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A Tribute to Carl Sagan:
Carl Sagan & Edward Teller

An Uneasy Alliance Over
Defending the Earth
David Morrison

Dr. David Morrison is a senior scientist at the NASA Astrobiology Institute. He received his Ph.D. in Astronomy from Harvard University, and until he joined NASA he was Professor of Astronomy at the University of Hawaii. He is a recent recipient of the Carl Sagan Medal of the American Astronomical Society for contributions to public understanding of science.

Carl Sagan and Edward Teller were bitter opponents in national security debates about issues such as “Star Wars” and nuclear test bans, but ironically they agreed on defending the Earth against asteroids — an agreement that neither, however, was ready to admit in public. They drew very different conclusions from the impact threat — Sagan saw it is a justification for space exploration, Teller as a reason to build bigger nuclear bombs.

As a consequence of the astounding success of the Cosmos television series, as well as his bestselling books and cover stories in Newsweek and Time, Sagan achieved a celebrity in the 1980s enjoyed by few academics. His fame provided a platform for public opposition to President Reagan’s Strategic Defense Initiative (SDI) or “Star Wars,” and he rallied objections from the academic community that questioned both the technical basis for SDI and its potential destabilizing effect on the balance of nuclear deterrence. In 1982, an additional opportunity presented itself to campaign for nuclear disarmament, thanks to research involving two of his former students, Brian Toon and Jim Pollack.

Toon and Pollack, who had developed atmospheric circulation models to analyze global dust storms on Mars, realized that smoke from petrochemical fires would have a much greater effect on global climate than naturally occurring dust. Sagan joined in a collaboration that generated the now-famous TTAPS paper on nuclear winter published in 1983 (Turco,R.P., Toon, A.B., Ackerman,T.P., Pollack, J.B., Sagan,C. [TTAPS]) 1990. “Climate and Smoke: An Appraisal of Nuclear Winter,” Science, volume 247, pp. 167–168, January). The TTAPS authors concluded that even a less-than-full-scale nuclear exchange, especially if directed against cities, could cause global cooling and collapse of agriculture.

Sagan argued that these new findings rendered nuclear war obsolete. But the pro-nuclear forces in the United States counter-attacked vigorously, vilifying Sagan in the process. The National Review called nuclear winter “a fraud” and titled one cover story “Flat-Earth Sagan Falls off the End of the World.” Edward Teller, who at age 73 was prehaps the second best known scientist in the U.S., debated Sagan on nuclear winter before a special session of Congress. These confrontations generated deep personal animosity between them. Years later Teller told me about an airport breakfast that he and Sagan shared at this time, where (to Teller’s obvious distaste) three strangers came up to ask Sagan for his autograph, but no one seemed to recognize Teller.

The early 1980s also saw the revolutionary research by Luis and Walter Alvarez that showed the end-Cretaceous mass extinction of 65 million years ago to be the result of an asteroid or comet impact. In addition to its profound effect on geology, paleontology, and evolutionary theory, the impact hypothesis stimulated new thinking about the contemporary hazard from asteroid impacts. Toon used the same basic software developed for nuclear winter to determine the threshold at which the consequences of an impact became global, triggering what is sometimes called an “impact winter.”

The impact hazard attracted the interest of the nuclear defense communities in both the U.S. and U.S.S.R., including Edward Teller. In 1994 this shared U.S./U.S.S.R. interest in asteroid defenses provided the occasion for Teller to visit one of the closed “nuclear cities” in Russia — Chelyabinsk 70 — an experience that I shared with him. By this time, Teller had become a public advocate for developing and testing asteroid defense systems, including nuclear options. Sagan was also concerned about the impact danger, noting that this was the greatest known natural hazard, capable of destroying an intelligent species just as easily as it had the dinosaurs. But he was equally concerned about the risks inherent in developing a nuclear defense against asteroids.

To illustrate this concern, Sagan turned to the ancient history of the Greek city-states in Sicily, writing about the “Marsh of Camarina” in Parade magazine — read by millions of Americans — and in a chapter in his 1994 book, Pale Blue Dot. To illustrate the danger of unintended consequences, he told the story of how the Camarinans drained their marsh to improve public health, only to be wiped out when their enemies realized that the marsh that had protected the Camarinans from their enemies no longer existed. These considerations led to what Sagan called the “asteroid deflection dilemma”, asking whether the danger of developing the technology to alter asteroid orbits for protection of the Earth might not provide a super weapon that could someday be turned against us.

Sagan saw no way out of this dilemma other than to defer asteroid defense to a future time when we might be better able to handle the responsibility. He concluded his essay thus: “Since hazards from asteroids and comets must apply to inhabited planets all over the Galaxy, if there are such, intelligent beings everywhere will have to unify their home worlds politically, leave their planets, and move small nearby worlds around. Their eventual choice, as ours, is spaceflight or extinction.”

By 1993, public and scientific interest in this “new” natural hazard had reached a point where Tom Gehrels, an early asteroid researcher at the University of Arizona, felt ready to organize a major international scientific meeting on the subject. Gehrels invited both Sagan and Teller to speak (albeit in separate sessions). Sagan remarked to me at the time how curious it was that he and Teller were on the same side of this issue, although he continued to express concern about the defection dilemma while Teller still advocated testing nuclear asteroid defenses. Both agreed, however, on the reality of the impact hazard and the need for action, beginning with a survey of potentially hazardous asteroids. Sagan suggested that this partial reconciliation was a reflection of the end of the cold war. Teller, for his part, refused to meet with Sagan, and he rejected the suggestion of the conference organizers that there be a joint Sagan-Teller paper in the published book Hazards Due to Comets and Asteroids. In the end, I was nominated to write the joint paper with Teller — but that is another story.

Sagan and Teller are both gone — two of the best-known and most influential scientists of the second half of the 20th century. Both sought and sometimes succeeded in influencing public policy in the nuclear confrontation between the U.S. and the U.S.S.R. Each accepted some credit for the end of the cold war — Teller for pursuing the defense buildup (including “Star Wars”) that bankrupted the U.S.S.R., Sagan for showing the futility of nuclear war due to nuclear winter. They agreed in part on the impact hazard, but were never reconciled on more basic issues of war and peace.

This article can be found in
Skeptic volume 13 number 1

volume 13 number 1
The Legacy of Carl Sagan

this issue includes: An Interview with Ann Druyan; Science, Religion & Human Purpose; An excerpt from Conversations with Carl; Tributes to Carl Sagan…
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