe to have been motivated in part by professional jealousy, that scientific views contrary to those that Sagan took (such as on the severity of nuclear winter) were not being sufficiently presented to the public.

Sagan's arguments against Velikovsky's catastrophism have been criticized by some of his colleagues. Dr. Robert Jastrow of NASA's Goddard Institute of Space Studies wrote: "Professor Sagan's calculations, in effect, ignore the law of gravity. Here, Dr. Velikovsky was the better astronomer." His comments on the Kuwait oil well fires during the first Gulf War were shown later to be in error; Sagan himself acknowledged his error in print.

Late in his life, Sagan's books developed his skeptical, naturalistic view of the world. In The Demon-Haunted World: Science as a Candle in the Dark, he presented tools for testing arguments and detecting fallacious or fraudulent ones, essentially advocating wide use of the scientific method. The compilation, Billions and Billions: Thoughts on Life and Death at the End of the Millennium, published after Sagan's death, contains essays written by Sagan, such as his views on abortion, and Ann Druyan's account of his death as a non-believer.

Personality
In 1966, Sagan was asked to contribute an interview about the possibility of extraterrestrials to a proposed introduction to the film 2001: A Space Odyssey. According to an uncited anecdote in The Independent, Sagan "responded by saying that he wanted editorial control and a percentage of the film's takings, which was rejected."

In 1994, Apple Computer began developing the Power Macintosh 7100. They chose the internal code name "Carl Sagan," in honor of the astronomer. Though the project name was strictly internal and never used in public marketing, when Sagan learned of this internal usage, he sued Apple Computer to use a different project name — other projects had names like "Cold fusion" and "Piltdown Man", and he was displeased at being associated with what he considered pseudoscience. Though Sagan lost the suit, Apple engineers complied with his demands anyway, renaming the project "BHA" (Butthead Astronomer). Sagan sued Apple for libel over the new name, claiming that it subjected him to contempt and ridicule. Sagan lost this lawsuit as well; still, the 7100 saw another name change: it was lastly called "LAW" (Lawyers Are Wimps).

Sagan is regarded by most as an atheist, agnostic, or pantheist observing statements such as: "The idea that God is an oversized white male with a flowing beard, who sits in the sky and tallies the fall of every sparrow is ludicrous. But if by 'God,' one means the set of physical laws that govern the universe, then clearly there is such a God. This God is emotionally unsatisfying... it does not make much sense to pray to the law of gravity."

Sagan married three times; the famous biologist, Lynn Margulis (mother of Dorion Sagan and Jeremy Sagan) in 1957, artist Linda Salzman (mother of Nick Sagan) in 1968, and author Ann Druyan (mother of Sasha and Sam) in 1981, to whom he remained married until his death.

Sagan and UFOs
Sagan had some interest in UFO reports from at least 1964, when he had several conversations on the subject with Jacques Vallee. Though quite skeptical of any extraordinary answer to the UFO question, Sagan thought that science should study the phenomenon, at least because there was widespread public interest in UFO reports.

Dr. Stuart Appelle notes that Sagan "wrote frequently on what he perceived as the logical and empirical fallacies regarding UFOs and the abduction experience. Sagan rejected an extraterrestrial explanation for the phenomenon but felt there were both empirical and pedagogical benefits for examining UFO reports and that the subject was, therefore, a legitimate topic of study."

In 1966, Sagan was a member of the Ad Hoc Committee to Review Project Blue Book. The committee concluded that the U.S. Air Force's Project Blue Book had been lacking as a scientific study, and recommended a university-based project to give the UFO phenomenon closer scientific scrutiny. The Condon Committee (1966-1968), lead by physicist Dr. Edward Condon, and their still-controversial final report, formally concluded that there was nothing anomalous about UFO reports.

Dr. Ron Westrum writes that "The high point of Sagan's treatment of the UFO question was the AAAS's symposium in 1969. A wide range of educated opinions on the subject were offered by participants, including not only proponents as McDonald and Hynek but also skeptics like astronomers William Hartmann and Menzel. The roster of speakers was balanced, and it is to Sagan's credit that this event was presented in spite of pressure from Edward Condon." With physicist Thornton Page, Sagan edited the lectures and discussions given at the symposium; these were published in 1972 as UFO's: A Scientific Debate

Jerome Clark writes that Sagan's perspective on UFO's irked Condon: "... though a skeptic, (Sagan) was too soft on UFOs for Condon's taste. In 1971, he considered blackballing Sagan from the prestigious Cosmos Club". (Clark, p. 603)

Some of Sagan's many books examine UFOs (as did one episode of Cosmos) and he recognized a religious undercurrent to the phenomenon. However, Westrum writes that "Sagan spent very little time researching UFOs ... he thought that little evidence existed to show that the UFO phenomenon represented alien spacecraft and that the motivation for interpreting UFO observations as spacecraft was emotional." (Westrum, 37)

It is sometimes noted that Sagan's generally hostile attitude to UFOs conflicted sharply with his views in a 1966 book he wrote with Russian astronomer and astrophysicist I.S. Shklovskii, Intelligent Life in the Universe. Here Sagan instead argued that technologically advanced alien civilizations were common and he considered it very probable that Earth had been visited many times in the past.

Yet only a few years later in UFO's: A Scientific Debate, Sagan was now highly skeptical of interstellar visitation. As to the physical possibility of interstellar travel, Sagan brought up the proposed Bussard ramjet as an interstellar vehicle. While not terribly practical, Sagan thought such proposed propulsion systems were nevertheless important because they demonstrated that there were conceivable ways of accomplishing interstellar travel "without bumping into fundamental physical constraints. And this suggests that it is premature to say that interstellar space flight is out of the question." But to this Sagan added, "I believe the numbers work out in such a way that UFO's as interstellar vehicles is extremely unlikely, but I think it is an equally bad mistake to say that interstellar space flight is impossible."

Sagan was likewise inconsistent in his views on interstellar travel in his 1980 Cosmos series. When scoffing at UFOs, he maintained that the distance between stars was too great to make interstellar travel feasible for aliens. Yet in another episode, he said the stars would "beckon" to humanity, and then again described the Bussard ramjet as one way humans might do it.

Legacy
After a long and difficult fight with myelodysplasia, Sagan died at the age of 62, on December 20, 1996, at the Fred Hutchinson Cancer Research Center in Seattle, Washington. Sagan was a significant figure, and his supporters credit his importance to his popularization of the natural sciences, opposing both restraints on science and reactionary applications of science, defending democratic traditions, resisting nationalism, defending humanism, and arguing against geocentric and anthropocentric views.

The landing site of the unmanned Mars Pathfinder spacecraft was renamed the Carl Sagan Memorial Station, in honor of Dr. Sagan on July 5, 1997. Asteroid 2709 Sagan is also named in his honor.

The 1997 movie, Contact (see above), based on Sagan's novel of the same name, and finished after his death, ends with the dedication "For Carl."

In an episode of Star Trek: Enterprise entitled "Terra Prime", a quick shot is shown of the relic rover Sojourner, part of the Mars Pathfinder mission, placed by a historical marker at Carl Sagan Memorial Station on the Martian surface. The marker displays a quote from Sagan: "Whatever the reason you're on Mars, I'm glad you're there, and I wish I was with you."

In 2004, the electronic music group Sagan released the CD/DVD "Unseen Forces." The music was accompanied by a DVD which featured humorous music video format homages of many of the historical sketches from "Cosmos."