A billion kilometres from the Earth,
a small probe uncoupled from an interplanetary spacecraft
and headed for its target.
It had a date with Jupiter.
In an encounter that would break all records
for speed and violence,
it accelerated to the largest planet in the solar system,
plunged into its fearsome clouds
and was swallowed by its atmosphere.
Without its atmosphere,
the Earth would not be a home to us.
Rain on our faces,
wind in our hair,
the very air we breathe.
Atmosphere is what turns our planet into our world.
To understand the other planets,
we have to understand their atmospheres.
The journey of discovery began here on Earth.
The first man to explore the limits of the atmosphere
was retired US Air Force colonel Joe Kittinger.
At the age of 72, he is still flying.
In 1960, he attempted something no one had tried before:
to leave the atmosphere behind.
'It was the first time anyone went to such a high altitude.'
The first time man had been in a space environment,
so I was concerned about what would happen.
Project Excelsior was an attempt to send a man to 100,000 feet
in a helium balloon.
If successful, Joe would be the first human in space.
At 63,000 feet, unprotected, a man's blood actually boils
because of lack of pressure.
So, for man, space is 63,000 feet.
Any place above that, you die without a pressure suit.
'The balloon was overhead.'
At the signal, they cut the straps and off I lifted.
'I had confidence in myself, the equipment and the team,
'but there's always that unknown,
'something you may not have covered.'
At 50,000 feet, Kittinger hit a snag.
The glove on his rudimentary space suit sprung a leak.
Now, what that meant was that the blood would pool in my hand,
just continuously pool and pool.
'I didn't tell my flight surgeon,
'because I didn't want to worry him and felt if I did,
'he would make me abort the flight.'
I was certain I could survive,
not having a pressure suit glove on my right hand.
So I continued on.
'Overhead, the sky was black as it could be.
'I couldn't see any stars, it was that black.'
An hour and a half after lift-off,
Kittinger reached 103,000 feet.
'I was up on a porch that was the highest step in the world,
'looking down on our planet.
'I could see the clouds below
'and the atmosphere, the haze layer.'
It was a very profound feeling I had,
the realisation of just how hostile that environment is,
and it's only 20 miles above our Earth.
'The air outside looks the same, but there's no air there.
'It's like being in an environment that's cyanide -
'you take one whiff and you're dead.
'Just 20 miles down, there was safety and comfort,
'and an environment that man is used to.'
Kittinger spent just 11 minutes in that netherworld,
on the brink of space.
And then he prepared to return.
So I took a final look around, said my silent prayer,
'hit the button to start the cameras working,
'and I jumped from the gondola.
'I fell face to Earth for a little ways.'
And I had no sensation of falling,
because I had no visual reference of anything,
so I thought I was really suspended in space.
'I turned on my back about this time and looked up,'
and the balloon was racing into the heavens.
I mean, at a fantastic rate.
To me, it was just flying away.
'What it was, the balloon was standing still,
'and I was the one that was falling so rapidly.'
Kittinger fell to Earth at the speed of sound,
but with no air around him it was a silent fall.
'I had no ripple of the fabric on the pressure suit.'
'It was a very weird sensation.'
After four minutes of falling through space,
Kittinger began to re-enter the protective blue haze,
and was hit by the familiar world of cloud, sky and air.
Joe Kittinger had gone beyond the edge of the sky,
and lived to tell the tale.
'15 minutes before, I'd been at the edge of space,
'and now I was in the Garden of Eden.
'We don't appreciate what a beautiful planet we have.'
'The atmosphere is a tiny layer, it's insignificant,
'in terms of mass, compared to the planet.
'And yet it is so key to our experience
'and our very existence here on this planet.'
Dave Grinspoon is an atmospheric scientist.
'When I first got excited about the planets,
'it was pictures of the surface of a planet,
'the most obvious, tangible thing that grabs you.
'But as I studied planets,
'I learned about the frontiers of knowledge,
'some of the real mysteries,'
crucial to understanding how the solar system became how it is,
how Earth achieved its unique status.
And I found that a lot of the problems that interested me
had to do with the evolution of the atmosphere.
As far as early astronomers knew,
other planets were just like the Moon: airless balls of rock.
But in 1761,
a Russian star-gazer observed an unusual event.
From an observatory in St Petersburg,
Mikhail Lomonosov saw the planet Venus
pass across the face of the sun.
He noticed that the edge of the planet
wasn't crisp like the Moon.
Venus seemed fuzzy.
As the planet crept past the sun,
he was astonished to see it surrounded by a thick halo.
It was a sure sign of an atmosphere.
For two hundred years,
what lay below the clouds remained a mystery.
Was Venus a world like Earth?
The Russians sent a probe to find out.
As Venera 4 headed for a splash-down on Venus,
the designers' main worry
was their spacecraft might sink and radio contact would be lost.
In October 1967,
radio dishes across Russia were trained on Venus,
eager for news from the world beneath the clouds.
Under a pressure 15 times that on Earth,
and when it was still 15 miles up, Venera 4 was crushed.
The probe didn't splash into an ocean. It never even came close.
What kind of hell lay below the clouds of Venus?
The Russians were determined to land a probe there.
They tested their probes to the limit.
The Soviets set about recreating Venus on Earth.
To mimic the severe atmospheric conditions,
they built the world's biggest pressure cooker.
After four years, they had a craft tough enough
to survive the crushing inferno of Venus: Venera 7.
Just before Christmas 1971, Perminov and his team
saw the probe's faint signal reporting touch-down.
They had made it.
'The first glimpses we got'
of how it looks on the surface were from the Russian pictures
of small pieces of strange volcanic landscapes on Venus.
'Those pictures were mostly of the ground.
'But just in the upper corner, you could see a bit of sky,
'this glowing - but featureless because it's cloudy - sky.'
That was really neat, being on the surface of Venus,
catching a glimpse of what it's like to see the sky of Venus.
But what had the probe detected as it fell through that sky?
'If you were plummeting through the atmosphere of Venus,'
first you'd have to make it through the extensive clouds.
They're not like clouds on Earth.
They cover the entire planet, over ten-miles thick,
so would take a long time to get through.
'When you're actually in them, they're diffuse, more like fog.
'But what would it be like to stand on the surface of Venus?'
First you would scream, and nothing else would happen.
You'd be instantly consumed by the hot, noxious atmosphere.
But if you had a good suit
and walked out onto Venus,
the first thing you'd notice would be the murky red light.
'On Venus, in the middle of the day,
'it's about as light as on a deeply overcast day on Earth.'
You'd never see the sun from Venus,
but sunlight filters through the clouds.
Venus is a world of unchanging and extreme weather.
In the high clouds is a constant drizzle of sulphuric acid.
And far below, close to the surface,
the probes detected an unending stream of electrical discharges.
(Eerie monotone humming)
Why there is lightning so far below the clouds,
where it's too hot for rain, is a mystery.
This hot, high-pressure, corrosive atmosphere
held one more surprise.
In 1990,
an American spacecraft pierced Venus's clouds with radar.
It saw that some of the highest mountain tops were very bright,
as if they were covered in something reflective.
'The high mountain peaks seemed to be coated
'with some kind of metallic or shiny frosting,
'something reflecting radar energy,
'and we don't know what it is.'
It may be coated with tellurium,
a trace metal on Earth, which may be common on Venus,
and seems to have the right properties.
Below a certain temperature, above a certain altitude,
it may be frosting all the high peaks of Venus.
Mountain tops on Earth are covered with frozen water.
On Venus, the snowcaps are more exotic.
It seems there's metallic snow on the highest peaks of Venus,
which is a strange thing to think about.
Venus turned out to be stranger than was ever imagined.
Would our other near neighbour be more similar to Earth?
In 1971, two Russian spacecraft were on their way to Mars.
As the probes barrelled in,
Mars was suddenly enveloped in a giant dust storm.
After transmitting for just 15 seconds, the craft fell silent.
The flight controllers had no doubt
that the violent dust storm was to blame.
But this was the first picture from the surface of Mars.
Incomplete and fuzzy - but there was a horizon,
and beyond it, the shadow of a dark sky.
Five years later, another spacecraft headed for touchdown
on the surface of Mars.
This time it was American.
It was called Viking.
'OK, maybe we could take this opportunity
'to summarise where we are today?'
To date, we have, of course, landed on the surface of Mars
and taken pictures immediately after landing, and since then.
'At the present time, we are slowly watching the build-up
'of data that's going to be built into a colour picture.'
And the way we're going to do it is bring up, first the red,
which you see now.
'Here comes the green, it'll be the second,
'and then finally the blue.
'With the last one coming down, you're now going to see
'the surface of Mars as best we can configure it in colour
'at the present time.
'You can see a number of different...'
'If you douse the lights... Oh, my gosh!'
'I wasn't even seeing all the colours. Look at that!'
'That's pretty spectacular. It does have a reddish hue to it.'
'And look at that sky. Light blue sky.'
But there'd been a mistake.
Because they expected the sky to be its familiar colour,
the engineers had unwittingly filtered in too much blue.
The next day, they showed Mars the way it really is.
This world has a pink sky.
Its thin atmosphere is tinged with red dust.
Mars is a desert world.
From day to night,
the temperature swings by hundreds of degrees.
On Mars, the colours of the heavens
are turned on their heads.
Red skies at noon and blue skies at sunset.
So, how did Mars and Venus,
both rocky planets like the Earth,
get such different atmospheres from our own?
It's a puzzle that atmospheric scientists
are just beginning to solve.
'A big part of my study is understanding the divergence
'of Earth's atmosphere
'from our neighbouring planet atmospheres, Mars and Venus.'
And I say divergence,
because the three probably started out very similar,
much more than they are today.
Four and a half billion years ago,
the planets formed from a giant cloud of dust and ice.
Mars, Venus and Earth were worlds of seething lava,
pummelled by meteorites and surrounded by a veil of steam.
Mars didn't have enough gravity to hold on to that atmosphere.
But while the Martian air was drifting away,
Earth and Venus remained covered in a thick layer of fog.
After that, Earth settled down and formed oceans. It rained.
'The same may have happened on Venus.'
Venus probably had oceans when it was young,
but that steam in the atmosphere,
with enhanced sunlight, being closer to the sun,
led to what we call a "runaway greenhouse".
Venus never lost its steamy clouds,
and like a thermal blanket, they trapped the heat in forever.
The hotter it got, the more steam boiled off the oceans.
The planet became locked in a global greenhouse effect,
and it meant disaster.
Eventually, Venus's oceans just kept boiling, and boiled away.
On Earth, we have a small greenhouse effect,
because of the CO2 and water in our atmosphere.
and it's a good thing.
The greenhouse effect gets a bad rap
because of global warming.
We worry about too much effect.
But if we didn't have one, we wouldn't be able to live here.
'We get 30 degrees of warming from Earth's greenhouse,
'just enough to keep it in the temperate range it's in.'
For planetary scientist Andy Ingersoll,
the delicate balance of Earth's climate is fascinating.
'I would say that we've learned there's a narrow habitable zone
'around every star.
'Mars is too cold, Venus is too hot.
'Earth is just right, and the extremes are frightening.'
For me, I'm a meteorologist,
the fascinating thing about Earth compared to other planets
is it has the most unpredictable weather in the solar system.
The dominant weather on Venus seems to be
just the circulation of the atmosphere.
'It'd be probably pretty boring if you could stand the heat,
'day after day of temperatures hot enough to melt lead.'
That's predictable.
'On Mars, the dominant weather you'd notice
'would be the day /night cycle,
'like a desert or semi-desert area on Earth.'
It's another sunny day and it will get cold at night.
'And that's predictable.
'The Earth is sort of in between,'
and that's where the weather is unpredictable.
There is another world with weather,
and it's written on a colossal scale.
The giant planet Jupiter.
'The whole planet is atmosphere because it's a gas ball.'
And there we have examples of storms that last for 300 years,
or maybe even longer than that.
I knew there were 300-year-old storms,
and I had this picture of these storms calmly spinning.
And when Voyager started taking close-up pictures,
we realised that at the smaller scales, it was chaos.
'Everything was changing.
'The tiny storms were coming and going every few days,
'yet we had these large storms that were very stable.
'And it deepened the mystery of why the large storms endured.'
Jupiter's Great Red Spot has existed for three centuries,
but its future is by no means certain.
The Great Red Spot is three times bigger than Earth,
but it's only one of a whole class of large storms.
The next largest class was three white ovals
that formed about 60 years ago
'and had been keeping their distance from each other.
'But last year two of them merged,'
so it was a historic event for Jupiter weather men.
What forces could be driving the giant storms?
Voyager could only look at Jupiter's surface.
To find out more,
a probe would have to dive right into those swirling clouds.
NASA put one of its most experienced engineers
to work on the design.
'For Jupiter, the central problem was'
that the planet is so massive, so gigantic,
that in falling into it from a great distance, as a probe does,
as a probe approaches a planet it falls into the gravity field,
'and falling into Jupiter brought you up to speeds
'of sixty kilometres per second.
'If you figure that in miles per hour, it's a big number.'
120,000, 140,000 miles per hour, something like that.
In the 1960s, Al Seiff had designed the heat shields
that saved pioneers like John Glenn and Neil Armstrong
from burning up on re-entry.
Seiff had a hand in almost every spacecraft NASA ever built.
'Spacecraft is stable.
'Galileo is on its way to another world.'
But Jupiter was his biggest challenge.
'We worked, as it turned out,
'for nearly 20 years in getting that experiment'
mounted and ready to go,
and finally to fly out into the solar system to that distance.
And it was the least certain of success of any mission,
because of the challenging entry conditions.
In December 1995,
Galileo began its kamikaze dive into Jupiter.
'The heat shield itself was made out of carbon phenolic.'
Carbon heats up to about 4,000 Kelvin
and then it starts to vaporise, and that's what this shield did.
'The heat shield was about 80% vaporised, about 20% remaining.
'It was white hot, white hot.'
When the fiery entry was over,
the probe began drifting gently into the planet's atmosphere.
It was hoped that Galileo would detect clouds and rainfall,
and uncover the secrets of Jupiter's weather.
But Andy Ingersoll was frustrated.
I was surprised at how little water they found,
because water is so important to the weather on Earth,
and it presumably was important to the weather on Jupiter.
Although Jupiter is covered with clouds,
Galileo had the bad luck to fall into a narrow gap between them.
It appears Galileo went into a desert of Jupiter,
and that if we'd gone somewhere else, we would have found rain.
As it sank ever deeper into the atmosphere,
Galileo detected the winds picking up
and the weather becoming more turbulent.
The probe was approaching the source of the giant storms.
But it could give no more clues.
Three hours after it began its descent,
Galileo ventured too far into Jupiter's boiling interior.
'There was no buoyancy to stop it from going down,
'so the high temperatures in the interior'
ultimately caused the probe to melt and vaporise,
and today the Galileo probe is part of Jupiter's atmosphere.
We have actually slightly contaminated this giant planet.
Was there another world to compare to the Earth?
For centuries, astronomers had watched
a tiny object circling near the rings of Saturn.
It was bright, like Venus.
It almost seemed to glow.
Could this world have an atmosphere too?
Astronomers were captivated.
It was the moon Titan.
'I think of the astronomers of yesteryear -
'like Christiaan Huygens who discovered Titan
'and those after him, who spent their lives
'looking through telescopes, trying to eke out information
'about the solar system, and about Titan in particular -
'and feel a tremendous sense of sympathy for them.'
Modern-day planetary scientists like myself,
know that we can hypothesise and come up with theories,
but eventually we'll know exactly what the truth is.
The first close-up pictures of this enigmatic moon
came in 1980, when Voyager flew by.
'It was clear it was a satellite with an atmosphere,
'which made it unique,
'because till then we associated atmospheres with planets.'
The composition of Titan's atmosphere,
the fact that it's 90% to 95% molecular nitrogen,
along with the fact that its atmosphere is pretty dense,
surface pressure is comparable to that of Earth - a bit more -
makes it kind of an analogue with the Earth,
which is surprising, because no one expected years ago
you'd find an analogue of Earth out near Saturn.
The surface of Titan remained hidden from Voyager's cameras.
But since then, the Hubble Space Telescope
has managed to peer through the thick clouds.
There are tantalising signs
of what look like continents and oceans.
Some say this mysterious moon has seas of liquid methane.
It is a world planetary scientists
are more eager than ever to explore.
In a matter of a few years, once we get to Titan
and once we get to Saturn, we'll find out a great deal
about what makes this object tick.
'Now we have an entire mission devoted to the study of Saturn,
'and Titan is a major target.'
It's the Cassini mission, launched in October '97.
I, of course, was there.
It was the most spectacular thing I'd ever witnessed.
'It was also very emotional because I and my colleagues
'have spent seven years developing instrumentation
'in the space craft itself to go back to Saturn,
'and now we have a six-year wait for it to get there.'
Cassini will arrive at Titan in November 2004,
and release a probe that will embark on a journey
down to the shrouded surface of this distant moon.
Parachutes will deploy, the air shell will be jettisoned
and the probe, instrumented with six scientific investigations,
will begin a two-and-a-half-hour descent through the atmosphere.
'On the way down it will measure the pressure, the temperature,
'the composition of the atmosphere,
'and will continue down until it finally breaks through
'what we think to be the lower cloud deck.'
The probe will slowly sink into an alien atmosphere,
taking pictures throughout its descent.
'We don't know what kind of material it might land in.'
The surface of Titan is likely to be ice, covered with debris
that has rained out of the atmosphere.
There may also be liquid on the surface, seas and lakes.
With pools of methane, oily fog, maybe even rain,
Titan could bear a frigid resemblance to the Earth.
'Here is a body with a surface and an atmosphere.
'You could, properly equipped, walk on the surface of Titan,
'and see the results of wind erosion and flowing liquids,
'and seas and waves and winds.'
It has almost a feeling of home to it, and I think
that's a tremendously powerful feeling for us.
Titan has brought back the spirit of the early days
of planetary exploration.
There's a new world out there to discover.
'Our current knowledge of Titan is comparable to our knowledge
'of Venus in the '50s and '60s.'
In those days, when we were trying to study Venus,
we imagined Earth-like scenarios,
and I suppose we're doing the same for Titan.
We're imagining lakes,
although we imagine they're made of hydro-carbons,
but still we feel comfortable with that concept.
It may turn out Titan is dry as a bone, just like Venus.
'You learn humility.
'You study Earth and think you understand it,
'then say "OK, I have these principles,
'"I'll apply them to another planet. Let's go there."
'You're always surprised.
'It's always not what you expect,
'and that teaches you how little you know,
'and how much there is to know.'