Orbit: Earth's Extraordinary Journey

At this moment, you're speeding around the Sun at 67,000 miles an hour.

In the next year, you will travel 584 million miles, to end up exactly where you started.

Three things: the orbit, spin and tilt of the Earth...

...have created and continue to shape the planet, and us, every minute of each day of every year.

Since the beginning, the Earth has made this trip around the Sun over four billion times.

The orbital path around the Sun is the ultimate Earth environment.

It's a dangerous place, bathed in solar radiation, deathly cold and crossed by asteroids and comets.

But despite the dangers, the orbit of the Earth is in just the right place.

Just right for life.

For most of human history, the idea of Earth orbiting anything at all was a bizarre concept.

The Earth, it was thought, sat at the centre of everything,

with everything else revolving around it.

Discovering that the Earth actually orbits the sun, spinning as it goes, solved some mysteries.

Day and night and the year suddenly made a lot more sense.

Today we know a lot more about the planet's annual trip around the Sun.

We've discovered that the orbital journey influences almost everything on the Earth...

...in ways that our ancestors could never have imagined.

The ancient Egyptians knew there was a special relationship between the Earth and the Sun,

and they got some of the science right -

but not everything.

Part shrine to the Sun god Amun Ra, and part stone calendar,

the architects built it specifically with December 21st in mind.

This day is the winter solstice.

It's the shortest day of the year in the northern hemisphere.

The people who built this temple didn't know why it was the shortest day.

But they did know that it happened on the same day every year.

So the temple was designed to line up perfectly with the Sun on this one special morning.

We know now that the regularity of this phenomenon is not the gift of the gods,

but because of the Earth's orbit.

Our planet will be here in the same place at the same time every year.

Planet Earth was born over four billion years ago,

from the planet-forming cloud of debris orbiting the new-born Sun.

It was a violent birth, for both the planet and its orbit.

As it grew, the young Earth was battered by other debris orbiting the Sun.

It even collided with a smaller planet which knocked Earth over,

leaving it spinning on an axis 23 degrees off the vertical.

Today, Earth's epic annual trip round the Sun may seem serene.

In reality it's still fraught with danger.

Though our planet's course is constant, the swirling soup of space is ever-changing.

Its path is littered with asteroids, meteorites and comets that could spell disaster.

This hole in the ground is Barringer crater in Arizona.

NASA impact specialist Dan Durda has come here to see what happens...

...when our planet comes into contact with something else.

This is what happens when we cross orbits with another object in the solar system.

About 50,000 years ago,

a relatively large meteorite landed on Earth, about maybe 40-45 metres across or so.

Slammed out of the sky at 17 or 20 kilometres a second.

The explosive energy that excavated this crater five or ten megatons,

something like that, is -

it's a little less than a thousand Hiroshima's worth of explosive energy.

So it's - it's far larger than anything human beings have ever witnessed.

You could put the Washington Monument down there in the bottom...

...and the top of it wouldn't quite make it to the - to the height of the rim of the crater.

So when - when objects like that hit the Earth they unleash a lot of energy.

The crater is more than three quarters of a mile across, and is considered a small one.

To get an idea of the force that created it, you only have to look at the rock it left behind.

So here I've got a - pretty much a pretty pristine piece of the Coconino sandstone,

a piece of rock from the basement of the crater.

In my other hand, I'm holding effectively the same stone, the same rock,

but this material has been pulverised,

crushed by the immense pressures produced in the impact even itself.

This stuff here is - is crushed to the point where I can break it apart in my hand,

and it gives you an idea of the damage done to even solid rock by an impact like this.

Scientists have calculated that the temperature in this crater reached 6,000 degrees Celsius,

hotter than the surface of the Sun.

Barringer crater is a little over a kilometre wide.

Incredibly, all this was done by an asteroid just 45 metres long.

Here's a nice piece of ejecta, a few meters across.

It came to rest here on the - on the rim of the crater,

but there are rocks just like it that were hurled for -

for miles in every direction, landing across the desert.

I can see block out there the size of small buildings.

And those weren't the only ones.

There were ones deposited out in the desert.

You can see the remains of them eroding today.

So it's - it's an immense amount of energy...

that can hurl rocks the size of buildings across the desert for miles in every direction.

Incredibly, all this was done by an asteroid just 45 metres across.

But not all are as small as the one that created Barringer crater.

65 million years ago, an asteroid six kilometres long...

...hit shallow seas just off the coast of the Yucatan peninsular in Mexico.

So put ourselves near the point of impact 65 million years ago.

It's just a few seconds to impact.

The asteroid is glowing far brighter then the sun in the sky.

Disappears over the horizon.

And then we would have seen the immense flash of the impact itself.

Followed 10 or 20 seconds later.

Boom! We're blasted flat by this shock wave of the blast from the impact itself,

which was strong enough to have actually laid flat forests for,

you know, 800 miles or so in every direction.

This is an immense impact.

It made a crater well over 180 kilometres wide,

and blasted so much hot debris into the atmosphere that almost the whole planet caught fire.

The impact was so catastrophic, it helped wipe out the dinosaurs.

Earth's orbit regularly takes it into the path of asteroids and comets.

But Earth has a secret weapon to defend itself.

It has an atmosphere.

Most objects along the Earth's orbit are small.

The atmosphere is very effective at burning up anything under 35 metres wide.

When we see shooting stars in the night sky,

they are actually small asteroids or bits of comet burning up as they hit the atmosphere.

Still, over the ages many large objects have struck the Earth - and will again.

Asteroids and comets are obvious, if occasional, dangers along the orbital path of the Earth.

But the Sun itself poses the greatest danger.

In the northern sky during winter you can sometimes see the Earth protecting itself.

For thousands of years, people have marvelled at the spectacular light displays...

...that sometimes appear in the night sky.

They've wondered what on Earth they could possibly mean.

The Vikings believed them to be the reflections of dead maidens.

Native Americans called them 'the dance of the spirit.'

What we see as the northern lights and the southern lights,

the aurora borealis and australis, are manifestations here on the Earth of living...

...within kind of the outer extension of the Sun's atmosphere.

Every second, the Earth flies through millions of tons of radioactive particles...

...blasted out from the Sun's surface.

The auroras are caused by those particles smashing into our atmosphere.

The Sun emits a continuous flow of charged particles known as the 'solar wind'.

This streams outwards in a blizzard of radiation.

When it reaches the Earth, it encounters a barrier.

The Earth's magnetic field deflects the particles and funnels them towards the poles.

Here they collide with atoms of nitrogen and oxygen in the atmosphere.

These collisions emit energy in the form of light, giving us the aurora.

From the international space station,

you get a better sense of the awesome scale of the aurora.

You're watching the Sun's atmosphere colliding with Earth's.

Without our protective atmosphere and magnetic field,

we'd be living on a much more harsh and alien and inhospitable planet.

We'd - we'd look like maybe the surface of Mercury,

baked by UV radiation and x-rays and blasted by solar flares.

So we live in this lovely zone with our protective atmosphere and magnetic field.

Earth's orbit is full of risk, not just from asteroids, but also from the Sun's harmful radiation.

But for all the danger from the Sun, being close to it provides us with one critical benefit.

Our planet enjoys a precise and very narrow band of temperatures...

that means water can exist in all three of its states.

As a solid, as a liquid, and as gas.

Each of those states behaves very differently.

And it's those differences that drives the climate system on Earth.

This is the cloud forest of Calilegua in northern Argentina.

Sitting in the foothills of the Andes, it's a ghostly place.

The trees and hills are shrouded in clouds.

The heat of summer has evaporated water,

so the air is laden with vapour, the gaseous form of water.

Dr Kristen Rasmussen has come hereto see...

...how this water vapour creates some of the most dynamic storms on the planet.

The exciting part about these storms is we've identified this region as...

...being a hot spot of very, very intense thunderstorms.

The thunderstorms in this part of the world bring a lot of hail, lightning, tornadoes, flooding,

a lot of things that can be very hazardous to the local population.

Throughout the day the land has absorbed more and more heat from the Sun.

It's approximately three o'clock right now.

The - the land surface has been heating all day, since the sun rose.

And all of that warm buoyant energy from the surface is now rising very quickly.

You kind of see a bubbling effect of that:

the cloud parcel rising up and condensing water as it forms the cloud.

As heat evaporates water and forces the wet vapour high into the atmosphere,

there it hits colder air and condenses back into liquid water,

forming towering cumulus clouds.

The huge mat of clouds pushes up into the atmosphere.

These storms are growing very, very quickly.

You can actually sit here for about five minutes and watch almost the whole thunderstorm develop.

The updrafts keep the storm growing up to ten miles high,

and at this point high winds spread the clouds out sideways to form the anvil shaped thunderhead.

By early evening, the storm has grown to epic proportions,

enveloping the entire mountain range of Calilegra.

For Dr Rasmussen it's a chance to experience one of the great forces of nature at first hand.

A phenomenon that can only happen...

...because Earth's orbit means its temperature is rising for liquid water.

This is actually the cloud base where the storm - where the storm initiates.

You cans see some lightning off here in the distance.

This means that there's ice the clouds.

They're very, very tall, they're very intense,

and they have the capability of producing large amounts of rain, and hail as well.

The extraordinary fact, the Earth orbits the Sun at the just the right distance,

gives rise to the very ordinary phenomenon of rain.

Water evaporates from the surface as a gas, condenses into a liquid, as it rises,

and eventually falls back to earth, where the cycle starts again.

It's this very cycle, unique to the Earth in our solar system,

which supports life in all corners of the planet.

This is incredible.

This is the thing I've come here for.

It's - I study these storms all clay long every day, and it's just amazing to see them in person.

If Earth's orbit around the sun were just a little different,

temperatures on our planet might be either too cold or too hot for water as a liquid and a gas.

Life as we know it, and the weather we enjoy, simply couldn't happen.

But within the narrow band of temperatures our planet experiences,

there is room for water's third state.

The edge of Lake Ontario sees...