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

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

RUSLAN SAFAROV (translated): 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.

People had no idea what was about to hit.

MARK BOSLOUGH: My first reaction was, "This is the big one."

Probably something bigger than we ever expected to see in our lifetimes.

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.

(man speaking Russian) Captured in frightening detail by dozens of digital cameras.

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 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...

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

NARRATOR: ...and new clues on the ground... BOSLOUGH: Crust formed, it exploded in the atmosphere, and 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.

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 15,000 to 20,000 tons of TNT.

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.

NARRATOR: 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.

(translated): I saw a small dot, and it became bigger and bigger.

It became a really bright white.

SAFAROV (translated): It was so bright it was blinding and I had to look down.

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

(translated): Everyone ran outside to look at it.

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.

(man speaking Russian) NARRATOR: Then, nearly three minutes later... (explosion) A series of explosions and shock waves reverberates... ...battering the city and injuring more than a thousand people.

Everyone who sees it wants to know-- what is it?

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?

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.

BOSLOUGH: Someone had just posted a Russian news article about this big explosion, this big event in Chelyabinsk.

And there was a YouTube video attached, and so I clicked on that and I watched it.

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 lifetimes.

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

The first videos were so unbelievable, the hairs stood up on the back of my neck, sort of thing, thinking, "Wow, this is a once-in-a-century event."

BROWN: It almost seemed surreal in some sense, sort of 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 put it on a collision course with Earth.

It was immediately obvious to me that this was a meteor and that it was 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?

WOMAN: We will have more stories coming up at 7:00 on Good Morning America.

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

Breaking news this morning-- a shocker from out of 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.

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.

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

NARRATOR: A 50-kiloton blast is equivalent to about 100 million pounds of TNT-- 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.

BROWN: So 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.

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

Infrasound are sound waves that are too low for us to hear.

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 tone of the shock wave or the tone of the sound gets lower and lower and lower, until it's below 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.

BROWN: In this particular case the signal was very obvious.

It was huge, and the most startling characteristic was the fact that it was a very, very low tonal frequency, much lower than anything I'd ever seen before.

CAMPBELL-BROWN: They can travel a long distance in the atmosphere without being absorbed.

And that's why when a car passes you, you can hear the sort of throbbing bass 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.

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.

BROWN: So the Earth really was ringing for a period 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 ten, to nearly 500 kilotons, larger even than most American hydrogen bombs.

The atomic bomb dropped on Hiroshima was about 15 to 20 kilotons, so 15,000 to 20,000 tons of TNT.

So, this event, Chelyabinsk, is about 20 to 30 times that event, which is what makes it so unusual but also so destructive.

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 had 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?

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 the 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.

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.

Some of them have been deflected into inner orbit-- orbits that cross the Earth's orbit-- and that can be on potential collision courses.

NARRATOR: In fact, meteors 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 they offer about the origin of our solar system and our planet.

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, 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.

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: 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 ten megatons of TNT equivalent.

And it's that energy that was responsible for excavating this enormous hole in the ground and spewing that rock into the desert around us.

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.

This is called the Barwell meteorite.

It's quite sort of rubbly looking.

One of the clues is that this is heating up to very, 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 the fireball.

That is so powerful that it actually smoothes 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, form 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 debris 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.

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.

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 subtle phenomena that we never would get.

It's going to be a huge bounty to science.

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

GREENWOOD: We've got a unique event taking place in an environment in which 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 which 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.

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.

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 angle was about 20, or even less than 20 degrees.

And that immediately said to me that the entry had to be quite shallow.

NARRATOR: Next, he tries to calculate its speed.

BROWN: By just looking at the duration, from the time it first appeared until most of the flaring and the breakup finished, was about ten 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 total distance traveled was about 180 kilometers in ten seconds, and from that come up with a very rough idea of about 18 kilometers a second.

NARRATOR: 18 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.

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 an event like this.

NARRATOR: 7,000 metric tons is over 15 million pounds, about the weight of the Eiffel Tower.

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

But Peter hopes to go even further.

He contacts his colleague Mark Boslough and together they make a plan.

Yeah, so it looks like with this fireball, there's lots of video.

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.

BROWN (on computer): We got the videos selected.

I'll get those off to you.

Other than that, we're good to go.

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

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 GPS 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.

BOSLOUGH: When I initially heard that people were looking for fragments, I didn't expect anybody to find anything because the asteroid 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.

I would just love to find some, and really these are optimal conditions because they're dark, they're covered with this dark fusion crust.

And it hasn't snowed 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.

The Russian scientists are trying to zero in on the most likely landing spot by retracing the direction that the meteor traveled.

To save time, they question local eyewitnesses.

(translated): It passed almost directly overhead.

(translated): 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 traveled through the atmosphere.

Mark is hoping to get his hands on something at least five centimeters wide.

These little meteorites, when they hit the snow, they make a hole.

And so if you... if you excavate around them and make it smaller and smaller, they should fall out, you should see 'em.

Very, very fine, tiny hole in snow.

NARRATOR: Dmitry spots a possible impact point.

And when he digs to the bottom... success.

You can see... yeah.

NARRATOR: Their first glimpse of this scientifically invaluable meteorite that fell from space.

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

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 gold mine.

(translated): 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.

Yeah, that's what I want.

Five centimeter...

I need five centimeter.

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.

If we find more, we'll get more... 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.

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 altitude.

There's a crack here.

A crack?

Yes.

What's the scale on that?

About three microns.

NARRATOR: The scans reveal that the fragment is only about 10% 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.

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 scans reveal that the meteorite fragments have significant internal cracks-- weaknesses that would have contributed to the sudden, explosive breakup of the rock.

When the asteroid was in space, it probably had cracks in it and it had big fractures.

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 shock wave that hit Chelyabinsk nearly three minutes later.

In England, Richard Greenwood is poring over videos, analyzing the shock wave's effects on the ground.

People had no idea what was about to hit.

Now this is a lovely bit of footage.

It's a normal day in the office, or that's what they think, anyway.

And 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 shock wave, which spreads out from that point.

So it's effectively an explosion in the atmosphere.

NARRATOR: Essentially, the explosion pushes air at high speed and pressure outwards from the center of the blast.

The reason people were caught by surprise was that the shock wave originated about 15 miles up and took nearly three minutes to arrive in the city.

There's a huge delay between the time that you actually see the event and the time it actually hits you.

The vision is traveling at the speed of light and the shock wave will be traveling at a much slower pace.

(men speaking Russian) 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.

(explosion) (woman screams) 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 shock wave-- 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 shock wave 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.

They had a position at one point where they knew the meteor was overhead.

The analogy here is with a sundial.

In this case, the source of the light is the meteor.

It's casting a shadow, and as the 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.

Well, I am doing a stellar calibration.

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 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 GPS 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.

So if the stars show up on the digital camera, we can get those angles and then calibrate 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 GPS 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.

We'll have a very precise trajectory as it streaked through the atmosphere, so we can backtrack that to get the pre-impact orbit.

So it'll help 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.

BROWN: And assuming that it's 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% 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.

BOSLOUGH: I think this building kind of gives us a sense of the scale of the asteroid size.

The asteroid was a piece of rock that weighed 10,000 tons.

When something going that fast slams into even really thin air, there's tremendous forces on it that just tear it apart and vaporize it, and that's where the explosion comes from.

That amount of mass at that speed has as much kinetic energy as a 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.

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.

BOSLOUGH: I started off at about 40 kilometers above the surface and I give 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.

BOSLOUGH: It gets down to the altitude of 23 1/2 kilometers above the surface and then it explodes.

This is where it dumps all its energy.

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

And you get this enormous fireball.

And that fireball continues to move downward and it pushes a shock wave 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 downward.

You can see that it's descending.

It's down to about ten 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.

What we see is something very different.

I allow it to explode at 23 1/2 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 shock wave moving across the 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.

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 1,000 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 ground-- proof of the far-reaching effect of the blast.

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 towards 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.

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

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

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 DA-14, 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.

Even many days beforehand, it was so close to the sun that no telescope on Earth could have 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 1/2 billion years ago, within the dust cloud that formed our early solar system.

BROWN: The first solids condensed.

Some of those solids would have formed an object kilometers or tens of kilometers in size at least.

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

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

NARRATOR: Fracturing at least once.

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 the 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... That sent it into a different orbit... And moved towards the inner solar system.

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

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.

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

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

As it was moving over the Pacific and then mainland China, coming in closer to the Earth about 100 kilometers altitude... BOSLOUGH: At 18 kilometers per second.

NARRATOR: 40,000 miles an hour...

Causing huge pressures to build up on the leading edge.

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.

And so the object gets really, really bright.

NARRATOR: Hot enough to vaporize rock.

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

The last of the energy is released.

BOSLOUGH: Then it exploded.

NARRATOR: Releasing a powerful shock wave.

The object itself just completely comes apart, and what you're left with are a few bigger pieces that by chance have managed to survive.

And it ended its 4 1/2-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.

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.

By far the highest priority is you've got to find them.

If you can't find 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.

SCHWEICKART: The telescopes that had been used for the last ten years and that 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.

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 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.

Sentinel will discover something on the order of 50% of the sort of city-buster objects and it will discover close to 100% of objects that are about 140 meters in diameter.

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

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.

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.

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