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She has traveled nearly

four billion kilometers.

Delivered a probe to the surface of a toxic moon.

And produced science nine years longer

than originally planned.

She discovered flowing liquids where none were expected.

Phantasmagorical structures on a planet's icy rings.

A weirdly breathing magnetosphere.

And a possible abode of life.

On a tiny world with a startling atmosphere.

Now, after 20 years in space, controllers have sent

the Cassini spacecraft into a daring spiral

to end her remarkable journey.

Streaking along Saturn's upper atmosphere

at more than 124,000 kilometers per hour,

Cassini streams data and images back to Earth,

until heat and turbulence tear her apart.

Three, two, one.

And liftoff of the Cassini spacecraft

on a billion mile trek to Saturn.

On the evening of October 15th, 1997,

a powerful Titan 4B booster leaves

launch complex 40 at the Kennedy Space Center.

All systems go.

The product of two decades

of intense collaboration between NASA

and the European Space Agency,

in particular the Italian Space Agency.

And the solid rocket boosters

have been jettisoned.

Some on Earth have protested this moment.

They fear that if the launch fails, Cassini's nuclear

electric generator will spread radioactive plutonium.

No nukes in space, no nukes in space.

As she glides outward on a nearly seven-year

transit to Saturn, scientists organize

their investigations based on past probes' findings.

Their questions nest within one another,

like the giant planet's concentric rings.

Does Saturn have a solid surface?

Or do its clouds thicken down to extreme densities,

perhaps creating exotic forms of matter?

Like metallic hydrogen.

Is there a rocky core?

Why are the rings so bright?

How old are they?

What's going on beneath the thick orange clouds of Titan,

Saturn's largest moon?

How are Saturn's moons and rings related?

What will this giant planet tell us

about the evolution of solar systems, and of life?

At its distance of nearly a billion and half kilometers

from the sun, Saturn's orbit lasts 29.5 Earth years.

Over the past Saturn year,

Earth has added some 2.5 billion people.

Average life expectancy rose from age 64 to almost 72.

The internet has exploded.

It now has almost four billion users,

with the half the world's population now online.

In the time of Cassini, our world has been consumed

with new ways of communicating,

connecting,

working,

living,

and seeing each other.

Saturn is the Roman name for Chronos,

the Greek God of time, who ate his children

in a fit of jealousy.

In myth, as in the ever-changing universe,

time is the devourer of all things.

To catch up with Saturn, mission planners have sent

Cassini on a series of fly-bys,

each time hitching a ride on the planet's gravity

to speed her on her way.

Pioneer 11 flew by Saturn in 1979.

Voyager One in 1980.

And Voyager Two in 1981.

Cassini will become the first craft to orbit Saturn,

captured by Saturn's gravity on June 30th, 2004.

Cassini's prime mission was to be four years,

but she will remain healthy far longer.

Her assignments will be extended twice,

an additional two-year equinox mission,

and a seven-year solstice mission.

Yet Cassini's entire exploration

will span less than half a Saturn year.

From late winter

through early summer.

The Saturn system lies 9.5 astronomical units from the sun.

That's nearly 10 times farther out than Earth.

The whole grand clockworks speeds along

at 34,821 kilometers every hour,

more than 10 times faster than a bullet fired from a gun.

The planet spins fast, rotating about its axis

once every 10 hours and 39 minutes.

So while a year on Saturn lasts only Earth 11,000 days,

it's about 24,500 Saturn days.

Made up mostly of hydrogen and helium,

Saturn is the least dense of all the planets.

It's often said, if you could find a bathtub big enough,

Saturn would float.

That's not quite accurate, but its density is lower

than that of the liquid water in Earth's oceans.

And it's big.

You could pour 764 Earths into the volume of Saturn.

This mission is officially named Cassini-Huygens,

honoring two remarkable men of 17th-century science.

The French-Italian Giovanni Domenico Cassini,

also known as Jean Dominique Cassini,

and the Dutch Christiaan Huygens.

Key players in the scientific revolution,

they collaborated and competed throughout their lives.

In the process, transforming astrology into astrophysics.

As director of the Paris Observatory,

Cassini built and refined long-tube refractor telescopes.

But Huygens found a way to remove the cumbersome tube,

inventing the so-called air telescope,

and its better engineered eyepiece.

In 1665, Cassini co-discovered Jupiter's great red spot,

and used it to time the big planet's rotation.

Huygens, applying Galileo's ideas,

invented the pendulum clock

and developed a balanced spring watch,

both pivotal innovations in the study of space time.

Cassini published maps of surface markings on Mars,

though Huygens had actually seen them first.

And Huygens pegged the length of the Martian day

at 24 and one-half hours,

just seven minutes shy of the correct figure we use today.

By getting parallax observations of Mars, Cassini was

the first to estimate the size of our solar system.

And Cassini used Galileo's method of measuring time

by watching eclipses of Jupiter's moons

to lay out lines of longitude on Earth.

Endeavoring to perfect the geometry of optical lenses,

Huygens developed the wave theory of light,

setting the stage for today's quantum mechanics.

Cassini explained the so-called zodiacal light,

seen by astronomers as the reflection of sunlight

off dust in the plane of the solar system.

Toward the end of his life, Huygens determined

liquid water to be a key factor in searching

for extraterrestrial life, a quest now undertaken

by the Cassini spacecraft and its Huygens probe.

We've known Saturn as the ringed planet since 1655,

when Cristiaan Huygens explained what Galileo

had seen 45 years earlier.

But if you were standing on most of Saturn's moons,

you would have a hard time actually seeing the rings.

Because most moons ride in the ring plane,

you could only view them edge on.

From the right, watch tiny Prometheus,

just 86 kilometers across, pass behind big Tethys,

1,062 kilometers side.

Little Pandora scoots through from left to right.

Here, medium sized Mimas, traveling 14 kilometers

per second, glides in front of the larger Rhea,

at eight kilometers per second.

When Cassini launched,

18 moons were known to orbit Saturn,

and 13 more were suspected.

By the end of Cassini's mission,

astronomers will confirm 53,

with another nine marked as conditional.

Only one of them is huge.

Titan.

In 1655, Christiaan Huygens was the first to observe Titan.

Giovanni Domenico Cassini discerned Iapetus in 1671,

Rhea in 1672,

Dione in 1684,

and Tethys in 1684.

The skywatching siblings William and Caroline Herschel

found Mimas and Enceladus in 1789.

In 1848, Hyperion was spotted

by the father-son team of William and George Bond

in the U.S. and William Lassell in the U.K.

Phoebe was the first moon to be discovered

through photography, by the American astronomer

William Henry Pickering in 1898.

Some of Saturn's moons are probably captured objects,

particularly those in retrograde orbits like Phoebe.

They orbit backwards, in the opposite direction

from the rotation of the planet.

Each Saturnian moon is a unique jewel

in the majestic planet's elaborate necklace.

Before Cassini, no one suspected Enceladus

to be anything other than a dead ball of rock and ice.

But scientific sleuthing led to a remarkable discovery.

Enceladus is about 482 kilometers in diameter.

It would fit neatly between Los Angeles and San Francisco.

The first good closeup, taken by Voyager Two in 1981,

showed the globe was remarkably bright

and free of craters.

In fact, Enceladus is the whitest body in the solar system.

The first hints of a more complex story

come from Cassini's magnetometer.

The moon's magnetic signature

looks more like that of a comet.

Field lines are bent around Enceladus' south pole.

A closer fly-by shows the southern polar region

has been recently resurfaced.

Cassini's infrared spectrometer indicates the area

is about 100 degrees warmer than the rest of the moon.

And that heat signature aligns with four prominent

tiger stripes appearing across the moon's southern terrain.

They turn out to be fissures,

cracks in the icy surface.

Each about 150 kilometers long and about one kilometer wide.

Scientists name them Alexandria,

Cairo, Baghdad and Damascus.

From each, jets of water vapor and water ice

shoot out into space.

Cassini eventually locates more than 100 of these geysers.

Mission planners arrange more fly-bys

and in October of 2015, navigate within 50 kilometers

of Enceladus' South Pole, and right through the jets,

giving the instruments a chance to taste and smell them.

It's mostly water ice.

There's also some ammonia, methane and carbon dioxide,

key compounds associated with life.

And hydrogen gas, which microbes, if there are any,

could be using as a source of energy.

Some of the particles are salty, like frozen sea spray,

flavored by sodium and potassium.

The science team imagines that a large subsurface ocean

lies beneath the icy crust.

As it sloshes around, carbon dioxide makes it fizzy,

like bottled soda.

When it finds a path to lower pressure,

through the cracks of the tiger stripes,

it sprays out into space like champagne when the cork pops.

Most of these salty water particles fall back

to the ground as snow, decorating and repairing

the surface of Enceladus.

But some escape Enceladus' weak gravity.

So Enceladus rides a misty ring of its own creation.

Were the moon to shut off its water jets,

this e-ring would dissipate completely

within a few hundred years.

Tracking and modeling icy tendrils made by interacting

geyser jets lets researchers calculate

how much mass Enceladus is pumping into Saturn's rings.

Enceladus orbits in resonance with the moon Dione.

Their dance is thought to heat their rocky cores.

It's likely that Dione too has a subsurface ocean.

Dione's big crater, Creusa,

paints bright rays at least 500 kilometers

across the moon's surface, giving researchers

a marker of geologic time.

Craters younger than Creusa disturb the rays.

Older ones underlie them.

Water is pulled down into the hot cores of these moons,

reemerging as steam through the sea floor.

We know this because Cassini has detected

tiny silicate particles

that can only form in boiling water.

In Earth's oceans, hydrothermal vents, called black smokers

and white smokers, are oases for life.

The churning hot water provides energy

and sets nutrient particles swirling around.

Is the same thing happening beneath the ice crystal crusts

of Saturn's snow globe moons?

To find out, future missions must look for amino acids,

fatty, acids, long carbon chain molecules,

and perhaps microorganisms hitchhiking on the ice crystals,

blasted far from the undersurface ocean depths.

Many astrobiologists are fascinated by Titan,

because it is very cold,

about minus 180 celsius, yet it is wet.

If anything lives on or inside Titan,

it is almost surely not related to life on Earth.

Titan is about the same diameter and mass

as the planet Mercury.

And Titan's surface gravity is about the same

as Earth's moon, but unlike Mercury or the moon,

Titan has a thick atmosphere, even thicker than Earth's.

And scientists don't yet know why.

It's the second-largest moon in the solar system.

At 5,150 kilometers in diameter,

only Jupiter's Ganymede is wider.

Titan is so much larger than the rest of Saturn's moons,

it may have formed from the collision

of several of its siblings.

By mission's end, Cassini will have flown

by Titan 127 times.

Each pass gives the craft

a gravitational acceleration equivalent

to almost 1,000 kilos of extra fuel.

When the two Voyagers sailed past in 1980 and 1981,

they saw only this orange, smog-enshrouded globe.

American and European scientists were so intrigued,

they began sketching out what would become

the Cassini-Huygens mission.

23 years later, an international team

commands the release of the Huygens probe,

to land on Titan.

January 14th, 2005.

The European-built Huygens probe

aero brakes into Titan's atmosphere with its heat shield,

deploys a parachute

and floats down

for over two and a half hours

through a nitrogen-laden atmosphere.

Its camera reveals an eerie landscape.

Huygens lands softly in a slushy bank of frozen methane,

along the shoreline of a hydrocarbon lake.

Huygens continues transmitting data for another half-hour

after landing, sending back images of rounded pebbles

probably made of hard frozen water ice.

Humanity has placed its imprint on yet another world.

The flowing liquid on Titan is methane,

which acts there as water does on Earth.

In the range of Titan's temperatures,

methane can exist in all three states,

solid, liquid and gas.

It is not impossible that a form of life

could be evolving on this moon,

feeding on the constant rain of hydrocarbons,

a true second Genesis,

completely separate from our earthly tree of life.

Since Voyager first glimpsed Titan,

researchers have speculated that liquid ocenas

grace its surface.

But dense orange smog has withheld the truth.

This haze collects when the sun's ultraviolet light

breaks up methane in the high atmosphere,

forming long molecular chains that grow large enough

to fall from the sky.

These methane byproducts form snow drifts

into a huge network of dunes around Titan's equator.

Cassini's camera was outfitted with near-infrared filters

to peer through the orange fog to verify bodies of liquid.

Images come back of relatively flat areas,

but are they truly oceans?

Or perhaps just methane slush?

Or thick tar?

The answer comes from Cassini's visible

infrared-mapping spectrometer.

It catches a specular reflection,

a bright spot of sunlight bouncing off

a clearly liquid surface.

With Cassini's radar, flat surfaces,

lakes and seas, appear dark,

unless they are ruffled by waves.

Titan indeed has oceans, ponds and pools

of liquid hydrocarbons, ethane, methane and propane.

Strangely, they form mostly in Titan's northern

hemisphere, near the pole.

Saturn's eccentric orbit is to blame.

Northern summer is warmer than southern summer on Saturn,

but because of the way Titan is tipped,

its southern summer is more severe,

so all the southern liquid methane evaporates

and rains out in the north.

Over time, the whole system flips.

In 15,000 years, Titan's lakes will all

be in the Southern Hemisphere.

This one, named Ligeia Mare, is larger

than Lake Superior on Earth.

It's probably 150 meters deep in spots.

Mysterious, frothy islands appear,

bubbling up from under the surface.

Titan holds a store of hydrocarbons

about 10 times larger than all Earth's

petroleum and natural gas reserves combined.

Hovering 300 kilometers over Titan's South Pole,

a huge persistent toxic cloud of frozen hydrogen

cyanide rotates ominously.

This vortex seems to develop when Titan's

Southern Hemisphere enters its autumn season.

Titan thrusts its methane up into space

at a frightening rate.

The atmospheric supply can't last longer

than about 20 million years.

Something must be replenishing it, but what?

Cassini's radar spots the answer.

Mountains, some up to a kilometer tall.

These may be cryovolcanos.

Some could be active today, spewing methane

from within the moon.

Cassini's radio science tools show Titan's

whole body flexing in response to Saturn's gravity.

That makes it warm enough inside to melt ice.

Unlike the subsurface oceans of Enceladus,

with its rocky ocean floors, Titan's ocean

is probably sandwiched between layers of water ice.

So if there's an underground marine layer,

it may be sealed off from the hot chemistry

of geothermal vents and any chance of nurturing life forms.

Even after Cassini-Huygens, we still don't know

how old Titan is, where it came from,

or how long it's had an active atmosphere.

Clues point to a cataclysmic event in the Saturn system,

perhaps 100 million years ago,

that either created Titan from smaller bodies,

or energized a barren, frozen Titan

into an active system.

Possibly like the primitive Earth.

To find out, we'll have to go back,

float longer through the haze,

sail the hydrocarbon seas

and soar through the nitrogen skies

of Titan.

To Galileo in 1610, the ringed planet

appeared as three stars, with two small ones

touching one large, prompting Galileo to declare

that Saturn must have ears.

But in 1655, using more advanced optics,

Christiaan Huygens unmasked the apparition

as a flat ring circling a spherical planet.

20 years later, Giovanni Domenico Cassini

discerned that it was not one ring, but several,

with gaps between them.

The widest gap was later named the Cassini Division.

We now know that it's 4,800 kilometers wide

and far from empty.

In 1859, Scottish mathematician James Clark Maxwell

showed that the rings are not solid discs

but composed of many smaller granules.

Each tiny fleck, pebble and boulder

revolves around the planet in its own orbit,

jockeying for position among trillions of others.

Though all are flying several kilometers per second,

they jostle one another only very gently.

The concentric rings are given letters.

A through G, in their order of discovery.

Material riding close into Saturn's cloud tops

in the D ring whips around Saturn in about four hours.

But particles in the distance F ring

take at least four times longer.

It wasn't until the Cassini's inspection that we learned

just how flat the rings are.

Incredibly, they are only 10 meters thick on average.

From top to bottom, only about as tall

as a three-story building.

These rings are 30 million times wider than they are high.

And ring material can ripple up and down,

crafting temporary corrugations.

Many intricate wave patterns can be seen

across the broad ring plane.

Each one is conjured by a specific gravitational

resonance with a Saturnian moon.

Though the basic physics of the ring system is simple,

it's mostly the product of gravity and collisions,

the intricate structure of the rings

is hard to mathematically model,

even after 13 years of Cassini observations.

The pressure of sunlight, and forces from Saturn's

magnetic field, can push or pull smaller,

less massive ring particles around.

The mission team flies a series of special orbits

that put the rings between Cassini and Earth.

By beaming radio signals of three different wavelengths

through the ring material, researchers build up

a color map of particle sizes.

Red areas hold more big particles.

Blue regions contain mostly smaller grains.

Radio waves, shown in the bottom half of this image,

reveal finer structure than photos taken in visible light.

Most particles are small, from tiny crystals

smaller than a dust grain, up to pebble sized.

Less common are ice rocks and large boulders,

up to the size of houses.

A few are the size of mountains.

Moons in the act of forming, or falling apart.

Grabbing an average size ring particle would feel

something like closing your hand around

a thick layer of very cold whipped cream

to find a glob of solid ice cream deep inside.

The packing of particles varies between rings.

Very little light gets through the B ring

of crowded snowballs.

An astronaut would have to dig her way through it.

The A ring is less congested.

And the G ring is very diffuse, like fog or smoke.

The Cassini mission confirms the rings are made

of almost pure water ice.

But tiny quantities of simple organic compounds

tint and shade them.

The astronomer Carl Sagan called

these colorful chemicals tholins, Greek for muddy.

Crossing Saturn's dark side,

Cassini's camera picks up the true colors

and intricate complexity of the ring plane's variations.

Subtle gravitational effects fashioned by the many moons

sculpt the ring material.

Small moons, this one's named Pan, orbit within the rings.

They seem to sweep clear their lanes of travel.

The moon Mimas, gravitationally combing the ring's

particles, causes the large Cassini Division.

Other rings are herded into form by so-called

shepherd moons, like Prometheus and Pandora.

Tiny Daphnis, just eight kilometers across,

pushes a wave of ring material

like the bow wave of a boat on a lake.

Daphnis bobs up and down like a merry-go-round horse,

gravitationally pulling particles into dune-like

mounds three kilometers tall.

It has plowed a 35-kilometer-wide furrow in the ring field,

called the Keeler Division.

The larger Pan forges the 325-kilometer Encke Gap

in Saturn's A ring, giving it a wavy edge.

And Pan's gravity inspires a spiral wake of particles.

Based on that gap, researchers predicted Pan's existence

in the late 19th century, long before it was first seen.

But no on anticipated Pan's remarkable shape.

And it's not unique.

The slightly wider moon Atlas has a similar structure.

Both wear skirts of gravitationally attracted icy crystals.

Certain small satellites on eccentric orbits

repeatedly crash through the F ring,

leaving behind sets of disruptive splashes

as evidence of their passing.

Other minuscule moonlets, too small to be seen,

fashion propellor-shaped resonant waves

in the snowy rings falling around Saturn.

The ever-evolving complexity of these ring circles

moves like a musical score,

playing in silence.

Once every 14 years, the ring plane lies

exactly edge on to the sun.

Cassini was present for one such equinox.

In essence, the sunlight on the rings

has been switched off, and they cast

no shadows on Saturn.

They are only visible because Saturn reflects

some light on them,

here digitally multiplied many times.

Only the F ring, which is slightly tilted

from the others, catches any sunlight.

But because the rings are so thin,

any tall structure within them

throws its shadow onto the ring plane.

Thus, in the outermost edge of the B ring,

a surprise appears.

Hundreds of long shadows mark a field of icy spikes.

Geometry dictates these points must range

up to a kilometer or so in height.

Like icebergs growing in a sea of mostly empty space,

these thorn-like clumps may be molded

by the invisible fingers

of a gravitational resonance with one of the moons.

Or they may be drawn out by static electricity

inspired by Saturn's magnetic field.

Some of Saturn's rings are constantly morphing,

while others are stoically consistent year after year.

How old are these bright, icy rings?

They may have formed in the last 100 million years,

when icy moons or comets collided

or were torn apart by Saturn's gravity.

Or they might be as old as Saturn,

the leftover debris of the solar system's birth.

Cassini may yet find the true answer.

Looking down at Saturn's North Pole,

Cassini gets a spectacular view of the globe's jet stream.

Some resonance of the wind energies banks the stream

into a six-sided shape, the hexagon.

Apparently, an eastward-flowing shallow jet stream

is bumped by occasional storms.

The jet reacts by settling into the most

stable geometric shape, six roughly sides.

This polar monsoon is about 32,000 kilometers wide.

You could fit two and a half Earths into it.

Its winds blow faster the closer they get to the pole.

Up to 540 kilometers per hour.

Why kill Cassini in a ball of atmospheric fire?

The international scientific community has agreed

to avoid any possibility of contaminating

Saturn's moons, especially Enceladus and Titan.

Cassini is warm inside, essentially room temperature.

Any microbes that found their way in during assembly

could have easily survived.

And Cassini's RTG will remain hot for decades.

If she was allowed to accidentally crash on Enceladus,

Cassini might easily melt her way down

through the ice into the underground ocean.

Back in 2009, studies had showed Cassini had enough fuel

to go beyond Saturn to explore the ice giants

Uranus or Neptune.

She could have been sent back to Jupiter.

Or outward to visit a population of comet-like

asteroids known as the Centaurs.

But to get all the science possible from the Saturn system,

Cassini would have to die here.

That led trajectory designers at JPL

to develop a jewel of orbital dynamics,

the Solstice Mission.

The plan optimizes repeated fly-bys of Titan.

The route features high orbits to look

at the poles and the rings.

Long loops to map the planet's magnetic fields.

Flattened equatorial orbits to check out moons.

And ending at the peak of northern summer

to mirror Cassini's arrival in southern summer.

Between the two, scientists can infer the full cycle

of Saturn's seasons.

The science based on Cassini's data

will go on for decades.

Nudging Cassini's speed by just eight meters per second

is enough to begin her death spiral.

The craft makes repeated dives between the innermost

ring material and the roiling atmosphere.

These maneuvers would have been far too dangerous

to contemplate when Cassini was designed.

Along the way, the spacecraft maps fluctuations

in Saturn's gravity and magnetic fields.

And samples icy particles flowing in from its rings.

Until the very end, Cassini beams science data

and images directly to Earth.

Finally, on September 15th, 2017,

as the Cassini spacecraft streaks along Saturn's clouds,

its high-gain antenna shifts away from Earth.

The signal fades.

And she is gone.