“Aww, Fritz!” Humason rumbled. “Leave the goddam moon to the lovers!”
Walter Baade retired to Germany, where he died of natural causes. Fritz Zwicky continued to work at Caltech into the 1970s. He ended up in a basement office in the Robinson building on the Caltech campus, down among the astronomy graduate students. Watching them pass by his office, every once in a while Zwicky would roar out, “Who in the hell are you?” He died in 1974. The asteroids named Zwicky and Baade drift in the Main Belt today and are not likely to collide with each other anytime soon.
The Shoemakers decided to take a break for coffee and apples. They walked over to the dome slit and looked at the sky, which had developed into one of those autumn nights when the stars grow needle-sharp, immanent.
“Andromeda galaxy’s up,” Gene said. “Right at the top of the sky.”
“I can never find it, Gene.”
They both leaned out of the dome slit, silhouetted against the Milky Way. “Look at the Horn of Plenty,” he said.
“Got it.”
“See the mouth of the Horn?”
“Sure.”
“Two stars near the end. Now go up from those two stars—”
“Which way?” she asked.
“That way. See a diffuse patch? Andromeda galaxy’s bigger than the full moon.”
“Oh! It looks different, Gene.”
“What? We’ve been looking at it in the guide scope.”
“Well, it looks different from here.”
Gene turned around and propped his elbows on the ledge of the dome and bit his apple. “What a beauty of a night,” he said.
“Gene, we need a pipeline to God,” she said, “to tell us where the asteroids are.”
Maarten Schmidt, who liked to philosophize on the catwalk of the Hale dome, remarked there on one occasion, “I cannot believe that people become astronomers in order to earn money, or to become well respected. In so many observatories they work alone. In the freezing, freezing cold. Everywhere one finds these people studying just one star. Just one star! What it means, I don’t know.” He paused. “It means they are all slightly mad.” He fell silent. Suddenly a meteor cut through the sky.
“Nice!” Maarten whirled around.
The meteor moved rapidly until it burst in a flash that left a spot before the eyes. A piece trailed off and faded out. Maarten put his hand up to his ear, listening. “We might hear a sonic boom,” he said. A minute went by. “Ja, well,” he said, “the time is getting on.” He descended to the data room, where Jim Gunn and Don Schneider were watching galaxies traverse the screens. “We saw a very bright, beautiful meteor,” Maarten said.
“Fantastic,” said Jim. “Did you hear anything?”
“No. We listened.”
They passed around a glass jar of Oreos and discussed the meteor. Maarten said, “The sound would have taken five minutes to reach us. It exploded. A little piece shot off.” He spread out his fingers. “Like this—pop!”
They had seen the destruction of an earth-crossing minor planet the size of a golf ball. When it hit the atmosphere, the resulting flash had been visible for upward of three hundred miles. “The Shoemakers,” Jim Gunn remarked, “are into one of the few areas of astronomy that may have a real impact.”
Sometime before the year 1664, a shower of meteorites fell into Milan, Italy, killing some sheep and a friar. A curious doctor who opened the dead friar found that a small stone had shot the friar through the femur, shattering the bone and causing his death. In 1856, a bright light passed over the ship Joshua Bates, covering it with black-glass dust. On the morning of June 30, 1908, a fireball passed over Siberia, making “even the light of the sun appear dark,” according to a witness. In the minutes that followed, something terrible happened in a deserted swamp in the valley of the Middle Tunguska River. A brilliant mushroom cloud boiled up into the stratosphere. Fifty miles from ground zero, a shock wave picked up a tent full of Evenkian nomads and tossed it like a purse through the air. A man sitting on a porch at the Vanovara trading station, seventy miles from ground zero, experienced a bath of radiant heat, followed by a concussion wave that plucked him from the porch and threw him ten feet through the air, knocking him senseless. The blast incinerated and flattened forests over hundreds of square miles, and the roar broke windows and crockery up to six hundred miles away. A pressure wave traveled twice around the earth. The following night, the sky glowed so brightly over Europe that a person in London could read a newspaper outdoors at midnight; and the cause of it all was the impact of a comet or an Apollo asteroid about two hundred feet across.
One afternoon in 1912, at Holbrook, Arizona, the section boss of the Santa Fe Railroad and his family were eating dinner. They heard a “terrific crash.” One of the boys ran outside. He said, “It’s raining rocks out here!” Father went out. As they looked, the entire plain for a mile eastward filled with puffs, reminding them of bullets “kicking up dust.” Fourteen thousand meteorites landed.
In Johnstown, Colorado, on July 6, 1924, a funeral was taking place in the graveyard behind the little church at Elwell, when there came a sound like machine-gun fire, and then a thug as a meteorite hit the road where the funeral procession had just passed. Mr. Clingenpeel, the undertaker, dug it up.
On April 28, 1927, in Aba, Japan, Mrs. Kuriyama’s five-year-old daughter was playing in the garden when she cried out. She had been struck in the head by a meteorite the size of a mung bean, which her mother found resting on her neckband. This stone now sits in a museum in Japan. It is called the Aba.
One bright day in 1931, Mr. Foster of Eaton, Colorado, was leaning on his hoe in his garden. Something hummed past his ear and whopped into the soil. He pulled out a coppery nugget shaped like his pinky. It was the Eaton. He sold it to a collector for five dollars.
In Benld, Illinois, Mrs. Carl Crum was working in her backyard on the morning of September 29, 1938, when she heard what sounded like an airplane going into a power dive, followed by a sound of breaking boards coming from her neighbor Mr. Ed McCain’s garage. “Just one of those things,” she thought.
That afternoon, Mr. Ed McCain thought he would “go uptown” in his Pontiac coupe. He went into his garage. He opened his car door. There was a hole in the seat. He called to his neighbor, Mr. Carl Crum: “Come over here, Carl, and see what the rats have done to my seat cushion.”
Mr. Crum came over and studied the damage.
Mr. McCain went on, “I knew the rats were getting thick around here, but I never supposed they could do that much damage.”
“Ed, no rats ever made that hole.”
Then they saw a hole in the roof of the car. They backed the car out of the garage. They lifted up the seat cushion, and there, tangled in the seat springs, they found the Benld.
An Air Force pilot flying at high altitude over Alabama on November 30, 1954, saw a bright light, like a falling star, heading in the general direction of the town of Sylacauga. Meanwhile in Sylacauga, across the street from the Comet Drive-In Theater, Mrs. E. Hulitt Hodges had just fallen asleep on her couch, when a loud noise woke her up. She jumped to her feet. At first she thought the gas heater had exploded. Then she felt a pain in her side. The falling star had destroyed her roof, bounced off her radio, and bruised Mrs. E. Hulitt Hodges viciously over the hip. Now, lying insolently on her rug, was the Sylacauga—eight and a half pounds of hypersthene stone, fresh from out near Mars.
After the nuclear test ban treaty of 1963, the United States Air Force established a secret global network of air-pressure sensors in order to detect any clandestine aboveground nuclear blasts. Within a year or two the network had detected a number of tremendous air shocks, including one over the South Atlantic that registered half a megaton. Either somebody was cheating or—Air Force scientists realized—meteoroids exploding in the upper atmosphere were releasing the energy of a nuclear bomb.
April 8, 1971. Wethersfield, Connecticut, dawn. A witness saw a bright bomb bursting over the town. An hour later Mr. P
aul Cassarino went into his living room and found plaster on the floor. He looked up and discovered the Wethersfield One embedded in the ceiling.
On August 10, 1972, something came in from space over Utah. For two minutes it moved northward over Idaho and Montana, going at least Mach 20. It may have skipped off the atmosphere over Canada, or, Gene Shoemaker suspects, it may have coasted to a relatively gentle impact somewhere in the Canadian forest. Mr. James Baker, vacationing at Jackson Lake, Wyoming, took an extraordinary photograph of it. In the picture his wife is standing on a dock. She is obviously startled. She looks toward the Grand Tetons, where, high over the peaks, a fireball is leaving a ruler-straight smoke trail through the upper atmosphere. This was almost certainly an Apollo object somewhat bigger than a diesel locomotive, overtaking the earth from behind. Had it come in at a steeper angle, Mr. Baker might have photographed a two-kiloton mushroom cloud boiling abaft the Tetons.
Wethersfield, Connecticut, again. One evening in 1982, Mr. and Mrs. Robert Donoghue were watching M*A*S*H in the breezeway of their home when they heard “a sound like a truck coming through the front door.” The Wethersfield Two entered their living room and battered the walls as it ricocheted around, overturning furniture. The Donoghues left in a hurry. Firemen found the stone under the living room table.
Perth, Australia, September 30, 1984. At Binningup Beach two sunbathers heard a whistle and a thump. A meteorite had missed them by twelve feet and burrowed into the sand. Three months later, in Claxton, Georgia, Mr. Don Richardson was just stepping out of his trailer when a whine that reminded him of mortar fire in Vietnam made him flinch and he saw a hostile incoming chondrite destroy Mrs. Carutha Barnard’s mailbox.
Gene and Carolyn Shoemaker are cosmic weather forecasters. The chance of falling stones, they warn, is 100 percent. Gene estimates that about once a year, somewhere over the earth, a meteoroid vaporizes in the upper atmosphere during a Hiroshima-force “event,” as he likes to call such happenings. Since water covers two thirds of the earth, many airbursts happen unwitnessed over the sea. Every twenty-five years, perchance, there is an event that nudges a megaton—the power of a hydrogen bomb. An event like the Siberian Tunguska blast will occur roughly once every three centuries. Gene gives 5 to 20 percent odds that an impact twice as powerful as Tunguska will occur during the next seventy-five years. He figures upon only a 1½ percent chance that a 670-megaton blast will happen in the next seventy-five years. That could probably snuff Belgium. “I’m not losing any sleep,” Gene said. “The odds are much better that we will do it to ourselves first.”
What if Carolyn actually found an object heading for the earth? “I might not notice it in the films,” Carolyn said from the control desk. “If it were heading right at us, it wouldn’t appear to move. It would look like a star. Not until it was on top of us, when it would suddenly appear to move very fast.” But by then, she added, it might be too late to tell anyone.
Gene dragged the shortest stool under the telescope and contorted himself in order to fit on it. “Naw, this isn’t going to work,” he said. He kicked the stool aside and sat on the floor. “This is a terrible angle,” he said, peering into the guide scope. “Suppose,” he said, “that a twelve-megaton fireball went off over a politically unstable region. Suppose that happened over Pakistan, and suppose Pakistan had the bomb. The heat, the light, getting knocked on your fanny by the airwave—a large number of people would swear they had been nuked. The political leadership might say, ‘Oh, those S.O.B.’s! They’ve nuked us!’ And respond with a real nuclear attack on someone else.”
Over geologic time, sooner or later, there would come to pass what Gene liked to call a “major event.” That was to distinguish it from an “event.” Major events happened roughly once every hundred thousand years, and many happened in the sea, of course. If the Tunguska horror had been made by an object the size of a building, then a responsible scientist had to consider what might happen if a projectile the size of the summit cone of the Matterhorn clobbered the earth. Gene knew what would happen. A Matterhorn coming in vertically, he said, would punch through the atmosphere in one second. He said, “A bow wave in front of it opens a hole in the atmosphere, and the atmosphere burns, making nitrogen oxides.” He turned on the lights and studied a dial on the wall. He said, “I think this clock is slipping again.” He pointed a flashlight at the base of the telescope and fiddled with some metal parts. After he had gotten things under control he explained that when a giant asteroid moving at nine miles per second hits the ground, it explodes. “I’m ready,” he said.
Carolyn counted down. He started an exposure looking to the west, where the Trojans were now setting, because dawn was approaching. He said that when the asteroid lands, a hypersonic compressional wave rips through the asteroid, transforming it into a liquid mass that tries to splash but turns into a fireball. The rocks at ground zero compress to one-third their normal size, and a tremendous flash of light floods out of ground zero. “The radiant heat,” Gene said, “would set buildings on fire sixty miles away.” If the asteroid hit the sea, it would start tsunamis (tidal waves) cresting all over the planet. If a major event occurred in the sea off Long Island, then—never mind Manhattan—forget New Jersey. Forget Boston, Washington, Miami, Lisbon, and Dakar, for the spreading tsunamis would obliterate many cities sitting on the rim of the Atlantic Ocean.
The impact would squeeze out a ring of debris—a conical sheet of crushed, molten, and vaporized rock known as the cone of ejecta. The leading edge of the cone of ejecta expands at hypersonic velocity upward into the atmosphere, like a blossoming flower. It superheats the atmosphere into a bubble of gas mixed with molten and vaporized glass. The bubble bursts through the top of the atmosphere into outer space, and the glass keeps going. Thumb-sized pieces of glass might soar on ballistic suborbital trajectories halfway around the earth to reenter the atmosphere at a concentration zone opposite the point of impact, causing fire storms of molten glass to rain down over a region the size of Australia. The blast might inject enough dust into the atmosphere to blot the sun over the entire planet, perhaps causing a temporary winter. Nitrogen oxides (the burned atmosphere) would turn into nitric acid rains. Gene suspected that during the last million years a spike of powerful impacts had occurred—a pulse of perhaps as many as thirty major events, including as many as twelve continental impacts. This suggested the possibility that our species had come of age during a mild comet shower. It seemed that Homo sapiens had already survived a dozen natural versions of nuclear war—with the important difference that impact holocausts do not leave radioactive fallout in their wakes.
There are asteroids much larger than the Matterhorn traveling on earth-crossing orbits. Sisyphus and Hephaistos, two earth-crossers, are each about six miles in diameter. Either could hit the earth. If either did, the result would be what Gene called a “global catastrophe” to distinguish it from a mere “major event.” The first strong evidence that life on earth might have endured a global catastrophe came in 1980, when Luis and Walter Alvarez and coworkers analyzed an unusual layer of grayish and reddish claystone found near the medieval town of Gubbio, Italy. This claystone dates from about sixty-five million years ago. It is thin—less than an inch thick—but it marks a sharp boundary between beds of rocks below it, containing Cretaceous fossils from the age of dinosaurs, and rocks above it, containing younger Tertiary fossils from the age of mammals. Now known as the K-T boundary layer, it contains abnormally large amounts of rare metals, such as iridium, which are found in much higher abundances in meteorites and comets than in crustal rocks on the earth. The K-T layer has been collected from more than seventy locations everywhere on earth, including the ocean basins. It resembles a coating of paint that was once laid down over the entire planet. The K-T layer also contains microscopic spherules of minerals that may once have been molten glass, shocked mineral grains, and carbon soot. Sixty-five million years ago the sky misted glass, forests on several continents burned, and the planet grew a skin
of clay.
The Alvarez group proposed that the impact of a large Apollo asteroid had thrown up enough dust to cause an impact winter, during which global temperatures dropped for months or years, halting photosynthesis in plants. At roughly the same time at least half of the species on earth vanished, both plants and animals. “If you put that thin layer of clay up in the atmosphere as dust,” Gene remarked, “it wouldn’t let in any more light than a slab of wet clay half an inch thick. The only light at the surface of the earth would have been from luminescent organisms and from fires.” Darkness had triggered the death of much single-celled life in the sea, dynamiting the pyramid of life at its base, causing mass extinctions to ripple upward.
Gene had teamed up with Cesare Emiliani and Eric Kraus to try an idea: that a single projectile, about six miles across, had dropped into the Pacific Ocean. Erupting with the force of a thousand nuclear wars, a shock bubble blew thousands of cubic miles of ocean, atmosphere, and crustal rock into outer space, leaving a hole filling with lava erupting from the earth’s mantle. In one hour a blanket of dust, spreading in free-fall through space, cloaked the earth. The sea returned over the crater as a tidal wave several miles high, which boiled when it hit the lava. The crater might still be preserved in a deep ocean basin—you might find rings on the sea floor. “But we just haven’t explored the oceans that well,” Gene said. The rock and seawater thrown into space came down as mud rains or mud snows over the whole planet, mixed with nitric acid, but a considerable amount of water vapor stayed aloft. Water vapor traps heat from the sun. Global temperatures, after plunging during the time of darkness, bounced upward when the skies cleared. A greenhouse effect had cooked the dinosaurs and turned shallow seas into hot tubs. Small mammals, able to shed heat by virtue of their small bodies, had managed to maintain breeding populations. “That’s one of the ideas you could put into the pot,” Gene said.