“Thunderstones” were held in awe and venerated as thunderbolt weapons that were hurled at this world by angry gods. In his book Natural History, Pliny wrote that a comet appeared in 44 BCE just after Julius Caesar was assassinated and during athletic games he had sponsored. Pliny reported that it was seen everywhere as a bright star in the north for seven days. It was widely believed that the comet was the soul of Caesar on its way to the sanctum, where the immortal gods lived. The emblem of a star was therefore added to a bust of the slain emperor that was dedicated in the forum.3
Comets soaring across the night sky, trailing their long luminescent tails, excite us because we take them to be elegant visitors from the infinite, black universe—space—that engulfs us. (And since vacuum is defined as a place absolutely devoid of matter, it does not apply to space, which is full of stars, planets, moons, asteroids, comets, and all manner of cosmic debris that are the size of pebbles and grains of sand.) But that take is new and is the product of modern astronomy. For centuries, comets were taken to be fiery messengers, flaming arrows, that were shot at Earth from angry gods. Unlike sunrise, sunset, the stages of the Moon, and other celestial occurrences, comets were unpredictable, and that caused widespread fear and superstition among people who found security in the predictability of nature. The sudden appearance of a comet was taken to be a harbinger of doom; a message from their god, who was angry and warning that a war, a natural disaster, a plague, a famine, or some other catastrophe was imminent. The Babylonian Epic of Gilgamesh, the most ancient surviving myth, warned that a comet would bring fire, brimstone, and a flood. The Sibylline Oracles, who were Roman prophets, referred to comets as a “great conflagration from the sky, falling to earth.”4
Chinese astronomers as early as the Han Dynasty kept extensive records on the appearances, paths, and disappearances of hundreds of comets and associated different shapes with specific disasters.5
The beginning of serious, modern astronomy that started when Copernicus decided that the Sun, not Earth, was the center of this planetary system took a colossal leap forward when Galileo began peering through that first telescope—a long, narrow, wooden tube that was covered with paper and had a small lens at either end—in 1609. Myth was replaced by hard reality, and a revolution started that slowly but surely gained momentum as more of the devices were built and used by men who were enthralled to be able to finally get much closer views of the heavens; to, in effect, get off here and go out there. One of them was Johannes Kepler, a German astronomer and astrologer (they were often considered indivisible) who used his celestial magnifier to study planets and then published the three laws that describe their orbits and that bear his name.
One night in 1682, seventy-three years after Galileo trained that crude cylinder on the Moon's battered surface, a twenty-six-year-old Englishman named Edmond Halley who had a deep interest in natural science, including oceanography and meteorology, in addition to mathematics, noticed a comet flash across the sky and instinctively began plotting its positions when it reappeared on successive nights. In August 1684, he visited the University of Cambridge to see an obscure professor named Isaac Newton. Halley told Newton about the frustration he and some friends had had in trying to understand the force that kept the planets traveling around the Sun and that grew weaker with distance. Objects moved faster when they were closer to the Sun, Halley explained, and slower when they were farther away.
“I have calculated it,” Newton told Halley. “I worked it out seventeen years ago. I wrote the full solution down five years ago. It's around here someplace,” the genius told his visitor. Then history's most famous absent-minded professor shuffled through some papers.
“You can't be serious,” said Halley. “You explained the force that controls the planets and you never told anybody about it?”
“It explains the Moon in orbit around the Earth and the tides also. And how dropped objects fall to the ground. Let's see. I think I put it in this drawer. No, not here, either,” the man who conceived the law of universal gravitation told his awed visitor.6
It was eventually calculated that the comet Halley had noticed and then took pains to plot returned every seventy-six years and, because of its brightness, it (like other comets) was seen, tracked, and studied. In keeping with tradition, the comet Halley studied so intently was named after him. And sure enough, as predicted, it returned on Christmas night in 1758, sixteen years after the man who immortalized it had passed on. It last crossed Earth's path in February 1986, and astronomers were able to see that it is made of water ice, carbon monoxide, carbon dioxide, ammonia, methane, various other chemicals, and some iron. It was appreciated by Earthlings who understood that they were witnessing what for almost all of them was a once-in-a-lifetime event that was savored. It is due back in 2062.
Astronomers and space scientists certainly appreciate comets for their appearance, but these are the people on the most-wanted list of those responsible for “mitigating” comets’ potential to do terrible damage on impact; that is, they are in charge of finding and stopping a comet before a collision that is bound to be terrible. That is why Clark Chapman, Daniel Durda, and Robert Gold wrote The Comet/Asteroid Impact Hazard: A Systems Approach. Their concern was shared by many in the space community, who organized the groups that are described in chapter 5: “The Other Salvation Army.” Scientists, including Gene Shoemaker, Clark Chapman, Donald Yeomans, Steven J. Ostro, Jon D. Giorgini, and David Morrison (a stalwart among the near-Earth-object fraternity who is at the NASA Ames Research Center in Northern California), have found comets so interesting that they specialized in studying them. Yeomans, as noted, was at NASA's Jet Propulsion Laboratory (JPL), where he headed the Near-Earth Object Program Office; so were Ostro and Giorgini. JPL ran most of NASA's Solar System Exploration Program in the 1960s and ’70s, most notably Voyager 1's encounter with Jupiter, Saturn, and several of their moons and Voyager 2's phenomenal “Grand Tour” of Jupiter, Saturn, Uranus, Neptune, and many of their moons. It is important to note that, in addition to sending home a veritable library of new scientific information about those distant worlds—that Jupiter's moon Europa has an active volcano; that the planet's fabled rings are made of an infinite number of rocks of all sizes and pebbles; and that Neptune has rings, too—the stream of imagery that came back to JPL from every close encounter with a solid body showed impact craters that were made by collisions with asteroids of all shapes and sizes. It was the pockmarked moonscape, the eternal target of sheer violence, repeated infinitely. The wandering rocks themselves, which were known to be an important cause of destruction in the Solar System, were studied with obsessive interest by many astronomers and planetary scientists, who then produced hundreds of scholarly papers and books, such as Comet/Asteroid Impacts and Human Society: An Interdisciplinary Approach edited by Peter T. Bobrowsky and Hans Rickman and Asteroid: Earth Destroyer or New Frontier? by Patricia Barnes-Svarney.
The ubiquitous (and mellifluous) astronomer Carl Sagan, who was unfairly denounced by some members of scienceworld for being a dreaded “popularizer,” wrote Comet, Revised with Ann Druyan, which was published in 1997. Two chapters were devoted to asteroids and comets hitting Earth and other targets and featured a ride on one (possibly sending a message to Michael Bay, Jerry Bruckheimer, Bruce Willis, and the rest of the cast and crew of Armageddon, which was released the following year). That was followed by Cosmos, a book that became the television series that made Sagan famous. Yeomans and three colleagues published Mitigation of Hazardous Comets and Asteroids in 2004, and he wrote Near-Earth Objects: Finding Them before They Find Us, a serious, readable primer on the subject (and on space in general), almost a decade later. And Duncan Steel, an Australian astronomer, weighed in with Rogue Asteroids and Doomsday Comets: The Search for the Million Megaton Menace That Threatens Life on Earth.
Not to be left out, a Russian astronomer and one from Belarus weighed in with news about a huge comet that came out of the Oort Cloud, which is believed to envelo
pe this Solar System and sends in every comet. This large comet, they predicted, will visit the neighborhood but not pose a threat to Earth. It is variously called Comet C/2012 S1, Comet ISON (for the International Scientific Optical Network, one of whose telescopes was trained on it), and Nevski-Novichonok for the two men who discovered it.
On at least one occasion, the dramatic specter of a comet streaking across the sky like a shot from God's sling caused a bizarre tragedy. The comet was Hale-Bopp, which was discovered in July 1995 by two independent American sky watchers after whom it was named: Alan Hale and Thomas Bopp. It caught the attention of the public when it became a spectacular sight as it passed Earth at distance of only seventy-six thousand kilometers in March 1997 and was acclaimed the Great Comet of 1997 by millions of people around the world who could see the spectacular show it put on just by looking at it without telescopes.
Thirty-nine of those who looked up lived in San Diego and belonged to a doomsday religious cult called Heaven's Gate, which was founded in the early 1970s by a recovering heart-attack victim named Marshall Applewhite and his nurse, Bonnie Nettles. Both Applewhite and Nettles believed that Earth was about to be recycled, which is to say wiped clean of the creatures that lived on it and then renewed. The two claimed to be from “somewhere else” in the literal, not figurative sense—extraterrestrials—and insisted that the only way to avoid the apocalypse and find salvation at the Next Level (of existence) was to shed every attachment to the planet, starting with ending all relationships with others, including family and friends, and forgoing individuality, jobs, possessions, money, and sexuality so their spirits would not be encumbered by earthly baggage for the voyage to the Next Level. The cult rented a 9,200-square-foot mansion in a gated, up-scale community and spent $10,000 on alien-abduction insurance to cover up to fifty people. Hale-Bopp became visible to the naked eye in May 1996, and the leader of Heaven's Gate was soon telling his devoted followers that the comet was being followed by a UFO that would transport their spirits to the Next Level, but only if they shed the excess baggage they called their bodies. The followers did that by swallowing phenobarbital, cyanide, and arsenic and washing them down with vodka over the course of three successive days, with one group carefully laying the “embarked” deceased in their own bunk beds, their faces and torsos covered with a square purple cloth, before it took its turn with the poison. All thirty-nine decomposing corpses were dressed in identical black shirts and sweat pants, wore armband patches that said “Heaven's Gate Away Team,” and carried a five-dollar bill and three quarters, which they apparently believed was the toll that had to be paid to get to the Next Level.7 Such is the often-perverse romance of comets, however dangerous they can be.
Monster asteroids are far more dangerous than comets because the latter are relatively soft and there are far fewer of them. The little prince may have loved the asteroid he lived on, but the rocks that roam inside the Main Belt and randomly cross Earth's path do not inspire affection from most of the planet's inhabitants, especially those who know firsthand what they do as meteorites, meteoroids, or bolides, as they are called when they get closest to Earth. (Bolides are loosely defined as fireballs.) Ask anyone in Chelyabinsk. One colossal object—a projectile roughly the size of Mars—is thought to have smashed into Earth during its formative stage with such force—a “giant impact”—that it knocked off a chunk that became the Moon. The author of a book on that subject gave it a title that is as vivid as it is imaginative: The Big Splat.8 Earthlings don't like and, indeed, fear asteroids because it is they, not comets, that have had the overwhelming number of collisions with this planet.
Earth's oceans and shifting terrain hide, obscure, or erase many impact sites. Yet more than 160 remain on all seven continents, many of them chillingly massive. The Sudbury crater in Ontario, Canada, is slightly more 130 kilometers across, and the Vredefort ring in South Africa is 190 kilometers in diameter and looks like an immense flat canyon when observed from high in the air or from space. The Acraman crater in Lake Acraman, South Australia, is a scar that was made about 580 million years ago and measures 90 kilometers across, while the Woodleigh crater in Western Australia was made roughly 364 million years ago. The Manicouagan crater in Quebec, which is 100 kilometers wide, resulted from an impact made 215 million years ago. And a crater was discovered in Chesapeake Bay, 201 kilometers from the nation's capital, in the early 1980s. Its estimated age is 35 million years, so it cannot be blamed on an incompetent administration, Republican or Democrat.9
There are innumerable examples all over Target Earth. The Hoba meteorite is a sixty-ton slab of iron that slammed into what is now Namibia about eighty thousand years ago, and since no one in that country seems to have the inclination or resources to move it, it has become a national monument to be appreciated and studied. The Willamette meteorite, which is a ten-foot-tall mass of pitted iron that weighs around fifteen tons, is thought to be the worldly remains of the iron core of a planet that crashed into what is now this one billions of years ago. It was discovered on Native American land in 1902, the local indigene's pleas to leave it alone because they believed it had healing power were ignored, and it was unceremoniously taken away and wound up on display at the American Museum of Natural History in New York. Another one, ALH 84001, is one of the most intriguing artifacts from out of this world. Its dull name notwithstanding, it comes from Mars, was discovered in Antarctica in 1984, and, after scientists saw on close examination, contains the fossilized remains of what may have been very small bacteria. If they really were living microscopic organisms, it would be further proof that Martians, however tiny, have indeed lived on the Red Planet.
Bombardment by near-Earth objects (NEOs) is widely associated by the public with the demise of the dinosaurs and, therefore, with prehistoric and ancient “history.” But Tunguska and Chelyabinsk were dramatic demonstrations that the attacks are still underway, and those “events” were in fact only two among thousands that have occurred since records of the rain of rocks and ice started being kept in the eighteenth century.
Like every other body in the Solar System, Earth is under constant bombardment from objects ranging in size from tiny grains of sand to enormously large rocks, many of which are laced with iron. Most of them come from the Asteroid Belt between Mars and Jupiter and are the remnants of a planet that failed to form because of Jupiter's and Saturn's gravitational pulls (and with a little help from Mars). The asteroids in the belt collide with each other constantly and break into fragments that head in all directions, including this one, at velocities that can reach fifty thousand miles an hour. The small rocks range from the size of a beer can to a Winnebago. Those in the medium category start at the size of a two-car garage and can be as large as a small building in the average town. The big ones would sit in a professional football stadium the way a hard-boiled egg would sit in a porcelain cup. And the really big ones start at a kilometer across and go upward to perhaps six to twelve kilometers. The good news is that the size of the asteroids is inversely proportional to their number, meaning that there are relatively few of the really big ones in the kilometer-or-larger class—also known as the Doomsday rocks. That means the neighborhood is fairly infested with thousands that could demolish downtown Peoria, Rangoon, or Nairobi, but the ones that are large enough to end civilization show up only about once every one hundred years. Some are round, some look like dog bones, and some like Idaho potatoes. Their composition varies, as well. The overwhelming majority of Gene Shoemaker's bullets are solid rocks, but there are also relatively soft ones that are of a substance like sand held together with glue, while still others are made mostly of iron.
The bullets speed around the planet in all directions and at all altitudes all the time. That is why there were major impacts on or explosions directly over Brazil, Canada, the United States (Michigan), Italy, Spain, the Indian Ocean, the Eastern Mediterranean, and Indonesia between 1930 and 2009.10 The explosion over Brazil happened above the Amazon in the northwestern par
t of the country, just after sunrise on August 13, 1930, and left physical and emotional scars that are still there. As women began to wash clothes in the Curuçá River and their men fished or tapped rubber trees, the Sun turned bloodred. Then the region was enveloped in darkness. Suddenly there were three very loud whistling sounds, followed by three loud explosions in rapid succession. The blasts turned the jungle into an inferno that burned for several months, which sent many villagers in the region, fearing imminent death, fleeing with as many belongings as they could carry. But others stayed in their huts in an area of several hundred square kilometers. Terrified children tried to hide in corners while some of their parents listened to what sounded like artillery shells and looked at the sky to see what was happening. The ground trembled as it would in an earthquake. Five days later, an Italian monk named Father Fedele d'Alviano began his annual missionary visit to the area and found people who were still shaken by the event. He tried to reassure them that they had suffered from meteorites coming from the sky, not from the wrath of God. It was subsequently called a possible “Brazilian twin” to the Tunguska explosion.11
And at least two impacts on the homes of individual Americans were offbeat enough to be reported by the news media as just plain quirky. On November 30, 1954, Mrs. Hodges was severely bruised when a five-kilogram rock came crashing through her roof in Sylacauga, Alabama, and hit her while she was napping in her living room. She got off with a severe bruise. (Mrs. Hodges happened to live across the street from—you can't make it up, as some in the news business say—the Comet Drive-In, a movie theater.) And thousands saw another rock as it sped over Kentucky and headed northeast before making a sonic boom and breaking into more than seventy fragments on October 9, 1992. One football-sized fragment that weighed nearly two pounds landed on Michelle Knapp's Chevrolet Malibu that was parked in her driveway in Peekskill, New York, that day. Its trunk was scrunched, but the $69,000 that a consortium of three dealers paid her for it more than took care of the repairs.12 As the dealers in Chelyabinsk were to learn twenty-one years later, souvenirs from heaven are valuable collector's items.
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