ZACHARY QUINTO: A billion miles from the sun... (rocks thudding) Past Mars and Jupiter... A planet like no other.

MICHELE DOUGHERTY: All you can see is this beautiful vision of the rings around it.

♪ ♪ QUINTO: Saturn.

Of all the distant worlds... EDGARD RIVERA-VALENTIN: 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.

(wind whipping) QUINTO: ...it's the most mysterious and alluring.

(lightning crashes) ♪ ♪ It's just a puzzle you want to solve.

QUINTO: We've visited three times.

Pioneer and Voyager offered a passing glimpse.

We started taking pictures several weeks out.

CAROLYN PORCO: It's a frenzied period of lots of activity and sleepless nights and discovery and conquest.

And then it's all over.

QUINTO: But Cassini came to stay... And what a wonderland we found.

QUINTO: ...and to astonish.

(thunder rumbles) KEVIN BAINES: We found water ice, and that basically blew our minds.

♪ ♪ PORCO: 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.

♪ ♪ JONATHAN LUNINE: 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.

QUINTO: And in the end a heartbreaking choice to save a world.

♪ ♪ JULIE WEBSTER: I was supposed to call end of mission.

I couldn't get the words out.

It's still hard.

♪ ♪ QUINTO: "The Planets: Saturn."

Right now, on "NOVA."

♪ ♪ ("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 ♪ ("The Void" ends) ♪ ♪ QUINTO: 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.

PORCO: 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.

♪ ♪ QUINTO: Saturn is a planet of mysteries.

(humming, electricity crackling) 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.

PORCO: 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.

(wind blowing, thunder rumbling) (lightning crashes) ♪ ♪ QUINTO: Picture a place with no surface on which to stand... (thunder rumbling) only endless atmosphere.

(crackling, rumbling) ♪ ♪ 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.

(rocks thudding) 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.

LEIGH FLETCHER: 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.

LUNINE: Now, what that snow line does is it puts a large amount of additional solid material into this disc.

(thudding) 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.

♪ ♪ QUINTO: 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.

(rock rumbling, crashing) 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.

(rocket engine igniting) ♪ ♪ The Voyager probes are two almost identical spacecraft built to explore the solar system's distant worlds.

♪ ♪ PORCO: 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.

♪ ♪ LINDA SPILKER: 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.

♪ ♪ JEFF CUZZI: They're all these ringlets and structure and bright and dark, beautiful, beautiful sight.

(whirring) ♪ ♪ CARL SAGAN: 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.

♪ ♪ PORCO: It's a frenzied period of lots of activity and sleepless nights and, and discovery and conquest, and then it's all over.

♪ ♪ QUINTO: 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.

♪ ♪ LUNINE: 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.

♪ ♪ BAINES: 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.

♪ ♪ QUINTO: 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?

♪ ♪ PORCO: 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.

♪ ♪ QUINTO: 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.

DOUGHERTY: 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.

♪ ♪ PORCO: All of us spent a lot of effort to get to that moment, uh, so there's a lot of emotion attached to it.

(rocket ignites) 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.

♪ ♪ QUINTO: 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.

(hissing, small explosion) 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.

QUINTO: 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.

CUZZI: 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.

♪ ♪ WEBSTER: 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.

MAN (on radio): The Doppler has flattened out.

(cheers and applause) WEBSTER: So there was a great deal of relief.

(cheers, laughter) QUINTO: When Cassini arrives, the images it sends back to Earth are breathtaking.

CUZZI: 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.

♪ ♪ CARL MURRAY: 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.

♪ ♪ PORCO: 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.

♪ ♪ QUINTO: Thanks to Cassini's powerful instruments, Saturn's billion-year-old mysteries start to become clearer.

(low humming) 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.

BAINES: 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.

♪ ♪ QUINTO: 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.

FLETCHER: 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.

♪ ♪ QUINTO: What began as a rocky, icy world is now a fully fledged gas giant.

And within its gaseous atmosphere, incredible mechanisms are at play.

♪ ♪ (thunder rumbling) ♪ ♪ 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.

♪ ♪ BAINES: 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.

FLETCHER: 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.

QUINTO: 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.

♪ ♪ (thunder rumbling) (wind blowing, lightning crashes) QUINTO: Looking deep inside the planet reveals a heat source shaped by extreme pressure.

(wind blowing, thunder rumbling) Within huge clouds of water, Cassini records lightning 10,000 times more powerful than any on Earth.

(thunder rumbling, lightning crackling) (crackling) This lightning transforms methane gas into enormous clouds of soot.

♪ ♪ (crackling) 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... (gems tinkling) 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.

♪ ♪ CUZZI: 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.

♪ ♪ PORCO: 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.

QUINTO: But the sparkling brilliance of Saturn's rings are a mystery.

CUZZI: 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.

♪ ♪ BROOKS: 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.

♪ ♪ CUZZI: 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.

PORCO: 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.

♪ ♪ QUINTO: 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.

SPILKER: 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.

♪ ♪ LUNINE: 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.

♪ ♪ QUINTO: 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.

PORCO: 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.

♪ ♪ QUINTO: 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.

♪ ♪ SPILKER: 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.

♪ ♪ QUINTO: A leading theory suggests that just beyond Saturn's atmosphere, an ice moon approaches close to or even just inside its Roche limit.

♪ ♪ (rumbling) As Saturn's immense gravitational force pulls it apart, the moon begins to rupture catastrophically... ♪ ♪ (rumbling) a world ripped apart by its proximity to a giant.

♪ ♪ (rumbling continues) ♪ ♪ (distant rumbling) Up to 17,000 trillion tons of ice... (rumbling) 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.

♪ ♪ PORCO: The images we returned, they were phenomenal.

They were resolution factor of maybe 20 times better than anything we had had before.

♪ ♪ QUINTO: 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.

PORCO: 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?

(chuckles) Really I've, I've said over and over again, they should put that in a movie.

♪ ♪ QUINTO: 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.

PAPPALARDO: 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.

(crackling sound) ♪ ♪ PORCO: 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.

♪ ♪ QUINTO: 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.

DOUGHERTY: 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.

♪ ♪ PORCO: 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.

(Cassini whooshing by) We flew through the plume in about a dozen of those.

♪ ♪ QUINTO: 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.

SPILKER: We were able to sample directly material coming out of Enceladus's plume.

And what a wonderland we found.

LUNINE: 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.

PAPPALARDO: 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.

QUINTO: 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.

♪ ♪ DOUGHERTY: 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.

♪ ♪ CUZZI: 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.

PORCO: 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.

♪ ♪ QUINTO: 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.

LUNINE: 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.

♪ ♪ QUINTO: But for Cassini's scientists, the exotic world under the ice of Enceladus is a bittersweet discovery.

♪ ♪ DOUGHERTY: 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.

MAN (on radio): This is ACS One, we just had transition to high rate mode.

QUINTO: 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.

DOUGHERTY: 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 whooshing) QUINTO: Cassini survives its approach to Saturn intact and continues on towards its demise.

♪ ♪ DOUGHERTY: 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.

MAN (on radio): Okay, we call loss of signal in one-one-five-five-four-six.

FLETCHER: 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.

(wind blowing) MAN: We have just heard the signal from the spacecraft is gone, and within the next 45 seconds, so will be the spacecraft.

(hissing, rumbling) DOUGHERTY: And then it lost control, it began to tumble.

(whooshing) QUINTO: 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.

(applause) WEBSTER: 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.

♪ ♪ QUINTO: 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.

♪ ♪ CUZZI: 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.

♪ ♪ SPILKER: 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.

♪ ♪ ZACHARY QUINTO: The mysterious outer worlds... the solar system's strangest realm.

(lightning crackles) DERRICK PITTS: This is a region of the solar system that we really had very little information about.

♪ ♪ ALAN STERN: A small planet should run out of energy and stop evolving billions of years ago.

Pluto didn't read any of those textbooks.

(chuckles) QUINTO: "The Planets: Ice Worlds"-- next time on "NOVA."

♪ I'm losing sight of our reality ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪