Nova (1974–…): Season 44, Episode 19 - Death Dive to Saturn - full transcript

For over a decade, the Cassini space probe has been sending dazzling images of Saturn's breathtaking rings and mysterious moons. Join Nova on a suspenseful ride during Cassini's final hours as it dives into Saturn's atmosphere.

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A milestone in space exploration

almost a billion miles
from Earth.

The spacecraft Cassini

transforms our understanding
of Saturn.

The Cassini Mission
is the most successful

interplanetary mission
in history.

Cassini told us
the Saturn system

might have life in it.

Exploring mysteries
of its beautiful rings,

and more than 60
intriguing moons.

More amazing, more exciting,
more unbelievable



than any theory
you could've put together.

This is our legacy.

Now, the spacecraft begins
its final daring mission.

This is the capstone to one
of the most phenomenal missions

anybody has ever done.

As Cassini's 20-year journey
comes to a fiery end.

The sense of an impending end
is the hardest thing

I've had to experience
in a long time.

It's a fitting end

to an amazing mission.

But how will it end?

What final secrets
will Cassini reveal

as it makes its death dive
to Saturn?

Right now, on "NOVA."



Pasadena, California.

Here, at the Jet Propulsion
Laboratory,

the mood is tense.

A group of scientists...

Along with their families
and friends...

Have gathered to share
a milestone in the history

of space exploration.

These are the last days
of a 20-year mission

in the life of a 5,000-pound
spacecraft named Cassini.

Almost a billion miles
from home,

Cassini is beyond
the terrestrial planets...

Mercury, Venus, Earth,
and Mars...

Beyond the asteroid belt
and mighty Jupiter.

The spacecraft is at Saturn.

Saturn is, I think,

by anyone's estimation

the most strikingly
beautiful planet.

If you asked anybody what planet
they'd love to see close up,

you'd love to see Saturn.

There is something special
about Saturn

that sets it apart even
from the other giant planets

in our solar system.

It has this extraordinary
ring system.

It has 62 moons
of great diversity.

In some sense, Saturn, more than
any of the other giant planets,

resembles its own
planetary system.

For more than a decade,

Cassini has made discovery after
discovery about the planet,

its magnificent rings,

and perhaps
the biggest surprise of all,

one tiny moon
with the possibility of life.

I think this is our best
opportunity to study

an extraterrestrial
habitable zone.

Tonight, the spacecraft begins
a dangerous series of maneuvers

that could result in an abrupt
and premature death.

I'm starting to get nervous.

Yeah, I'm a little sweaty, too.

We got 30 minutes to go.

Anxiety is in the air,

as the team waits to hear back
from their beloved spacecraft.

I am really excited,
but I am nervous,

because neither the spacecraft
nor the instruments

were designed to do this.

Throughout this daring mission,

the Cassini team has faced
risk after risk.

You cannot be afraid.

It's an adventure.

This is Cassini's final mission.

Its grand finale.

We are going through an area
no one has gone through before.

Cassini must pass through
a gap between the planet

and its innermost ring.

So we think

this 2,000-kilometer gap
is in fact a gap,

and it's clear, right?

But we're not positive.

Whether it's large dust
or small dust,

any of those that you hit

at 76,000 miles per hour...

Can kill your spacecraft.

It only takes one.

It only takes one larger
ring particle

to hit the spacecraft

that could stop you cold
in your tracks.

But the risk is worth it,

because if all goes well
for the next five months,

Cassini will peer inside
the gas giant as never before,

to answer a puzzling question:

what is Saturn's interior
made of?

What lies beneath
its mysterious atmosphere?

We don't know
what we are going to find,

but our expectations are high.

We think we will be able
to determine

whether it has a core
and how big this core is.

The mysteries we want to solve
with the grand finale

mostly have to do with revealing
Saturn from the inside out.

Understanding Saturn's
internal structure

will shed light
on how the gas giant formed.

Though we think we have
a picture of how giant planets

like Saturn or Jupiter formed,

it's rather incomplete.

And there's another mystery
the team hopes to unravel.

There's still quite a debate.

Are the rings young?

Hundreds of millions of years?

Or are they four-and-a-half
billion years old?

As old as the solar system.

The answer could finally
be revealed.

That is, if Cassini passes
through the gap intact.

It's about 15 or 20 minutes
from when we will get

that signal back
that the spacecraft is fine.

We're waiting with bated breath.

I tried not to sit in there
and bite my nails

because I'm a bit nervous
right now.

The signal takes about
80 minutes to travel

almost a billion miles
across the solar system.

By midnight,
the room is so quiet,

you can hear a pin drop.

It's time for Cassini
to phone home.

I think of it
as the "Hi, Mom" moment,

where the spacecraft
goes through and says,

"I made it safely.

I'm ready to get on
with the rest of the trip."

And finally, the call arrives.

Cassini Ace, I can confirm

we have a good lock
on the 66360 bit rate data.

Cassini has survived its first
close encounter with Saturn,

but this is just the beginning
of its final mission.

Once a week
for the next five months,

it will plunge between
the planet and the rings

again, and again, and again.

Seven seconds of terror

every seven days as we go
through that little gap.

Each dive is dangerous.

As the orbits progress,
we meet closer and closer

to Saturn's atmosphere.

But you don't want to get
very close to the planet

until you're prepared
to lose the spacecraft.

Which is why
we're only doing this now

at the end of the mission.

Even if Cassini successfully
completes all of its dives,

the mission and the spacecraft
are still destined

to come to an end.

It's on a path towards death.

That's sealed at this point.

On its last dive,
Cassini will plunge

into the cloud tops
of Saturn's atmosphere

where it will burn up,

and a 20-year mission
will come to an end.

I feel emotional even now
when I think

what will happen
on September 15.

We're extended all the way out
to Saturn

and we're going to miss that.

Oh, wow.

That is so cool!

Saturn has long been an object
of curiosity and wonder

for Earth-bound observers.

I see it, I see it.

Getting a glimpse
of the gas giant

is an unforgettable experience.

Oh, that's amazing!

Especially if you can see
its majestic rings.

Oh, yeah, I can see the rings!

That's crazy.

Pretty awesome.

Awesome is right.

Saturn is named
for the powerful Roman god

of agriculture and wealth.

It is the second-largest planet
in our solar system,

with a volume more than
700 times that of Earth's.

It doesn't have a surface
you can walk on...

It's mostly a mixture
of hydrogen and helium,

and that makes it
incredibly light.

In fact if there were a lake
large enough,

Saturn could float on it.

Its stormy atmosphere rages
with winds that blow

a thousand miles an hour,

some of the fastest ever
recorded in the solar system.

As for its temperature,

the gas giant is a frigid
288 degrees Fahrenheit.

At its northern pole
is a massive structure,

a six-sided jet stream
called the hexagon.

You could take two Earths
and plunk them into that space

and they would not fill it up.

Since 2004,
the Cassini imaging team,

lead by Carolyn Porco,

has been taking hundreds
of thousands

of mind-blowing images.

Just such a surreal-looking
planet.

Really, it's, it's...

It wins the beauty contest
in the solar system,

that's for sure.

Exploring Saturn
for over a decade

has given the Cassini team
a rare opportunity...

To watch as the color
of the hexagon shifts

from the blue of winter
to summer,

when its atmosphere
reacts to sunlight,

and forms a golden haze.

It also gave the team
a front-row seat

to an historic event.

There have been six giant storms
since the late 19th century.

Astronomers have observed them
from Earth,

and so, when Cassini was
in orbit around Saturn,

we had sort of
a one-in-three chance

of seeing one of these storms

with all of our instruments
ready and working.

And we caught one.

We got to watch it
from its birth,

which was December 5, 2010,
all the way into late 2011.

It ending up circling
the entire planet,

and the tail ended up meeting
its head

and eventually it dissipated.

The scientists on Cassini
are still studying this monster.

Over the last 13 years,
the imaging team

has been documenting
Saturn's turbulent atmosphere.

They even made the first movie
of lightning on another planet.

This thunderstorm is larger than
the continental United States,

and these lightning bolts,

a thousand times stronger
than the ones we have on Earth.

The weather on Earth actually
after a while gets boring

and then you go to some other
place and you see, "My God,

look at what is possible."

It puts the Earth
into this larger perspective

and helps us understand

the universe of possible
configurations

that a planetary atmosphere
could have.

Once we can do that,
then we can go back and say,

"Oh, okay, I look at what I see
on the Earth

in a little different way than
I was looking at it before."

After almost 20 years in space,

Cassini is on its final chapter,

flying closer to the gas giant
than ever before.

Now, just two days after
Cassini's first daring dive

between the planet
and its innermost ring,

the team gathers at JPL
to make a critical decision...

Should they put their spacecraft
in harm's way, again?

On its first dive, just how much
of a beating did Cassini take?

It's William Kurth's job
to figure it out.

Kurth is in charge

of the radio and plasma wave
science instrument.

This is the one instrument
that can detect the impact

of the dust that is hitting
the spacecraft.

He translates those hits
into audio signals we can hear.

First, he plays a recording
made in 2016,

when Cassini flew just outside
one of Saturn's rings,

called the F-Ring.

This is the sound of dust
hitting Cassini's antenna.

So how does that sound
compare to what he heard

during Cassini's first dive

between the rings
and the planet?

He hears a hiss,

the sound of charged particles
in the environment,

but not much dust.

I really expected

to see a lot more dust
than we did.

In fact, the team now calls this
1,200-mile gap the Big Empty.

But it doesn't mean
it's danger-free.

If we're surprised,
the show's over early,

and we don't want that
to happen.

In my business, we call them
the unknown unknowns.

You don't know
what to worry about,

but you know there might be
something out there.

Far from public view,
in a basement at JPL,

a group of engineers
is hard at work

making sure Cassini's mission
doesn't end

before it's supposed to.

We are responsible
for the health and safety

of the spacecraft.

This is the Cassini
integrated test lab,

also known as the ITL.

The ITL is our representation

of the spacecraft on the ground.

Throughout this clean room

are dozens of
vintage circuit boards,

exact copies of the guts
of the Cassini spacecraft,

along with the ground-based
equipment needed to run it.

The equipment's 30 years old.

Everybody talks about
gigabits these days,

we're down in kilobits.

The memory and computing power
here

is less than in your smartphone,

but it's kept Cassini running.

We were built to be bulletproof.

And we are go for
orbit trim maneuver 467 test.

Right next door,
Julie Webster's team

uses this vintage hardware
to test contingency maneuvers

for Cassini's final dives.

No red alarms and we are go
for orbit trim maneuver 467.

The timing of everything
is highly choreographed.

The accelerometer
has powered on at this time.

Because we are doing something

almost every second
on the spacecraft,

but certainly every minute.

Wind roll turn has started.

To have either an anomaly
on the spacecraft

or a sequence
that isn't quite right,

there's very little time

to figure out what's wrong,
fix it, clean it back up,

put the sequence back on board
the spacecraft.

We don't have a lot of time
to recover at that point.

We're game on.

At this time, the wind roll
turn has completed.

How does the team keep track
of a spacecraft

a billion miles away?

We actually carry about
a 2,400-star map

and those stars are the same
stars that sailors used

with a sextant.

Just like a navigator
sailing the seas

hundreds of years ago,

Cassini looks at the stars
around it

and compares their positions

to those on a star map
it carries on board.

It's just built
into the flight software.

That's how we know
where we are in space.

Today, we've developed
sophisticated technology

to expand our view
of the solar system.

We can even see galaxies
far beyond our own.

But the effort to see
beyond Earth

began with a handful
of 17th-century astronomers

who strived to develop
a revolutionary new tool:

the telescope.

At the Dutch National Museum

for the History
of Science and Medicine,

one of the few early telescopes

that has survived
the test of time

is taken out of storage.

What you see actually
is a black iron tube.

This telescope must be handled
with extreme care.

The material's pretty thin,

so I will carefully
take out piece by piece

till I have the whole tube.

When fully extended,
it's almost 15 feet long.

The lens is mounted
by a metal spring.

Beautiful craftsmanship
they used to make this.

That's perfect.

This telescope was built
by Christiaan Huygens,

the 17th-century
Dutch astronomer.

At his home in the Netherlands,

Huygens and his brother
Constantin designed and built

some of the best telescopes
of the day.

They even ground
their own lenses.

The better the lens,
and the longer the telescope,

the more detail he could see.

One, two...

But using a long, fragile tube
was anything but easy.

If you have two or three
supporting people,

then you can look
through the tube.

But you have
to hold it still, also.

With a telescope
a lot like this,

Huygens set out to solve
a mystery...

One that even baffled Galileo,

the great astronomer
who first observed Saturn.

Galileo couldn't figure out
what was sticking out

of the planet's sides.

As Huygens searched
for the answer,

he filled his notebooks with
drawings of his observations.

Here again Saturn,
another night.

In 1656, at the ripe old age
of 27,

Huygens finally reached
the conclusion:

that Saturn is surrounded
by a ring.

He also discovered
its largest moon, Titan.

One, two, three moons.

And the number two is his moon.

He adds the word "meus",
my moon.

The others were discovered
by Cassini.

Jean-Dominique Cassini

ran the newly built
Paris Observatory.

There, he discovered
six more moons of Saturn,

and that instead
of a single ring,

Saturn was surrounded by
several rings separated by gaps.

Over time, telescopes got
better, and better, and better.

And by the 20th century,

we accomplish the unimaginable:

we take our telescopes
into space.

In 1979, Pioneer 11 flies
by Saturn, sending back images

that bring us closer
to the cosmos than ever before.

A year later, Voyager's
state-of-the-art cameras

arrive at Saturn
and exceed our expectations.

We thought during Voyager days,
this was, like...

Nothing could top this.

Then comes Cassini,

built with a suite
of scientific instruments

designed to explore Saturn,
its rings, and its moons

in extraordinary detail.

Cassini's Battlestar Galactica,

because it's just festooned
with instruments.

It's a flagship.

And liftoff
of the Cassini spacecraft

on a billion-mile trek
to Saturn.

19 countries pool
their talent and resources

to get this massive effort
off the ground.

Science is international
by definition.

I never represented my country
with Cassini.

Cassini is a family.

Seven years later,
Cassini arrives at Saturn,

and the team gets ready
to explore

one of the most intriguing
objects in the solar system:

Saturn's largest moon, Titan.

Before Cassini arrived,

Titan was Terra Incognita

and that was a big draw,
of course.

This "unknown land"
has an atmosphere so thick

that 25 years earlier,
the spacecraft Voyager

could not see
beneath Titan's haze,

but it did detect
one surprising characteristic.

The main constituent
of Titan's atmosphere

is, like on Earth, nitrogen.

The similarity
to Earth's atmosphere

was too intriguing to ignore.

Scientists just wanted
to go back.

It sounds like a cliché,
but every space mission

raises more questions.

Scientists want to go back
so badly

that the European Space Agency
takes on the challenge

of building a probe to land on
the surface of this alien world.

Called Huygens, it fits snugly
on Cassini's side.

Huygens was there to teach us
about Titan's atmosphere,

take pictures
of the surface also,

and then it would
either crash or sink,

but it would have
a disastrous end.

We did not know anything

about the surface of Titan.

It was really very difficult
to design for the unknown.

So do their efforts pay off?

As Huygens makes its way
through Titan's hazy atmosphere,

in a control room
in Darmstadt, Germany,

scientists anxiously wait
to hear back from the probe.

And so, at the right time,

we were starting to be
a little bit nervous.

Then they get a signal.

The Huygens probe has survived
its descent to Titan.

Soon data comes streaming in,

including the echo
of Huygens' radar

as it bounces off
the moon's surface.

I think this is the best techno
music we've ever heard.

During its descent,

the probe measures wind speeds

of 270 miles per hour,

and confirms Titan's atmosphere

is made of mostly nitrogen,
along with methane and ethane.

Finally, Huygens
gives the world our first look

beneath Titan's dense haze.

I thought it was a joke.

It looked so much like an image
of the surface of Mars.

My colleague says,
"This is Titan."

I look at it and said,
"That can't be Titan.

There's pebbles on it,
it looks like... it looks red."

He says, "It's Titan, you better
get used to it."

Over the next decade,

Cassini continues
to explore Titan,

making more than 100 flybys.

In the upper atmosphere,
there was this organic factory

that was producing huge amounts

of very complex organic
compounds,

and those molecules
were then falling down

through the atmosphere,

ending up on the surface
and forming these organic dunes.

Massive dunes, as large as ones
found in the Sahara Desert,

next to long,
river-like channels.

Now we've mapped
the whole world,

and it's, in many ways,
almost as diverse as the Earth.

Radar and infrared cameras
paint a picture

of a moon with dozens of lakes.

In fact, Titan is the only
celestial object

in our solar system

known to have liquid
on its surface,

except, of course, planet Earth.

What would it be like
to stand by a lake on Titan?

How would it compare to Earth?

We're here on the shore of
Cayuga Lake in upstate New York,

because the view that we have
here would not be too different

from the view you might see
from the shores of a Titan lake.

Behind me,
we have a beautiful blue sky

that's reflecting
off the surface of the lake,

making it look blue.

And, on Titan,
that sky would be a hazy orange.

And the lake would be reflecting
that hazy orange sky,

and upon closer inspection,

we'd notice
it's not made of water.

It's a mixture of methane
and ethane

that's similar
to liquid natural gas.

In fact, all of Titan's lakes
and seas combined is 300 times

the volume of all the proven
gas reserves here on Earth.

Now, they create a landscape
that's similar

to the rocky deserts of
the southwestern United States.

The ground is going to be
a broken-up mixture

of water, ice, and organics

with the texture
of plastic shavings

or Styrofoam beads.

Sometimes, it'll rain,
and when it rains on Titan,

those raindrops are going
to fall slower

than we might expect

and they're going to make
large splashes...

because the gravity on Titan
is 1/7 that of Earth's,

similar to what the astronauts
felt on the moon's surface.

And finally, because we have
this colder, denser atmosphere,

sound travels faster
than it does here on Earth,

and the sound of the waves will
be coming to us more compressed

and perhaps be perceived
as slightly pitched.

My voice itself will sound

a little bit more alien
and complex.

But I hope you're
holding your breath,

because while Titan's atmosphere
is predominantly nitrogen,

just like Earth,

there is very little
to no oxygen to speak of.

And if you can visualize
all of this,

I'd like to welcome you to...
the shores of a lake on Titan.

It's so Earth-like:

rain, lakes, rivers,
erosion, clouds.

To me, there's a romance to that
that I just can't escape.

I'm from Scotland,

which is a place that has a lot
of cold rain,

and I don't like rain,

but I like to think
that I'm privileged to live

on one of only two worlds
in the solar system

where rain falls onto the
surface and flows in rivers.

And the other one is Titan.

But Titan is not the only moon
to pique the team's interest.

Saturn has 62 moons,
and counting.

From Prometheus,
which looks like a potato,

Janus, which resembles
a meatball,

and Mimas,
which has an uncanny resemblance

to the Death Star in Star Wars.

Let's not forget lapetus,
the yin-yang moon,

because it has one side
as dark as night

and the other as bright as snow.

And Pan, a tiny moon
shaped like a ravioli.

And then there's Enceladus,

which is just such
a total surprise.

Saturn's moon Enceladus is a
tiny, bright, white ball of ice,

the most reflective object
in the solar system.

Enceladus is a small body.

It's less than the distance

between Los Angeles
and San Francisco.

And everybody expected
to be a dead body.

Little Enceladus
poses a great mystery.

Its north pole is pockmarked
with craters,

the remnants of asteroid impacts
that were common

billions of years ago.

But its south pole
is surprisingly smooth,

almost craterless.

The question is, why?

Early in the mission,
the magnetometer,

an instrument that measures
magnetic fields,

picks up on something strange.

It was really spectacular.

It was like a plume
of water vapor

coming off from the south pole.

Around the same time,
Cassini's cameras capture

a mind-boggling image.

This is what we saw.

We saw dozens
of fine jets shooting off

the south pole of Enceladus.

More instruments,
designed to detect

the chemical composition
of gases and dust,

find that these jets, or plumes,
are made of water ice.

Enceladus spews out
1,000 tons of it every hour.

Some of it comes back down
like freshly fallen snow,

and create the moon's smooth,
white surface.

It's pretty startling.

We started to try to figure out,
well, why is this happening?

This moon is supposed
to be cold and dead.

In subsequent flybys,
Cassini gets even closer,

and the instruments reveal
even more clues.

We have tasted this material

coming out of Enceladus.

It has organic molecules,
carbon-bearing molecules,

nitrogen-bearing molecules.

And one mineral no one ever
expected to find:

salt.

That told us these ice grains
were frozen seawater.

Frozen seawater coming out
of enormous fractures

in the south pole,

and these fractures
are radiating heat.

This is the south pole.

There's more energy coming
from these fractures

than anywhere else on Enceladus.

It would be as if on Earth,

we found there was more heat
coming from Antarctica

than there was from the Equator.

Plumes of frozen seawater
gushing out

of massive fractures.

So, could there be liquid water

beneath the frozen moon's
surface?

It took about ten years
of Cassini data

to answer that question.

An instrument
which measures gravity

helped solve the mystery.

From the gravitational field,

we can unpeel
what the interior is like.

And what they find
inside Enceladus is stunning.

The gravity data revealed

something denser than ice
under the surface: water.

We provided one important clue

that there was
an underground ocean.

Enceladus has an ocean about
20 miles beneath its icy crust.

The ocean under the surface
of Enceladus

is not limited
to the southern polar region,

but is global.

But there's still more.

We found tiny particles
of nano-silica.

What's so amazing is
that those nano-silica grains

could only form
in really hot water.

All of a sudden, the pieces
started to fall into place,

and so we're thinking, maybe
you have hydrothermal vents

on the sea floor of Enceladus.

Not only is there liquid water,
but it's hot.

On Earth,
at the bottom of the ocean,

there are hydrothermal vents,

openings in the sea floor
where heated water flows.

Although it's dark and cold,

life thrives.

Biochemists have suggested that
it's in environments like this

where life might've got going
billions of years ago.

To learn more, Cassini flies
even closer to the plumes,

and makes another
startling discovery.

On the last flyby
that we operated,

we were able to detect
native H2 in the plume.

H2 is a form of hydrogen gas

found around Earth's
hydrothermal vents.

H2 is a nutrient
for microbial systems.

So it's, like, microbial candy.

Could this be food for life?

It doesn't get
any better than this,

to go to Saturn
and come away having discovered

what we think might be the best
place in the solar system

to go to search for life.

Nobody can say that there
is life on Enceladus,

but it's a possibility.

Not only do these plumes
from Enceladus

offer tantalizing clues
of life beyond Earth,

but their icy particles
also create

one of Saturn's
most majestic features,

its outermost, or E-Ring.

The E-Ring is the only
known ring

that is basically made up
of samples of a moon.

This E-Ring is not

like the classical rings
of Saturn.

It's not sharp and dramatic.

You can't see it through
a home-built telescope.

But it's there.

While all the gas giants

of the outer solar system
have rings,

they're faint.

Except, of course, for Saturn's.

Saturn's rings are composed
of billions of particles

made mostly of water ice.

The rings are mostly ice,
and then there's a veneer

of contaminant schmutz.

We don't really know what it is.

Ring particles come in a wide
variety of shapes and sizes.

From the size of a grain of sand
to the size of a house.

Saturn's rings are
the biggest and most massive

in the solar system.

Extending about 90,000 miles
into space.

On the other hand,

if you were to look
at them on edge,

they are a flat disc
of particles

only tens of meters thick.

Within this shallow band
of ice and dust,

Cassini discovered
a dynamic world.

Just look at these shadows,
they're just wild.

Long shadows cast by the sun
reveal particles

reaching miles
above the ring plane.

We get to see lots of places
just really densely packed,

where the particles
are protruding

two miles above the ring plain.

I mean, it's astonishing.

I've often loved imagining
that I'm in a shuttle craft,

flying low over the rings.

And from a low elevation
above the rings,

the rings would look to you
like they extended to infinity,

because they're really very big.

So, flying low over the rings,
it's only 30 feet thick,

and you're passing
all these particles,

and then eventually coming upon
a wall of rubble two miles high.

I mean, come on...
That's like science fiction.

Glorious,
just absolutely glorious.

They do look calm and beautiful
and tranquil from afar,

but they're really a kind of
an explosion of events

that are going on.

The rings are fluid.

Ring particles
run into each other,

forming mysterious waves
hundreds of miles long.

To see all these little waves,

these little bow waves
going through,

just a phenomenal picture.

Cassini's cameras reveal
what's causing

these strange formations:

Saturn's collection
of tiny moons.

We have grades of moons.

We have little things
we call ring moons,

and we have smaller things
we call moonlets,

embedded within the rings

and sprinkled around very close
to the rings.

In addition,
there's a whole class of objects

we call propeller objects,
swarms...

Hundreds or thousands
or more of these things.

The rings are full
of little propellers,

just festooned with them.

A young researcher
on the imaging team,

Matt Tiscareno,
is the first to notice them.

What I saw

was two horizontal dashes
oriented in the same direction

as each other,

about 100 meters across,

about the size
of a football field.

Someone might have dismissed
that as cosm ray hits

or imperfections
in the camera or something,

but he noticed something
that was very common

among all these streaks.

They all looked
like little propellers.

At the center of each streak,

a tiny moon tugs and tosses
ring particles about.

They clump together
along the edges

of what looks like
a distinctive shape.

These little
two-bladed propellers

orbiting Saturn's rings

can, some scientists suggest,

shed light
on a mysterious process

that took place
billions of years ago:

the formation
of our solar system.

Four-and-a-half billion
years ago,

this is what
we believed happened.

We have a very large cloud
of material, gas, but also dust.

A blast wave...

We actually believe
it's this nearby supernova...

Rippled through this gas cloud.

Things end up in a disk
and then they start to form.

As they orbit
around the young sun,

gas and dust stick together.

Pebbles turn into boulders,

boulders into
mountain-size comets.

Comets turn into protoplanets.

Protoplanets become planets.

Over hundreds of millions of
years, the planets take shape.

The best place
in our solar system

to observe a process like this
in action

is in Saturn's rings.

So, watching the propellers,

other clumping we've seen
in the rings,

perhaps we can better understand

how the planets
might have formed

from a disc of gas and dust

to form the larger objects.

The propellers are kind of
like the seeds

that came together
from which the planets formed.

These propellers can't grow

into full-fledged planets.

They're kind of frustrated in,
they can't form,

they can't go on
to complete the process

because they're too close
to Saturn.

The power of Saturn's gravity
pulls them apart.

It actually prevents things
from coalescing.

It's like the formation
of baby planets,

but it's an arrested state
of development.

They don't get very big.

So we are seeing here
an initial process.

It went to completion
in the solar system

billions of years ago.

But we get to see it
in Saturn's rings,

and that's what's
so special about it.

That was the golden opportunity
we had

in going back to Saturn's rings
with Cassini.

In the last 13 years, Cassini
has made startling discoveries

about Saturn's
extraordinary rings,

but there's
a fundamental question

the team has yet to answer:

how old are the rings?

Did they come together
alongside the gas giant

4.5 billion years ago,
when the solar system formed?

Or are they much younger?

There are two schools
of thought.

We're thinking that Saturn's
rings may not be as old

as the solar system.

They may be a recent addition,

maybe 400
or 500 million years ago.

My inclination is to believe

that they are probably older
rather than younger.

As Cassini completes one daring
dive after another,

the team is hoping to resolve
this question once and for all.

In June 2017, team members
from around the world gather

at the European Space Agency
in Noordwijk, Holland.

Well, we've just had
our seventh plunge

in between the rings
and the planet,

and there are all kinds
of surprises for Cassini.

Every week, we get
a new dataset and we go, "Oh."

Everything is speeding up.

The spacecraft is speeding up;
the data is speeding up.

And the discoveries.
There's never a dull moment.

Especially when it comes
to probing Saturn's interior.

A key instrument to do this
is the magnetometer,

which measures the magnetic
field of a planet.

What that magnetic field does,

if you can measure it outside,

it allows you to understand
what's going on in the inside.

So far, what the magnetometer is
showing is downright baffling.

We've got an internal planetary
field

that's been generated,

but it's really weird.

It's different from any other
planetary fields we've seen.

On Earth,

our magnetic field is produced
deep inside the planet

by the motion
of churning liquid metal

surrounding a hard inner core.

It works a lot like
a large generator or dynamo.

According to a theory called

planetary dynamos,

you get permanent
magnetic fields

when you have electrically
conducting fluids inside planets

that are circulating
in these twisting motions

that will then generate
the magnetic field.

These twisting motions
in the center of the Earth

create a magnetic field
on a tilt

from Earth's axis.

But Michele Dougherty
can't find that tilt on Saturn.

I'm only gonna tell you this.

I haven't told anyone else
on the Cassini project yet.

There are five members
of my team who know this.

But we think

the rotation axis of the planet

and the magnetic axis
of the planet,

at this stage,
as far as we can tell,

are lying on top of each other.

The problem is,

this doesn't fit
planetary dynamo theory,

the current idea
of how magnetic fields work.

If we're seeing
what I think we're seeing,

the internal field is generated
in a different way

than people think
or the field is dying.

So what does that tell us
about what's inside Saturn?

Though the planet is made mostly
of hydrogen and helium,

scientists believe
that Saturn, like Earth,

has a solid core.

So we think Saturn has
a small core at the very center.

But maybe the more interesting
part of Saturn

is that it would be surrounded
by a metallic hydrogen envelope.

That metallic hydrogen
is likely the source

of Saturn's magnetic field.

Here on Earth,

the magnetic field protects us
from the sun's solar wind,

which could erode
our atmosphere,

making life on Earth impossible.

Without a magnetic field,

Earth could look a lot
like Mars,

which was once a warmer
and wetter place.

But its magnetic field weakened,

leaving it a barren desert.

Cassini's data raises
the possibility for Michele

that Saturn's magnetic field
might also be dying.

If this actually happened
at Saturn,

the gas surface
would be stripped away.

And I don't know what
will happen as a result of that.

It's crazy.

But that's why you do science.

You know, I said to someone.
Someone said to me,

"Ooh, do you understand
your data?" I said, "Nah."

And they said, "That's great,
isn't it?" I said, "Yeah."

I think
scientists are disappointed

whenhey're always right.

This is a field where you can
take delight in being baffled.

My favorite time is when we find
out that we're profoundly wrong.

And one thing
they could be wrong about

is how Saturn formed

from the disk
of the early solar system.

They've assumed that,
like other planets,

Saturn came together through
a clumping of dust and debris,

then attracting hydrogen
and helium gas around it.

Now they're hoping to discover

in the billions of bytes of data
yet to be analyzed

whether this idea
is right or wrong.

I think it's too early
for us to say

whether it's changing our view
of planet formation.

But what it's saying is

we have to confront the data
directly

and see what the observations
are telling us.

And the observations
might finally be telling them

about the age of Saturn's rings.

Crucial clues may come from an
instrument that measures mass.

What will really seal the deal
is to find out from Cassini

just how massive the rings are.

The older the rings are,

the more debris
they may have gathered up,

becoming more massive over time.

Less massive rings
point to a younger age.

So far, we have hints

that maybe the rings are not
as massive as we thought.

My hunch has been young,

and my hunch is still young.

On September 15, 2017,

after 20 years, the final hour

of the mission has arrived.

We are bringing the mission
to a close,

because, basically,
we are out of gas.

With its fuel tanks
almost empty,

Cassini must be destroyed to
reduce the risk of contaminating

Saturn's moons
with earthly microbes.

And so, in the early hours
before dawn, the team gathers

to witness Cassini's
final moments together.

The probe is traveling
more than 70,000 miles an hour

as it enters
Saturn's upper atmosphere.

Thrusters position
the spacecraft

so its antenna
can send data back to Earth

for as long as possible.

As it dives,
pushing against the atmosphere,

the temperature rises.

Cassini can no longer maintain
stability,

and communication with Earth
comes to an end.

The signal
from the spacecraft is gone and,

in the next 45 seconds,
so will be the spacecraft.

All alone,
nearly a billion miles away,

Cassini begins to break apart,

as temperatures reach those
on the surface of the sun.

The propellant tanks explode.

The instruments atomize
in the intense heat.

What remains sinks
into the cloud tops,

and Cassini becomes part

of the distant world
it explored.

This has been
an incredible mission,

an incredible spacecraft.

And you're all
an incredible team.

I've worked on Cassini
for almost 30 years,

and that's the time it takes
for Saturn to go around the sun.

I have a mixture of feelings.

A feeling of sadness...
This group of people

I've worked together
with for decades

will now be scattering
and going their separate ways.

At the same time,
a tremendous sense of pride.

We've rewritten the textbooks
on Saturn.

Cassini told us the Saturn
system might have life in it,

that it has moons that are
every bit as interesting

as the Earth and Mars,

the Saturn ring system
may be relatively young.

And so it's told us
such an incredible amount

about this one planetary system
within our solar system.

It was perfect.

It was truly perfect.

It-it did everything
we asked it to do.

Including sending back to Earth
a final gift:

these images
of Saturn's atmosphere,

closer than ever before.

It's just been
an amazing machine.

We've used every bit of it,

and it's been phenomenal.

So, I... You know,
it's-it's time to say good-bye.