Original subtitles

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.

Music

Even so, messenger still approaches Mercury moving so

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

Slowing on each pass.

Music

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.