All language subtitles for The.Planets.UK.2019.S01E05.1080p.BluRay.x264-SHORTBREHD.he_auto

Lift-off of Messenger
on NASA's mission to Mercury.

Far, far away...

...beyond Mars...

...past the storms of Jupiter
and the rings of Saturn...

...conditions become very different.

Temperatures plummet...

...and distances between worlds

are measured not in millions
but in billions of kilometres.

Here lie the most mysterious planets
of them all.

Uranus.

A pale blue marble hanging
in the dark, frozen depths of space.

And further out...

...the solar system's final true planet.

Neptune.

Beyond, we thought we'd only ever find
tiny, lifeless worlds...

...frozen to the core.

How wrong we were.

We live at a time

when our civilisation
has unprecedented reach.

And we have a near permanent presence
at Mars, both in orbit and on the surface,

and we've had spacecraft orbiting
Mercury, Venus, Jupiter

and Saturn for extended periods.

But once we get beyond Saturn,
things become much more difficult.

The distances are vast.

Travel times are measured
not in months but in years, decades even.

And that means that we've only visited
the outer planets once.

Each visit is a voyage into the unknown

as we strive to reach
the most distant planetary systems

ever visited.

And when we do get there,
our spacecraft are travelling so fast

that they fly straight through,
only spending a few hours in the system.

But those few hours of precious data

have allowed us to begin to understand
the mysteries that lie there,

on the outer edges of the solar system.

We have ignition
and we have lift-off.

Reports coming back indicate
those twin large solid motors

are functioning perfectly.

Our first mission into the dark
was only possible

thanks to some help
from the planets themselves.

Each planet orbits at a different speed.

And for most of the time,

they're scattered around the Sun.

But once every 175 years...

...something rather wonderful happens.

The outer planets align...

...creating a path...

...that leads from Earth,
past Jupiter and Saturn...

...all the way to Uranus and Neptune.

Voyager 2 had been launched
at precisely the right moment

to take advantage
of this rare planetary alignment.

It arrived at Jupiter
in a little under two years.

Two and a half times more massive
than all the other planets combined...

...Jupiter gave Voyager
a gravitational kick.

Another two years on,

and Voyager witnessed
one of the most beautiful sights

in the solar system.

The icy rings of another gas giant.

Saturn.

And then, on and on...

...further into the darkness.

Almost nine years after leaving Earth,

Voyager approached
an entirely new class of planet.

Just like Jupiter and Saturn,

the planet's upper atmosphere

is composed mostly
of swirling hydrogen and helium gas.

And hidden beneath

lies an exotic icy mix
of methane, ammonia and water.

But unlike the other gas giants,
Uranus is almost featureless.

For all the time
that Voyager stared at the planet,

it saw just ten cloud formations.

And we soon discovered why.

Uranus, at minus 224 degrees Celsius,

is the coldest planet in the solar system.

The first of the ice giants,

in a permanent state of deep freeze.

Voyager spent just six hours with Uranus.

And as its gaze widened,
it took in the entire system.

Just like Saturn, Uranus has rings.

The rings are so dark, so faint

that they're very difficult to see
from Earth.

In fact,
we didn't discover them until 1977.

They must be made
of some kind of material

that doesn't reflect
a lot of sunlight back.

And we don't know what that is.

It can't be water ice,
like the rings of Saturn,

because they would be much brighter.

They're also extremely delicate and thin.

And again, that's a great mystery
cos you'd expect the particles to collide

and the rings to spread out over time -
that doesn't happen.

So something must be holding those rings
in place.

And the answer can be seen
in this photograph taken by Voyager 2.

You can see a bright thin ring,
which is known as the Epsilon Ring,

and above it and below it,
you can see two moons.

The inner one is called Cordelia,
and the outer one is called Ophelia.

Now, the thing you need to know
about orbits -

a moon or a ring particle
that's orbiting closer to a planet

is moving more slowly

than a moon or a ring particle
that's orbiting further away.

And so, if there's a particle
in the Epsilon Ring

that maybe has a collision,
loses a bit of energy

and starts to drop down,

then Cordelia tends to accelerate that
particle back up into the ring again,

to speed it up
by its gravitational influence.

Whereas, if there's a particle
on the outer edge of the Epsilon Ring

and it gets a bit more energy
and raises up in altitude,

then Ophelia tends to slow it down,

and so it falls back into the ring again.

And by that mechanism,
these moons keep the ring

nice and neat around Uranus.

And for that reason,

these moons have become known
as shepherd moons.

Now, we haven't discovered
any more shepherd moons.

I mean, Voyager was only in the system
for a few hours.

But we assume that they're there,

orbiting around Uranus,

keeping the other rings
precisely in place.

The missing moons
are not the only mysterious thing

about the planet.

Uranus orbits the Sun in a unique way.

A consequence of events
that happened long ago.

Uranus was born
from the vast cloud of material

that surrounded the young Sun.

Over time, this material drew together

under the influence of gravity...

...forming each of the planets...

...all orbiting
in the same anticlockwise direction

as the primordial cloud of gas and dust.

This rotation remains to this day.

Almost all the planets spin on their axes

in the same anticlockwise direction.

But for reasons
we don't yet fully understand,

Venus and Uranus spin on their axes
in the opposite direction.

And Uranus is even more curious.

The entire planet is on its side.

It's not known for certain
why Uranus spins in this way,

but it seems likely that, at some point,
probably in the distant past,

it was hit by another planet,

possibly the size of Earth,
or even larger,

which knocked the planet over.

And indeed,
modern computer simulations suggest that,

when you do that to a planet,

all the moons follow,

and you end up with a planet
and its system of moons,

and now its rings,

sort of corkscrewing around the Sun
on its side.

Voyager had taken us

to the most distant planet
we had ever visited.

And now, an almost unthinkable distance

of 1.6 billion kilometres
of icy cold space

lay between the spacecraft
and its next encounter.

The size and scale of the solar system
is impossible to imagine.

But if we scale everything down,
then it becomes easier to visualise.

The Sun is 1.4 million kilometres
in diameter.

But scale that down
by a factor of around 600 million,

and it becomes the size of that light.

And the Earth, Mars and the rocky planets

are about that big on this scale.

And the distances are, I think,
even more remarkable.

So Mercury,
the closest planet to the Sun,

would be all the way over there
by the sea.

So this would be where Mercury orbits.

And Mercury is 96 metres away.

It's about just under a centimetre
in diameter.

And the next planet, Venus,
is somewhere over there.

We'd fly past Venus
about 180 metres away from the Sun...

...and then head onwards and outwards
towards the Earth.

Here, at the Earth's orbit,

the true scale of our fragility
and isolation becomes apparent.

The Sun, there, 250 metres away,

with the twinkling stars
of the galaxy beyond.

And here's the Earth -
two centimetres across.

On that pebble,
all of human history played out.

At around 380 metres, we pass by Mars,

the last of the rocky planets,

and then onwards, into the darkness.

Crossing the harbour takes us
to the fringes of the outer solar system.

And now the distances and the sizes
are beginning to get really big.

Jupiter, the largest planet
in the solar system,

is over ten times the diameter
of our Earth.

And, I mean, there's the Sun,
1.3 kilometres across the harbour.

At almost twice the distance
from the Sun to Jupiter,

we reach Saturn...

...2.4 kilometres on our scale.

And we really are now entering
the twilight of the solar system.

Light levels are like early morning
or late evening here on Earth.

But we can still see Saturn
in the night sky

because the planet, and its rings,
are highly reflective.

As far again as all the rocky planets,

and Jupiter and Saturn,

we reach Uranus.

Three billion kilometres from the Sun,

and we begin to fully appreciate
the distances Voyager had travelled...

...on its journey to the farthest reaches
of the solar system.

Journey's end - Neptune,

a ball of gas
about seven and a half centimetres across,

7.4 kilometres away from the Sun,

on this scale.

And we are still down there,
on the island, in Reykjavik Harbour.

Little microscopic ants crawling
around on the surface of a pebble.

It had taken 12 years,

but Voyager had finally crossed
the great expanse of space

between Earth
and the most distant planetary system.

Neptune.

17 times the mass of the Earth

and even more massive than Uranus.

But in contrast to its sister planet,

Neptune's atmosphere
is bursting with activity.

What we found was a planet
of extreme weather,

where high-altitude winds
whip white methane clouds around

at speeds
of over 2,000 kilometres per hour.

Those are the highest wind speeds
anywhere in the solar system.

And Voyager saw a great dark spot,
not unlike Jupiter's Great Red Spot.

A storm system the size of Earth that,

when we looked
only four or five years later,

we found had vanished.

But over the years,

we've observed more storms
raging over the planet.

It's one of the great mysteries
of planetary exploration -

why a planet so far from the Sun,

with so little energy falling
into its atmosphere from sunlight,

can have the most extreme winds
in the solar system.

And Voyager made
yet another puzzling discovery.

Although further from the Sun,
the planet is warmer than Uranus.

The source of this extra heat
remains a mystery.

But for some reason,

Neptune emits over two and a half times
the heat it receives from the Sun.

And this internal heat
helps to explain the ferocious storms.

As the heat makes its way
from the core of the planet

and out into space...

...it churns up the entire atmosphere...

...creating winds unlike anything
seen elsewhere in the solar system.

The probable reason
that wind speeds can be so high

is because there's no solid surface
on a planet like Neptune.

It's exotic liquids further down
and gases further up,

but there's no rocky surface.

There are no mountains and continents
to break up the flow of atmospheric gases.

And so, the winds can just whip
around the planet at supersonic speeds.

Voyager 2 had almost completed
its grand tour of the solar system.

But before it began
its lonely journey out,

into interstellar space...

...it had one last world to visit.

Triton.

A vast moon covered
in a sheen of frozen nitrogen.

We expected it
to be a silent, still world,

but Voyager was in for one last surprise.

But when Voyager arrived,

we saw geological activity
on that frozen world.

We saw geysers erupting up into space,

eight kilometres high,

and then carrying dark material
a hundred kilometres downwind.

Now, the key to understanding
what process caused those eruptions

was that we noticed
that the geysers tended to erupt

on a place on Triton's surface
below the faint Sun,

even though it's so many billions
of miles away.

What's happening is

the sunlight is falling
on the thin layer of nitrogen ice,

going through,

then heating up a layer of darker
particles a metre below the surface.

A difference of just four degrees Celsius

is enough for the heat to radiate up...

...and vaporise the frozen nitrogen,
creating gas.

That gas is under pressure,

and ultimately
it punched through the nitrogen ice,

carrying those dark particles up
and making the geysers of Triton.

In the furthest reaches
of the solar system,

the faint light of the Sun is still enough

to power the most distant
of geological wonders.

But this can't explain
the rest of Triton's mysterious features.

Terrain on this scale
could only be created

by a much more powerful force.

And a clue seems to be lurking
in Triton's odd orbit.

Unlike every other large moon
in the solar system,

Triton orbits in the opposite direction
to the spin of the planet.

That means that it's highly unlikely

that Triton and Neptune
formed at the same time.

If you formed a planet and a moon

out of the same collapsing cloud
of gas and dust,

then they tend to spin and orbit
in the same direction

as the spin of the initial dust cloud.

So the most likely explanation to that

is that Triton is a visitor
to the Neptunian system

that, unlike Voyager, never left.

One theory is that billions of years ago
Triton was not a moon at all.

It grew up in a region of space
way beyond Neptune.

The Kuiper Belt.

Here, trillions of frozen lumps of water,

ammonia and methane circle the Sun.

The frozen leftovers from the formation
of the solar system.

Perhaps Triton ended up
on the inner edge of this region...

...and ventured close enough to Neptune
to be drawn in by its gravity...

...until, eventually,
it was plucked from the Kuiper Belt

and forever trapped in orbit
around the distant blue planet.

Today, Triton orbits
in a nice, regular circle around Neptune.

But it wouldn't have started out that way.

When it was first captured,
it would have glanced past the planet

and ended up in a wide elliptical orbit,

sometimes being far away from the planet
and sometimes close.

That means that,
as Triton orbited around Neptune,

the gravitational pull
was constantly changing.

And that stretches
and squashes the moon,

and heats up the interior, by friction.

It's a process called tidal heating.

The molten interior would have exploded up
through cracks and faults

in Triton's crust...

...creating the ragged surface
we see today.

And what you're left with
is a cooling moon orbiting in a circle,

the wrong way around the planet,

the dramas of its past hidden from view.

All that remains are the geysers,

powered by the faint light of the Sun,

painting dark streaks
across the now quiescent surface.

Voyager had fulfilled its mission

and now headed onwards,
out of the solar system...

...where it would never encounter
another planet.

But elsewhere,
another world awaited our arrival...

...far out in the darkness.

We have ignition and lift-off
of NASA's New Horizons spacecraft

on a decade-long voyage to visit
the planet Pluto, and then beyond.

The New Horizons spacecraft,

on its way to the very edge
of our solar system.

Our ambition to learn more
about a small, intriguing world

depended on the success of New Horizons.

As the tiny probe,
less than three metres in length,

was sent out into the abyss...

...most of its electronics
and all non-essential systems

were shut down to save power.

It would hibernate for almost
its entire 4.8 billion kilometre trek.

Whilst New Horizons continued
its voyage to Pluto,

a controversy arose here on Earth
about the definition of Pluto itself.

See, we always knew that Pluto is small,
smaller than Neptune's moon Triton,

but it was discoveries made
in the vicinity of Pluto

by the Hubble Space Telescope

that really challenged
its status as a planet.

From Earth, Pluto, far away
on the inner edge of the Kuiper Belt,

is so distant that, even with Hubble,

it appears as nothing more
than a fuzzy image.

But Pluto was not alone.

We began to detect other distant objects.

Then, Hubble discovered that another world

is almost the same size as Pluto.

The discovery of these new worlds
has forced us to reconsider

what we mean by the word planet.

The International Astronomical Union
has a three-part definition of a planet.

Firstly, it has to orbit the Sun,

and Pluto certainly does that.

It goes round once every 248 years.

Secondly, it has to be massive enough

for gravity to sculpt it
almost into a sphere,

and Pluto indeed is almost spherical.

But thirdly, it has to clear its own orbit
around the Sun.

And that is essentially
a mass constraint as well.

It has to have enough gravity
to throw things out from its orbital path.

And that's where Pluto falls down.

Now, why that extra idea?

Well, ultimately, it's because
if Pluto is big enough to be a planet,

then Eris is big enough to be a planet

and Makemake is big enough
to be a planet.

And tens, or even potentially hundreds,
of objects out there

are big enough to be planets.

Now, does it really matter?

Well, I don't think so.

Ultimately, Pluto is a world,

and therefore we should explore it.

And in July 2015,
that's exactly what we did.

After nine years of flight,
New Horizons awoke.

Copy that. We are in lock
with telemetry with the spacecraft.

We had our first glimpse
of the most distant world ever visited.

Pluto is beautiful.

Far from being
a frigid, featureless world,

it turns out
that it has all the characteristics

of a dynamic, living planet.

And, yes, it is cold on the surface,

about minus 230 degrees Celsius -

I'd be walking over solid nitrogen -

but the blotches that could be made out

in the grainy Hubble Space Telescope
images

turned out to be a tremendous variety
of geological form and structure.

But by far the most recognisable feature

is the region known as Tombaugh Regio,

or to give it its more popular name,
Pluto's Heart.

The western lobe of the Heart
is called Sputnik Planitia.

A giant plain of frozen nitrogen,
methane and carbon monoxide...

...that stretches
for a million square kilometres.

And at its edge lies a range of mountains
made of pure frozen water ice...

...that rise up to six kilometres
above the plain.

But there's something very strange
about the region,

something that sets it apart
from the rest of this dwarf planet.

The surface of Pluto is covered
in craters -

the scars of impacts that have taken place
over many billions of years,

just like the surface of our moon -

except if you look on Sputnik Planitia.

It is absolutely smooth.

There are no craters there at all,
not a single one.

From space,
the lack of craters is striking.

And the closer you look,
the stranger the mystery gets.

Detailed imagery beamed back
by New Horizons

revealed a network of hexagon
and pentagon shapes...

...that crisscross
the frozen nitrogen surface.

A clue to what might be happening
can be seen in these images.

And those kind of patterns
are found elsewhere in nature.

Here, for example,
on the surface of the Sun.

Or here, in a liquid
that's been heated from below.

These patterns
are characteristic of convection.

There's a heat source,
which is causing material to rise up,

and then it cools and falls back again.

And in those circulating
convection currents,

you tend to get patterns like this.

So what could be happening
below the surface of Sputnik Planitia

is that there is a heat source deep down

which is melting the nitrogen ice

and causing it to rise, cool
and then fall again.

And those convection currents
are constantly resurfacing the area.

That a small world like Pluto
is still active was a huge surprise.

Our best theory of how this could be

is that somewhere deep in its interior...

...there are radioactive elements
that generate heat as they decay.

This heat warms
a sunless, half-frozen ocean of water

that has existed for billions of years

beneath the surface of Pluto.

Now, the evidence from Sputnik Planitia
that there's still an internal heat source

and studies of other geological features
on Pluto

have led some scientists to suggest

that that ocean of water
might still be there.

And the relatively warm ocean

could explain the lack of craters
on Sputnik Planitia.

The entire area
is constantly being repaved,

as the nitrogen surface
is slowly turned over.

But why all this activity here
and nowhere else?

Perhaps Pluto's Heart
is the site of a huge impact...

...that long ago punched a large hole
in the surface,

almost down to the vast ocean beneath.

A hole that slowly filled
with soft nitrogen ice...

...that now gently churns
just above a warmer ocean.

Imagine that, an ocean,
billions of miles away from the Sun.

On the frozen frontier
of the solar system,

on this most surprising of worlds,

there may still be an ocean of water.

New Horizons' closest encounter
with Pluto lasted just hours...

...during which only one hemisphere
was visible...

...leaving the other side a mystery.

Pluto held a final surprise.

As New Horizons turned its camera back
for one last look...

...it captured an image
of Pluto's atmosphere glowing in the dark.

A thin blue sky
over a hidden ocean of water...

...4.8 billion kilometres from Earth.

We've come a long way.

After just half a century
of space exploration...

...we've reached every one of the planets
in the solar system...

...and begun to tell their story.

What's emerging is a tale
of never-ending change...

...of dramatic origins...

...moments of hope...

...and loss...

...where just eight motes
of rock, ice and gas...

...set against the dark backdrop of space

have conspired together to produce life
on at least one world.

"Why do we explore?" some people ask.

"Shouldn't we deal with the problems
here on Earth before committing

"time and energy and resources
to exploring the stars?"

Well, I think focusing entirely
on our mote of dust

would be a profound mistake.

It would mean that we'd taken the decision

to sit huddled in a tiny corner
of the solar system

wondering what we're doing here.

It would mean that we'd taken the decision

to fight amongst ourselves
for ever more precious resources

confined below a thin shell of air
on a small rock

rather than following
the three-dimensional path

marked out by the lights in the night.

We live in a solar system of wonders,

of planets of storms and moons of ice,

and landscapes and vistas
that stir the imagination

and enrich the soul -

a system of limitless resources,

limitless beauty and limitless potential.

A system
that we've only just begun to explore...

...in a journey
that has already rewarded us with so much.

Pluto was the most distant world
that NASA had ever planned to visit.

Ten, nine, eight, seven...

The launch
of New Horizons was incredibly dramatic.

Ignition and lift-off of NASA's...

And it just shoots up so fast.

It's the fastest launch ever from Earth.

About ten miles per second.

In order to ensure
that it was going to last for the decade

across space and actually work there,
they built in a lot of redundancy.

There were sort of two of everything,

computers and the guidance systems,

and they figured out how to hibernate.

The first spacecraft that hibernated.

To put the spacecraft largely asleep,

let the spacecraft just silently coast...

...then we could extend
the life of the electronics.

No spacecraft mission
had ever really used hibernation

as a day-to-day way
to cross the solar system.

As the spacecraft slept,

it sped away from Earth
towards its destination...

...to a world that had lain hidden
amongst the stars

for almost all of human history.

The discovery of Pluto was in 1930.

Clyde Tombaugh,

a Kansas farm boy
who was using the telescope

at the Lowell Observatory in Arizona.

Every clear night, Clyde would go out

and take images of the sky
on photographic plates.

And when it was cloudy or during the day,

he would compare those plates.

And Clyde Tombaugh
noticed that there was this little speck

moving across the stars.

And then, in 1930,
Clyde Tombaugh discovered Pluto.

But Tombaugh
would never see Pluto up close.

He died nine years
before the launch of New Horizons,

at the age of 90.

He had asked
if a mission ever did get launched,

some of his ashes could be sent
on the journey.

And to think that Clyde's ashes,
you know,

flew by Pluto and, you know,

he was finally visiting the place that
he discovered, that was pretty cool.

No, I mean, Pluto was a shock
and a revelation in so many ways.

Our jaws just dropped to the floor.

This amazing world

that we would never have known about

unless we sent a spacecraft out there.

But New Horizons didn't stop
after its flyby of Pluto.

It continued on for three more years,

ever deeper into the Kuiper Belt.

The temperatures out there
are almost absolute zero,

so everything is so well preserved.

The Kuiper Belt
completely changed everything.

It's an archaeological dig, if you will,

into the early history
of our solar system,

so it's a scientific wonderland.

On New Year's Day, 2019,

New Horizons had reached
the centre of the region...

...and was about to encounter
the most distant object ever visited.

This object may be
the most primitive object

ever encountered by a spacecraft.

By looking at Ultima Thule today,

we think we're looking back in time
to the origin of the solar system.

Ultima Thule's formation

was perhaps just one
of the countless collisions

that formed the planets themselves.

But somehow it escaped

and was flung far out
into the Kuiper Belt.

The worlds out there,
which are billions of miles from the Sun,

there are so many mysteries kept
in these objects,

and we can really go out and explore them

and start to understand these mysteries.

Like every mission before it,

New Horizons has helped write the story
of the planets.

A story
that we're only just beginning to tell.

Planets of our solar system,
they're in our back yard.

We have the technology to be able to go

and stand on these surfaces.

To be able to go out there
and be able to continue doing something

that's very natural and intrinsic
to all of us,

and that is answering the big questions

and exploring the unknown.

I feel as though
we're absolutely just at the beginning

of planetary exploration.

There's a lot of exploration left for us.