Horizon

A billion miles from home.

Running low on fuel.

Almost out of time.

We're putting down a fully functioning spacecraft.

After 13 years orbiting Saturn, the spacecraft Cassini

is about to plunge to a fiery death, becoming part of the

very planet it's been exploring.

And at Nasa's Jet Propulsion Laboratory, the entire mission

team has gathered from across the globe to say goodbye to a great

explorer, and to celebrate an extraordinary mission.

Cassini was not so much of a mission as a way of

life.

I've spent 20 years of my life on this.

27 years I've worked on the project.

What we found was spectacular.

It was like a Jules Verne adventure come true.

Cassini sent back dramatic visions of strange new worlds.

Nothing of what we had imagined came close to

the reality.

It revealed methane lakes, electric dunes, buried

oceans.

That Moon has an ocean in it that could harbour life.

It's the end of a mission that has not only

unveiled the beauty and probed the mysteries of Saturn,

but has changed our view of the solar system forever.

Playback is starting to run.

Systems lead, mission planning.

Go ahead, mission planning.

Spacecraft has just crossed 30 degrees north

latitude, attitude is 6,000 miles.

The dead of night, Friday the 15th of September.

In the control room of Nasa's Jet Propulsion Laboratory,

the flight director and her team of engineers

are presiding over the

death of a spacecraft that they have nurtured for nearly three decades.

In less than an hour, they will watch as Cassini sends its final transmission.

It's sad.

It was 27 years of the investment of many lives into this.

Nearby, at the California Institute of Technology,

is the vast family of scientists from around the world who run the 12

instruments on Cassini, measuring everything

from infrared to radar, magnetic fields, and cosmic dust.

It's a big family, indeed.

This is one of the aspects of this mission

that I will miss more.

In some ways, I just want it to be over now.

You know, you're sort of waiting for it

to end and it's really weird.

Very mixed emotions at the moment.

Really strange.

All have played a vital role in exploring Saturn.

But it's Cassini's skills with a camera that

have been truly inspirational.

Head of the camera team and guardian of

the Cassini image gallery is Carolyn Porco.

Carolyn has been responsible for designing the camera

systems, deciding when, where, and which pictures to take, and

delivering and processing hundreds of thousands

of images of Saturn, its rings, and moons.

This is one of my favourites.

The reason why I love it is because because we're seeing the

rings edge on and you just really can tell how thin they are.

You can get the sense of three dimensionality, because we see the

dark shadowing.

It so beckons for you to go there.

And this is the famous hexagon, OK?

This is the feature that drives everybody crazy

because they don't expect to see something

that is straight sided in an atmosphere.

It is nothing more than a jet stream like our jet stream, OK?

Really, you can relax.

This is not the end of the world.

Back at the rings, I mean, have you ever, ever

seen anything just so magnificently regular and beautiful?

With its two on-board cameras, Cassini has

unveiled the Saturn system as a strange, mysterious, and unique

place.

Saturn just captivates people because it looks so supernatural.

It engenders the immediate feeling that

it can't possibly be real.

But we're lucky to have it.

I think we're dead lucky to have Saturn.

I'm glad I grew up in a solar system that had

a planet like Saturn.

But taking and downloading such amazing pictures,

when your camera is a billion miles away, through the bleak void of

outer space, is no mean feat.

In charge of remotely managing the spacecraft from Nasa's Jet

Propulsion Laboratory is Julie Webster.

You know scientists and engineers are true nerds, and you'll

see a lot of nerd stuff.

Julie is Cassini's chief engineer.

She's been caring for the intrepid explorer

since before it left home, and uses a scale

model of the spacecraft to

demonstrate its capabilities.

This is a quarter sized model, so this is

a quarter sized me.

I'd like to be this weight, not this height.

So you start at the top.

This is a four metre antenna.

It was also designed for radar, so this is the composite

infrared spectrophotometer.

This is the ultraviolet spectrophotometer.

This is the VIMS, visual and infrared, and these

are the imaging cameras, the narrow angle and wide

angle, that are used for the actual photos that you see.

So this is the ion and neutral mass spectrometer

that is the most important data that we're going to get in the last few

hours.

And here's part of the magnetometer instrument, and here's

the other half of the magnetometer instrument.

The trick is...

The reason you want to put it out on a

boom is you do not want to measure the magnetic field of the

spacecraft.

You want to measure the magnetic field of Saturn, or

whatever you're at at the time.

Let's see.

That's the nuclear battery.

I actually sat inside this part during

the build, so when I close my eyes and think of Cassini, I actually see

the wiring inside the spacecraft and the things

switching on and off, and

when it goes in, that's what I'm going to see in my mind is the

aluminium melting on the structure.

I just never thought it would happen.

And now it is.

So now we're headed back up to the first floor to my office.

Cassini divides its time between gathering

data, which it stores on

its internal memory, and then sending it back home.

A typical day, we're out taking pictures,

collecting data, storing it on the solid-state recorder,

and then we'll turn back to Earth and we'll play it

back for nine hours.

We only have four gigabits of data that we can

load up, and it takes nine plus hours to play that data back.

For nine hours at a time, up to six times a week, Cassini sends its

precious images of Saturn on the billion mile

journey back to Earth.

Our fascination with Saturn stretches back to the very first

images that revealed it as a planet like no other.

It was here at the Paris Observatory that Giovanni

Cassini, after whom the spacecraft is named, first observed the main

division in the rings.

Astronomer Pascal Descamps has set up this

telescope to mimic the view seen by Cassini.

This view may seem blurry and indistinct, but it was a

dramatic improvement from the dot of light seen with the naked eye.

And it was enough to allow Cassini to

spot that the rings were not a solid disc.

And it's the nature of these rings that has driven scientists

ever since.

In the 342 years since Giovanni Cassini's blurry but historic

image, our view has improved dramatically, largely thanks

to spacecraft Cassini.

And there is one image from Cassini that may have

done more than any other to alter our perception

of our own place in the solar system.

This, of course, is something I'll be boasting about

for the rest of my days.

It's something I call the day the Earth smiled.

And it is taken when the sun is behind Saturn.

This, of course, you can't do from the surface of the

Earth.

We're on the other side of Saturn in this picture.

But if you look really close over the shoulder

of Saturn, between the E ring and the G ring,

you can see our own planet Earth, a billion miles in the distance.

We have cleared the tower.

And the Cassini spacecraft is on its way to Saturn.

So what does it take to get a spacecraft the size of a bus

with 12 delicate instruments all the way to Saturn and to keep

operating for 20 years?

And the solid rocket boosters have been jettisoned.

So we'll go into the dark room.

Inside the dark room, the mission support

area is where the whole flight team gathers for critical events

like the final plunge, launch, or orbit insertion.

It took seven years to get to Saturn, a spiralling journey that

involved four gravity assists,

close flybys past planets that boost the speed like a slingshot.

It's a flight path designed by head of navigation Duane Roth.

We had a Titan 4B.

Titan 4B.

So that was to get us away from the Earth,

and it still wasn't quite enough to get

all the way to Saturn, so we

had two Venus flybys, gravity assists, from

each of those, an Earth

gravity assist, and finally we had a Jupiter gravity assist,

and that got us to Saturn.

But credit where credit is due.

We built on Galileo.

Galileo built on gravity assists and every mission has

taken it to a new level.

Once Cassini was approaching Saturn, it had to perform possibly

its most critical manoeuvre of the mission.

A perfectly timed engine burn that would slow it down enough