All language subtitles for B5EABF1E26D031C2D588B183D131AA6C_eng

The fate of the planets is
intricately tied with that of the

star that they orbit. So for
our own solar system, the sun

dictates our fate. As our
sun has changed, so has the

potential for life on our
neighboring planets. Their history

shows that habitability
is a delicate balance

that doesn't always last.

There is only one planet
that has retained its water and

habitability. And that's Earth.
What's remarkable about

Earth is the stability of those
conditions, that Earth has

been able to maintain
oceanic conditions at its surface

throughout its entire history,
through billions of years,

and that's what's facilitated
the very rich biological

evolution of Earth. Earth is
a very special place. It's the

only place in the universe
where we know definitively that

there is life. For that to
happen, you need not just liquid

water appearing on a planet,
but liquid water staying on a

planet, and that's the magic of Earth.

Thanks to the size and geology
of our planet, the atmosphere

has remained stable enough
for billions of years, protecting

the precious water that has
enabled complex life to evolve.

Life has woven itself into
the fabric of the planet,

and has shaped the
continents and the oceans

so that now, it's time
for us to go back to

Earth. Life itself maintains
the very atmosphere

that protects our fragile ecosystems.

But as the sun continues to grow hotter,

temperatures on
Earth rise, and so does

Earth's atmosphere.
Upsetting weather patterns,

raising great storms across the planet,

and devastating droughts.

As plants around the world
die out, oxygen levels plummet.

Around a billion years from
now, the age of complex life on

Earth will finally draw to a
close. Earth will ultimately be

like Venus and Mars, just
mostly CO2. There'll be nothing

else really in the atmosphere,
and it will remain in that

state as a sort of hot, oven
planet until ultimately the

sun, when it uses up its
hydrogen and goes into a different

phase, will become what
we call a red giant. As we've

seen in other stars throughout
our galaxy, throughout the

universe, our sun will
continue to get hotter and hotter

over time. And at that point,
it will expand greatly to the

point where it will nearly
engulf the Earth entirely, and

at that point, the planets will
lose their atmospheres and

they'll just sort of swept away.

As it exhausts its hydrogen
fuel, the sun's outer edge inflates,

as our star enters
its red giant phase,

expanding millions of
kilometers out into space.

Mercury is the first to be engulfed.

Then Venus' fate is sealed.

Some models predict that Earth may barely
escape the fiery fate of its neighbors,

hanging on beyond the edge
of the dying star with Mars.

But the long era of the four
terrestrial planets will be over.

The lives lived on one of them,
nothing more than a distant memory.

Poor Uranus, it's got
rather boring clouds, truth be

told. So there wasn't a
lot to see with the cameras.

Passing close to
the planet, Voyager

sensors reveal why
Uranus is so featureless.

Temperatures here are the coldest
of any world in the solar system.

There's simply not enough
heat to drive the storm seen

on Jupiter and Saturn. Uranus
is an entirely new class of

planet,

an ice giant.

And more surprises were to come.

Uranus has rings.

We knew long ago from telescopic
observations the rings were

there at Uranus. We didn't
know their configuration or their

shape.

Voyager begins to explore the ring system
and spots something curious in the shadows.

Two moons, Cordelia on the
inner edge of the brightest ring

and Ophelia on the outer edge,
help to organize the system.

Particles within the rings
can be moved inwards or

outwards as these tiny objects
called shepherd moons whose

gravitational force can shape
and sculpt the rings themselves.

But it's the orientation of
the orbits of these moons

and the rings themselves
that singles out Uranus as the

strangest of worlds.

Uranus looks bizarre. The
rings, rather than looking like

that when you look through
a telescope, they're actually

orbiting up and over the top
of the planet. The whole thing

is tipped like a spinning
top. This bizarre orientation

hints at an unusual past.

Since the beginning,
everything in the solar

system has been circling
in the same direction.

It begins with a vast cloud of
material that orbits the young sun.

Over time, the sun begins to move
and over time, the material draws together

forming each of the planets.

Today, they all orbit the sun
and spin on their axes in the

same counterclockwise
direction as that primordial cloud.

Except for Venus and Uranus
which spin in the opposite

direction. But Uranus is even
stranger because the entire

planet is on its side. This
was a really strange system.

So why is Uranus so tipped
on its side? If a roughly Earth

-sized object smashed into
Uranus late in its formation,

then that would have had the
proper amount of momentum to

basically knock it on its side.

And the collision may
also help to explain another

of Uranus' mysteries.
Why the planet is so cold.

That impact presumably
relinquished all of that internal

energy that the planet had
when it first formed and has left

it as a relatively dead and sluggish world.

Much is learnt during this brief encounter.

But Voyager's strangest
discoveries are yet to come.

What powers weather is
very different depending

on where you find
yourself in the solar system.

The Earth's atmosphere and
the weather on Earth is driven

largely by one thing, and
that's the sun. And the sun pours

its energy down through
the atmosphere and it hits the

surface and the surface of
the Earth then warms up. With

Earth, you've got all the
energy coming in from the sun

heating up the surface, creating
the atmosphere and creating

thermals that then drive the
atmospheric flows that we see.

Somewhere like Saturn doesn't
have a surface to be heated

up. So when you compare the
weather systems on these giant

planets to those that we have
on Earth, you have to sort of

tear up the rule book a
little bit. The sun is the great

controller of Earth's atmosphere.
But in the outer reaches

of the solar system where
Saturn lives, sunlight is 100

times weaker. It means some
other heat source must be

driving Saturn's incredible weather.

As Cassini studied the cloud
tops of Saturn, it was able to

infer a huge amount about
the truly strange world that must

lie beneath and the energy
source that helps power this

planet. We had a whole variety
of scientific goals at Saturn

and one of the main ones was
to understand the meteorology

of the Saturn atmosphere and
what energizes the winds that

we see on it and so on. And
we have affirmed now the belief

that atmospheric systems on
Saturn are actually powered by

energy from below from an
internal heat source on Saturn.

They're not powered by
sunlight like we have on the Earth.

Looking deep inside the planet reveals a
heat source shaped by extreme pressure.

Within huge clouds of water,
Cassini records lightning

10 ,000 times more
powerful than any on Earth.

This lightning transforms methane
gas into enormous clouds of soot.

At 8,000 kilometers deep,
the pressure of the atmosphere

is 80 times greater than that
at the bottom of our deepest

oceans. Enough to transform
this graphite rain into diamonds.

But even these diamonds are likely
destroyed by the pressures of Saturn,

eventually dissolving.

30,000 kilometers down and
Saturn's heat source is revealed.

Here, pressures are so intense
that the atmosphere behaves

like a liquid metal, able to
conduct electricity. In this

state, molten helium falls
like rain. And the heat source is

so intense and as it merges
with the surrounding material,

kinetic energy is released
as incredible amounts of heat.

And it's this extraordinary heat
source that helps drive Saturn's weather.

Sent to look for
conditions where life could

have begun is NASA's
Mars Reconnaissance Orbiter.

MRO sends back more data than
all other Mars missions combined.

MRO has three cameras on
board. The first is the Marsy

Weather Camera. It sees
horizon to horizon on every orbit.

So it builds up a map of
the entire planet every day

so you can see a global
weather map every day on Mars.

The second camera is the
Context Camera. It provides high

resolution and it's covered
about 99% of the surface.

MRO has made more than 60,000 orbits.

Its high resolution
cameras revealing Mars in

unprecedented detail.
Discovering polar avalanches.

Shifting sand dunes.

And what look like seasonal flows
of sand or even liquid meltwater.

Then, in 2017, MRO turns its
gaze to one of the red planet's

oldest features, the Aridania
Basin. Thought to have been an

ancient sea. Aridania Basin
is a huge basin in some of the

most ancient crust on Mars.
It formed about 3.8 billion

years ago and it held more
water than 10 times that of the

Great Lakes or three times
that of the Caspian Sea on Earth.

And it is on the ancient sea bed
that MRO sees something remarkable.

MRO saw a massive
400-meter-thick deposit formed from a

mineral that forms in deep
sea hydrothermal environments

such as one that might have undersea vents.

Mars not only had the same
ingredients for life as Earth, it

also had an active environment
to spark it into action. The

Aridania Basin

Aridania Basin was an ancient
sea 3.7 to 3.8 billion years

ago and that's about the
same time when life was first

emerging on Earth. This
might have been a place where

life could have existed because
those hydrothermal vents

underneath that sea might
have created a very conducive

environment for life.
These initial conditions in the

history of both planets
look so similar that it seems

reasonable to expect that
this could eventually lead to

life.

These actively fertile conditions
are thought to survive in

places like the Aridania
Basin for hundreds of millions of

years.

But then, 3.7 billion years
ago, something happens

that transforms prospects
for life on Mars... forever.

Mars underwent a fairly
stupid process of substantial

transformation in its climate.
The climate got colder. What

liquid water there was either
soaked into the ground and

froze or froze at the surface.
A lot of it ultimately would

get transported to the poles where it forms
these big thick ice caps that we see today.

At the same time as the
temperature plummets,

Mars becomes more volcanically active.

Leading to catastrophic flooding.

Water rages down from
the southern highlands

until, in a place known as Echis Chasma, it
plunges over cliffs four kilometers high.

Creating the largest waterfall
the solar system has ever seen.

Cascading into a spectacular
canyon ten kilometers wide

by a hundred kilometers
long. Mars is the largest volcano

Once the floods subside,
the water disappears. The only

trace it ever existed etched
into the planet's surface.

Neptune.

An ice giant. Seventeen times the
mass of Mars and the mass of Earth.

Over one and a half billion
kilometers farther from the

Sun than Uranus, Neptune
is almost chemically identical.

Yet bizarrely, it couldn't
be more different.

Neptune, in a way, was
almost a relief because it wasn't

featureless. You know, it
was like, oh, good, a planet with

clouds and features again.

Neptune was a great excitement. It's
got swirling clouds, really strong winds.

At 1,500 miles an
hour, incredibly fast. The

fastest winds that we've
got in the solar system.

One of the things that
Voyager discovered was

an immense dark vortex
within the atmosphere.

A huge swirling dark spot.

We say it's a giant storm,
but just those words don't fully

convey what's going on. We're
talking about a storm the size

of a planet Earth or larger.

And the surprises keep coming.
Even though it's much farther

out, Voyager discovers
Neptune is warmer than Uranus.

And the source of the heat is
another oddity of this strange

planet. In fact, when we
look at this dynamic activity in

these extraordinarily cold
regions, we really are at a loss

to understand just what's
going on here. One theory is that

the buildup of pressure
beneath the thick layers of cloud

turns the carbon and the
methane into a rain made of

diamonds. They then melt
as they fall into the interior of

the planet, producing the extra heat.

As the heat makes its way
out into the frozen cold of space,

it churns the entire atmosphere,
whipping up winds around

the globe. There are no
mountain ranges, no valleys, no

continental boundaries. To
get in the way of the perfect

fluid dynamical flows.

What that means is that when
you start a weather pattern

going, when you start a vortex
spinning or you start a plume

rising, there's very little
to get in the way of it. So

those winds that develop and
go around, they just keep going

around and around. There's nothing to slow
them down, no friction with the surface.

And the extreme cold here
makes the atmosphere less viscous,

creating supersonic winds that outpace
anything seen on Jupiter or Saturn. Music

Most surprising is Mercury.

The least explored of the
rocky planets. Because of the

difficulty of getting into orbit
around a planet so close to

the sun.

Five, four, three, main
engine start, two, one and zero

and liftoff of messenger on
NASA's mission to Mercury.

A planetary enigma in our
inner solar system. Music

Now going through the
sound barrier. Following a direct

route to Mercury would be
impractical. A spacecraft would

arrive at such high velocity
it would need to haul a

prohibitive amount of heavy
fuel in order to slow down

enough. We just had spacecraft separation.

Groundward solid have jettison.

So messenger controls its
speed by stepping from one

planet to the next. Using
their gravity to slow itself.

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Even so, messenger still
approaches Mercury moving so

fast that it is forced to fly
past the planet three times.

Slowing on each pass.

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Until after nearly seven years of flawless
navigation it arrives safely in orbit.

At last able to begin its mission.
Mapping Mercury's surface.

That first image ever acquired
from orbit around Mercury

was amazing. When it finally
came in it looked perfect. It

looked exactly like we
expected it to be. For me that was

the real moment when I
realized that we had successfully

made it into orbit and
everything was working. This

pioneering voyage has led to
a completely new idea of how

Mercury may have formed.

Music

Mercury is the most cratered
planet in the solar system.

With many puzzling features
that hinted a violent past.

Mercury has a very unusual
orbit. If you were on the surface

of the planet you would actually
have to go around the sun

twice to get one full Mercury
spacecraft. It's a solar day.

Also highly unusual is the
planet's disproportionately large

core. Mercury is basically a
big ball of metal surrounded

like a tiny bit of rock. How
do you end up with a planet

that has so much metal inside
and then almost none on its

surface? That was a huge
mystery. And the major question is

why? Why is it like this when
the rest of the planets aren't?

A clue to understanding these
strange features was found in

Mercury's chemistry. So very
soon after we got into orbit we

started to get chemical
data back from the

surface and we immediately
got some surprises.

The tiny probe detects
volatile chemical elements in

concentrations that no one
had thought possible this close

to the sun.

These are elements
that go into rocks but

that evaporate at
relatively low temperatures.

It immediately told us that
some of the older ideas of

how Mercury formed could
not be correct. What Messenger

discovered about Mercury could suggest a
new twist in the story of the solar system.

Just a few million years
after its formation the young

Mercury is seething with
the heat of its violent birth.

Slowly a crust forms
rich with volatile elements.

If it had been close to the
sun these elements would have

evaporated before the rock
hardened. So what could explain

why they're so abundant on
Mercury today? The orbits of the

planets and their current
locations have changed over the

history of the solar system
so where things are today

doesn't mean that's necessarily
where they formed they could

have and probably did move
from different locations to where

they are now. One possible
thing is that Mercury didn't form

where it is today it formed
much closer to the other planets

maybe even outside of Venus
or Earth or somewhere in between

there. If Mercury had
formed near Earth and

remained at a more
comfortable distance from the sun

its destiny could have been very different.

But it wasn't to be. So what
turned it into the strange sun

scorched world we see today?
We don't know. We don't know

the full story. What we have
right now are theories and as

we have new observations we
are trying to change everything

to fit those observations.

Messenger's evidence of the
volatiles on Mercury's surface

and the unusual size of its
core suggest an interesting new

theory.

It's possible Mercury began
life as much as 160 million

kilometers further from
the sun than it is today.

In the region of space where
the young Earth is also forming

a region with scores of
planetary embryos all fighting for

position.

In the chaos it's possible
that something large

pushes Mercury off orbit
and in towards the sun.

Mercury brushes against
another planetary embryo.

This glancing blow removing
much of its crust and mantle.

Now, little more than a
metallic planetary core

Mercury continues toward the sun.

Ending up in the peculiar elliptical
orbit we see that Mercury can't see today.

Before we sent the Messenger
mission to Mercury I think we

had a very simplistic idea
of what it was going to be like

and Mercury turns out to be
a more complex place with a

more interesting and complex
history than we had previously

imagined. But of course all
things have to come to an end

and once we were out of fuel
we could no longer burn our

engines to keep from crashing into Mercury.

After four years of observations
Messenger's fuel finally runs out.

And the spacecraft adds
yet another crater to this tiny

world. Where any prospects
for life were scorched away when

it was thrown too close to the sun.

The sun's outer corona
burns at a scorching one million

degrees releasing a barrage
of charged particles that travel

at around 400 kilometers per second

the solar wind. The sun's outer corona

This onslaught would strip
away our atmosphere but for the

powerful force that protects
us the Earth's magnetic field.

Solar wind is this stream of
charged particles that comes

streaming out from the sun
and at Earth which has a powerful

magnetic field when those
charged particles begin to get

close to the sun and to Earth
they get diverted around Earth

by interactions with that
magnetic field. That protection

keeps solar wind and other
ionizing radiation off of the

surface. So on Earth where
we have this really great

magnetic field we are nice,
safe, and sound inside the shell

of that protected from all
that radiation. The magnetic

field of Earth effectively forms
a protective bubble around

the Earth's atmosphere. And
when the sun dips below the

horizon there are times when
Earth's protective force field

is visible.

The aurora is a stunning display
of Earth's magnetic field in action.

It's best seen at the
poles but across Earth it's

protecting our atmosphere
and all life on our planet.

This vital protective shield
is generated deep within.

The way a magnetic field is
generated inside a planet is

when you have convective
motion in a fluid that is capable

of conducting electricity.
And in the Earth that

electrically conducting fluid
is liquid iron. And the molten

portion of the Earth's
core is a place where these

motions take place and it
can set up a magnetic field.

Just like Earth, Mars
once had a molten metallic

core generating a magnetic
field around the planet.

The auroras danced above Mars' poles

protecting its atmosphere and seas below.

But the field didn't last.

In the oldest rocks on Mars
you see evidence of a once

powerful magnetic field. You
get to the younger rocks rocks

that are 3 billion 2 billion,
1 billion years old no

evidence of a magnetic field
whatsoever and there is no

intrinsic magnetic field on Mars today.

Half a billion years after it formed,
Mars' magnetic field dies out.

The bright auroras above
its poles slowly fade away.

As the shield that protects
the planet shuts down for good.

Once it stops then what
happens is all the atmospheric

components, things like
hydrogen and oxygen that make up

water, they get stripped away
because you don't have the

shield the magnetic shield
anymore so the high energy

particles that come in from
the Sun and from outer space,

they begin to strip away the
components that make up water.

Without its magnetic
field to protect it Mars'

atmosphere and then
water slip away into space.

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So why did Mars lose its protective shield?

What happened deep beneath its surface that
stopped Mars from developing like Earth?

The answer lies at the beginning
of Mars' story. At its very creation.

4.6 billion years ago when the
planets were forming from the

dust clouds circling the Sun
early differences between Mars

and Earth set the young
planets on very different paths.

Mars forms further from
the Sun where crucially

there is less rocky
material to build a planet.

Mars is different because
it's not just further out it's

actually much smaller if a
planet gets to be too small it

just freezes all the way through.

Mars is just half the diameter of Earth
meaning its core cooled more quickly

and so it lost the
heat that powered the

dynamo that generated
its protective shield.

Mars' small size
condemning the planet to die.

4.6 billion years ago when the
planets were forming from the

dust clouds circling the Sun
early differences between Mars

and Earth

millions of years ago
Saturn has an extra moon

perhaps 400 kilometers across
and formed almost entirely of ice.

But this moon is doomed.
It's orbiting just too close

to resist the immense
forces of Saturn's gravity.

The rings probably formed
from an object that got too close

to Saturn. There's this
invisible boundary around Saturn

called the Roche limit and
that's the limit depending on

what you're made of where
Saturn's gravity is strong enough

that it will actually pull you
apart that the gravity on the

side closer to Saturn is strong
enough that compared to the

gravity on the other side
that will literally rip you apart

you don't have enough gravity
of your own to stay together.

A leading theory suggests
that just beyond Saturn's

atmosphere an ice moon
approaches close to or even just

inside its Roche limit.

As Saturn's immense
gravitational force pulls it

apart the moon begins
to rupture catastrophically.

A world ripped apart
by its proximity to a giant.

Saturn's gravitational
force The moon's

gravitational force pulls
it apart. pulls it apart.

The moon's gravitational force pulls
it apart. The moon's gravitational force

pulls it apart. Up to

17,000 trillion tons of ice breaks
apart in orbit around Saturn.

And thanks to the speeds
this material is traveling it's

likely that in just a few
days it spreads out to encircle

the great giant.

Saturn's iconic ring is now in place.

But as Cassini turns
its instruments towards

it it sees a single
ring transformed.

The images we returned they
were phenomenal. They were

resolution a factor of maybe
20 times better than anything

we had had before.

Cassini reveals how Saturn's
giant rings have evolved.

This debris now forms a
disk wider than Jupiter yet on

average just 10 meters thick.
Within, moon-sized chunks of

ice orbit the structure
clearing great voids.

Turning one ring into many.

But it's as Cassini captures
images with the sun directly

above the equator that the
most surprising feature of the

rings emerges.

We knew this was going to
be a time for us to investigate

the third dimension. Something
that you don't get to see

when you just look at a picture of Saturn's
rings and what we found was staggering.

Just right out of the gate
we saw vertical structures. I

just can't tell you how
surprised we were to see this. It's

just the spectacle of it was
just unanticipated. I have just

imagined flying along in a
shuttlecraft across the ring,

right? Close to the ring so
to my perspective it would be

almost like it was an infinite
sheet of gleaming debris and

I'm flying along and flying
along and flying along and

suddenly I come upon a wall
of rubble that's two miles high.

I mean, is that cool or
what? Really, I've said

over and over again they
should put that in a movie.

This once tiny world of rock
and ice that has seen the

most dramatic transformations
is today the solar system's

greatest jewel. It's the most amazing thing

ever. After more than a decade
in orbit, Cassini has forever

deepened our understanding
of Saturn. But its mission is far

from over.

As the young Jupiter
circles the Sun it clears a path

through the gas cloud that
envelops the early solar system.

But that process causes it
to do something alarming.

Jupiter begins to spiral inwards

plowing straight through the region of
space that would become the asteroid belt.

When the young Jupiter
moves through the primordial

asteroid belt, things get
scattered around, things get

gravitationally deflected,
and as a consequence of all

of this gravitational interaction,
more than 99% of the

original mass that was
there is basically now gone.

The asteroid belt has been
disrupted over time by the

movement of the giant planet
and this has acted to throw

material from the asteroid belt out of the
asteroid belt into the outer solar system.

These protoplanetary cores
like Ceres were never able to

graduate to full-fledged planets
because there was just not

enough material in the
orbital neighborhood.

When Jupiter passed
through the primordial asteroid

belt it starved Ceres of
material halting its growth.

And the dwarf planet was condemned to
live out its life as a cold, barren rock.

But Ceres isn't the only world in our solar
system whose development was cut short.

Mars would suffer a similar
fate at the hands of Jupiter.

After marauding through what
would become the asteroid belt

Jupiter enters the region of
space where Mars is forming.

Continuing its journey
spiraling towards the sun.

As Jupiter bulldozes inwards

its immense gravity scatters
material in all directions.

Some is sent careering into the sun.

And some is thrown
out into interstellar space.

It is because of the
gravitational clearing of this

neighborhood by Jupiter that
Mars was unable to grow to more

than 10% of that of the Earth.
This is why Mars is small.

And by clearing material out
of the inner solar system the

giant planet may also have
prevented the formation of the

super Earths we see in other systems.

And if it had continued moving inward
our planet too might never have formed.

But then just as it looks
like Jupiter will sweep

everything away the giant
planet stops in its tracks.

Because in the shadows of the outer
solar system another planet is forming.

The solar system's second gas giant

Saturn.

And its arrival changes everything.

As Jupiter moved inwards, Saturn
moved inwards and caught up

to Jupiter's orbit. When this
happened the two locked into a

special configuration known as
a mean motion resonance. This

is where the planets begin
to interact gravitationally in a

very coherent manner. And
now locked as a unit the two

reversed their migration
course and moved back out.

Saturn caused Jupiter to

Leaving behind just enough material
from which the inner planets could form.

Venus

and our home.

Earth is in exactly the
right location to provide the

environment for life to
develop. So if Jupiter had moved

around even more, who
knows maybe life never would

have developed in the
solar system as we know it.

And as its voyage across the
solar system draws to a close

Jupiter helps to provide
our living world with its most

precious ingredient.

Today, two thirds of our
planet's surface is covered by

ocean. More than 1.3 billion
cubic kilometers of water.

Each drop teeming with life.

One of the big questions
about the evolution of our solar

system is how did we get
water here on this planet? So we

think that the inner planets
didn't naturally form with a

lot of water. Water is primarily
in the outer solar system.

But if you end up having Jupiter
and Saturn moving outwards,

then that is able to suddenly
deliver more water into the

inner solar system.

Locked in a gravitational
dance with Saturn, Jupiter moves

back through the asteroid
belt. And as it does so, water

-rich material is flung
inward, where it is incorporated

into the growing terrestrial
worlds. So the movement of the

giant planets have actually
played an important role in the

delivery of water to the inner
solar system. In a sense, it

is Jupiter's outward motion
through the asteroid belt that

we have to thank for the
delivery of water to the surface of

the Earth. The Earth
would be quite a different

planet if not for Jupiter's
primordial dance.

Around 100 million
years ago, an asteroid 10

kilometers across is
ejected from the asteroid belt.

Dislodged from its orbit by Jupiter, it
is now on a collision course with Earth.

When it strikes, the impact
generates a fireball so hot

anything within a 1,000
kilometer radius dies instantly.

The impact throws some 300 billion
tons of sulfur up into the atmosphere.

And during the nuclear winter
that follows, 75 percent of

species, plant and animal,
are wiped from the face of the

Earth. Including the largest
creatures ever to walk on its surface.

By driving the dinosaurs
to extinction, Jupiter

cleared the way for
us to inherit the Earth.

And although it still occasionally
flings asteroids our way,

Jupiter also protects us.

For objects heading toward
Earth from the far reaches of the

solar system, the giant planet's
immense gravity acts as a

shield.

Around 100 years ago, a
lump of rock and ice 4 kilometers

across is hurtling towards
the inner solar system. But

Jupiter stands in its way.
Five, four, three, two, one.

We have ignition and liftoff
of Atlantis in the Galileo

spacecraft bound for Jupiter.
Seventy years later, a mission

launches that will witness
just how effective a shield

Jupiter can be.

On board the space shuttle Atlantis,

the Galileo orbiter.

Three years after
Galileo launches, Comet

Shoemaker-Levy 9 has been
captured by Jupiter's gravity.

And now, locked in
the giant planet's deadly

embrace, tidal forces
begin to tear the comet apart.

Its journey into the inner
solar system cuts short. What

happens next presents the
Galileo scientists with a unique

opportunity.

And so everyone was
observing it and they realized two

things. One is that it had just
passed very close to Jupiter

and probably had gotten pulled
apart by Jupiter's tides. And

the other was that it was
going to crash into Jupiter two

years later. The discovery
that a comet was about to

hit Jupiter was a huge
surprise and very exciting.

After crossing the inner
solar system, Galileo is

approaching the spot where
it will witness the comet's final

moments. We knew Galileo
would be able to see the flashes

directly. We knew that the
impacts were going to occur on

the night side and that Galileo
was going to be able to see them.

On July 16th, Galileo
sees the first fragments

of the comet enter Jupiter's
southern hemisphere.

At 60 kilometers per second
the fragments really heated the

atmosphere, red hot almost
to the temperatures of the sun.

And the pieces hit the planet
one after the other. Pieces of

Comet Shoemaker-Levy 9
strike Jupiter over the course of six

days. They looked like
big flashes on Jupiter.

The pictures of the
impact were fantastic.

These tiny points of light
captured from 240 million

kilometers away are the only
time a comet strike has ever

been witnessed.

The most destructive impact
releases energy equivalent to 6

million megatons of TNT.
Leaving behind a giant dark cloud

12,000 kilometers across.

By capturing objects in its
orbit and incinerating them on

impact, Jupiter protects us
from bodies that might otherwise

collide with Earth.