by Bill Bryson
When asteroids were first detected in the 1800s—the very first was discovered on the first day of the century by a Sicilian named Guiseppe Piazzi—they were thought to be planets, and the first two were named Ceres and Pallas. It took some inspired deductions by the astronomer William Herschel to work out that they were nowhere near planet-sized but much smaller. He called them asteroids—Latin for “starlike”—which was slightly unfortunate as they are not like stars at all. Sometimes now they are more accurately called planetoids.
Finding asteroids became a popular activity in the 1800s and by the end of the century about a thousand were known. The problem was that no-one was systematically recording them. By the early 1900s, it had often become impossible to know whether an asteroid that popped into view was new or simply one that had been noted earlier and then lost track of. By this time, too, astrophysics had moved on so much that few astronomers wanted to devote their lives to anything as mundane as rocky planetoids. Only a few, notably Gerard Kuiper, the Dutch-born astronomer for whom is named the Kuiper belt of comets, took any interest in the solar system at all. Thanks to his work at the McDonald Observatory in Texas, followed later by work done by others at the Minor Planet Center in Cincinnati and the Spacewatch project in Arizona, a long list of lost asteroids was gradually whittled down until by the close of the twentieth century only one known asteroid was unaccounted for—an object called 719 Albert. Last seen in October 1911, it was finally tracked down in 2000 after being missing for eighty-nine years.
A meteor shower in 1872 recorded by the artist Amédée Guillemin. Meteor showers often occur when the Earth’s orbit passes through the drifting tails of defunct comets. (credit 13.4)
So, from the point of view of asteroid research the twentieth century was essentially just a long exercise in book-keeping. It is really only in the last few years that astronomers have begun to count and keep an eye on the rest of the asteroid community. As of July 2001, 26,000 asteroids had been named and identified—half in just the previous two years. With up to a billion to identify, the count obviously has barely begun.
In a sense it hardly matters. Identifying an asteroid doesn’t make it safe. Even if every asteroid in the solar system had a name and known orbit, no one could say what perturbations might send any of them hurtling towards us. We can’t forecast rock disturbances on our own surface. Put those rocks adrift in space and what they might do is beyond guessing. Any asteroid out there that has our name on it is very likely to have no other.
Think of the Earth’s orbit as a kind of motorway on which we are the only vehicle, but which is crossed regularly by pedestrians who don’t know enough to look before stepping off the verge. At least 90 per cent of these pedestrians are quite unknown to us. We don’t know where they live, what sort of hours they keep, how often they come our way. All we know is that at some point, at uncertain intervals, they trundle across the road down which we are cruising at over 100,000 kilometres an hour. As Steven Ostro of the Jet Propulsion Laboratory has put it, “Suppose that there was a button you could push and you could light up all the Earth-crossing asteroids larger than about ten metres, there would be over a hundred million of these objects in the sky.” In short, you would see not a couple of thousand distant twinkling stars, but millions upon millions upon millions of nearer, randomly moving objects—“all of which are capable of colliding with the Earth and all of which are moving on slightly different courses through the sky at different rates. It would be deeply unnerving.” Well, be unnerved, because it is there. We just can’t see it.
Altogether it is thought—though it is really only a guess, based on extrapolating from cratering rates on the Moon—that some two thousand asteroids big enough to imperil civilized existence regularly cross our orbit. But even a small asteroid—the size of a house, say—could destroy a city. The number of these relative tiddlers in Earth-crossing orbits is almost certainly in the hundreds of thousands and possibly in the millions, and they are nearly impossible to track.
The first one wasn’t spotted until 1991, and that was after it had already gone by. Named 1991 BA, it was noticed as it sailed past us at a distance of 170,000 kilometres—in cosmic terms the equivalent of a bullet passing through one’s sleeve without touching the arm. Two years later, another, somewhat larger asteroid missed us by just 145,000 kilometres—the closest pass yet recorded. It, too, was not seen until it had passed and would have arrived without warning. According to Timothy Ferris, writing in the New Yorker, such near misses probably happen two or three times a week and go unnoticed.
An object a hundred metres across couldn’t be picked up by any Earth-based telescope until it was within just a few days of us, and that is only if a telescope happened to be trained on it, which is unlikely because even now the number of people searching for such objects is modest. The arresting analogy that is always made is that the number of people in the world who are actively searching for asteroids is fewer than the staff of a typical McDonald’s restaurant. (It is actually somewhat higher now. But not much.)
While Gene Shoemaker was trying to get people galvanized about the potential dangers of the inner solar system, another development—wholly unrelated on the face of it—was quietly unfolding in Italy with the work of a young geologist from the Lamont Doherty Laboratory at Columbia University. In the early 1970s, Walter Alvarez was doing fieldwork in a comely defile known as the Bottaccione Gorge, near the Umbrian hill town of Gubbio, when he grew curious about a thin band of reddish clay that divided two ancient layers of limestone—one from the Cretaceous period, the other from the Tertiary. This is a point known to geology as the KT boundary1 and it marks the time, 65 million years ago, when the dinosaurs and roughly half the world’s other species of animals abruptly vanish from the fossil record. Alvarez wondered what it was about a thin lamina of clay, barely 6 millimetres thick, that could account for such a dramatic moment in the Earth’s history.
At the time, the conventional wisdom about the dinosaur extinction was the same as it had been in Charles Lyell’s day a century earlier—namely, that the dinosaurs had died out over millions of years. But the thinness of the clay layer clearly suggested that in Umbria, if nowhere else, something rather more abrupt had happened. Unfortunately, in the 1970s no tests existed for determining how long such a deposit might have taken to accumulate.
In the normal course of things, Alvarez almost certainly would have had to leave the problem at that; but luckily he had an impeccable connection to someone outside his discipline who could help—his father, Luis. Luis Alvarez was an eminent nuclear physicist; he had won the Nobel Prize for physics the previous decade. He had always been mildly scornful of his son’s attachment to rocks, but this problem intrigued him. It occurred to him that the answer might lie in dust from space.
Even a quite substantial asteroid probably wouldn't be noticed until it was within a few days of impact.
A conventional but misleading view of the asteroid belt. Though asteroids number in the hundreds of millions and form a loose belt in the great emptiness between Mars and Jupiter, there is on average a million miles of space between any two of them, so they hardly constitute a jumble, as shown here and almost always elsewhere. (credit 13.5)
Every year the Earth accumulates some 30,000 tonnes of “cosmic spherules”—space dust, in plainer language—which would be quite a lot if you swept it into one pile, but is infinitesimal when spread across the globe. Scattered through this thin dusting are exotic elements not normally much found on Earth. Among these is the element iridium, which is a thousand times more abundant in space than in the Earth’s crust (because, it is thought, most of the iridium on Earth sank to the core when the planet was young).
Luis Alvarez knew that a colleague of his at the Lawrence Berkeley Laboratory in California, Frank Asaro, had developed a technique for measuring very precisely the chemical composition of clays using a process called neutron activation analysis. This involved bombarding samples with neutrons in a small
nuclear reactor and carefully counting the gamma rays that were emitted; it was extremely finicky work. Previously Asaro had used the technique to analyse pieces of pottery, but Alvarez reasoned that if they measured the amount of one of the exotic elements in his son’s soil samples and compared that with its annual rate of deposition, they would know how long it had taken the samples to form. On an October afternoon in 1977, Luis and Walter Alvarez dropped in on Asaro and asked him if he would run the necessary tests for them.
The American Nobel laureate Luis Alvarez, who became gripped with the problem of explaining dinosaur extinctions after a conversation with his geologist son Walter. (credit 13.6)
It was really quite a presumptuous request. They were asking Asaro to devote months to making the most painstaking measurements of geological samples merely to confirm what seemed entirely self-evident to begin with—that the thin layer of clay had been formed as quickly as its thinness suggested. Certainly no-one expected his survey to yield any dramatic breakthroughs.
“Well, they were very charming, very persuasive,” Asaro recalled in an interview in 2002. “And it seemed an interesting challenge, so I agreed to try Unfortunately, I had a lot of other work on, so it was eight months before I could get to it.” He consulted his notes from the period. “On June 21, 1978, at 1.45 p.m., we put a sample in the detector. It ran for 224 minutes and we could see we were getting interesting results, so we stopped it and had a look.”
The results were so unexpected, in fact, that the three scientists at first thought they had to be wrong. The amount of iridium in the Alvarez sample was more than three hundred times normal levels—far beyond anything they might have predicted. Over the following months Asaro and his colleague Helen Michel worked up to thirty hours at a stretch (“Once you started you couldn’t stop,” Asaro explained) analysing samples, always with the same results. Tests on other samples—from Denmark, Spain, France, New Zealand, Antarctica—showed that the iridium deposit was worldwide and greatly elevated everywhere, sometimes by as much as five hundred times normal levels. Clearly something big and abrupt, and probably cataclysmic, had produced this arresting spike.
After much thought, the Alvarezes concluded that the most plausible explanation—plausible to them, at any rate—was that the Earth had been struck by an asteroid or comet.
The idea that the Earth might be subjected to devastating impacts from time to time was not quite as new as is now sometimes suggested. As far back as 1942, a Northwestern University astrophysicist named Ralph B. Baldwin had suggested such a possibility in an article in Popular Astronomy magazine. (He published the article there because no academic publisher was prepared to run it.) And at least two well-known scientists, the astronomer Ernst Öpik and the chemist and Nobel laureate Harold Urey had also voiced support for the notion at various times. Even among palaeontologists it was not unknown. In 1956 a professor at Oregon State University, M.W. de Laubenfels, writing in the Journal of Paleontology, had actually anticipated the Alvarez theory by suggesting that the dinosaurs may have been dealt a death blow by an impact from space, and in 1970 the president of the American Paleontological Society, Dewey J. McLaren, proposed at the group’s annual conference the possibility that an extraterrestrial impact may have been the cause of an earlier event known as the Frasnian extinction.
As if to underline just how un-novel the idea had become by this time, in 1979 a Hollywood studio actually produced a movie called Meteor (“It’s five miles wide…It’s coming at 30,000 m.p.h.—and there’s no place on Earth to hide!”) starring Henry Fonda, Natalie Wood, Karl Malden and a very large rock.
So when, in the first week of 1980, at a meeting of the American Association for the Advancement of Science, the Alvarezes announced their belief that the dinosaur extinction had not taken place over millions of years as part of some slow inexorable process, but suddenly in a single explosive event, it shouldn’t have come as a shock.
A year before the Alvarezes announced their controversial theory, Hollywood provided an unwitting reminder that the idea of devastating impacts from space was hardly new. In fact, it had been around since 1942. (credit 13.7)
But it did. It was received everywhere, but particularly in the palaeontological world, as an outrageous heresy.
“Well, you have to remember,” Asaro recalls, “that we were amateurs in this field. Walter was a geologist specializing in palaeomagnetism, Luis was a physicist and I was a nuclear chemist. And now here we were telling palaeontologists that we had solved a problem that had eluded them for over a century. It’s not terribly surprising that they didn’t embrace it immediately.” As Luis Alvarez joked: “We were caught practising geology without a licence.”
But there was also something much deeper and more fundamentally abhorrent in the impact theory. The belief that terrestrial processes were gradual had been elemental in natural history since the time of Lyell. By the 1980s, catastrophism had been out of fashion for so long that it had become literally unthinkable. For most geologists the idea of a devastating impact was, as Eugene Shoemaker noted, “against their scientific religion.”
Nor did it help that Luis Alvarez was openly contemptuous of palaeontologists and their contributions to scientific knowledge. “They’re really not very good scientists. They’re more like stamp collectors,” he wrote in the New York Times, in an article that stings yet.
Opponents of the Alvarez theory produced any number of alternative explanations for the iridium deposits—for instance, that they were generated by prolonged volcanic eruptions in India called the Deccan Traps (“trap” comes from a Swedish word for a type of lava; “Deccan” is the name of the area today)—and above all insisted that there was no proof that the dinosaurs disappeared abruptly from the fossil record at the iridium boundary. One of the most vigorous opponents was Charles Officer of Dartmouth College. He insisted that the iridium had been deposited by volcanic action even while conceding in a newspaper interview that he had no actual evidence of it. As late as 1988, more than half of all American palaeontologists contacted in a survey continued to believe that the extinction of the dinosaurs was in no way related to an asteroid or cometary impact.
The one thing that would most obviously support the Alvarezes’ theory was the one thing they didn’t have—an impact site. Enter Eugene Shoemaker. Shoemaker had an Iowa connection—his daughter-in-law taught at the University of Iowa—and he was familiar with the Manson crater from his own studies. Thanks to him, all eyes now turned to Iowa.
Geology is a profession that varies from place to place. In Iowa, a state that is flat and stratigraphically uneventful, it tends to be comparatively serene. There are no alpine peaks or grinding glaciers, no great deposits of oil or precious metals, not a hint of a pyroclastic flow. If you are a geologist employed by the state of Iowa, a big part of the work you do is to evaluate Manure Management Plans, which all the state’s “animal confinement operators”—pig farmers, to the rest of us—are required to file periodically There are 15 million pigs in Iowa, so a lot of manure to manage. I’m not mocking this at all—it’s vital and enlightened work; it keeps Iowa’s water clean—but with the best will in the world it’s not exactly dodging lava bombs on Mount Pinatubo or scrabbling over crevasses on the Greenland ice sheet in search of ancient life-bearing quartzes. So we may well imagine the flutter of excitement that swept through the Iowa Department of Natural Resources when in the mid-1980s the world’s geological attention focused on Manson and its crater.
Trowbridge Hall in Iowa City is a turn-of-the-century pile of red brick that houses the University of Iowa’s Earth Sciences Department and—way up in a kind of garret—the geologists of the Iowa Department of Natural Resources. No-one now can remember quite when, still less why, the state geologists were placed in an academic facility, but you get the impression that the space was conceded grudgingly, for the offices are cramped and low-ceilinged and not very accessible. When being shown the way, you half expect to be taken out onto a roof ledge and
helped in through a window.
Ray Anderson and Brian Witzke spend their working lives up here amid disordered heaps of papers, journals, furled charts and hefty specimen stones. (Geologists are never at a loss for paperweights.) It’s the kind of space where if you want to find anything—an extra chair, a coffee cup, a ringing telephone—you have to move stacks of documents around.
“Suddenly we were at the centre of things,” Anderson told me, gleaming at the memory of it, when I met him and Witzke in their offices on a dismal, rainy morning in June. “It was a wonderful time.”
I asked them about Gene Shoemaker, a man who seems to have been universally revered. “He was just a great guy,” Witzke replied without hesitation. “If it hadn’t been for him, the whole thing would never have gotten off the ground. Even with his support, it took two years to get it up and running. Drilling’s an expensive business—about thirty-five dollars a foot back then, more now, and we needed to go down three thousand feet.”
Eugene Shoemaker was one of the first scientists to be convinced of the potential—and in the long run inevitable—dangers of comets and asteroids colliding with Earth. (credit 13.8)
“Sometimes more than that,” Anderson added.
“Sometimes more than that,” Witzke agreed. “And at several locations. So you’re talking a lot of money. Certainly more than our budget would allow.”
So a collaboration was formed between the Iowa Geological Survey and the US Geological Survey.
