54 pages • 1 hour read
Carl SaganPale Blue Dot: A Vision of the Human Future in Space
Nonfiction | Book | Adult | Published in 1994A modern alternative to SparkNotes and CliffsNotes, SuperSummary offers high-quality Study Guides with detailed chapter summaries and analysis of major themes, characters, and more.
Chapters 6-9Chapter Summaries & Analyses
Chapter 6 Summary: “The Triumph of Voyager”
The next four chapters cover the Voyager missions in detail. The sixth chapter begins with a discussion of NASA’s nonscientific value: to generate visions of the future that motivate and inspire. Sagan acknowledges that as he writes Pale Blue Dot, the purpose of NASA is in question due to recent expensive failures (including the deadly Challenger mission). But Sagan argues that there have been many triumphs too. He lists scientific breakthroughs and the growth of a satellite communications network. He also notes that many of the successes have been due to missions without men or women at risk. He raises the question of whether human spaceflight is worth pursuing but, for now, argues that robot spacecraft have been worth the cost.
Sagan then describes the history of the Voyager program: its inception in the 1960s, funding in 1972, and final mission parameters after reaching Saturn. He describes the probes: as big as a house, with a plutonium power source and various instruments attached. Due to limited propulsion technology, the probes used gravity assists to reach as far as they have. One or both Voyager spacecraft explored Jupiter, Saturn, Uranus, Neptune, and their moons. They reached Jupiter in 1979 and Saturn in 1980 and 1981. Voyager 2 reached Uranus in 1986 and Neptune in 1989, and by 1992, both crafts had left the immediate solar system. When the program launched in 1977, humans were wholly ignorant about most of the planets of the solar system. With Voyager’s help, we have studied volcanos outside of Earth, rings around Jupiter, and many other wonders. And both spacecraft continue to transmit reams of data.
Sagan explains how astounding the Voyager programs are. NASA’s Jet Propulsion Lab could only afford to build spacecraft that would work reliably as far as Saturn. But because engineers kept learning after the launch, they were able to keep the ships working through visits to Uranus, Neptune, and beyond. This effort, Sagan explains, was a combination of ingenuity and luck. Because no repairs could be made to the probes after launch, engineers needed to troubleshoot from afar. When Voyager 2’s main receiver stopped working, and when one of the platforms which aimed the instruments jammed, the mission might have failed if not for engineering heroics. Even when the probes are so far from Earth that the signal is incomprehensibly faint, these machines sensitively rotate toward their objects of study to allow their cameras ample exposure time while hurdling at high speeds.
Chapter 7 Summary: “Among the Moons of Saturn”
The seventh chapter focuses on Voyager’s flyby of Saturn’s moon Titan. Sagan frames the discussion of Titan with what is known of the origins of life on Earth: the oldest known fossils and the assumption that, about four billion years ago, life began in the form of tiny microbes and persisted to become more diverse and complex over time. We know that for life to develop there needs to be organic, carbon-based molecules: amino acids (the building blocks of protein) and nucleotide bases (the building blocks of nucleic acids). On Earth, the atmosphere used to have more hydrogen and less oxygen than it does now. And so, to study what primitive Earth was like, it would be helpful to study a planet that has a similar makeup. The closest thing we have is Titan, which resembles primitive Earth despite being half the size and much farther from the Sun.
Titan was discovered by Christianus Huygens in 1655. Its methane atmosphere was discovered in 1944 and its red clouds were discovered in 1970. But there wasn’t much more that could be learned about Titan until the Voyagers passed by the moon in 1980 and 1981. Voyager determined how high the cloud layer is, that the atmosphere is mostly nitrogen, and confirmed the presence of methane. The atmosphere includes hydrocarbons and nitriles, like primitive Earth, and enough chemical activity to suggest a future where more complex molecules may appear. Sagan has participated in laboratory studies that suggest an atmosphere like Titan’s, if it interacts with water, would lead to the formation of amino acids, nucleotide bases, and other building blocks of life.
As the Voyagers passed by, there was no break in the clouds. Sagan cites experiments with radio pulses that contradict theories that Titan’s surface might be all rock or all oceans of liquid hydrocarbons (93). The temperature suggests that any water would be too frozen to catalyze possible life. But Sagan also imagines asteroid or comet collisions that temporarily break up Titan’s theoretical surface ice. He posits that life has a chance on Titan and that humans have a chance to unlock mysteries about the origins of life on Earth by further studying this moon of Saturn.
Chapter 8 Summary: “The First New Planet”
The eighth chapter is about Uranus. Sagan opens by noting that, before humans developed the necessary instruments to see farther into the solar system, it was believed that there were only five other planets: “five bright points of light that grace the night sky [that] break lockstep with the others over a period of months” (98). These points of light acted more like the Sun and the Moon than other stars, and these seven heavenly bodies form the basis for many aspects of human culture today: for instance, the days of the week.
Returning to the themes of the last few chapters, Sagan notes that humans have been swayed by this numerology for centuries. Galileo stopped looking for moons around Jupiter because four seemed like the right number. G. D. Cassini stopped looking for moons after delivering the discovery of a fourteenth orb (six planets and eight moons) in the solar system to Louis XIV. It was significant when William Herschel, a musician, discovered Uranus in 1781. Uranus was the first planet that was unknown to the ancients and, if there was one planet previously not known, there may be more.
Before the Voyager program, it was known that Uranus had an atmosphere and clouds made of hydrogen, helium, methane, and other hydrocarbons. We have since discovered that below the clouds there is an atmosphere that includes ammonia, hydrogen sulfide, and water, as well as a rocky planet surface. Unlike other planets (and like Earth), Uranus has an internal heat source. Unique in our solar system, the planet lies sideways, with one of its poles facing the Sun, and its magnetic axis and the axis of rotation do not match. Uranus has rings and moons named after characters from Shakespeare and Alexander Pope. But much of the planet remains a mystery to us. Many theories suggest there was a major planetary collision epochs ago.
The innermost moon, Miranda, was discovered as recently as 1948. Then there was “a revolution in our understanding of the Uranus system” (106) when Voyager 2 flew near Miranda on January 24, 1986. Voyager discovered that Uranus was surrounded by an intense radiation belt as well as 10 new moons. It determined the length of a day on the planet (approximately 17 hours) and that Miranda was surprisingly full of geological activity for a moon so far from the sun. Readings even suggest that Uranus might have a vast, deep ocean. Sagan emphasizes the speed by which we have learned about Miranda: “In only half a lifetime it has gone from an undiscovered world to a destination whose ancient and idiosyncratic secrets have been at least partially revealed” (108).
Chapter 9 Summary: “An American Ship at the Frontiers of the Solar System”
The ninth chapter is about Neptune and the most distant edges of the solar system. At the time of Voyager 2’s arrival in 1989, Neptune was the farthest planet from the Sun. (Pluto overtook that honor in 1999). Because of this distance, very little was known about the planet before Voyager reached it. Now we know that, like Uranus, Neptune is a large rocky planet surrounded by a gaseous exterior. It is four times the size of Earth, is chemically like Uranus, has rings, is very blue, and has a “Great Dark Spot” like Jupiter’s “Great Red Spot.”
Neptune also has a unique moon called Triton that revolves around the planet backward (in the reverse of Neptune’s rotation), suggesting it was not created at the same time as the planet but arrived after. The moon Triton has a nitrogen-rich atmosphere like Titan’s, but a thinner cloud layer revealing a rock surface of many kinds of ice, pristine impact craters, valleys, and plains of snow. The snow appears as patches of different colors depending on its chemical makeup and the way the seasons shift the sediment beneath. Nothing is alive, but Triton, too, has the makeup of a place where life can one day happen.
Sagan emphasizes Neptune’s (and the Voyager probe’s) distance from the Sun: roughly 40 astronomical units away, where one astronomical unit is the distance from Earth to the Sun. Neptune is so far away that the Sun appears as tiny as other stars in the sky. Light takes five hours to reach Earth and, since Neptune was first discovered in 1846, it has yet to complete a revolution around the Sun. Beyond the orbit of Neptune is an uncountable number of asteroids and inactive comets kept at bay by the gravities of Neptune and Uranus. These make up the Kuiper Belt. Most of them, and most everything beyond Neptune and Pluto, are too far away to be studied from Earth.
Sagan then discusses the search for planets beyond our solar system. He notes that while we cannot see planets that far away, we can see that some other stars are in the center of rings of dust (just as our star is at the center and, beyond Neptune, there is a field of tiny objects). Vega and Epsilon Eridani, for instance, are stars surrounded by rings of dust around 40 AU away, the same distance as Neptune from the Sun. Thus, we might guess that those stars, too, have planets whose gravities have cleaned up the inner dust. Seeing planets that far away is difficult without better telescopes. However, using the pulsar timing method, Alexander Wolszczan was able to confirm that other stars do have planets by measuring gravitational interactions in the star’s flicker. The pulsar designated B1257+12 has at least three planets in its orbit: one 2.8 times, one 3.4 times, and one .015 times the size of Earth.
Sagan ends the chapter with a summary of the Voyager missions’ failure to find evidence of life. They found no oxygen atmospheres, no atmospheres with chemical imbalances possibly explained by life, no geometric patterns on surfaces, and no radio waves beyond cosmic noise. He repeats that the moons Titan, Miranda, and Triton are lifeless. But, in each case, he ends the paragraph with “But of course we might be wrong” (121). He imagines two possible futures: one in which news of life elsewhere will become routine, and one in which the Voyager probes never encounter anything in their travels and end up circling the Milky Way forever, bearing memories of Earth.
Chapters 6-9 Analysis
On every page of Sagan’s discussion of the Voyager program, the reader is meant to look up at these two robots with awe and wonder. Sagan paints the probes as romantic heroes: “They are the ships that first explored what may be homelands of our remote descendants” (72). He emphasizes the sheer luck and fragility of the missions, casting them as underdogs to further underline how amazing their success has been. Sagan taps into the strategies that were successful in Cosmos to reinvigorate enthusiasm for NASA.
The chapters have tended to follow (and will continue to follow) a certain structure: first, Sagan discusses what ancient humans thought of a particular celestial object. He summarizes how scientific discovery reshaped what humans know of that object. Then, he describes in more detail any recent discoveries and their implications. This structure illustrates how recent most scientific progress has been and how quickly our technology is accelerating. Often, Sagan seems to suggest that the current moment is merely a prologue to a future where news of new planets and even alien life will start to feel mundane: “announced over the morning coffee” (122). This techno-optimistic view of Voyager serves to sell the idea of future unmanned NASA missions to the public. It is also a leap of faith on Sagan’s part and often eschews the possible drawbacks and dangers of machine intelligence and drone technology.
Some of the data reported by Voyager has been superseded by later missions, such as Cassini, a joint US and European spacecraft launched in 1997 toward Saturn, which sent a probe into Titan’s atmosphere. Cassini revealed that Titan has rain, lakes, rivers of hydrocarbons, and an ocean of salt water beneath its surface. Telescopes such as the James Webb Space Telescope are now powerful enough to take over the search for exoplanets. That said, the exciting potential that Sagan exploits remains unchanged. In Pale Blue Dot, Sagan repeatedly focuses on what might be possible, what might have happened (for instance, Uranus’s collision with another planet), and what could happen next (for instance, life on Titan or Triton). The repeated refrain, that “we might be wrong,” energizes the duller reality that, so far, everything discovered in space has been largely cold, still, and lifeless.
