• The Good: Accessible science for the wonder and awe of our existence
  • The Bad: A little outdated
  • The Literary: Brings together history and biography to tell the story of our astrophyiscal visitors

Comets grace our skies, changing the heavens for a brief time, bringing uncertainty and doubt, death, and maybe life itself.

This book is structured in such a way that reading it is a journey of discovery. The introduction is from the point of view of a narrator astride a comet hurtling through the galaxy. From there, humanity’s myths about comets progress through the ages, and it’s wonderful to see emphasis placed on both eastern and southern (in addition to western) interpretations. The !Kung tribe of the Upper Omuramba in what is now Namibia were unique in their view of a comet as a guarantee of good times ahead. The Chinese are unparalleled in their systematic records since 1400 BC, only missing the return of Halley’s comet once. Seneca debated against Aristotle’s belief that comets were sub-lunar weather-phenomena, but that view held firm for two thousand years in the west.

When the age of the Renaissance and Enlightenment gave rise to the field of science in its infancy, the authors show each theory and discovery in turn. The tides turned with scientist Edmond Halley, who as a boy witnessed two comets—one in 1664, popularly associated with the Great Plague of London, the other in 1665, connected with the Great Fire. Halley’s wonder for science and experimentation flourishes, and as a plucky eighteen-year-old, he wrote to John Flamsteed, England’s first Astronomer Royal, informing him that the authoritatively published tables on the positions of Jupiter and Saturn were in error.

Halley seems to have been an all around cool guy, getting along with all the big egos of the day, remaining unbiased, and often chosen by the Royal Society to perform repeat measurements of controversial findings. Halley convinced recluse Isaac Newton to publish his incredible theories about gravity. Halley accepted a demotion to become the Royal Society’s secretary, a lowly job without the honor of wearing a wig, because no one else was willing to do it. Among his many interests, he invented, developed, and tested one of the first practical diving bells. He wanted to study the work of the ancient mathematician Apollonius, but due to the burning of the Library of Alexandria, no copies in Greek survived, so, at the age forty-nine, Halley taught himself Arabic. But most famously, Halley catalogued, studied, and concluded that the slight differences in the orbital elements of the comets of 1531, 1607, and 1682 were in fact visitations of the same comet. He may have been the first ever scientist to prophesize an event 50 years in the future, and be right!

Immanuel Kant, writer and philosopher, also studied the skies and was the first to postulate that the Milky Way was only one of innumerable other galaxies, a perspective not fully demonstrated until the 1920s. Because of his many radical views, Kant dedicated his book to Prussian emperor Frederick the Great, so he’d be likely to stay in good graces with the government. Later in life Kant strongly supported the American and French antiroyalist revolutions, and Frederick’s successor gave Kant a cabinet order deploring his “misuse” of philosophy. One of the things I love about this book the ability to bring in just enough historical context to enrich the scientific story.

In the early 20th century, astronomer Camille Flammarion predicted of Halley’s comet that “the cyanogen gas would impregnate the atmosphere [of the Earth] and possibly snuff out all life on the planet.” Together with the fears of chemical warfare of the time, and a pervading dread of comets throughout human history, a global pandemonium ensued that was completely unnecessary.

But back to comets; I love learning about their life cycle. Even as we have perceived them primarily as portents of doom, they are themselves extinguishing their own matter each time they pass the sun. Composed primarily of different types of ice and dust all mixed up, the sun vaporizes the ice in spurts, creating the tails. Water ice has a specific rigid structure, in which foreign molecules are physically trapped in a molecular cage. A typical cometary nucleus contains about as much snow as falls each year the Northern United States. Most comets rotate and have days similar to those of Earth. Comets “die” in one of several ways—disintegration, hitting the sun, moons, or planets, or injection into the solar system.

Now, we probe comets with different types of light through spectroscopy. Some comets are composed of mostly silicates like rocks. Those that traverse close to the sun feature metals like chromium, nickel, and copper. We classify two types of comet tails. Type I emanate a blue light because of the fluorescence of CO+. Type II appear yellow because they reflect sunlight back to us. A comet tail always points away from the sun, blown back by solar wind radiation, whether the comet is making its way toward the sun or away from it.

Dense cratering of moons and other planets that have no active geological activity supports the theory that space boulders and icebergs were once extremely numerous, and indeed planets originally formed by all sorts of crashing into things. Now most of the comets reside in the Oort cloud just outside of our solar system. The number of comets in the Oort Cloud is thus larger than the number of stars in the Milky Way Galaxy, so you’d think they’d still be crashing into each other all the time, but the Oort Cloud is so big, current evidence suggests that the distance between comets is about the distance from Earth to Uranus.

Back here on Earth, a foray into geology provides the evidence for the theory of a comet impact as the inciting incident for the Cretaceous extinctions. In between two limestone layers, a clay layer is infused with iridium, a common comet ingredient. From the thickness of the clay layer, it follows that the iridium was not deposited instantaneously, but over 10,000 or even 100,000 years. Multiple comets are likely to have been involved, and particles ejected into the atmosphere by impact would have settled very slowly. The atmospheric particles blocked out the sun, dropping the average temperature on Earth from warmer than it is now to below freezing.

Later chapters of the book tend toward theory and speculation. Some scientists argue there is evidence that supports a mass extinction or a comet crater every 30 million years. In addition, the sun has its own periodicity, bobbing up and down through the galactic plane about every 30 million years. I will have to follow up on the current research. Could comets be the cause of our recurring extinctions? Could they also have deposited the necessary ingredients for life on Earth? Could we one day colonize and pilot them as interplanetary spaceships? What will humanity accomplish before the next visit of Halley’s comet in 2061?

Highly recommended for everyone! Hail Sagan!