Sagan, 62, was at the Fred Hutchinson Cancer Research Center at the time of his death. He had received a bone marrow transplant from the center in April 1995 for the treatment of myelodysplasia, a pre-leukemic syndrome. Sagan continued to supervise undergraduate and graduate students and do research while recuperating from his illness, but returned unexpectedly to the Seattle hospital this month.

He attended the University of Chicago attaining a Bachelor's Degree in Physics in 1955, a Master's in Physics in 1956, and a Ph. D in Astronomy & Astrophysics in 1960. He worked at Harvard until 1968, when he accepted the title of David Duncan Professor of Astronomy and Space Sciences as well as Director of the Laboratory for Planetary Studies.

He taught at Harvard University in the early 1960s before coming to Cornell, where he became a full professor in 1971.

Sagan played a leading role in NASA's Mariner, Viking, Voyager and Galileo expeditions to other planets. He has received NASA Medals for Exceptional Scientific Achievement and twice for Distinguished Public Service and the NASA Apollo Achievement Award.

His research has focused on topics such as the greenhouse effect on Venus; windblown dust as an explanation for the seasonal changes on Mars; organic aerosols on Titan, Saturn's moon; the long-term environmental consequences of nuclear war; and the origin of life on Earth. A pioneer in the field of exobiology, he continued to teach graduate and undergraduate students in courses in astronomy and space sciences and in critical thinking at Cornell.

The breadth of his interests were made evident in October 1994, at a Cornell-sponsored symposium in honor of Sagan's 60th birthday. The two-day event featured speakers in areas of planetary exploration, life in the cosmos, science education, public policy and government regulation of science and the environment -- all fields in which Sagan had worked or had a strong interest.

Sagan was the recipient of numerous of awards in addition to his NASA recognition. He received 22 honorary degrees from American colleges and universities for his contributions to science, literature, education and the preservation of the environment and many awards for his work on the long-term consequences of nuclear war and reversing the nuclear arms race.

Among his other awards have been: the John F. Kennedy Astronautics Award of the American Astronautical Society; the Explorers Club 75th Anniversary Award; the Konstantin Tsiolkovsky Medal of the Soviet Cosmonauts Federation and the Masursky Award of the American Astronomical Society. He also was the recipient of the Public Welfare Medal, the highest award of the National Academy of Sciences, "for distinguished contributions in the application of science to the public welfare."

Sagan was elected chairman of the Division of Planetary Sciences of the American Astronomical Society, president of the Planetology Section of the American Geophysical Union and chairman of the Astronomy Section of the American Association for the Advancement of Science. For 12 years he was editor of Icarus, the leading professional journal devoted to planetary research.

He is co-founder of The Planetary Society, a 100,000-member organization and the largest space-interest group in the world. The society supports major research programs in the radio search for extraterrestrial intelligence, the investigation of near-Earth asteroids and, with the French and Russian space agencies, the development and testing of balloon and mobile robotic exploration of Mars. Sagan also was Distinguished Visiting Scientist at the Jet Propulsion Laboratory in California and was contributing editor of Parade magazine, where he published many articles about science and, most recently, about the disease that he has battled for the past two years.

Astronomer, educator and author, Sagan was perhaps the world's greatest popularizer of science, reaching millions of people through newspapers, magazines and television broadcasts. He is well-known for his work on the PBS series Cosmos, the Emmy- and Peabody-award-winning show that became the most watched series in public-television history. It was seen by more than 500 million people in 60 countries. The accompanying book, Cosmos (1980), was on The New York Times bestseller list for 70 weeks and was the best-selling science book ever published in English.

He received 22 honorary degrees from American colleges and universities for his contributions to science, literature, education, and the preservation of the environment, as well as awards for his work on the consequences of nuclear war and reversing the nuclear arms race.

He received numerous awards for his scientific achievements, such as the Public Welfare Medal, the highest award of the National Academy of Sciences. In a posthumous award to Dr. Sagan, the National Science Foundation declared that "his research transformed planetary science - his gifts to mankind were infinite."

His books include Contact and Billions and Billions. The year he died - he published The Demon-Haunted World: Science as a Candle in the Dark , which became Sagan's eighth New York Times bestseller.

Sagan has published more than 600 scientific papers and popular articles and is author, co-author or editor of more than 20 books, including The Dragons of Eden (1977), for which he won the Pulitzer Prize in 1978. The U.S. paperbound edition of his book Pale Blue Dot: A Vision of the Human Future in Space appeared on best-seller lists worldwide and was selected as one of the "notable books of 1995" by The New York Times. His reading of an abridged audiocassette version was nominated for a Grammy and was cited by Publisher's Weekly as one of the "two best audiobooks of the year."

Many things are revealed in the biographies that were not known outside a close circle of friends. For much of his adult life Sagan used marijuana and believed that it gave him many of his best ideas.

Married three times, his last book was written with his third wife Ann Druyan, his equal in expression.

It is a remarkable series of essays called "The Demon-Haunted World: Science as a Candle in the Dark".

It was published in 1996, the year he died. It arrived at a time when many had become suspicious of, if not hostile to, science because they believed its view of the world was a sterile impersonal one of logic in which humans had no special place.

Sagan knew that we only live on a spec of dust floating in a sunbeam but he also knew that humans were a special part of the universe. We are the way the cosmos knows itself.

His lifelong obsession was the idea that there could be life on other worlds in our solar system and beyond. It is sad that he never lived to see the discovery of life in space. He would have been our ambassador.

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