On the science of Apocalypse


I don't really think that the end can be assessed as of itself, as being the end, because what does the end feel like? It's like trying to extrapolate the end of the universe. lf the universe is indeed infinite, then what does that mean? How far is all the way and then if it stops what's stopping it and what's behind what's stopping it? So "What is the end?" is my question to you.
--David St. Hubbins, This Is Spinal Tap

APOCALYPSE

We have all read it, or think we have. It does not take long to read: forty-five minutes, by my reckoning. The Bible's concluding chapter begins directly, as a letter, quickly sashays into garish incoherence, occasionally breaks into startling verse, and above all promises to show "what must soon take place." It has been used most often, by religious charismatics, as a kind of divine itinerary: Jesus catching the 7:15 out of heaven, arriving in flames at Jerusalem's holy Mount anon. Although America enjoys an early tradition of apocalyptic thinkers (the saintly Roger Williams among them), a visiting Anglican priest named John Nelson Darby was the first to smuggle into the New Canaan an organized vision-known as Premillennial Dispensationalism - centered around the book's theology of blessed annihilation. That was in 1862. An American apocalypse, perhaps not entirely coincidentally, was in the process of erupting. Despite chronic humiliations, Premillennial Dispensationalists are with us still. A recent poll disclosed that 59 percent of the American public believes that the events projected by one John of Patmos will come true. Ancient Christians apparently shared as strong a belief in the sturdiness of John's prophecies. An annoyed Augustine of Hippo advised that "those who make calculations" based on John's auguries should "relax your fingers and give them a rest."

The vernacular name by which this queer book is often known, Revelations, is wrong.
It is Revelation, or The Revelation to John (and note that preposition). The book's Greek title is Apokalypsis, or Apocalypse, which at its peaceable etymological nativity meant simply "unveiling." Revelation comes to us from the Vulgate Bible, the translators of which bowed Apokalypsis into Revelatio, the nearest Latin equivalent. Drawing partly on the "small" apocalypse in the book of Daniel, partly on Jesus' premillennial dispensationalist discourse on the Mount of Olives, and partly on God knows what, Apocalypse has arguably influenced the Christian worldview more than any other book in the

Bible-surprising for a gloomy phantasmagoria that the Good Book's fourth-century compilers very nearly omitted. Yet here we are, still helplessly attempting to clear away its two millennia of murk. The same bewildering poll that unveiled a 59 percent rate of American acceptance of Apocalypse also revealed that a quarter of Americans believe that their Bible-despite a conspicuous textual absence of airplanes, skyscrapers, or Muslims-predicted the horrific but hardly apocalyptic attacks of two Septembers ago.

In Apocalypse, most biblical scholars now agree, John of Patmos is actually describing a situation that, through archaeology and textual analysis, we are more or less able to piece together. Some churches are "falling asleep," others are behaving licentiously, and a great beast (probably Nero Caesar, whose name adds up to 666 when Hebrew letters are given numerical values; this is called gematria) doth loom astride the scattered Christians' beleaguered world. Apocalypse is an angry, fatherly chastisement that combines the "cosmic battle" scenario familiar throughout the ancient world with a more newly minted Christian persecution narrative. In other words, Apocalypse makes us privy not to the future but to John of Patmos's recent past and immediate present-a trick of perspective, often used by biblical authors, called vaticinia ex eventu, history disguised as prophecy. Apocalypse's numerological idée fixé - the Alpha and Omega, the slain Lamb's seven horns, seven seals, seven bowls, four horsemen, 666 itself - is symptomatic of nothing more than the ancient world's obsession with "whole" and "incomplete" numbers. Rest easy, Christians. Relax your fingers.

Lest we rest too easy, however, here is a numerical sequence that does have bearing on the end of the world: 1950 DA. This is the designation given to an asteroid that after fifteen more near-Earth passes may eventually collide with our planet. If this rock fulfills its most dreadful cosmic destiny, John's prediction of a sky vanishing like a scroll rolling up into itself, his stars falling to Earth, his black sun and earthquakes and lakes of fire and bloodred moon, will all come quite horribly to pass, with one exception: there will be very few Christians, very few people, left to exult in this much-delayed fulfillment of prophecy.

BANG, WHIMPER, FIRE, ICE

Not excepting Robert Frost's famous eschatological either-or, the world will end in fire and ice. Our sun, now enjoying the solar equivalent of middle age, will, somewhere between two and five billion years from now, run out of fuel and "go nova." It will first become what is known as a red giant, a faltering star with a surface temperature much lower than that which it currently has. This process will unfortunately cause the sun to expand many times beyond its present size, incinerating everything between it and Jupiter. The sun's burning wave of helium will be traveling tens of thousands of miles per second, and on Earth the sky will pass from blue to yellow to the fire of atmospheric ignition in roughly the time it takes to blink. Once this terminal solar spasm has run its course, the sun will collapse into a small, cool, extremely dense star known as a white dwarf. The remains of the inner solar system's flash-fried orbital matter will then be encased within icy spherical coffins hundreds of miles thick. Since very few species have ever managed to achieve a life span anywhere near even one billion years, it is extremely unlikely that recognizable inheritors of humanity's mantle will be witness to this celestial endgame. "Going nova" is, for now, an intellectual indulgence for astrophysicists and neurotic civilians whose pessimistic affirmations are most comfortably expressed in cosmic terms.

What, then, do we mean by The End? If we mean The End of the World Itself, we need not concern ourselves. Setting aside the aftereffects of our sun's demise fifty million centuries hence, Spaceship Earth is stubbornly resilient. In geologic time, it is probably indestructible. Do we mean The End of Life? If so, of what kind of life? Three and a half billion years ago the primordially molten Earth cooled enough to allow for the stabilization of its chief chemical components. The result was swarms of prokaryotic cells that held planetary sway for two billion years. One and a half billion years ago these simpletons became eukaryotic cells, replete with fancy new cytoplasm and nuclei. Another billion years passed before the occurrence of anything resembling multicellular development. The story of life on Earth is largely the story of stromatolites, organisms not likely to have believed that the god of their understanding reserved a special place for each of them in heaven. Do we mean by The End of the World, then, The End of Multicellular Life? If so, the world has, by any practical definition, already ended. Two hundred and fifty million years ago, during the later Permian period, 95 percent of all Earth's life was suddenly eradicated. Do we mean something as anthropocentric as The End of Human Life? If so, how many of us have to die to signal the collapse of the human endeavor? Perhaps we should ask the ghosts of European Jewry. For the citizens of Hiroshima and Nagasaki, the world certainly appeared to end on August 6 and August 9, 1945. For the people of China the world ended with equally inarguable force in 1556, when the worst earthquake in recorded history tore open, lifted up, and smashed back into place the Shensi province, in the process snuffing out 800,000 lives. More dramatically, 73,500 years ago what is known as a volcanic super-eruption occurred in modern-day Sumatra. The resultant volcanic winter blocked photosynthesis for years and very nearly wiped out the human race. DNA studies have suggested that the sum total of human characteristics can be traced back to a few thousand survivors of this catastrophe.

Earth itself is the most ambitious mass murderer in the galaxy. Its pleasant, cloudy face should be slapped onto a WANTED poster, and soon: this lunatic is out to get us. As the volcanologist Bill McGuire notes in A Guide to the End of the World, an elegant volume of nervous-breakdown-inducing persuasiveness, "The Earth has been around just about long enough to ensure that anything nature can conjure up it already has." As recently as 1902, Mount Pelee in Martinique exploded, vaporizing the town of St. Pierre with a glowing avalanche of skin-melting gas and lava flowing as fast as a cheetah at full sprint. Thirty thousand people died; the blast's two survivors were likely to have regarded any notion of the world's end with seared, unblinking eyes. Earth is cratered with somewhere between 1,500 and 3,000 volcanoes, and at least one explodes each day. Most of North America's volcanoes have been dormant for centuries--one particularly angry specimen detonated 450 million years ago and evaporated an area the size of Egypt-and this is certainly good news. The bad news is that the volcanoes that have gone speechless for the longest periods of time tend to have the most to say when they finally break their silence. The long-dormant Indonesian volcano Tambora blew off its peak in 1815, with an explosion heard 900 miles away. Twelve thousand Indonesians died as a direct consequence of the eruption, and another 80,000 perished due to famine. The atmosphere was crowded with so much ash and sulfur that 1816 became known as the "year without a summer." England saw its heaths frosted through July, allowing Mary Shelley the proper frame of mind to complete the wintry Frankenstein.

That such natural disasters seem, in the Western mind, exclusive to primitive people in far-off lands is mostly a fluke of our current distributional good fortune. This quietly unapologetic stance, which braids laissez-faire eugenics with Western exceptionalism, is in fact a highly callous form of disaster denial, for yesterday's village-erasing lava flow in Sumatra is tomorrow's super-eruption in British Columbia. Human civilization as a whole has grown amid a long breather from the kinds of natural calamities that, 10,000 years ago, left our freshly de-Ice Aged planet broiling beneath junglish centigrade from Ellesmere Island to Admundsen-Scott Station. It would probably require a psychiatrist magically privy to the workings of the mass mind to explain why we as a species seem resistant to believing that such disasters could recur, especially when many feel so ominously near. The planetary conditions we are today seeing unfold -alarming rises in human population, sea level, and temperature (in 100 years Earth will be hotter than it has been at any time in the last 1,500 centuries) - could very well be the overture to nature's awful new symphony.

Yes, the old doomsday warhorses of overpopulation and global warming (which actually should be called anthropogenic warming) are still bedeviling us. Little more can be said of global warming, other than to note that it is well under way and that those who maintain otherwise are the meteorological equivalent of creationists. In the case of overpopulation, no one can deny that the world's inhabitants doubled between 1960 and 2000. Whether this is merely bad or catastrophically bad is still unclear. Equally unclear is whether overpopulation (a term too often used as code for too many brown and yellow people) will result in a long-feared drain of world resources. But the truth remains that this sudden explosion of population is, most basically, unprecedented in human history.

One runs the risk of sounding like an idiot, or worse, when pondering the sandwich-board finalities of The End. Consider the most famous Cassandra of the 197Os, Hal Lindsey, author of The Late Great Planet Earth, the best-selling book of that deeply regrettable decade. (It still, somehow, sells around 10,000 copies a year.) His predictions of the rise of a single world religion, a Soviet-Ethiopian invasion of Israel, and the obliteration of Tokyo, London, and New York, all systematically unfulfilled, have made him a laughingstock along the lines of pet rocks. It is with a sense of near torment, then, that I report Lindsey's Reagan-era sinecure as a consultant on Middle Eastern affairs. Our current decade's disseminators of global doom, Jerry B. Jenkins and Tim LaHaye, authors of the projected twelve-novel cycle Left Behind, are hardly improvements. The series' ninth installment, the accurately titled Desecration, was the best-selling novel of 2001. These jaw-droppingly substandard books peddle a religion of fear and despair (passenger-laden airliners, whose faithly pilots are Raptured up to heaven, are allowed to crash), partake of some truly shocking Jew-baiting (Nicolae Carpathia, the books' antichrist, lately of the United Nations, is helped along by a sinister cabal of "bankers"), and pile up body counts with slasher-flick glibness (the nuclear immolation of Chicago merits a single paragraph). And just as Hal Lindsey filled the mental shoals of Ronald Reagan with his insights into the Middle East ("If you stumble over some of these unpronounceable ancient names, glance at your daily newspaper and notice the tongue , twisters assigned to people today"), Tim LaHaye served as the co-chairman of Jack Kemp's 1988 presidential campaign.

Apocalypticism, it hardly need be said, drags out of humanity all that is small and terrible and mean. It also drags out what is worst about God, for whom love seems an infrequent mood. But the apocalyptic vision speaks to us, and has spoken to us, for thousands of years. It has allowed the quatrain-scribbling Nostradamus to live indefinitely off the obscurantist residuals of human anxiety. It has allowed some in the Muslim world to regard figures as disparate as Pope Innocent III and George W. Bush as the long-feared Dajjal. It has allowed George W. Bush, arguably the worst president in American history, a surreally nonexistent pretext for world war. It seems we are all going a little nova. In July of 2002, 13,500 men and women of the army, navy, air force, and marines engaged in a war game called Millennium Challenge. That exercise, the largest of its type ever conducted, was based on a rogue military commander staging a coup in an earthquake-riven Middle Eastern nation. Make that millennium-challenged: the rogue military commander won, until the war game's rules were reset.

Is a war game rigged to ensure victory a reasonable depiction of The End? Or does The End consist merely in extrapolating the size of the volcano needed to turn Jakarta into a cinder or calculating how many feet of water New England will be sitting under once the polar ice caps melt? Is it discerning the arctic climate of Great Britain once the Gulf Stream vanishes or projecting the megadeath of a nuclear exchange between Islamabad and Delhi? It will be no easy thing to eradicate six billion human beings, the most wide-ranging, adaptable, and notoriously intelligent creatures to which the planet has ever played host. But whereas human beings are tough, the civilizations they form are appallingly delicate. Here is the buried psychological certainty - I may die, but human culture will endure - that the apocalyptic vision upends.

Thus we have entertained, and still entertain, The End at the hand of God. We have been plagued, and will be plagued, by The End at the hand of the planet itself. What seems most likely is that The End of the World will mean the reduction of humanity to a dangerously puny number, whereupon our planet will become another planet altogether, something we will live upon but that will no longer belong to us. Perhaps that future will be simpler and more peaceful, perhaps toothier and rougher. Star Trek or Mad Max? If we do proceed apace and engage in a global swap of ICBMs, who among us could argue that we did not have it coming? But imagine the different, far less personal end embodied by objects such as 1950 DA. Who among us would argue that we had that coming?

This new End means the loss of God, and not only because it creates a kind of philosophical yard sale in which everything must go. No, God will be discarded long before The End truly begins. We have already rummaged off not a few versions of Him. The hirsute old thunderbolt-hurlers to whom we long paid tribute - priapic Zeus, testy Yahweh-cannot map the genome, or tell us our past, or even explain our future. Only we can do that. "Playing God" previously meant the ability to take life, a feat we too can now achieve with spectacularly divine wrath. The old God, whatever his alias, is dead, and the God we are left with today is of little discernible help. Or perhaps God is nothing more than a mile-wide chunk of cosmos-wandering silicate, serenely floating right toward us. Was it all a mistake?

I HEARD THE LEARN'D ASTRONOMER

NASA's Deep Space Network antennae at the Goldstone Deep Space Communications Complex are spread out along twenty-six miles on the grounds of Fort Irwin, a United States Army training center in the desert wastes between Los Angeles and Las Vegas. I am traveling to the complex, which is fenced off from the surrounding military base, with Steve Ostro, a radar astronomer who works out of NASA's Jet Propulsion Laboratory (JPL), in Pasadena, California. Ostro is a handsome, southern-California-fit man in his early fifties. His resemblance to a more dashing version of Russell Johnson's Professor from Gilligan's Island is spoiled very slightly by his glasses, which although not unflattering are as thick as bulletproof glass. As we chat we pass by small road signs that say things such as TANK XING, billboards that read THE ABCS OF SAFETY: AlWAYS be CAREFUL, and hard, dried-up lakebeds that serve as landing strips. Often, Ostro tells me, he has seen military exercises, some of them live-fire, playing along these bleak horizons: Black Hawk helicopters cruising 100 feet off the ground at 130 miles per hour, tank columns advancing upon coyotes and gophers while huge volleys of artillery boom across the vacant desert sky.

Goldstone's centerpiece antenna, the DSS-14, was built in 1966 to receive communications from, and plot the movements of, early NASA missions to Mars. The monolithic dish dominates the surrounding, studies-in-brown terrain due to both its twenty-four-story height and (despite a crow infestation) its gleaming polar whiteness. When pointed straight up, as it is now, it strongly resembles a chandelier. The nine-million-pound antenna rests atop a round pedestal and has a pointing precision measured in something called millidegrees. Small, anonymous buildings wreathe the antenna, some housing dozens of tall metal lockers that contain telemetry equipment used to communicate with deep-space vehicles, others crammed with chugging generators that provide the antenna enough wattage to power a small town. Inside these buildings it is kept very cold-a little less than fifty degrees-to prevent the computers from overheating. Not surprisingly, a low and vaguely doomed hum pervades Gold-stone's every nook and turn, rather like what one hears aboard a cruising 747, only much louder, and many who work here complain of headaches. After two days I will be complaining of one myself.

For the last several years Ostro has been coming to Goldstone to map by radar our solar system's larger known asteroids. The DSS-14 sends out a beam far too thin to discover new asteroids. That duty is handled by the Near-Earth Asteroid Tracking project at JPL, which reports its findings to the Minor Planet Center, in Cambridge, Massachusetts. If the observed object is a new asteroid - occasionally older asteroids go missing and are mistaken on reappearance for new ones - MPC gives the object a number based on the year and month and order in which it was discovered. (1950 DA, for instance, was discovered in 1950 during the second half - each half-month is given a letter - of February. The A indicates that it was the first asteroid found in that half-month.) NEAT observes the new asteroid until its orbit can be reasonably determined, at which point Ostro takes over. Powered by a half-million watts, the antenna sends out into space a beam with the angular resolution of the human eye. The beam hits the targeted asteroid uniformly, scattering in every direction; only a small portion of the beam's energy ever makes it back to Goldstone. But this tiny fraction provides Ostro and his colleagues with enough information to determine the asteroid's velocity, size, and likely structural components. Often enough, they are able to use the collected data to create a grainy but nevertheless beautifully revealing image of the object.

As we walk into the lab housed in the antenna's pedestal to meet with Ostro's colleagues, he tells me he finds radar astronomy "an unbeatable experience." What before had been only an infinitesimal point of light attached to an ungainly number suddenly becomes a tiny, detailed world. I begin to understand his remark, made way back on Interstate 15, that he stopped reading the sci-fi novels he loved as a teenager when the science he was involved in became more interesting to him than fantasy.

Inside the pedestal I meet Lance Benner, Jon Giorgini, and Ray Jurgens, JPL scientists highly adept in the different areas of astronomy and planetary science that allow the team to apply an astonishing interpretive breadth to the antenna's radar readings. Jurgens, the oldest of the men, seems to be providing a good deal of support merely by standing on the lab's fringes and quietly observing. All of them are distracted, as the antenna's transmit-receive cycle is about to begin. I stand to the side, staring at a few taped-up computer-generated images of Goldstone-mapped asteroids.

Very little is known about asteroids. The spin states, shapes, geological compositions, surface characteristics, and collisional histories are, for the vast majority of identified objects, still a mystery. Only with radar imaging is much of this data, accomplished one object at a time, finally becoming clear. The asteroids' irregular shapes give each something resembling a personality: Kleopatra is shaped like a dog bone, Eros like a rutabaga, Geographos like a turd. Some rotate evenly, others like badly thrown footballs.

The lab's equipment appears, to me, strangely antiquated. Somehow our technology improves but gets no closer to the touch-screen sleekness of cinematic futurism. Little red and white lights blink on the hulking computers' faces alongside small screens active with greenish waveforms. A spray of cables hangs from seemingly every panel. Jurgens, who has worked at Goldstone since the 1970s walks over and explains that much of this equipment is twenty years old. Later inquiries as to how adequately NASA funds Goldstone's radar astronomy work-it is about one ten-thousandth of NASA's overall budget-will result in meaningful silences.

Ostro escorts me to another computer at the room's far end, the real-time sawtooth display of which will soon show us the electromagnetic Doppler frequency the antenna is receiving back from the asteroid. This information will be used, Ostro explains, to calculate the asteroid's orbit uncertainty. "We go through this process where we have a projected orbit. We see how good the prediction was, and then we make a better orbit and a new prediction of uncertainty. There's always uncertainty, and that's one of the really interesting domains of this whole problem. What is the uncertainty, and how do we reduce it? That's why we're here." 

I am here, I remind him, to find out about the chances of one of these asteroids colliding with Earth. But this is where the uncertainty comes in, he tells me. Every asteroid travels along a path we can determine using orbital trajectories, but within that trajectory there exists an error ellipse in which we cannot be sure where the asteroid will be. This ellipse can be many hundreds of thousands of miles in width, which makes reducing the uncertainty of an object that passes near Earth that much more important. Asteroid 1997 XFll, the asteroid we will be observing today, has a curious history of uncertainty. Five years ago, 1997 XFll was predicted to have a small but nonzero chance of hitting Earth in 2028. Whether this was due to hasty "back of the envelope" calculations or a real uncertainty in its error ellipse is still debated. What is known is that the media frenzy was so immediate ("Killer Asteroids!") that NASA created the Near-Earth Objects Office to handle future impact threats. It was later determined that 1997 XFll had no chance of hitting Earth, and before the day ends we will know everything else there is to know about this defanged rock. "

A lot of the confusion about this topic," Ostro goes on, "ultimately comes down to miscommunication. All of this is unfamiliar and intrinsically arcane and inaccessible and beyond the experience of humanity." (A sample from a paper Ostro gave me: "This is simply saying that a survey system with a limiting magnitude m_lim = 20 will achieve the same completeness of absolute magnitude H = 20 objects as a system with m_lim = 19 will achieve of H = 19 objects.") I ask Ostro if he personally worries about the day they discover an asteroid that has a high probability of hitting Earth. He is silent for such a long time that I ask again. "Let me rephrase your question," he says. "Is there a God?" After some uncomfortable laughter on my part, Ostro tells me about 1950 DA, the only large asteroid currently known that has a nonzero chance of colliding with Earth before the next millennium. If it does strike, it will impact the North Atlantic just off the U.S. coast in March of 2880. "It's a little bit of a stretch to say it might hit the Earth-the probability is 1 in 300-but it's the most dangerous object we know. Now, do we care about that? Should anybody care at all about the fact that an asteroid might hit Earth nearly a millennium from now?"

I imagine standing in a room with 300 people, then being told that one of us will be taken outside and shot. I tell Ostro that I think I can care about that.

"What if I had said, 'Well, we found an object that has a pretty good chance of hitting the Earth in 500,000 years.' Would that concern you? Should we care about that?"

I admit that I have a hard time gathering the emotional momentum that allows my concern to travel ahead a half million years.

Ostro nods. "What it comes down to is that this is a very new kind of topic, and it's hard to get one's bearings thinking about it, much less for society to decide whether to worry and spend money on it. And if so, how much, and how?'

The lab's small, encaged red light begins to flash in alert: the antenna is finally transmitting its half-million-watt, pencil-thin beam of energy seven million miles into deep space. Its round-trip time back to Goldstone will take a little under eighty seconds. As we wait, I find myself thinking of Whitman's "When I Heard the Learn'd Astronomer." In the poem Walt grows so "tired and sick" of an astronomer's "charts and diagrams" that he goes out "In the mystical moist night-air, and from time to time,/Look'd up in perfect silence at the stars." I wonder what poem he might have written had he known that some of those stars had the potential to end poetry and everything else, for all time.

How did we get here? In 1178. a monk in everything else, for all time. Canterbury, England, recorded the testimony of two men who witnessed a "flaming torch" spring up off the face of the moon, which "writhed, as it were, in anxiety," then "took on a blackish appearance." What these men saw, some scientists believe (the issue is debated), was the formation by an asteroid collision of the moon's youngest known crater, Giordano Bruno, named in honor of a defrocked Italian philosopher-priest. The explosion had the estimated force of 120,000 megatons, equal to 120 billion tons of TNT. Hiroshima was a mere 15 kilotons. The greatest man-made explosion in history, a Soviet nuclear test on the Arctic island Novaya Zemlya in 1962, was 60 megatons. If every nuclear device on the planet were somehow to explode at the same moment, no more than 20,000 megatons would be unleashed. The formation of Giordano Bruno, if that is indeed what the two witnesses saw, marked perhaps the first time in recorded history that human beings observed what is now known as a large-body impact. The next would occur more than 800 years later, when two dozen fragments of a shattered comet would explode on the surface of Jupiter. Not even the intervening centuries of scientific advancement would allow us any true comprehension of the destructive potential of large-body impacts. Faced with the effects of 20 megatons of explosive energy for every man, woman, and child on Earth, the mind is quickly beaten into something misshapen and medieval.

The term "asteroid" means "like stars," stars being what earlier humankind most often mistook asteroids for. When an asteroid breaches Earth's atmosphere it becomes a meteor; when it strikes Earth's surface it is called a meteorite. A "shooting star" is typically envisioned as a midsized burning chunk of speeding rock, when in fact most shooting stars are no bigger than a grain of sand. The speed at which they travel, and the opposing force of Earth's atmosphere, cause the particles to explode. The streaks of light we see in the night sky are these particles' violently released energy.

The solar system's primary asteroid repository whirls in a formation known as the asteroid belt, which lies between Mars and Jupiter, the latter possessing our system's second most powerful gravitational force, after the sun. Most asteroids are the fragmentary remains of the same cosmic deus ex nihilo that discharged rock and matter across the galaxy several billion years ago. In our solar system alone, as many as a trillion pieces of debris once floated around the perpetually contained hydrogen explosion we know as the sun, most of which were bashed into oblivion by collisions. Only nine of these space rocks were large enough to form atmospheres, and only one is known to have developed complex forms of life. This did not make them invulnerable. A Mars-sized object slammed into the nascent Earth billions of years ago, for instance, and the drama of its effect can be appreciated by the fact that it threw off a huge, wounded, molten glob that froze, was captured by Earth's gravity, and eventually became the moon. Because of this ancient demolition derby, the solar system is presently a much more open place, and collisions are far less frequent.

The first identified asteroid, Ceres, was found in 1801. By 1900 astronomers had located 462 more. All but one (Eros, discovered in 1898) were asteroid-belt objects. In the last 100 years, 150,000 more have been pinpointed and the orbits of 52,000 accurately determined. Most of this orbital surveying was accomplished after 1968, when the asteroid Icarus passed within four million miles of Earth and first caused astronomers to ponder the possibility, if not the likelihood, of large-body impacts.

Asteroids are categorized into several classes. S-types, which dominate the inner half of the asteroid belt, are composed of stone and silicate materials. C-type asteroids, which take up most of the outer half, are dark rocks rich with complex organic compounds called carbonaceous chondrites. P- and D-type asteroids are the farthest away and, consequently, the most composition-ally mysterious. The belt's nearest asteroids are M-types, highly reflective iron-nickel objects that scientists believe are tantalizing atavists of planetary cores. In Ostro's office at the Jet Propulsion Laboratory he handed me a black-and-gold chunk of a billion-year-old M-type asteroid. Although it was only a little larger than a compact disc, my hand nearly hit the floor. Its density, Ostro explained, was twice that of any object of terrestrial origin, and to cut it open would reveal an interior as bright as freshly forged steel. The smallest known M-type asteroid, 3554 Amun, with a radius of only 500 meters, is thought to contain $1 trillion worth of nickel, $800 billion worth of iron, and $700 billion worth of platinum. Since towing it back to Earth would be counterproductively expensive, and likely to result in the collapse of the world market for fine metals, these figures are, for now, lit with little more than the neon of sci-fi dreams. 

In its journey around the sun Earth passes through the orbits of twenty million asteroids. Many of these Earth-crossers are called near-Earth asteroids. NEAs much smaller than 100 meters wide are basically undetectable but for a fluke of stargazing luck; unfortunately, an object of only, say, 90 meters possesses the collisional capability of roughly 30 megatons of explosive energy, a figure that is dreadful but globally manageable. NEAs larger than 100 meters are thought to number 100,000, a fraction of which have been located; in the event of an impact these could effect serious global climate change. Around 20,000 NEAs are large enough, individually, to annul a country the size of the Czech Republic. The number of NEAs bigger than one kilometer in diameter is currently thought to be around 1,000. At astronomers' current rate of detection-roughly one a day-a survey of the entire population of one-kilometer NEAs will be complete within the next decade.

This one-kilometer threshold is important, for asteroids above it are known as "civilization- enders." They would do so first by the kinetic energy of their impact, striking with a velocity hitherto unknown in human history. The typical civilization-ender would be traveling roughly 20 kilometers a second, or 45,000 miles per hour - for visualization's sake, this is more than fifty times faster than your average bullet - producing an impact fireball several miles wide that, very briefly, would be as hot as the surface of the sun. If the asteroid hit land, a haze of dust and asteroidal sulfates would enshroud the entire stratosphere. This, combined with the soot from the worldwide forest fire the impact's thermal radiation would more or less instantaneously trigger, would plunge Earth into a cosmic winter lasting anywhere from three months to six years. Global agriculture would be terminated, and horrific greenhousing of the climate and mass starvation would quickly ensue, to say nothing of the likely event of world war - over the best caves, say. In the event of a 10-kilometer impact, every- thing within the ocean's photic zone, including food-chain-vital phytoplankton, would die, but this would hardly matter, as the deadly atmospheric production of nitrogen oxides, which would fall as acid rain, would for the next decade poison every viable body of water on Earth. Chances are, however, that the impact would be a water strike, as 72 percent of meteorite landings are thought to have been. This scenario is little better. A one-kilometer impact would, in seconds, evaporate as many as 700 cubic kilometers of water, shooting a tower of steam several miles high and thousands of degrees hot into the atmosphere, once again blotting out incoming solar radiation and triggering cosmic winter. The meteorite itself would most likely plunge straight to the ocean floor, opening up a crater five kilometers deep, its blast wave cracking open Earth's crust to uncertain seismic effect. The resultant tsunami, radiating outward in every direction from the point of impact, would begin as a wall of water as high as the ocean is deep. If a coastal dweller were to look up and see this wave coming he or she would be killed seconds later, as it would be traveling as fast as a 747. Of course, these are all projections based in physics, and can be scaled either slightly up or slightly down in their potential for global destruction. As the paleontologist David M. Raup puts it, "The bottom line is that collision with a. . . one-kilometer body would be most unpleasant."

Although one-kilometer impacts (at least several thousand megatons) are thought to occur once every 800,000 years, with 200-meter objects (1,000 megatons) striking once every 100,000 years and 40-meter objects (10 megatons) striking once every 1,000 years, only a handful of professional and amateur astronomers are currently watching the skies. Nearly half of the asteroids believed capable of destroying one quarter of humanity remain uninventoried. Not until 1998 did the U.S. Congress direct NASA to identify, by 2008, 90 percent of all asteroids and comets greater than one kilometer in diameter with orbits approaching Earth. Unfortunately, the government agency - of any government, anywhere - that would react to and be expected to deal with the likelihood of an asteroid impact does not currently exist. The impact threat is what Ostro calls "low probability and high consequence," and bureaucracies scatter like roaches from the kitchen-bright possibility of severe consequences. We need only to consider the disgraceful games of administrative duck-duck-goose played in the aftermath of comparatively smaller disasters, such as the terrorist attacks of September 2001, to recognize the federal unwillingness to counter its own congenital laxity.

Nonetheless, as I wait with Ostro, Benner, Giorgini, and Jurgens for 1997 XFll's first measurements to appear on-screen, I experience something like patriotism. The United States is currently the only nation in the world doing anything about the possibility of asteroid impacts. I am standing with a group of interstellar Paul Reveres. When I mention this to Ostro he shrugs. "The world owes a great deal more to the United States than is commonly supposed," he says, staring at the screen. "And the United States should be very proud of itself for supporting the research it does, and being the first to take this seriously."

The bandwidth begins to come alive, and after a little while Ostro is excitedly pointing things out. "It looks like it's a fast rotator; it looks like it's more or less spheroidal; it's not an elongated object. We guessed its size at a little more than a kilometer, and that looks to be about right." While Benner begins the slow process of creating a pixelated image of 1997 XFll, Giorgini sits down at a computer console to exploit the new radar information to reduce the uncertainties of the asteroid's orbit by a factor of 5,000, using its current position to integrate its orbit backward and forward in time. Jurgens laughs and says that twenty years ago this would have required two years of computation. When I ask if that resulted in more or less error, Jurgens says, dryly, "There was a lot of error."

Giorgini's computer is essentially putting to use every practical thing that human beings have learned about mathematics and physics over the last 1,000 years. While we wait for it to provide a table of the asteroid's journey through time and space, I ask Giorgini if he ever worries about impacts. "It's unlike almost all other natural disasters," he says. "We can't do much about a hurricane, and we can't do much about volcanoes, but there's a predictability to asteroid impacts that will give us an interval, and the interval is comparable to a human lifetime. Some guy working alone in his basement could design some killer bacteria without anybody knowing about it. Whether an asteroid hits us before then, I don't know. You can't worry about everything." When I ask about the day 1950 DA's nonzero likelihood of impact came up on Goldstone's screens, he gives his head one brisk shake. "That was very exciting."

Suddenly Ostro tells me that if 1997 XFll's impact hazard unexpectedly comes up nonzero I will be escorted into the desert and left for the coyotes. I ask whether impact "cover-ups" fall under the heading of right-wing or left-wing conspiracy. "Both wings flap together," Ostro says.

The information we have been waiting for begins to unscroll in several columns down Giorgini's screen. "So here we have the close-approach table," he says. I determine, privately, to call it the Holy Shit Table. "Notice these are all zeroes from the year 1900 up through the year 2100. And here in 2028"-its former impact year-"you can see it's about two and half lunar distances from Earth." He executes a few quick keystrokes and brings up another table showing the asteroid's close approaches, or "planetary encounters," every year from 1627, a year before Salem was founded on Massachusetts Bay, to 2228. Nothing but zeroes in the Holy Shit Table, not only for Earth but for the moon, Venus, and Mars as well. Until then, at least, we are safe from 1997 XFll. This, Ostro says happily, is as close to time travel as we are likely to get. Thanks to one radar reading we have been awarded the virtual travel diary and itinerary of an object seven million miles away. We are safe.

THE GIGGLE FACTOR

Impact theories are not new. One of the first scientists to argue in favor of them was Dr. Grove Karl Gilbert, in 1893, though he went only as far as placing past asteroid impacts on the moon. The resistance to dealing with the implications of Earth-based impacts, however, is almost as old as science itself. The planetary scientist John S. Lewis, author of Rain of Iron and Ice (somewhat plaintively subtitled The Very Real Threat of Comet and Asteroid Bombardment), credits this resistance to the "giggle factor," a "half-suppressed hysteria that arises from an emotional inability to deal with the truth." Whereas Isaac Newton was obsessed with the end of the world, dusting relentlessly for the sulfuric cultural fingerprint of the antichrist, he dismissed the (even then) growing evidence for large-body impacts, believing that God had put everything in its proper celestial place. When a meteorite struck Weston, Connecticut, in 1807, two Yale professors verified the strike as extraterrestrial. Thomas Jefferson, after parsing their report, allegedly said that he "would find it easier to believe that two Yankee professors would lie, than that stones should fall from the sky."

Earth is home to more than 170 known impact craters, some as old as two billion years. Erosion has undoubtedly erased dozens more. For many veers these craters, including the startlingly well Preserved Meteor Crater in Arizona, were said to have resulted from "crypto-volcanic" (hidden volcanic) activity, though some argued that this was impossible. On account of the controversy surrounding Meteor Crater's origins, maps made of Arizona prior to its statehood in 1912 omitted it altogether, no small feat of obfuscation for a formation nearly one mile in diameter. Since the few impact studies being conducted at that time saw researchers dropping marbles into bowls of oatmeal and recording the ratio of displaced mush vis-à-vis the size of the offending cat's-eye, it is easy to understand why the crater's enormity and almost perfect roundness proved so baffling.

Since 18 12, when a twelve-year-old girl named Mary Anning found a seventeen-foot-long Ichthyosaurus fossil along the cliffs of Dorset, humankind has been forced to come to terms with the upsetting evidence that many hundreds of thousands of startling and, sometimes, frightening genera came before it. The secondary recognition-that something caused these creatures to go extinct on a massive scale-soon followed. Mass extinctions are clearly evident in the fossil record and were duly noticed by nineteenth-century geologists (or "undergroundologists," as they were then known). They made these extinctions the basis of division (Cambrian, Devonian) within the geologic time scale. The question about mass extinctions was one of agency. Georges Cuvier, a brilliant French paleontologist and until Charles Darwin the most famous scientist of the nineteenth century, settled upon a "Doctrine of Catastrophes," which envisioned violent "revolutions" that all but swept clean ancient worlds of life. The events were "so stupendous," Cuvier wrote in 1821, that "the thread of Nature's operations was broken by them and her progress altered." Delighted Christians took this as proof of the Noachian deluge.

The opposing view of the 'world's prehistory was represented by the equally brilliant Scottish geologist Charles Lyell, who began as an admirer of Cuvier. Lye11 believed that "we are not authorized in the infancy of our science, to recur to extraordinary agents." Lyell's view of the fate of species, most forcefully explained in his Principles of Geology (1830), was derived from a theory of slowly accumulating processes. Extinctions, then, were gradual affairs, and not subject to fantastical caprice. Lyell's view became known as uniformitarianism and proved so convincing that, within decades, to espouse any version of Cuvier's catastrophism became the scientific equivalent of wearing a tinfoil hat and claiming that streetlamps were issuing death rays. The gradualist interpretation of the world - which has, in most disciplines, served science well - came to dominate the study of mass extinction. In this century the demise of the dinosaurs has been associated with causes ranging from small ratlike mammals eating Tyrannosaurus eggs to gamma-ray bombardment from an exploding supernova to dinosaurs becoming too big to mount their partners to gradual climate change. (I remember vividly, as a young boy, weeping over an educational cartoon filmstrip that showed Diplodocus staggering through a desert and collapsing.)

But on June 30, 1908, something happened that would slowly begin to change the terms of this debate. Over the skies of a Siberian riverine area known as Tunguska (a word that has "9/l l"- like resonance among astronomers), a small stony meteor no wider than sixty yards punched through the upper atmosphere and, due to aerodynamic pressure, detonated four and a half miles above the surface. The explosive force was that of 800 Hiroshimas and shook the Earth with the ferocity of a magnitude-eight earthquake. Seven hundred square miles of Russian woodlands were incinerated in seconds. That evening Europe's sky was so bright that there were reports of games of midnight cricket being played by uneasy Londoners. Amazingly, only two people were reported to have died in the Tunguska blast, as the area was mostly uninhabited. Had the asteroid been delayed by four hours, the explosion would have occurred over St. Petersburg (and, perhaps, prevented Soviet Communism). Russia was under considerable upheaval at the time - Czar Nicholas II had dissolved two successive parliaments in the preceding two years - and no one journeyed to Tunguska to investigate the event before 1927. Not until years after Hiroshima, Nagasaki, and the repeated irradiation of New Mexico and Kazakhstan were the physics of massive explosions properly understood, and the strangely craterless Tunguska site was subsequently adduced to have been caused by an asteroidal airburst, an event more common than one might suspect. The U.S. Defense Support Program's comprehensive satellite system, ostensibly used to provide for the global tracking of enemy bombers and missile launches, detects a dozen ten- to twenty-kiloton explosions in the upper atmosphere every year. A 1963 blast of 500 kilotons above Antarctica was initially mistaken for a nuclear test by South Africa. Until the popular emergence of UFO sightings, these explosions were commonly observed and reported by civilians.

Nonetheless, the danger posed by large-body impacts was discounted well into the 1950s a time when a theory as elementary as continental drift was seeing its first acceptance. Nuclear weapons were still in their nativity, after all, and humankind had several thousand silos' worth of exterminating angels to worry over. By 1964 it was commonly believed that Earth possessed only six verified impact craters, and the impact-mass extinction link was ridiculed. In 1970 the Canadian paleontologist Digby McLaren went public with his belief that the possible culprit of the mass extinction at the end of the Devonian period, 365 million years ago, the fourth most intense mass extinction of all time, was impact-related. The Nobel laureate Harold Urey made another claim for impacts and mass extinctions in the journal Nature, in 1973, but since Urey was a chemist his research was thought suspect. Then, in 1980, Luis Alvarez, Walter Alvarez, Frank Asaro, and Helen Michel published in the journal Science an article entitled "Extraterrestrial Cause for the Cretaceous-Tertiary Extinction."

The Cretaceous-Tertiary extinction, which occurs at what is known as the K-T boundary (C having already been secured by the Carboniferous period), marks the sixty-five-million-year-old point at which the dinosaurs go AWOL from the fossil record. Even at their acme of diversity (small children should probably stop reading now) no more than fifty species of dinosaurs, a decidedly trivial portion of Mesozoic Era life, were alive at one time. At the K-T boundary, as few as twenty-five saurian species were left. None survived past it. Of the mammalian species, perhaps ten or fifteen survived the K-T mass extinction, commonly thought to be the second most profound of its kind. Similar losses are mirrored in the fossil record of every species alive at the time.

The Alvarez group determined that the K-T boundary clay had an anomalously high incidence of the element iridium. Since iridium is roughly 5,000 times more abundant in extraterrestrial objects than in Earth's accessible crust, it seemed clear that something cataclysmically extraterrestrial in origin occurred at the K-T boundary, an event that showered Earth with the element. These findings were attacked for several years, leading the New York Times, in 1985, to issue a now famously dyspeptic editorial. "Astronomers," the Times scolded, "should leave to astrologers the task of seeking the cause of earthly events in the stars." (Stephen Jay Gould brilliantly ridiculed the Times by writing a fictitious editorial dated 1663: "Now that Signor Galileo . . . has renounced his heretical belief in the earth's motion, perhaps students of physics will.. . leave the solution of cosmological problems to those learned in the infallible sacred texts.") Not until the discovery, several years later, that the "shock metamorphism" of large-body impacts (and nuclear explosions) can form two separate minerals, stishovite and coesite, and that many suspected impact sites had high concentrations of both, did the hypothesis begin to win converts. A mid-1980s poll revealed that 90 percent of American scientists quizzed accepted that impacts occurred, though only 4 percent accepted an impact as the explanation for the K-T mass extinction. Previous beliefs that large-body impacts affected only the "lethal area" in the direct vicinity of the strike were, however gradually, abandoned. Impacts may happen, it was at last conceded, but they were freakish, remote events that did not script the fate of biology.

In 1990 the eminent late astronomer Eugene Shoemaker presented a paper to the Geological Society of America that demonstrated, as never before, the sheer number of Earth-crossing asteroids in our solar system, concluding that "Earth resides in an asteroid swarm." With these six words, Shoemaker finally, viscerally stated what scientists had been willing to accept only with decorous academic distance: millions of pieces of rock, some of them massive, were flying past Earth at staggering speeds, and sometimes these rocks hit us. All we had to do was have a look around at our beat-up planet for the well-documented proof. Opponents of the impact hypothesis suffered further attrition when, under the tip of the Yucatan Peninsula, near the Mexican port city of Progreso, a sixty-five-million-year-old crater 120 miles in diameter was linked decisively to the K-T mass extinction. The crater, called Chicxulub, had been found as long ago as 1978, but because its discoverer, an oil-industry geologist named Glen Penfield, was not an academic, converts were hard-won. By the early 1990s Chicxulub had become widely accepted as the scar of a large-body impact that finished off the dinosaurs and 75 percent of all other life on Earth.


Currently almost 1,000 potentially civilization-ending NEAs are known to have orbits passing within five million miles of Earth. It is statistically likely that most of us will live to see another impact, however small or large. In 1996, asteroid 1996 JA1 came within 200,000 miles of hitting Earth. We were provided with three days' notice. A week later, asteroid 1996 JG came within 2,000,000 miles of hitting Earth. We did not even see it until it had already passed. Both were twice as large as the Tunguska asteroid, and either could have killed one percent of Earth's human population; that is, 60 million people.

With the acceptance of the impact hypothesis has come another, related area of study: the frighteningly prominent role smaller impacts are now thought by some to have played in known human history. Asteroids themselves have traditionally been subject to great, if puzzled, veneration. In ancient China, for instance, people used to grind up and eat meteorites, and other cultures took the metal from M-type meteorites and smithed them into weapons. Simply put, it is very likely that smaller catastrophic impacts have occurred far more often than most people realize. A l,000-megaton event in Argentina resulted in mile-wide craters not thought to be more than about 10,000 years old. A small (not quite mass) extinction occurred 10,000 years ago, which finished off, primarily in the Americas, larger terrestrial mammals such as the woolly mammoth and the saber-toothed tiger. If we project forward another 6,000 years, we watch several fairly advanced civilizations, such as that of Ur, fall and vanish. We find that bookkeeping of the skies becomes, quite suddenly, a matter of some cultural importance. The literature of the ancient cultures that proceeded from Ur, including that which became the Judeo-Christian, contains much lore of a vast flood, lost continents, vanished oceans, a world gone meteorologically mad. We find Egypt, which traced its origins to Pythom, apparently a falling object of some kind, and we find a strange and seemingly offhanded Egyptian memory, now forever enshrined in Exodus, of a darkness lasting three days. We find an ancient city in Arabia, Wabar, supposedly destroyed by another falling object, an event that happens to coincide with the rise of Babylonian astrology. There are counter explanations to all of these developments, of course, just as heaven is a counterargument to death-it exists primarily because we need it to. If it is true that small though still catastrophic impacts-caused by asteroids not big enough to find with existing technology -are statistically timed to occur roughly every few thousand years, this means that the woolly mammoth and Ur fell victim to separate impacts, and it means that another is due, well, any day now.

Unlike ancient human beings, however, we actually can do something about this other than devise cosmologies of desperate emotional necessity. There are several theoretical defenses to asteroid impacts. One method involves landing a rocket-booster device on the collision-course asteroid and then attempting to steer it from Earth's path. Another proposes placing solar panels on an incoming asteroid to create what is called a "solar-powered mass driver," thereby altering its trajectory in a fuelless, less-expensive way. Yet another envisions painting asteroids black in the hopes that the change in absorbed solar radiation would gradually shift the object's orbit. But the most imperative theories involving asteroid defense are nuclear. It seems beautifully, if demonically, apt that the weapons that have terrified us all for half a century might turn out to possess the cleansing holy fire we were all promised they did. How effective they would be against the unideological phenomenon of asteroids is another matter. Simply hitting the rocks with missiles would accomplish nothing; they are traveling too fast, and the warheads' tonnage would only be absorbed. The most useful deployment of nuclear force against an unopposable asteroidal object would be to explode a powerful nuke at the asteroid's surface near the closest point its orbit takes to the sun, where the leverage of deflection is greatest. Then our planet would wait. If this first explosion did not alter the asteroid's course enough (chances are it would not), one would try another explosion, and perhaps another, thus "herding" the asteroid into a different orbit. Problems with this method, and there are many, include the possibility that the asteroid in question has been weakened by an older collision, or is loosely bound together (a rubble pile, this is called), or has an insecure interior riddled with open space. Nuclear weapons, used against such unstable bodies, might only transform one asteroid coming our way into several asteroids, perhaps hundreds-pieces of which would be large enough to breach Earth's atmosphere. The other problem would be an enlivened nuclear-weapons industry, one busily developing warheads not as a deterrent but as devices intended to be used. No one would object, of course, if we were to keep a few of these newer-line missiles-which would, no doubt, be better than ever-on high alert, or deployed a few more here or there. There are maniacs out there. Maniacs with nuclear weapons! China, for its part, has explained its resistance to the anti-nuclear-proliferation Comprehensive Test Ban Treaty on the grounds that it would like to keep its missiles in the event of an asteroid-impact threat. No doubt Iran and Iraq and Libya and Algeria would like to step up their own programs, too. Just in case.

HELPLESS

Reading about asteroid impacts will undoubtedly cause many people distress. I feel bad about that, and I would like to say that although these threats are terrifying all is not lost. Concerned, dedicated people are working on the asteroid-impact threat, and one need not be a deluded idealist to believe that they may succeed. Hope, after all, takes as its foundation not likelihood but possibility. There is, however, another threat to ourselves and our civilization, one that cannot be stopped or avoided. You readers who find yourselves already traumatized, let me entreat you here, please, to stop reading.

Comets differ from asteroids in several ways. Consensus holds that they are "dirty snowballs" made up of ice and carbon-bearing rock. A 1986 "flyby" mission to Halley's comet beamed back data suggesting that its nucleus is less dense than water, 50 percent of it a warren of cracked and empty networks. How this pertains to other comets is not known. Before the comet Hyakutake was found in 1996, only five previous comets had been detected, by radar. Traveling like frozen freight trains along the loneliest edges of the solar system, comets occasionally enter the inner solar system, the neighborhood of Earth, at twenty-six miles a second, leaving behind them a long tail of dust and gas crystals that can stretch back as far as sixty million miles. Replacement dust accumulates on cometary surfaces; when the dust layer becomes thick enough, comets gain an excellent shield against solar heating. An icy skein builds, and they get bigger. One cometlike object, Quaoar, recently found floating out around Pluto, is 800 miles across. As comets approach the sun, however, their frozen gases expand and form makeshift jets that can alter their course. This makes predicting accurate orbits of newly discovered comets nearly impossible. But discovering them is also challenging. They are too fast, not typically seen until they pass near the sun, and in any event their gas- and dust-obscured passage across the universe's dark starry backfield is often difficult to discern.

We know of two types of comets. The closest to Earth, called short-period comets, are found just beyond Neptune in the Kuiper Belt, named after the astronomer Gerard Kuiper. Long-period comets make up what is known as the Oort cloud-named in honor of the astronomer Jan Oort, who first hypothesized its existence-an envelope of as many as a trillion comets that travel around the sun far beyond Pluto. The Kuiper Belt and Oort cloud are both remnants of the early physical conditions of the primitive solar nebula. The Oort cloud was most likely formed by gravitational ejecta from the Uranus and Neptune regions, and some astronomers believe that the Kuiper Belt is merely the innermost edge of the Oort cloud. Jupiter's massive gravitational force shields Earth from many long-period comets, but its movement is also thought to be the key mechanism for injecting the few comets that manage to get past it, often in shattered form, into short-period Earth-crossing orbits. Almost all long-period comets have orbital periods of 100,000 years or more, making it all but certain that there are literally millions of comets with periodic near-Earth passes we know nothing about. We could have as little as three months' 'warning when a comet on a collision course with Earth appears in the sky, Most are too big to stop with nuclear weapons, which does not much matter, as their meddled-with, chaotic trajectories make intercepting them fantasy.


Edmund Halley, in his A Synopsis of the Astronomy of Comets (1705), was the first scientist to wonder if comets might impact Earth. Halley's astonishing foresight was only negligibly explored until the 1970s. In the 199Os, however, comets stepped to the forefront of scientific thinking about apocalyptic mass extinction. On March 24, 1993, three years after concluding that the "Earth resides in an asteroid swarm," Eugene Shoemaker, along with his wife, Carolyn, and the amateur (though highly respected) astronomer David Levy, detected upon a photographic plate a strange smear of pearly light. What they had found was twenty-one pieces of a comet that had been torn apart by Jupiter's orbit the previous July. The pieces of this comet, now called Shoemaker-Levy 9, were predicted to impact the planet's surface the following year. Sadly, the impact would occur on Jupiter's dark side, viewable only at a great distance by the Voyager and Galileo probes. Many worried that humankind's first opportunity to observe a large-body impact in at least 800 years (or since what may have been the formation of the moon's Giordano Bruno crater, in 1178) would be spoiled.

They needn't have worried. Upon impact, the larger pieces of Shoemaker-Levy blew fireballs thousands of kilometers high into Jupiter's atmosphere and were plainly visible through telescopes on Earth. When Fragment G collided with the King of Planets two days after the first impact, the flash was so bright that infrared scopes all over Earth were momentarily fried. The scar left by Fragment G was larger than Earth itself, and the explosive energy released was the equivalent of a Hiroshima-sized nuclear bomb exploding every second for thirteen years. Shoemaker-Levy's volatile swan song was quickly and accurately called the astronomical event of the century, and has proved the starkest challenge so far to humanity's sense of its own inviolability.

Scientists are divided on whether the K-T mass extinction sixty-five million years ago was caused by a comet or an asteroid. The severity of the event-miles of evaporated ocean; the very high chance that a hundred trillion tons of molten rock were thrown into space, frozen, and then pulled back down to the surface of Earth in the form of more impacting meteorites; an ozone layer so shredded that any creature peeking out of its cave even a year after the impact would have found its skin on fire in the ultraviolet spring; the sheer number of extinctions-points to a comet. It is empirically inarguable that every few dozen million years a mass extinction is visited upon Earth. Various arguments place these mass extinctions, the extent and agencies of which are still debated, at intervals ranging from twenty-six to thirty million years. A theory called the "Shiva Hypothesis," named after the Hindu god of destruction, holds that mass extinctions occur in startling simultaneity with the movement of our solar system through the galactic plane, a passage that is thought to perturb millions of Oort cloud comets into our path. Comets, and the mass extinctions they cause, might very well be the piston that drives Earth's biological processes. If the Shiva Hypothesis is correct, we are all just marking time until the next comet arrives.

Before leaving the Jet Propulsion Laboratory, I stopped to visit Don Yeomans, a man commonly regarded as one of the world's key figures in near-Earth object studies, to talk about comets. Yeomans struck me as a former nerd who has aged extremely well. He radiated a thoughtful, deceptively low-key intelligence. That he is well liked was clear from the boxed Yeomans action figure someone had given him, and from the fact that the writers of a doomsday television movie named their impacting comet after him. When he went through his files to find the film's title, I noticed that one of his drawers was labeled "NEOs and Things That Go Bump in the Night . . ."

"We're hit by tons of material every day," Yeomans told me after his futile file search, "but it's all dust. We're all walking around with comet dust in our hair. What really interests me is that comets and their impacts with Earth are - apart from the sun - the only bodies that have a direct effect on evolution, on life, on the origin of life. They probably brought the materials of life to Earth - carbon-based molecules and water. They're not just something in the sky that looks nice, like the outer planets or stars or clusters. That's all very interesting but has no real effect on us. When you come right down to it, the guy in the street wants to know, 'What's in it for me? Why should we be studying these things?' And comets, I always claim, pass the brother-in-law test."

"The brother-in-law test?" 

Yeomans smiled. "My brother-in-law doesn't believe in the space program that much. And he says, 'Hey, why are you guys spending millions of dollars to do this and that?' But after I explained comets and the origin of life and evolution, he said, 'Maybe that makes sense."'

Yeomans was addressing the notion that impacts upon the ancient atmosphere, before the origin of life on Earth, generated a stew of simple organic molecules that eventually resulted in amino acids and nitrogen bases able to serve as the keystones of life. Impacts are high-energy processes not unlike ultraviolet light, cosmic-ray irradiation, and lightning discharges. The atmospheric violence caused by impacts can also create a high incidence of these potentially life-creating processes-a case of God appearing in the whirlwind, hurricane, earthquake, and tsunami.

Human beings are not a goal that this stew evolutionarily pushed toward; they are only one entity within evolution. Contrary to the worries of certain Kansas school boards, evolution does not, in fact, push toward anything. The Darwinian evolution attacked by creationists is not even Darwinian but an inflexible version of natural selection promoted by Darwin's followers that Darwin himself would not have recognized. Today, Darwinian natural selection is an acknowledged fact of life's micro development, but on a macro scale natural selection is moot. How does an organism adapt when its world is hit without warning by a ten-mile-wide ball of ice traveling 70,000 miles per hour? Evolution is not even good for the planet, based as it is on speciation and phyletic transformation. If these processes were allowed to go on indefinitely the planet would be overrun and all life would go extinct. Thus mass extinction serves as speciation's necessary foil, and the victims run well into the millions: today's extant species account for less than one percent of the total number to have ever lived. "Mass extinctions," wrote Stephen Jay Gould, "can derail, undo, and reorient whatever might be accumulating during the 'normal' times" between "imparting a distinctive, and perhaps controlling, signature to diversity and disparity in the history of life." 

"These objects are weird," Yeomans concluded. "And the history of these objects is weirder still. Comets are unlike anything else. They show up unexpectedly, they disappear unexpectedly, they have different shapes and sizes and characteristics. There's nothing celestial-looking about them. They are the wild cards."

By the time I left Yeomans's office, The End of the World and its wild-card causes had begun to seem less like a force of impartial terror than something far more complicated-something necessary . Life is a huge blackboard tilled with a million marks of chalk. Every thirty million years that chalkboard is forcefully wiped clean, leaving only a few small smudges in the comers, whereupon life begins again without regard to perfection of adaptation, what has come before it, or the miserable consciousnesses of those few creatures able to wonder why they are here. For reasons no one yet understands, smaller animals seem to have a survivalist edge in the aftermath of most mass extinctions. One such (most likely shitlessly frightened) animal, a primate, survived the K-T mass extinction. Possibly it lived in a wet, boggy area, the only part of the planet that would not have burned up in the global firestorm. Say its name: Puratorius. We may have this tiny creature to thank for our current civilization. So impacting comets giveth, clearing the way for new species, and they taketh away. This most unstoppably terrifying vision of the world's end is also the most comforting, for it forms an iron law no organism, including ours, can hope to step around. Life cannot win. "Behold," the Lord says in Isaiah 65:17, "I create new heavens and a new earth: and the former shall not be remembered, nor come into mind." There is freedom in that recognition, a sense of knowing where we stand in fighting to stave off oblivion, and of knowing where we surrender. As everything must.