Meteor Strike

Meteor Strike

PBS Airdate: March 27, 2013

NARRATOR: It came from outer space, searing the sky over a densely populated Russian city.

(Meteor Witness): I went to the window, and I looked outside, and I just saw this giant streak across the sky.

CAB DRIVER (Meteor Witness/Translation): It was so bright, it was blinding, and I had to look down.

NARRATOR: A white-hot fireball, as big as a building, weighing 10,000 tons, headed for Earth.

RICHARD GREENWOOD (The Open University, U.K.): People had no idea what was about to hit.

MARK BOSLOUGH (Sandia National Laboratories): My first reaction was, "This is the big one, probably something bigger than we ever expected to see in our lifetimes."

CAROLINE SMITH (Natural History Museum, London): This is a once in a century event.

NARRATOR: It is the biggest recorded meteor strike since 1908, when another monster from space flattened 800 square miles of Siberian forest. But unlike that blast, which took place in an isolated wilderness, this meteor is witnessed by a multitude of stunned onlookers,…

I'm filming.

NARRATOR: …captured in frightening detail, by dozens of digital cameras.

RICHARD GREENWOOD: We've got a unique event taking place in an environment which people have filmed it extensively. It's absolutely incredible.

PETER BROWN (University of Western Ontario): It's going to be a huge bounty to science.

NARRATOR: Now, the race is on to find out what really happened. Through unprecedented video evidence, dramatic eye-witness accounts,…

MICHAEL GARNETT (Canadian Resident of Chelyabinsk/Meteor Witness): I had never seen anything like that before in my life.

NARRATOR: …and new clues on the ground,…

MARK BOSLOUGH: Two weeks ago, it exploded in the atmosphere. Here I am, holding it in my hand.

NARRATOR: …scientists piece together the complete story of the blast heard round the world and the even bigger disaster that could have been.

MARK BOSLOUGH: When something going that fast slams into even really thin air, there's tremendous forces on it.

PETER BROWN: The atomic bomb dropped on Hiroshima was about 15,- to 20,000 tons of T.N.T. This is about 20 to 30 times that event.

NARRATOR: Will we be this lucky again? Is there any way to predict the next Meteor Strike? Right now, on NOVA.

Late winter in Siberia; the Russian city, Chelyabinsk: at 9:20 a.m., on a Friday morning in February, a strange light appears in the east. In a heartbeat, it flares to brilliance and tears across the sky, far faster than any plane.

(Meteor Witness): I saw a small dot, and it became bigger and bigger. It became really bright white.

CAB DRIVER (Meteor Witness/Translation): It was so bright, it was blinding, and I had to look down.

MICHAEL GARNETT: I went to the window, and I looked outside and saw this giant streak across the sky.

CAB DRIVER (Meteor Witness/Translation): Everyone went outside to look at it.

MICHAEL GARNETT: I had never seen anything like that before in my life.

NARRATOR: It vanishes without a sound.

Astonished witnesses, around the city, are drawn to gape at the smoky trail left by this fleeting apparition.

(Meteor Witness/Translation): Are you filming?

NARRATOR: Then, nearly three minutes later, a series of explosions and shockwaves reverberates and batters the city, injuring more than 1,000 people. Everyone who sees it wants to know, "What is it?"

NARRATOR: Is it a sneak attack from a distant enemy or a plane coming apart in the air? A piece of space junk or a satellite? And why was there no warning?

(Meteor Witness/Translation): What was it?

NARRATOR: In this internet and YouTube age, pictures are soon uploading around the world, images instantly recognizable to physicists like Mark Boslough, who first hears about it online in far off Albuquerque, New Mexico.

MARK BOSLOUGH: Someone has just posted a, a Russian news article about this big explosion, this big event in Chelyabinsk. And there was a, a YouTube video attached, and so I clicked on that, and I watched it. And when I heard that boom and that it set off car alarms, I knew this is the big one, probably something bigger than we ever expected to see in our lifetime.

NARRATOR: Other scientists around the world are equally excited and astonished.

CAROLINE SMITH: The first videos were so unbelievable. The hairs stood up on my neck, sort of thing, thinking, "Wow! This is a once in a century event."

PETER BROWN: It almost seemed surreal, in some sense, or like a parallel universe. And after I watched a few of the videos and saw some of the damage that was being shown, it started to really sink in.

NARRATOR: Unlike the thousands of frightened and confused witnesses on the ground in Russia, the scientists immediately know exactly what they're looking at.

This streak of light in the sky is no bomb or piece of space junk. The culprit is an asteroid, a rock from outer space, whose orbit sent it on a collision course with Earth.

MARGARET CAMPBELL-BROWN (University of Western Ontario): It was immediately obvious to me that this was a meteor and an extremely large event.

NARRATOR: A meteor is the scientific term for the fantastic light show generated when a space rock smashes into and through Earth's atmosphere. Most people have seen tiny meteors—we call them shooting stars—but when a meteor is this big and bright, it's known as a fireball.

This one is not only big, but historic. In fact, it's the biggest fireball seen since 1908, when a huge meteor exploded in Russia near the Tunguska River, flattening 800 square miles of Siberian forest.

The record of that event is vague, but the meteor strike over Chelyabinsk holds out the promise of providing detailed answers about the real threat we face from outer space: How can a rock create this kind of blast in the sky? How big does it have to be to be dangerous? And, most importantly, are there more on the way?

UPSYNC: You're watching America This Morning.

NARRATOR: Soon, the whole world knows that a massive, devastating meteor strike has occurred in central Russia.

UPSYNC Newsreader: Breaking news this morning: a shocker from outer space, a sudden meteor shower raining down chunks of burning rocks on Russia.

NARRATOR: In Ontario, Canada, meteorite experts, such as Margaret Campbell-Brown, are bombarded with requests for information.

MARGARET CAMPBELL-BROWN: It was a very exciting day. It alternated between calls from the media, doing radio and television interviews; we were also talking to our colleagues all around the world, checking our numbers with them and trying to find out which things they had discovered.

NARRATOR: Astrophysicist Peter Brown is asked by NASA and others for an early estimate of the size of the blast.

PETER BROWN: My first estimate was 40 or 50 kilotons, based on no data, just, sort of, a, an estimate from experience, as we tried to get some indication, really, of, of what the overall character of the event was, so that people would be informed with, with accurate information that day.

NARRATOR: A 50-kiloton blast is equivalent to about one hundred million pounds of T.N.T., about three times the power of the bomb that flattened Hiroshima, in 1945, and killed at least 60,000 people in an instant. But it's just an estimate.

To get the number more precisely, Peter accesses a highly sensitive global system, designed to detect illegal nuclear bomb blasts as part of the nuclear weapons test ban treaty.

PETER BROWN: There are 46 of these stations, operating all over the world, and they're constantly monitoring the atmosphere for any sort of pressure fluctuations resulting from an explosion.

NARRATOR: Each location has several super-sensitive microphones, arranged to record the power and direction of extremely-low-frequency sound waves, known as infrasound.

MARGARET CAMPBELL-BROWN: Well, most people are familiar with…ultrasound, and ultrasound waves are sound waves that are too high for us to hear; infrasound are sound waves that are too low for us to hear.

PETER BROWN: We're listening to the very lowest, very deep bass tones, reflecting the huge amount of energy in this explosion. As the explosions get bigger, the, the tone of the, the shockwave or the tone of the, the sound gets lower and lower and lower, until it's below the, the, the level that human hearing is able to detect.

NARRATOR: It may be designed for nuclear bomb blasts, but the system also picks up natural sounds of volcanic eruptions, earthquakes, tsunamis or even exploding death rocks from space.

When Peter checks the infrasound data on the morning of February 15, he is shocked.

PETER BROWN: In this particular case, the signal was very obvious. It was huge, and the most startling characteristic was the fact that it was very, very low tonal frequency, much lower than anything I'd ever seen before.

MARGARET CAMPBELL-BROWN: They can travel a long distance in the atmosphere without being absorbed, and that's why, when a car passes, you can hear the, sort of, throbbing base of the music and not the, sort of, high notes in the music, because the low-frequency noise propagates farther than the high-frequency noise.

NARRATOR: This blast was so strong that its low-frequency sound waves were able to travel an incredible distance.

MARGARET CAMPBELL-BROWN: This particular meteor, the infrasound that was produced actually circled the world several times and was heard each time it passed an infrasound station.

PETER BROWN: The earth really was ringing for a period of, of almost a day, as the event just kept going around and around the world.

NARRATOR: Analyzing these sound waves from the blast, Peter is stunned to see that his original estimate of 40 to 50 kilotons wasn't even close. He immediately raises the official estimate of the explosion's power, by a factor of 10, to nearly 500 kilotons, larger than most American hydrogen bombs.

PETER BROWN: The atomic bomb dropped on Hiroshima was about 15 to 20 kilotons, so 15,- to 20,000 tons of T.N.T. So, this event, Chelyabinsk, is about 20 to 30 times that event, which is what makes it so unusual, but also so destructive.

MARGARET CAMPBELL-BROWN: When he told me what the energy was, I was really surprised, a really powerful explosion.

NARRATOR: The size of the blast is astounding, but that's not the only surprise. Amazingly, the Chelyabinsk monster appeared the very day another asteroid made an uncomfortably close pass with Earth.

That asteroid is called DA14, and NASA telescopes have been tracking it for a year. NASA predicted that its closest point would bring it to within 18,000 miles of Earth, well inside the orbit of communications satellites; a close shave, but a miss, nonetheless.

So was there any connection between the two space rocks hurtling toward Earth?

MARK BOSLOUGH: After having looked at a few of the YouTube videos, it was very clear that this one came from the east, and the DA14 was coming from the south; the Chelyabinsk object came from the daytime sky, DA14 was coming from the nighttime sky. So they're in very different orbits.

NARRATOR: Coming from different directions means the two objects couldn't possibly be related, so two celestial hammer blows, aimed at planet Earth, on the same day, turns out to be pure coincidence.

But given the huge number of asteroids in our solar system, it certainly won't be the last close encounter. The truth is space is far from empty. There are millions of asteroids tumbling around our solar system. The majority orbit the sun in a band between Mars and Jupiter, known as the asteroid belt, and they're made of the same stuff as Earth and the other planets.

MARK BOSLOUGH: We can think of the asteroid belt as filled with all the debris that didn't quite manage to form a planet.

NARRATOR: Most asteroids remain safely out of the way, but a random collision or a planet's gravity can sometimes cause an asteroid to break from its usual path and begin a long fall inward, towards the sun.

MARK BOSLOUGH: Some of them have, have been deflected into inner orbit, orbits that cross the earth's orbit and, and that can be on potential collision courses.

NARRATOR: In fact, space rocks rain down on Earth all the time, but most aren't very big, the size of trash cans or baseballs or pebbles. Every shooting star is a meteor, burning to dust high in the atmosphere. But, every now and then, a piece of one survives long enough to reach the ground. Those fragments are called meteorites.

The Natural History Museum, in London, holds one of Europe's largest collections of priceless space rocks. Caroline Smith studies them for the valuable clues that they offer about the origin of our solar system and our planet.

CAROLINE SMITH: Well, meteorites come in three different flavors, stones, stony irons or irons. These two that I have here, these are called stony iron meteorites and these are a mixture of rock and metal. Because they've got so much metal in them, they are much heavier than a normal rock from Earth. I mean that's a good, sort of, 12 to 15 kilograms.

And this is an iron nickel meteorite, and it's, you know, it's heavy. This is about three times as heavy as the one that I've just shown you.

NARRATOR: To make the point that the meteorite is nearly pure metal, the museum has milled this solid knob out of its body.

CAROLINE SMITH: And that's actually showing that it's crystalline metal, in here. And, because it's very dense, it does put into context the type of damage that you can get from something of that size.

NARRATOR: The most threatening space rocks are big and dense, made mostly of metal. These stand the best chance of surviving the brutal searing of Earth's atmosphere, so they can reach the surface and produce one of these.

This hole in the ground, nearly a mile across, is Meteor Crater, in Arizona.

DAN DURDA (Southwest Research Institute): Meteor Crater was formed about 50,000 years ago, when an iron meteorite slammed into the rock with most of its cosmic speed still intact at about 40,000 miles per hour. The energy released in that impact was something on the order of 10 megatons of T.N.T., and it's that energy that was responsible for excavating this enormous hole in the ground and spewing that rock into the desert.

NARRATOR: Most of the time, the smaller violent collisions with Earth leave no trace. Occasionally a few fragments are left behind.

Each of these meteorites has endured the shock of slamming into the earth's atmosphere at tens of thousands of miles per hour, an incredibly punishing ordeal that heats the rock to more than 3,000 degrees Fahrenheit and leaves deep, otherworldly scars on the intact fragments.

CAROLINE SMITH: This is called the Barwell meteorite. It's quite, sort of, rubbly looking. One of the clues is that it's heating up to very hot temperatures, so hot that the rock actually turns into a gas. That's what gives the really bright glow that you see from a fireball. That is so powerful, that actually smooths the surface of the rock. So, you can see, this one has got this, sort of, quite smooth texture.

We also see what looks like thumbprints. In fact, I can actually put my thumb into this one here. And these, we think, formed, as this object is hurtling through the atmosphere. You actually get little vortices of air, and it actually scours the rock away.

NARRATOR: Ordinarily, space rocks that become meteors don't survive to land as smooth or sculpted chunks on the ground. The extreme heat generated by entry is usually too much for the rock to endure, and it melts away.

DAN DURDA: Most of the heating actually comes from the shock of that meteorite passing through the air. The air is compressed ahead of the meteorite, and that heats that air, and that intensely heated plasma, just ahead of the meteorite, is what is radiating back to burn away the meteorite itself.

NARRATOR: With so little evidence left from past collisions, meteors, and how they inflict damage, have remained elusive and mysterious. But that is about to change.

The rock that lit up the Siberian sky, over Chelyabinsk, is no ordinary meteor. When it first wandered away from the asteroid belt and made its way to Earth, it was indistinguishable from countless other bits of space rock. But it became unique when its final moments were captured by dozens of cell phones and video cameras, creating a record of unprecedented detail: hundreds of images uploaded to the web, many captured because of Russia's odd addiction to car cams. Soaring insurance fraud, corruption and drunk driving have led to a boom in small dashboard cameras.

Mounted inside the driver's compartment they endlessly record the previous hour or so of activity. Anything happening on the road ahead gets caught on a camera—from a plane crash into an overpass to a massive roadside "cow-tastrophe"—nothing, though, to compete with the historic images those cameras caught on that February morning.

PETER BROWN: I don't think we could have done a better job if we had gone out and instrumented in a dedicated way. There's no way we could have had this many cameras, this many angles, this many views. We're going to be able to study these videos in excruciating detail, for years, and look at all the, the subtle phenomena that we never would get. It's going to be a, a huge bounty to science.

NARRATOR: Richard Greenwood is one of the first to use the footage for scientific analysis.

RICHARD GREENWOOD: We've got a unique event taking place in an environment where people have filmed it extensively. It's absolutely incredible. It's an absolute treasure trove for science.

So, we're going to look at a piece of footage that is probably unique, because it shows the fireball from beginning to end.

NARRATOR: What makes this so valuable to scientists is that the meteor was filmed by so many different people from so many different vantage points.

RICHARD GREENWOOD: This is a chap driving to work, he's got his camera on, it's sunrise, and, suddenly, the fireball comes out of nowhere. It's probably about 70 kilometers up, at this stage. First time that's ever been recorded, absolutely fantastic! And you see it coming right down into the lower atmosphere. You can see the head of the fireball starting to develop there. And what we have next is an explosion, as it penetrates into the denser part of the atmosphere.

NARRATOR: Peter Brown is also examining the videos, trying to make a preliminary estimate of the meteor's vital statistics. He's already calculated the size of the explosion, nearly 500 kilotons, but he also wants to know the rock's speed, direction and mass, because if there's one thing he knows about the destructive potential of flaming rocks: size does matter.

Peter soon finds a video online that gives him a clue. A dashboard camera caught a perfect side view of the meteor as it sped past.

First, he estimates the angle of the meteor's approach.

PETER BROWN: The fireball went right across the entire field of view. By just doing a quick, sort of, measurement on the screen, it was apparent that the, the angle was about 20, or even less than 20 degrees and that that immediately said to me that the entry had to be quite shallow.

NARRATOR: Next, he tries to calculate its speed.

PETER BROWN: By looking at the duration, from the time it first appeared until most of the, the, the flaring and the breakup finished, it was about 10 seconds.

So using those numbers, and just the back-of-the-envelope estimate of the entry angle, 20 degrees, I was able to guess, that the, the total distance travelled was about 180 kilometers in 10 seconds, and, from that, come up with a very rough idea of about 18 kilometers a second.

NARRATOR: Eighteen kilometers a second, or about 40,000 miles per hour, over 50 times the speed of sound!

Armed now, with the amount of energy released by the breakup of the meteor and the velocity, Peter is able to calculate the weight of the rock.

PETER BROWN: We could put those two pieces of information together, and that, uniquely, gives us the mass. And our first estimates of the mass were something like 7,000 metric tons, which is quite considerable for a, an event like this.

NARRATOR: Seven-thousand metric tons is over 15,000,000 pounds, about the weight of the Eiffel Tower.

No one has, ever before, so precisely measured the explosive power, speed and mass of a large meteor.

But Peter hopes to go even further. He contacts his colleague Mark Boslough, and together they make a plan.

PETER BROWN: So it looks like with this fireball there's lots of video. So I think…

NARRATOR: They hope to calculate the meteor's precise path as it seared the sky over Russia, and then retrace the asteroid's approach, so they can find out exactly where it came from—something that would normally be impossible to do after the fact, if not for the dozens of eyewitness videos available online.

PETER BROWN: Fantastic. We got the videos selected. I'll get those off to you. Other than that, we're good to go.

NARRATOR: To get the most out of the videos, they need more information about where they were shot.

MARK BOSLOUGH: The problem with the videos is they're not calibrated. So they show a patch of sky, they show an object crossing the patch of sky, but just from the video itself, we don't know how high above the horizon it was, we don't know precisely the direction the camera was pointed.

NARRATOR: So Mark will head to Russia and travel to the exact locations where several of the original videos were taken, to record the G.P.S. coordinates and to take pictures of the night sky, for reference.

This is the first step in building a 3D map of the sky that will allow them to plot the precise path of the meteor, as it passed over Russia.

The long journey will take him from Albuquerque, New Mexico, to Siberia, to the region of the Ural Mountains situated between Europe and Asia, to the city of Chelyabinsk.

When he arrives, he is surprised to hear reports that locals are finding meteorites. Somehow, fragments of the space rock have survived the blast and are being picked up off the ground.

MARK BOSLOUGH: When I initially heard that people were looking for fragments, I didn't expect anybody to find anything, because the asteroid was, was big, this was, this really was the biggest event that's ever been observed. And I thought they would completely be vaporized.

NARRATOR: So Mark heads out to join the meteorite hunters.

MARK BOSLOUGH: Oh, I would just love to find some. And, and, really, these are optimal conditions, because they're dark, they're covered with this dark fusion crust. And, it hasn't snowed since, since these things fell, and that means even a physicist like me should be able to find one.

NARRATOR: Samples of the rock would be invaluable pieces of the puzzle, something that the scientists hadn't dared hope for. Not only could they reveal what the original asteroid was made of, whether stone or metal or a mixture, but they might reveal crucial details about how and why the meteor exploded.

Mark teams up with some Russian colleagues on a kind of scavenger hunt: scientists Dmitry Badukov and Dmitry Sadailenko.

The potential debris field laid out before them is vast. They calculate that fragments could have fallen across an 800-square-mile area. Searching is like looking for a needle in a haystack, without being sure if the needle even exists.

RUSSIAN SCIENTIST: Nobody here to ask?

NARRATOR: The Russian scientists are trying to zero in on the most likely landing spot by retracing the direction that the meteor travelled. To save time, they question local eyewitnesses.

UPSYNC: How high was it? Was it exactly above your head?

(Meteor Witness): It was above these buildings.

NARRATOR: Though it wasn't his primary reason for traveling to Russia, Mark has become obsessed with finding his own piece of the asteroid. A good-sized chunk could tell them a lot about the behavior of the asteroid, as it travelled through the atmosphere. Mark is hoping to get his hands on something at least five centimeters wide.

MARK BOSLOUGH: Well, you know, these little meteorites, when they hit the snow, they make a hole. And so, if you, if you excavate around them and, and make it smaller and smaller, they should fall out, you should see them.

DMITRY SADAILENKO: Very, very fine, tiny hole and small.

NARRATOR: Dmitry Sadailenko spots a possible impact point. And when he digs to the bottom, success: their first glimpse of this scientifically invaluable meteorite that fell from space.

DMITRY SADAILENKO: Here, here!

NARRATOR: The team has hit on an area right under the flight path of the meteor.

DMITRY SADAILENKO: Let's see. Maybe we'll find another one.

NARRATOR: But they're not the only ones on the hunt. The area has been struck by meteor fever, as locals are aware they may be walking on a goldmine.

There are boys down the road with fragments packaged up. Ask them.

NARRATOR: Mark never manages to find a piece on his own, but his Russian colleagues make sure he has a chance to examine one.

MARK BOSLOUGH: Yeah, that's what I want, I need five centimeters.

What's amazing to me though, when you think about it, I mean, this is part of an asteroid that had been, you know, floating through space, orbiting the sun for billions of years. And two weeks ago, it exploded in the atmosphere, dropped to the ground, and here I am holding it in my hand! That's amazing.

NARRATOR: And the information locked inside the tiny fragment will help scientists figure out the composition of the original asteroid.

MARK BOSLOUGH: If we find more, we'll get more, a, a larger statistical sample, but I think, you know, the ones that have already been found are sufficient for at least determining what type of a rock it was.

NARRATOR: Tying composition to blast size is crucial, if we are ever going to understand meteor explosions and the potential destructive power they hold.

The next day, Mark is off to a lab in neighboring Yekaterinburg, where he hopes to get a look at his fragment, along with other pieces of the meteorite collected by the Russians, with the lab's scanning electron microscope.

First, they slice the rock open to reveal the interior, untouched by the fires of the atmospheric entry.

MARK BOSLOUGH: What matters the most is the strength and the density. A weak asteroid will break up higher up in the atmosphere and a low-density asteroid will also break up and explode higher up. A denser, stronger asteroid will make it deeper into the atmosphere and explode at lower altitudes.

There's a crack here.

NARRATOR: The scans reveal that the fragment is only about 10 percent metal. The rest is rock. Analyzing additional samples will tell them if this piece is representative of the original object.

While dense metal asteroids might survive the journey through the atmosphere, to smash into the ground below, rocky asteroids like this one are much less resilient.

MARK BOSLOUGH: The way we understand it is it hits the atmosphere going so fast that there's so much stress on it that it actually breaks the asteroid. It exceeds the strength of the asteroid. And that can happen very, very fast.

NARRATOR: So, in this case, the asteroid may have met its demise early in its entry. The microscope scans reveal that the meteorite fragments have significant internal cracks, weaknesses that would have contributed to the sudden, explosive breakup of the rock.

MARK BOSLOUGH: Yeah, well, so, so, when the asteroid was in space, it probably had cracks in it, and it had big fractures, and, and so, when it, when it hit the atmosphere, it was almost like hitting a brick wall. And once it hits the denser part of the atmosphere, there's kind of this mutual, mutually reinforcing cascade of failure, and it just explodes. It just goes all at once.

NARRATOR: The evidence points to a rocky weakened asteroid that exploded soon after hitting the earth's atmosphere. It was this violent breakup that caused the shockwave that hit Chelyabinsk nearly three minutes later.

In England, Richard Greenwood is pouring over videos, analyzing the shockwave's effect on the ground.

RICHARD GREENWOOD: People had no idea what was about to hit. Here's a lovely bit of footage. A normal day in the office—or that's what they think anyway—then, all of a sudden, her colleague gets thrown across the room at her. That's the power of an air blast.

That explosion dumps a huge amount of energy into the atmosphere, and that causes a shockwave, which spreads out from that point. So it's, effectively, an explosion in the atmosphere.

NARRATOR: The explosion pushes air at high speed, pressure and outwards from the center of the blast. The reason people were caught by surprise was because the shockwave originated about 15 miles up and took nearly three minutes to arrive in the city.

RICHARD GREENWOOD: There's a huge delay between the time that you actually see the event and the time that it actually hits you. The vision is travelling at the speed of light and the shockwave will be travelling at a much slower pace.

There's a fantastic piece of footage, because this shows you the aftermath, people are studying the trail here, it's what's left after the fireball's gone through, and they think it's all over.

It is now.

So, what we've got here is, basically, the detonations, which took about three minutes to get here from where…from the airburst.

NARRATOR: It was the shockwave, the blast of air following the explosion, that shattered windows, causing the hundreds of injuries around the city.

But they'll never fully understand the shockwave, unless they can reconstruct the exact trajectory of the meteor. One way to roughly estimate the path is to follow the shifting shadows cast by the moving fireball.

RICHARD GREENWOOD: They had a position, at one point, where the meteor was overhead. An analogy here is with a sundial; in this case, the source of the light is the meteor. It's casting a shadow, and that meteor is moving very fast, so the shadow is swinging around.

You can actually use the angle and the length of the shadow to define the unique direction towards the fireball, at any one particular time.

NARRATOR: The shadow method is ingenious, but from Mark's vantage point in Russia, there's a more accurate way of calculating the trajectory. And that's what he's now set out to do.

The first step is to travel around the region and find the precise locations where some of the best videos were shot. And he has to do it at night, when the stars are out.

MARK BOSLOUGH: Well, I am doing a stellar calibration. So, so we got…one of our videos was from a dash cam, from a car parked in this parking lot. And the fireball streaked across the sky, here—we're looking south—it went from left to right, and, and what we really want to do is determine the exact angles to the fireball, as seen from this location.

NARRATOR: The goal is to create a map of the sky and plot the exact trajectory of the meteor. Standing where the eyewitness video was originally taken, Mark records G.P.S. coordinates of the camera position and matches one of the video frames with his still camera, taking care to capture the star field in the background.

MARK BOSLOUGH: So, if the stars show up on the digital camera, we can get those angles and then calibrate that—that image that was taken from the dash cam—and know exactly the angles to the trajectory of the fireball.

NARRATOR: By lining up the star field photo and the video frame, Mark will be able to calculate the location of the fireball relative to those stars.

With at least two photos and their corresponding G.P.S. coordinates, Mark and Peter hope to use simple geometry to locate the fireball in three dimension space, building a model that shows exactly the angle and direction of its approach.

MARK BOSLOUGH: We'll have a very precise trajectory as it streaked through the atmosphere, so we can backtrack that to get the orbit, the pre-impact orbit. So, it'll help, you know, at the beginning, the middle and the end.

NARRATOR: Meanwhile, back in Canada, Peter Brown and his team are trying to figure out exactly how big the meteor was. Using the information about its composition and density, found in Russia, it's simple math to calculate the volume, the physical size.

PETER BROWN: Assuming that it's a, a sphere—we always assume, for simplicity, things are spheres—we could estimate the size. And, in that case, the size came out to be about 15 meters, if it's 7,000 metric tons.

NARRATOR: Eventually, Peter and NASA settle on a final estimate of the mass and size of the meteor: 10,000 tons and about 20 meters, or 65 feet, in diameter, 25 to 30 percent larger than originally calculated.

With those figures in mind, Mark spends his last day in Russia looking for something of a similar size for comparison.

MARK BOSLOUGH: I think this building kind of gives us a sense of the scale of the asteroid strike, the asteroid of rock that weighed 10,000 tons. Something going that fast slams into even really thin air, there are tremendous forces on it that tear it apart and vaporize it, and that's where the explosion come from. That amount of mass, at that speed, has as much energy as half-a-megaton bomb.

NARRATOR: The surprisingly large size of the meteor and the evident power of the blast raise a puzzling question: why wasn't the damage on the ground, bad as it was, worse?

To find out, Mark Boslough takes what's known about the meteor's composition, angle and speed and plugs this information into a supercomputer, normally used to model the behavior of nuclear weapons.

MARK BOSLOUGH: So what we're doing is we're setting up a three-dimensional problem, and we're looking at a cross-section.

NARRATOR: The computer simulation recreates the final moments of the meteor's trajectory through the atmosphere and shows the devastating blast wave.

MARK BOSLOUGH: I started off about 40 kilometers above the surface, and I gave it a speed of 18 kilometers per second.

NARRATOR: That's 40,000 miles an hour!

He's also figured out exactly where the meteor exploded.

MARK BOSLOUGH: It gets down to the altitude of twenty-three-and-a-half kilometers above the surface, and it explodes, and this is where it dumps all its energy.

NARRATOR: The massive explosion occurs about 15 miles above the surface of the earth.

MARK BOSLOUGH: You get this enormous fireball. And that fireball continues to move downward, and it pushes a shockwave ahead of it. So, these are like mushroom clouds, with two big, giant nuclear explosions at the bottom.

The shock from the explosion continues to push forward, and it starts to move downwards. You can see that it's descending. It's down to about 10 kilometers above the surface here, at ground zero.

NARRATOR: The simulation shows that, because the meteor was entering at a shallow angle, much of the energy of the blast wave was expended as it moved horizontally. The meteor was destructive, but it could have been far worse, if the blast wave had been focused more downward.

Mark's second simulation shows what would have happened if the meteor had struck at a sharper angle, plunging more directly towards the ground.

MARK BOSLOUGH: What we see is something very different: allowed to explode at 23-and-a-half kilometers above the surface, but it continues to move downward at very high speed. The blast wave, when it gets to the ground and reflects, and then you see this shockwave moving across this surface. And that would have blown down trees and structures. If the area directly under this were populated, it would cause a lot of casualties. It would have caused a lot of destruction.

NARRATOR: This steep angle is similar to what scientists believe happened in 1908 over the Tunguska region, also in Siberia, but the Tunguska meteor was probably twice the size and remained intact longer, before exploding violently.

MARK BOSLOUGH: The Chelyabinsk event is very similar to what we think happened at Tunguska, but the Tunguska explosion was much closer to the ground, and it was much more intense, in terms of energy release, and, therefore, the blast wave was much stronger. And that's why it blew down trees over this wide area, almost a thousand square miles.

NARRATOR: Notably, witnesses living far from the scene of the blast reported that they felt heat radiating down from the sky, even though the explosion was miles above the surface, proof of the far-reaching effect of the blast.

MARK BOSLOUGH: Broadly speaking, Chelyabinsk was a mini-Tunguska. It was very much like Tunguska, but smaller and higher up.

NARRATOR: That glancing blow was a remarkable lucky break for Chelyabinsk. They were spared because the asteroid's path brought it toward Earth at a shallow angle, like a car merging onto a freeway. The consequences of a Tunguska-style direct hit on a major modern city would be devastating.

MARK BOSLOUGH: I've seen maps of the Tunguska blast area overlaid with a map of Washington D.C., and it extends far beyond the Beltway, in every direction.

NARRATOR: A Tunguska-like airburst over Washington would take out the city and the surrounding suburbs.

MARK BOSLOUGH: If something like the Tunguska event happened now, it could kill hundreds of thousands or even a million people, if it happened right over a big city. An asteroid has more damage potential on the ground than a nuclear bomb of the same energy.

NARRATOR: The real nightmare would be a meteor made mostly of metal, rather than rock that comes crashing down at a steep angle like the Tunguska fireball did, hitting the ground instead of blowing up in the atmosphere. In that nightmare scenario, instead of having to contend with broken windows and collapsed walls, the entire city of Chelyabinsk would be wiped out, replaced by a giant crater like this.

Impressive though it is, that's nothing compared to the battering our planet has taken in the past. One collision hit the young earth so hard it's thought to have knocked off a huge fragment that became our moon. And another massive strike, something the size of Mount Everest, helped kill off the dinosaurs, 65 million years ago.

There is much we don't know about those long-ago events, certainly not compared to the growing body of knowledge about the Chelyabinsk meteor. For all we've learned so far, however, one mystery still remains: why did no one see it coming, particularly at a time when astronomers were tracking DA14, that 100-foot-wide asteroid that was 20,000 miles further away and presumably harder to spot?

Looking at the trajectory model, it becomes clear why this asteroid snuck up on us and couldn't be detected.

PETER BROWN: Even many days beforehand, it was so close to the sun that no telescope on Earth could've, could've detected it. So it hit, basically, from the daylight side of the atmosphere.

NARRATOR: And the trajectory model shows something else surprising. It might seem like the asteroid hit us, but the truth is, we hit the asteroid! It crossed Earth's path, and our planet rolled right over it.

Although no one saw it coming, thanks to Mark's calibrated photos and trajectory model, we can now tell where it came from and the likely route it took to Earth.

The life story of the Chelyabinsk meteor began 4.5 billion years ago, within the dust cloud that formed our early solar system.

MARK BOSLOUGH: The first solids condensed; some of those solids would've formed an object kilometers or tens of kilometers, in size, at least.

NARRATOR: It tumbled around the asteroid belt with millions of others,…

MARK BOSLOUGH: It circles the sun for a billion years, and then it gets in a wreck.

NARRATOR: …fracturing at least once.

PETER BROWN: The rock has lots of cracks in it, so it's pretty weak, actually. After that collision, forces from the sun would slowly allow that orbit to drift, until it reached a position where it was able to interact very strongly with the planet Jupiter.

NARRATOR: The gas giant's gravitational pull, tugging on the ancient rock…

MARK BOSLOUGH: That sent it into a different orbit.

PETER BROWN: And moved towards the inner solar system.

NARRATOR: For millions of years, it spiraled slowly sunward,…

MARK BOSLOUGH: So it had an orbit that crossed Earth's orbit, and it may have had near-misses with the earth many, many times.

NARRATOR: …until it finally rushed up towards our planet.

PETER BROWN: Right up until that mid-February day, in 2013, when the final act was played out.

NARRATOR: Caught by Earth's gravity, it accelerated.

PETER BROWN: As it was moving over the Pacific and then mainland China, coming in closer to the earth, about 100 kilometers altitude…

MARK BOSLOUGH: …at 18 kilometers per second.

NARRATOR: Forty-thousand miles per hour, causing huge pressures to build on the leading edge,…

PETER BROWN: …there's heat, there's light, there's lots of collisions. As more of the material begins to be exposed it burns faster and faster, so the object gets really, really bright.

NARRATOR: …hot enough to vaporize rock.

MARK BOSLOUGH: And all that deceleration and fragmentation and ablation and radiative heating and expansion, that all happened in a split second.

PETER BROWN: The last of the energy is released.

MARK BOSLOUGH: Then it exploded.

NARRATOR: Releasing a powerful shockwave.

PETER BROWN: And so the object just completely comes apart. What you're left with are a few bigger pieces that have, by chance, happened to survive. And it ended its journey, its four-and-a-half-billion year journey, in this impact, south of Chelyabinsk.

NARRATOR: A frightening display, and yet, it could have been so much worse. The shallow angle and high altitude explosion spared a city: only broken windows and a rain of tiny pebbles.

MARK BOSLOUGH: The next day, little kids picked them up.

NARRATOR: The question is, "Will we be as lucky next time?"

CAROLINE SMITH: There is absolutely the potential that at some point in the future, the earth will be hit, which would wipe out humankind.

NARRATOR: For former Apollo 9 astronaut, Rusty Schweickart, Chelyabinsk is a welcome warning shot. On his own trip to space he was struck by Earth's fragility. Today, he's on a quest to galvanize the world to meet the threat of the next big rock.

RUSSELL "RUSTY" SCHWEICKART (Apollo 9 Astronaut): By far, the highest priority is you've got to find them. If you haven't found them, there's no way you can protect yourself from them, so early warning…

NARRATOR: To provide some warning, NASA tracks known asteroids and searches for new ones every day. But they've focused primarily on the bigger rocks. Their Earth-based telescopes have found only a tiny fraction of the much smaller asteroids, capable of wiping out a city.

RUSTY SCHWEICKART: The telescopes that had been used for the last 10 years and have found about 10,000 near-Earth asteroids, have essentially reached their capacity. We really need to upgrade the search capability.

NARRATOR: There are plans on the drawing table to deploy a different kind of technology that would be more effective.

DONALD YEOMANS (Jet Propulsion Laboratory, NASA): The most efficient way to discover these near-Earth asteroids would be to have an infrared telescope in space, because these objects are dark and they radiate strongly in the infrared wavelengths.

NARRATOR: Known as Sentinel, the proposed instrument would take up a station in an orbit around the sun. Clear of atmosphere and packed with infrared sensors, it would allow us to detect asteroids far earlier and with greater accuracy.

RUSTY SCHWEICKART: Sentinel will discover something on the order of 50 percent of the city-buster objects, and it will discover close to 100 percent of objects that are about 140 meters in diameter.

NARRATOR: Not perfect, but a vast improvement over the situation we are in today.

RUSTY SCHWEICKART: Right now, to be honest with you, we're driving around the solar system without any insurance.

NARRATOR: Many scientists believe investing in this kind of insurance would be worth it, to scan areas of space invisible from Earth.

MARK BOSLOUGH: In principle, we could have discovered the Chelyabinsk asteroid in advance. If we had a space-based observing system that was surveying the sky for objects that were on collision courses that had the potential to run into the earth, even from the sunward side, we could have issued a warning in Chelyabinsk.

NARRATOR: But what good would a warning have been? Scientists estimate that a space rock the size of the Chelyabinsk meteor hits Earth once every 75 or 100 years. There's little doubt that another object like this one has got our planet in its sights and will hit its target in the next century or so.

But what if it's bigger? What if it blasts in at a steeper angle, over an even more heavily populated city? Many believe we need to take the potential danger more seriously and be prepared, not just to detect, but to deflect or even destroy asteroids.

One thing's for sure, the Chelyabinsk meteor has dramatically changed our thinking about the threat from space.