Is there an asteroid or comet out there with our name on it? NOVA scans the skies and the geological record on Earth, for evidence that giant rocks from outer space have struck before and will eventually plow into our planet again.
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00:01Tonight on NOVA.
00:03Massive rocks have pounded the planets for centuries.
00:06But humans have never witnessed an actual impact
00:09until nearby Jupiter was pummeled.
00:12Could it happen here?
00:13Could planet Earth be the next target?
00:16It's been treated as a little bit silly.
00:18We're not really being silly.
00:20Comets really do impact planets.
00:23And a big one could come in at any time.
00:26Look out.
00:27Here comes the Doomsday Asteroid.
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01:16By the Corporation for Public Broadcasting and viewers like you.
01:26The night sky seems like a protective blanket around the world.
01:32And yet, what if beyond this realm of peace and tranquility,
01:36a devastating asteroid were heading towards the Earth?
01:40That's exactly what happened almost 90 years ago.
01:47On a quiet June morning, in an isolated forest in Siberia,
01:52a cosmic intruder was about to disturb the calm of the day.
01:56An asteroid from space exploded in the atmosphere with a force hundreds of times greater than the Hiroshima bomb.
02:14Forests were devastated for nearly a thousand square miles.
02:18Had this been New York City, at least half a million people would have died.
02:24The shockwave was picked up by instruments as far afield as London.
02:36In cities all across Europe, one could read a book by the light of the midnight sky.
02:42But no one knew why those June nights were so eerily bright in the summer of 1908.
02:54The rugged Siberian terrain would keep anyone from exploring the area
02:58until a young scientist named Leonid Kulik heard of the mysterious blast
03:03and decided to investigate some of the local nomad stories.
03:08In 1927, Kulik journeyed 80 miles up the Tunguska River,
03:15following directions from the local people who for years had fearfully avoided the region.
03:21Charred trees guided the explorers as they made their way through the devastated forest.
03:30As Kulik began to investigate,
03:37he noticed that the fallen trees all lay with their tops pointing away from a central region.
03:46Kulik suspected the destruction was caused by the explosion of a large meteorite.
03:55Under harsh conditions, the team set to work dredging the swamp, hoping to find the meteorite.
04:02But although they searched year after year, all they ever found were microscopic spheres of metal and glass.
04:11It was not until recently that it was proven to be an asteroid.
04:17But was the Tunguska disaster a freak event?
04:22Or could it happen again, anywhere, anytime?
04:26For most people, rocks falling from the sky are a source of wonder.
04:36This one was videotaped by amateur sky watchers all the way from Canada to Texas.
04:45When it finally fell to earth, it was a local media sensation.
04:51Michelle Knapp's car is like a museum exhibit.
04:54Hundreds of people have been coming by since Friday to look at the hole in her trunk.
05:00What damaged her car was this rock.
05:03Already, she says, she has had several calls from museums and collectors wanting to buy it and the trunk of her car.
05:10A bucket full of starlight.
05:12Catch a falling star and put it in your mind.
05:15Never let it go.
05:19One town, Wethersfield, Connecticut, has had more than its fair share of meteorite falls.
05:26I just thought, I don't want the neighbors to hear about this.
05:30It was a very, very strong force that penetrated the house.
05:34We heard this tremendous crash and we looked up and saw this huge hole.
05:39We could see the stars.
05:41And...
05:42And we just looked at each other and asked a very intelligent question.
05:47I asked one and she asked me, what happened?
05:52It came through the ceiling of the living room.
05:54It bounced from there into the dining room, which was adjacent.
05:57Hit the ceiling, rolled and hit the ceiling a second time.
06:01And then rolled underneath the dining room table, where it was found by a young fireman.
06:05One of our volunteer firemen who came to the house after our call.
06:08Matter of fact, we celebrate each year.
06:10We get together for a dinner each year.
06:12There were six of us.
06:13On the anniversary.
06:14On the anniversary.
06:15So it's been an important event in our life.
06:21Amazingly, ten years before, another rock fell out of the sky on this house, less than a mile away.
06:28The residents of Wethersfield are proud of this distinction.
06:32At cocktail parties, we have something to talk about.
06:35Nobody can top us.
06:44Sometimes these objects are big and bright enough to give people advance warning.
06:49In 1986, millions turned out to watch the return of Halley's Comet.
06:56But despite the public's fascination, few understand the nature of these objects.
07:04Comets and asteroids are small bodies that are the remnants from the origin of the solar system.
07:10The current view about how the planets formed is that they were made out of little pieces of things
07:16that grew together into larger pieces by gently bumping into each other and growing.
07:21And most of the material that didn't go into the sun made the larger planets.
07:26But there were leftover icy and rocky materials that we call comets and asteroids.
07:32The rubble that forms asteroids tends to be rockier and is concentrated in an area called the asteroid belt in the inner solar system.
07:45Comets are icier.
07:47They are found mostly in the far reaches of the outer solar system.
07:51When they approach our sun, the ice evaporates, surrounding the comet in a bright cloud.
08:04As the Earth formed, it was bombarded by giant comets and asteroids which brought precious minerals and water.
08:12This violent era ended three billion years ago, giving life a chance to evolve.
08:21Tiny fragments of asteroids, which occasionally still fall to the Earth, are called meteorites.
08:29And scientists scour the world for them.
08:34Meteorites are a unique source of information about the earliest history of the solar system.
08:40Some of them have chemistries which are very similar to the chemistry of the sun.
08:46And clearly these are very primitive objects indeed.
08:51In most parts of the world, your chances of finding a meteorite would be pretty slim.
08:56But here on the Nullarbor, they're very high.
08:59Here it's been dry for tens of thousands of years at least, probably much longer than that.
09:05It also has a general lack of vegetation.
09:08And the local country rock here is made of limestone, a pale rock.
09:12Most meteorites are very dark rocks and so show up against a pale background.
09:17They're more easily recognized here than in most other places.
09:20Meteorites have been accumulating here for a long time, tens of thousands of years.
09:25And so that they are concentrated on this ancient surface that we're walking along.
09:30While rocks on Earth have been changed by geological processes, meteorites reveal a different history.
09:40In part because of their violent journey through the atmosphere.
09:43The Earth's gravitational attraction will cause it to enter the Earth's atmosphere at a minimum velocity of 11.2 kilometers a second.
09:53About 40 times the speed of sound.
09:56And 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.
10:03And that gives rise to a phenomenon we call the fireball.
10:08In the last few seconds of luminous flights of the atmosphere, the fireball goes out and the remaining melt on the surface solidifies to form a thin skin on the surface of the meteorite, which is called a fusion crust.
10:23The interior though, has remained virtually unaltered.
10:25Fortunately for us, most of the meteorites that fall to the Earth are quite small.
10:36Still, they prove that the space between the planets isn't entirely empty.
10:43There's a constant infall of extraterrestrial material.
10:47The bulk of this arrives as dust.
10:49Hundreds of thousands of tons of material actually falls each year.
10:52If that was all to arrive in Mongolia, clearly that would be catastrophic.
10:56But this dust falls gently or drifts gently down onto the surface of the Earth.
11:02Only very occasionally, larger objects, sufficiently large to survive passage through the Earth's atmosphere, land on the surface of the Earth's features.
11:15But how safe are we?
11:17Ancient myths and recent geological evidence suggest that rocks large enough to cause serious damage can still hit the Earth.
11:27In Australia, we have a group of 13 craters that was formed by an impact of a large iron meteorite around about 5,000 years ago.
11:34Now we know that Aboriginals were occupying Australia perhaps 40,000 years ago or more.
11:43And it's very likely indeed that they actually witnessed this impact event.
11:46This impact event.
11:47And their local mythology speaks of a very strange event.
11:51And they have a local dialect name for these craters.
11:56And translated, they talk about fire and devil rock.
12:00And all of these things tend to point to the fact that these ancient Aborigines actually witnessed the impact.
12:07And they were not the only ones.
12:13On the Pampas in Argentina, the Incas may have witnessed nature at its most savage.
12:18A 200-meter rock entered the atmosphere at a very shallow angle.
12:34A wall of flame erupted at the point of impact and rolled downrange for miles, incinerating everything in its path.
12:41The 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,
12:58caused as the asteroid broke and skipped across the Earth's surface.
13:02But despite this evidence of catastrophe, people today are skeptical about ideas of danger or even death from the sky.
13:19Those fears belonged to earlier times, before the age of science, when superstition made people frightened of the heavens.
13:32At Oxford University, a scientist named Victor Klub has gone back to the earliest written records he can find to understand the origins of this fear.
13:47The Chinese kept detailed records and, like cultures everywhere, blamed fireballs on angry gods.
13:53They clearly saw the sky as interactive with the Earth, and originally, the broad idea was that there were things impacting upon the Earth.
14:08God hurled his thunderbolts at us, and there were catastrophes.
14:17For centuries, the heavens were seen as a source of potential disaster.
14:21Then came the age of enlightenment, and this view of the heavens was to change radically.
14:30A major breakthrough would come with Isaac Newton's discovery that gravity keeps the planets revolving in predictable orbits around the sun.
14:40The 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.
14:58Contrary 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.
15:12They called this idea uniformitarianism.
15:15One of the great advocates for this viewpoint was a biologist named Thomas Huxley, who publicly attacked the Church's defense of catastrophism.
15:26The whole of this argument came to a head in the middle of the 19th century, with an argument between Bishop Wilberforce and Huxley, which Huxley won.
15:43And this was essentially a victory for science over everything.
15:48And it was science with catastrophism written out of it.
15:54Well into the 20th century, catastrophes had no place in mainstream science and were banished to the realm of science fiction.
16:02If our calculations prove to be correct, this will be the most frightening discovery of all time.
16:11These two bodies have traveled almost a million miles in two weeks.
16:21End of the world just around the corner!
16:25The fireball will head directly for our planet.
16:30It would be the greatest catastrophe of all, the biggest fireball ever to hit our Earth.
16:37And I have already seen a preview of this event.
16:42And it certainly will annihilate many, many nations.
16:46Fireballs 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 re-examined.
17:05One geologist suspected it was caused by a huge iron meteorite that had exploded on impact with the ground.
17:14But all that remained were tiny traces of iron.
17:19So he needed other proof.
17:22One was to study the structure, the deformation on the walls of the crater.
17:27And how rocks had been pushed out and then peeled up and overturned.
17:32And I could compare that deformation directly with experimental craters.
17:36Smaller craters formed by nuclear explosions.
17:38Shoemaker reasoned that since both nuclear and impact explosions are equally powerful, they ought to create the same sort of wound in the Earth.
17:54As he began to look at the deformation of the rock beneath suspected meteor craters around the world,
18:02Shoemaker discovered that they had the same characteristic shape as craters caused by nuclear explosions.
18:08The crater doesn't hold its original shape.
18:13The 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 donut-shaped ring.
18:25And then there's an uplift in the center.
18:26Shoemaker thought he was on to something, but he needed better proof.
18:34There was one more place he could look, and it was riddled with craters.
18:39The Moon.
18:41Maybe it looks obvious now that the craters on the Moon were formed by impact, but in fact,
18:45the vast majority of scientists who studied the Moon, astronomers in particular, at the time that I began this work,
18:53thought that these craters on the Moon were probably formed by volcanoes.
18:57That had been the prevailing idea for a century before.
19:01The Apollo mission revealed that the scars on the Moon had the same shape as the nuclear and impact craters.
19:09Better still, there were thousands of them.
19:11The beauty of going to the Moon is there's no rain, no atmosphere to speak of.
19:18And so a crater once formed lasts a very, very long time.
19:23So we have a perfect record going back three and a third billion years in which we could say,
19:30here is the rate at which these things have been formed, and compare that directly to the Earth.
19:34To his surprise, he found that large craters were still being formed on the Moon.
19:43Shoemaker's conclusion was inescapable.
19:46If asteroids are out there, still hitting the Moon, the Earth is also at risk.
19:52So 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.
20:06Shoemaker proved that the Arizona crater was a meteor crater.
20:12But if the Earth is battered as often as the Moon, where are all the other scars?
20:19Erosion takes its toll.
20:21Rain and wind and weathering of the rocks lead to the filling in of craters.
20:28Nature abhors craters.
20:29She fills them in on the surface rather quickly, so craters disappear.
20:32Still, it takes millions of years for erosion to wipe out a crater.
20:40And now that geologists know what to look for, they have identified almost 200 impact craters around the world.
20:53Our next step was to see what were the objects that made these craters.
20:57The 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.
21:09This 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.
21:22At the time we started, only slightly more than a dozen things had ever been discovered in the entire course of astronomical observation.
21:34This was not surprising since the rocks he found were tiny by astronomical standards.
21:40Faintly reflecting the light of our sun, they could just be picked up on photographs moving against the starry background.
21:48But the more Shoemaker looked, the more he saw.
21:54Although they look faint, these streaks represent mountains of rock weighing millions of tons.
22:01And every one of them has the potential to hit the Earth.
22:05While for decades astronomers probed the galaxies beyond our solar system, they had ignored the danger in our own backyard.
22:14But now they know that there are multiple sources of asteroids and comets concentrated in different locations in the solar system.
22:23Essentially, there are two catastrophic machines in the solar system which are directing missiles at us.
22:36And one of them is the asteroid belt, which is regularly perturbed by Jupiter in its orbit, close to the asteroid belt.
22:44And the other one is the so-called Oort cloud, which is perturbed by the galactic environment.
22:54Every now and then, a passing star disrupts this icy reservoir in the far reaches of the solar system, sending comets inward towards the planets.
23:04Comets orbiting in a newly discovered reservoir known as the Kuiper belt occasionally dislodge themselves as well.
23:13Eventually, any of these comets may wind up perilously close to the Earth.
23:18We're a kind of target amongst all these bodies milling around which have come from places further out in the solar system.
23:27Once scientists realized that the Earth is still vulnerable to giant comets and asteroids, the stage was set for re-examining one of the greatest mysteries of science.
23:46The disappearance of the dinosaurs some 65 million years ago.
23:50Until their demise, the dinosaurs had survived 1,500 times longer than the human race has existed.
24:02In the late 70s, a father and son team, Luis and Walter Alvarez, came up with an explanation for the dinosaur extinction,
24:11based on extraordinary evidence that Walter brought to his father's attention.
24:15In fossil beds across the globe, scientists were discovering something curious about the layer separating the time of the dinosaurs from the time of the mammals which followed.
24:31The layer contained a high concentration of iridium, an element rare on Earth, but common in asteroids.
24:39They 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.
24:52Without that critical proof, people found the theory hard to swallow.
24:56I think it was almost universally negative. One can find a few who were not negative, and of course this number increases with hindsight.
25:09But no, I think to a first approximation, we all found it an appalling idea.
25:15Most experts preferred traditional explanations for the extinction, like changes in the world's climate, which happened gradually over thousands of years.
25:26Paleontologists are accustomed to think in terms of long-term geologic processes affecting the evolution of life.
25:37And 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,
25:45they just don't like that. It's against their scientific religion, if you will.
25:49We didn't know anything about what had been learned through the 60s and 70s about meteorite impact and the impact rates.
25:58Because 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.
26:09And sure, meteor crater was probably an impact crater, but that's only one. It couldn't affect geologic history.
26:18While a small asteroid wouldn't cause global disruption, what if a big one hit?
26:27If we get to a really big event, say a 10-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 keep them up in a pile and set them off.
26:4010,000 megatons makes a 10-kilometer crater.
26:48A rock 10 kilometers wide would unleash a billion megatons.
26:53Even in this nuclear age, it's hard to imagine the devastation that such an explosion would inflict on the Earth.
27:00By a strange twist of fate, the evidence for such an apocalypse was soon to be found on the Yucatan Peninsula.
27:17A geologist named Glenn Penfield was on assignment for a Mexican petroleum company looking for oil deposits.
27:25His first step was an aerial survey to measure faint variations in the Earth's magnetic field, which would signal the presence of oil.
27:41But instead of oil, Penfield found a symmetrical ring of highly magnetic material more than a hundred miles across.
27:49Buried a mile beneath the surface of the Yucatan, it was dated to 65 million years.
27:58A 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.
28:13Finding the big buried crater in Yucatan was a major step because we found the smoking gun.
28:27Really, we found the smoking cannon that produced the catastrophe at the end of the Cretaceous.
28:32The environmental shock produced by a large comet or asteroid impact certainly is a good candidate.
28:51Because of simply, if nothing else, because of the energy levels.
28:56The debris which would go out from the impact point at ballistic velocities affect all hemispheres at the same, essentially the same time.
29:04As to this particular scenarios, I don't think we know enough yet to know whether it was darkness or increase or decrease in temperature.
29:12What the killing mechanism actually was is still elusive.
29:21There are many theories of what happened, all of them horrific.
29:25The impact could have blasted tons of sulfur rich rock and dust high into the atmosphere, encircling the globe and igniting the skies.
29:34This burning debris would rain back down upon the earth and the skies would be darkened for months, if not years to come.
29:45There is evidence of enormous lava flows at the time, which may have been triggered by shock waves from the blast.
29:58Lightning set off by volcanic ash falls contributed to raging wildfires and the temperature would have soared.
30:06The environmental devastation would have lasted for millennia.
30:19In the end, 90% of the world's biomass burned and two thirds of the world's species disappeared forever.
30:28Among them, the dinosaurs.
30:29The dinosaurs.
30:31Whether or not this catastrophe is what finally killed the dinosaurs is still hotly debated.
30:37But one thing is clear.
30:39The disastrous consequences of another impact from a huge asteroid or comet could obliterate our species.
30:48Many scientists believe that the question is not if, but when.
30:53If we're going to take the case of a global catastrophe, those only occur something like once every 100,000 years, maybe at the outside once every 200,000 years.
31:04So that's a very long time scale compared to the human lifetime.
31:08However, for the smaller objects, the maybe 100 meter fragments of asteroids which mostly blow up in the atmosphere but nevertheless cause widespread devastation underneath, those occur much more frequently.
31:18We certainly should be expecting at least one of those to occur over the next 50 or 100 years.
31:27But it happens like a game of cosmic darts.
31:32It happens at random.
31:34It could happen just as likely tomorrow as it could during some particular day 300,000 years from now.
31:42And a big one could come in at any time.
31:47Particularly, I can say that because we have not searched for them all.
31:52We only know a small fraction of the ones that exist.
31:55And 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.
32:02Which is about as far as we can reliably calculate the orbits.
32:04But the other 90% that we haven't found yet could hit any time.
32:11Despite this risk, surprisingly few astronomers hunt for asteroids and comets whose orbits cross the Earth's.
32:19In the past decade, a handful of astronomers around the world have joined with Shoemaker in searching the night skies.
32:28Until they finish 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.
32:43However, without more resources, they won't finish this daunting task any time soon.
32:49If we continue at our present rate, it will take us another five or six hundred years before we discover 99% of them.
32:57However, the total number of astronomers involved in looking for Earth-threatening asteroids is in fact less than the staff of something like a High Street McDonald's restaurant.
33:08That is the number of people worldwide that we're talking about looking for asteroids.
33:12So far, scientists have found only a small percentage of the total number.
33:20They estimate that there are a few hundred thousand Earth-crossing asteroids bigger than the one that exploded above Tunguska,
33:29and about 2,000 larger than half a mile.
33:32If any of these larger rocks were to hit us, it would cause a global catastrophe.
33:37Imagine for a moment if, instead of these objects being tiny and not visible to the naked eye, they were suddenly made visible.
33:48Suppose that there was a button you could push and you could light up all the Earth-crossing asteroids larger than about 10 meters.
33:55There would be over a hundred million of these objects in the sky, and you'd go outside at night.
34:04Instead of being able to see a few thousand bright stars, the sky would be filled with millions of these objects,
34:12all 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.
34:21While scientists would like to have this clearer picture of what's out there,
34:30very few telescopes are designed to look for objects so close to the Earth.
34:35Most people think that astronomers get out at night in observatories and scan the skies.
34:40That's not true. Almost all the telescopes we have in the world are designed to peer at very tiny little pieces of the sky,
34:47way off in the distance to see a quasar or hunt for black holes or look at a distant galaxy.
34:55The only real network of telescopes that scans the skies has been designed and built by the military.
35:01But the military has not been looking for comets or asteroids.
35:05They've been tracking enemy satellites, and most importantly, guarding against a possible nuclear attack.
35:11So whenever an explosion with no apparent cause is detected in the upper atmosphere, it is reason for alarm.
35:26Missile warning, Mrs. Beal, stand by for confidence reporting.
35:31SSCO, no malfunction.
35:32In 1978, such an explosion was detected over the ocean.
35:38Competence is high, I repeat, confidence is high.
35:42Some thought it was caused by a meteor whose blinding flash of light could trigger satellite sensors looking for nuclear airbursts.
35:52While this event remains a mystery, the military has begun to release data from other meteor explosions.
35:59Now some scientists believe there may be far more rocks out there than previously thought.
36:06And we have no defense against them.
36:10But in the 1980s, President Reagan proposed spending billions of dollars on another threat from the skies.
36:20A nuclear attack.
36:21What if free people could live secure in the knowledge that their security did not rest upon the threat of instant U.S. retaliation to deter a Soviet attack.
36:36That we could intercept and destroy strategic ballistic missiles before they reached our own soil or that of our allies.
36:42Reagan proposed a radical new defense that would destroy nuclear weapons before they struck.
36:52When 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.
37:02A program known as the Strategic Defense Initiative or SDI would have set up an elaborate missile defense system.
37:11It would be built around satellites in space armed with interceptor rockets that would track down and destroy nuclear warheads.
37:19The program was nicknamed Star Wars.
37:24I asked my daddy what this Star Wars stuff is all about.
37:29But the Cold War was almost over and the need for such a futuristic, expensive project was called into question.
37:36This has created the situation where the SDI no longer needs to exist.
37:43And I suspect this has been somewhat of an embarrassment to the SDI.
37:50And inevitably they were bound to look for a way of preserving themselves.
37:57With 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.
38:11But is such a feat even possible?
38:14If you discover an Earth crossing asteroid that was going to impact, say, 30 years in the future,
38:20then you could actually nudge it a long time ahead of time.
38:27You only have to change the velocity by an exquisitely small amount.
38:32Millimeters per second is a change in velocity needed to divert a body from a dead centered collision to a miss.
38:44Given enough warning, we could launch a rocket.
38:47But even so, the current Star Wars technology, which relies on lasers to destroy a target,
38:55is not powerful enough to affect a billion ton asteroid.
39:00Would a nuclear warhead do the job?
39:05A nuclear weapon carries a tremendous amount of energy per pound to lift off the Earth.
39:09And calculations show that a typical thermonuclear weapon would have the energy required to move the asteroid.
39:20Exactly how to use those weapons, whether to have a series of smaller blasts or to blast the thing into smiterines with one immense explosion,
39:29these details have not been worked out.
39:31But 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.
39:42Critics have pointed out that such a powerful technology could be abused.
39:46Well, if we have 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.
40:01This is intrinsically and extraordinarily dangerous technology.
40:08Moving a several-kilometer object into collision course with the Earth would be equivalent to engineering a mass extinction.
40:16I think it's a rather remote risk.
40:20And of course, if someone did try to change the orbit of an asteroid and aim it at the Earth,
40:27if we're on our toes, we could have a program to go back and change the orbit and send it somewhere else.
40:33But my own view is we should not attempt to just experiment until we're really confronted with a genuine threat to the Earth.
40:41Besides, we may not know enough about asteroids and comets to safely avert a collision.
40:51As we contemplate the possibility of having to deflect an object,
40:57perhaps the worst thing that could happen is if we do whatever we do, blast a bomb,
41:03and 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.
41:14So if we were to send spacecraft missions to asteroids and comets with this problem in mind,
41:20finding out how strong they are, just what are they like?
41:24If you want to put a bomb in the center to blast it to smithereens, can we in fact drill holes in these things?
41:31Are they made of ice? Are they made of solid metal?
41:35We have speculations about this on the basis of looking at them from a great distance with telescopes,
41:40but we don't really know until we get up there.
41:43Until we mount a mission to an asteroid, the best we can do from the ground is to take pictures with radar telescopes.
41:52This was the first image of our closest companions in space.
41:56The faint radar trace of Castalia, a huge dumbbell-shaped rock rolling over and over.
42:07Two years later, radar astronomer Steve Ostro captured the near-Earth asteroid Teutatus.
42:13It's obviously undergone an extraordinary history of collisions for the past, oh, at least 10 million years, several tens of millions of years.
42:27It's unlike any other object that we've ever seen.
42:31It'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.
42:40And it's clear that there are a lot of questions about where this object got its unusual properties, where did it come from?
42:48We'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.
43:03The gaseous planets, the solid planets, their moons, ring systems, and main-belt asteroids now.
43:12But we have no pictures of any Earth-crossing asteroids, because we've never sent a spacecraft to any Earth-crossing asteroid.
43:19NASA is now planning to send a satellite into orbit around one of the largest near-Earth asteroids called Eros.
43:3222 miles long, roughly twice the length of Manhattan, Eros may reveal the nature of these mysterious objects.
43:41But 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.
43:56In 1993, on the night of March 23rd, David Levy and Gene Shoemaker were photographing the skies near Jupiter, looking for comets and asteroids.
44:16Examining the pictures two nights later, team member Carolyn Shoemaker came across something unexpected.
44:23Jupiter was a big, overexposed-looking blob on one side.
44:30So I started to scan it, and I started at the top, and then as I moved across, about a little more than halfway down,
44:39I suddenly thought, I saw something.
44:44What she saw was a bar of light with a tail winging away.
44:48In the weeks that followed, more powerful telescopes tracked it,
44:51until the Hubble Space Telescope, floating high above the Earth,
44:56showed with startling clarity a string of 21 comets on a collision course with the planet Jupiter.
45:06This is the first time that anything in the solar system has crashed into another thing,
45:13and we've been watching.
45:14So we had no previous experience to go on as to what might happen.
45:18We knew how big the comet was, more or less, but it was far away, and we've never seen a comet up close.
45:25There's one or two fuzzy pictures taken of Halley's Comet by some spacecraft that got moderately close to Halley,
45:32but we really don't know what comets are like.
45:33We don't know what happens when explosions of the multi-million megaton variety happen,
45:42and people did their best to try to predict, but there were wide uncertainties.
45:47Despite their ignorance, scientists were eager to predict what these impacts might look like on Jupiter.
45:56Mystics were inspired to make predictions as well.
46:01The planet Jupiter is actually going to explode. The consequences will be far more serious than astronomers and scientists are admitting at the moment.
46:16Everyone should be warned to buy very dark sunglasses and wear them as I am wearing them now.
46:23People should stay indoors, keep pets indoors.
46:28Aeroplanes would be very dramatically affected.
46:35Scientists debated just what the damage would be, stymied by their inability to determine how big the comet was.
46:43Estimates varied from one to ten kilometers.
46:48A factor of ten in size is a factor of a thousand in energy,
46:51so it made a huge difference.
46:54And we weren't really certain what the right answer was.
46:58And the weight of opinion, in fact, among my astronomical colleagues who were thinking about this really was that it would be smaller.
47:06There were only a few of us holding out for the bigger fragments.
47:12On the night of July 16, 1994,
47:15telescopes around the world geared up to photograph this cosmic collision of the century.
47:30High-powered astronomers and backyard enthusiasts eagerly awaited the results of nature's great experiment.
47:37Floating hundreds of miles high above the Earth,
47:45the Hubble Space Telescope was poised to capture the impact of the first fragment called Nucleus A.
47:51As the images came in, astronomers weren't sure whether they were looking at a comet hitting Jupiter,
48:02or a Galilean satellite, what they call Jupiter's moons.
48:07What we saw was very unusual.
48:11We saw a little ball that seemed to be detached from Jupiter.
48:14At that point, Heidi said,
48:16tell me that's not a Galilean satellite.
48:18And so I ran over and grabbed the astronomical almanac off of the shelf,
48:22which has a nice diagram in it showing you a picture of Jupiter
48:25and the relative positions of all the Galilean satellites.
48:28And Melissa McGrath and I stood there and looked at it and said,
48:31no, this shouldn't be one of the Galilean satellites.
48:32But then the next image came in and you saw that little ball start to spread out.
48:37And then it became apparent to everybody that it was a plume associated with the impact.
48:42Look at that!
48:46The damage inflicted on Jupiter exceeded everyone's expectations.
48:52A few days later, the biggest fragment, Nucleus G, would smash into the planet
48:57with a force half a million times greater than the explosion at Tunguska.
49:03Tunguska was about a 12 megaton explosion.
49:08Nucleus G released about 6 million megatons.
49:15Or in other words, we're talking about a factor of about 500,000 difference in energy.
49:24It's just enormous.
49:25More than 100,000 pictures of Jupiter were taken during the week the comet struck.
49:33Scientists will be sifting through the fine points of the data for years.
49:39But one thing is clear.
49:41If a collision such as this had happened on Earth instead of Jupiter,
49:46it would have been a catastrophe for our planet.
49:52When the comet struck Jupiter, and beyond all expectation,
49:57any person could get a small telescope, cheap telescope in their backyard,
50:02and look at Jupiter and see these huge black spots.
50:04It makes it very real.
50:08And when you're told that these black spots are the size of our entire planet Earth,
50:13that black stuff created when a comet of just the size we're talking about hitting the Earth,
50:20a small comet, something very imaginable, a small mountain crashed into Jupiter,
50:25and produces this big black spot.
50:29And I think people who saw these spots with their backyard telescopes
50:33or heard about them realize suddenly that this is a real risk.
50:37Up to this point in time, because the odds are so low,
50:42it's been treated as a little bit silly.
50:45It is, after all, a much lower risk than the risk to human life and limb
50:49that's posed just by other natural hazards.
50:53Large storms, earthquakes, volcanic eruptions.
50:59But I think the fact that we really have now seen an impact,
51:04in fact a whole series of impacts, on Jupiter this week has changed a lot of minds.
51:10We're not really being silly.
51:13Yes, Virginia, comets really do impact planets, and the Earth really is at risk.
51:20In 1972, an amateur filmmaker captured the descent of a meteor through the atmosphere.
51:28Had this fireball hit the Northwest,
51:31it would have exploded with the force of five Hiroshima bombs.
51:36Fortunately, it grazed the atmosphere and went back out to space.
51:41It is incidents like this that reveal how ill-prepared we are.
51:46I hate the thought of us behaving like ostriches and stuffing our heads in the ground,
51:54pretending that there are no potential dangers around the corner.
51:58The reality is that these fireball increases will happen fairly suddenly when they happen.
52:04We have no means at the moment of predicting them.
52:07They may happen tomorrow, they may happen a hundred years hence, who knows.
52:12The fact is, we do not as a society, as a world society, have the means of handling this situation at the moment.
52:19The society is only beginning to realize that there is a threat from the skies.
52:27And I'm not sure how we ought to, as a society, regard it.
52:32It really does happen.
52:35It really could have immense, horrible, terrible consequences, even the end of civilization.
52:42And yet it is very unlikely to happen.
52:45And 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.
52:56And even though the odds are very low that that could happen,
53:03it seems prudent to look for these asteroids and comets,
53:08because with a fairly modest effort on the scale of many things our country does,
53:13we could find 90% of the things that we don't now know that could do us in.
53:20Astronomers now recognize that the universe is not the tranquil place we once thought.
53:27Our home within it, the Earth, is vulnerable.
53:32The only thing we know for sure is that someday the Earth will once again be hit by a devastating rock.
53:41But unlike most other natural disasters, a cosmic impact may be avoidable.
53:46At 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.
53:57Go online with NOVA for a cyber tour of our turbulent solar system.
54:12Our website features the latest evidence of asteroids and comets in space, on Earth, and on our sister planet, Venus.
54:17Link to NOVA online from the PBS homepage.
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