On January 2nd, 1959, with the space age barely a year old,
the Soviet Union launched Lunik - "little moon".
It was sent to plant a Soviet pennant on the moon.
Within hours of the launch, it became clear
that Lunik was going to miss its target.
As the Soviet scientists watched their tiny probe
sail out to join the planets
in an endless journey around the sun,
an inspired thought occurred to them.
They renamed their spacecraft Mechta - "The Dream".
In 1926, when this recording of Holst's Planets suite was made,
there were thought to be eight planets.
Then, in 1929,
a young man arrived at an observatory
in Flagstaff, Arizona, to start the search for a ninth.
At that time, little was known about the planets.
Closest to the sun lies Mercury,
a tiny world of iron and rock, barely discernible in the glare.
Then Venus, perhaps a second Earth,
hidden beneath a blanket of cloud.
Then Earth.
And beyond us, Mars, the Red Planet.
It has seasons, polar caps,
and the possibility of life.
Far beyond these rocky worlds are the distant giants.
Jupiter, over 1,000 times bigger than the Earth,
and Saturn, with its distinctive and dramatic rings.
The two remaining planets
are 15 times the size of the Earth, yet are so distant
that they appear as the faintest of stars.
Uranus - an aquamarine mystery.
And finally, Neptune,
a world that moved unevenly across the sky.
This irregular movement
suggested the presence of a more distant planet,
whose gravitational tug might be toying with Neptune's orbit -
Planet X.
'February 18th, 1930.
'Clyde Tombaugh, sitting in an office
'very near to where we are now,
'looking at the photographs he had taken of the night sky...'
With his eye at the eyepiece of the blink comparator back there.
'And he had been searching on the plates
'that were centred on a star
'in the constellation of Gemini, the Twins.'
He had started that morning.
He had moved slowly across,
click, click, seeing one image,
then the other, keeping on moving back.
'All these images were negative, all the stars.
'And anything else would be black on a white background.
'At 4pm, he crossed the plate's centre.
'He passed the area where the guide star was.
'The star Delta Geminorum -'
big, big bright star.
He moved a little bit more, a little bit more,
'and then he saw a very faint, a very faint black dot.'
Then he blinked to the other one on the other plate,
'and he saw it appear here and there.'
On his plates, taken several days apart,
Tombaugh noticed that a point of light had moved.
He knew instantly this was what he was looking for.
It was an historic moment.
'He took the walk from the comparator room,
'all the way down to the director's office,
'and he stopped, did his tie and combed his hair a little,
'and said "I wanted to appear a little nonchalant about this."
'Then he stepped into the office...
(Clears throat)
"Dr Slipher? I have found your Planet X."
Planet X was soon named Pluto.
It marks the end of the solar system.
A tiny world of ice, smaller than our moon,
now known to have its own satellite, Charon.
But Pluto patrols the outer edge of the solar system,
in the distant realm of giants.
Worlds of swirling water, like the azure Neptune,
and Uranus, which mysteriously orbits the sun
spinning on its back.
Pluto lies way beyond the gargantuan worlds,
the gas planets that have no landscapes:
Saturn, with wind reaching
thousands of kilometres per hour,
and Jupiter, that has an Earth-sized storm
that has lasted for centuries.
The closest worlds to the sun
are small islands of rock and iron.
Mars, with its faint atmosphere of carbon dioxide,
and Venus, smothered in clouds of sulphuric acid.
Then there is Mercury,
boiling in sunlight, and freezing at night.
Nine different worlds, with seemingly little in common,
save that they orbit a single sun
and are bound together by its gravity.
And then there is the Earth.
A small planet in the measure of the solar system.
It has a thin atmosphere that clings to a rocky surface.
But the Earth is different. It is special. It has life.
What process could create such a variety of different worlds?
Hal Levison is at the forefront of a branch of astrophysics
that is still struggling with the mystery
of how the planets formed.
'It's amazing,'
when you consider that
all planets in the solar system:
the Earth and the rocky planets,
the cores of the giant planets, Jupiter and Saturn,
and the majority of the outer planets,
Uranus, Neptune and Pluto,
formed from material that is very fine pieces of dust,
much finer than the dust I'm holding in my hand.
About the consistency or size
of particles of dust in cigarette smoke.
I was an astrophysicist,
interested in an obscure type of galaxy, when five years ago
I got the bug of trying to understand
how material like this can form the planets we see today.
By the 18th century, astronomers had discovered that galaxies
are filled with drifting clouds of gas called nebulae.
Perhaps these clouds were the raw materials of the planets.
Two men, the philosopher Immanuel Kant
and the mathematician Simon de Laplace,
looked at the uniform direction
of the orbits of the planets in the sky.
They suggested the planets were a relic of a cloud
of dust and gas that circled the sun during its formation.
In a single process, they concluded,
the solar system was born.
The idea was elegant, and quite brilliant,
but the complex details of their theory
lay centuries in the future.
Its proof had to wait for the arrival of the space age.
September 1944. London was under siege.
Mysterious weapons were raining down from the sky.
Hitler's vengeance weapon threw people into confusion.
Nothing had prepared them for a supersonic missile
that took just six minutes to travel from mainland Europe.
The technology behind these missiles was highly advanced.
It had been developed by a brilliant young engineer
called Wernher von Braun.
Von Braun's rocket was called the V-2.
Designed to save the war for the Nazis, eventually it became
the foundation of our journey to the planets.
When Germany fell,
American troops headed straight for the V-2 factories.
Before the dust had settled in Europe,
von Braun and his team of engineers found themselves
working for the United States Army.
In the deserts of New Mexico, the captured rocket parts
were reassembled by German and US engineers.
The modified V-2s soon flew beyond the range of cameras.
Engineers fixed astronomical telescopes
to anti-aircraft gun mounts.
The system was designed by Clyde Tombaugh,
the discoverer of Pluto, and his films still survive.
The Americans destroyed the German rocket factories
to keep von Braun's secrets from the advancing Red Army.
But when they arrived,
the Soviets found enough to take back to Moscow.
The man given the task of piecing together the rockets
was Sergei Pavlovich Korolev,
and Boris Chertok was his right-hand man.
While their brief was to develop rockets
which could reach America,
Korolev's eyes were firmly fixed on the planets.
But it was the Americans who made all the early running.
By the end of the decade, they were strapping film cameras
to rockets, and sending them high above the atmosphere.
The cameras had to endure an 18-mile plummet back to Earth.
Miraculously, some survived,
and astronomers got their first glimpse of the only planet
they couldn't see with their telescopes.
For the first time, we could see the curvature of the Earth.
The arcing horizon was a humbling reminder
that we were living on a gigantic ball of rock and iron.
How such a world could have grown from a cloud of dust
seemed more baffling than ever.
George Wetherill has dedicated his career
to the question of planet formation.
When he started, the science was dominated by one man.
No great scientist ever devoted his life
to understanding this problem.
It was sort of a hobby,
something they did on the side.
And I think the first person to really devote his life to this
was a Russian scientist named Victor Safronov,
who started working on these problems
shortly after World War II.
And he tried to identify what all the scientific problems are
that you need to understand and need to solve,
in order to understand the grand problem
of the formation of the solar system.
And to this day, his lists of problems are essentially
the same problems that we're working on today.
Victor Safronov revisited the 200-year-old idea
that the planets formed from a disc of gas and dust.
He set about structuring this complex process
into comparatively simple stages.
The first stage is still not fully understood.
Remember, we're starting off with very fine pieces of dust,
and the process of how you get from that
to something the size of a boulder, or even a mountain,
is actually not very well understood.
The party line of what most people think actually happened,
was that you had this disc of dust.
Dust settled into the mid plain of this protoplanetary nebula.
'And you got what's called gravitational instability
'that formed big clumps,
'things maybe the size of 100 metres in diameter.'
Safronov's second stage was less complex.
It was called accretion.
He calculated that in a remarkably quick time,
the clumps would gather together,
building the embryos of planets.
As they grew, a new force became significant - gravity.
'An amazing thing happens that Victor Safronov discovered.'
That is, as these things start to grow,
the bigger something gets, the more it can eat.
So you end up with this runaway situation,
where the bigger guys are getting bigger still,
and it's sort of a race to eat up the little guys.
And so you start off with an un-countable number
of objects the size of mountains.
And you end up with maybe 100,
in the inner part of the solar system,
objects the size of the Moon, going up to the size of Mars.
Competing worlds sucked in the surrounding debris
until there was simply no more to be had.
In the inner solar system, where there are now four planets,
there were once upwards of 100.
How that army of worlds became just four was still a puzzle.
But Victor Safronov had a hunch
that the process would leave those planets
spattered with the scars of impact.
Was this what we could see on the moon?
Unknown to the West, Safronov had taken a giant stride
towards a theory of planet formation.
Perhaps, somewhere in the solar system,
there might be a planet bearing the hallmarks of his theory.
In 1957, the Americans announced
that they were preparing to enter the space age.
They were about to launch
the world's first artificial satellite.
In the Soviet Union, Korolev acted immediately.
'For Korolev, it was the beginning
'of the race with Americans.'
And he wanted to be first,
ahead of the Americans,
like all of us.
And I think he wanted to do this
maybe 100 times more than any others.
'Then he called my father and told him,
'"I want to launch this first satellite.'
"Let's do this before the Americans, as soon as we can."
It would be a huge gamble,
but finally Khrushchev agreed to let him try.
Now Korolev had to convince his engineers they could do it too.
On October 4th, 1957,
while the Americans were still finalising their plans,
Sputnik was launched.
(Beeping)
40 years on,
Korolev's achievement is still celebrated in Russia.
'That evening, he was very proud,
'he realised it was a great achievement.'
And next day, he understood the reaction of the outside world
was much stronger than it was in our country,
and the feeling was much stronger than even his feeling,
specially in the United States.
Korolev's rockets had opened the door to space.
The planets were beckoning.
Bruce Murray is a veteran of the US space programme.
When his career started,
the planets seemed a very long way away.
He still remembers the first time
he saw Mars through a telescope.
And it just blew me away.
I was so taken with the fact
that here was a real object,
it was three-dimensional, or seemed to be.
'It was colourful, glowed, and really drove home to me'
there's a place out there, a real place,
not just something I studied in school somewhere.
As a young man, Bruce Murray was taken under the wing
of physicist Bob Leighton, who had developed a way
to make time-lapsed films of the planets.
The images were extraordinary because they could show
the planet rotating, you could time-lapse it, take one frame,
wait a minute, take another frame, and make a time-lapse.
And it brought to everybody the image of Mars
that the most dedicated astronomers only infer,
because they have to remember all those frames.
And he did it for fun.
Leighton's films brought the planets to life.
For the first time,
astronomers could see one of the moons of Jupiter
orbiting its giant parent.
'The outer planets,
'the ones that are huge masses of gas,
'in the case of Jupiter and Saturn,
'you could actually see some beautiful structure.'
The first thing, as in the inner solar system,
is diversity - "My Lord, everything is different."
But Mars, the Earth's smaller cousin,
was always the most tantalising.
Leighton could see mysterious dark patches
rotating with the planet.
But what would a close encounter with the surface reveal?
In 1964, the American probe Mariner 4
set off to send back the first pictures from another planet.
'Bob Leighton was charged with bringing back the images.'
..and blue clouds, oh, yes...
'He asked Bruce Murray to join him.'
'I was dragged or sucked along, however you want to look at it,'
into this wonderful experience,
of becoming the first experimenters to look at Mars
through a close-up camera.
'This is Mariner Control Center at JPL.
'The spacecraft is 134.217 million miles from Earth
'and 50,142 miles from Mars.'
After a journey of eight months,
Mariner 4 was homing in on its target.
'The first picture will cover an area of approximately
'176 miles square on the sunlit bit of the planet.'
I wish I was as sure as he is!
'About four minutes from now, we should be able to determine
'the camera shutter is operating
'and that the recorder is running.'
The anticipation of not just the scientists,
but the public and news media, was incredible,
because Mars was thought to have life, and in the popular mind,
maybe it had Martians, as far as we were concerned.
Mariner 4 was a fly-by.
It would get only one chance at the pictures.
'..the scan position for 5605 is 323. Congratulations.'
323! Exactly where they wanted her!
10,000 miles from the surface,
Mariner 4's cameras whirred into life.
These signals came back - if you think of one picture element,
one sample of light - the rate at which these came in from Mars
was one of these per second.
- Hey! Here we go. - There she goes. That's data!
And so it took three weeks for our 20 pictures to come back.
Give me Bruce Murray's phone number.
Where the devil are the Mars picture interpreters?
Yeah, data's coming in, boy. What are you doing in bed?
There we go.
We got some pictures.
The planet was not what they had expected.
There was no sign of life here. No vegetation.
Just picture after picture of a dull, flat landscape.
It wasn't until frame 12
that the first features became visible.
'What we could see were these huge craters.
'300 kilometres, 200-mile craters across, on Mars.'
Impact craters -
and that meant that Mars was preserving a signature
from its earliest times, 3 or 4 billion years ago.
So we had a major conclusion, stunning everybody,
from these very few pictures we got.
When the news filtered through to the Soviet Union,
one man wasn't as surprised as his Western rivals.
Craters were exactly what Victor Safronov expected.
Soon, Safronov's ideas were being discussed in the West,
where superior technology allowed George Wetherill
to take the accretion theory further.
'I'd called it the planetesimal problem.'
That says there's a lot of objects, small planets,
moving around the sun in orbits.
What you want to understand is how they accumulate together
to form large planets.
Wetherill's computers uncovered
a terrifying period of planet formation.
'What you find if you do the problem with the computer'
is that, as they grow, they start to perturb one another
into orbits which cross the orbit of another planet.
Soon, the neat orbits of Safronov's army of planets
became fatally disrupted.
As they started tugging each other off course,
the solar system was brimming with loose cannon.
World-shattering collisions were inevitable.
'George realised that it was like a wild frat party.'
All hell breaks loose in the inner solar system.
Stuff either hits the sun or gets thrown out to Jupiter,
and out of the solar system.
It's a very violent, happening party.
If Wetherill was right,
during this period, the inner solar system
must have been strewn with planetary debris.
The four surviving planets would have had to endure
a final stage of intense bombardment.
In 1973, George Wetherill got the chance to test his work.
Mariner 10 was on its way to Mercury.
78 million kilometres from Earth, beyond the scope
of the most powerful telescopes,
the surface of this planet was a total mystery.
'A few months before the Mercury mission,'
I was in a meeting where people discussed
what we might find on Mercury, to get thinking about Mercury.
A very distinguished planetary astronomer
in answer to a question, proclaimed that Mercury
would have no craters, or few.
The curious thing is that the craters on Mars
were also a surprise to most planetary astronomers.
After a journey that took in a fly-by of Venus,
by February, Mariner 10 was nearing its target.
'Subsequently, I was invited to JPL'
and sat in a little room above mission control
and saw the pictures coming in.
'The first pictures of Mercury showed just a fuzzy ball.'
You could imagine seeing craters, but then it got closer.
Soon, it started to look like the Moon.
Mercury was the most cratered planet in the solar system.
One impact was so great
that it left shock waves set in stone
on the other side of the planet.
It was proof of the final stage of the accretion theory.
I was just thrilled by this. I knew they were there,
but actually seeing them, that I'd been thinking about
all these years, and now here they are.
It made me very excited.
And all these military men around kept saying,
"Isn't that beautiful? It's just like a 52 drop in 'Nam."
Here, then, are the inner planets.
The survivors of a life-or-death struggle.
Mercury, Venus and Mars.
Each bearing the scars of creation.
But what of the Earth?
Surely our planet could not have survived unscathed?
In the field, Hal Levison gets a real sense of the violence
that rained down on the planets, including our own.
This hole in the ground was made in a matter of seconds.
Despite being an awesome sight,
'something that tells us the solar system is still active
'and that things are still running into each other,'
'It's a relatively insignificant hole in the ground.
50,000 years ago, a 50-metre fragment of a world blown apart
billions of years earlier careered into the Earth
in what is now Arizona.
Here is evidence of the final stages of accretion.
But what of the worlds that dwarf the inner planets?
How does the accretion theory account for the gassy giants
that rule the distant regions of the solar system?
We have different planets as we get farther from the sun,
because as you get farther from the sun, the temperatures drop.
About four times more distant from the sun than the Earth is,
we hit a point where water would condense and become a solid.
With water turning to ice, the amount of material available
to form the outer planets was far greater.
Jupiter and Saturn grew so large that they started sucking in
the primordial gases from the original dust cloud,
swelling them to hundreds of times the mass of the Earth.
This region had many more worlds than exist today.
Their orbits were also disrupted.
We can find no traces of impacts in their gassy atmospheres,
but evidence can be seen in their rotation.
It is believed that a world the size of the Earth
collided with Uranus.
Today, Uranus still rolls around the sun on its back.
When did these planet-building impacts come to an end?
'I've found a lot of comets.
'I've helped discover 21 of them.'
There is nothing like the night
we found the Shoemaker-Levy 9.
We had no idea how important that discovery was going to be.
'It made page 23 in the London Times,
'that Carolyn and Jean Shoemaker and I discovered this comet.'
Interest increased several months later,
when it was announced that Shoemaker-Levy 9
was on a collision course with Jupiter.
This was not page 23 of the London Times, it was page 1.
It was a different story.
Shoemaker-Levy 9 was going to show us what it's all about.
In all civilisation,
since Galileo first looked through a telescope, in 1609,
and since he first looked at Jupiter, in 1610,
this is the first time we've seen a comet strike a planet.
July 16th, 1994. Impact day.
And every available telescope is trained on Jupiter.
Look! Oh, my God! Look at that!
'It's how the solar system was built, comets hitting planets.
'Comets first hitting each other.
'Very slowly, almost an embrace rather than a collision,'
then these objects get bigger and their gravity gets bigger,
the speed gets higher, and it gets more violent.
The teenage solar system has become dysfunctional.
And finally, when does it end?
'What Shoemaker-Levy 9 taught us is it hasn't happened yet.'
Right then, in the summer of 1994,
around Jupiter, there's a big yellow police fence,
that says danger, keep out, solar system under construction.
It's still happening.
Jupiter grew a little bit during the week of July 16, 1994.
'Water was dumped on Jupiter.
'It had more carbon sulphide down there.'
It was as if nature had said,
"OK, guys, I'm going to show you how it works,
"and all you have to do is watch it."
Here, then, are the gas giants.
Jupiter and Saturn mark the current limit
of the plant builders' theories.
Far beyond these gargantuan worlds lie the ice giants,
Uranus and Neptune.
But out here, the accretion theory runs into trouble.
'The formation of Uranus and Neptune
'are the greatest mysteries in the solar system,'
because everything goes more slowly at greater distances
from the sun, so all these processes slow down.
When we try to run the same computer programs out there
that we did in the terrestrial planet zone,
we don't get planets forming.
No matter what we do, we can't form Uranus and Neptune
using these kind of models.
'I can't make Uranus and Neptune go away.'
They're there, and our models can't make them.
So we do indeed have a long way to go
before we really figure all this out.
How these worlds formed so quickly is a puzzle.
Scientists don't know enough about early conditions
this far from the sun.
What kinds of worlds went into the formation
of Uranus and Neptune?
In 1992, two astronomers were surveying the space
beyond Neptune when they found a substantial chunk of ice.
Since then, they have found many more.
Called the Kuiper Belt, it is now thought that they are
the building blocks of ice giants that never were.
'The Kuiper Belt is a region where the small ice mountains
'that we've talked about started accreting and building
'into larger things.
'To me, that's the region we need to look at,
'because planet formation started there,'
and it was frozen in
at some intermediate state.
Understanding that will tell us
in detail how accretion started,
but what shut it off is also going to be interesting,
and will tell us something about the process.
So to me, the future lies in the outer part of the solar system.
But there is a planet that lies
at the inner edge of the Kuiper Belt.
70 years after its discovery, the strange, tiny world of Pluto
may at last be making sense.
'Pluto was discovered in 1930,'
and it was the oddball of the solar system.
Most of the planets are in nice circular orbits. Not Pluto.
Most are set in this plane that represents the accretion disc.
Not Pluto.
And it was just an oddball, it was small and icy,
different to anything else that we knew about.
Could this small, icy world be a survivor of accretion?
A world that somehow escaped being swallowed up
by the growing Neptune, or being hurled out of the solar system?
Could Pluto be the missing link
in the formation of the ice giants?
'Turns out Pluto was just the largest known member
'of this population.
'It went from being this lonely remote oddball,'
to being essentially the grandfather of a population.
And the Kuiper Belt probably has more objects
than any other region in the solar system.
It's the most populous region
and yet we didn't know about it 10 years ago.
In the 40 years since Mechta broke free
from the Earth's gravity, we've sent probes to all the planets.
We've sampled the corrosive clouds of Venus,
and recorded planet-wide thunderstorms on its surface.
We've survived dust storms on Mars,
and seen canyons that could swallow countries.
We've mapped the icy moons of Jupiter,
and plunged into its atmosphere.
We've skimmed the rings of Saturn.
We've seen active geysers
on the most distant and freezing moon in the solar system.
But just as the first stage of our reconnaissance
of the planets draws to a close,
we have a new region to explore.
In 1992, Clyde Tombaugh got a request from NASA -
permission to visit his planet.
'Clyde was melted. He melted when he got that letter.
'He felt that all of his life's effort and work with Pluto,
'his work at White Sands, was coming to a head.'
He felt that letter was really a sign that NASA,
through their mission to Pluto,
was finally acknowledging him as the man that he really was.
Clyde Tombaugh died in 1997.
Pluto Express is planned to launch in 2003.
It will take 12 years to reach its goal.
After analysing Pluto's composition,
it will head out in search of a Kuiper Belt object.
Perhaps something in their cratering record or chemistry
will provide the final piece of the creation jigsaw.
Whatever the craft finds, Pluto's importance
in the grand order of the solar system is assured.
It will be a manned mission to Pluto in a very special sense.
It's not going to have a real living person,
but you can bet it's going to have Clyde's spirit on board
on its way to Pluto, to see what kind of a planet
that little guy really is.