Nova (1974–…): Season 46, Episode 15 - The Planets: Saturn - full transcript
NASA's Cassini probe explores Saturn's icy rings and moons, capturing ring-moon interactions and revealing ingredients for life on the moon Enceladus.
A billion miles from the sun...
Past Mars and Jupiter...
A planet like no other.
All you can see is
this beautiful vision
of the rings around it.
Saturn.
Of all the distant worlds...
Saturn is very weird.
Every time we think
we have Saturn pegged,
we get another piece of evidence
that tells us,
no, everything we thought
was wrong.
It's the most mysterious
and alluring.
It's just a puzzle
you want to solve.
We've visited three times.
Pioneer and Voyager offered
a passing glimpse.
We started taking pictures
several weeks out.
It's a frenzied period
of lots of activity
and sleepless nights
and discovery and conquest.
And then it's all over.
But Cassini came to stay...
And what a wonderland we found.
And to astonish.
We found water ice,
and that basically blew
our minds.
Just right out of the gate,
we saw vertical structures.
We just lacked the imagination
that it would require
to predict
what it would look like.
I think that Enceladus
may be the first place
where we will discover a form
of life
that's independent from life
on the Earth.
And in the end
a heartbreaking choice
to save a world.
I was supposed to call
end of mission.
I couldn't get the words out.
It's still hard.
"The Planets: Saturn."
Right now, on "NOVA."
Major funding for "NOVA"
is provided by the following:
("The Void" by Muse playing)
♪ They'll say
no one can see us
♪ That we're estranged
and all alone
♪ They believe
nothing can reach us
♪ And pull us out
of the boundless gloom
♪ They're wrong
♪ They're wrong
♪ They're wrong
Beyond the warm worlds
of the inner solar system...
beyond the gas giant Jupiter,
in the freezing region
far beyond the sun...
Lies Saturn...
A planet made unique
thanks to a nearly
45,000-mile-wide ring
of frozen water.
Here, trillions of pieces of ice
have been sculpted
by gravitational forces
into some of the solar system's
most stunning vistas.
Even from nearly
a billion miles away,
this planet inspires.
We were about,
13 or 14 years old,
you know, just tootling around
trying to find, you know,
this and that,
and then we saw Saturn,
and it's just so alien
and mysterious-looking.
All you can see is
this beautiful vision
of the rings around it.
It's a thin, flat disc
touching nowhere, you know?
It's, they're just fascinating.
And you just can't help
but wonder
what causes that structure.
Saturn is very weird.
Every time we think
we have Saturn pegged,
we get another piece of evidence
that tells us
no, everything
we thought was wrong.
Saturn is a planet of mysteries.
Why did this world become
so large?
What drives its strange weather?
And where did its beautiful,
bright rings come from?
Today our probes are beginning
to shed light
on these questions,
making discoveries which hint
that Saturn's true beauty
lies beyond its rings,
hidden within
its extraordinary moons.
We now have
what I consider to be
the most promising place to go
look for life.
Will we find life if we go back?
I don't know.
Nobody knows.
But we want to try.
Picture a place with no surface
on which to stand...
only endless atmosphere.
Compared to Earth,
Saturn is so alien
that it's hard to imagine
how it could have grown
from the same ingredients.
So how did this incredible,
mysterious planet come to be?
Saturn begins
as a tiny, ragged world,
tumbling chaotically
through space.
Just like
the inner, rocky worlds,
it clumps together
with other objects to grow.
But there is
a crucial difference.
Saturn is forming far
from the sun.
Among the planets
in the solar system,
Saturn and Jupiter were two
of the first ones to form.
They formed at a very unique
place in the solar system,
what is known as the snow line.
Now, the snow line is
the location
at which the temperature
of the original solar nebula
got cold enough for ice to form.
And that ice then goes on
to form the building blocks
for the larger planets
that are present further out.
Now, what that snow line does is
it puts a large amount
of additional solid material
into this disc.
And so that would accelerate
the growth of the giant planet,
so it would allow them
to form their cores more quickly
than if only rock were present.
As the infant Saturn sweeps
through its orbit,
it's able to scoop up huge
amounts of extra frozen matter
unavailable to the worlds
of the inner solar system.
Under the force of gravity,
this abundant ice
and rock collides and combines,
Helping the young Saturn grow
into a giant.
Just a few million years
after its birth,
Saturn has formed into
a great ball of rock and ice.
So how did this world become
the giant planet of gas
we know today?
The clues to what happened next
would be revealed
by one of the most ambitious
deep-space missions
ever attempted.
The Voyager probes are two
almost identical spacecraft
built to explore the
solar system's distant worlds.
Voyager was unlike anything
that had come before.
It was a long-duration,
tricky, uncertain journey
across the solar system
and beyond.
We started taking pictures
several weeks out,
and Saturn just grew larger
and larger in those images
and more and more detail.
I remember getting down to JPL,
and the whole place was full
of all these big RVs
from all the TV networks,
and I was amazed that
there was all this interest.
So I got in, and
people showed me these images,
and they were just spectacular.
You could start to see evidence
of the banded structure
around Saturn.
You could start to see detail
in the rings,
and it was just sort of like
watching this zoom in
to see Saturn.
They're all these ringlets and
structure and bright and dark,
beautiful, beautiful sight.
Just an amazing harvest
of pictures,
you know, there's tens of
thousands of photographs
of Saturn and its rings
and its moons
taken by the Voyager 2
spacecraft.
It's a frenzied
period of lots of activity
and sleepless nights
and, and discovery and conquest,
and then it's all over.
The Voyager probes
completely transform
our understanding of Saturn...
showing in great detail
the composition
of the upper atmosphere,
almost all of it made
of helium and hydrogen...
The very same gasses abundant
in the early solar system.
It's critical evidence
in piecing together
Saturn's story...
and its evolution
from a rocky world.
Just a few million years
after the formation of the sun,
Saturn is beginning
a radical transformation.
Many times more massive
than the worlds
of the inner solar system...
Its gravity is powerful enough
to draw in hydrogen
and helium gas.
Saturn got a large core,
perhaps 20 times
the mass of the Earth,
that was large enough to cause
all of that of hydrogen
and helium gas around it
to collapse onto the core.
It had enough gravity then
to pull in a lot
of the other gases,
and from that,
to grow this huge,
what we call gas-giant planet.
When you get to a certain size,
then you start grabbing
onto these clouds of water
and hydrogen and helium
that are still hanging around.
It's a runaway effect.
Once you grow a little bit,
now you're going to expand
really fast.
So that's how you grow from
a small, relatively small body
to a huge body
in only a few million years.
Just a few hundred million years
after its birth,
Saturn is now vast,
so big, it could contain
5,000 Earth-sized worlds.
Over time this gas will compress
and condense,
eventually becoming
the second-largest planet
in the solar system.
The Voyager probes reveal some
of the mysteries of Saturn,
but they raise more questions
than they answer.
What is happening beneath
Saturn's strange, hazy exterior?
Does the planet still have
a rocky core?
And what is powering
the massive storms
swirling across its surface?
After Voyager was over,
all of us were just really eager
to go back,
because everything we saw,
we just, you know,
we just skimmed the top,
we didn't have time to delve
into.
Just a few years
after Voyager's flyby,
planning begins on a new probe
loaded with instruments
to uncover Saturn's secrets.
Building a craft like this
is a huge challenge.
It's difficult to put a number
on how many people were involved
in Cassini,
um...
tens of thousands, I would say.
There's over ten miles of wiring
in the main harness alone.
And you have to check out
every wire.
This is a very, very
complex effort,
so it took us
two-and-a-half years
to put the thing together.
All of us spent a lot of effort
to get to that moment, uh,
so there's a lot of emotion
attached to it.
And then you see it launch,
and you know
that it's on a journey
to a very, very distant place.
And so you feel
like you're going too.
Cassini is one of the
heaviest interplanetary probes
ever launched,
weighing in at over six tons.
But its huge size complicates
its route to Saturn.
To be able to launch a
spacecraft as large as Cassini
with enough fuel to be able
to do exciting science out
at Saturn,
you can't use all that fuel up
in getting out to Saturn.
And so what is done is
we use flybys past
other planets.
It's almost like
two sort of billiard balls...
If the moving one bumps
into the stationary one,
it gives the stationary one
a bit of energy.
So it's an exchange of energy.
The spacecraft is flown
really close to a planet,
gets a bit of a kick,
and gets kicked out
onto the next one.
The journey to Saturn will
require two trips past Venus,
one past Earth,
and finally a speed boost
from Jupiter.
These maneuvers help Cassini
reach speeds
of over 60,000 miles per hour.
But the probe's most difficult
moment is yet to come.
The one critical sequence,
going into orbit around Saturn.
You can get there,
you throw a rock,
and the rock flies,
but when we get there,
we have to stop.
So we had to fire the rocket
at exactly the right time...
in exactly the right direction
for exactly
the right amount of time,
or after all that planning,
we just go right by.
The time to get data from Saturn
to the Earth is 90 minutes.
So if you send a command
to the spacecraft,
it takes 90 minutes to get
out there,
lets the command work it,
and then it takes 90 minutes
to get back.
Until you get there,
there's always this chance
in your mind
that something's not right.
The Doppler has flattened out.
So there was a great deal
of relief.
When Cassini arrives,
the images it sends back
to Earth are breathtaking.
I was sitting
in what they call the blue room
being interviewed while they
were beaming for the first time,
and then one of these pictures
came on,
and I just went,
"Wow, look at that."
That was really
the jumping-out-of-the-chair
moment for me,
and it was just the first day.
What we were seeing was
the highest resolution
that had ever been seen.
I saw phenomena
that I'd only seen
in kind of computer simulations
before,
and now I was seeing it
for the first time, um,
and realizing all,
all this was real.
And we were seeing details
in the rings
that were shocking, shocking.
We just lacked the imagination
that it would require
to predict
what it would look like.
Thanks to
Cassini's powerful instruments,
Saturn's billion-year-old
mysteries
start to become clearer.
Peering into
Saturn's tumultuous atmosphere
produces new insights about
the planet's distant past.
One of the biggest questions is
what lies within the clouds.
You see clouds of different
things floating around,
but what are
those clouds made out of?
We didn't really know
until we got there with Cassini.
Some of the clouds actually are
made of water ice.
We found water ice,
and that basically blew
our minds.
It showed there is water
down there.
The discovery of icy clouds
along with the measurements of
the gravitational field
of the giant planet
hint at what may have happened
to the young, rocky ice planet.
Data collected by Cassini
suggests that Saturn no longer
consists of a rocky core
surrounded by a vast atmosphere,
but rather it is a single object
with no distinct boundaries.
Saturn doesn't really have
a solid surface anywhere.
That core that presumably was
there in the first place
would effectively disintegrated
or been mixed in
with all of the other material
that was present.
So it may be today that
the material is still there,
but it's completely distributed
within the envelope
of Saturn itself.
What began as a rocky, icy world
is now a fully fledged
gas giant.
And within
its gaseous atmosphere,
incredible mechanisms
are at play.
Saturn's atmosphere is
characterized
by many unusual,
even bizarre features.
Huge weather systems that take
on strange forms,
persisting
for hundreds of years...
perhaps none more striking
than the formation nicknamed
the giant hexagon.
It's over 16,000 miles across,
big enough to swallow almost
four Earth-sized objects.
But what could be driving this
huge planetary weather system?
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, 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 thermals that then,
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 rulebook 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 weather,
something deep within...
Creating huge, complex forms
that arise in its atmosphere.
As Cassini studies
the cloud tops of Saturn,
it's 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 5,000 miles deep,
the pressure of the atmosphere
is 80 times greater
than that at the bottom
of our deepest oceans...
Enough to transform
this sooty, graphite rain...
into diamonds.
But even these diamonds are
likely destroyed
by the pressures of Saturn...
eventually dissolving.
18,000 miles down,
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.
As this strange rain 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.
Within just a few hundred
million years of its birth,
Saturn has witnessed
great drama.
Now it will remain
largely unchanged
for billions of years,
vast but still very different
from the planet we know today.
And in time,
its great size will lead
to its ultimate,
iconic transformation.
The rings have always made
Saturn kind of special.
If you ask a kid to draw
a picture of a planet,
they will draw a picture
of Saturn.
Saturn's rings are what
you would call a debris disc.
It's a collection
of icy particles
ranging in size
from the largest being the size
of small apartment buildings,
all the way down to this... the
tiniest little piece of dust.
But the sparkling brilliance of
Saturn's rings are a mystery.
They're so white and purely ice,
but we know that Saturn's rings
are being bombarded all the time
by meteorites
from the outer solar system,
which are not white,
they're dark,
they're sooty like charcoal.
We know that the rings are
exposed to in-falling material
and that they appear to be
contaminated
by this material.
I like to give the analogy
of fresh snow in the city.
Eventually over time, the snow
just kind of gets dirty.
So why are the rings so bright?
Thanks to a series of flybys,
Cassini is able to make
a startling discover.
They cannot stay
as bright and clean
as they are today
if they're much older
than one hundred million years.
At the end of the day,
you have to accept the evidence,
and the evidence says
that the rings are actually
much younger than we thought.
So it means that during
the time of the dinosaurs,
Saturn didn't have rings.
Some velociraptor with
a telescope looking at Saturn
would not have seen rings.
So if the rings are young,
then where did they come from?
Cassini would give us hints
that the answer lies
not with planet itself,
but with the worlds trapped
in orbit around it:
Saturn's moons.
The surfaces of these moons are
like storybooks,
telling us about the history
of how the Saturn system,
how these moons evolved.
Our job is to unravel the story
that that moon is telling us.
Saturn has 62 moons.
Some of them look
like flying saucers
or, or pancakes or potatoes,
very irregularly shaped.
Once a moon gets bigger,
its gravity tends
to round it out,
and you get more of
a rounded, nicely shaped moon.
The moons are
the archaeological debris
from planet formation,
and the, their size,
their orbits, uh,
even their composition,
tell us about the details
of the environment
within which they form.
So it's possible
in the case of Saturn
that we can use
the different moons
to understand different eras,
different epochs
of the evolution of Saturn.
As Cassini continues
its journey,
it reveals many moons
made almost entirely of ice.
And some of them take on
extraordinary forms
in and around the rings.
Cassini analyzes the ice moons
in ever-greater detail,
and it becomes apparent
many of them are made
of the same icy material
as the rings themselves,
which suggests
that Saturn's rings
may once have been a moon.
The satellite system that
we're seeing around Saturn now
is probably not the original
satellite system that it had,
because if the rings
were created
by the destruction of
one or two preexisting moons,
it means that
there were moons before
that don't exist any longer.
Millions of years ago,
Saturn has an extra moon...
Perhaps 250 miles 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
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,
it 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.
Up to 17,000 trillion tons
of ice...
breaks apart
in orbit around Saturn.
And thanks to the speeds
this material is travelling,
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 factor
of maybe 20 times better
than anything we had had before.
Today Cassini reveals
how Saturn's giant rings have
evolved.
This debris now forms a disc
wider than Jupiter...
Yet on average
just 30 feet 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 shuttle-craft
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, 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.
After more than decade in orbit,
Cassini has forever deepened
our understanding of Saturn.
But its mission is far
from over.
Because just beyond the rings
lies another treasure:
a tiny world
that may hold answers
to some of our deepest questions
about the possibility of life
in the solar system...
The ice moon Enceladus.
Enceladus is one the moons in
a regular orbit around Saturn.
It's quite small,
its diameter is 500 kilometers.
From far away it looks smooth.
When we look at it close up,
we see that those smooth areas
are really places
where there's been fracture
upon fracture,
fault upon fault,
destroying, disrupting
the surface.
We got to see these
five prominent fractures,
deep, long fractures crossing
this region.
And eventually, over time,
we were able to find
that there were geysers coming
from those fractures.
Cassini discovers this tiny moon
is alive with activity.
Its giant plumes eject over 500
pounds of ice and water vapor
into space every second.
I think for me and for
a lot of scientists on Cassini,
the biggest surprise was
that Enceladus was active.
There's this little moon
out there
which is supposed to be dead,
and it's spewing out this vast
amount of water vapor.
It became clear
once we had found the plume
that we needed to get
as deep into it as possible.
Cassini, over 13 years,
did 23 flybys.
We flew through the plume
in about a dozen of those.
Piloted from
nearly a billion miles away...
Cassini, on its closest flyby,
passes within just 30 miles
of the surface of Enceladus.
And what its instruments detect
is breathtaking.
We were able to sample directly
material coming out
of Enceladus's plume.
And what a wonderland we found.
We are sampling a subterranean
or subsurface ocean.
And indeed Cassini,
as it flew through the plume,
found, uh, not only water vapor
and water ice,
but salts.
Some of these particles,
the fact that they're salty,
tells us almost certainly
that they came
from an underground ocean.
Under that ice
there is an ocean,
and then beneath that ocean
is rock.
And it is there
because of Saturn.
As Enceladus moves
around the planet,
its vast gravitational force
pulls at the moon,
holding it in orbit.
But every couple of orbits,
another larger moon called Dione
draws Enceladus back.
This process repeatedly
stretches and squeezes its core,
warming and melting
the icy interior.
But it's what Cassini finds next
that changes everything.
As the plumes are analyzed
in ever greater detail,
scientists discover
complex organic compounds.
Once we got over our shock,
we then started focusing
on the fact that there was
organic material leaking out
from this little moon as well,
and then everyone got excited.
So here you have a hot,
liquid, salty, organic-rich,
subsurface ocean,
and a lot of people feel that
this is one of the better places
where life may even exist today
in the solar system.
I don't know what to think.
I just know that, you know,
Enceladus is a place
that we know more about.
It's a,
it's an alien environment
that meets
all our formal requirements
for a place
that could support life.
Deep beneath
Enceladus's icy shell,
hot rock is in contact
with water...
Almost certainly creating
hydrothermal vents.
On Earth, these types of vents
support a multitude of life,
and leading theories suggest
that these might have been
the setting
for the emergence
of life on Earth.
Which makes some wonder
if the same thing is happening
on Enceladus.
I think that Enceladus
may be the first place
where we will discover
a form of life
that's independent
from life on the Earth,
which would be one of
the most spectacular discoveries
that science has ever made.
If it doesn't have life,
that would be equally amazing
too,
because here you have
the conditions right for life,
and yet it didn't form.
So maybe you need something else
to get life started.
But for Cassini's scientists,
the exotic world
under the ice of Enceladus
is a bittersweet discovery.
As we were approaching
the end-of-mission phase,
we were essentially running out
of fuel,
we knew we had to end
the mission somehow.
And to protect,
in particular protect Enceladus,
because it would be pretty bad
to go back to Enceladus
and discover life,
and it turned out to be Earth
microbes on Enceladus, so...
We didn't sterilize Cassini.
We didn't know we had to.
You don't want to go
to a place like Enceladus
and find something
that the previous spacecraft
delivered there.
This is ACS One, we just had
transition to high rate mode.
Cassini ends its mission
in spectacular fashion,
making a series
of ever-closer orbits
to study the planet's rings
before a final dive
into Saturn's atmosphere,
a journey from which
it will never return.
Some of us were concerned
the spacecraft wouldn't even
survive the first orbit,
because we were going
through the gap
between the rings
and the atmosphere of Saturn.
We didn't know what was there.
We thought it was empty,
but we weren't sure.
Cassini survives its approach
to Saturn intact
and continues on
towards its demise.
We're out at JPL
watching the signal coming
from the spacecraft,
and there was a spike.
And it went away,
but it seemed to come back.
Because the spacecraft hadn't
been told its life was ending,
and so it had been programmed
to keep in touch with the Earth.
So it moved itself.
Okay, we call loss of signal
in one-one-five-five-four-six.
The last command
that my colleagues uploaded
to the spacecraft
was a list of the 200 or some
scientists and engineers
that had been involved
in the development
of one of the instruments,
which means that as Cassini was
literally disintegrating,
in its computer memory was
a final thought of home.
And I think that's just
a tremendous, tremendous thing.
We have just heard
the signal from the spacecraft
is gone,
and within the next 45 seconds,
so will be the spacecraft.
And then it lost control,
it began to tumble.
The world that had borne witness
to some of the greatest dramas
in the solar system's history
consumes the craft that has
given us its remarkable story.
Congratulations to you all.
I was supposed to call
end of mission.
"Thanks and Godspeed Cassini."
I couldn't get the words out.
And there was some applause
but mostly people just...
People were just looking
at each other.
It was, it was, it was so sad.
Really, really sad.
It's still hard.
We journeyed out to Saturn
to see this great beauty
up close,
to understand its deep history,
and explore its rings.
But in the end we were rewarded
with something
far more profound:
early hints
of a second home for life.
It's sad that Cassini is no
longer up there doing its thing,
but it has left us
a tremendous legacy of data
that is going
to keep us all busy for decades.
It's revealed so much,
and there's still secrets in
that data
that are going to become
unlocked.
I'm convinced graduate students
20 years from now
will be finding new things
in that data
that we didn't, didn't know were
there.
Now when I go out in the
night sky and look at Saturn,
I know Cassini is there,
that the bits of Cassini will
always be part of Saturn.
Saturn will never be quite
the same.
We landed a device
of our own making
on a world in...
in the outer solar system.
We spent 13 years, you know,
in the...
just methodically, quietly...
just monitoring, observing.
And... it's...
it, to me, it represents
the best that humanity has
to give.
It's really...
it's us at our finest.
Major funding for "NOVA"
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visit ShopPBS
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Past Mars and Jupiter...
A planet like no other.
All you can see is
this beautiful vision
of the rings around it.
Saturn.
Of all the distant worlds...
Saturn is very weird.
Every time we think
we have Saturn pegged,
we get another piece of evidence
that tells us,
no, everything we thought
was wrong.
It's the most mysterious
and alluring.
It's just a puzzle
you want to solve.
We've visited three times.
Pioneer and Voyager offered
a passing glimpse.
We started taking pictures
several weeks out.
It's a frenzied period
of lots of activity
and sleepless nights
and discovery and conquest.
And then it's all over.
But Cassini came to stay...
And what a wonderland we found.
And to astonish.
We found water ice,
and that basically blew
our minds.
Just right out of the gate,
we saw vertical structures.
We just lacked the imagination
that it would require
to predict
what it would look like.
I think that Enceladus
may be the first place
where we will discover a form
of life
that's independent from life
on the Earth.
And in the end
a heartbreaking choice
to save a world.
I was supposed to call
end of mission.
I couldn't get the words out.
It's still hard.
"The Planets: Saturn."
Right now, on "NOVA."
Major funding for "NOVA"
is provided by the following:
("The Void" by Muse playing)
♪ They'll say
no one can see us
♪ That we're estranged
and all alone
♪ They believe
nothing can reach us
♪ And pull us out
of the boundless gloom
♪ They're wrong
♪ They're wrong
♪ They're wrong
Beyond the warm worlds
of the inner solar system...
beyond the gas giant Jupiter,
in the freezing region
far beyond the sun...
Lies Saturn...
A planet made unique
thanks to a nearly
45,000-mile-wide ring
of frozen water.
Here, trillions of pieces of ice
have been sculpted
by gravitational forces
into some of the solar system's
most stunning vistas.
Even from nearly
a billion miles away,
this planet inspires.
We were about,
13 or 14 years old,
you know, just tootling around
trying to find, you know,
this and that,
and then we saw Saturn,
and it's just so alien
and mysterious-looking.
All you can see is
this beautiful vision
of the rings around it.
It's a thin, flat disc
touching nowhere, you know?
It's, they're just fascinating.
And you just can't help
but wonder
what causes that structure.
Saturn is very weird.
Every time we think
we have Saturn pegged,
we get another piece of evidence
that tells us
no, everything
we thought was wrong.
Saturn is a planet of mysteries.
Why did this world become
so large?
What drives its strange weather?
And where did its beautiful,
bright rings come from?
Today our probes are beginning
to shed light
on these questions,
making discoveries which hint
that Saturn's true beauty
lies beyond its rings,
hidden within
its extraordinary moons.
We now have
what I consider to be
the most promising place to go
look for life.
Will we find life if we go back?
I don't know.
Nobody knows.
But we want to try.
Picture a place with no surface
on which to stand...
only endless atmosphere.
Compared to Earth,
Saturn is so alien
that it's hard to imagine
how it could have grown
from the same ingredients.
So how did this incredible,
mysterious planet come to be?
Saturn begins
as a tiny, ragged world,
tumbling chaotically
through space.
Just like
the inner, rocky worlds,
it clumps together
with other objects to grow.
But there is
a crucial difference.
Saturn is forming far
from the sun.
Among the planets
in the solar system,
Saturn and Jupiter were two
of the first ones to form.
They formed at a very unique
place in the solar system,
what is known as the snow line.
Now, the snow line is
the location
at which the temperature
of the original solar nebula
got cold enough for ice to form.
And that ice then goes on
to form the building blocks
for the larger planets
that are present further out.
Now, what that snow line does is
it puts a large amount
of additional solid material
into this disc.
And so that would accelerate
the growth of the giant planet,
so it would allow them
to form their cores more quickly
than if only rock were present.
As the infant Saturn sweeps
through its orbit,
it's able to scoop up huge
amounts of extra frozen matter
unavailable to the worlds
of the inner solar system.
Under the force of gravity,
this abundant ice
and rock collides and combines,
Helping the young Saturn grow
into a giant.
Just a few million years
after its birth,
Saturn has formed into
a great ball of rock and ice.
So how did this world become
the giant planet of gas
we know today?
The clues to what happened next
would be revealed
by one of the most ambitious
deep-space missions
ever attempted.
The Voyager probes are two
almost identical spacecraft
built to explore the
solar system's distant worlds.
Voyager was unlike anything
that had come before.
It was a long-duration,
tricky, uncertain journey
across the solar system
and beyond.
We started taking pictures
several weeks out,
and Saturn just grew larger
and larger in those images
and more and more detail.
I remember getting down to JPL,
and the whole place was full
of all these big RVs
from all the TV networks,
and I was amazed that
there was all this interest.
So I got in, and
people showed me these images,
and they were just spectacular.
You could start to see evidence
of the banded structure
around Saturn.
You could start to see detail
in the rings,
and it was just sort of like
watching this zoom in
to see Saturn.
They're all these ringlets and
structure and bright and dark,
beautiful, beautiful sight.
Just an amazing harvest
of pictures,
you know, there's tens of
thousands of photographs
of Saturn and its rings
and its moons
taken by the Voyager 2
spacecraft.
It's a frenzied
period of lots of activity
and sleepless nights
and, and discovery and conquest,
and then it's all over.
The Voyager probes
completely transform
our understanding of Saturn...
showing in great detail
the composition
of the upper atmosphere,
almost all of it made
of helium and hydrogen...
The very same gasses abundant
in the early solar system.
It's critical evidence
in piecing together
Saturn's story...
and its evolution
from a rocky world.
Just a few million years
after the formation of the sun,
Saturn is beginning
a radical transformation.
Many times more massive
than the worlds
of the inner solar system...
Its gravity is powerful enough
to draw in hydrogen
and helium gas.
Saturn got a large core,
perhaps 20 times
the mass of the Earth,
that was large enough to cause
all of that of hydrogen
and helium gas around it
to collapse onto the core.
It had enough gravity then
to pull in a lot
of the other gases,
and from that,
to grow this huge,
what we call gas-giant planet.
When you get to a certain size,
then you start grabbing
onto these clouds of water
and hydrogen and helium
that are still hanging around.
It's a runaway effect.
Once you grow a little bit,
now you're going to expand
really fast.
So that's how you grow from
a small, relatively small body
to a huge body
in only a few million years.
Just a few hundred million years
after its birth,
Saturn is now vast,
so big, it could contain
5,000 Earth-sized worlds.
Over time this gas will compress
and condense,
eventually becoming
the second-largest planet
in the solar system.
The Voyager probes reveal some
of the mysteries of Saturn,
but they raise more questions
than they answer.
What is happening beneath
Saturn's strange, hazy exterior?
Does the planet still have
a rocky core?
And what is powering
the massive storms
swirling across its surface?
After Voyager was over,
all of us were just really eager
to go back,
because everything we saw,
we just, you know,
we just skimmed the top,
we didn't have time to delve
into.
Just a few years
after Voyager's flyby,
planning begins on a new probe
loaded with instruments
to uncover Saturn's secrets.
Building a craft like this
is a huge challenge.
It's difficult to put a number
on how many people were involved
in Cassini,
um...
tens of thousands, I would say.
There's over ten miles of wiring
in the main harness alone.
And you have to check out
every wire.
This is a very, very
complex effort,
so it took us
two-and-a-half years
to put the thing together.
All of us spent a lot of effort
to get to that moment, uh,
so there's a lot of emotion
attached to it.
And then you see it launch,
and you know
that it's on a journey
to a very, very distant place.
And so you feel
like you're going too.
Cassini is one of the
heaviest interplanetary probes
ever launched,
weighing in at over six tons.
But its huge size complicates
its route to Saturn.
To be able to launch a
spacecraft as large as Cassini
with enough fuel to be able
to do exciting science out
at Saturn,
you can't use all that fuel up
in getting out to Saturn.
And so what is done is
we use flybys past
other planets.
It's almost like
two sort of billiard balls...
If the moving one bumps
into the stationary one,
it gives the stationary one
a bit of energy.
So it's an exchange of energy.
The spacecraft is flown
really close to a planet,
gets a bit of a kick,
and gets kicked out
onto the next one.
The journey to Saturn will
require two trips past Venus,
one past Earth,
and finally a speed boost
from Jupiter.
These maneuvers help Cassini
reach speeds
of over 60,000 miles per hour.
But the probe's most difficult
moment is yet to come.
The one critical sequence,
going into orbit around Saturn.
You can get there,
you throw a rock,
and the rock flies,
but when we get there,
we have to stop.
So we had to fire the rocket
at exactly the right time...
in exactly the right direction
for exactly
the right amount of time,
or after all that planning,
we just go right by.
The time to get data from Saturn
to the Earth is 90 minutes.
So if you send a command
to the spacecraft,
it takes 90 minutes to get
out there,
lets the command work it,
and then it takes 90 minutes
to get back.
Until you get there,
there's always this chance
in your mind
that something's not right.
The Doppler has flattened out.
So there was a great deal
of relief.
When Cassini arrives,
the images it sends back
to Earth are breathtaking.
I was sitting
in what they call the blue room
being interviewed while they
were beaming for the first time,
and then one of these pictures
came on,
and I just went,
"Wow, look at that."
That was really
the jumping-out-of-the-chair
moment for me,
and it was just the first day.
What we were seeing was
the highest resolution
that had ever been seen.
I saw phenomena
that I'd only seen
in kind of computer simulations
before,
and now I was seeing it
for the first time, um,
and realizing all,
all this was real.
And we were seeing details
in the rings
that were shocking, shocking.
We just lacked the imagination
that it would require
to predict
what it would look like.
Thanks to
Cassini's powerful instruments,
Saturn's billion-year-old
mysteries
start to become clearer.
Peering into
Saturn's tumultuous atmosphere
produces new insights about
the planet's distant past.
One of the biggest questions is
what lies within the clouds.
You see clouds of different
things floating around,
but what are
those clouds made out of?
We didn't really know
until we got there with Cassini.
Some of the clouds actually are
made of water ice.
We found water ice,
and that basically blew
our minds.
It showed there is water
down there.
The discovery of icy clouds
along with the measurements of
the gravitational field
of the giant planet
hint at what may have happened
to the young, rocky ice planet.
Data collected by Cassini
suggests that Saturn no longer
consists of a rocky core
surrounded by a vast atmosphere,
but rather it is a single object
with no distinct boundaries.
Saturn doesn't really have
a solid surface anywhere.
That core that presumably was
there in the first place
would effectively disintegrated
or been mixed in
with all of the other material
that was present.
So it may be today that
the material is still there,
but it's completely distributed
within the envelope
of Saturn itself.
What began as a rocky, icy world
is now a fully fledged
gas giant.
And within
its gaseous atmosphere,
incredible mechanisms
are at play.
Saturn's atmosphere is
characterized
by many unusual,
even bizarre features.
Huge weather systems that take
on strange forms,
persisting
for hundreds of years...
perhaps none more striking
than the formation nicknamed
the giant hexagon.
It's over 16,000 miles across,
big enough to swallow almost
four Earth-sized objects.
But what could be driving this
huge planetary weather system?
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, 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 thermals that then,
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 rulebook 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 weather,
something deep within...
Creating huge, complex forms
that arise in its atmosphere.
As Cassini studies
the cloud tops of Saturn,
it's 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 5,000 miles deep,
the pressure of the atmosphere
is 80 times greater
than that at the bottom
of our deepest oceans...
Enough to transform
this sooty, graphite rain...
into diamonds.
But even these diamonds are
likely destroyed
by the pressures of Saturn...
eventually dissolving.
18,000 miles down,
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.
As this strange rain 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.
Within just a few hundred
million years of its birth,
Saturn has witnessed
great drama.
Now it will remain
largely unchanged
for billions of years,
vast but still very different
from the planet we know today.
And in time,
its great size will lead
to its ultimate,
iconic transformation.
The rings have always made
Saturn kind of special.
If you ask a kid to draw
a picture of a planet,
they will draw a picture
of Saturn.
Saturn's rings are what
you would call a debris disc.
It's a collection
of icy particles
ranging in size
from the largest being the size
of small apartment buildings,
all the way down to this... the
tiniest little piece of dust.
But the sparkling brilliance of
Saturn's rings are a mystery.
They're so white and purely ice,
but we know that Saturn's rings
are being bombarded all the time
by meteorites
from the outer solar system,
which are not white,
they're dark,
they're sooty like charcoal.
We know that the rings are
exposed to in-falling material
and that they appear to be
contaminated
by this material.
I like to give the analogy
of fresh snow in the city.
Eventually over time, the snow
just kind of gets dirty.
So why are the rings so bright?
Thanks to a series of flybys,
Cassini is able to make
a startling discover.
They cannot stay
as bright and clean
as they are today
if they're much older
than one hundred million years.
At the end of the day,
you have to accept the evidence,
and the evidence says
that the rings are actually
much younger than we thought.
So it means that during
the time of the dinosaurs,
Saturn didn't have rings.
Some velociraptor with
a telescope looking at Saturn
would not have seen rings.
So if the rings are young,
then where did they come from?
Cassini would give us hints
that the answer lies
not with planet itself,
but with the worlds trapped
in orbit around it:
Saturn's moons.
The surfaces of these moons are
like storybooks,
telling us about the history
of how the Saturn system,
how these moons evolved.
Our job is to unravel the story
that that moon is telling us.
Saturn has 62 moons.
Some of them look
like flying saucers
or, or pancakes or potatoes,
very irregularly shaped.
Once a moon gets bigger,
its gravity tends
to round it out,
and you get more of
a rounded, nicely shaped moon.
The moons are
the archaeological debris
from planet formation,
and the, their size,
their orbits, uh,
even their composition,
tell us about the details
of the environment
within which they form.
So it's possible
in the case of Saturn
that we can use
the different moons
to understand different eras,
different epochs
of the evolution of Saturn.
As Cassini continues
its journey,
it reveals many moons
made almost entirely of ice.
And some of them take on
extraordinary forms
in and around the rings.
Cassini analyzes the ice moons
in ever-greater detail,
and it becomes apparent
many of them are made
of the same icy material
as the rings themselves,
which suggests
that Saturn's rings
may once have been a moon.
The satellite system that
we're seeing around Saturn now
is probably not the original
satellite system that it had,
because if the rings
were created
by the destruction of
one or two preexisting moons,
it means that
there were moons before
that don't exist any longer.
Millions of years ago,
Saturn has an extra moon...
Perhaps 250 miles 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
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,
it 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.
Up to 17,000 trillion tons
of ice...
breaks apart
in orbit around Saturn.
And thanks to the speeds
this material is travelling,
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 factor
of maybe 20 times better
than anything we had had before.
Today Cassini reveals
how Saturn's giant rings have
evolved.
This debris now forms a disc
wider than Jupiter...
Yet on average
just 30 feet 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 shuttle-craft
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, 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.
After more than decade in orbit,
Cassini has forever deepened
our understanding of Saturn.
But its mission is far
from over.
Because just beyond the rings
lies another treasure:
a tiny world
that may hold answers
to some of our deepest questions
about the possibility of life
in the solar system...
The ice moon Enceladus.
Enceladus is one the moons in
a regular orbit around Saturn.
It's quite small,
its diameter is 500 kilometers.
From far away it looks smooth.
When we look at it close up,
we see that those smooth areas
are really places
where there's been fracture
upon fracture,
fault upon fault,
destroying, disrupting
the surface.
We got to see these
five prominent fractures,
deep, long fractures crossing
this region.
And eventually, over time,
we were able to find
that there were geysers coming
from those fractures.
Cassini discovers this tiny moon
is alive with activity.
Its giant plumes eject over 500
pounds of ice and water vapor
into space every second.
I think for me and for
a lot of scientists on Cassini,
the biggest surprise was
that Enceladus was active.
There's this little moon
out there
which is supposed to be dead,
and it's spewing out this vast
amount of water vapor.
It became clear
once we had found the plume
that we needed to get
as deep into it as possible.
Cassini, over 13 years,
did 23 flybys.
We flew through the plume
in about a dozen of those.
Piloted from
nearly a billion miles away...
Cassini, on its closest flyby,
passes within just 30 miles
of the surface of Enceladus.
And what its instruments detect
is breathtaking.
We were able to sample directly
material coming out
of Enceladus's plume.
And what a wonderland we found.
We are sampling a subterranean
or subsurface ocean.
And indeed Cassini,
as it flew through the plume,
found, uh, not only water vapor
and water ice,
but salts.
Some of these particles,
the fact that they're salty,
tells us almost certainly
that they came
from an underground ocean.
Under that ice
there is an ocean,
and then beneath that ocean
is rock.
And it is there
because of Saturn.
As Enceladus moves
around the planet,
its vast gravitational force
pulls at the moon,
holding it in orbit.
But every couple of orbits,
another larger moon called Dione
draws Enceladus back.
This process repeatedly
stretches and squeezes its core,
warming and melting
the icy interior.
But it's what Cassini finds next
that changes everything.
As the plumes are analyzed
in ever greater detail,
scientists discover
complex organic compounds.
Once we got over our shock,
we then started focusing
on the fact that there was
organic material leaking out
from this little moon as well,
and then everyone got excited.
So here you have a hot,
liquid, salty, organic-rich,
subsurface ocean,
and a lot of people feel that
this is one of the better places
where life may even exist today
in the solar system.
I don't know what to think.
I just know that, you know,
Enceladus is a place
that we know more about.
It's a,
it's an alien environment
that meets
all our formal requirements
for a place
that could support life.
Deep beneath
Enceladus's icy shell,
hot rock is in contact
with water...
Almost certainly creating
hydrothermal vents.
On Earth, these types of vents
support a multitude of life,
and leading theories suggest
that these might have been
the setting
for the emergence
of life on Earth.
Which makes some wonder
if the same thing is happening
on Enceladus.
I think that Enceladus
may be the first place
where we will discover
a form of life
that's independent
from life on the Earth,
which would be one of
the most spectacular discoveries
that science has ever made.
If it doesn't have life,
that would be equally amazing
too,
because here you have
the conditions right for life,
and yet it didn't form.
So maybe you need something else
to get life started.
But for Cassini's scientists,
the exotic world
under the ice of Enceladus
is a bittersweet discovery.
As we were approaching
the end-of-mission phase,
we were essentially running out
of fuel,
we knew we had to end
the mission somehow.
And to protect,
in particular protect Enceladus,
because it would be pretty bad
to go back to Enceladus
and discover life,
and it turned out to be Earth
microbes on Enceladus, so...
We didn't sterilize Cassini.
We didn't know we had to.
You don't want to go
to a place like Enceladus
and find something
that the previous spacecraft
delivered there.
This is ACS One, we just had
transition to high rate mode.
Cassini ends its mission
in spectacular fashion,
making a series
of ever-closer orbits
to study the planet's rings
before a final dive
into Saturn's atmosphere,
a journey from which
it will never return.
Some of us were concerned
the spacecraft wouldn't even
survive the first orbit,
because we were going
through the gap
between the rings
and the atmosphere of Saturn.
We didn't know what was there.
We thought it was empty,
but we weren't sure.
Cassini survives its approach
to Saturn intact
and continues on
towards its demise.
We're out at JPL
watching the signal coming
from the spacecraft,
and there was a spike.
And it went away,
but it seemed to come back.
Because the spacecraft hadn't
been told its life was ending,
and so it had been programmed
to keep in touch with the Earth.
So it moved itself.
Okay, we call loss of signal
in one-one-five-five-four-six.
The last command
that my colleagues uploaded
to the spacecraft
was a list of the 200 or some
scientists and engineers
that had been involved
in the development
of one of the instruments,
which means that as Cassini was
literally disintegrating,
in its computer memory was
a final thought of home.
And I think that's just
a tremendous, tremendous thing.
We have just heard
the signal from the spacecraft
is gone,
and within the next 45 seconds,
so will be the spacecraft.
And then it lost control,
it began to tumble.
The world that had borne witness
to some of the greatest dramas
in the solar system's history
consumes the craft that has
given us its remarkable story.
Congratulations to you all.
I was supposed to call
end of mission.
"Thanks and Godspeed Cassini."
I couldn't get the words out.
And there was some applause
but mostly people just...
People were just looking
at each other.
It was, it was, it was so sad.
Really, really sad.
It's still hard.
We journeyed out to Saturn
to see this great beauty
up close,
to understand its deep history,
and explore its rings.
But in the end we were rewarded
with something
far more profound:
early hints
of a second home for life.
It's sad that Cassini is no
longer up there doing its thing,
but it has left us
a tremendous legacy of data
that is going
to keep us all busy for decades.
It's revealed so much,
and there's still secrets in
that data
that are going to become
unlocked.
I'm convinced graduate students
20 years from now
will be finding new things
in that data
that we didn't, didn't know were
there.
Now when I go out in the
night sky and look at Saturn,
I know Cassini is there,
that the bits of Cassini will
always be part of Saturn.
Saturn will never be quite
the same.
We landed a device
of our own making
on a world in...
in the outer solar system.
We spent 13 years, you know,
in the...
just methodically, quietly...
just monitoring, observing.
And... it's...
it, to me, it represents
the best that humanity has
to give.
It's really...
it's us at our finest.
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