PDF Summary:Cosmos, by Carl Sagan

What’s out there in the vast reaches of space? Are we alone in the universe? Renowned astronomer Carl Sagan offers some insight into these questions and many more in his widely acclaimed book, Cosmos.

Sagan is a Pulitzer Prize-winning author and perhaps the greatest astronomer of our age. In this book, he provides a comprehensive description of the science, philosophy, and history of astronomical discovery, from ancient Ionia to the time of the book’s publication in 1980.

In this guide, we’ll delve into Sagan’s theories about the possibility of interstellar travel, the search for extraterrestrial life, and man’s quest for understanding the mysteries of the universe. As we go along, we’ll update the information with the most recent scientific discoveries and provide perspectives from other experts in related fields.

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Scientists in this period also proposed that the orbits of the planets were elliptical rather than circular, and that gravitational forces were at work in their orbiting of the sun.

(Shortform note: Sagan takes a Western historical perspective in the timeline he presents. In fact, the earliest known suggestion that the Earth revolves around the sun comes from an ancient Vedic text from India. An Indian astronomer named Yajnavalkya, proposed the heliocentric model of the universe around 800 BC, more than 500 years before Aristarchus. In that text, he states, "The sun strings these worlds—the earth, the planets, the atmosphere—to himself on a thread." It’s unclear why Sagan doesn’t acknowledge this, which leaves us to question whether it’s because of a bias toward Western history or if he was unaware (or skeptical) of this text.)

According to Sagan, Aristarchus’ “great legacy” is the argument that we’re not special. He argued that humans are not the center of the universe, nor is our planet. This idea can be extended to many aspects of life—Aristarchus challenged the prevailing anthropocentric (“human-centered”) worldview. Sagan explains that this was a bold challenge to the social order, as it conflicted with a religious worldview that asserted that humans are uniquely created by the Gods, as well as with the idea of natural social hierarchies.

(Shortform note: Challenging an anthropocentric worldview isn’t necessarily in contradiction with all religions. Most indigenous religions are animistic belief systems that see humans as a part of the natural world just like everything else. These belief systems don’t assume humans are superior to, or have dominion over, anything else in nature, and put a strong emphasis on respect and honor for all non-human beings.)

Alexandria

By the 3rd century AD Alexandria, Egypt had become the new center of scientific inquiry in the West. Around 285 BC, the Great Library of Alexandria was established as the world’s greatest repository of scientific knowledge and literature, though Sagan laments that most of the documents were lost when it was destroyed. He says that he has often wondered where we might be today (scientifically) if all of that knowledge hadn’t been lost.

(Shortform note: The Great Library of Alexandria is believed to have contained in its collections over half a million written works on every subject of human knowledge and inquiry, from cultures across the world. Some scholars believe the Library was burned to the ground in 48 BC during Julius Caesar’s occupation of Alexandria, while others believe it suffered a more gradual destruction due to long periods of political unrest in the area. There is evidence that the buildings were converted into churches and mosques at different periods. Whatever the case, the vast majority of the Library’s contents have been lost to history.)

Sagan explains that it was in Alexandria that Eratosthenes (276-194 BC) made the discovery that the Earth was round. And although this was resisted by many at the time, he points out that it was crucial knowledge for an accurate scientific understanding of how the celestial bodies operate, as well as solidifying a view of Earth as one among other spherical celestial bodies. This also paved the way for explorers who would brave the circumnavigation of the globe centuries later.

However, Sagan explains that much of the astronomical theory coming out of this time period was rejected in favor of the more-accepted Ptolemaic worldview. Ptolemy (100-170 AD), maintained that the Earth was the center of the universe, and the sun, moon, other planets, and stars moved around it, attached to invisible spheres. This model, Sagan says, was ultimately supported by the Church in the coming centuries, and because of that institution's powerful reach, the Ptolemaic model lingered through the Middle Ages.

Astronomy During the Middle Ages

Sagan’s timeline skips from around the time of Ptolemy to the 16th century. The large gap in history is often attributed to the suppression of science in the Western world by the Roman Catholic Church throughout the Middle Ages (5th-15th centuries).

However, there’s ample documentation of important astronomical research during this historical period, especially in the Islamic world. Islamic scholars were particularly interested in accurate astronomical explanations because the Qur’an instructs Muslims to pray five times a day facing toward the city of Mecca (the birthplace of Muhammad), as well as giving instructions for fasting that require knowing the exact times of sunrise and sunset.

European Renaissance

When Europe began to emerge from what is known as the Dark Ages, the astronomical models we know to be correct today were developed. Here we’ll look at a brief timeline of discovery, as presented by Sagan, during the 16th and 17th centuries in Europe.

(Shortform note: The concept of the “Dark Ages” came from Renaissance-era scholars who believed ancient Greece was the pinnacle, up to that point, of human intellectual achievement, and that the subsequent historical era dominated by the Church in Europe was antithetical to progress. Sagan’s timeline in Cosmos contributed to solidifying this view in the popular imagination of the late 20th century. However, although it’s clear that the Church had a negative impact on scientific research, most scholars don’t use the term “Dark Ages'' anymore, and consider the concept itself to be a “myth.”)

In 1543, Polish mathematician and astronomer Nicolaus Copernicus proposed his heliocentric model. In 1616 the Catholic Church placed Copernicus’ work on their banned books list, where it remained until 1835.

In 1571, German astronomer Johannes Kepler also came out in support of the Copernican heliocentric model. According to Sagan, Kepler was a religious man, but he thought a heliocentric model was more in line with religious beliefs. In his view, the sun was a metaphor for God, around which everything revolves. Sagan says that Kepler also calculated the elliptical orbits of the planets to explain their previously unexplainable movements. Because of these theories, Kepler was excommunicated by the Lutheran Church.

In the late 1500s, Italian astronomer Galileo Galilei made advancements to the existing telescope technology. Due to his more powerful designs, he was able to discover the four largest moons of Jupiter, and the craters of the moon. Galileo also adhered to the heliocentric model.

(Shortform note: In 1633 Galileo was investigated and tried by the Roman Inquisition, charged with heresy, forced to recant, and spent the last decade of his life on house arrest. While in there, however, he wrote some of his most important work.)

In the 1600s, Isaac Newton discovered the law of gravity, which Sagan tells us was able to solidify the explanation of how the planets revolve around the sun, and the moons around the planets. Sagan says that Newton’s discoveries and the laws of nature derived from them, laid the foundation for our current understanding of the universe.

Absence of Evidence Is Not Evidence of Absence

Sagan points out that it was actually quite rational to believe that the Earth is flat and that the sun revolves around us, because from our perspectives that’s exactly how it appears. The idea that the Earth is round, rotating, and orbiting the sun is contrary to what our sense perceptions tell us. So, even setting religious beliefs aside, it’s natural that humans strongly resisted these ideas for a long time, especially in the absence of the kind of technology we have now—for example, satellite photos.

This stresses the importance, though, of not conflating “absence of evidence” with “evidence of absence.” In other words, being unable to prove something exists is not the same as being able to prove it doesn’t exist. It was only the unrelenting persistence of these scientists in their attempts to explain why the planets moved irregularly relative to the stars that led to their revolutionary discoveries.

Part 3: Exploration of Space

Now that we have a solid working model of the universe, and have been able to directly observe much of it via telescopes, we move into the age of physical exploration. We’ll begin with discussing what modern astronomers have discovered about the celestial bodies in our own solar system, then look at the potential that lies beyond our solar system. Sagan offers some insightful views on the potential for interstellar travel and for discovery of life on other worlds.

Our Solar System

On July 20th, 1969, humans made the first trip to a destination off the planet Earth and set foot on the moon. The moon is the nearest celestial body to us, and it remains the only one humans have ever traveled to. However, Sagan explains that we’ve been able to create vehicles that can travel much further, enduring the inhospitable climates elsewhere in the solar system, and send information to us.

At present, Sagan says that none of the atmospheres and climates of the other bodies in our own solar system are inhabitable by anything like the kind of life we have here on Earth. But he reminds us that life could take any number of forms, and because it would have evolved in a different atmosphere than ours, it could take forms that we might not expect.

(Shortform note: Astronomers now believe that Venus may have once had an atmosphere more like Earth’s, but that greenhouse gasses accumulated and eventually made it the volcanic inferno it is now. Venus is the hottest planet in our solar system, with a surface hot enough to melt lead. Its previous oceans were boiled away, and its atmosphere became dense carbon dioxide, with sulfuric clouds. Astrobiologist Vikki Meadows believes it’s an example of an alternate planetary evolution that could serve as a warning for Earth about “how terrestrial planets die.”)

Looking for Life on Mars

So far, Sagan explains, we’ve been able to fairly extensively study the Moon, Mars, Venus, Jupiter and its moons, and Saturn and its moons. Sagan says that Mars has been the primary focus of study because its polar caps are covered with ice—which means there’s the possibility that it was once liquid water, which in turn denotes the possibility for life.

(Shortform note: In 2016, NASA’s Mars Reconnaissance Orbiter provided images that astronomers believe suggest liquid water is presently flowing on the surface of Mars. They believe the water would be briny, containing high levels of salt, which would allow it to remain liquid in otherwise freezing temperatures. Other scientists have contested this evidence, arguing that the features are more likely created by sand.)

As of the writing of this book, Sagan says that no compelling evidence for life has been found on Mars. However, he says we’ve been able to map the entire planet with orbiting satellites, and the US’s Viking lander has successfully landed on its surface and collected and tested soil samples. While some of the microbiology tests showed promise of chemical reaction, Sagan says there was no definitive evidence of the kind of life we know.

(Shortform note: The Curiosity rover landed on Mars in 2012 and is still exploring the planet as of 2022. The major mission of Curiosity is to discover whether Mars may have once had the right environmental conditions to support simple life forms. In 2022, it returned data from an analysis of rock samples that contain a form of carbon associated with biological life. Scientists say they would still need more evidence to support a claim that there was once life on Mars, but the finding is compelling.)

Sagan speculates on what we should do if we did discover any form of life on Mars. He strongly feels that if we discover even the smallest microbial life we should leave Mars alone entirely, and do nothing more to interfere with it. However, he says if we conclude that there is no life on Mars, he can imagine possibilities for humans to colonize and settle there, after altering the atmosphere.

Mission to Mars

One of the major contemporary proponents of colonizing Mars is entrepreneur and investor Elon Musk. Musk founded the private spaceflight company SpaceX, which is developing a rocket called Starship that would be able to transport humans to Mars. Musk imagines colonization of Mars to be essentially a “life insurance” policy in the face of potential catastrophic events on Earth. His vision includes a process of “terraforming” the planet to make the atmosphere more suitable for human habitation, similar to Sagan’s suggestion, but in the shorter term it would have to involve living in an encapsulated artificial environment.

Although tests have been conducted on prototypes of Starship, the cost of a mission to Mars may be a prohibitive factor. However, NASA has said that they envision a crewed mission to Mars by around 2040.

The Space Probes

Spacecraft called Voyager I and II were designed to explore beyond our immediate neighbors. Launched in 1977, their mission was to fly beyond Mars, travel among the moons of Jupiter and Saturn, and then ultimately drift off into interstellar space forever.

(Shortform Note: This website shows the current locations of Voyager I and II, 44 years after their launch. Voyager II went on to explore Uranus and Neptune, and it’s still the only spacecraft to have visited those outer planets. They’ve both now flown beyond the solar system, into interstellar space, and are still in operation.)

Sagan tells us that Voyager I took pictures of three of Jupiter’s four largest moons, and Voyager II was able to capture photos of the fourth one, Europa. Europa is unique, he says, in that there are visible geometric lines on its surface that look like nothing else we know of, other than man-made canals. (Shortform note: See a video and photo of Europa here.)

Another one of Jupiter’s moons, Io, is unique in that the Voyager images show active volcanoes over much of its colorful surface. Sagan says this appears to be a newer planet still actively forming, with a rapidly changing landscape. (Shortform note: See a photo and more in-depth description of Io here.)

Beyond Our Solar System

Even with the ability for travel within our solar system being limited, Sagan points out that humans still dream of going further, and he thinks we always will. Since we haven’t found any evidence of a habitable environment within our solar system, might there be some in other solar systems?

We don’t know how many planetary systems might exist in the cosmos. Sagan says the evidence suggests that almost all single stars like the sun should have satellite planets, but we can’t see them because the light of the stars is too bright. He says scientists are working on telescope technology that could dim the light so they might be able to see the much smaller objects orbiting the stars. Also, he says that in addition to “planets” orbiting stars, Jupiter, Saturn, and Uranus all have satellite “moons” that are planet-like, so many other planets could have such satellites as well. So there are innumerable possibilities for other worlds that could sustain life.

Discovery of Exoplanets

The technology that’s needed to observe exoplanets (planets orbiting stars outside of our solar system) now exists. The first exoplanet was observed in 1995, orbiting a star called Pegasus 51, about 50 light-years away from Earth. Since then, astronomers have discovered at least 4,000 more exoplanets in the Milky Way galaxy. Astrophysicist Hakeem Oluseyi says that the more scientists discover about the universe, the more likely they believe it is that life exists elsewhere. Scientists now estimate that there are about 2 trillion galaxies in the universe; Oluseyi believes this makes the odds very high that there is intelligent life out there somewhere. However, he still thinks we’re a very long way away from being able to travel or even communicate across this kind of distance.

Interstellar Travel

Alpha Centauri is the closest “star” to us, being 4.3 light-years away, but Sagan explains that it’s actually a triple star system. Considering the vast distance, will it ever be possible to travel to other solar systems? Sagan suggests that nuclear fusion technology might make it possible to power spacecraft fast enough. However, he explains that such technology would require the spacecraft to be launched from Earth’s orbit, rather than from land, because of the dangers involved with the nuclear fusion reaction.

Nuclear Fusion Technology

In 2022, a panel of experts speaking at a meeting of the International Atomic Energy Agency discussed how advancements in nuclear fusion technology could soon be used to propel faster space travel. They say that rockets would have to lift off from earth using traditional fuel, but then once in space, the nuclear power could take over, and the technology currently available could cut travel time to Mars by 25%. Currently it takes about 7 months to reach Mars.

Additionally, this nuclear technology could be used as a sustainable power source for extraterrestrial settlements, should we have the ability to create a long-term human habitation on Mars or another planet. Potentially, this kind of technology could power such a station for decades.

Sagan says the technology to build a ship that can travel anywhere close to light-speed is probably thousands of years away, but a ship using nuclear fusion technology may be able to travel about 10% the speed of light. With Alpha Centauri being 4.3 light-years away, a voyage there would take 43 years, meaning it would be feasible for humans to travel there within a lifetime. But he proposes that to travel any further, we might need “multigenerational” spacecraft, where generations of humans would be born and live their lives, so that subsequent generations might reach other solar systems.

Considering the scientific discoveries being made in Ionia over 2,000 years ago, Sagan believes that we might be there right now if that scientific inquiry had not been suppressed by religious forces for centuries.

Generation Ships

The hypothetical concept of multigenerational space travel is something astronomers have pondered for decades, and some have undertaken serious research into the idea. They call such a vehicle a “generation ship.” A 2019 article by an international team of scientists addressed some of the practical questions concerning the development of a generation ship, such as how many people it would require, how large the ship would need to be, and how food would be produced.

They determined that such an endeavor would require a crew of at least 98 people, to avoid genetic issues with inbreeding; they suggested around 500 people optimally. Based on a proposed cylindrical shape, they said the ship would need to be around 1,050 feet long and 735 feet in radius, which they point out is still smaller than the world’s largest building.

The experts also devised a plan for food involving cultivation in an artificially created plot of land that would need to be about 0.17 square mile in size. The only practical issue they haven’t yet worked out is how to have sufficient water. But, when that’s solved, they say the idea is quite feasible—given an adequate budget, which would need to be enormous.

The Search for Extraterrestrial Life

The idea of intelligent life existing on other planets is probably the most powerful driving force of humanity’s exploration of the cosmos. It’s the theme of countless works of human imagination, from books and poetry to movies and television. Sagan believes the evidence would suggest it’s far more likely than not that there’s some kind of life out there in the universe; he says there could realistically be a hundred billion planetary systems in the galaxy, and each and every one of them would be unique.

According to Sagan, the same kind of evolutionary process that happened on Earth should logically operate everywhere in the universe. But with the number of possibilities of how that would unfold in a hundred billion unique environments, he thinks any life that may have evolved would probably be nothing like humans, or any kind of life we know.

(Shortform note: Carl Sagan’s 1985 novel Contact, which was used as the basis for a movie of the same name in 1997, explores this question. In one scene, a character asks another “Do you think there are people on other planets?” to which the reply is, “The universe is a pretty big place…so if it's just us, seems like an awful waste of space.” While Sagan makes it clear that he doesn’t think life on other planets would be “people,” like us, this sentiment clearly reflects his own thoughts about the potential.)

Scientists have been intentionally trying to communicate with other potential life in the universe since the 1970s. Sagan says that we have the technology to communicate over interstellar distances; it’s called radio astronomy. Radio waves travel at the speed of light. The Arecibo Observatory in Puerto Rico has been searching continuously for any radio waves coming from outer space, and Sagan says that it once broadcast a message into space, directed toward a distant star cluster called M13.

Sending Messages to the Stars

That first interstellar message was broadcast in 1974 and contained binary-coded information, including a description of DNA and graphics of a stick-figure person, our solar system, and the telescope that sent the message. Several other messages in different forms have been sent since that time.

The Arecibo telescope collapsed in 2020. In 2022, astronomers created an updated message that they’ve proposed to send out later in the year via two radio telescopes—one in China and one in northern California. That message includes a more sophisticated drawing of humans, a number of mathematical operations, a description of the Earth, and a detailed stellar map that shows the location of Earth—that last portion is controversial, as many believe it’s risky to broadcast our location without any idea of what/who might receive it.

Humans have created many imagined scenarios in film and literature depicting the arrival of alien life on Earth. In pondering this possibility, Sagan says that one thing is clear: There’s no cause to worry about what we would do if extraterrestrial beings arrived here on Earth. Because if that happened, they’d be so technologically beyond us that there would be nothing we could do. He also hypothesizes that any species that has survived long enough to develop such technology has probably learned how to live effectively and peacefully with themselves and others, or they would have destroyed themselves already, as humans may be in danger of doing.

Breakthrough Listen

Physicist Stephen Hawking was both an enthusiastic supporter of searching for extraterrestrial life and very cautious about the idea of actually making contact. In 2015, Hawking helped to launch Breakthrough Listen, the most comprehensive program ever undertaken to explore interstellar space for messages or signs of life. The project aims to survey the 1 million closest stars to Earth, using radio and optical telescope technology.

On the question of what we should do if we discover signs of life, Hawking advocated for quietly collecting as much information as possible, without revealing ourselves. He didn’t necessarily agree with Sagan’s sentiment about how extraterrestrials might approach us. He thought an extraterrestrial visit to Earth would likely have the same result for us that European colonization of the Americas had on the Native peoples. He said "we only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet."