First Light: The Search for the Edge of the Universe (15 page)

Astronomers used to think that the asteroid belt might be the rubble of an exploded planet. Now they think it is the leftover material from a planet that never formed. Jupiter, the heaviest planet in the solar system, disturbed a ring of planetesimals in the region now occupied by the asteroids, preventing that ring from accreting into a planet. Jupiter’s gravity raked those planetesimals, mixed them up, tossed them around. They could not stick together. Every time two planetesimals collided, they broke into fragments, and Jupiter pulled the fragments everywhere, causing more collisions and the production of more fragments. The asteroids are shattered bits of planetesimals that never congealed into a world; they are the bones of an accretion disk. Jupiter is still churning the Main Belt; accidents still happen. Most asteroids appear to be pieces of broken objects. Hammered by repeated impacts, asteroids are covered with a layer of dust and rubble, and some may even be piles of bashed fragments barely clinging together under their own gravity. Jupiter has already thrown most of the mass of the asteroid belt off into deep space. “If you took all the asteroids in the Main Belt and wadded them up into a ball,” Gene said, “you would get something about a tenth of the mass of the moon. A
spit in the bucket.” Jupiter is still gradually grinding up the Main Belt and throwing its fragments away.

While most Belt asteroids are on stable orbits that do not come near the earth, it seems pretty clear to scientists who trace the entanglements of orbits that the Belt must be pumping asteroids into earth-crossing orbits. The Main Belt is itself gathered into rings, separated by clear lanes called the Kirkwood Gaps. Jupiter sweeps those lanes clean. Any fragment that falls by chance into a Kirkwood Gap enters a resonating dance with Jupiter, which can flip the asteroid away. Nothing can remain for long inside a Kirkwood Gap. Orbital specialists believe that the Kirkwood Gaps, and other unstable areas in and around the Main Belt, are a source of many earth-crossing asteroids. For example, two asteroids can collide in the Belt. A fragment can drift into a Kirkwood Gap. Jupiter can pull the fragment from the Kirkwood Gap and throw it into an orbit near Mars. If the asteroid happens to have a close encounter with Mars during the next few million years or so, then Mars can throw the asteroid inward toward the earth. As a result, the supply of earth-crossers is constantly being renewed. Jupiter drags asteroids from the Kirkwood Gaps and hands them to Mars, and Mars hands them to the earth. Saturn can also pull an asteroid from a Kirkwood Gap and throw it directly at the earth.

“A lot of astronomers call asteroids the vermin of the skies,” Carolyn said.

Gene laughed, while his figure moved vaguely, outlined by a red reading lamp at the control desk.

“Gene and I,” Carolyn went on, “regard
galaxies
as the vermin of the skies.”

“There are far too damn many galaxies,” Gene said. “Carolyn has nearly reported galaxies to the Minor Planet Center.”

“They’re confusing,” she said. “The fainties can look like comets. I get so excited. Then I find out it’s only a galaxy.”

G
ene’s father, George Shoemaker, bought a farm in the 1930s along the North Platte River in Wyoming, where he raised navy beans. Beans were a lucrative crop during the Depression, and George’s only problem was that his wife, Muriel, could not take farming beans. “My mother was gone like a shot,” Gene said. “I guess if she had been able to stand it, I would be a farmer right now.” Muriel Shoemaker left for Buffalo, New York, to teach school. Despite their differences over farming, George and Muriel stayed in love with each other and remained married. Gene would spend the winter in Buffalo and then take a train to Wyoming to spend the summer with Dad on the bean farm. Tiring of beans, his father went to Hollywood, where he eventually found work as a grip in a movie studio and where Muriel joined him again.

Gene went to high school in Los Angeles, where he became interested in radioactive minerals. He majored in geology at Caltech during the years following World War II. “Caltech,” he said, “has always been a haven for space freaks.” The tendency, he said, began with the Hale Telescope. He liked to stand in the viewing gallery of the Caltech optical shop and watch Marcus Brown’s men, in white tennis shoes, work a polishing machine that traced Lissajous figures across the biggest piece of glass the world had ever seen. A few miles away, in Arroyo Seco, plumes of smoke occasionally erupted and a rumble shook the surrounding towns: Professor Theodore von Kármán and his students at the Jet Propulsion Laboratory were testing rocket motors. Then, during the summer of 1948, fresh out of Caltech, Gene found himself working for the United States Geological Survey, mapping uranium-bearing
formations in the Paradox Valley of westernmost Colorado. The Geological Survey put him in a bunkhouse in a mining settlement. He would drive into the town of Naturita each day for breakfast, five miles on a dirt road through the Paradox Valley. One morning, when he was pounding along in a Jeep on the way to breakfast, a strange thought flooded over Gene. As he tells it, “I started thinking about von Kármán and those rocket motors. I also knew what was happening at the White Sands Proving Grounds. Wernher von Braun was down there, firing off a bunch of captured German V-2 rockets. All of a sudden I got this feeling in my bones. I said, By God, they are going to build a rocket—
they are going to build a rocket and take men to the moon with it!
What a thing! What an unbelievable thing! To be the first man on the moon! And what other person to explore the moon but a geologist? I decided right there that when they took applications, I was going to be standing at the head of the line.” He saw a flaw in his plan, which was, as he put it, “If you had told anybody in 1948 that you wanted to be a geologist walking around on the moon, they would have considered you a prime candidate for the lunatic asylum.” He swore an oath to do whatever he could to get himself to the moon but to keep his mouth shut about his ambition. At twenty years of age in Paradox Valley, something terrible happened to Gene Shoemaker. He became a geologist obsessed with the sky.

A lunar geologist would have to know something about the holes on the moon. In the late 1940s, prevailing opinion said that these holes had been made by volcanoes. Gene taught himself explosive volcanism. The earth’s surface concealed many enormous, ringlike features known as crypto-volcanic structures—believed to be the remains of superexplosive volcanic eruptions. He studied cryptovolcanoes. He also walked around Meteor Crater, a hole in the ground nearly a mile across, outside Flagstaff. Despite its name, “the majority of geologists,” Gene said, “were equivocal—skeptical, perhaps—that it was of impact origin.” Some thought that Meteor Crater might be a collapsed salt dome or a hole left by a volcanic steam explosion. Not many professional geologists accepted a theory first proposed by Daniel Moreau Barringer in 1906, that a nickel-iron meteorite had exploded on impact there. Gene set out to make a geologic map of Meteor Crater for his Ph.D. thesis.
Barringer had drilled a series of holes in the floor of the crater, hoping to find a nickel-iron asteroid under the crater, which he never found. Gene examined Barringer’s old core samples and discovered they contained a lot of shattered rock, which was full of microscopic droplets of quartz glass saturated with particles of meteoritic iron. Around the lip of the crater Gene found layers of sedimentary rock peeled back from the rim, “like the petals of a flower blossoming.” He discovered that these layers of ejected rock had been deposited in reverse order. No volcano would lay down ejected debris in such an orderly fashion. For comparison he mapped craters formed by nuclear bombs in the Nevada desert—the Jangle U crater and the Teapot Ess. There he found thumb-sized blebs of shock-melted glasses blown into deeply shattered rocks, and sediments peeled back like flower petals from the lip of the crater, laid down in reverse order. The resemblance between nuclear and meteor craters seemed eerie to him. The evidence came to this: Meteor Crater had been made by an asteroid.

He stayed with the Geological Survey after receiving his degree. In 1960, Gene, Edward Chao, and Beth Madsen, all of the Geological Survey, discovered a natural mineral that they named coesite, found in the rocks of Meteor Crater. Coesite is a polymorph of silica that can form under shock—a wave of extreme pressure must rip through the rock, crushing the silica’s molecular lattice into coesite. No known event at the surface of the earth other than the impact of a giant meteorite could do that. As Gene would later say, “We had discovered a fingerprint for impact.” That brought him to Germany.

The Ries Basin is a circular depression seventeen miles across, north of the city of Augsburg, on the western border of Bavaria. Most geologists had assumed it to be an old volcano. “My German wasn’t good,” Gene said, “but the more I read about the Ries Basin, the more I became convinced it was an impact crater.” He believed that with the coesite fingerprint test he could prove it. On July 27, 1960—six days after the discovery of coesite was first published—he and Carolyn drove into the Ries in a new Volkswagen bus. Around sunset they found a quarry—it belonged to a cement factory, and the workers had gone home—and climbed down inside. Gene broke a few pieces of rock with his hammer and looked at
them in the fading light. At that moment, a scientific field, impact geology, came of age.

“The rock was shocked, melted, crushed,” he said, “full of blobs of dark glass. I just knew instantly there was coesite in it.” During the following days Gene and Carolyn explored the Ries. Gene walked through villages in a daze, with a rock hammer dangling from his hand. He found blasted, shocked rock everywhere, even cut into blocks and built into walls and houses. The Ries was a tremendous impact crater, populated with farms and towns. Near the center of the Ries they came across St. George’s Church, in the town of Nördlingen, built of the Ries rock—a shattered and sintered granite speckled with oozy droplets of black glass. The medieval stonecutters had unknowingly put up a church to the God of the Apocalypse. Fifteen million years ago, during Miocene times, something had come in from space and exploded on impact. Crustal rocks had offered this object the resistance that a tub of lard would give to a concussion grenade. Rock blown from the lip of the Ries Basin had soared or slithered for miles across Bavaria. The Ries amounted to a Kepler or a Tycho—essentially a lunar crater in Europe.

That was the first proof that a giant impact crater existed on the earth. Gene’s discovery opened the question of just how many impact craters the earth conceals, and it also opened the question of whether many of the so-called crypto-volcanic structures might actually be the eroded roots of impact craters. At last count, geologists have identified more than one hundred likely impact structures, including the sacred Lake Bosumtwi in Ghana; Lake Manicouagan in Quebec; dozens of eroded craters in the United States, including structures called Crooked Creek, Decaturville, Flynn Creek, Upheaval Dome, and the Manson Structure; the Serra de Cangalha in Brazil; the Rouchechouart in France; and Gosses Bluff in Australia. Gene thinks that perhaps as many as a thousand impact craters will eventually turn up, “provided we don’t cover the earth with nuclear craters first.” In 1960, when he walked into the Ries, the debate over whether the moon craters had been made by volcanoes still lingered; but if a big impact crater could be found on the earth, then those holes and rings in the moon would be impact craters too. Galileo had seen them the first time
he looked through a telescope, but to show that they were made by asteroids and comets, and that the earth was pockmarked with similar rings, took three more centuries and Gene Shoemaker with a hammer.

He founded the United States Geological Survey’s Branch of Astrogeology, now located in Flagstaff, dedicated to the geologic study of other worlds. He grew to prominence in the American space program, working first on the Ranger lunar probes, then as Principal Investigator for the imaging cameras on the Surveyor lunar lander, and finally as the Principal Investigator in charge of the geological fieldwork done during the Apollo manned lunar landings. But he never achieved escape velocity; he never left the earth. The adrenal glands on his kidneys progressively failed in 1962, killing forever his chances of going into space. “The irony of it,” he said, “is that I chaired the committee that recommended the names of the first astronaut candidates to NASA.” He would never forget the night launch of
Apollo 17
—the last of the manned lunar missions. He and Carolyn watched enthralled at Cape Canaveral as their friend and colleague from the Geological Survey, the geologist Harrison H. Schmitt, pulled away from the earth riding on a Saturn V rocket, which brightened and dimmed as it cut through cloud decks, a machine as tall as a thirty-story office building and already going at supersonic speed as it leaned to begin its roll downrange, while Gene studied, with the detachment of a scientist, the pain of the unfulfilled hope that had started that summer in 1948, in Paradox Valley, and which had delivered him to an open field in Florida, witnessing the launch of the first and last geologist to walk on the moon.

He would eventually leave the Apollo space program for other things, but he could not keep his eyes on the ground. Ever since Meteor Crater and the Ries, he had been wondering about rocks that fall from the sky. How many of them are out there? If you went looking for them with a telescope, what would you find? Could finding rocks in space give you a better estimate of how often the earth takes a hit? In 1972, when he began seriously thinking about a program to search for earth-crossing asteroids, the orbits of only three earth-crossers were accurately known: Icarus, Geographos, and Toro. Apollo had been lost. Hermes, the asteroid
that had ambushed the earth in 1937, had also been lost (it still is). Astronomers had shown more interest in looking for exploding galaxies than loose cannonballs near the earth. Yet the bombardment of the earth was quite obviously a continuing natural process.

He began working with a geophysicist from Caltech’s Jet Propulsion Laboratory, Eleanor Helin. Like Gene, Eleanor had begun to suspect that the number of earth-crossers might be enormous. She probed the Caltech archives looking for sightings of lost minor planets. She traveled to Germany to decipher the logbooks of dead astronomers—of Max Wolf and Karl Reinmuth—trying to recover the orbits of vanished earth-crossers. In 1973, Shoemaker and Helin founded the Palomar Planet-Crossing Asteroid Survey. She carried on the bulk of the telescopic work during the program’s early years, spending long nights on the eighteen-inch and the forty-eight-inch Schmidt telescopes on Palomar. They endured gambler’s luck. Right at the beginning, a huge Apollo object swept by, 1973 NA, now lost. “I said, ‘Hot damn! We are on to something!’ ” Gene recalled. But then came a long dry spell with no discoveries. Then a burst of discoveries. Then another dry spell. “There were times when I was almost ready to give it up, but Eleanor Helin just would not quit.”

She discovered Aten and Aristaeus, and codiscovered Ra-Shalom, all earth-crossers. She also discovered a large number of asteroids known as Amors, on unstable orbits near Mars—objects that could either hit Mars or be flipped into earth-crossing orbits in the future. Shoemaker and Helin defined three classes of earth-crossers. The Aten objects spend most of their time inside the earth’s orbit. The Amor objects spend most of their time out around Mars, moving inward to brush the earth’s orbit once in a while. The Apollo objects slash deeply back and forth. Gene has estimated that there are a total of about two thousand big Apollos, Atens, and Amors out there—asteroids able to collide with the earth now or at some time in the future—two thousand drunken mountains driving the freeways, most of which we have never seen. Smaller objects—the size of the Great Pyramid at Giza, for example—are exceedingly more numerous but exceedingly difficult to find. The odds are slim that something large might hit the earth during a human lifetime. From the human perspective, major impacts are
rare. “Human civilization,” Gene said, “is essentially instantaneous.” From an astronomical perspective, hypervelocity planetoids slam into the earth rather frequently. We live in an asteroid swarm.

Shoemaker and Helin eventually decided to divide their program. Helin founded the International Near-Earth Asteroid Survey—a program to coordinate sightings of earth-crossers all over the world. Shoemaker opted for a small but intense program on the Little Eye. Having neither the time nor the patience to scan films for asteroids, he needed an assistant.

It had not taken Gene long to propose marriage to Carolyn, but after they had been married, it had taken Gene about two years to work up his courage to tell her that he intended to go to the moon. She was frightened. She wondered if her husband was unstable. On second thought, that did not seem too bad, considering that she had always wanted to go to the moon herself, since those summer nights in Chico during her childhood; and so their hope became a mutual affair. She would explain that during the 1960s, “I thought that travel to the moon would become so common that even someone like me would be able to go.” Neither of them had made it into space, but at least there was no harm in going up on a mountain each moonless part of the month between September and May, to photograph a dome of jewels out of reach and to swear at a telescope.