Asteroids - The Good, the Bad and the Ugly Horizon

High above us, out in space, there are millions of very strange,

but very special chunks of rock tumbling between the planets.

Each one has a different story to tell.

And those stories are important to understanding the story of the solar system.

These are the asteroids...

debris from an extraordinary event...

..The birth of our solar system, 4.5 billion years ago.

Asteroids ARE fossils of the early solar system.

They were accumulated from some of the starting materials from which everything else was made.

But asteroids continue to present a threat to the very future of our planet.

If one of hits, every man woman and child on the planet could die.

Yet asteroids are about far more than destruction.

Around the world there are scientists working to uncover

what these messengers from the solar system tell us about our place in the universe.

They essentially created the solar system we live in and the planet that we live on.

And what they're finding is that while asteroids may not be beautiful,

they do hold a surprising power over life and death on our planet.

Are we alone in the universe?

Are there other unknown planets in the outer solar system?

Are the laws of nature the same everywhere?

Is the solar system stable?

High up above the clouds, Professor Dave Jewitt dares to challenge forces of the unknown.

I like mysteries.

I like to think about the things that are really not understood.

And if I can see some way to do something to address a problem,

that other people haven't really followed through with, then that's what I wanna do.

And right now, he's believes there is nothing more intriguing and mysterious

than the tumbling rocks of the solar system.

If somebody told me 30 years ago I'd be studying asteroids,

I would've said, "Yeah, you're nuts. All the hot science is elsewhere."

Why would I spend my time on an object that basically is not going to go anywhere in my lifetime?

So, how wrong can you be?

And Dave Jewitt is not alone.

Oh, Gosh. I just love asteroids. I suppose it makes me geeky, right?

I love the motion, I love the equations of motion. I love the way all that works.

We've seen very few of them up close,

but every time we see a new one, we learn something new, we find something we weren't expecting.

Asteroids are the debris left over from the solar nebula.

They contain the raw material that never quite made it to form a planet.

In a way, asteroids are fossils of the early solar system.

On the Earth, all the materials we see have been processed

by being sucked into the mantle and blown out of volcanoes,

and so there's no material on the Earth which remembers what it was like when the Earth still formed.

Asteroids are time capsules that contain information

about the earliest times in Solar System history,

information that's been lost from the other planets,

that's been lost from the Earth, lost from the Moon.

Asteroids have been around, and they've seen it all.

Asteroids offer tantalising clues into the earliest moments of our solar system.

But, for these scientists, the problem is, how do you get at them?

For the vast majority of asteroids, we have no information at all,

except the existence of the object and a guess as to how big it is.

This fuzzy image helps explain the problem.

This is what an asteroid looks like through the most powerful optical telescope on earth.

So how do you begin to study an object you can hardly see?

Well, one way is to study these.

Tiny fragments of asteroids that have fallen to earth and broken apart, called meteorites.

This is it's the oldest thing you can hold in your hand, really a piece of history back to the time,

even before the Earth was formed, way, way before we were ever formed.

Almost everything we know about what asteroids are actually made of comes studying these kind of fragments.

Each one has its own history of the solar system, they're like a puzzle that we're trying to understand.

We think some asteroids are made of iron, or at least they were so large

that, when they formed, they heated and melted.

They could get all the iron to their core,

just like the Earth has an iron core.

And when we take a big iron meteor, like this, and slice it open,

the quality of the metal is really quite amazing. It's a very pure metal,

nickel iron, some of the oldest metal in the solar system, in fact. More than 4. 5 billion years old.

So out there in space, there are gigantic boulders, ranging in size from 900 kilometres

to just a few metres and made of primordial metal and dust.

But the more scientists have examined the remains of asteroids, the stranger they get.

It is probably a complete zoo.

And we find the meteorites have a huge variety of types and compositions.

And it's telling us that the asteroids must have a wide variety of compositions as well.

There is one type in particular that has opened a door on the strange and unfamiliar world

that asteroids inhabit out there in the coldness of space.

We think that most asteroids are probably like this,

very stony, like the kind of things we'd find on Earth.

But they have a completely different chemistry than Earth rocks.

They're put together like little bits of rocks all reheated, re-melted and glued together,

And it tells us that the asteroid belt is a place with an incredible impact history,

asteroids colliding into each other, breaking apart, reforming.

And so when we see these meteorite samples,

it's telling us about that amazing collision history in the asteroid belt.

Over 90% of asteroids are found in an orbit between Jupiter and Mars, called the main belt.

Almost 200 million kilometres across, it is home to millions of these orbiting rocks.

But perhaps the most pressing question is whether any of them

are on a collision course with planet Earth.

Arizona's arid desert air

makes it the perfect place for a very special kind of job.

This is the where people come to hunt asteroids.

I started out hunting asteroids about 12 years ago, as an amateur.

I had read an article in a popular magazine,

and that got me really interested in the field because not too many people were working there.

It might not seem it, but Richard Kowalski is in the front line of defending planet Earth.

Every night when I come up to the telescope, I do have it in the back of my head

that every person on the planet does have a vested interest in what I'm doing.

If one hits, there's the potential that every man, woman and child on the planet could die.

Richard wants to discover any asteroids that could be on a collision course with Earth.

This is our largest telescope it's a 60-inch or 1.5m F2.

It's the telescope that we've been using for approximately five years now.

We discover as many as 3,000 new asteroids every night.

But what Richard fears

is that one of them could create destruction like this, or worse.

Baringer Crater is just a few hundred kilometres

from Richard's telescope.

It is 1 kilometre across and 200 metres deep.

It was made when a 300,000-tonne asteroid smashed into Earth, 50,000 years ago.

The Earth has been hit in the past and will be hit again in the future.

What we'd like to do is to be able to discover these objects before they hit the Earth.

So, one of the great challenges for scientists is to understand

what would happen if an asteroid were to strike planet Earth...now.

Pete Schultz wants to understand the unique nature of the explosion caused

if an asteroid were to impact with the Earth's surface.

It takes a truly odd piece of equipment.

OK. We're getting close.

This was serial number one, it was built during the Apollo time,

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

but this is the first one and the last one and it's 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.

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

to simulate an asteroid hitting the Earth.

So for this experiment, we are going to fire this tiny quarter-inch aluminium sphere at very high speeds,

up around 5km per second, and we'll see what kind of crater it produces.

The target it will hit is made of sand.

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

Outside of the impact chamber are special, super hi-speed cameras

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

capturing every detail of the impact and the aftermath for analysis.

OK, lights out. Everything good?

OK, we're out of here.

We have high voltage, warning lights.

And...rolling.

The ball travels 15 times faster than the speed of sound.

And it incinerates, exactly like some asteroids would.

Perfect. Perfect.

Now we're seeing the fire ball come in,

it's brighter than the sun and then, kapow, it hits the surface, geez.

This whole region down range 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 down range.

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 simply by the footprint of the crater alone.

It means they are far more complex, and dangerous,

than many had previously thought.

One of the enduring puzzles,

ever since asteroids were first discovered 200 hundred years ago,

is why they come anywhere close to the Earth in the first place.

Most of the time, asteroids

are in a stable orbit between Mars and Jupiter.

But some go wandering, leaving their orbit to propel themselves

through space, and coming under the influence of Jupiter's gravity...

a force that accelerates them towards Earth.

Scientists have been hunting for

an explanation for this strange behaviour.

Steve Chesley of NASA's Jet Propulsion Lab in California

has made a study of a 200 billion tonne asteroid called Golevka.

This is a model of Golevka, it's actually about

500 metres across, say the size of a football stadium.

Um, 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, what's called the Yarkovsky effect.

The Yarkovsky effect is a very small acceleration of the asteroid,

and what it is, is, if you take a model,

you see the sun is hitting the asteroid,

warming the surface. As the asteroid rotates,

that hot surface radiates the heat out

in a different direction into space

and that causes an acceleration, 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 effect upon orbit of the asteroid

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's one of the most powerful telescopes in the world.

And it uses radar to map 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 million kilometres out in space.

We knew that it would be in one place

if the Yarkovsky effect wasn't acting on it,

and 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 the Yarkovsky effect.

So these very precise radar observations, allowed us to see

the twelve kilometre displacement caused by the Yarkovsky effect.

So photons, those elementary, massless, particles of light,

really can create a tiny force.

The force is about

one ounce on Earth - say the weight of a shot glass

is - that's the force

on this huge asteroid the size of a football stadium.

Even for me, it's truly remarkable, it's dramatic, that a force

so slight can have such dramatic changes

on individual asteroids' orbits over millions of years.

Steve Chesley's research

means that, for as long as the sun is shining,

there will be a force that could send one of those asteroids

hurtling on a journey towards Earth.

Needless to say, this isn't good news.

But with a threat like this, what can you do?

Well, for now, there's really only one thing you can do...

and that's to keep an eye out for them.

Watching for asteroids is what Richard Kowalski does,

night after night, at his observatory in the Arizona desert.

What you can see on this screen is we have divided

the sky into thousands of areas,

we then choose a number of these areas into a single block

which will then tell the telescope

to observe each individual area in succession.

Once it's gotten to the last area,

it then goes back to the first area and repeats the process.

Over the course of an hour,

the telescope repeatedly scans the same areas of the sky.

While the stars appear stationary, the telescope can spot

any other objects that change position...

which could be asteroids.

As you can see, on this screen is the sequence of four images

that came from the telescope.

These objects around the screen are not moving

so we know that they are stars,

but this object in the centre is moving and thus we know

that is an asteroid.

The importance of surveying for near Earth asteroids

is asteroid impact on the Earth is truly the only natural disaster

that we can actually predict before it happens.

So whenever Richard finds an asteroid he thinks could be

on a collision course with Earth, he immediately files a report.

It goes to the central body whose job is to monitor

possible asteroid impacts.

Just outside Boston, is the home of the Minor Planet Centre.

It's director is Tim Spahr

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

This is the nerve centre

of the entire asteroid field.

If somebody discovers something, it has to come through here

and our job is to then distribute that to the rest of the world.

Asteroids' elusiveness is part of the thrill.

In some cases,

if you're studying an asteroid

that's moving extremely fast, er, we ambush it.

We go ahead of where we think it will be in and set the telescope up

in that area and then hope that it comes through the field like that.

So, you know, it's actually ambushing.

And then when you get the asteroid, then you chase it down and follow it.

Not surprisingly, keeping track of thousands of objects in the sky

isn't something that you can do in your head.

Thankfully, help is at hand.

This is really the brains of the Minor Planet Centre right in here.

This computer system has information about where asteroids are,

where they will be in the future.

All the observations all the software is in here.

We definitely need it to be running all the time, we need it to be safe.

We need everything working here.

-<

-Do you feel a sense of responsibility?

I definitely feel a sense of responsibility

for keeping a track of the asteroids. I feel like it's our duty,

it's our task to do that and I do feel personally responsible for it.

And the task facing Tim, is growing rapidly.

In 1999, only 10,000 asteroids were known of.

Since then, hundreds of thousands more,

of all shapes and sizes, have been discovered.

Tim has developed a map to visualise their location.

And on that map, there's one class of asteroid

he's concerned with above all -

those near-Earth asteroids closest to our planet.

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

The Sun in the centre, and the third planet up there would the Earth.

The red dots in here are actually near-Earth asteroids,

the green ones are the regular main belt asteroids.

There are over 7,000 near-Earth asteroids,

but there's one type they are particularly concerned to locate.

Those asteroids that are over one kilometre in diameter.

These are the monsters of the skies.

An Earth impact with one of these

would spell catastrophe for the planet.

If a one-kilometre diameter asteroid were to hit, say, New York City,

that would very likely affect people in you know, 100 miles away.

It might kill people 100 miles away. So you're talking, really,

a catastrophe, instantaneously, as soon as it hits.

Tim's data reveals

that there are 900 asteroids bigger than a kilometre

in those dangerous near-Earth orbits.

But the big question -

are any of them on a collision course with Earth?

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 impacts of those objects for at least 100 years.

But there are still smaller asteroids

than one kilometre that we have not yet discovered.

So I can't say we're safe from them

because we don't know where they are, just yet.

So, for now, we are safe from a catastrophic asteroid impact.

Even if the thousands of smaller asteroids might still pose a threat.

However another group of scientists have a very different mystery

about asteroids to investigate.

One that may help solve one of the greatest quandaries

about life on Earth.

This is Maunu Kea

in Hawaii. It is home

to some of the most powerful telescopes in the world.

For 30 years, Professor Dave Jewitt

has used them to probe deep into the solar system.

And once Dave interrogates deep space,

it's rarely ever the same again.

In 1992,

I discovered the first objects found beyond Neptune

since Pluto.

The biggest discovery in the solar system

since the discovery of the asteroids.

It was a discovery that led to Pluto losing its status as a planet,

something the world had taken for granted for over 60 years.

It established Dave's reputation as a pioneering astronomer.

It's important not to work on things that other people are working on.

All you'll do is get the same result as everybody else. You won't make

any discoveries, you'll just confirm what is already known.

Dave's desire to journey where others fear to tread

has led him to this.

This bright dot with a long hazy tail is called Elst-Pizarro.

It was found alongside all the other asteroids in the asteroid belt.

The problem was, it just didn't look like an asteroid.

So why did it seem so out of place?

it didn't look like the other asteroids so it was a freak.

And it got a lot of attention straightaway,

because it was such a remarkable object.

Nobody had seen anything like that before.

What had got them excited was that to astronomers,

asteroids normally look like this.

Just a point of light.

No dust cloud, and definitely no tail.

For years Elst-Pizarro, with its orbit of an asteroid

but strange fuzzy appearance, left scientists baffled.

Until finally, someone suggested an explanation.

Finally, a paper came out saying

it must be due to the collision between two asteroids.

So two asteroids slammed into each other

with high speed, and shattered

and produced a cloud of dust.

So the strange tail was thought to be the debris from a collision

between Elst-Pizarro and another asteroid.

And, very quickly,

most of the scientific world forgot about Elst-Pizarro.

But Dave didn't.

He had a hunch that there was more to this puzzling little light in the sky than at first appeared.

A hunch that, if proved correct, might help solve one of the great mysteries of life here on Earth.

Dave decided to investigate, and began looking for someone to work with.

Somebody with a head for the challenge.

When Dave suggested that I look at this object, I didn't actually know anything about it.

Nothing had really been said about it in the last 6 years since it's been discovered,

so I just decided, OK, it's just an interesting thing to take a look at.

Dave and Henry knew that if Elst-Pizarro's fuzzy tail really had been caused by a collision

the debris should have dispersed by now,

and Elst-Pizarro should look like a normal asteroid again.

But when they looked again, what they saw was that the tail was still there.

It was strong evidence the collision theory was wrong.

Collisions are very, very rare.

Either Elst-Pizarro is the unluckiest asteroid in the solar system, that keeps getting whacked

and producing dust in that way, which doesn't make any sense,

or there's another mechanism for producing the dust.

Elst-Pizarro's appearance remained an anomaly.

Dave and Henry realised that if they were going to make any real sense of it

they needed to find another example of an asteroid behaving in the same strange way.

Dave and Henry's problem was that, in the 200 years since asteroids were discovered,

Elst-Pizarro was the only one like it.

Finding another one could be a complete wild goose chase.

Using the giant telescopes on Mauna Kea, Dave and Henry began to hunt through the asteroid belt.

For four years, they scanned the skies.

They studied 300 more asteroids.

All of them looked identical...

..except for one.

When we saw these images, I didn't know what to think actually.

Maybe this is what we've been looking for all this time.

But we were maybe just a bit nervous. You know, we may be on the... the cusp of something big.

What they'd seen was an asteroid

sporting a tiny, faint fan-shaped tail.

Just like with Elst-Pizarro, they were convinced it was impossible

this tail was created by a collision between asteroids.

They had another explanation that to many seemed unthinkable.

This is an image of a comet.

They are objects that are thought to have been born

in the freezing outer reaches of the solar system.

They have long, elliptical orbits

that bring them towards the sun and the Earth.

And in comets the tail is a sign of something very special inside the centre.

Ice.

Their appearance is due to the vaporisation of the ice, that blows material off to make a tail.

So they have this distinctive appearance, basically of having a long tail of dust.

For 200 years the asteroid belt was thought to be

an orbiting collection of dry lumps of rock and metal.

Dave and Henry's new idea was that those asteroids they had observed

might look fuzzy and have tails because they too actually had ice inside them.

It was a radical suggestion, because scientists had always thought

asteroid orbits were far too close to the sun for them to be icy.

People were uncomfortable,

because of this prevailing idea that the asteroids are rocky,

and the comets are icy, and there should be nothing in-between.

The reason why ice in the asteroids mattered so much

is that it could help explain something that makes our planet unique in the solar system.

Our beautiful blue planet is the only one to have an abundant supply of liquid water.

Around 70% of the Earth's surface is covered by the oceans.

But there has always been a mystery as to where all this water actually came from.

For a decade, Dave Jewitt has been investigating this problem,

because scientists have established that when Earth formed, over 4.5 billion years ago,

it used to be a very different kind of place.

The early Earth was really hot. It formed from hot material in orbit around the sun.

So hot that we think the entire surface of the Earth

was covered by liquid lava for the first 100 million years,

a bit like the land that we see behind us.

Dave believes the searing heat of molten rock

would have had a profound effect on the Earth's early climate.

Because it was so hot, we also think the early Earth was very dry.

It's like putting something in the oven and baking it for too long.

It comes out bone dry. We think the Earth was bone dry when it formed.

That would means that the lush, wet climate that we enjoy today

must be the result of some dramatic events long after the Earth was born.

The Earth got its water some time after it had formed and cooled down,

by being hit by objects that carried water from somewhere else in the solar system.

If Dave and Henry were right, a constant stream of icy asteroids hitting the early Earth

could have played a vital role in bringing our planet its water.

But for all their observations, they hadn't actually seen ice on an asteroid.

So the one problem with our observations is that they only told us what the object looked like

and with that information we knew... we thought we could only explain it with the presence of ice

but we couldn't actually prove that that was the case.

The last piece of the jigsaw finally arrived early this year,

with help from the mighty telescopes of Maunu Kea.

Andy Rivkin makes the invisible visible,

by using a NASA telescope to look at objects using infrared light.

The infrared part of the spectrum is useful

because it contains information about the composition of asteroids and other objects,

and so by observing there you get a better handle on the composition

than you would if you observed only in the visible.

Andy studies the shape of the infrared spectrum reflected off the surface of asteroids,

because tiny differences in the peaks and troughs can reveal what the surface is made of.

Andy became interested in an asteroid called 24 Themis.

The shape of its spectrum

meant something very odd must be happening at its surface.

We started by comparing it to other materials and objects that we thought might be similar.

We tried comparing it to other asteroids, but it didn't look like any of the other asteroids.

We tried comparing it to meteorites, and it didn't look like any other meteorites.

So we knew we had to come up with some other explanation.

Finally, in April this year, Andy and his team published their explanation

as to why 24 Themis gives off such a strange kind of light.

We found that water ice was actually the best choice, and that was really exciting, because

it was the first time, certainly that we knew of, that anyone had found water ice out in the asteroid belt.

Even though it had been suspected for some time

that it could be out there, no-one had ever seen it.

Andy had finally proved an asteroid really could be icy.

It now seems certain the strange behaviour and tails

seen by Dave and Henry on their asteroids was caused by ice too.

DAVE: I think any time you make a discovery it's exciting.

Any time you find a new thing, it's a big thrill.

Definitely a big thrill, yeah.

Cos it's hard.

It means that asteroids could have played

one of the most important roles in creating the Earth we see today.

We know that asteroids did hit the Earth, for billions of years.

The question is what the asteroids brought with them.

We previously thought mostly rock and metal.

Now we understand that the asteroids would also have brought

a lot more water and ice than we'd previously suspected.

These discoveries are starting to change our understanding of the solar system.

Water and ice really are abundant in the asteroid belt.

And that maybe water and ice

is more abundant throughout the entire inner solar system.

Finding the water in the asteroid belt is the key to starting to change

our thinking about where Earth's water may have come from.

Astronomers still don't know how much of Earth's water came from asteroids

and how much from other sources of ice such as comets.

Without that water, of course, life on Earth could not exist.

Which provokes what is perhaps the most intriguing question of all.

Did asteroids play a role in the creation of life?

Not far from San Francisco, California,

there are scientists pondering this very question.

Scott Sandford wants to investigate whether the basic chemicals of life could have been formed in space,

perhaps even on an asteroid.

So he's created the conditions of deep space in a machine.

This machine has been developed to allow us to simulate environmentsthat are out in space,

either in the interstellar medium, events where stars form,

or the environments, let's say, in the icy satellites of planets in the outer solar system.

Environments that have low temperatures, no air, so vacuum, and high radiation fields.

He wants to see if the complex carbon molecules that are essential to life

could be created from the much simpler chemicals found in space.

In this chamber is a sample probe covered in a tiny layer of water, methanol and pyrimidine

that is frozen to just 20 degrees above absolute zero, and exposed to intense ultraviolet light.

In this particular experiment we're looking at whether certain conditions will form one of the nucleobases,

so one of the molecules that makes up our DNA.

And from his analysis of samples from experiments like this,

Scott has made a remarkable discovery.

By processing ices of the type we see out in space, we can make

some of the building blocks that we see in biology on the Earth today.

We're making the building blocks of life, that's what we're finding.

Just because you can create these building blocks of life in a lab,

it doesn't mean it really happens on an asteroid.

So Scott has carefully examined meteorite samples to see if they contain traces of these chemicals.

In some classes of meteorites, which we think

come from asteroids, we find a variety of organic compounds.

And these include things that some people are familiar with,

like amino acids, the building blocks of proteins in our bodies,

but also materials like the nucleobases, the building blocks of DNA.

So hidden within the rock could have been the materials

that made possible the emergence of life on Earth.

And that means that when asteroids struck Earth billions of years ago

they could have completely transformed our planet.

Asteroids could have played an important role in getting life started on Earth

by delivering the raw starting materials that we need to get everything going to get life started.

It seems the story of life on Earth is inextricably linked to the story...

of asteroids.

The possibility that asteroids hold the key to some of the deepest mysteries about our planet explains

why scientists have always dreamt of reaching out into space and bringing back a pristine asteroid sample.

And earlier this year, that wish may finally have come true.

In June, one of the strangest space missions in history came to an end.

It might look like a firework display, but this

is actually a Japanese spacecraft re-entering the Earth's atmosphere.

Seven years after it first left the Earth, the Hayabusa probe landed in the Australian desert.

The scientific team were careful to handle the crashed probe with extreme caution.

Because within this small container is what scientists hope will be the

first ever asteroid sample collected directly from space.

The sample consists a lot of little grains, and some of the grains are as small as ten microns,

so ten millionths of a metre across, so this is a particle smaller than the width of a human hair.

Even in a microscope it looks like a dot, OK, and so, um,

the analyses of such small samples is obviously complicated.

Obviously our hope is that some of that material really is from

the asteroid, but at this point we don't know for sure one way or the other.

It may be months or even years, before the team discovers what if anything these grains can reveal.

While many scientists are excited about what

asteroids might tell us about the beginnings of life on Earth,

new research suggests that it is how asteroids might put an end to life that should really concern us.

On the 6th of October 2008

asteroid hunter Richard Kowalski saw something that would

help change the assessment of the threat 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 Center.

I got up in the morning, about 7 o'clock.

I had a message from 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 Center saying we had an

impacter coming in, in less than 24 hours.

So that woke me up.

NASA's expert on asteroid orbits, Steve Chesley, immediately started to verify the data.

Steve The first thing I saw was a 1.000,

100% probability of impact and erm, in less than a days' time.

This 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 be mistaken for a nuclear bomb.

We wanted folks to know that this was a natural event by mother nature

rather than some sort of a man-made event like a missile or something dreadful.

Information passed rapidly up the chain of command.

So, NASA headquarters notified the Whitehouse

that this was coming.

Everyone wanted to know where it would strike

NASA predicted a remote area of the Nubian desert.

AT quarter to three 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.

As dawn broke, the smoke trail it left behind was still visible from the ground.

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 that would hit the Earth later that day.

What makes this impact so worrying is that this asteroid was too small

for anyone to see until it was very, very close to the Earth

For one scientist, it's was a salutary reminder that

we cannot afford to ignore the threat posed by small asteroids.

Physicist Mark Boslough uses one of the world's most powerful

supercomputers to study the hazards facing our planet, from climate change to nuclear explosions.

But for years, he's been fascinated by a strange event at the beginning of the last century,

and what it might tell us about the threat of asteroid impacts.

On June 30th 1908, without warning,

a massive explosion wiped out

over 1,500 square kilometres of Siberian forest.

Millions of trees were destroyed.

Scientists thought it had been caused by an asteroid 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 an asteroid attack of a very particular kind.

The explosion was caused by an asteroid that entered the atmosphere,

got close to the surface and exploded before it hit the ground.

That explosion created a blast wave with hurricane-force winds

that knocked trees over for thousands of square miles.

Scientists call it an air burst - a massive explosion in the atmosphere rather than on the ground.

As it enters the atmosphere at speeds of up to 20 kilometres per second

the air resistance decelerates the

asteroid so fast it breaks apart in a huge explosion.

And crucially, it is small asteroids that are most likely to explode in this way.

Most of the damage from an explosion like this is actually the blast waves, it's the very high winds.

Based on the physics of nuclear explosions,

the original air burst model estimates the Tunguska explosion

must have been 1,000 times bigger than the nuclear bombs at Hiroshima and Nagasaki.

But crucially, the air burst model suggests the

asteroid would have packed this huge destructive force

even though it was as small as 100 metres in diameter.

But Mark realised there was another problem.

The model was ignoring a crucial difference between nuclear bomb air bursts, and asteroids.

Asteroids are extremely heavy and move so fast that they carry huge momentum

He created a new simulation to investigate the effect this would have on their destructive power.

In this simulation I include more of the physics to be more realistic, you can see that the main shockwave

doesn't come out of the point of the explosion, but it comes out from the point where the fireball descends to.

so by the time the shockwave hit the ground it's much stronger

than it would otherwise be so there is more damage on the ground,

because the destructive power was carried downward.

Based on Mark's new calculations, the devastation at Tunguska could have been caused

by an asteroid only one third as large as previous estimates.

Perhaps as small as 30-50 metres in diameter.

And for him 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.

Mark's work means scientists may have to redraw the asteroid threat map.

If a Tunguska scale asteroid exploded over London or New York it

would be very destructive, it would be as destructive as a nuclear bomb exploding over one of those cities.

Scientists estimate that there could be over a million of these kinds of asteroids up in space.

But nobody knows where they are, or where they are headed.

A 2010 report by the American National Academies of Sciences,

was so concerned about the potential threat

to Earth from the smallest kind of asteroids, that it has called for a new survey to track them down.

The problems is that even for dedicated asteroid hunters like

Richard Kowalski, they are extremely hard to find.

Many of them are as dark as a charcoal briquette and we see them by reflected sunlight.

So you can imagine a 100-metre charcoal briquette out in space is going to be kind of hard to see.

And that means that if an asteroid like this is heading for Earth,

we might only see it when it is very close, with very little warning.

There's a reasonable chance that you'll see it for the first time, on

it's terminal trajectory, just days, or weeks, before the impact.

If that were to happen, there is nothing that anyone could to stop it from hitting the Earth.

For the public authorities, the only option, would be to try to get

the thousands or even millions of people out of the impact zone.

I think Katrina, Hurricane Katrina,

really illustrated how hard it is to evacuate a large area.

It's not set up that we have an asteroid evacuation plan

in place right now.

I know it's been discussed at the UN, but if we were to be

issued 3 days' warning, I really don't know what would happen.

And it wouldn't be very good.

I'm sure we're not ready for that yet.

However far away they may be,

and however difficult to find,

scientists now understand that Earth's past

and its future cannot be separated from these tiny rocks of destiny.

The quest to understand them will continue on Earth and from space.

NASA currently has a spacecraft en route to visit two of the largest asteroids

in the solar system.

It will arrive in July 2011.

And President Obama has challenged NASA to send astronauts to an asteroid by 2025.

200 years after they were first discovered,

solar system science has finally entered the age of the asteroids.

The good, the bad and the ugly.

Are asteroids bad, good or ugly?

I'd say all of the above.

I wouldn't say that asteroids are good or bad.

They essentially created the solar system we live in

and the planet that we live on.

They've shaped the Earth in ways, it's safe to say, humans wouldn't be around

but for the asteroid impact.

I take asteroids like people - I take them as I find them

and try to learn their individual foibles.

I think ultimately asteroids will be our friends

because they have the capability of giving us resources for use as we try to explore

and extend our reach into space.

Asteroids are certainly not ugly. Asteroids are beautiful.

The ones that we don't understand, I think that makes them more beautiful.

That's the beauty of science. That's why we keep doing it.

It's to try to learn more things and when we get a curve ball thrown in,

that's part of the process. That's the fun.

I have an asteroid named after myself. It's (2956) Yeomans.

That's quite a hoot.

I do have an asteroid and this is it.

(17857) Hsieh 1998KR1

I have an asteroid. It's called (6434) Jewitt. It makes me feel like I'm part of the cosmos.

I have an asteroid named after me. Pete Schulz. It looks like I've got a bullet with my name on it.

HE LAUGHS

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