The Comet's Tale

On the edge of space,

halfway to the nearest star, there is a vast cloud of debris,

lumps of rock and ice

that have drifted on the edge of our solar system for four billion years.

They are among the most mysterious objects in the universe.

Sometimes they are sent into the heart of our solar system,

where they are transformed into the blazing stars we call comets.

For centuries, our ancestors were in awe of comets.

They were messengers from the gods...

..carrying the power of life and death.

It's only now, as we have the power to uncover the comet's secrets,

that we are discovering that those ancestors were right all along.

The comet's tale is a story

that really CAN tell us about life, the universe and everything.

About once every ten years, a really bright comet lights up the skies.

Some are bright enough to be seen in broad daylight.

Others unfurl their tails across half the sky.

They appear from nowhere, and just as suddenly disappear.

It's little wonder that throughout history

people have tried to explain their significance

and the effects they have on the Earth.

As far back as the second century BC,

the Chinese had taken the trouble to classify comets

into 29 separate types.

The shapes they saw were so striking

they have penetrated human consciousness.

Many other societies explained the sudden appearance of comets

through their mythology.

Comets marked the presence of gods in the skies.

Comets are recorded in myths globally.

Almost every population in the world talks about dragons...

..things that fall from the sky, stones and iron falling from the heavens...

..giants with a single eye in their head...

..wizards and things like that, that can be interpreted as comets.

If you look hard enough,

comets can even be found in the most famous folk stories.

When you peel away all of the layers from the King Arthur story,

what you're left with is a local lord

who was left in Britain after the Romans had retreated.

And that doesn't explain why or how

he became such a huge figure in English folklore.

There's another possibility, and this is that Arthur may have been a comet.

When Arthur has a battle, he takes his sword out,

and that sword shines with the light of 30 torches.

And, generally, Arthur's battles occur

at times of known meteor showers.

And in those battles, there's widespread destruction.

In one of the battles, 11 countries get destroyed,

and that's characteristic of what you might expect from a comet,

and it's difficult to explain in human terms.

Was King Arthur really a comet?

It's an idea that requires quite a stretch of the imagination.

But comets were often depicted as fiery swords.

Perhaps the myths were created to help people

interpret events in the sky that they couldn't otherwise explain.

Whether you believe all the interpretations or not,

there is no doubt that comets were the subject of deep superstition.

The reason was simple.

In times when the movement of the stars and planets

was used to predict the future,

the sudden appearance of something unusual was a dangerous omen.

In the medieval world,

philosophers and astronomers thought that everything further away from us

than the moon is was perfect.

Out there, beyond the moon, nothing ever changed.

And comets just don't look like that.

Comets look like fiery signs moving across our sky.

They were threats to the idea

of a perfect, calm, orderly universe.

Perhaps they were guided by God,

in order to send signs to humanity of God's purposes.

When comets passed,

astrologers almost automatically predicted great transformation,

the death of kings, war, invasion, plague or famine.

Not everyone viewed them with foreboding.

Genghis Khan saw comets as a personal message

telling him to fulfil his destiny and wage war across Asia and Europe.

To William the Conqueror, the appearance of a comet in 1066

was such an auspicious sign, it was immortalised on the Bayeux Tapestry.

But for the Saxons, the comet was definitely a harbinger of doom.

For centuries, all explanations of comets

relied on superstition and astrology.

It finally took the greatest mind of them all

to make scientific sense of them.

In 1680, a particularly bright comet

caught the attention of Isaac Newton.

It was a fascination that was to change the way we understand the universe.

"On November 19th, at half past four in the morning in Cambridge,

"the comet was seen by some young man.

"And on the same day, at five in the morning,

"at Boston in New England,

"the comet was also observed."

What was extraordinary about this comet was that

it was visible in the sky, it was very bright,

you could see it and October and November of 1680,

and then it disappeared behind the sun.

And then another comet appeared from behind the sun,

in December, at the end of the year,

and was visible right through to March of 1681.

So the big question is, is this one comet or two?

If it's one comet, then it's bent a lot near the sun.

Here's a diagram that Newton himself made

of the path of the comet, if it's one comet.

You see here that it was approaching the orbit of the Earth and the sun

in October and November of 1680,

and then it disappears behind the sun, and reappeared in December

and it stayed visible through February and March of 1681.

For the sun to bend the path of a comet this much,

Newton realised that there had to be an unseen force at work.

He called it gravity...

..an idea which didn't come from an apple falling from a tree,

but from a comet passing behind the sun.

It was by thinking about this puzzle,

that Newton began to formulate the idea of gravitation.

Before 1681, Newton had no notion of universal gravitation.

Newton, between 1681 and 1684, began to suppose that comets come back,

that they're therefore like planets, because they move in closed orbits,

that you should be able to calculate the shape of the orbit,

that you could, in fact, predict when comets come back.

And that is exactly what he and his friend Edmund Halley set out to do.

If they could use the theory of gravitation

to predict the return of a comet,

they would not only prove Newton's theory,

but also show that comets were not omens or supernatural signs,

but predictable, orbiting bodies like the planets.

Halley collated tables of all the comet observations he could find,

looking for similarities in their behaviour -

what direction they came from,

how close they got to the sun.

At last he found three comets

whose descriptions were almost identical,

one recorded in 1531, one in 1607,

and one he had observed himself in 1682.

Each appearance was separated by 75 years.

Convinced that these three sightings were of the same comet,

Halley made a public prediction that it would return again

in late 1758 or early 1759.

It was an extraordinary piece of scientific bravado for the time.

Neither Newton or Halley lived to see the prediction fulfilled.

But when, on Boxing Day 1758,

a comet was spotted in Germany, it was greeted with jubilation

and as a triumph of science over the supernatural.

Tracing back through the historical records,

Comet Halley could be found reappearing like clockwork.

This Babylonian tablet records its appearance in 164 BC.

The comets that Genghis Khan and William the Conqueror saw

weren't messengers from God, but the regular appearances of Comet Halley.

'A once-in-a-lifetime meeting tonight, live on BBC1.

'After its latest visit to Earth, Halley's Comet is already on course

'back towards the icy wastes of outer space. In an attempt...'

The last time it visited in 1986, it was greeted like an old friend.

Around the world, people were transfixed, as the probe Giotto

beamed back the first live pictures of a comet nucleus.

It was the last we'll see of Comet Halley until 2061.

Perhaps the most surprising thing about comets

is that, 300 years after Newton and Halley,

despite all that we now know about them,

comets still have the power

to capture and control people's imaginations.

In 1997, a bright comet called Hale-Bopp

suddenly appeared in the skies.

It captivated sky-watchers around the world.

It was particularly keenly watched

by a cult in California known as Heaven's Gate.

They believed that the comet was concealing a spaceship

that had come to take them to another, better world.

At this point, this is considered a mass suicide investigation.

So deep was their conviction,

that, as the comet came closest to the Earth,

all 39 members poisoned themselves,

believing that in taking their own lives,

they would free their souls to board the spaceship.

It seems no matter how far science progresses,

comets can still hold the power of life and death.

What is true, as we are now discovering scientifically,

is that comets have had a profound effect on life on Earth.

One of the reasons they retain so much power over us

is that they seem to appear out of nowhere.

On the island of La Palma in the Canaries,

Alan Fitzsimmons is trying to unlock the comet's secrets.

Here we have the William Herschel Telescope,

4.2 metres of astronomical loveliness,

all ready and waiting to spot a few of these comets.

It may seem odd to use such a powerful telescope to observe comets,

the brightest objects in the night sky,

but the scientists want to study the comet's nucleus.

To do that they must see them before they start producing their tails,

when they are further away than Jupiter.

The nucleus is tiny, just a few kilometres across,

and in the vastness of space, they are incredibly difficult to spot.

They only reflect, on average, 4% of the light that hits them,

and a piece of coal reflects 8% of the light that hits it, so...

a comet nucleus is twice as dark as a lump of coal.

So we're trying to see these things from the feeble amount of sunlight

they reflect out beyond Jupiter. It's not an easy thing to try and attempt to do.

Oh, my God!

That's it. That's it! HE LAUGHS

-Oh, fantastic stuff!

-Another one.

Yep, yep, another one in the bag! Happy days.

I do love this job, you know.

This tiny moving blob of light is the comet's nucleus,

a delicate lump of rock and ice floating slowly through space.

The most powerful telescopes pick them up

as they drift through the outer planets.

But their orbits suggest they come from much further into space,

far beyond the realm of the planets.

Finding exactly where they come from hasn't been easy.

Since the 1940s, astronomers have been looking

for a huge reservoir of comets called the Kuiper Belt,

which they believed lay beyond Neptune.

The only problem was, no-one could find it.

No matter how hard they looked,

the only object anyone could see out there

was the ex-planet Pluto.

By the end of the 1980s,

almost everyone had given up looking,

apart from two maverick astronomers on Hawaii.

I don't intentionally do things that are different,

but I do...

try to be sceptical of everything,

because I think that's essential for science.

So if you just buy into the prevailing wisdom,

then all you can ever do is confirm what's already known.

In 1987, Dave Jewitt and Jane Luu

set up at the Mauna Kea observatory in Hawaii

to stare into empty space...

using up precious telescope time, finding nothing at all.

At first, I couldn't get money to support any of this,

because it seemed, I guess, so speculative or so crazy,

and so instead I used money from other sources, probably illegally.

Everyone, even their colleagues, thought they were mad.

They started asking, "When are you going to stop?

"You've been doing it for years and still haven't found anything.

"Maybe there isn't anything out there."

Eventually, their bloody-minded persistence paid off.

After five years and over 400 hours observing,

there was a small, moving anomaly on one of their pictures.

He said, "Jane, come take a look at this,"

and we saw something and it was moving at about the right speed

we would expect a distant solar-system object would move.

We were pretty excited about that,

because we hadn't seen any other candidate object so good.

It was just the first positive sign that we'd got after five years.

This was it. At last they'd found the first Kuiper Belt object.

They named it, catchily, "1992 QB1".

Since then, over 1,000 Kuiper Belt objects have been found,

quietly orbiting in the dark reaches beyond Neptune.

But there are thought to be up to six billion potential comets here,

orbiting in a belt three billion kilometres wide.

Occasionally they collide...

..and one is sent tumbling into the inner solar system,

to become what we call a "short-period comet".

Short-period comets orbit the sun frequently,

and their orbital paths bring them back around the sun

perhaps once every 10 or 20 years or so.

And these are the objects that we believe

have come from this trans-Neptunian region,

just beyond the realm of the giant planets.

Comet Halley is the most famous of the short-period comets.

But there is another sort of comet,

one whose orbit can be thousands, or even millions of years long.

They come from far beyond the Kuiper Belt,

from an almost mythical place called the Oort Cloud.

The long-period comets, as the name suggests, take a really long time

to go round the sun, perhaps anywhere between 200 years

and a million years to go around once.

And these are the objects that tell us that the Oort Cloud exists,

stretching out tremendous distances towards the nearest stars.

This is the very edge of the solar system,

halfway to the nearest star,

50,000 times further from the sun than the Earth.

Here, trillions of comets cling tenuously to the sun's gravity.

We now think that the Oort Cloud was formed

during the violent formation of the solar system.

As the planets formed and their gravity increased,

they catapulted chunks of debris into space.

Some of it collected in the Oort Cloud,

where it has been ever since.

They've been out there in deep freeze, in deep space,

and so, by studying comets,

we're studying the formation and the evolution of our solar system.

But studying the comets isn't easy.

The Oort Cloud is so far away,

and the objects in it so tiny, that we'll never be able to see it.

But luckily for the astronomers,

sometimes something happens that sends them tumbling towards us.

All cloud objects become comets,

not through the action of anything to do with our sun,

but through the action of other stars, generally.

Our sun is just one of 100,000 million stars

that form our Milky Way galaxy,

the Milky Way that you can see on a clear, dark, moonless night.

And they're all moving in slightly different directions

with slightly different velocities, and occasionally,

a star will come close enough to our Oort Cloud

that it will perturb some of those comets out there.

Some of them will be thrown into interstellar space, never to return.

But some fraction of them will be nudged inwards into our solar system.

Until recently, it was assumed that our solar system

was isolated from the rest of space

by the vast distances surrounding it.

It's because we see comets that we now know

what happens around the Earth

is driven by the movement of distant stars.

This is one of the great discoveries of the last 20 years.

Astronomers as a whole

have increasingly begun to recognise that the solar system,

not just the Earth, is open to its galactic environment.

That's a very important change of mindset

than our ancestors, even our parents probably had.

Comets coming from the Oort Cloud are bringing with them

a message from outer space, quite literally.

Once pushed out from the Oort Cloud,

these long-period comets start the two-light-year journey

towards the sun.

They drift through space, silently and unseen,

for tens, even hundreds of thousands of years.

But as they pass Jupiter, just 400 million miles away,

they light up, and start to produce their tails.

All of a sudden, we can see them.

And we can also reach them.

A number of missions have been sent

to investigate comets at close quarters.

The probe Deep Impact was sent

to rendezvous with the comet Tempel 1,

testing its strength and density by crashing into it.

The Stardust mission flew past the Comet Wild 2...

capturing grains of dust from its tail

that it brought back to Earth.

CONFIRMATION ON RADIO

APPLAUSE

'All stations, the main chute is open, we're coming down slowly.'

These bits of dust have been unchanged

since the formation of the solar system,

and can tell us exactly what comets are made of.

Perhaps surprisingly, the basic building blocks of comets

aren't at all exotic. In fact, they're remarkably mundane.

I'm going to show you a really neat demonstration

of how to make a cometary nucleus

using some relatively simple things you can find in the average house.

To start with, I'll add some water.

The second major constituent of a comet is the dark organic materials,

the rubble and the crud and so on. To replicate that, I've got some soil.

We also need to think about other ingredients that are in there.

There are quite complex chemicals, organic chemicals.

We can replicate that by putting in this soy sauce.

No-one's suggesting there's actually soy sauce in a comet,

but again, it's really nice analogue for it.

So I'll give it a good glug, more than you'd use in an average dish.

We can add a little ammonia, in this case, in these smelling salts.

Just bash a few crystals in. What it really does, more than anything,

is just make the comet a little smellier.

There we go.

Now, the other ingredient we're going add into this comet

is carbon dioxide, or what you more commonly know as dry ice.

You get a fairly violent reaction,

because the dry ice is meeting water,

which is about 80 or probably 100 degrees warmer.

You can see the carbon dioxide turning to gas.

Just give that a good squeeze and see how we're doing.

That's really bubbling away violently now.

And that looks good.

This dirty snowball

is a surprisingly accurate model of a comet nucleus.

And as it warms up, it starts to behave exactly as real comet does.

It produces a tail.

You can see a beautiful - one of the best I've seen doing this -

a beautiful jet coming out here, and that's exactly what happens

when a cometary nucleus gets near the sun.

Near the surface particularly, you see ice turning to gas,

because there's no pressure in space to allow it to exist as a liquid.

And so the frozen carbon dioxide, the frozen water,

the other chemicals in a comet become gas-like and they jet out.

In the vacuum of space, the escaping gas blows dust from the surface,

and the debris builds up into a vast halo, called a coma.

A single comet nucleus can produce a coma

over a million kilometres across, bigger than the sun.

It would expand for ever,

if it wasn't for the pressure waves emitted from the sun

that shape the coma into the comet's dramatic twin tails,

one made of dust, and one made of gas.

Which is why, no matter what direction the comet is travelling,

the tails always point away from the sun.

The longest tails can be half a billion kilometres long,

and stretch halfway across the solar system.

It's a grand display, one of the greatest shows in the universe,

though it is a sign that, after four billion years,

the comet is dying.

The fact that we see them now as active comets

means they have only a few thousand years left in their current orbits.

So you can imagine, if you likened that

to the lifetime of a human being,

we're seeing the last few seconds of the lifetime of the comet.

Every second, they are losing tonnes of dust and ice.

Each time it goes round the sun,

the comet nucleus shrinks.

Most only have enough fuel for 1,000 revolutions around the sun.

Even a large comet like Halley will only last another 150,000 years.

And then, as their fuel supplies are exhausted, they die.

Some perish as they crash into the sun.

And some disintegrate,

eventually turning into dust,

dust we see as shooting stars as they enter the atmosphere.

But not all of them disintegrate to dust.

Some of them leave a more substantial skeleton

drifting through space.

We predict there should be hundreds, or maybe even thousands,

of dead comets which still haven't yet been found.

If the comet fades to dust and nothing else,

then in a way they're not dangerous.

But if they leave behind big chunks, hundreds of metres across possibly,

then those objects could come through the atmosphere

with some devastating consequences on the ground.

Is the Earth really in danger from comets?

Although big, they are incredibly fragile,

no more substantial than cigarette ash.

Still, some think a comparatively small chunk of comet

could cause a global catastrophe.

Mike Baillie is a paleoecologist,

who's found the evidence of an ecological disaster

in the fields of Northern Ireland.

20-odd years ago, the farmer, when this field was being drained,

found that these oaks were effectively floating to the surface,

the land was shrinking round them.

So he got a digger in, pulled them out and pushed them into a heap.

And then in a flat area like this, it's a handy windbreak

for sheltering animals, so that the farmers leave them,

and it means that we have access to some ancient trees.

Some of these oaks are thousands of years old,

preserved perfectly in the oxygen-free conditions of the Irish peat bogs.

By sampling tens of thousands of trees,

Professor Baillie has built a year-by-year record

of the Irish environment for the past 7,500 years.

What he has found encoded in the annual rings

is that the trees sometimes mysteriously stopped growing.

This tree went from a period of extended, perfectly good,

what we would regard as normal growth.

When we get to this year, which is the year 540,

there is a damage scar, and its growth

was radically changed in character thereafter.

When the same pattern emerged in many of the Irish trees,

it was initially dismissed as the effects of savage storm

that struck Ireland in 540.

What caused a complete turnaround in this story

was when we found that this same dated event

occurs from Mongolia, Siberia, Northern Sweden,

across Northern Europe, North America and South America.

This was the realisation that this was a global event.

Here was evidence of a worldwide catastrophe in 540 AD.

What could have caused such a widespread disaster?

There was one obvious suspect.

The biggest volcanic eruptions

blast so much ash into the upper atmosphere

that it spreads around the world,

blocking out the sun's rays.

The eruption of Mount Pinatubo in 1991

caused global temperatures to fall

by over half a degree for the next two years.

It's the same pattern that is seen in the 540 event.

But volcanic eruptions also leave another trace...

a layer of sulphuric acid

that is frozen into the Greenland ice sheets.

But the samples taken from deep within the ice

show no acid spike in 540.

Whatever stopped the trees growing, it wasn't a volcano.

If you've a global environmental event

and it wasn't caused by a volcano,

what's the next most likely explanation?

And you're allowed to ask the question,

"Could it have been some sort of loading from space?

"Could it be an extra-terrestrial event?"

And as soon as you start even asking that question,

you realise that the most likely sort of event you'd be talking about

would be some sort of brush with a comet.

In theory, a lump of comet just 300 metres across

could affect the global climate.

As it hit the atmosphere at over 20,000 kilometres an hour,

it would burst into flame....

and blow itself to bits while still kilometres above the ground,

spreading a shroud of dust around the world

that could block out the sun.

It's an event they call an airburst.

And there is much more recent evidence

that suggests these events do happen.

On June 30th 1908, a huge explosion

tore through the forests of Tunguska, Siberia.

No-one witnessed the explosion itself...

..but it threw up clouds of dust that reflected so much light

they could be seen as far away as London.

It was so bright, the story goes, people played cricket at midnight,

and people were able to read newsprint.

That gives you an idea of how much dust must have been liberated

very, very high into the atmosphere, which was illuminating the ground,

even as far away as London is from where the impact actually happened.

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

What they found astounded them.

60 million trees across an area the size of London

had been levelled.

Puzzled, the Russian scientists performed a set of experiments

to try and figure out what could have caused such devastation.

The only explanation was that the explosion was caused

by an extraterrestrial object, just 60 metres across,

running into the upper atmosphere.

This was just a small airburst,

but it showed the amount of damage that could be done.

All that energy of motion is converted into heat,

it's just like the explosion of a nuclear weapon

in the Earth's atmosphere.

One can only imagine the effects

if an airburst exploded over a populated area.

If this is the devastation comets can cause,

perhaps our ancestors had good reason to be afraid of them.

'The comet called Shoemaker-Levy, photographed...'

'..About to smash into the planet Jupiter...'

'..What could be the most dramatic astronomical event of the century...'

'When the comet pieces strike, it will be a massive collision...'

'..Crash into the planet with the force of millions of nuclear weapons.'

If anyone was left in any doubt of the destructive power of comets,

an event in 1994 would change their minds.

It happened 400 million miles away,

but, in solar system terms, it was in our backyard.

Comet Shoemaker-Levy 9 was discovered in 1993.

It had had been torn into 21 separate pieces

by the gravity of the planet Jupiter,

creating a line of debris, known as the String of Pearls,

that was on a collision course with the giant planet.

This was a unique astronomical event.

No-one had witnessed two solar system objects collide before.

As the scientists planned their observations,

less rational comet-fever took hold once more.

At the time of the comet impact,

we may see the bio-field around people change.

Some people will feel excited,

some may feel more depressed, we don't know.

We have a piece of software that tells us what the comet's impact

will mean for you as an individual.

So is it going to happen to me on that date, the 16th?

Maybe discard some old ways that were holding you back.

-July 16th, I should beware, right?

-Yeah.

No-one knew what effect, if any, the comet fragments,

just a few kilometres across, would have on such a huge planet.

As the comets approached,

the world's astronomers watched the pictures coming in,

and held their breath.

Isn't that incredible?

That's amazing!

Look! Oh, my God!

That's it.

Look at that!

THEY CHATTER EXCITEDLY

The scale of the explosions surprised everyone.

OK, we just blew up a section of the planet.

This is the southern pole here.

You can see there's a bright streak. See that bright streak?

And around the edge of the streak, there's some other stuff.

It was not there the day before.

It's a new feature on Jupiter.

As the comet struck the Jovian atmosphere,

fireballs 3,000km high erupted from the surface of the planet.

The biggest impact released 600 times more energy

than the entire world's nuclear arsenal.

Scars bigger than the Earth were left on the surface of Jupiter.

These were some of the biggest explosions

that had ever been witnessed.

It delivered, how about it? Yoo! HE LAUGHS

Perhaps this was not the right time to celebrate.

The Jupiter collision showed the enormous power

that could be released by a full blown impact with a comet.

Just imagine if it had hit the Earth.

It's not just fantasy.

Impacts this size have happened to the Earth.

The last time was 65 million years ago.

It spelled the end of the dinosaurs,

and wiped out half of the rest of the species on the planet.

Awakened to the danger, we now have projects to scan the skies

looking for objects that are a threat to the Earth.

But most of the sky surveys concentrate on finding asteroids.

Unlike comets, which originate in deep space,

asteroids mostly come from the belts between Mars and Jupiter,

from where they are periodically deflected

into an orbit that crosses the Earth's.

But these rogue asteroids are easy to spot,

and their trajectories can be plotted hundreds of years into the future...

..giving scientists plenty of time

to devise ways to divert them so they miss us.

Unfortunately, avoiding an incoming comet

would be a very different proposition.

Comet impacts are likely to be more rare than asteroid impacts,

but they're going to be much harder

to mitigate against because a comet will be coming in from deep space,

we'll have a few years at best, a few months at worst,

knowledge before impact. And that may not be time enough

to launch a spacecraft to, to deflect the orbit.

People might remember a few years ago,

we had a fantastic comet called Comet Hale-Bopp.

Suddenly came out of nowhere.

It was about 3,000 years since it had last passed by.

And that was a very bright comet. And suddenly it was there in our midst.

We've had, recently, a comet called Comet McNaught,

which was a comet you could see with the naked eye.

Nobody had predicted the appearance of these comets.

You might get two, three, maybe six months' notice.

So, it's possible that there could be a catastrophic collision

with the Earth any time.

It's a scary prospect,

but perhaps not one to keep you awake at night.

It's estimated that the Earth will endure a Tunguska-sized impact

that could destroy a city, once every thousand years.

And an extinction level event,

like the one that wiped out the dinosaurs, every 100 million years.

And rather than being afraid of comet impacts,

perhaps we should be grateful for them.

They might be responsible for us being here at all.

MUSIC: "The Blue Danube Waltz" by Johann Strauss

Way back at the beginning of the solar system,

the planets were formed from the disc of debris

that the early sun had thrown out around it.

As the disc was stirred...

bits of dust stuck together and became boulders...

boulders stuck together and made asteroids...

..which stuck into larger clumps...

that became the planets.

It was a hot, violent process,

and when it was finished,

it left the Earth a barren and sterile place.

Yet within a few hundred million years,

the Earth was covered in oceans,

and life was thriving.

Where can it all have come from?

There is only one obvious source of water

and organic molecules in the solar system -

comets.

But to explain how the Earth was transformed

from a sterile boulder into the blue planet,

astronomers have had to propose a massive hailstorm of comets,

3.8 billion years ago.

They call it the "late heavy bombardment".

The solar system wasn't as we see it today,

there is still a lot of the stuff left over,

the builders' rubble from the building of the planet.

And Jupiter and all the other gas giants out there

were doing a very good job at throwing stuff inwards to the sun.

Some of which, unfortunately, we got in the way of,

or, rather, I should say fortunately for us,

if they really did bring the water that we see today.

We're talking about a bombardment of the, if you like, the proto-Earth.

That takes millions, or tens of millions of years,

with an impact rate that may have been

1,000 or 10,000 times what it is today.

The bombardment hasn't finished, it's still going on today.

We're not quite so aware of it,

because that bombardment is by tiny, tiny dust particles,

that float down through the atmosphere every day,

about 60,000 tonnes over the whole of the Earth, every year,

comes down as dust particles.

Water is bound into those mineral grains.

So, we're still being bombarded

by grains that carry water and carbon, even today.

It's an amazing thought that all the water and organic molecules

that make up every living thing on the planet

may have started out on a comet beyond the edge of the solar system.

But there are people who have taken the idea even further.

They believe comets brought actual living creatures to the Earth.

Earth life is essentially alien life,

it is not a life that is indigenous to the Earth, by any means.

And if we evolved from that life, then I think we are the products

of evolution from alien life.

In the 1960s, astronomers Fred Hoyle and Chandra Wickramasinghe

proposed that the Earth was colonised by bacteria from space.

It was an idea called panspermia, and it's never quite gone away.

I think it's true to say, isn't it, that if your theory is correct,

life not only MAY be spread throughout the universe, it MUST be?

It would really be very surprising if life is not...

if the galaxy is not just teeming with life.

Since then there have been a number of strange discoveries

that may, or may not, back up the theory.

In 2001, the Indian state of Kerala was shocked by loud explosion,

followed by storms of blood-red rain over the next two months.

Local scientist Doctor Godfrey Louis

collected samples from all over the state and analysed them.

What he found was that the rain was being turned red by cells

the like of which he had never seen before.

It's quite exciting to see this.

They look like red blood cells but they're not.

Because these are having a very thick cell wall.

And that cell wall is not there in blood cells.

What made the cells even more mysterious

was that he could find no trace of DNA in them.

If that was the case, the cells were like nothing else on Earth.

The staggering claim is that

this is possibly extra-terrestrial. That's a big claim, I know,

but all the experiments are supporting this claim.

Come in, please.

This is some of the samples we have collected.

-Pretty good sample, we have...

-So, this is the real stuff?

-Yeah, this is the real stuff.

-It's red, for sure.

For Chandra Wickramasinghe, this was vital evidence

to back up his theory.

A fragment of comet must have blown up in the upper atmosphere

and rained extra-terrestrial bacteria down on the Earth.

If it is true that these are alien bugs from space,

then it is an absolute clear cut proof of ongoing panspermia,

and that would be absolutely fascinating, and I'd be over the moon!

But unfortunately for Wickramasinghe, there was bad news.

On closer inspection, the cells in the red rain

didn't seem so alien after all.

This is DNA from the red rain.

Wow, there is no doubt about it.

-No doubt about it.

-Absolutely conclusive, as far as I can see.

-Yeah.

-The material, when examined by astrophysicists

and people who are looking for evidence to support a view...

doesn't appear to be anything that they've ever seen before.

But people who actually have seen things before say,

"It looks like red algae to me."

I would tend to go with the people who have seen more things

in the biological kingdom, rather than those

who are looking to support their own ideas

about how the world should work without the data to back it up.

The journey towards the truth is always a rewarding one, I think,

and I saw this as a journey toward discovering a truth

that was quite plain to me and is becoming plainer and plainer

to other people now, after 30-odd years.

Whatever their actual role,

it seems comets were vital in the evolution of life on Earth.

And they maintain the ability to destroy it.

It seems our ancestors were right all along,

comets do carry the power of the gods.

Maybe that is why they still cause so much excitement

when they appear in the sky.

What's more, when astronomers turn their telescopes to other stars,

they see clouds of dust, the tell-tell signs of comet activity.

Just as comets helped bring life to our solar system,

they may be doing the same elsewhere.

Perhaps that is the next message that comets will bring us,

that we are not alone.

Subtitles by Richard J Boyle Red Bee Media Ltd

Email [email protected]