The Truth About Meteors: A Horizon Special Horizon

For the residents of the Russian city of Chelyabinsk,

the morning of Friday, February 15th, 2013

began like any other.

As they set off to work, in what has become a craze throughout Russia,

many recorded their journeys.

But these cameras, usually used for capturing

minor traffic incidents, were about to record history.

A fireball brighter than the sun appeared from nowhere...

..before exploding with the power of 30 Hiroshimas.

A minute later,

a shockwave blew in the windows of 4,000 buildings across the region.

The broken glass accounting for most of the 1,200 injured.

The people of Chelyabinsk had just experienced the most powerful

meteor strike for more than a century.

The meteor that exploded over Chelyabinsk is a spectacular

reminder of just how exposed our world is.

Earth is this tiny planet in a vast, violent cosmos.

It is also a reminder of the powerful impact that these

alien rocks can have on the fate of our planet

and on us.

This is not the first time it has happened.

Over the last few years, scientists have examined many other

devastating impacts in the earth's past.

Using this knowledge, I want to answer the key questions

that the Chelyabinsk meteor strike raises.

Where did this alien rock come from? When will the next one strike?

And can we do anything to protect ourselves?

A fortnight after the impact,

the meteor strike is still big news in Russia.

In Chelyabinsk there is a popular new winter pastime -

hunting for any fragments of the meteorite that remain.

Scientists have also been out in force,

particularly around Lake Cherbarkul

where there is evidence of an impact in the ice.

So many fragments have been found here,

it has been called the Cherbarkul meteorite.

They are trying to piece together exactly what happened,

because the fact is no-one in the scientific world saw this coming.

I was shocked. I was truly shocked.

I never thought I would see an event like this over a major

city during my lifetime.

We could not predict this was going to happen.

The piece of rock that entered the atmosphere was relatively small,

maybe only a few metres across, and so we could not see

this before it entered.

When something like this happens, there is no doubt about it,

it is frightening.

But I have to admit, as a geologist

witnessing a once-in-a-lifetime event,

it is utterly thrilling.

You only had to look at social media to see that scientists

all over the UK and around the world were getting very,

very excited about this as the news broke.

It was exciting. It was exciting for me as a meteoriticist

because you immediately want to know, what is it?

What has landed? Is it a bit of Mars or a bit from an asteroid?

I am almost ashamed that I had such great excitement about seeing

this event and knowing that meteorites had fallen,

because people had been injured.

Chebarkul was the biggest meteorite to strike the Earth

since we've had the technology to measure them.

From its journey through the atmosphere

to its spectacular end, every moment was captured.

One of the best documented 16 seconds in science ever.

'Professor Alan Fitzsimmons is one of the scientists'

who has been examining the meteorite footage frame by frame.

These are amazing images,

but what can you get out of these as an expert?

What it shows us, first of all, is a great record

of the entry of the object into the Earth's atmosphere,

so you see it right from the moment it really penetrated and there it is.

-That is the edge of the atmosphere?

-That is it and it is coming down at a fairly shallow angle

probably and as we play the movie on, what we see is, bang, there,

it suddenly got brighter. So something has happened to the object.

It is starting to break apart and as it breaks apart, it releases

some of its orbital energy and that is causing that big flare up there.

Is that because the atmosphere is denser, it is harder?

That's right, it is finding it harder and harder to punch through the atmosphere.

As we roll on, we suddenly get this bang, this huge flare up where

suddenly the whole object is starting to fragment and break apart.

That is where the majority of the energy is being released.

If we look here.

There are just little bits falling off.

There is another flare up here and there is another flare up there,

and now we can still see it's glowing, incandescent,

some major fragment of the object is still falling down through the Earth's atmosphere.

Underneath that trajectory you are going to have showers

of bits of asteroid, essentially, falling down,

and then finally 16.5 seconds later,

-and what we are left with is this contour trail.

-The shockwave.

The shockwave coming towards us, that is right.

It is about a minute later that it has gone and reached the ground.

-This guy driving doesn't know yet that the shockwave is on its way here.

-That is right.

He is still happily listening to the radio on his drive to

work in the morning.

The explosion generated a shockwave

so massive it was detected over 15,000 kilometres away.

The low-frequency waves were picked up by monitoring stations.

This is kind of like a listening network around the world.

That is right. They're not set up for fireball or asteroid impacts,

but set up to listen for nuclear explosions.

What the monitoring stations picked up were some of the largest

infrasonic waves ever recorded.

Here they have been modified to make them audible.

It has been detected down in Antarctica, we've got records of it

up there in Alaska, so the pressure wave from the entry of the object

and the explosive fragmentation was found, seen all over the world.

So from the data that is coming in, it is early days, obviously,

but from the data that is coming in, what is your best guess at the size of that rocky lump?

Well, from the infrasound we know the energy released

was something like 500,000 kilotons of energy, which is huge.

-I was thinking it sounded a lot.

-That is right.

And because we know it came in,

from the video footage, at about 17.5 kilometres per second,

we can combine that energy with that velocity to get a mass of the object.

From that mass, we can get a size

and it is probably about 15 metres across or so.

That is a rarity, isn't it?

We think these things come in maybe, in once every 50 or 100 years,

that is all randomly, so this is a really special and really rare event of course.

Meteor strikes as big as this may be rare

but scientists have a surprisingly detailed knowledge of what

meteorites are and where they come from.

Long before the meteorite reached its explosive finale in full

view of Chelyabinsk's dash cams, it had a very different existence

and going by a very different name.

Meteorites begin life in deep space,

as part of much larger bodies called asteroids.

These can range in size

from just a few metres to more than 900 kilometres.

The leftovers from the nebula that created our solar system

some 4.6 billion years ago.

And millions of them

circle the sun in a trail known as the asteroid belt.

Here, collisions create smaller fragments

and when these fall towards Earth, they take on one of two forms.

The smallest pieces will burn up in the atmosphere to become meteors,

what we call shooting stars.

Only the larger fragments that make it all the way to the earth's

surface are called meteorites.

The meteorite is a piece of rock from space,

or a piece of metal from space, that falls through our atmosphere

and actually hits the ground to be recovered.

Technically, scientists love their words, it is

not a meteorite before it is actually found and discovered.

By collecting and comparing meteorites,

scientists have been able to piece together a picture of how they form

and these studies have revealed some of the most remarkable rocks

in the solar system.

Few places in the world have got as many

meteorites as the Natural History Museum, meteorites like this.

It is a cracker, isn't it?

The ones that are out here on display are just a fraction.

The bulk of the collection is behind the scenes

and that is where the science goes on.

So all these are meteorites in some shape or form?

They are all either meteorites.

'Professor Sara Russell is expert at decoding the messages hidden

'within these fragments of space rock.'

This looks quite rocky, what about that one?

It looks like a humble rock but if you hold it -

be very careful of this one, this is older than the Earth, it is

the oldest thing you will ever hold.

-This is older than what, 4.6 billion years?

-Yes.

We have this number of 4.6 billion years of the age of the solar system,

we know that from meteorites like this one,

and from looking at the age of the components within it.

If you can see, it has these rounded objects in it, which are 1mm to 1cm

in size, these are called chondrules, and these were once free-floating.

Before there were planets, these were free-floating in the solar system

around the very young sun and then they slowly coalesced

to make asteroids and larger and larger objects,

until eventually planets were formed.

These were the building blocks of planets.

So the Russian meteorite, any news on what kind it is?

Well, the early reports are that it is an ordinary chondrite

and that means it will be similar to this one, so this is really exciting

for us as scientists because we want to know how the planet is formed,

what was around before the planets, what the environment was like

and how the material that made up the planets first came together,

and the chondrites are the best way of finding that out.

Is this the most common in the solar system?

It is the most common type to fall down to Earth.

There is almost certainly a bias that the only material that we get

to Earth is stuff that happens to cross the Earth's orbit.

It has to be going in a slightly odd direction to cross the Earth

anyway, so there is some kind of selection bias.

This is a really special thing for you to kind of have in your career.

Yes, if only something like this would happen in Britain

so we could go and get it.

I don't think there's too many people watching this programme

that will be saying, "I wish it happened in the UK."

-Obviously somewhere uninhabited.

-OK.

How much of this stuff comes to us every year?

Actually, huge amounts. The Earth is growing by

at least 40,000 tonnes a year, so a huge amount of material is falling

to Earth but we don't really notice most of it

because the vast majority of it comes in the form of dust.

Although several thousand meteorites actually land on Earth every year,

most of those actually go unnoticed.

They fall just too far away from people.

If a meteorite falls maybe 15 feet away from you,

you probably won't notice it.

It will make a dull thud and that will be it, unless it is very large.

This event is special because it was so large.

There was no way you could not notice this meteorite falling.

It really wanted to get noticed. It said, "Ta-da! I am here."

Those events are spectacular and they give us scientists these

important pieces of rock from which we can learn about the solar system.

It is remarkable how we are able to build up this picture of what

is going on millions of miles away in the solar system.

It is one of the joys of science really, almost like a detective

picking up on those tiny clues to tell a bigger story.

So that the big question, the one that really needs answering,

is why do some of these asteroids suddenly head straight towards us?

Over 95% of asteroids are found in an orbit between Jupiter

and Mars, called the main belt.

It's almost 200,000,000 kilometres across

and home to millions of these orbiting rocks.

These asteroids have been following the same path for millions of years.

So long as they remain here, they pose no threat to Earth

but occasionally, one goes astray.

Collisions are one of the reasons why this might happen.

But in the last decade,

we have learned that just a few rays of light are enough, because one

scientist has tracked the orbit of just one of these millions of rocks.

Steve Chesley of NASA's Jet Propulsion Lab in California

has made a study of 200,000,000 tonne asteroid called Golevka.

This is a model of Golevka.

It is actually about 500 metres across,

say the size of a football stadium.

It rotates in this direction.

As you can see, it has a very angular shape to it.

He set out to investigate a 100-year-old theory that said

asteroids were powered by the sun itself.

It was called the Yarkovsky effect.

The Yarkovsky effect is a very small acceleration and acts on asteroids

and what it is is,

if you take a model, the sun is hitting the asteroid,

warming the surface, and as the asteroid rotates,

that hot surface radiates the heat out in a different

direction into space and that causes an acceleration, a very slight

acceleration coming from the photons that are emitted from the asteroid.

The idea is that this acceleration,

slight as it is, can have significant effects upon

the orbits of asteroids over millions of years.

It was an intriguing idea.

What sent asteroids out of their orbit and on a path towards

Earth was photon propulsion, but what was lacking was proof.

The Arecibo telescope is over 300 metres in diameter.

It is one of the most powerful telescopes in the world and it

uses radar to mark the precise position of objects in deep space.

It was this telescope that would allow Steve Chesley to detect any

tiny alterations in the orbit of asteroid Golevka,

more than 15,000,000 kilometres out in space.

We knew that it would be in one place

if the Yarkovsky effect wasn't acting on it, and it would be over here

if it was acting and our models were correct.

When Steve and his team studied the data, the results were unequivocal.

We knew from the radar measurements where Golevka was within a few

tens of metres and yet it was actually 12 or 15 kilometres

away from where it was predicted to be without Yarkovsky effect,

so these very precise radar observations allowed us to see

the 12-kilometre displacement caused by the Yarkovsky effect.

So photons, those elementary mass-less particles of light,

really can create a tiny force.

The force is about one ounce on earth,

say that the weight of a shot glass, that is

the force on this huge asteroid, the size of a football stadium.

Even for me it is truly remarkable, it is dramatic that a force

so slight can have such dramatic changes on individual asteroids'

orbit over millions of years.

The Yarkovsky effect is subtle.

It takes many millions of years to gently nudge an asteroid

out of its regular orbit.

But once that orbit has been disturbed,

the consequences can be profound.

Now it can come increasingly under the influence of the solar system's

largest planet - Jupiter.

Jupiter has a mass 300 times bigger than Earth's

so there is a huge gravitational field.

Often that works to our benefit.

Stray objects can be swept up in Jupiter's gravity,

drawing them into the planet.

We've actually observed Jupiter acting as a shield in this way.

This photograph, from the Hubble Space Telescope,

shows the fragments of a comet torn apart by Jupiter's gravity...

as the pieces were drawn to the planet's atmosphere

the impacts left blast scars - some as big as the Earth...

..but there is a downside to Jupiter...

it can also deflect asteroids into orbits that cross the Earth's path.

The Chelyabinsk meteor appears to be one of these typical

Earth-crossing events.

The likelihood is that it was thrown out of its regular orbit

by either one or a combination of the known causes -

collision, the Yarkovsky effect, Jupiter's gravity.

It continued its new orbit for hundreds, thousands,

even millions of years before meeting its fateful end.

We can even begin to trace the exact path that the Chelyabinsk meteor

took on its collision course with Earth.

Within the 16 seconds of action are all the clues we need.

Now, from just one vantage point it's not clear...

exactly how far up it is or how far away it is

but that's what we get from looking at other vantage points.

So, here we are, again, at a different angle.

The object is coming in, almost out of the sun, there,

and by combining this video clip with the other video clips,

what we can do is trigonometry.

Basically, you can figure out how high up the object was

and how far away it was.

And if you catch the object early enough

then you actually know where it was in the atmosphere

the first time you saw it.

So, you're kind of, triangulating to get that fixed position

-and it changes over time so you get the trajectory?

-That's right.

In the first part of the trajectory, what you've got there

is a path that is relatively unaffected by the Earth's atmosphere.

So, we can use that part of the video footage to track back

and figure out where this object came from in the solar system.

I love watching this because I now know where it is going to come

and you see it just hitting the edge of the atmosphere.

It's going to be...just...

Come on, come on...

-There it is!

-Yup.

It's about 90 kilometres up, at that stage,

travelling at 17.5 kilometres per second.

Using the different camera positions,

scientists have pinpointed the exact position

at which the meteor entered the atmosphere...

and, by tracking the speed and angle of the shadows

that the meteor casts,

they've calculated its velocity.

Together this is enough to track back the asteroid's path

from deep space.

Although the asteroid and Earth orbits are different durations

and at angles to one another

their clockwork regularity means that we were bound to collide.

So, this shows, speeded up, obviously, three and a half hours,

the last three and a half hours of the life of this little rascal.

Yeah, it's nice to see it from the asteroid's point of view.

The thing to remember is that this asteroid has been in its orbit,

going around the sun, roughly once every two years, we believe...

-Minding its own business.

-Absolutely.

..and, unfortunately, on February 15 it found a planet in the way.

Sure enough,

at 09:20 hours the neat yet entered our atmosphere above Siberia.

On this path and at time

it was Chelyabinsk that took the full impact...

..but could there have been another scenario?

The meteorite landed at a latitude of 55 degrees north,

had it arrived just a few hours later

we would have been directly in its flight path.

So, was this a near miss for us?

If the asteroid had been in a different part of its orbit,

so it didn't hit this year but it hit next year,

it would have still hit us on February 15th

but instead of coming in over Russia

it would have come in over the UK and Ireland

and would have entered the Earth's atmosphere,

in fact, entered the North Atlantic Ocean.

In order for the meteorite to strike anywhere near Britain

our paths through space would have had to be fundamentally different.

So we know where asteroids come from

and the forces that shape their date with destiny...

but what exactly happens next?

The moment that a meteor strikes?

And what determines just how devastating that strike will be?

When the Chelyabinsk meteor reached our atmosphere

it was travelling at more than 65,000 kilometres per hour...

and measured more than 15 metres across.

Apart from some unconfirmed reports

of craters at the bottom of Lake Chebarkul

there's surprisingly few signs of an impact.

Little of the 7,000 tonnes of space rock that entered the atmosphere

have been recovered...

..perhaps 300 fragments...

..and yet, the effects were felt over 3,000 square kilometres.

The question is how can apparently so little do so much harm?

There's a clue from the last time

Earth experienced a meteor strike on this scale.

On June 30th, 1908,

a huge explosion tore through the forest of Tunguska, Siberia.

It was 20 years before the Russians mounted an expedition to the site.

What they found astonished them...

..60 million trees across an area the size of London

had been levelled.

Scientists thought it has been caused by a meteorite strike...

..but then why was there no sign of any kind of impact crater?

The answer is that the devastation had to be caused by a meteor attack

of a very particular kind.

Physicist Mark Boslough has been fascinated

by how so much destruction can be caused

without any apparent direct contact.

The explosion at Tunguska was caused by an asteroid

that entered the atmosphere, got close to the surface

and exploded before it hit the ground.

And that explosion created a blast wave with hurricane force winds

that knocked trees over for thousands of square miles.

Scientists call it an airburst -

a massive explosion in the atmosphere,

rather than on the ground.

As it enters the atmosphere at speeds of up to 24 metres per second

the air resistance decelerates the asteroid so fast

it breaks apart in a huge explosion.

Most of the damage from an explosion like this is actually the blast wave,

it's the very high winds.

Mark created a simulation to see what size

an asteroid would need to be to generate such destructive power.

In this simulation I include more of the physics to be more realistic.

We can see that the main shockwave

doesn't come out of the point of the explosion

but it comes out of the point where the fireball descends to.

So, by the time the shockwave gets to the ground

it's much stronger than it would otherwise be

and there's more damage on the ground

because the destructive power was carried downward.

Based on Mark's calculations, the devastation at Tunguska

could have been caused by an asteroid,

perhaps as small as 30 to 50 metres in diameter...

..and this carries a worrying implication.

Smaller asteroids are more dangerous than we used to think

and because there are so many more smaller asteroids

than bigger asteroids

we need to take that risk more seriously than we used to.

The lesson of Tunguska helps explain why in Chelyabinsk

there's so much damage but very little meteorite to be found.

If we go back to the video footage and we see the object coming in,

when it's in the high atmosphere it suffers very little effect

but just here you get this huge flare-up

and that's because the atmosphere has become so dense

that it's almost impossible for it to push through any more.

And, basically, something's got to give, and the asteroid gives,

and it, basically, just breaks apart

in a huge catastrophic fragmentation effect,

and that is what creates a shockwave,

which we hear as this sonic boom.

EXPLOSION

Really it's a balance between the size of the object,

its speed into the atmosphere and, critically,

the altitude at which it explodes.

Too high, if it's too small and it explodes too high

the shockwave has little effect on the ground.

If it's...quite low in the atmosphere, it's a large object,

then that shockwave is completely devastating.

Actually seeing it in real life really brings home to you

the energy that these things carry

and, even though it exploded tens of kilometres, perhaps, up in the air,

so, quite a long way from the ground, the force of the explosion,

the shockwave, was able to damage buildings over a huge area and injure people,

and that was quite a shocking thing to see.

The destructive power of an air blast is immense

but, in a way, the people of Chelyabinsk are lucky

because out there in the cosmos is a different kind of asteroid,

one that poses an even greater threat.

I've seen the evidence of what one of those can do,

the damage that it leaves behind,

and what you realise is the Earth's own destructive forces -

you know, the great earthquakes, the volcanic eruptions -

seem trivial in comparison.

This is Barringer Crater, Arizona...

..the 50,000-year-old remnant of a massive meteorite impact.

'This place really gives you a sense of the destructive power'

of incoming meteorites.

The blast here would have vaporised a city larger than London

but the lump of rock that did it measured barely 15 metres across.

Down on the ground the scale of the impact is even more breathtaking...

..the crater is more than a kilometre across

and nearly 200 metres deep.

The forces here were enormous,

the impact turned this solid rock

into this pulverised mush.

It just...bursts out in your hand.

I mean, look at that.

They started out as the same kind of rock.

The meteor that struck here was about the same size

as the one that flattened Tunguska

but there is a critical difference...

at Barringer the meteor didn't explode in the atmosphere,

it struck ground.

So, this is just a fragment of the true devastation unleashed here.

Fortunately, to understand exactly why ground strikes

are so very destructive

we don't have to wait for another Barringer to happen...

because today we can simulate this kind of impact.

And that's thanks to the research of Pete Schultz...

and one very special piece of equipment.

So, so, this was serial number one, it was built during the Apollo time.

I guess because they thought there would be several of them made

but this is the first one and the last one.

And is the only one like it, in the world.

This is NASA's Vertical Gun Range.

It was built to study how impacts affected the moon

as the astronauts prepared to make the first lunar landing.

We are armed, gated and reset.

Today, Professor Pete Schultz uses it to model precisely

the dynamics of an asteroid impact.

We know that these...asteroid impacts are bad

but you want to understand really how bad.

Peter uses the NASA gun to fire projectiles at very high speed

to simulate an asteroid hitting the Earth.

So, for this experiment we're going to fire

this tiny quarter-inch aluminium sphere at very high speeds,

up to around five kilometres per second,

and then we will see what type of crater it produces.

The target it will hit is made of sand.

So, we use sand because it records the shock affects very clearly.

Outside of the impact chamber are super high-speed cameras

that can film at up to 1,000,000 frames per second,

capturing every detail of the impact and the aftermath.

-OK, lights out. Everything good?

-Yeah.

-OK, we're out of here.

We have high voltage, the paddle is in, the warning lights...

and...rolling.

ALARM BUZZES

Oh, perfect. Perfect, perfect.

Now we're seeing the fireball come in - it's brighter than the sun

and then, "Kapow!", it hits the surface. Jeez!

This whole region, downrange, would have been incinerated.

It would have been incinerated just by this plasma,

this exploding vapour plume engulfing everything.

There would have been winds that would have been going so fast

it could pick up houses and spread them hundreds of kilometres away.

This would have been Armageddon.

Experiments like this reveal several important things.

One is that it's not just the impact,

it's all that vapour that runs downrange.

In fact, you can see areas, here,

where there was so much wind it actually carved out

pieces of this landscape.

So, what these experiments help us do,

they actually allow us to witness the event -

see it in real time -

and try to understand the processes that are going on.

It's really complex but we have to see it to understand it.

So, asteroid impacts unleash a trail of destruction far greater

than suggested by the footprint of the crater alone.

Comparing the effects of an airburst with a ground strike,

it seems the Chelyabins got away lightly.

It's estimated that the largest piece to hit the ground

weighed 500 kilos,

a fraction of the asteroid's original mass of 7,000 tonnes.

Now if a piece of rock that big had hit that area of Russia,

it would have produced a huge impact crater.

Then that kinetic energy is then delivered into the ground

and we see things like seismic shock.

So, you get... People would feel earthquakes on the ground.

So, the fact that it was an airburst actually limited the consequences

for the people on the ground.

So, yes, still quite dramatic,

still, you know, obviously, causing injuries

but it could have been a lot worse, had it survived down to ground.

Ground strikes are amongst the most destructive

natural hazards we know of.

When viewed from space,

Earth's encounters with giant asteroids in its deep history

are revealed.

And there is evidence from our planet's past

of a truly devastating meteorite strike

that decisively altered the course of life on Earth.

Today, millions of years after the impact,

the evidence for that crater is well hidden.

SHE SHOUTS

This is a gateway to the cenotes,

the unique cave system of Mexico's Yucatan Peninsula.

Wow!

Look at the size of this!

This is magnificent!

That is beautiful.

'This cave may be stunning,

'but it provides the evidence for one of the greatest catastrophes

'in the Earth's history.'

And that water, it's so clear!

Lower the gear, please!

There's actually much more to this amazing cavern

than first meets the eye.

But to understand the scale of what happened here,

you have to go deeper still.

Underwater.

OK?

I'm not sure if I'm ready for this.

I've got all the equipment, but...

there's something about going down into water

when you're not quite sure where your exit is...

But I trust Bernadette completely here.

HE CHUCKLES

She knows what she's doing.

So I'm as ready as I'll ever be.

-Ready?

-All right.

Descending into the depths of the cenote is like entering a new world.

Fewer people have visited some of these drowned caverns

than the surface of the moon.

As divers have explored further,

they've discovered the cenotes are actually part of a huge complex

of tunnels and caves.

In fact, when you look from above,

you can see there are cenotes

scattered across hundreds of kilometres.

And when they're mapped,

it becomes clear that they follow

a distinctive circular course through the jungle.

They mark out the rim of a giant crater.

Scientific instruments show

the structure of the underlying rock has been deformed,

revealing the boundaries of a colossal meteorite impact crater.

This amazing cavern is part of a bigger story, a much bigger story.

65 million years ago, THIS was the site

of one of the most catastrophic impacts in Earth's history.

What became known as the Chicxulub meteorite landed here.

And THAT triggered the extinction of the dinosaurs.

The meteorite was 15 kilometres across,

enough to cause utter devastation

across the whole planet.

It exploded with a force of 100 million million tonnes of TNT.

The blast sent a giant plume of vaporised rock out into space.

A crater was punched 30 kilometres into the Earth's crust.

It was above this rim of weakened rock that these cenotes formed,

millions of years later.

The blast would have been ferocious.

But it was what happened next

that made the impact a global catastrophe.

The blast plume that shot into space fell back to Earth.

Billions of molten particles superheated the air

to a temperature of hundreds of degrees.

Fires swept the planet,

choking the atmosphere with soot and dust.

The dinosaurs, and most other creatures, were doomed.

That discovery, back in the 1980s,

about what happened at Chicxulub,

changed everything.

Up until then, we thought

that the Earth had changed only through grindingly slow processes,

but now we knew that there was also sudden, violent catastrophes

that made the Earth the way it was.

Of course, what that meant

was that something like this could happen again.

At any moment.

Luckily, the very biggest asteroids are few and far between.

But there are still plenty of rocks out there

that represent a significant danger to us.

So, at the summit of an extinct Hawaiian volcano,

Professor Nick Kaiser and his colleagues

are searching the skies for killer asteroids.

Each night, using a revolutionary billion-pixel sensor,

the team scans a vast swathe of the sky.

Follow me up to the next floor,

you'll see a better view

of the telescope itself.

They are looking for any unidentified objects

that could be heading our way.

By capturing several images of the same patch of sky,

separated by several minutes,

the team can see if anything's changed

against the background of stars.

You can see that there's a dark thing and a white thing.

What that means is, in these two exposures,

there was an asteroid, which was here in the first exposure

and there in the second one.

It's kind of cute, here's another one in the same image.

And, in fact, we'll detect hundreds of asteroids in a single exposure.

Their observations are collated

at the nerve centre of asteroid detection -

the Minor Planet Centre,

just outside Boston.

Its director is Tim Spahr.

And his job is to keep track of every asteroid in the solar system.

Tim has developed a map to visualise their location.

And, on that map, the most important are the Near-Earth Asteroids,

the ones closest to the planet.

On the screen here is a map of the solar system.

And I've got the sun in the centre

and the third planet out here would be that of the Earth.

The red dots in here are actually Near-Earth Asteroids,

the green ones are the regular Main-Belt Asteroids.

There are over 9,000 Near-Earth Asteroids.

But there's one type they're particularly concerned to locate...

..those asteroids that are over one kilometre in diameter.

An Earth impact with one of these would spell disaster.

Tim's data reveals

that there are 900 asteroids bigger than a kilometre

in those dangerous near-Earth orbits.

But he has some good news.

Right now, there's no information

that any of those large objects will hit the Earth in the next 100 years,

so we're safe from impact of those objects for at least 100 years.

So there are no catastrophic asteroid impacts on the horizon.

But there are still dangers out there.

On 6th October 2008,

asteroid hunter Richard Kowalski saw something that would change

the assessment of threats presented by asteroid impacts.

The night was proceeding normally

and up on the screen came another asteroid.

As I continued to make observations throughout the night,

it appeared to be moving slightly faster.

And this indicates that the object is close to the Earth.

As with any other asteroid,

Richard reported what he'd found to the Minor Planet Centre.

I got up in the morning about seven o'clock

and I had a message on the computer saying,

"Could not compute an orbit for a particular object."

I grabbed the observations of this object and I computed an orbit

and it was immediately apparent, right then,

that that object was going to hit the Earth.

And, sort of ominous fashion,

it said it was in 19 hours.

Following a strict written protocol,

Tim quickly reported the findings to NASA's asteroid investigation team,

in California.

We got a call from Tim Spahr, at the Minor Planet Centre,

saying we had an impacter coming in, in less than 24 hours.

That woke me up.

NASA's expert on asteroid orbits, Dr Steve Chesley,

raced to verify the data.

The first thing I saw was a 1.000,

a 100% probability of impact

in less than a day's time.

I'd never seen anything like this

outside of simulations and software testing.

An asteroid strike would create a huge explosion.

NASA feared this might even be mistaken for a nuclear bomb.

We wanted folks to know this was a natural event,

by Mother Nature rather than some sort of man-made

event like a missile or something dreadful.

Information passed rapidly up the chain of command.

NASA headquarters notified the White House that this was coming.

Everyone wanted to know where it would strike.

NASA predicted a remote area of the Nubian Desert.

At 2:45 in the morning, NASA were proved right.

The explosion created a vast fireball burning as hot as the sun.

It was so big and so hot this image was captured by a weather satellite.

And yet the object that caused it was only four metres across.

Smaller than the asteroid which exploded over Chelyabinsk.

I definitely think the impact was a wake-up call.

I have to admit I never thought I'd see that in my career,

where we would discover something and it would hit the Earth later that day.

What was worrying about that impact was that the asteroid was too

small to detect until it was very, very close to the Earth.

Of course, for Chebarkul, it wasn't even spotted until it was already here.

But we are getting better at spotting smaller asteroids.

On the same day that Chebarkul was hit, another asteroid,

similar in size to the object that created the Barringer Crater,

came within just 28,000 kilometres of the Earth.

Approaching from beneath the planet, asteroid 2012 DA14,

passed inside the orbit of our geostationary satellites

before heading off to the north.

This asteroid had been successfully tracked for a year.

Despite its proximity, scientists knew that it posed a threat.

So we know we are safe for at least 100 years from most near-Earth asteroids over a kilometre in size.

We are better at detecting objects down to 50 metres across,

like DA14.

But for asteroids smaller than that, like the one which exploded over

Chelyabinsk, we still have little or no warning.

There are still some we haven't found.

So there's this unknown bit of the equation where we are still looking

for some, we know they are there but we don't know where they are.

So this is a threat, but hopefully as technology moves on,

we'll always have a much better idea whether one's going to pose a risk to the Earth.

We could see an event tomorrow or in 10 or 20 years time,

that we hadn't previously detected.

That is always the risk we face.

Until we can catalogue and identify all the hazardous

objects in the solar system, that risk will always remain.

And there's one other factor that can make it particularly hard to spot an incoming object.

It's the reason why no-one saw the asteroid that was hurtling towards Chelyabinsk.

It came in in the daytime sky out of the sun.

Right.

We've got telescopes looking out there for these objects,

but they only work at night.

Radar doesn't help either, because to really use radar,

to find these objects, you have to know exactly where to look.

If you don't know what's coming in, you don't know where to look.

Because of that then, this thing and objects like this,

if they come in at that particular direction they're always going

to take us by surprise at the moment with our current survey system.

But even if we can spot an asteroid heading towards us

and in good time to prepare,

what if anything can we do?

There's different options for deflecting asteroids and it is a bit sci-fi at the moment.

The idea of shooting it out of

the sky with a nuclear weapon

would really be a dreadful idea.

It would just shower us with radioactive debris,

and it would just be... do more harm than good.

What would be much better would be to push it, nudge it

slightly off its course so that it wasn't then going to collide.

So how do you gently nudge an asteroid?

There's lots of different techniques to push it.

So... The one I love is called a mass driver.

There's a machine, which sits on the asteroid and throws off rocks,

so it is accelerating rocks that way

and that makes the asteroid gradually move in the opposite direction.

You can paint one side of the asteroid white.

That reflects the sun and there's this weird effect that makes the asteroid gradually drift across.

We can launch a mission now, which is essentially can impact an asteroid

and then deflect it - a bit like a billiard shot or a snooker shot.

We just hit the asteroid extremely fast with a spacecraft

and that small impact is sufficient to just alter its course

so that it misses the Earth.

When you consider Earth's history, stretching over billions of years,

it's clear that meteorite impacts, far from being unexpected,

are just a normal part of the life cycle of our planet.

But that is not how they seem to us.

The Chebarkul meteorite is a reminder of something

we would probably rather not think about too often -

how a sudden, apparently random event could have devastating consequences.

EXPLOSION AND SCREAMING

But this time we have been lucky.

Although it was terrifying for those who witnessed it,

this meteor struck without causing any fatalities.

And close enough to be captured on multiple cameras.

So it's given us a huge amount of information to help us

prepare for the next one.

I think perhaps the real lasting legacy of the Russian meteor

will be the effect it has had on the popular consciousness and perhaps on politicians.

Scientists have been saying for decades now that these things do happen from time to time,

that they could be dangerous if they happened over populated area.

But now we have actual proof, we have an event we can point to.

We know it could've been worse than this.

So, I think if this leads to more vigilance and perhaps,

the detection of future impacting events, that'll be a good outcome.

When a bit of an asteroid, comes through the atmosphere and lands on

the Earth as a meteorite, it reminds us that the solar system is a dynamic place.

It's... It's not finished.

It's still working. It's still evolving and still changing.

So next time you look up at the night sky, spare a thought

for those thousands of rocky lumps whizzing across our path.

A few of them have got our name on them,

but the thing is by analysing in detail the data from the meteor,

it means that next time, and there will be a next time,

we will be much better prepared.

Subtitles by Red Bee Media Ltd