Carl Sagan

Chapter 1 · 1934 – 1947 The Stars Are Suns

Carl Edward Sagan was born on November 9, 1934, in Bensonhurst, a working-class neighborhood of Brooklyn, New York. His father, Samuel Sagan, born in 1905 in Kamianets-Podilskyi, Ukraine, then part of the Russian Empire, had come to the United States as a boy; he worked in the garment trade. During the worst years of the Great Depression he was a movie theater usher. His mother, Rachel Molly Gruber, born in New York in 1906, was a homemaker. The boy was named after his maternal grandmother, Chaiya Clara, who had died in childbirth. The family, Reform Jews, lived in a modest apartment.

Skepticism and Wonder

His parents were not scientists; they knew almost nothing about science. But Sagan would later say that they had given him, at the same time, the two uneasily cohabiting modes of thought at the heart of the scientific method: skepticism and wonder. His mother, very poor in her youth, whose intellectual ambitions had been thwarted, poured into her only son dreams she had not been able to live. His father gave him his sense of wonder. When the boy announced that he wanted to become a scientist, his father advised him first to learn a profession so that he could support himself in his science “hobby.”

The 1939 Fair

In 1939 his parents took him to the New York World’s Fair. He was four. He would keep vivid memories of it: the great model of a city of the future crisscrossed by highways; a flashlight shining on a photoelectric cell that set off a crackling sound; the sound of a tuning fork turned into a wave on a screen; the beginnings of television. He also saw the burial of a time capsule filled with objects from the 1930s, to be opened in the distant future. As an adult he would send similar messages out toward the stars.

The Right Book

He wondered what the stars were, and no one around him could really tell him. As soon as he was old enough, his parents gave him his first library card. He told the story: he asked for a book about stars, the librarian first brought him a picture book of movie stars; he complained, she smiled and came back with the right book. There he learned that the stars are suns, but very far away. He understood then that the universe was far larger than he had imagined. It was, he would say, a kind of religious experience, a sense of grandeur that never left him.

At six or seven he went with a friend to the American Museum of Natural History and the Hayden Planetarium. His parents bought him chemistry sets and books. And he read Edgar Rice Burroughs’s novels whose hero, John Carter, is transported to Mars. In 1947 he discovered the magazine Astounding Science Fiction, which he awaited eagerly every month. He rated the stories he read on scorecards of his own devising, as he did for baseball; many, he judged, asked interesting questions but answered them poorly.

Sources: English-language Wikipedia article “Carl Sagan” (birth, neighborhood, parents and their dates, name, religion, father’s job, the remark on skepticism and wonder, 1939 Fair, library, museum, Burroughs, Astounding; tertiary source, drawing on the biographies by K. Davidson and R. Spangenburg and on Sagan’s writings); D. Morrison, Carl Sagan 1934-1996, Biographical Memoir, National Academy of Sciences, 2014 (father’s origin, mother, the father’s advice on the “hobby”). Sagan’s recollections are summarized rather than quoted, because his books are under copyright.

Chapter 2 · 1948 – 1960 Chicago

In 1948 his father’s work took the family to Rahway, New Jersey. Carl Sagan was a brilliant, bored student there: his classes did not challenge him. He was president of the chemistry club and set up a laboratory at home. One day he discovered, with delight, that astronomers were paid to do what he loved: perhaps he could do astronomy full-time.

An Essay on First Contact

Before finishing high school he entered an essay contest. In it he argued a bold idea for a high-school student of the time: contact with advanced extraterrestrials might be as disastrous for humans as the arrival of Europeans had been for Native Americans. The judges gave him first prize. He graduated in 1951.

A University That Took a Sixteen-Year-Old

That same year the University of Chicago accepted him, at sixteen, with a full scholarship. It was one of the few colleges that admitted students so young. Its chancellor, Robert Hutchins, had built a curriculum around the great books, discussion and comprehensive examinations. Sagan discovered there that science is part of culture: an aspiring physicist, he was told, should know Plato and Shakespeare.

His teachers were among the best of the century. He studied in a physics department orbiting around Enrico Fermi, learned mathematical elegance from Subrahmanyan Chandrasekhar, talked chemistry with Harold Urey, spent summers studying biology with the geneticist Hermann Muller at Indiana University, and learned planetary astronomy from Gerard Kuiper, then, in his words, its only full-time practitioner. In 1952, in Chicago, Stanley Miller and Harold Urey produced amino acids, the building blocks of proteins, by passing sparks through a gas mixture imitating the atmosphere of the early Earth. The experiment awakened in Sagan an interest that would never leave him: the origin of life.

He earned a bachelor of liberal arts, then in 1955 a bachelor of science in physics, in 1956 a master’s in physics, and in 1960 a doctorate in astronomy and astrophysics under Kuiper, with a thesis titled Physical Studies of the Planets. Kuiper wrote a sentence about him that became famous: “Some persons work best in specializing on a major program in the laboratory; others are best in liaison between sciences. Dr. Sagan belongs in the latter group.”

Lynn Margulis

In 1957, during his graduate studies, he married a biology student, Lynn Margulis, who had entered the University of Chicago at fourteen. She would become one of the great evolutionary biologists. They had two sons, Dorion and Jeremy. The marriage did not last: they divorced in 1964 according to Wikipedia, in 1965 according to David Morrison, Sagan’s former student who wrote his memoir for the U.S. National Academy of Sciences.

A Bomb on the Moon

In 1958, with Kuiper, the young Sagan worked on a secret U.S. Air Force project, Project A119, which studied the possibility of detonating a nuclear weapon on the Moon and observing its effects. The project went nowhere. In 1999 the journal Nature revealed that Sagan had listed the titles of two classified project reports in a 1959 scholarship application.

Sources: English-language Wikipedia article “Carl Sagan” (Rahway, chemistry club, essay contest, Chicago at 16, Hutchins, teachers, the 1952 Miller-Urey experiment, degrees and dates, thesis, Kuiper’s quotation, Margulis and the 1964 divorce, Project A119 and the 1999 Nature article; tertiary source); D. Morrison, Biographical Memoir, NAS, 2014 (full scholarship, Kuiper’s quotation, Margulis entering at 14, sons, divorce in 1965). The description of the Miller-Urey experiment belongs to the general history of science.

Chapter 3 · 1956 – 1962 The Furnace of Venus

Until the mid-1950s, Venus was thought of as Earth’s twin. It is nearly the same size; its thick clouds hide its surface, and one could imagine swamps, oceans, a tropical climate there. In 1956 radio astronomers discovered that the planet emits far too many radio waves for a temperate world. Either its surface was scorching, or the waves came from the upper atmosphere. Many considered a surface at nearly seven hundred degrees above absolute zero physically impossible.

A Giant Greenhouse

Carl Sagan’s doctoral thesis offered an explanation. A thick atmosphere of carbon dioxide and water vapor lets sunlight in but traps the heat the ground gives off: the greenhouse effect. On Venus it would be pushed to the extreme. Sagan later coined a phrase to describe how a planet starting from conditions close to the Earth’s can tip into such an extreme state: the “runaway greenhouse effect.” In 1961 he published a major article on the planet Venus in the journal Science. He argued that Venus is dry and very hot, far from the balmy paradise others had imagined.

Mariner 2

On December 14, 1962, the American probe Mariner 2 flew past Venus. It was the first spacecraft to complete a flyby of another planet. Sagan was among those who had worked with its team. Its measurements confirmed that the heat did indeed come from the surface. For years afterward, with his former student James Pollack, Sagan built ever more refined models of the atmosphere of Venus; David Morrison calls them the definitive studies of such a large greenhouse effect.

This first scientific victory already says much about his way of working. He reasoned quickly, from the simplest physical laws. Kuiper had taught him back-of-the-envelope calculations: when an explanation occurs to you, scribble a few approximate equations on an old envelope and see whether the answer comes anywhere near; if not, look for another explanation. And he connected worlds to one another: Venus would remain, all his life, his cautionary example for the Earth. In 1985 he would testify before the U.S. Congress that the greenhouse effect would change the Earth’s climate.

Sources: D. Morrison, Biographical Memoir, NAS, 2014 (the 1956 discovery, debate over the hot surface, nearly 700 K, thesis on the greenhouse effect, “runaway greenhouse effect,” role on Mariner 2, work with Pollack 1965-1976 and Morrison’s judgment; bibliography: “The planet Venus,” Science, 1961); English-language Wikipedia article “Carl Sagan” (dry and hot Venus, Mariner 2 in 1962, back-of-the-envelope calculations learned from Kuiper, testimony before Congress in 1985; tertiary source). The date of Mariner 2’s flyby and the principle of the greenhouse effect belong to general history and physics.

Chapter 4 · 1960 – 1964 An Astronomer Among Biologists

From 1960 to 1962 Carl Sagan was a researcher at the University of California, Berkeley, on a Miller Institute fellowship. He felt he lacked biology. So he did postdoctoral work in that field at Stanford and Berkeley. According to David Morrison, he received more encouragement at that time from biologists than from astronomers, many of whom considered planetary studies to lie on the fringes of respectable science, and exobiology to be beyond the pale.

Oases on Mars

His first papers dealt with possible life in the solar system. One, with Stanley Miller, studied the making of organic molecules in simulated planetary atmospheres. Another, published in 1962 with the Nobel-laureate biologist Joshua Lederberg, was titled “Microenvironments for Life on Mars.” The idea is simple and has lasted: even if Mars as a whole is hostile, there might be small favored places, warmer or wetter, where life could have held on. Fifteen years later, when Viking’s experiments found nothing conclusive, these oases would come up again.

The Green Bank Equation

In 1961 Sagan was one of the eleven scientists gathered at the National Radio Astronomy Observatory in Green Bank, West Virginia, where the astronomer Frank Drake presented his equation for estimating the number of detectable civilizations in our galaxy. He also worked, at a distance, with the Soviet astrophysicist Iosif Shklovsky. What was to be a translation of Shklovsky’s book on intelligent life in the universe became a joint work, more than twice as long as the original, in which the two authors’ contributions were set out as a dialogue. Morrison dates the book to 1961; the American edition, Intelligent Life in the Universe, appeared in 1966.

“Earth Chauvinism”

In these years Sagan popularized a phrase that became a tool for thinking: “Earth chauvinism.” He meant by it the tendency to assume that living beings anywhere else would necessarily resemble those on Earth. NASA’s official history of Mars exploration uses it as soon as it turns to the first designs for life-detection instruments: building a machine to find life on Mars forces one to guess what that life would be like, and so risks looking only for what we already know.

Sources: D. Morrison, Biographical Memoir, NAS, 2014 (postdoctoral biology at Stanford and Berkeley, reception by biologists and astronomers, papers with Miller and with Lederberg, Green Bank and its 11 participants, collaboration with Shklovsky and the date of 1961; bibliography: Lederberg and Sagan, “Microenvironments for life on Mars,” PNAS, 1962); English-language Wikipedia article “Carl Sagan” (Miller Fellow at Berkeley 1960-1962; tertiary source); E. C. Ezell and L. N. Ezell, On Mars: Exploration of the Red Planet, 1958-1978, NASA SP-4212, 1984, chapter III (“Earth chauvinisms,” a term popularized by Sagan) and chapter XI (Sagan and Lederberg’s microenvironments after Viking). The 1966 publication date of Intelligent Life in the Universe belongs to general bibliography.

Chapter 5 · 1963 – 1968 Planets Are Places

After his 1961 article on Venus, two Harvard astronomers, Fred Whipple and Donald Menzel, invited Carl Sagan to give a colloquium, then offered him a lectureship. Sagan asked for more: an assistant professorship. He got it. From 1963 to 1968 he taught at Harvard and also worked at the Smithsonian Astrophysical Observatory in Cambridge.

A Divisive Professor

His public lectures were titled “Planets Are Places.” The phrase sums up what was changing: Mars, Venus and Jupiter were ceasing to be points of light and becoming worlds to be visited. Students admired him, and so did the public. Many of his astronomy colleagues, Morrison writes, demurred: his outgoing personality and interests outside the traditional curriculum set him apart.

He was already working for NASA. He was a member of the science team of Mariner 4, the first probe to photograph Mars up close, in July 1965; its pictures showed a surface covered with craters, like the Moon. As early as 1963 he was one of twenty scientists brought together by General Electric to review its studies for a large Mars program, then called Voyager, along with Melvin Calvin, Joshua Lederberg, Wolf Vishniac and Harold Urey. In 1965 he published, with Paul Swan, a paper on choosing landing sites on Mars. That work, a decade ahead, foreshadows what he would do for Viking.

In 1966 he also sat on the ad hoc committee reviewing Project Blue Book, the U.S. Air Force’s UFO investigation. The committee found the investigation scientifically lacking and recommended a university-based study.

The Refusal

In 1968 Harvard denied him tenure. Sagan would say the decision came as a great surprise. Wikipedia cites several reasons: interests judged too scattered, at a time when a researcher was expected to become the expert in a narrow field; a public fame that some colleagues equated with self-promotion. His former teacher Harold Urey, a Nobel laureate, had written to the tenure committee strongly recommending against him. Urey would later change his mind: in 1977 he wrote to Sagan how much he had liked one of his books.

A Cornell astronomer, Thomas Gold, had long been courting him. Sagan accepted his offer. He left for Ithaca, New York, where he would stay nearly thirty years.

Sources: English-language Wikipedia article “Carl Sagan” (invitation from Whipple and Menzel, assistant professorship, Smithsonian, Blue Book committee of 1966, denial of tenure and the reasons cited, Urey’s letter and his change of mind, Thomas Gold; tertiary source); D. Morrison, Biographical Memoir, NAS, 2014 (“Planets are Places” lectures, reception by students and colleagues, member of the Mariner 4 team); NASA SP-4212, chapters III and IX (General Electric group of 1963, Swan and Sagan’s 1965 paper on landing sites). The Mariner 4 images (July 1965) are described in the biography of Percival Lowell on this site.

Chapter 6 · 1966 – 1970 The Wave That Was Only Wind

For a century one argument kept coming back in favor of life on Mars: the “wave of darkening.” Each Martian spring, as the polar cap melts, the planet’s dark regions seem to deepen, and the change appears to move toward the equator. Percival Lowell and others saw it as the growth of vegetation, like the greening of forests in spring on Earth. Carl Sagan, who had been drawn to science by the dream of a habitable Mars, would help demolish that argument.

Another Explanation

From 1966 to 1970, with James Pollack, Sagan developed a rival hypothesis: the wave of darkening could be caused by dust raised and moved by winds that change with the seasons. To test it, they used the first measurements of the very low atmospheric pressure on Mars and of apparently large elevation differences between regions, and built realistic models of the global circulation of the Martian atmosphere. Their papers bore titles such as “Secular changes and dark-area regeneration on Mars” (1966), “Elevation differences on Mars” (1968) and “Windblown dust on Mars” (1969).

Morrison notes the irony: the man who kept a strong interest in the search for life on Mars undercut one of the main arguments for it. Sagan drew a logical conclusion. If nothing could be concluded from Earth, one had to go and look, with instruments able to detect microbes in the soil. He became an enthusiastic advocate of Viking’s biology experiments.

“Go There”

In 1969 the Mariner 6 and 7 probes flew past Mars and showed an atmosphere that was 95 percent carbon dioxide, with very little nitrogen and traces of oxygen and water vapor. This was hardly encouraging for those hoping to find life. Sagan, NASA’s official history reports, repeated what had become his refrain: the only way to make such a determination was to go there and check out the planet.

The Viking orbiters, from 1976 on, would provide definitive support for the dust hypothesis, and show how wind streaks could be used to reconstruct seasonal winds. In his 1976 article for NASA, Sagan wrote that the changes observed in the past were “almost certainly” due to the redistribution of bright and dark dust by Martian winds.

Sources: D. Morrison, Biographical Memoir, NAS, 2014 (the wave of darkening as the main argument, Sagan and Pollack’s hypothesis of 1966-1970, pressure and elevation measurements, circulation models, the irony, support for Viking’s experiments, confirmation by the Viking orbiters; bibliography: Icarus 1966, J. Geophys. Res. 1968, Nature 1969); NASA SP-4212, chapter VIII (results of Mariners 6 and 7 and Sagan’s refrain); C. Sagan, “The Search for Life on Mars,” first of the Six Articles by Carl Sagan, NASA News Release 76-74, 1976 (Lowell, vegetation, dust).

Chapter 7 · 1968 – 1971 Ithaca

At Cornell, Carl Sagan found what Harvard had not given him: freedom. The university president, Frank Rhodes, and the director of the Center for Radiophysics and Space Research, Thomas Gold, valued what today would be called his “out of the box” thinking. He quickly became a campus star. Associate professor in 1968, he was a full professor in 1970 and directed the Laboratory for Planetary Studies.

A Way of Working

Morrison, who was his student, describes him as an “idea person” and a master of intuitive physical arguments and back-of-the-envelope calculations. He usually left the details to others; most of his papers were collaborations, often with students, many of whom went on to become leaders in planetary science. For years he edited Icarus, which became the foremost professional journal of this new science; NASA’s official history calls it the single most important journal for studies related to Mars, the one in which many of the issues in planning flights to the planet were first debated. He was a founding member of the American Astronomical Society’s Division for Planetary Sciences.

In 1968 he married a New York artist, Linda Salzman. Their son Nick was born in 1970.

Debating in Public

Sagan believed one could show the public how science works by organizing fair debates with advocates of beliefs it did not share. In 1969 he organized, at the annual meeting of the American Association for the Advancement of Science, a symposium on whether UFOs could be alien spacecraft. The two sides were reasonably well balanced; Sagan, who as a boy had fervently hoped UFOs were real, gave a low-key presentation emphasizing the absence of scientifically acceptable evidence. He would publish the proceedings in 1972.

Budgets

These were also the years when space money grew scarce. At the end of 1969, a budgetary error and a press article saying that scientists favored cutting NASA’s science program nearly wiped out the Viking project in four hours. Space programs were criticized for not helping to solve pollution or hunger. Sagan, NASA’s official history reports, pointed to the Defense Department as the real source of budget misallocations. Viking survived, postponed to 1975.

Sources: D. Morrison, Biographical Memoir, NAS, 2014 (Rhodes and Gold, “campus star,” laboratory, “idea person,” collaborations, Icarus, Division for Planetary Sciences, Linda Salzman and Nick, the 1969 AAAS UFO symposium); English-language Wikipedia article “Carl Sagan” (associate then full professor in 1970, proceedings published in 1972; tertiary source); NASA SP-4212, chapter VII (budget crisis at the end of 1969 and Sagan’s position) and bibliography (importance of Icarus). Wikipedia limits Sagan’s editorship of Icarus to 1975-1979; Morrison extends it through the 1970s and into the 1980s. The postponement of Viking to 1975 is reported by NASA SP-4212.

Chapter 8 · 1971 – 1972 A Planet in a Storm

On November 12, 1971, two days before the Mariner 9 spacecraft went into orbit around Mars, five men discussed the red planet in public at the California Institute of Technology: the writer Ray Bradbury, the science-fiction author Arthur C. Clarke, the planetary scientist Bruce Murray, the New York Times science editor Walter Sullivan, and Carl Sagan. Their conversation, later published as Mars and the Mind of Man, marked a historic moment: for the first time an Earth spacecraft was about to orbit another planet.

Learning as You Go

Sagan was a member of the Mariner 9 science team. He was also part of the team preparing Viking’s landing four years later, and he and the geologist Harold Masursky were charged with telling the Mariner people what the Viking team hoped to learn from their mission. What they wanted, NASA’s official history reports, was not in the written plans: they wanted to be able to learn as they went along and modify their plans accordingly. In NASA shorthand this was called the “adaptive mode.” At Sagan’s request, the problem of processing Mariner’s data too slowly was raised before Viking’s Science Steering Group, which agreed to fund part of the work.

He was also named, with Geoffrey Briggs, Michael Carr, Hugh Kieffer, Conway Leovy and Harold Masursky, to analyze the Mariner 9 images relevant to the future landing sites.

The Storm

When Mariner 9 arrived, Mars was entirely shrouded by a planet-wide dust storm. Almost nothing could be seen. But the spacecraft, controlled from Earth, could wait. This was exactly the case Sagan had in mind: an “adaptive” mission. In one of his 1976 articles he contrasted this success with the failure of the Soviet Mars 3, which reached Mars at the same time, in the midst of the storm: its whole program had been loaded into its computer before launch, and nothing could be changed as the extent of the storm became clear. Its lander set down, returned a twenty-second fragment of a featureless television picture, then fell silent. Winds of more than 200 miles an hour, revealed by Mariner 9, had probably tipped it over.

When the dust settled, Mariner 9 revealed a planet no one expected: immense volcanoes, a gigantic canyon system, thousands of sinuous channels resembling dried-up riverbeds. Schiaparelli’s and Lowell’s “canals” were not there. Sagan would write that the spacecraft, in orbit for a full year, had photographed the planet pole to pole with a discrimination of fine detail a hundred times better than the astronomers of Lowell’s time could have managed.

For his studies of Mars with Mariner 9, Sagan received the NASA Medal for Exceptional Scientific Achievement. Shortly afterward, in 1972 and 1973, the Pioneer 10 and 11 probes left for Jupiter and Saturn carrying an engraved plaque he had sponsored: a message for any intelligent beings who might one day find the craft. His wife Linda drew the man and woman on it.

Sources: English-language Wikipedia article “Mars and the Mind of Man” (panel of November 12, 1971 and participants; tertiary source); NASA SP-4212, chapter IX (Masursky and Sagan before the Mariner team, “adaptive mode,” data processing raised at Sagan’s request, site analysis team); C. Sagan, Six Articles, NASA, 1976: first, fourth and fifth articles (Mariner 9 a year in orbit, resolution a hundred times better, absent canals, channels, pre-programmed Mars 3, twenty seconds of picture, winds of more than 200 miles an hour, adaptive mode and Mariner 9’s discoveries) and NASA’s introduction (medal for Mariner 9, Pioneer plaques); D. Morrison, Biographical Memoir, NAS, 2014 (Mariner 9 team, plaque drawn by Linda Salzman Sagan). The 1971 storm and Mariner 9’s orbit insertion on November 14, 1971 belong to general history.

Chapter 9 · 1970 – 1975 Where to Land?

Choosing where to set a spacecraft down on Mars, without ever having seen the place up close, was one of the hardest problems of the Viking mission. From 1970 on, Carl Sagan was a member of the working group in charge of landing sites. David Morrison writes that he was one of three scientists, with the geologist Harold Masursky and the astronomer Tobias Owen, who made the critical recommendations on where the two landers should be targeted.

The Criteria

In December 1970 Sagan drafted “provisional criteria” for site selection. In them he flagged a danger the engineers had underestimated: their model predicted Martian winds of less than 90 meters per second, but newer calculations indicated the possibility of winds of 140 to 200 meters per second; if such winds were encountered during landing, the survivability of the spacecraft would be very much in question. He noted that the landing ellipse, the zone in which the craft might come down, then measured 400 by 840 kilometers. And he set down a rule of common sense: a lander “is not very useful even if it did crash in the most interesting part of the planet.” Before any lander was committed to a site, there had to be extensive Mariner 9 data, not only images; certification would require the sharper pictures of the Viking orbiter.

He listed his six favorite sites. He was particularly interested in the uncratered terrain photographed by Mariner 4: if it had been recently reworked, perhaps by tectonic activity, it was more likely to have had recent outgassing, and to interest both geologists and biologists.

The Pole or Safety

Some scientists wanted to send the second lander near the north pole, where there might be more water, and so a better chance of life. Sagan came down on the side of safety. On January 12, 1973, he wrote to the mission director, Tom Young: “When I total up the pros and cons I find that the scientific advantage of a polar landing site, while real, is far outweighed by the risks.” He put the chances of a successful polar landing at much less than fifty-fifty; he compared the risk to losing two hundred million dollars, the projected cost of a future probe to Jupiter and Saturn.

Radar and Quicksand

On February 3, 1973, he wrote to the project manager, James Martin, a letter in which NASA’s official history finds hyperbolic prose but specific steps. He denounced “serious shortcomings in the landing site selection procedures.” The orbiter’s photographs would not show the one-meter boulders that could tip a lander over; a surface smooth at the hundred-meter scale might be rough at ten centimeters. Some colleagues read areas that reflected radar poorly as sand or dune fields; he feared something else, and reminded Martin that the Soviets had suggested quicksand as the cause of Mars 3’s failure. He recommended serious theoretical work on radar, comparisons of Martian echoes with those from known terrestrial terrains, and support for the great radio telescopes at Arecibo, Haystack and Goldstone to examine Mars. Radar, properly understood, would in his view be more useful in site certification than all the photographs.

Martin and Young took his recommendations seriously. On March 23, 1973, Martin announced three actions, including nearly simultaneous radar observations of Mars from Arecibo, Goldstone and Haystack. Sagan, the official history writes, had once again acted as a catalyst.

Sources: NASA SP-4212, chapters IX and X (working group of 1970, “Provisional Criteria for Viking Landing Site Selection” of December 1970, winds of 140 to 200 m/s, 400 × 840 km ellipse, the sentence on the crashed lander, Mariner 71 data, six favorite sites, uncratered Mariner 4 terrain, letter to Young of January 12, 1973 and quotation, comparison with $200 million, letter to Martin of February 3, 1973, quicksand, recommendations, Martin’s actions of March 23, 1973, “catalyst”); D. Morrison, Biographical Memoir, NAS, 2014 (decisive role with Masursky and Owen).

Chapter 10 · Spring 1976 “Nobody Knows”

On April 30, 1976, NASA sent newspapers a series of six articles written by Carl Sagan, which any editor could print as they stood. The two Viking spacecraft were on their way to Mars. In them Sagan explained to the general public what the probes would do, why, and what could go wrong. These texts, rarely cited, show his manner better than any portrait.

The Telegram

He opened the series with a legend. Some fifty years earlier, he recounts, a celebrated newspaper publisher had wired a famous astronomer: “Wire collect immediately 500 words on whether there is life on Mars.” The astronomer dutifully replied 250 times: “Nobody knows.” And no one, Sagan adds, paid any attention to this confession of ignorance: ever since, we have heard authoritative pronouncements from those who think they have deduced life on Mars and from those who think they have excluded it.

He then told, in a few lines, the story of Schiaparelli’s and Lowell’s canals, and their end: they do not exist, and the most generous assessment is to put them down to the imprecision of the human hand-eye-brain combination. But life was not excluded for all that. Terrestrial microbes could survive on Mars. And nothing, in his view, ruled out even organisms large enough to be seen. He offered a thought experiment: had Mariner 9 orbited the Earth a hundred thousand years ago, before cities and fields, it would have photographed no clear signs of life whatever.

The Machines

He described the landers as hands and eyes, ears and noses and mouths set down on Mars and connected to us by a radio link tens of millions of miles long. He imagined the evening weather report in the summer of 1976, with the temperatures at Chryse and Cydonia after those at Needles and Duluth. He explained the three biology experiments, one of which offers the Martian soil a nutrient broth labeled with radioactive carbon, in the hope that the little Martians would like to eat it, while admitting they might find it unpalatable and meanwhile be placidly munching on the spacecraft’s paint. The cameras, for their part, make no assumptions about the chemistry of life: if organisms ranging in size “from ants to giraffes” passed by, they would have a fair chance of seeing them.

Calamity and Catastrophe

The fourth article was about danger. He quoted the British prime minister Benjamin Disraeli, asked the difference between a calamity and a catastrophe: if his opponent Gladstone fell into the Thames, that would be a calamity; if someone pulled him out, that would be a catastrophe. If Viking failed to find life because it did not land in a wettish place, Sagan concluded, that would be a calamity. If Viking failed to return any data at all, that would be a catastrophe.

Viking in Reverse

The last article turned the situation around. Imagine Martians sending a Viking to Earth, to the same coordinates as the sites chosen on Mars. The first would land in the middle of the Atlantic and sink, “blub, blub, blub,” and would be considered a mysterious failure. The backup site would be in the Gulf of Tonkin. The second prime site would be in the North Atlantic. But the second backup site would be at the southwest entrance to Yellowstone National Park: there one would find not only microorganisms but bears, rangers and tourists. Two points on a planet, he concluded, are not enough to know it.

He confessed his nightmares. In one, footprints appear around the lander every morning, but no one is ever seen making them: a searchlight for night-time television would have been needed, and it cost too much. In the other, a purple geometric patch appears a little more than three meters from the craft, just beyond the reach of its arm. Viking does not move. The next step, he wrote, must be a rover that would choose each day where to go, and that the whole world would follow. He also warned of the danger of bringing Martian samples back without precautions: no one knew whether any Martian microbes might be dangerous on Earth. And he closed by recalling that 1976 marked the five-hundredth anniversary of a young Genoese sailor’s first glimpse of the Atlantic, Christopher Columbus; Viking, if it succeeded, would count for as much as his three ships.

Sources: C. Sagan, Six Articles by Carl Sagan, NASA News Release 76-74, April 30, 1976, revised May 10, 1976 (NASA Technical Reports Server 19760069204): “The Search for Life on Mars” (telegram, canals, macrobes, Mariner 9 around the Earth), “Viking and Life on Mars” (hands and eyes, weather report, experiments, paint, ants and giraffes), “Landing on the Planet Mars” (Disraeli), “The Future Exploration of Mars” (Viking in reverse, nightmares, rover, back contamination, Columbus). These texts are summarized and quoted only briefly. The Columbus anniversary is Sagan’s; we report it without discussion.

Chapter 11 · June – July 1976 The Pink Sky of Chryse

On June 19, 1976, Viking 1 went into orbit around Mars. The landing was planned for July 4, the bicentennial of American independence. But the first photographs of the chosen site, in the plain of Chryse, showed rougher terrain than expected. There began, at the Jet Propulsion Laboratory in Pasadena, a series of tense meetings in which dozens of specialists had to decide, from photo mosaics barely pasted together, where to risk a spacecraft costing almost a billion dollars.

Abstaining

In one vote of the landing site group, thirty specialists favored a site to the northwest of the planned zone, two another site. Three abstained, Carl Sagan, the radar specialist Len Tyler and the geologist Henry Moore, because there were not enough data to make a decision. Under questioning, Sagan said that if he had to, he would vote for science. Later, as the group worried about the Arecibo radar observations due on July 4 and 5, he distinguished two possible situations: good-quality pictures with mediocre radar, or good-quality radar with mediocre pictures. For safety, he wanted both of good quality.

The July 4 landing was postponed. Viking 1 finally set down in the plain of Chryse on July 20, 1976. It was the first spacecraft to operate for any length of time on the surface of Mars. The project scientist, Gerald Soffen, had formed a small advisory group, Joshua Lederberg, Bradford Smith, Tobias Owen and Carl Sagan, to follow the deliberations; but, NASA’s historians note, events moved too fast; there was no time to reflect.

The Sky

Sagan was a member of the lander camera team. When the first color photograph came in, a geologist on the team, Tim Mutch, enthused publicly over a light blue sky and a reddish ground, a surprisingly terrestrial-like desert scene. But to borrow Carl Sagan’s phrase, write NASA’s historians, to see the picture in terms of deserts on our own planet was an “Earth chauvinism”: the photo was of Mars, and the sky should have been red. When James Pollack of the imaging team told a press conference on July 21 that the Martian sky was pink, he was greeted with some friendly boos and hisses. The next day Sagan chided the reporters in his own way: “The sort of boos given to Jerry Pollack’s pronouncement about a pink sky reflects our wish for Mars to be just like the Earth.”

A Manager Who Listened

Afterward Sagan paid an unexpected tribute to the project manager, James Martin. He had been prepared, he said, for resistance when he proposed postponing the landing or taking a closer look at the radar data. Martin had instead been “remarkably willing to listen.” “It sounds like a reasonable thing for a project manager to do,” he added, “but that’s not always been the case in past missions.”

Sources: NASA SP-4212, chapters X and XI (the vote, abstentions of Sagan, Tyler and Moore, “vote for science,” demand for good radar and good pictures, Soffen’s advisory group, Mutch’s reaction, “Earth chauvinism,” Pollack’s press conference of July 21 and Sagan’s reply on the 22nd, interview of September 15, 1976 about Martin); C. Sagan, Six Articles, NASA, 1976 (planned date of July 4, plain of Chryse, cost of almost a billion dollars); NASA SP-4212, appendix D (Sagan on the lander imaging team). The dates of orbit insertion (June 19) and landing (July 20, 1976) are given by Sagan and by the general history of Viking; see also the history of NASA on this site.

Chapter 12 · 1976 – 1977 Neither Yes Nor No

Both Viking landers worked. Viking 2 set down in turn, in September 1976, in the plain of Utopia. The arms dug, the ovens heated, the biology experiments ran one after another. And the results said neither yes nor no.

Reactions Without Life?

Some biology experiments produced reactions that, on Earth, would have been attributed to microbes. But the chemical analysis instrument, designed to detect organic molecules at concentrations of one part in a million, found practically none. For the project scientist, Gerald Soffen, positive biology results could not be interpreted as indicating life in the absence of organics. He reluctantly concluded that life on Mars was unlikely. Most scientists attribute the observed reactions to an unusual, highly oxidizing chemistry of the Martian soil.

Others argued that Viking might have landed in the wrong places. Nearer the poles, where soil and air are moister, life might exist. Or, as Carl Sagan and Joshua Lederberg had suggested as early as 1962, there might be Martian microenvironments, small oasis-like areas where life had evolved and survived. Soffen thought this unlikely: winds and dust storms would have spread any organic material all over the planet. The apparent absence of life deeply divided the biology team.

No Giraffes

The cameras, on whose team Sagan served, showed no giraffes or ants, no footprints: rocks, sand, a pinkish sky. Sagan had warned, before the landing, that Viking was not a definitive search for life on Mars but a significant first step. He had also written that if, after a long series of post-Viking investigations, no sign of life was found, we would have discovered something of considerable importance about the unlikelihood of life, a finding that would underscore the preciousness and rarity of what has happened here on our small Earth.

The rest of the Viking program proved him right on another point. The orbiters photographed the planet for years and provided definitive support for his windblown-dust hypothesis. The landers measured the composition of the atmosphere and the gas argon, which, Sagan had explained, would tell whether Mars had once had a dense atmosphere. The debate about life, for its part, would remain open for decades.

Sources: NASA SP-4212, chapter XI (Soffen’s position on organics, his conclusion, the wrong-site hypothesis, Sagan and Lederberg’s microenvironments, Soffen’s objection, divisions in the biology team); C. Sagan, Six Articles, NASA, 1976 (sensitivity of one part in a million, “significant first step,” the sentence on the rarity of life, argon); D. Morrison, Biographical Memoir, NAS, 2014 (confirmation of the dust hypothesis by the orbiters). Viking 2’s landing in Utopia Planitia in September 1976 and the majority interpretation of an oxidizing chemistry belong to the general history of Viking.

Chapter 13 · 1972 – 1979 A Record for the Stars

In 1977 two Voyager spacecraft left for the giant planets, and beyond, out of the solar system. Each carried a golden record. Carl Sagan led the team that designed it. It holds photographs and sounds of Earth, and music: Bach, Beethoven, Chuck Berry. Sagan told the story of the record in a book published in 1978, Murmurs of Earth, written with, among others, his wife Linda and the novelist Ann Druyan.

Messages in a Bottle

It was the second time. In 1972 and 1973 the Pioneer 10 and 11 probes had left with the engraved plaque whose design he had sponsored and which Linda had drawn. These four craft are the first human objects to leave the solar system entirely. No one knows whether anyone will ever find them.

The Most Famous Scientist in the World

These years made him a celebrity. His first major book for the public, The Cosmic Connection, appeared in 1973. A reviewer compared it to a collection of dinner conversations with Sagan; that was no coincidence, Morrison relates: he had dictated the first draft on a weeklong cross-country drive. He dictated all his books and articles, which gave his writing the tone of conversation, and his talks the organization of written text.

His next book, The Dragons of Eden, on the evolution of human intelligence, won the Pulitzer Prize (in 1977 according to Morrison, in 1978 according to Wikipedia). He became a regular guest on Johnny Carson’s Tonight Show. In 1977 Newsweek put him on its cover and presented him as “the leading spokesman and salesman for the new science of exobiology.” A phrase was attributed to him that he never said, by his own account and Morrison’s: “billions and billions,” which came from a Carson parody. He ended up using it as the title of his last book.

That same year, 1977, NASA awarded him its Distinguished Public Service Medal. He did not neglect science: from 1972 to 1981 he was associate director of Cornell’s Center for Radiophysics and Space Research; in 1976 he became the David Duncan Professor of Astronomy and Space Sciences, a chair he held until his death.

Sources: D. Morrison, Biographical Memoir, NAS, 2014 (Voyager record and the team led by Sagan, Murmurs of Earth and its coauthors, plaque drawn by Linda Salzman, the four first craft to leave the solar system, Cosmic Connection dictated, dictation method, Pulitzer in 1977, Carson, Newsweek cover and quotation, “billions and billions”); English-language Wikipedia article “Carl Sagan” (music on the record, Pulitzer in 1978, NASA medal in 1977, Cornell posts; tertiary source). Morrison describes the plaque as a coded greeting from humans to any alien species that might one day find the craft.

Chapter 14 · 1978 – 1985 Cosmos

In 1978 Carl Sagan moved to Los Angeles. He wanted to follow closely the Voyager encounters with Jupiter and Saturn, and he began producing a thirteen-hour series for American public television. It was called Cosmos. He wrote it with Ann Druyan. It was broadcast in 1980.

Hundreds of Millions of Viewers

The success was immense. More than four hundred million people in dozens of countries would see the series according to Morrison; at least five hundred million in sixty countries according to Wikipedia. It won two Emmys and a Peabody Award. The companion book stayed on the New York Times bestseller list for seventy weeks and made Sagan wealthy as well as famous. Time put him on its cover in 1980 and called him the “prince of popularizers.” His face became one of the best known in the world. Morrison relates that a Texas state trooper once tore up his speeding ticket on learning he had been Sagan’s student, and launched into an enthusiastic discussion of Cosmos.

Ann Druyan

After an acrimonious divorce from Linda Salzman, he married Ann Druyan, whom Morrison calls the love of his life; in 1980 according to Morrison, in June 1981 according to Wikipedia. They settled in Ithaca in a house on the edge of a cliff, modeled on an Egyptian temple. They had two children, Alexandra, known as Sasha, and Samuel. Ann Druyan, Morrison writes, made him more interested in philosophy and international politics.

The Planetary Society

In 1980, with Bruce Murray, director of the Jet Propulsion Laboratory, Sagan founded the Planetary Society, a public-membership organization supporting planetary exploration. He was its first president. The idea was simple: if space exploration depends on public money, the public must understand it and ask for it. Sagan often said so: since science is largely paid for by the public, the public has a right to know how the money is spent; and sharing the excitement of discovery is a joy in itself.

In 1985 he published a novel, Contact, the story of an astronomer who picks up a signal from the star Vega. To send his heroine on her journey, he asked his friend the physicist Kip Thorne for advice on “wormholes,” hypothetical shortcuts through space-time; the question led Thorne to original research. The novel became a film in 1997.

Sources: D. Morrison, Biographical Memoir, NAS, 2014 (Los Angeles in 1978, Voyager, 13-hour series, more than 400 million viewers, the book 70 weeks on the New York Times list, wealth, Time cover and “prince of popularizers,” the speeding ticket, divorce, marriage to Ann Druyan in 1980, house modeled on an Egyptian temple, children, Ann Druyan’s influence, Planetary Society with Bruce Murray, Contact in 1985 and the 1997 film); English-language Wikipedia article “Carl Sagan” (co-writing with Ann Druyan, 500 million viewers in 60 countries, Emmys and Peabody, marriage in June 1981, cliffside house, first president of the Planetary Society, reasons to popularize, Kip Thorne and wormholes; tertiary source). Bruce Murray’s post as director of JPL from 1976 to 1982 belongs to general history.

Chapter 15 · 1983 – 1990 From Martian Dust to Nuclear Winter

One of the most consequential ideas of Carl Sagan’s life was born on Mars. With James Pollack, Brian Toon, Peter Gierasch and Richard Turco, he had studied how small Martian dust storms can grow rapidly to cover the whole planet. These models of dust transport and atmospheric circulation had then been used on Earth to understand how ash from great volcanic eruptions spreads through the upper atmosphere and cools the climate.

Nuclear Winter

Applied to the soot from the fires of cities struck by nuclear weapons, the same models led in 1983 to a frightening conclusion. A full-scale nuclear war between the United States and the Soviet Union would plunge the Earth into lasting darkness and cold. This is “nuclear winter,” described in a 1983 article by Turco, Toon, Ackerman, Pollack and Sagan, known by their initials as TTAPS. The term was coined by Turco according to Wikipedia, by Sagan according to Morrison.

The hypothesis set off fierce disputes. The physicist Edward Teller, father of the American hydrogen bomb, ended up calling Sagan an excellent propagandist. Morrison thinks these debates may have contributed to the end of the Cold War, as Sagan believed, but that they also made him many enemies. His biographers judge that, scientifically, this was a low point of his career, and that a very uncertain hypothesis had been spread too quickly through the media.

Activist

Sagan opposed the arms race and President Reagan’s missile-defense project, “Star Wars,” which he judged technically unachievable and destabilizing. In 1986 and 1987 he was arrested twice climbing the fence of the Nevada nuclear test site, during protests against American tests. A colleague on the Galileo spacecraft team recalls that at the end of a meeting, when each member described his plans for the coming year, Sagan explained that he could no longer work on Galileo: all his time was committed to “saving the world from nuclear war.” His teammates conceded that this was indeed a worthwhile objective.

Mars, Together

His most ambitious idea linked his two battles. With Roald Sagdeev, director of the Space Research Institute in Moscow, he promoted a joint U.S.-Soviet crewed mission to Mars. He saw in it, Morrison writes, both a way to defuse the arms race and the fulfillment of his dream: the exploration of the planet most likely to harbor life. The collapse of the Soviet Union ended that dream. At home, after the explosion of the space shuttle Challenger in 1986, NASA’s robotic missions were cut back.

Sources: D. Morrison, Biographical Memoir, NAS, 2014 (Martian dust storms with Pollack, Toon, Gierasch and Turco, application to volcanoes then to soot, nuclear winter of 1983, the term attributed to Sagan, effects on the Cold War and enemies, opposition to “Star Wars” and debates with Teller, the Galileo anecdote, the joint mission with Sagdeev, end of the dream, cuts after Challenger; bibliography: Science 222, 1983); English-language Wikipedia article “Carl Sagan” (TTAPS, term coined by Turco, Teller’s remark, the biographers’ judgment, arguments against “Star Wars,” arrests in Nevada in 1986-1987; tertiary source). Edward Teller’s titles belong to general history.

Chapter 16 · 1990 – 1995 A Pale Blue Dot

On February 14, 1990, the Voyager 1 spacecraft, more than six billion kilometers away, turned around and photographed the planets of the solar system. In one of the images, the Earth takes up less than a pixel: a tiny dot in a band of sunlight scattered in the camera. That photograph exists because Carl Sagan had asked for it.

Ten Years of Insistence

Sagan had proposed the idea as early as 1980, when Voyager 1 passed Saturn and completed its primary mission. He acknowledged that such a picture would have little scientific value: the Earth would be too small for any detail to be made out. But it would offer, he thought, a perspective on humanity’s place in the universe. There were fears that pointing the camera so close to the Sun would damage it. It was not until 1989 that the idea was set in motion, and the NASA administrator, Richard Truly, had to intervene for the picture to be taken. Sagan made it the subject and title of a 1994 book, Pale Blue Dot.

The Academy

In 1992 his nomination to the U.S. National Academy of Sciences was rejected. According to Wikipedia, his media activities had made him unpopular with many scientists. Two years later, in 1994, the same Academy awarded him its highest honor, the Public Welfare Medal, for his popularization of science as well as his research.

A Candle in the Dark

Sagan worried about the rise of anti-science attitudes. In 1995 he published The Demon-Haunted World: Science as a Candle in the Dark. Morrison counts it among his finest books. It defends a simple idea: pseudo-science flourishes in exact proportion as real science is misunderstood; science is far from a perfect instrument of knowledge, but it is the best we have. In it he popularized a “baloney detection kit,” a set of tools for examining a claim, and a maxim he had made famous: extraordinary claims require extraordinary evidence.

Asteroids

In the early 1990s he joined another debate. He supported the search for asteroids that might one day strike the Earth, but opposed military proposals, American and Russian, to develop nuclear weapons to destroy or deflect them. In 1994, with Steven Ostro, he published in Nature a short paper on the “dangers of asteroid deflection”: whoever can steer an asteroid away from the Earth can also steer one toward it. At a major meeting in Tucson that same year, Teller refused to sign a joint communiqué with him.

Sources: English-language Wikipedia article “Pale Blue Dot” (date, distance, size of the Earth in the image, Sagan’s proposal in 1980, low scientific value, fears for the camera, 1989, Richard Truly’s intervention, the 1994 book; tertiary source); D. Morrison, Biographical Memoir, NAS, 2014 (rejection of 1992, medal of 1994, The Demon-Haunted World and its central idea, asteroids, the 1994 paper with Ostro, the Tucson meeting and Teller’s refusal); English-language Wikipedia article “Carl Sagan” (reason for the Academy rejection, “baloney detection kit,” maxim on extraordinary claims, deflection dilemma; tertiary source). The ideas of Sagan’s books are summarized, not quoted.

Chapter 17 · 1994 – 1997 The Carl Sagan Station

For the last two years of his life, Carl Sagan fought a rare bone-marrow disease, myelodysplasia. He received three bone-marrow transplants from his sister, Carol. He kept working. At the time of his death, Morrison writes, he was deeply involved in the filming of his novel Contact, he had just joined the board of trustees of the SETI Institute, which searches for extraterrestrial signals, and he was working on joint publications with his last graduate student, Christopher Chyba.

December 20, 1996

He died of pneumonia on December 20, 1996, at the Fred Hutchinson Cancer Research Center in Seattle. He was sixty-two. He was buried in Lake View Cemetery in Ithaca. Among the most touching tributes, Morrison notes, were those from people who had never met him but who credited him with transforming their lives. His last book, Billions and Billions, appeared in 1997, after his death.

On Mars

On July 4, 1997, seven months after his death, the American Mars Pathfinder spacecraft landed on Mars, in the Ares Vallis region. It carried a small wheeled robot, Sojourner, the first mobile vehicle to roll on another planet: exactly what Sagan had called for in 1976. The next day, July 5, the lander was renamed the Carl Sagan Memorial Station.

The film Contact, released the same year, ends with a dedication: “For Carl.” An asteroid, number 2709, bears his name. NASA’s Ames Research Center dedicated in 2001, on what would have been his sixty-seventh birthday, the site of a center for the study of life in the cosmos named after him.

His former student Steven Squyres would lead the science team of the two robots Spirit and Opportunity, which landed on Mars in January 2004 and roamed it for years, searching for traces of past water.

Sources: D. Morrison, Biographical Memoir, NAS, 2014 (myelodysplasia for two years, work in progress at his death, SETI Institute, Christopher Chyba, tributes, Carl Sagan Memorial Station); English-language Wikipedia article “Carl Sagan” (three transplants from his sister, death from pneumonia on December 20, 1996 at the Fred Hutchinson Cancer Research Center, age 62, Lake View Cemetery, renaming on July 5, 1997, Contact dedication, asteroid 2709, Ames center in 2001, Squyres and the robots; tertiary source); English-language Wikipedia article “Mars Pathfinder” (Carl Sagan station; tertiary source). His sister’s first name (Carol) is given by Wikipedia; Pathfinder’s landing date (July 4, 1997), Ares Vallis, Sojourner and the dates of Spirit and Opportunity belong to the general history of Mars exploration; see also the biographies of Rob Manning and Steve Squyres on this site.

Chapter 18 · After 1996 What He Did for Mars

Carl Sagan discovered no moon, no volcano and no trace of life on Mars. Yet his Martian work is among the most important of the twentieth century, because it changed the way we go to look. Taken in order, it shows that he almost always pushed in the same direction.

Undoing the Illusions

First he helped undo the last illusions inherited from Lowell. With James Pollack he showed that the “wave of darkening,” the last great argument for Martian vegetation, could be explained by dust and wind, which the Viking orbiters confirmed. He gave a name, “Earth chauvinism,” to the error of imagining other worlds on the model of our own, and he recalled it publicly even before the pink sky of Chryse.

Keeping the Question Open

But he never concluded too quickly in the other direction. In 1962, with Lederberg, he proposed that life might survive in small oases. In 1976 he refused to exclude organisms visible to the naked eye, for lack of evidence against them. After Viking he recalled that two landing points are not enough to know a planet. His position fits in the telegram he liked to tell about: until one has gone to look, the only honest answer is “nobody knows.”

Preparing the Missions

He shaped the very design of missions. He called for the “adaptive mode,” the ability to change plans in light of what is discovered, for Mariner 9 as for Viking. He warned about Martian winds, about boulders invisible to the cameras, about the role of radar. He preferred safety to the temptation of a richer but riskier polar site. He called, as early as 1976, for mobile vehicles that would choose their route each day: twenty-one years later, Sojourner was rolling near the station that bears his name; twenty-eight years later, two robots led by his former student Steven Squyres were crossing the planet. And he warned against contaminating Mars with terrestrial microbes, and the Earth with possible Martian ones, questions at the heart of the “planetary protection” rules that govern sample-return missions today.

Making People Want the Journey

Finally, and above all, he made people want the journey. Through his articles, his books, the Planetary Society and Cosmos, he convinced hundreds of millions of people that planets are places, and that their exploration concerns them. He thought that people of a distant future would look back on the Viking era the way Americans look back on the first voyages to the New World. He also thought, with Sagdeev, that Mars could be a common project rather than a race.

Sources: synthesis of the preceding chapters, whose sources are given there: D. Morrison, Biographical Memoir, NAS, 2014; NASA SP-4212; C. Sagan, Six Articles, NASA, 1976; English-language Wikipedia articles. The notion of “planetary protection” and its link with sample return belong to general space culture.