So from the point of view of asteroid research the twentieth century was essentially just along exercise in bookkeeping. It is really only in the last few years that astronomers havebegun to count and keep an eye on the rest of the asteroid community. As of July 2001,twenty-six thousand asteroids had been named and identified—half in just the previous twoyears. With up to a billion to identify, the count obviously has barely begun.
The meteorite itself would vaporize instantly, but the blast would blow out a thousand cubickilometers of rock, earth, and superheated gases. Every living thing within 150 miles thathadn’t been killed by the heat of entry would now be killed by the blast. Radiating outward atalmost the speed of light would be the initial shock wave, sweeping everything before it.
In the normal course of things, Alvarez almost certainly would have had to leave theproblem at that, but luckily he had an impeccable connection to someone outside hisdiscipline 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 mildlyscornful of his son’s attachment to rocks, but this problem intrigued him. It occurred to himthat the answer might lie in dust from space.
Of course, Witzke and Anderson added hastily and more or less in unison, there wouldhave been terrible devastation across much of the Earth, as just described, and completeannihilation for hundredsof miles around ground zero. But life is hardy, and when the smokecleared there were enough lucky survivors from every species that none permanentlyperished.
“At the time we started, only slightly more than a dozen of these things had ever beendiscovered in the entire course of astronomical observation,” Shoemaker recalled some yearslater in a television interview. “Astronomers in the twentieth century essentially abandonedthe solar system,” he added. “Their attention was turned to the stars, the galaxies.”
He shrugged. “Who knows? Anyway, it was a pretty good insight into how unattractivescience can get when you’re playing at a certain level.”
One week before the impact, Nature ran an article, “The Big Fizzle Is Coming,” predictingthat the impact would constitute nothing more than a meteor shower.
“It was pretty stunning,” recalls Anderson. “I mean, we had this thing that was reallyimportant and then suddenly we didn’t have it anymore. But even worse was the realizationthat the people we thought we’d been collaborating with hadn’t bothered to share with us theirnew findings.”
Conveniently a natural test of the theory arose when the Shoemakers and Levy discoveredComet Shoemaker-Levy 9, which they soon realized was headed for Jupiter. For the first time,humans would be able to witness a cosmic collision—and witness it very well thanks to thenew Hubble space telescope. Most astronomers, according to Curtis Peebles, expected little,particularly as the comet was not a coherent sphere but a string of twenty-one fragments. “Mysense,” wrote one, “is that Jupiter will swallow these comets up without so much as a burp.”
Anderson and Witzke no longer had the crater that killed the dinosaurs, “but we still hadthe largest and most perfectly preserved impact crater in the mainland United States,”
Trowbridge Hall in Iowa City is a turn-of-the-century pile of red brick that houses theUniversity of Iowa’s Earth Sciences department and—way up in a kind of garret—thegeologists of the Iowa Department of Natural Resources. No one now can remember quitewhen, still less why, the state geologists were placed in an academic facility, but you get theimpression 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 outonto a roof ledge and helped in through a window.
For those outside the zone of immediate devastation, the first inkling of catastrophe wouldbe a flash of blinding light—the brightest ever seen by human eyes—followed an instant to aminute or two later by an apocalyptic sight of unimaginable grandeur: a roiling wall ofdarkness reaching high into the heavens, filling an entire field of view and traveling atthousands of miles an hour. Its approach would be eerily silent since it would be moving farbeyond the speed of sound. Anyone in a tall building in Omaha or Des Moines, say, whochanced to look in the right direction would see a bewildering veil of turmoil followed byinstantaneous oblivion.
When asteroids were first detected in the 1800s—the very first was discovered on the firstday of the century by a Sicilian named Giuseppi Piazzi—they were thought to be planets, andthe first two were named Ceres and Pallas. It took some inspired deductions by theastronomer William Herschel to work out that they were nowhere near planet sized but muchsmaller. He called them asteroids—Latin for “starlike”—which was slightly unfortunate asthey are not like stars at all. Sometimes now they are more accurately called planetoids.
The amount of soot and floating ash from the impact and following fires would blot out thesun, certainly for months, possibly for years, disrupting growing cycles. In 2001 researchers atthe California Institute of Technology analyzed helium isotopes from sediments left from thelater KT impact and concluded that it affected Earth’s climate for about ten thousand years.
An asteroid or comet traveling at cosmic velocities would enter the Earth’s atmosphere atsuch a speed that the air beneath it couldn’t get out of the way and would be compressed, as ina bicycle pump. As anyone who has used such a pump knows, compressed air grows swiftlyhot, and the temperature below it would rise to some 60,000 Kelvin, or ten times the surfacetemperature of the Sun. In this instant of its arrival in our atmosphere, everything in themeteor’s path—people, houses, factories, cars—would crinkle and vanish like cellophane in aflame.
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.
To most people in Manson the biggest thing ever to happen was a tornado that rolled upMain Street in 1979, tearing apart thebusiness district. One of the advantages of all thatsurrounding flatness is that you can see danger from a long way off. Virtually the whole townturned out at one end of Main Street and watched for half an hour as the tornado came towardthem, hoping it would veer off, then prudently scampered when it did not. Four of them, alas,didn’t move quite fast enough and were killed. Every June now Manson has a weeklong eventcalled Crater Days, which was dreamed up as a way of helping people forget that unhappyanniversary. It doesn’t really have anything to do with the crater. Nobody’s figured out a wayto capitalize on an impact site that isn’t visible.
Luis Alvarez never knew of the discovery of the Chicxulub crater or of the Shoemaker-Levy comet, as he died in 1988. Shoemaker also died early. On the third anniversary of theShoemaker-Levy impact, he and his wife were in the Australian outback, where they wentevery year to search for impact sites. On a dirt track in the Tanami Desert—normally one ofthe emptiest places on Earth—they came over a slight rise just as another vehicle wasapproaching. Shoemaker was killed instantly, his wife injured. Part of his ashes were sent tothe Moon aboard the Lunar Prospector spacecraft. The rest were scattered around MeteorCrater.
I asked them about Gene Shoemaker, a man who seems to have been universally revered.
The first one wasn’t spotted until 1991, and that was after it had already gone by. Named1991 BA, it was noticed as it sailed past us at a distance of 106,000 miles—in cosmic termsthe equivalent of a bullet passing through one’s sleeve without touching the arm. Two yearslater, another, somewhat larger asteroid missed us by just 90,000 miles—the closest pass yetrecorded. It, too, was not seen until it had passed and would have arrived without warning.
“No idea,” I replied.
“At least we thought it was a collaboration,” said Anderson, producing a small painedsmile.
The idea that the Earth might be subjected to devastating impacts from time to time was notquite as new as it is now sometimes presented. As far back as 1942, a NorthwesternUniversity astrophysicist named Ralph B. Baldwin had suggested such a possibility in anarticle in Popular Astronomy magazine. (He published the article there because no academicpublisher was prepared to run it.) And at least two well-known scientists, the astronomerErnst ?pik and the chemist and Nobel laureate Harold Urey, had also voiced support for thenotion at various times. Even among paleontologists it was not unknown. In 1956 a professorat Oregon State University, M. W. de Laubenfels, writing in the Journal of Paleontology, hadactually anticipated the Alvarez theory by suggesting that the dinosaurs may have been dealt adeath blow by an impact from space, and in 1970 the president of the AmericanPaleontological Society, Dewey J. McLaren, proposed at the group’s annual conference thepossibility that an extraterrestrial impact may have been the cause of an earlier event knownas the Frasnian extinction.
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 chemicalcomposition of clays using a process called neutron activation analysis. This involvedbombarding samples with neutrons in a small nuclear reactor and carefully counting thegamma rays that were emitted; it was extremely finicky work. Previously Asaro had used thetechnique to analyze pieces of pottery, but Alvarez reasoned that if they measured the amountof one of the exotic elements in his son’s soil samples and compared that with its annual rateof deposition, they would know how long it had taken the samples to form. On an Octoberafternoon in 1977, Luis and Walter Alvarez dropped in on Asaro and asked him if he wouldrun the necessary tests for them.
I asked them how much warning we would receive if a similar hunk of rock was comingtoward us today.
“Oh, about once every million years on average,” said Witzke.
Shoemaker-Levy 9 had been orbiting Jupiter in a fairly conspicuous manner since 1929, but ittook over half a century before anyone noticed.
Still, many people didn’t quite grasp what an impact could do. As Stephen Jay Gouldrecalled in one of his essays: “I remember harboring some strong initial doubts about theefficacy of such an event . . . [W]hy should an object only six miles across wreak such havocupon a planet with a diameter of eight thousand miles?”
“Oh, probably none,” said Anderson breezily. “It wouldn’t be visible to the naked eye untilit warmed up, and that wouldn’t happen until it hit the atmosphere, which would be about onesecond before it hit the Earth. You’re talking about something moving many tens of timesfaster than the fastest bullet. Unless it had been seen by someone with a telescope, and that’sby no means a certainty, it would take us completely by surprise.”
But that’s just the initial shockwave. No one can do more than guess what the associateddamage would be, other than that it would be brisk and global. The impact would almostcertainly set off a chain of devastating earthquakes. Volcanoes across the globe would beginto rumble and spew. Tsunamis would rise up and head devastatingly for distant shores. Withinan hour, a cloud of blackness would cover the planet, and burning rock and other debriswould be pelting down everywhere, setting much of the planet ablaze. It has been estimatedthat at least a billion and a half people would be dead by the end of the first day. The massivedisturbances to the ionosphere would knock out communications systems everywhere, sosurvivors would have no idea what was happening elsewhere or where to turn. It would hardlymatter. As one commentator has put it, fleeing would mean “selecting a slow death over aquick one. The death toll would be very little affected by any plausible relocation effort, sinceEarth’s ability to support life would be universally diminished.”
But there was also something much deeper and more fundamentally abhorrent in the impacttheory. The belief that terrestrial processes were gradual had been elemental in natural historysince the time of Lyell. By the 1980s, catastrophism had been out of fashion for so long that ithad become literally unthinkable. For most geologists the idea of a devastating impact was, asEugene Shoemaker noted, “against their scientific religion.”
The story begins in the early 1950s when a bright young geologist named EugeneShoemaker paid a visit to Meteor Crater in Arizona. Today Meteor Crater is the most famousimpact site on Earth and a popular tourist attraction. In those days, however, it didn’t receivemany visitors and was still often referred to as Barringer Crater, after a wealthy miningengineer named Daniel M. Barringer who had staked a claim on it in 1903. Barringer believedthat the crater had been formed by a ten-million-ton meteor, heavily freighted with iron andnickel, and it was his confident expectation that he would make a fortune digging it out.
Nor did it help that Luis Alvarez was openly contemptuous of paleontologists and theircontributions to scientific knowledge. “They’re really not very good scientists. They’re morelike stamp collectors,” he wrote in the New York Times in an article that stings yet.
“Well, you have to remember,” Asaro recalls, “that we were amateurs in this field. Walterwas a geologist specializing in paleomagnetism, Luis was a physicist and I was a nuclearchemist. And now here we were telling paleontologists that we had solved a problem that hadeluded them for over a century. It’s not terribly surprising that they didn’t embrace itimmediately.” As Luis Alvarez joked: “We were caught practicing geology without alicense.”
By the time Shoemaker came along, a common view was that Meteor Crater had beenformed by an underground steam explosion. Shoemaker knew nothing about undergroundsteam explosions—he couldn’t: they don’t exist—but he did know all about blast zones. Oneof his first jobs out of college was to study explosion rings at the Yucca Flats nuclear test sitein Nevada. He concluded, as Barringer had before him, that there was nothing at MeteorCrater to suggest volcanic activity, but that there were huge distributions of other stuff—anomalous fine silicas and magnetites principally—that suggested an impact from space.
“None,” he said, with a strange air of satisfaction. “Not one.”
What Shoemaker and his colleagues found was that there was more risk out there—a greatdeal more—than anyone had ever imagined.
Ray Anderson and Brian Witzke spend their working lives up here amid disordered heapsof papers, journals, furled charts, and hefty specimen stones. (Geologists are never at a lossfor paperweights.) It’s the kind of space where if you want to find anything—an extra chair, acoffee cup, a ringing telephone—you have to move stacks of documents around.
The results were so unexpected, in fact, that the three scientists at first thought they had tobe wrong. The amount of iridium in the Alvarez sample was more than three hundred timesnormal levels—far beyond anything they might have predicted. Over the following monthsAsaro and his colleague Helen Michel worked up to thirty hours at a stretch (“Once youstarted you couldn’t stop,” Asaro explained) analyzing samples, always with the same results.
The one thing that would most obviously support the Alvarezes’ theory was the one thingthey didn’t have—an impact site. Enter Eugene Shoemaker. Shoemaker had an Iowaconnection—his daughter-in-law taught at the University of Iowa—and he was familiar withthe Manson crater from his own studies. Thanks to him, all eyes now turned to Iowa.
But it did. It was received everywhere, but particularly in the paleontological community,as an outrageous heresy.
Intrigued, he began to study the subject in his spare time.
Alvarez wondered what it was about a thin lamina of clay, barely a quarter of an inch thick,that could account for such a dramatic moment in Earth’s history.
Think of the Earth’s orbit as a kind of freeway on which we are the only vehicle, but whichis crossed regularly by pedestrians who don’t know enough to look before stepping off thecurb. At least 90 percent of these pedestrians are quite unknown to us. We don’t know wherethey live, what sort of hours they keep, how often they come our way. All we know is that atsome point, at uncertain intervals, they trundle across the road down which we are cruising atsixty-six thousand miles 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-crossingasteroids larger than about ten meters, there would be over 100 million of these objects in thesky.” In short, you would see not a couple of thousand distant twinkling stars, but millionsupon millions upon millions of nearer, randomly moving objects—“all of which are capableof colliding with the Earth and all of which are moving on slightly different courses throughthe sky at different rates. It would be deeply unnerving.” Well, be unnerved because it isthere. We just can’t see it.
“And it seemed an interesting challenge, so I agreed to try. Unfortunately, I had a lot of otherwork on, so it was eight months before I could get to it.” He consulted his notes from theperiod. “On June 21, 1978, at 1:45 p.m., we put a sample in the detector. It ran for 224minutes and we could see we were getting interesting results, so we stopped it and had alook.”
“Suddenly we were at the center of things,” Anderson told me, gleaming at the memory ofit, when I met him and Witzke in their offices on a dismal, rainy morning in June. “It was awonderful time.”
PEOPLE KNEW FOR a long time that there was something odd about the earth beneathManson, Iowa. In 1912, a man drilling a well for the town water supply reported bringing up alot of strangely deformed rock—“crystalline clast breccia with a melt matrix” and “overturnedejecta flap,” as it was later described in an official report. The water was odd too. It wasalmost as soft as rainwater. Naturally occurring soft water had never been found in Iowabefore.
Opponents of the Alvarez theory produced any number of alternative explanations for theiridium deposits—for instance, that they were generated by prolonged volcanic eruptions inIndia called the Deccan Traps—and above all insisted that there was no proof that thedinosaurs disappeared abruptly from the fossil record at the iridium boundary. One of themost vigorous opponents was Charles Officer of Dartmouth College. He insisted that theiridium had been deposited by volcanic action even while conceding in a newspaper interviewthat he had no actual evidence of it. As late as 1988 more than half of all Americanpaleontologists contacted in a survey continued to believe that the extinction of the dinosaurswas in no way related to an asteroid or cometary impact.
If you are a geologist employed by the state of Iowa, a big part of the work you do is toevaluate Manure Management Plans, which all the state’s “animal confinement operators”—hog farmers to the rest of us—are required to file periodically. There are fifteen million hogsin Iowa, so a lot of manure to manage. I’m not mocking this at all—it’s vital and enlightenedwork; it keeps Iowa’s water clean—but with the best will in the world it’s not exactly dodginglava bombs on Mount Pinatubo or scrabbling over crevasses on the Greenland ice sheet insearch of ancient life-bearing quartzes. So we may well imagine the flutter of excitement thatswept through the Iowa Department of Natural Resources when in the mid-1980s the world’sgeological attention focused on Manson and its crater.
Hildebrand traveled to the site and decided fairly swiftly that they had their crater. By early1991 it had been established to nearly everyone’s satisfaction that Chicxulub was the impactsite.
How hard an impactor hits depends on a lot of variables—angle of entry, velocity andtrajectory, whether the collision is head-on or from the side, and the mass and density of theimpacting object, among much else—none of which we can know so many millions of yearsafter the fact. But what scientists can do—and Anderson and Witzke have done—is measurethe impact site and calculate the amount of energy released. From that they can work outplausible scenarios of what it must have been like—or, more chillingly, would be like if ithappened now.
“It was a real learning curve for us,” Witzke went on. “There was actually quite a lot of badscience going on throughout the period—people rushing in with results that didn’t alwaysstand up to scrutiny.” One of those moments came at the annual meeting of the AmericanGeophysical Union in 1985, when Glenn Izett and C. L. Pillmore of the U.S. GeologicalSurvey announced that the Manson crater was of the right age to have been involved with thedinosaurs’ extinction. The declaration attracted a good deal of press attention but wasunfortunately premature. A more careful examination of the data revealed that Manson wasnot only too small, but also nine million years too early.
“And remember,” added Anderson, “this was a relatively minor event. Do you know howmany extinctions were associated with the Manson impact?”
At the library in Manson they are delighted to show you a collection of newspaper articlesand a box of core samples from a 1991–92 drilling program—indeed, they positively bustle toproduce them—but you have to ask to see them. Nothing permanent is on display, andnowhere in the town is there any historical marker.
So when, in the first week of 1980, at a meeting of the American Association for theAdvancement of Science, the Alvarezes announced their belief that the dinosaur extinctionhad not taken place over millions of years as part of some slow inexorable process, butsuddenly in a single explosive event, it shouldn’t have come as a shock.
“Very occasionally we get people coming in and asking where they should go to see thecrater and we have to tell them that there is nothing to see,” says Anna Schlapkohl, the town’sfriendly librarian. “Then they go away kind of disappointed.” However, most people,including most Iowans, have never heard of the Manson crater. Even for geologists it barelyrates a footnote. But for one brief period in the 1980s, Manson was the most geologicallyexciting place on Earth.
The Manson impact was the biggest thing that has ever occurred on the mainland UnitedStates. Of any type. Ever. The crater it left behind was so colossal that if you stood on oneedge you would only just be able to see the other side on a good day. It would make the GrandCanyon look quaint and trifling. Unfortunately for lovers of spectacle, 2.5 million years ofpassing ice sheets filled the Manson crater right to the top with rich glacial till, then graded itsmooth, so that today the landscape at Manson, and for miles around, is as flat as a tabletop.
Anderson said. (A little verbal dexterity is required to keep Manson’s superlative status. Othercraters are larger—notably, Chesapeake Bay, which was recognized as an impact site in1994—but they are either offshore or deformed.) “Chicxulub is buried under two to threekilometers of limestone and mostly offshore, which makes it difficult to study,” Andersonwent on, “while Manson is really quite accessible. It’s because it is buried that it is actuallycomparatively pristine.”
Interestingly, because these things are so difficult to compute and must incorporate such asignificant margin of error, even if we knew an object was heading our way we wouldn’tknow until nearly the end—the last couple of weeks anyway—whether collision was certain.
Geological Survey.
At the time the conventional wisdom about the dinosaur extinction was the same as it hadbeen in Charles Lyell’s day a century earlier—namely that the dinosaurs had died out overmillions of years. But the thinness of the clay layer clearly suggested that in Umbria, if1It is KT rather than CT because C had already been appropriated for Cambrian. Depending on which sourceyou credit, the K comes either from the Greek Kreta or German Kreide. Both conveniently mean “chalk,” whichis also what Cretaceous means.
As if to underline just how un-novel the idea had become by this time, in 1979 aHollywood studio actually produced a movie called Meteor (“It’s five miles wide . . . It’scoming at 30,000 m.p.h.—and there’s no place to hide!”) starring Henry Fonda, NatalieWood, Karl Malden, and a very large rock.
So a collaboration was formed between the Iowa Geological Survey and the U.S.
The limestone that elsewhere gives Iowa its hard mineralized water was obliterated andreplaced by the shocked basement rocks that so puzzled the water driller in 1912.
Tests on other samples—from Denmark, Spain, France, New Zealand, Antarctica—showedthat the iridium deposit was worldwide and greatly elevated everywhere, sometimes by asmuch as five hundred times normal levels. Clearly something big and abrupt, and probablycataclysmic, had produced this arresting spike.
“Well, they were very charming, very persuasive,” Asaro recalled in an interview in 2002.
nowhere else, something rather more abrupt had happened. Unfortunately in the 1970s notests existed for determining how long such a deposit might have taken to accumulate.
Working first with his colleague Eleanor Helin and later with his wife, Carolyn, andassociate David Levy, Shoemaker began a systematic survey of the inner solar system. Theyspent one week each month at the Palomar Observatory in California looking for objects,asteroids primarily, whose trajectories carried them across Earth’s orbit.
Even if we did manage somehow to get a warhead to the asteroid and blasted it to pieces,the chances are that we would simply turn it into a string of rocks that would slam into us oneafter the other in the manner of Comet Shoemaker-Levy on Jupiter—but with the differencethat now the rocks would be intensely radioactive. Tom Gehrels, an asteroid hunter at theUniversity of Arizona, thinks that even a year’s warning would probably be insufficient totake appropriate action. The greater likelihood, however, is that we wouldn’t see any object—even a comet—until it was about six months away, which would be much too late.
The search moved elsewhere. By chance in 1990 one of the searchers, Alan Hildebrand ofthe University of Arizona, met a reporter from the Houston Chronicle who happened to knowabout a large, unexplained ring formation, 120 miles wide and 30 miles deep, under Mexico’sYucatán Peninsula at Chicxulub, near the city of Progreso, about 600 miles due south of NewOrleans. The formation had been found by Pemex, the Mexican oil company, in 1952—theyear, coincidentally, that Gene Shoemaker first visited Meteor Crater in Arizona—but thecompany’s geologists had concluded that it was volcanic, in line with the thinking of the day.
Within minutes, over an area stretching from Denver to Detroit and encompassing what hadonce been Chicago, St. Louis, Kansas City, the Twin Cities—the whole of the Midwest, inshort—nearly every standing thing would be flattened or on fire, and nearly every living thingwould be dead. People up to a thousand miles away would be knocked off their feet and slicedor clobbered by a blizzard of flying projectiles. Beyond a thousand miles the devastation fromthe blast would gradually diminish.
Finding asteroids became a popular activity in the 1800s, and by the end of the centuryabout a thousand were known. The problem was that no one was systematically recordingthem. By the early 1900s, it had often become impossible to know whether an asteroid thatpopped into view was new or simply one that had been noted earlier and then lost track of. Bythis time, too, astrophysics had moved on so much that few astronomers wanted to devotetheir lives to anything as mundane as rocky planetoids. Only a few astronomers, notablyGerard Kuiper, the Dutch-born astronomer for whom the Kuiper belt of comets is named,took any interest in the solar system at all. Thanks to his work at the McDonald Observatoryin Texas, followed later by work done by others at the Minor Planet Center in Cincinnati andthe Spacewatch project in Arizona, a long list of lost asteroids was gradually whittled downuntil by the close of the twentieth century only one known asteroid was unaccounted for—anobject called 719 Albert. Last seen in October 1911, it was finally tracked down in 2000 afterbeing missing for eighty-nine years.
Unaware that the meteor and everything in it would have been vaporized on impact, hewasted a fortune, and the next twenty-six years, cutting tunnels that yielded nothing.
According to Timothy Ferris, writing in the New Yorker, such near misses probably happentwo or three times a week and go unnoticed.
It would certainly be the most interesting few months in the history of the world. And imaginethe party if it passed safely.
An object a hundred yards across couldn’t be picked up by any Earth-based telescope untilit 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 ismodest. The arresting analogy that is always made is that the number of people in the worldwho are actively searching for asteroids is fewer than the staff of a typical McDonald’srestaurant. (It is actually somewhat higher now. But not much.)While Gene Shoemaker was trying to get people galvanized about the potential dangers ofthe inner solar system, another development—wholly unrelated on the face of it—was quietlyunfolding in Italy with the work of a young geologist from the Lamont Doherty Laboratory atColumbia University. In the early 1970s, Walter Alvarez was doing fieldwork in a comelydefile known as the Bottaccione Gorge, near the Umbrian hill town of Gubbio, when he grewcurious about a thin band of reddish clay that divided two ancient layers of limestone—onefrom the Cretaceous period, the other from the Tertiary. This is a point known to geology asthe KT boundary,1and it marks the time, sixty-five million years ago, when the dinosaurs androughly half the world’s other species of animals abruptly vanish from the fossil record.
For most of the time of the object’s approach we would exist in a kind of cone of uncertainty.
Which is of course why no one has ever heard of the Manson crater.
The first Anderson or Witzke learned of this setback to their careers was when they arrivedat a conference in South Dakota and found people coming up to them with sympathetic looksand saying: “We hear you lost your crater.” It was the first they knew that Izett and the otherUSGS scientists had just announced refined figures revealing that Manson couldn’t after allhave been the extinction crater.
“Sometimes more than that,” Witzke agreed. “And at several locations. So you’re talking alot of money. Certainly more than our budget would allow.”
Asteroids, as most people know, are rocky objects orbiting in loose formation in a beltbetween Mars and Jupiter. In illustrations they are always shown as existing in a jumble, butin fact the solar system is quite a roomy place and the average asteroid actually will be abouta million miles from its nearest neighbor. Nobody knows even approximately how manyasteroids there are tumbling through space, but the number is thought to be probably not lessthan a billion. They are presumed to be planets that never quite made it, owing to theunsettling gravitational pull of Jupiter, which kept—and keeps—them from coalescing.
This was actually used as evidence to support the notion that the extinction of dinosaurs wasswift and emphatic—and so it was in geological terms. We can only guess how well, orwhether, humanity would cope with such an event.
The trauma to Manson’s geology had come not from within the Earth, but from at least 100million miles beyond. Sometime in the very ancient past, when Manson stood on the edge of ashallow sea, a rock about a mile and a half across, weighing ten billion tons and traveling atperhaps two hundred times the speed of sound ripped through the atmosphere and punchedinto the Earth with a violence and suddenness that we can scarcely imagine. Where Mansonnow stands became in an instant a hole three miles deep and more than twenty miles across.
One second after entering the atmosphere, the meteorite would slam into the Earth’ssurface, where the people of Manson had a moment before been going about their business.
“So how often does something like the Manson impact happen?” I asked Anderson andWitzke before leaving.
In a sense it hardly matters. Identifying an asteroid doesn’t make it safe. Even if everyasteroid in the solar system had a name and known orbit, no one could say what perturbationsmight send any of them hurtling toward us. We can’t forecast rock disturbances on our ownsurface. Put them adrift in space and what they might do is beyond guessing. Any asteroid outthere that has our name on it is very likely to have no other.
But let’s assume we did see the object coming. What would we do? Everyone assumes wewould send up a nuclear warhead and blast it to smithereens. The idea has some problems,however. First, as John S. Lewis notes, our missiles are not designed for space work. Theyhaven’t the oomph to escape Earth’s gravity and, even if they did, there are no mechanisms toguide them across tens of millions of miles of space. Still less could we send up a shipload ofspace cowboys to do the job for us, as in the movie Armageddon; we no longer possess arocket powerful enough to send humans even as far as the Moon. The last rocket that could,Saturn 5, was retired years ago and has never been replaced. Nor could we quickly build anew one because, amazingly, the plans for Saturn launchers were destroyed as part of aNASA housecleaning exercise.
“Why not?”
Every year the Earth accumulates some thirty thousand metric tons of “cosmicspherules”—space dust in plainer language—which would be quite a lot if you swept it intoone pile, but is infinitesimal when spread across the globe. Scattered through this thin dustingare 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 isthought, most of the iridium on Earth sank to the core when the planet was young).
“Sometimes more than that,” Anderson added.
Altogether it is thought—though it is really only a guess, based on extrapolating fromcratering rates on the Moon—that some two thousand asteroids big enough to imperilcivilized 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 isalmost certainly in the hundreds of thousands and possibly in the millions, and they are nearlyimpossible to track.
The impacts began on July 16, 1994, went on for a week and were bigger by far thananyone—with the possible exception of Gene Shoemaker—expected. One fragment, knownas Nucleus G, struck with the force of about six million megatons—seventy-five times morethan all the nuclear weaponry in existence. Nucleus G was only about the size of a smallmountain, but it created wounds in the Jovian surface the size of Earth. It was the final blowfor critics of the Alvarez theory.
And in all likelihood, remember, this would come without warning, out of a clear sky.
Though Manson’s strange rocks and silken waters were matters of curiosity, forty-oneyears would pass before a team from the University of Iowa got around to making a trip to thecommunity, then as now a town of about two thousand people in the northwest part of thestate. In 1953, after sinking a series of experimental bores, university geologists agreed thatthe site was indeed anomalous and attributed the deformed rocks to some ancient, unspecifiedvolcanic action. This was in keeping with the wisdom of the day, but it was also about aswrong as a geological conclusion can get.
Geology is a profession that varies from place to place. In Iowa, a state that is flat andstratigraphically uneventful, it tends to be comparatively serene. There are no Alpine peaks orgrinding glaciers, no great deposits of oil or precious metals, not a hint of a pyroclastic flow.
“He was just a great guy,” Witzke replied without hesitation. “If it hadn’t been for him, thewhole thing would never have gotten off the ground. Even with his support, it took two yearsto get it up and running. Drilling’s an expensive business—about thirty-five dollars a footback then, more now, and we needed to go down three thousand feet.”
It was really quite a presumptuous request. They were asking Asaro to devote months tomaking the most painstaking measurements of geological samples merely to confirm whatseemed entirely self-evident to begin with—that the thin layer of clay had been formed asquickly as its thinness suggested. Certainly no one expected his survey to yield any dramaticbreakthroughs.
By the standards of today, crater research in the early 1900s was a trifle unsophisticated, tosay the least. The leading early investigator, G. K. Gilbert of Columbia University, modeledthe effects of impacts by flinging marbles into pans of oatmeal. (For reasons I cannot supply,Gilbert conducted these experiments not in a laboratory at Columbia but in a hotel room.)Somehow from this Gilbert concluded that the Moon’s craters were indeed formed byimpacts—in itself quite a radical notion for the time—but that the Earth’s were not. Mostscientists refused to go even that far. To them, the Moon’s craters were evidence of ancientvolcanoes and nothing more. The few craters that remained evident on Earth (most had beeneroded away) were generally attributed to other causes or treated as fluky rarities.
The good news, it appears, is that it takes an awful lot to extinguish a species. The badnews is that the good news can never be counted on. Worse still, it isn’t actually necessary tolook to space for petrifying danger. As we are about to see, Earth can provide plenty of dangerof its own.
