Space

Take a walk on planet Earth.

It's a wonderful place.

But don't take it for granted, because one day it might just vanish - forever.

This is the story of the universe we live in...

..and the dangers we face just by being here.

We'll fly with the comets and asteroids which threaten all life on our planet.

Here on Earth, we'll recreate our sun's headlong plunge around the galaxy

and watch disasters unfold which could spell the end for us all.

Our planet is under threat.

Our mission...

..is to stay alive.

If you're a gambler, this is the planet to live on,

because just by being here, we're all taking a risk.

And the odds are stacked against us.

Do you feel lucky? You should do.

You may not realise it, but we are all winners.

We human beings are lucky to be here,

because the universe keeps trying to wipe us out.

Trying...

..and failing.

We have survived every disaster the universe has ever thrown at us - so far.

But our run of luck won't last forever.

When it ends, our planet and our entire species will cease to exist...

..unless we can do something about it.

Could our whole planet really be in danger?

What could threaten a globe 13,000 km across, or 6 billion people?

To understand the threat, you have to see it first hand.

I want to show you planet Earth as it was in its earliest days.

This is where it all begins.

We're lucky there's even a planet here for us to live on,

because it was very nearly blown apart before life had even begun.

This is planet Earth, billions of years ago, at the very beginning of its life.

As you can see, it is under attack.

These are asteroids, rocks left over from the creation of the planets themselves.

Some of them were vast, and one of them was on a collision course with Earth.

This collision nearly smashed our Earth to pieces.

We were hit by a ball of molten rock the size of Mars travelling at colossal speed.

The impact ripped huge chunks out of our planet.

Somehow it survived. But what about the future?

Smaller chunks of rock are still flying around our solar system today.

We call them asteroids,

and they could put an end to human civilisation.

Asteroids range in size from pebbles to mountains, or even bigger.

And they're dangerous. They're flying around at incredible speeds.

To get an idea of the damage they can do, just take a trip to the moon.

Three, two one, zero. All engines running.

Man, a species born and who's lived all his life on Earth,

moves with this journey out into the solar system.

What can you say about a sight like that?

You are go to continue power descent.

In 1969, for the first time ever,

human beings walked on the surface of another world...

(BANG, FOLLOWED BY ASTRONAUT'S VOICE)

..the moon.

Tranquillity Base here. The eagle has landed.

Roger, Tranquillity, we copy.

And what a world it was - airless, barren, hostile.

And bearing the disturbing evidence of a terrifying threat.

The moon is scarred with craters,

the marks of countless asteroids that have slammed into it.

The moon is under attack.

And travelling through space, alongside the moon, is another planet,

the one we call home.

So why aren't we taking the same pounding?

There's a simple answer to that...

we are.

This crater was formed in a few seconds about 49,000 years ago.

It's almost 1.2 kilometres in diameter. It's almost 200 metres deep.

It was caused by an asteroid about 40 metres in diameter.

It came in with a velocity of about 25 km/second,

and it exploded with the force of a 15-megaton hydrogen bomb.

This is Meteor Crater in Arizona.

Geologist Jeff Wynn thinks that what happened here could happen again, soon.

Local effects would have been devastating.

Anything within the circumference of the crater would have been obliterated.

If you were an animal grazing within a few kilometres, you would have been killed.

If you were beyond that, depending on where the wind was blowing,

the ejector coming down would have killed you.

This is just one example of what could happen if one of these things did hit.

Geologists are discovering more and more evidence of asteroid impacts here on Earth.

Meteor Crater would be hard to miss.

Other impacts are more difficult to spot.

But we have seen them. We've photographed them from space.

These are the remains of titanic collisions, some of them vast, 20 or 30 km across.

The Earth is being pounded all the time.

Siberia, 1908, an entire forest was flattened.

There was an impact in Saudi Arabia in 1933.

This was caught on camera in Canada in 1972.

A near miss - it flew back out into space. If it had crashed on Earth,

it would have exploded with more force than an atom bomb.

Amazingly, in all these events, no one at all was killed.

But even if an asteroid doesn't hit us, it could do something even more dangerous.

It could hit the sea.

If a big asteroid smashes into the ocean, it could create a huge deadly wave

that could wipe out entire cities.

I think there's a realistic possibility

that something like this could cause serious problems to human civilisation

some time in the next century or two.

It's not an appealing thought, but the threat is real.

Whole cities could be obliterated from space.

Whether it's your city comes down to luck.

But before you feel too lucky, think about this.

It may not matter where you live.

There are things out there which could wipe all life from the face of the planet.

Space contains dangers that make asteroids...

..seem utterly insignificant.

Our telescopes have captured images of truly colossal disasters.

This photograph shows a star a hundred times bigger than our sun

blasting incandescent gas out into space.

And this is a black hole

spewing out jets of super-hot matter at close to the speed of light.

If these had been near us, there's almost no chance that we'd have survived.

It looks as though fortune has been on our side.

At least it has so far.

But for how much longer?

Professor Mike Rampino thinks the odds for long-term survival of life on Earth are slim.

Luck plays a big role in the existence of life on the planet Earth.

You have to realise that the galaxy is a very violent place.

If we get unlucky, that could put an end to complex life on the planet.

Here's the problem.

We don't notice it, but planet Earth is on the move.

While we get on with our lives,

the world is hurtling around our galaxy at 230 kilometres a second.

And high speed means high risk - some you win, some you lose.

We may not realise it, but we're on the ride of our lives.

Think of the Earth's orbit as a kind of cosmic roller-coaster ride.

As we move in this orbit, we're moving past gas clouds.

We can come close to black holes.

We can get close to stars.

We can come close to supernova explosions.