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"The Doomsday Asteroid"

PBS Airdate: April 29, 1997

ANNOUNCER: Tonight on NOVA. Massive rocks have pounded the planets for centuries, but humans have never witnessed an actual impact until nearby Jupiter was pummeled. Could it happen here? Could Planet Earth be the next target?

GENE SHOEMAKER: It's been treated as a little bit silly. We're not really being silly. Comets really do impact planets.

CLARK CHAPMAN: And a big one could come in at any time.

ANNOUNCER: Look out! Here comes "The Doomsday Asteroid."

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The Corporation for Public Broadcasting. And viewers like you.

PETER THOMAS: The night sky seems like a protective blanket around the world, and yet, what if beyond this realm of peace and tranquility a devastating asteroid were heading towards the earth? That's exactly what happened almost ninety years ago. On a quiet June morning in an isolated forest in Siberia, a cosmic intruder was about to disturb the calm of the day. An asteroid from space exploded in the atmosphere with a force hundreds of times greater than the Hiroshima bomb. Forests were devastated for nearly a thousand square miles. Had this been New York City, at least half a million people would have died. The shock wave was picked up by instruments as far afield as London. In cities all across Europe, one could read a book by the light of the midnight sky. But no one knew why those June nights were so eerily bright in the summer of 1908. The rugged Siberian terrain would keep anyone from exploring the area until a young scientist named Leonid Kulik heard of the mysterious blast and decided to investigate some of the local nomads' stories. In 1927, Kulik journeyed eighty miles up the Tunguska River, following directions from the local people, who for years had fearfully avoided the region. Charred trees guided the explorers as they made their way through the devastated forest. As Kulik began to investigate, he noticed that the fallen trees all lay with their tops pointing away from a central region. Kulik suspected the destruction was caused by the explosion of a large meteorite. Under harsh conditions, the team set to work dredging the swamp, hoping to find the meteorite. But although they searched year after year, all they ever found were microscopic spheres of metal and glass. It was not until recently that it was proven to be an asteroid. But was the Tunguska disaster a freak event, or could it happen again, anywhere, any time? For most people, rocks falling from the sky are a source of wonder. This one was videotaped by amateur sky watchers all the way from Canada to Texas. When it finally fell to earth, it was a local media sensation.

TV CORRESPONDENT: Michelle Knapp's car is like a museum exhibit. Hundreds of people have been coming by since Friday to look at the hole in her trunk. What damaged her car was this rock. Already, she says, she has had several calls from museums and collectors wanting to buy it and the trunk of her car.

PETER THOMAS: One town, Wethersfield, Connecticut, has had more than its fair share of meteorite falls.

WANDA DONAHUE: And I just thought, I don't want the neighbors to hear about this.

ROBERT DONAHUE: It was a very, very strong force that penetrated the house.

WANDA DONAHUE: We heard this tremendous crash, and we looked up and saw this huge hole. We could see the stars, and. . .

ROBERT DONAHUE: And we just looked at each other and asked a very intelligent question. I asked Wanda and she asked me, "What happened?" It came through the ceiling of the living room, and it bounced from there into the dining room, which was adjacent, hit the ceiling, rolled, and hit the ceiling a second time. . .

WANDA DONAHUE: And then rolled underneath the dining room table, where it was found by a young fireman, one of our volunteer firemen, who came to the house after our call.

ROBERT DONAHUE: Matter of fact, we celebrate each year. We get together for a dinner each year. There are six of us.

WANDA DONAHUE: On the anniversary.

ROBERT DONAHUE: On the anniversary. So, it's been an important event in our life.

PETER THOMAS: Amazingly, ten years before, another rock fell out of the sky on this house, less than a mile away. The residents of Wethersfield are proud of this distinction.

WANDA DONAHUE: At cocktail parties, we have something to talk about. Nobody can top us.

PETER THOMAS: Sometimes, these objects are big and bright enough to give people advance warning. In 1986, millions turned out to watch the return of Halley's comet, but despite the public's fascination, few understand the nature of these objects.

CLARK CHAPMAN: Comets and asteroids are small bodies that are the remnants from the origin of the solar system. The current view about how the planets formed is that they were made out of little pieces of things that grew together into larger pieces by gently bumping into each other and growing. And most of the material that didn't go into the sun made the larger planets, but there were leftover icy and rocky materials that we call comets and asteroids.

PETER THOMAS: The rubble that forms asteroids tends to be rockier and is concentrated in an area called the asteroid belt, in the inner solar system. Comets are icier. They are found mostly in the far reaches of the outer solar system. When they approach our sun, the ice evaporates, surrounding the comet in a bright cloud. As the earth formed, it was bombarded by giant comets and asteroids, which brought precious minerals and water. This violent era ended three billion years ago, giving a life a chance to evolve. Tiny fragments of asteroids, which occasionally still fall to the earth, are called meteorites, and scientists scour the world for them.

ALEX BEVAN: Meteorites are a unique source of information about the earliest history of the solar system. Some of them have chemistries which are very similar to the chemistry of the sun, and clearly, these are very primitive objects, indeed. In most parts of the world, your chances of finding a meteorite would be pretty slim, but here on the Nullabor, they're very high. Here, it's been dry for tens of thousands of years, at least. Probably much longer than that. It also has a general lack of vegetation, and the local country rock here is made of limestone, a pale rock. Most meteorites are very dark rocks, and so show up against a pale background. They're more easily-recognized here than in most other places. Meteorites have been accumulating here for a long time, tens of thousands of years, and so that they are concentrated on this ancient surface that we're walking along.

PETER THOMAS: While rocks on earth have been changed by geological processes, meteorites reveal a different history, in part because of their violent journey through the atmosphere.

ALEX BEVAN: The earth's gravitational attraction will cause it to enter the earth's atmosphere at a minimum velocity of 11.2 kilometers a second, about forty times the speed of sound, and at such high velocities, friction in the atmosphere causes the surface of the body to melt and the air around it to become electrically charged. And that gives rise to a phenomenon we call the fireball. In the last few seconds of luminous flight through the atmosphere, the fireball goes out, and the remaining melts on the surface solidifies to form a thin skin on the surface of the meteorite, which is called a fusion crust. The interior, though, has remained virtually unaltered.

PETER THOMAS: Fortunately for us, most of the meteorites that fall to the earth are quite small. Still, they prove that the space between the planets isn't entirely empty.

ALEX BEVAN: There's a constant infall of extraterrestrial material. The bulk of this arrives as dust. Hundreds of thousands of tons of material actually falls each year. If that was all to arrive in one go, clearly, that would be catastrophic. But, this dust falls gently or drifts gently down onto the surface of the earth. Only very occasionally, larger objects, sufficiently large to survive passage through the earth's atmosphere, land on the surface of the earth as meteorites.

PETER THOMAS: But how safe are we? Ancient myths and recent geological evidence suggest that rocks large enough to cause serious damage can still hit the earth.

ALEX BEVAN: In Australia, we have a group of thirteen craters that was formed by an impact of a large iron meteorite around about five thousand years ago. Now, we know that aboriginals were—aborigines were occupying Australia perhaps forty thousand years ago or more, and it is very likely, indeed, that they actually witnessed this impact event, and their local mythology speaks of a very strange event, and they have a local dialect name for these craters. And translated, they talk about "Fire" and "Devil Rock," and all of these things tend to point to the fact that these ancient aborigines actually witnessed the impact.

PETER THOMAS: And they were not the only ones. On the pampas in Argentina, the Incas may have witnessed nature at its most savage. A two hundred-meter rock entered the atmosphere at a very shallow angle. A wall of flame erupted at the point of impact and rolled downrange for miles, incinerating everything in its path. The only record of this terrible event was discovered three years ago by a pilot when he spotted these shallow, teardrop depressions in the modern pampas, caused as the asteroid broke and skipped across the earth's surface. But despite this evidence of catastrophe, people today are skeptical about ideas of danger or even death from the sky. Those fears belong to earlier times, before the age of science, when superstition made people frightened of the heavens. At Oxford University, a scientist named Victor Clube has gone back to the earliest written records he can find to understand the origins of this fear. The Chinese kept detailed records, and like cultures everywhere, blamed fireballs on angry gods.

VICTOR CLUBE: They clearly saw the sky as interactive with the earth, and originally, the broad idea was that there were things impacting upon the earth. And God hurled his thunderbolts at us, and there were catastrophes.

PETER THOMAS: For centuries, the heavens were seen as a source of potential disaster. Then came the Age of Enlightenment, and this view of the heavens was to change radically. A major breakthrough would come with Isaac Newton's discovery that gravity keeps the planets revolving in predictable orbits around the sun.

VICTOR CLUBE: The picture that emerged was clearly the one that Newton himself was picturing where the solar system was seen as a giant clockwork machine, and the image was that this clockwork would function forever and forever.

PETER THOMAS: Contrary to the biblical record of catastrophism, the new science argued that the earth's landscape formed over billions of years by gradual processes like wind and rain. They called this idea uniformitarianism. One of the great advocates for this viewpoint was a biologist named Thomas Huxley, who publicly attacked the church's defense of catastrophism.

VICTOR CLUBE: The whole of this argument came to a head in the middle of the nineteenth century, with an argument between Bishop Wilberforce and Huxley, which Huxley won. And this was essentially a victory for science over everything, and it was science with catastrophism written out of it.

PETER THOMAS: Well into the twentieth century, catastrophes had no place in mainstream science and were banished to the realm of science fiction.

1ST ACTOR, "SCIENTIST": If our calculations prove to be correct, this will be the most frightening discovery of all time. These two bodies have traveled almost a million miles in two weeks.

2ND ACTOR, "NEWSBOY": End of the world just around the corner!

SOFIA RICHMOND: The fireball will head directly for our planet. It would be the greatest catastrophe of all, the biggest fireball ever to hit our earth. And I have already seen a preview of this event, and it certainly will annihilate many, many nations.

PETER THOMAS: Fireballs did not become a subject for mainstream science until the 1950s, when the origin of a mysterious mile-wide crater in Arizona, long thought to be an extinct volcano, was finally reexamined. One geologist suspected it was caused by a huge iron meteorite that had exploded on impact with the ground. But all that remained were tiny traces of iron, so he needed other proof.

GENE SHOEMAKER: One was to study the structure, the deformation on the walls of the crater, and how rocks had been pushed out and then peeled up and overturned, and I could compare that deformation directly with experimental craters, smaller craters formed by nuclear explosions.

PETER THOMAS: Shoemaker reasoned that since both nuclear and impact explosions are equally powerful, they ought to create the same sort of wound in the earth. As he began to look at the deformation of the rock beneath suspected meteor craters around the world, Shoemaker discovered that they had the same characteristic shape as craters caused by nuclear explosions.

GENE SHOEMAKER: The crater doesn't hold its original shape. The walls collapse downward, the floor comes up, and what we've learned to recognize as an impact structure is a ring structural depression where the rocks have been depressed in a doughnut-shaped ring, and then there's an uplift in the center.

PETER THOMAS: Shoemaker thought he was onto something, but he needed better proof. There was one more place he could look, and it was riddled with craters: the moon.

GENE SHOEMAKER: Maybe it looks obvious now that the craters on the moon were formed by impact, but in fact, the vast majority of scientists who study the moon, astronomers in particular, at the time that I began this work, thought that these craters on the moon were probably formed by volcanoes. That had been the prevailing idea for a century before.

PETER THOMAS: The Apollo mission revealed that the scars on the moon have the same shape as the nuclear and impact craters. Better still, there were thousands of them.

GENE SHOEMAKER: Well, the beauty of going to the moon is there's no rain, no atmosphere to speak of, and so a crater, once formed, lasts a very, very long time. So, we have a perfect record going back three and a third billion years, in which we can say, "Here is the rate at which these things have been formed," and compare that directly to the earth.

PETER THOMAS: To his surprise, he found that large craters were still being formed on the moon. Shoemaker's conclusion was inescapable. If asteroids are out there still hitting the moon, the earth is also at risk.

GENE SHOEMAKER: So, it was extremely important in that first intellectual step to recognize that, yes, indeed, very large objects do fall out of the sky and make holes in the ground.

PETER THOMAS: Shoemaker proved that the Arizona crater was a meteor crater. But if the earth is battered as often as the moon, where are all the other scars?

GENE SHOEMAKER: Erosion takes its toll. Rain and wind and weathering of the rocks lead to the filling in of craters. Nature abhors craters. She fills them in on the surface rather quickly, so craters disappear.

PETER THOMAS: Still, it takes millions of years for erosion to wipe out a crater, and now that geologists know what to look for, they have identified almost two hundred impact craters around the world.

GENE SHOEMAKER: Our next step was to see what were the objects that made these craters. The only way to answer that is to go to the telescope and scan a large part of the sky and see what's coming by. This is the first time ever that anyone had attempted to go and systematically survey the space near the earth for earth-crossing bodies, asteroids and comets that could hit the earth. At the time we started, only slightly more than a dozen of these things had ever been discovered in the entire course of astronomical observation.

PETER THOMAS: This was not surprising, since the rocks he found were tiny by astronomical standards. Faintly reflecting the light of our sun, they could just be picked up on photographs moving against the starry background. But the more Shoemaker looked, the more he saw. Although they look faint, these streaks represent mountains of rock weighing millions of tons, and every one of them has the potential to hit the earth. While for decades, astronomers probed the galaxies beyond our solar system, they had ignored the danger in our own back yard. But now they know that there are multiple sources of asteroids and comets concentrated in different locations in the solar system.

VICTOR CLUBE: Essentially, there are two catastrophic machines in the solar system which are directing missiles at us. One of them is the asteroid belt, which is regularly perturbed by Jupiter in its orbit close to the asteroid belt, and the other one is the so-called Oort Cloud, which is perturbed by the galactic environment.

PETER THOMAS: Every now and then, a passing star disrupts this icy reservoir in the far reaches of the solar system, sending comets inward towards the planets. Comets orbiting in a newly-discovered reservoir known as the Kuiper belt occasionally dislodge themselves, as well. Eventually, any of these comets may wind up perilously close to the earth.

VICTOR CLUBE: We're a kind of target amongst all these bodies milling around which have come from places further out in the solar system.

PETER THOMAS: Once scientists realized that the earth is still vulnerable to giant comets and asteroids, the stage was set for reexamining one of the greatest mysteries of science: the disappearance of the dinosaurs some sixty-five million years ago. Until their demise, the dinosaurs had survived fifteen hundred times longer than the human race has existed. In the late seventies, a father and son team, Luis and Walter Alvarez, came up with an explanation for the dinosaur extinction, based on extraordinary evidence that Walter brought to his father's attention. In fossil beds across the globe, scientists were discovering something curious about the layers separating the time of the dinosaurs from the time of the mammals which followed. The layer contained a high concentration of iridium, an element rare on earth, but common in asteroids. They surmised that a giant asteroid had hit the earth and killed the dinosaurs, but despaired of finding the crater so many millions of years later. Without that critical proof, people found the theory hard to swallow.

DAVID RAUP: I think it was almost universally negative. One can find a few who were not negative, and of course, this number increases with hindsight, but—No, I think to a first approximation, we all found it an appalling idea.

PETER THOMAS: Most experts preferred traditional explanations for the extinction, like changes in the world's climate, which happened gradually over thousands of years.

GENE SHOEMAKER: Paleontologists, of course, are accustomed to think in terms of long-term geologic processes affecting the evolution of life. And if you now suddenly tell them, "Well, occasionally, a stone the size of a mountain falls out of the sky and produces a global catastrophe," they just don't like that. It's against their scientific religion, if you will.

DAVID RAUP: We didn't know anything about what had been learned through the '60s and '70s about meteorite impact and the impact rates, because we had grown up with our textbooks, which said that there indeed were bombarding asteroids and comets, but this was all in the early days, the so-called "early bombardment" of the earth, and it was all over. And sure, a meteor crater was probably an impact crater, but that's only one. It couldn't affect geologic history.

PETER THOMAS: While a small asteroid wouldn't cause global disruption, what if a big one hit?

GENE SHOEMAKER: If we get to a really big event, say, a ten-kilometer crater, which is still small compared to the biggest, that's equal to the energy of all of the nuclear weapons in the world, if you heaped them up in a pile and set them off. Ten thousand megatons makes a ten-kilometer crater.

PETER THOMAS: A rock ten kilometers wide would unleash a billion megatons. Even in this nuclear age, it's hard to imagine the devastation that such an explosion would inflict on the earth. By a strange twist of fate, the evidence for such an apocalypse was soon to be found on the Yucatan peninsula. A geologist named Glen Penfield was on assignment for a Mexican petroleum company, looking for oil deposits. His first step was an aerial survey to measure faint variations in the earth's magnetic field, which would signal the presence of oil. But instead of oil, Penfield found a symmetrical ring of highly magnetic material more than a hundred miles across. Buried a mile beneath the surface of the Yucatan, it was dated to sixty-five million years. A map measuring minute variations in the density of the rock revealed an uplifted region in the very center of this enormous ring, an unmistakable mark of an impact crater.

GENE SHOEMAKER: Finding the big buried crater in Yucatan was a major step, because we found the smoking gun. Really, we found the smoking cannon that produced the catastrophe at the end of the Cretaceous.

DAVID RAUP: The environmental shock produced by a large comet or asteroid impact certainly is a good candidate, because it simply, if nothing else, because of the energy levels, the debris which would go out from the impact point at ballistic velocities, affect all hemispheres at the same—essentially, the same time. As to the particular scenarios, I don't think we know enough yet to know whether it was darkness or increase or decrease in temperature. What the killing mechanism actually was is still elusive.

PETER THOMAS: There are many theories of what happened, all of them horrific. The impact could have blasted tons of sulfur-rich rock and dust high into the atmosphere, encircling the globe and igniting the skies. This burning debris would rain back down upon the earth, and the skies would be darkened for months, if not years, to come. There is evidence of enormous lava flows at the time, which may have been triggered by shock waves from the blast. Lightning, set off by volcanic ashfalls, contributed to raging wildfires, and the temperature would have soared. The environmental devastation would have lasted for millennia. In the end, ninety percent of the world's biomass burned, and two thirds of the world's species disappeared forever, among them, the dinosaurs. Whether or not this catastrophe is what finally killed the dinosaurs is still hotly debated, but one thing is clear. The disastrous consequences of another impact from a huge asteroid or comet could obliterate our species. Many scientists believe that the question is not "if," but "when."

DUNCAN STEEL: If we're going to take the case of a global catastrophe, those only occur something like once every one hundred thousand years, maybe at the outside, once every two hundred thousand years. So, that's a very long time scale compared to the human lifetime. However, for these smaller objects, the maybe hundred-meter fragments of asteroids which mostly blow up in the atmosphere but nevertheless cause widespread devastation underneath, those occur much more frequently. We certainly should be expecting at least one of those to occur over the fifty or a hundred years.

CLARK CHAPMAN: But, it happens like a game of cosmic darts. It happens at random. It could happen just as likely tomorrow as it could during some particular day three hundred thousand years from now, and a big one could come in at any time. Particularly, I can say that because we have not searched for them all. We only know a small fraction of the ones that exist, and we happen to know from calculating their orbits that those particular ones are not going to hit any time in the next hundred years or so, which is about as far as we can reliably calculate the orbits. But the other ninety percent that we haven't found yet could hit any time.

PETER THOMAS: Despite this risk, surprisingly few astronomers hunt for asteroids and comets whose orbits cross the earth's. In the past decade, a handful of astronomers around the world have joined with Shoemaker in searching the night skies. Until they've finished this enormous survey, we must live with the possibility that a huge rock could devastate the earth just as easily tomorrow as in another million years. However, without more resources, they won't finish this daunting task any time soon.

DUNCAN STEEL: If we continue at our present rate, it will take us another five or six hundred years before we discover ninety-nine percent of them. However, the total number of astronomers involved in looking for earth-threatening asteroids is, in fact, than the staff of something like a High Street McDonald's restaurant, OK? That is the number of people, worldwide, that we're talking about looking for asteroids.

PETER THOMAS: So far, scientists have found only a small percentage of the total number. They estimate that there are a few hundred thousand earth-crossing asteroids bigger than the one that exploded above Tunguska, and about two thousand larger than half a mile. If any of these larger rocks were to hit us, it would cause a global catastrophe.

STEVE OSTRO: Imagine, for a moment, if instead of these objects being tiny and not visible to the naked eye, they were suddenly made visible. Suppose that there was a button you could push and you could light up all the earth-crossing asteroids larger than about ten meters. There would be over a hundred million of these objects in the sky, and you'd go outside at night, and instead of being able to see a few thousand bright stars, the sky would be filled with millions of these objects, all of which are capable of colliding with the earth, and all of which are moving on slightly different courses through the sky at slightly different rates.

PETER THOMAS: While scientists would like to have this clear a picture of what's out there, very few telescopes are designed to look for objects so close to the earth.

CLARK CHAPMAN: Most people think that astronomers get out at night in observatories and scan the skies. That's not true. Almost all the telescopes we have in the world are designed to peer at very tiny little pieces of the sky way off in the distance to see a quasar or hunt for black holes or look at a distant galaxy. The only real network of telescopes that scans the skies has been designed and built by the military.

PETER THOMAS: But the military has not been looking for comets or asteroids. They've been tracking enemy satellites, and most importantly, guarding against a possible nuclear attack. So, whenever an explosion with no apparent cause is detected in the upper atmosphere, it is reason for alarm. In 1978, such an explosion was detected over the ocean. Some thought it was caused by a meteor whose blinding flash of light could trigger satellite sensors looking for nuclear air bursts. While this event remains a mystery, the military has begun to release data from other meteor explosions. Now, some scientists believe there may be far more rocks out there than previously thought, and we have no defense against them. But in the 1980s, President Reagan proposed spending billions of dollars on another threat from the skies: a nuclear attack.

RONALD REAGAN: What if free people could live secure in the knowledge that their security did not rest upon the threat of instant US retaliation to deter a Soviet attack, that we could intercept and destroy strategic ballistic missiles before they reached our own soil, or that of our allies?

PETER THOMAS: Reagan proposed a radical new defense that would destroy nuclear weapons before they struck.

3RD ACTOR: When the network detects the launch of a Soviet ICBM, computers acquire the missile track and send interceptors to destroy the missile during its initial boost phase.

PETER THOMAS: A program known as the Strategic Defense Initiative, or SDI, would have set up an elaborate missile defense system. It would be built around satellites in space, armed with interceptor rockets that would track down and destroy nuclear warheads. The program was nicknamed "Star Wars." But the cold war was almost over, and the need for such a futuristic, expensive project was called into question.

VICTOR CLUBE: This has created the situation where the SDI no longer needs to exist, and I suspect this has been somewhat of an embarrassment to the SDI, and inevitably, they were bound to look for a way of preserving themselves.

PETER THOMAS: With sharp cuts in the military budget, weapons designers wondered if they could apply their skills to a related problem, saving the earth from an asteroid impact. But, is such a feat even possible?

GENE SHOEMAKER: If you discover an earth-crossing asteroid that was going to impact, say, thirty years in the future, then you could actually nudge it a long time ahead of time. You only have to change the velocity by an exquisitely small amount. Millimeters per second is the change in velocity needed to divert a body from a dead-centered collision to a miss.

PETER THOMAS: Given enough warning, we could launch a rocket. But even so, the current "Star Wars" technology, which relies on lasers to destroy a target, is not powerful enough to affect a billion-ton asteroid. Would a nuclear warhead do the job?

CLARK CHAPMAN: A nuclear weapon carries a tremendous amount of energy per pound to lift off the earth, and calculations show that a typical thermonuclear weapon would have the energy required to move the asteroid. Exactly how to use those weapons, whether to have a series of smaller blasts or to blast the thing into smithereens with one immense explosion, these details have not been worked out, but there is enough energy there, and we have the technology to put them in a rocket and launch them, if we're given enough warning time.

PETER THOMAS: Critics have pointed out that such a powerful technology could be abused.

STEVE OSTRO: Well, if we had a standing asteroid deflection capability, then maybe someday, people will start trying to deflect objects that are non-threatening into collision course with the earth. This is intrinsically an extraordinarily dangerous technology. Moving a several-kilometer object into collision course with the earth would be equivalent to engineering a mass extinction.

GENE SHOEMAKER: I think it's a rather remote risk. Of course, if someone did try to change the orbit of an asteroid and aim it at the earth, if we're on our toes, we could have a program to go back and change the orbit and send it somewhere else. But, my own view is we should not attempt to just experiment until we're really confronted with a genuine threat to the earth.

PETER THOMAS: Besides, we may not know enough about asteroids and comets to safely avert a collision.

CLARK CHAPMAN: As we contemplate the possibility of having to deflect an object, perhaps the worst thing that could happen is if we do whatever we do, blast a bomb, and the thing comes apart in several pieces that we can't control, and the several pieces rain down on our planet instead of the thing moving, as we had hoped. So, if we were to send spacecraft missions to asteroids and comets with this problem in mind, finding out how strong they are, just what are they like, if want to put a bomb in the center to blast it to smithereens, can we, in fact, drill holes in these things? Are they made of ice? Are they made of solid metal? We have speculations about this on the basis of looking at them from a great distance with telescopes, but we don't really know till we get up there.

PETER THOMAS: Until we mount a mission to an asteroid, the best we can do from the ground is to take pictures with radar telescopes. This was the first image of our closest companions in space, the faint radar trace of Castalia, a huge, dumbbell-shaped rock rolling over and over. Two years later, radar astronomer Steve Ostro captured the near-earth asteroid Toutatis.

STEVE OSTRO: It's obviously undergone an extraordinary history of collisions for the past, oh, at least ten million years, several tens of millions of years. It's unlike any other object that we've ever seen. It's highly irregular. It has craters on it. It's jagged. It's in an unusual rotation state, so it's not just spinning around, but it's almost as if it's tumbling. And it's clear that there are a lot of questions about where this object got its unusual properties, where did it come from. We've never seen an optical picture of any earth-crossing asteroid, and this is a little bit ironic, because we do have pictures of all other classes of major solar system body, the gaseous planets, the solid planets, their moons, ring systems, and main belt asteroids, now, but we have no pictures of any earth-crossing asteroids, because we've never sent a spacecraft to any earth-crossing asteroid.

PETER THOMAS: NASA is now planning to sent a satellite into orbit around one of the largest near-earth asteroids called Eros. Twenty-two miles long, roughly twice the length of Manhattan, Eros may reveal the nature of these mysterious objects. But even before this exploration, three scientists were about to discover a natural experiment in the making that would begin to answer many of our questions. In 1993, on the night of March 23rd, David Levy and Gene Shoemaker were photographing the skies near Jupiter, looking for comets and asteroids. Examining the pictures two nights later, team member Carolyn Shoemaker came across something unexpected.

CAROLYN SHOEMAKER: Jupiter was a big, overexposed-looking blob on one side. So, I started to scan it, and I started at the top, and then as I moved across about a little more than halfway down, I suddenly thought I saw something.

PETER THOMAS: What she saw was a bar of light with a tail winging away. In the weeks that followed, more powerful telescopes tracked it, until the Hubble space telescope, floating high above the earth, showed with startling clarity a string of twenty-one comets on a collision course with the planet Jupiter.

CLARK CHAPMAN: This is the first time that anything in the solar system has crashed into another thing and we've been watching. So, we had no previous experience to go on as to what might happen. We knew how big the comet was, more or less, but it was far away, and we'd never seen a comet up close. There's one or two fuzzy pictures taken of Halley's comet by some spacecraft that got moderately close to Halley, but we really don't know what comets are like. We don't know what happens when explosions of the multi-million-megaton variety happen, and people did their best to try to predict, but there were wide uncertainties.

PETER THOMAS: Despite their ignorance, scientists were eager to predict what these impacts might look like on Jupiter. Mystics were inspired to make predictions, as well.

SOFIA RICHMOND: The planet Jupiter is actually going to explode. The consequences will be far more serious than astronomers and scientists are admitting at the moment. Everyone should be warned to buy very dark sunglasses and wear them as I am wearing them now. People should stay indoors, keep pets indoors. Aeroplanes would be very dramatically affected.

PETER THOMAS: Scientists debated just what the damage would be, stymied by their inability to determine how big the comet was. Estimates varied from one to ten kilometers.

GENE SHOEMAKER: A factor of ten in size is a factor of a thousand in energy, so it made a huge difference, and we weren't really certain what the right answer was. And the weight of opinion, in fact, among my astronomical colleagues who were thinking about this really was that it would be smaller. There were only a few of us holding out for the bigger fragments.

PETER THOMAS: On the night of July 16, 1994, telescopes around the world geared up to photograph this cosmic collision of the century. High-powered astronomers and backyard enthusiasts eagerly awaited the results of nature's great experiment. Floating hundreds of miles high above the earth, the Hubble space telescope was poised to capture the impact of the first fragment, called Nucleus A. As the images came in, astronomers weren't sure whether they were looking at a comet hitting Jupiter or a Galilean satellite, what they call Jupiter's moons.

HAL WEAVER: What we saw was very unusual. We saw a little ball that seemed to be detached from Jupiter, and at that point, Heidi said, "Tell me that's not a Galilean satellite." And so I ran over and grabbed the astronomical almanac off of the shelf, which has a nice diagram in it showing you a picture of Jupiter and the relative positions of all the Galilean satellites, and Melissa McGrath and I stood there and looked at it and said, "No. That shouldn't be one of the Galilean satellites." But then, the next image came in, and you saw that little ball start to spread out, and then it became apparent to everybody that it was a plume associated with the impact.

ASTRONOMER: Look at that! Look at that!

PETER THOMAS: The damage inflicted on Jupiter exceeded everyone's expectations. A few days later, the biggest fragment, Nucleus G, would smash into the planet with a force half a million times greater than the explosion at Tunguska.

GENE SHOEMAKER: Tunguska was about a twelve-megaton explosion. Nucleus G released about six million megatons, or in other words, we're talking about a factor of about five hundred thousand difference in energy. It's just enormous.

PETER THOMAS: More than a hundred thousand pictures of Jupiter were taken during the week the comet struck. Scientists will be sifting through the fine points of the data for years, but one thing is clear. If a collision such as this had happened on earth instead of Jupiter, it would have been a catastrophe for our planet.

CLARK CHAPMAN: When the comet struck Jupiter, and beyond all expectation, any person could get a small telescope, cheap telescope in their back yard and look at Jupiter and see these huge black spots. It makes it very real, and when you're told that these black spots are the size of our entire planet earth, that black stuff created when a comet of just the size we're talking about hitting the earth, a small comet, something very imaginable, a small mountain crashed into Jupiter and produces this big, black spot. And I think people who saw these spots with their backyard telescopes or heard about them realized suddenly that this is a real risk.

GENE SHOEMAKER: Up to this point in time, because the odds are so low, it's been treated as a little bit silly. It is, after all, a much lower risk than the risk to human life and limb that's posed just by other natural hazards, large storms, earthquakes, volcanic eruptions. But I think the fact that we really have now seen an impact, in fact a whole series of impacts, on Jupiter this week has changed a lot of minds. We're not really being silly. Yes, Virginia, comets really do impact planets, and the earth really is at risk.

PETER THOMAS: In 1972, an amateur filmmaker captured the descent of a meteor through the atmosphere. Had this fireball hit the Northwest, it would have exploded with the force of five Hiroshima bombs. Fortunately, it grazed the atmosphere and went back out to space. It is incidents like this that reveal how ill-prepared we are.

VICTOR CLUBE: I hate the thought of us behaving us like ostriches and stuffing our heads in the ground pretending that there are no potential dangers around the corner. The reality is that these fireball increases will happen fairly suddenly, when they happen. We have no means at the moment for predicting them. They may happen tomorrow. They may happen a hundred years hence. Who knows? The fact is, we do not, as a society, as a world society, have the means of handling this situation at the moment.

CLARK CHAPMAN: The society is only beginning to realize that there is a threat from the skies, and I'm not sure how we ought to, as a society, regard it. It really does happen. It really could have immense, horrible, terrible consequences, even the end of civilization. And yet, it is very unlikely to happen. And the risks, on a strict numerical, Las Vegas-style odds, is the same, roughly speaking, as the risk of dying from an airplane crash for a typical American. And you know, the odds are very low that that could happen. It seems prudent to look for these asteroids and comets, because with a fairly modest effort on the scale of many things our country does, we could find ninety percent of the things that we don't now know that could do us in.

PETER THOMAS: Astronomers now recognize that the universe is not the tranquil place we once thought. Our home within it, the earth, is vulnerable. The only thing we know for sure is that someday, the earth will once again be hit by a devastating rock. But unlike most other natural disasters, a cosmic impact may be avoidable. At the very least, we have the means to search the sky, to see whether we or our children face this risk in the next century.

How can comets tell us the history of our solar system? How did they impact life on earth? Find out at NOVA's website. Log on to pbs.org.

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