It was almost like a dispatch from another planet: the invitation to the young astronomer to leave Brooklyn and visit the lakes and gorges of upstate New York.

"A letter shows up in my mailbox from Carl Sagan," said Neil deGrasse Tyson, recalling the 1975 invitation at a recent Library of Congress event celebrating Sagan. "I couldn't believe it. Famous people don't write out of the blue to strangers."

But the invitation was real. In response to his Cornell application, Tyson met the famous professor on a college visit soon after. Sagan offered to let the 17-year-old astronomer camp out at his house if a snowstorm knocked out his bus ride home.

Tyson eventually ended up at Harvard instead of Cornell, but he now hosts Cosmos, the 13-part remake of the original series (airing on the National Geographic Channel and Fox on Sundays at 9 p.m. EDT/8 p.m. CDT) that shot Sagan's celebrity sky high.

The original invitation, the visit, and the connection were typical Carl Sagan.

Life in the Cosmos

"He worked very hard for his students, got them jobs, worried about their education, many of them very well placed now," says William Poundstone, author of Carl Sagan: A Life in the Cosmos. "If you talk to the people he inspired, who knew him, they are uniformly effusive."

"Sagan was certainly the most famous U.S. scientist of the 1980s and early 1990s," says science journalism expert Declan Fahy of American University in Washington, D.C. Fahy says, "After Cosmos reached half a billion viewers in 60 nations, his fame reached another level. The book of the series spent more than 70 weeks on the bestseller list."

But who was Carl Sagan? Scientist, celebrity, writer, professor, skeptic, and free-thinker, he was much more than the narrator of a TV series.

"Part of what made him great was the number of things he pursued," says NASA's David Morrison, director of the Carl Sagan Center for the Study of Life in the Universe, at the SETI Institute in Mountain View, California. Morrison marveled both at Sagan's breadth of accomplishments and his lack of self-importance.

View Images Sagan, here in 1974, was a professor of astronomy at Cornell University in Ithaca, New York, for much of his career. PHOTOGRAPH BY SANTI VISALLI INC., GETTY

Scientist

"He worked very hard, 18-hour days. He had a tremendous appetite for his work," says Poundstone. "He was made for television, sure, and he looked very relaxed and normal in jeans when other scientists didn't. But there was a lot more to him."

As a scientist, Sagan made a real mark on planetary science in the early 1970s as a young Harvard professor, "at a time when planetary science was a bit of a backwater," Poundstone says.

Sagan first predicted that the greenhouse effect made the atmosphere of Venus hot enough to melt lead, at a time when some scientists still speculated that its clouds might hide oceans, says Morrison.

Sagan also identified dark-shaded regions on Mars as highlands and identified lighter areas as desert plains marked by dust storms. Those storms later bedeviled NASA's Mars Viking landers in the 1970s.

"He was a really great big-picture scientist, great with back-of-the envelope calculations, who could see the fundamental premises of science and observations," says Morrison.

On the two Voyager missions launched in 1977 to explore Jupiter, Saturn, Uranus, and Neptune, Sagan was a member of the imaging science team. "That was really before the crest of his fame," says Morrison, a former student of Sagan's. "He was not a superstar then, just one of us."

Celebrity

Sagan led the team that put together the "golden records" attached to the Voyager mission's two spacecraft. The records (sent along with phonograph needles) included cuts of everything from Bach to "Johnny B. Goode," along with greetings and natural sounds from Earth.

To a large extent, Poundstone says, Sagan benefited from filling a scientific niche, planetary science, that was set to explode with new knowledge as a result of NASA's line of planetary probes exploring the solar system starting in the 1960s.

Reporters gravitated toward Sagan on those missions, Poundstone says. "They knew who could explain things." Sagan ended up as a regular on the Tonight Show (as Tyson now is on Comedy Central's Colbert Report), a guest favorite of Johnny Carson.

Parodied by Carson for his consonant-rolling pronunciation of "bill-ions and bill-ions" in the series, Sagan indeed thought big, even opening a line of Cosmos-themed stores that anticipated the museum-themed stores in malls today.

After the 1980 publication of Cosmos and the premiere of the PBS series, "things changed for Carl. He was getting death threats; he had to travel in limousines and keep a closed schedule," Morrison said. "People don't remember that."

View Images Sagan testified at a congressional hearing in 1985 on the climatic, biological, and strategic effects of nuclear war. PHOTOGRAPH BY KARL SCHUMACHER, TIME LIFE PICTURES/GETTY

Skeptic

The death threats partly derived from Sagan's work on the U.S. Air Force's Project Blue Book, which investigated UFOs in the 1950s and 1960s. "He started out with an open mind, but came to the conclusion that there wasn't any evidence for aliens visiting Earth," Poundstone says. Sagan was a big proponent of the likelihood of life existing elsewhere in the universe, however.

Sagan also waded into one of the wellsprings of today's global warming debate, as the senior author on a 1983 Science journal study of "nuclear winter." The group's climate model found worldwide subzero temperatures an inevitable consequence of the dust clouds resulting from a nuclear exchange between the superpowers.

He had angered both UFO fans and belligerent critics of the nuclear winter report. Morrison recalls that after Sagan received threats, "they hid his office number at Cornell and he used a back door to get to work." In the era of the "Unabomber" mailing explosives to professors, the threats were taken seriously.

"Science is more than a body of knowledge. It's a way of thinking, a way of skeptically interrogating the universe with a fine understanding of human fallibility," Sagan said in his last, 1996 interview with PBS's Charlie Rose. He died that same year from cancer.

The power of skeptical thinking infused his 1995 book, The Demon-Haunted World: Science as a Candle in the Dark, which sought to explain the scientific method to general readers. It included a "baloney-detection kit" for skeptics.

Sagan wasn't skeptical about everything: He was also a marijuana advocate, appearing as "Mr. X," a successful pot smoker, in a book by Harvard's Lester Grinspoon. And he advocated for medical marijuana in the years before his death.

View Images Sagan talks about the Voyager 2 spacecraft at the Jet Propulsion Laboratories in Pasadena, California, in 1986. PHOTOGRAPH BY LENNOX MCLENDON, AP

Spokesman

"Do something meaningful," said Ann Druyan, Sagan's third wife, speaking at the Library of Congress event, where Sagan's papers were donated to its collections. "That was at the heart of his whole life."

Druyan, Sagan's third wife and frequent collaborator, was a co-writer on the original Cosmos and is part of the production team on the new series.

Sagan wrote 20 popular books (dictating, not typing them) and hundreds of scientific studies. Nevertheless, his fame brought him criticism from other scientists, and a snub from the National Academy of Scientists, when he was nominated for membership but not accepted.

"That's just human nature: envy and resentment," Poundstone says.

Today, Cosmos is remembered for popularizing thinking big about space and inspiring young scientists. And the memory of Sagan as a prominent defender of science is perhaps his greatest legacy.

"His influence can be seen now in that almost every scientist with a prominent media profile cites him as an influence," Fahy says. "When scientific organizations want to increase public interest in science, one of their first ideas is something like: 'We need more Carl Sagans.'"

If he were around today, Poundstone says, Sagan would likely be speaking out forcefully for science in his inimitable way, regardless of the slights.

"For at least 100 years we have had scientific celebrities," he says. "But Sagan was the first one with a personality made for the age of television, and that was where he found a home."