NOVA

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