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On January 2nd, 1959, with 
the space age barely a year old,

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the Soviet Union launched 
Lunik, or "little moon".

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It was sent to plant 
a Soviet pennant on the moon.

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Within hours of the launch,
it became clear

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that Lunik was going 
to miss its target.

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As the Soviet scientists 
watched their tiny probe

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sail out to join the planets

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in an endless journey 
around the sun,

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an inspired thought 
occurred to them.

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They renamed their spacecraft 
Mechta - "The Dream".

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(Music: "The Planets" 
by Gustav Holst)

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In 1926, when this recording of 
Holst's Planets suite was made,

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there were thought 
to be eight planets.

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Then, in 1929,

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a young man arrived 
at an observatory

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in Flagstaff, Arizona, 
to start the search for a ninth.

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At that time, little 
was known about the planets.

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Closest to the sun lies Mercury,

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a tiny world of iron and rock, 
barely discernible in the glare.

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Then Venus, 
perhaps a second Earth,

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hidden beneath 
a blanket of cloud.

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Then Earth.

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And beyond us, Mars, 
the Red Planet.

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It has seasons, polar caps,

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and the possibility of life.

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Far beyond these rocky worlds 
are the distant giants.

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Jupiter, over 1,000 times 
bigger than the Earth,

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and Saturn, with its distinctive 
and dramatic rings.

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The two remaining planets

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are 15 times the size of the Earth, 
yet they are so distant

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that they appear 
as the faintest of stars.

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Uranus - an aquamarine mystery.

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And finally, Neptune,

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a world that moved 
unevenly across the sky.

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This irregular movement

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suggested the presence 
of a more distant planet,

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whose gravitational tug might be 
toying with Neptune's orbit -

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Planet X.

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February 18th, 1930.

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Clyde Tombaugh, 
sitting in an office

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very near to where 
we are sitting right now,

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looking at the photographs that 
he had taken of the night sky...

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Sitting with his eye at the eyepiece 
of that blink comparator back there.

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And he had been 
searching on the plates

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that were centred on a star

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in the constellation 
of Gemini, the Twins.

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He had started that morning.

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He had moved very closely, 
very slowly across,

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click, click, seeing one image,

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then the other, then the other, 
keeping on moving back.

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All these images were 
negative, all the stars.

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And anything else would be 
black on a white background.

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At 4 o'clock that afternoon, 
he crossed the very centre of the plate.

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He passed the area 
right where the guide star was.

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The star Delta Geminorum -

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big, big bright star.

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He moved a little bit more, 
a little bit more,

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and then he saw a very faint, 
a very faint black dot.

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And then he blinked to the other one 
that appeared on the other plate,

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and he saw it appear 
here and appear there.

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On his plates, 
taken several days apart,

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Tombaugh noticed that 
a point of light had moved.

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He knew instantly that this was 
what he was looking for.

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It was an historic moment.

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He took the walk 
from the comparator room,

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all the way down 
to the director's office,

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and he stopped, then he did his tie 
and combed his hair a little bit,

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and he said "I wanted to appear 
a little nonchalant about this."

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Then he stepped 
into the office...

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"Dr Slipher? I have 
found your Planet X."

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Planet X was soon 
named Pluto.

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It marks the end 
of the solar system.

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A tiny world of ice, 
smaller than our moon,

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now known to have 
its own satellite, Charon.

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But Pluto patrols the outer edge 
of the solar system,

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in the distant realm of giants.

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Worlds of swirling water, 
like the azure Neptune,

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and Uranus, which 
mysteriously orbits the sun

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spinning on its back.

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Pluto lies way beyond 
the gargantuan worlds,

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the gas planets 
that have no landscapes:

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Saturn, with wind reaching 
thousands of kilometres per hour,

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and Jupiter, 
that has an Earth-sized storm

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that has lasted for centuries.

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The closest worlds to the sun

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are small islands 
of rock and iron.

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Mars, with its faint atmosphere 
of carbon dioxide,

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and Venus, smothered 
in clouds of sulphuric acid.

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Then there is Mercury,

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boiling in sunlight, 
and freezing at night.

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Nine different worlds, with 
seemingly little in common,

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save that they orbit 
a single sun

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and are bound together 
by its gravity.

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And then there is the Earth.

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A small planet in the measure 
of the solar system.

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It has a thin atmosphere 
that clings to a rocky surface.

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But the Earth is different. 
It is special. It has life.

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What process could create such 
a variety of different worlds?

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Hal Levison is at the forefront 
of a branch of astrophysics

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that is still struggling 
with the mystery

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of how the planets formed.

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It's amazing, when you consider

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that all planets in the solar system:

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the Earth and the rest: 
the rocky planets,

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the cores of the giant planets, 
Jupiter and Saturn,

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and the majority of the outer planets, 
Uranus, Neptune and Pluto,

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formed from material that is 
very fine pieces of dust,

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much finer than the dust 
I'm holding in my hands.

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About the consistency or size

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of particles of dust 
in cigarette smoke.

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I was an astrophysicist,

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interested in, sort of, 
an obscure type of galaxy,

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when about five years ago

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I got the bug 
of trying to understand

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how material like this can form 
the planets that we see today.

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By the 18th century, astronomers 
had discovered that galaxies

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are filled with drifting clouds 
of gas called nebulae.

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Perhaps these clouds were the 
raw materials of the planets.

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Two men, the philosopher 
Immanuel Kant

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and the mathematician 
Simon de Laplace,

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looked at the uniform direction

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of the orbits 
of the planets in the sky.

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They suggested the planets 
were a relic of a cloud of dust and gas

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that circled the sun 
during its formation.

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In a single process, 
they concluded,

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the solar system was born.

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The idea was elegant, 
and quite brilliant,

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but the complex details 
of their theory

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lay centuries in the future.

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Its proof had to wait for 
the arrival of the space age.

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September 1944. 
London was under siege.

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Mysterious weapons were 
raining down from the sky.

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Hitler's vengeance weapon 
threw people into confusion.

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Nothing had prepared them 
for a supersonic missile

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that took just six minutes to travel 
from mainland Europe into the heart of Britain.

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The technology behind these 
missiles was highly advanced.

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It had been developed 
by a brilliant young engineer

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called Wernher von Braun.

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Von Braun's rocket 
was called the V-2.

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Designed to save the war 
for the Nazis,

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eventually it became the foundation 
of our journey to the planets.

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When Germany fell,

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American troops headed 
straight for the V-2 factories.

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Before the dust 
had settled in Europe,

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von Braun and his team 
of engineers

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found themselves working 
for the United States Army.

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In the deserts of New Mexico, 
the captured rocket parts

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were reassembled by German 
and US engineers.

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The modified V-2s were soon flying 
way beyond the range of conventional cameras.

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To record their progress,

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the engineers fixed astronomical telescopes 
to anti-aircraft gun mounts.

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The system was designed 
by Clyde Tombaugh,

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the discoverer of Pluto, 
and his films still survive.

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Before they left the German 
rocket factories,

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the Americans destroyed 
as much as they could

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to prevent von Braun's secrets from falling 
into the hands of the advancing Red Army.

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But when they arrived,

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the Soviets found just enough 
to take back to Moscow.

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The man given the task of 
piecing together the rockets

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was Sergei Pavlovich Korolev,

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and Boris Chertok 
was his right-hand man.

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While their brief was to develop rockets 
which could reach America,

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Korolev's eyes were firmly 
fixed on the planets.

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But it was the Americans 
who made all the early running.

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By the end of the decade, 
they were strapping film cameras to rockets,

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 and sending them 
high above the atmosphere.

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The cameras had to endure 
an 18-mile plummet back to Earth.

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Miraculously, some survived,

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and astronomers got their 
first glimpse of the only planet

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they couldn't see 
with their telescopes.

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For the first time, we could 
see the curvature of the Earth.

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The arcing horizon 
was a humbling reminder

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that we were living on 
a gigantic ball of rock and iron.

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How such a world could have 
grown from a cloud of dust

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seemed more baffling 
than ever.

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George Wetherill 
has dedicated his career

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to the question 
of planet formation.

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When he started his work, the science 
was dominated by one man.

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No great scientist 
ever devoted his life

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to understanding this problem.

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It was sort of a hobby, 
something they did on the side.

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And I think the first person to 
really devote his life to this

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was a Russian scientist 
named Victor Safronov,

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who started working on these problems 
shortly after World War II.

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And he tried to identify what 
all the scientific problems are

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that you need to understand 
and need to solve,

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in order to understand 
the grand problem

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of the formation 
of the solar system.

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And to this day, his lists 
of problems are essentially

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the same problems 
that we're working on today.

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Victor Safronov 
revisited the 200-year-old idea

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that the planets formed 
from a disc of gas and dust.

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He set about structuring 
this complex process

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into comparatively 
simple stages.

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The first stage 
is still not fully understood.

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Remember, we're starting off 
with very fine pieces of dust,

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and the process of how you get 
from something like that

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to something the size of 
a boulder, or even the size of a mountain,

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is actually 
not very well understood.

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The party line of (the ... you know) what most 
people think actually happened,

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was that you had 
this disc of dust.

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Dust sort of settled into the mid plain 
of this protoplanetary nebula of this disc.

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And you got what's called 
gravitational instability

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that formed big clumps,

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things the size of 
maybe 100 metres in diameter.

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Safronov's second stage 
was less complex.

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It was called accretion.

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He calculated that in 
a remarkably quick time,

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the clumps 
would gather together,

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building the embryos 
of planets.

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As they grew, a new force 
became significant - gravity.

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A really amazing thing happens, though, 
that Victor Safronov discovered.

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00:18:49,840 --> 00:18:53,520
And that is, as these things 
start to grow,

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the bigger something gets, 
the more it can eat.

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00:18:57,920 --> 00:19:00,920
So you end up 
with this runaway situation,

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where the bigger guys are getting bigger 
still faster than the other guys are,

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and it's sort of a race 
to eat up all the little guys.

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And so you start off 
with an uncountable number

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of objects that are 
the size of mountains.

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And you end up 
with maybe 100,

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in the inner part 
of the solar system,

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00:19:18,960 --> 00:19:23,280
objects about the size of the Moon, 
or maybe going up to the size of Mars.

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Competing worlds sucked 
in the surrounding debris

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until there was simply 
no more to be had.

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In the inner solar system, where 
there are now four planets,

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there were once 
upwards of 100.

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How that army of worlds became 
just four was still a puzzle.

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But Victor Safronov 
had a hunch

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that the process 
would leave those planets

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spattered with 
the scars of impact.

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Was this what we could 
see on the moon?

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00:20:01,200 --> 00:20:04,240
Unknown to the West, Safronov 
had taken a giant stride

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towards a theory 
of planet formation.

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00:20:07,760 --> 00:20:10,480
Perhaps, somewhere 
in the solar system,

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00:20:10,600 --> 00:20:14,920
there might be a planet bearing 
the hallmarks of his theory.

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00:20:20,440 --> 00:20:23,280
In 1957, the Americans announced

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00:20:23,360 --> 00:20:26,000
that they were preparing 
to enter the space age.

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00:20:29,680 --> 00:20:30,880
They were about to launch

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00:20:30,960 --> 00:20:33,680
the world's first 
artificial satellite.

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00:20:36,320 --> 00:20:40,080
In the Soviet Union, 
Korolev acted immediately.

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00:20:40,160 --> 00:20:43,000
For Korolev, 
it was the beginning

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00:20:43,080 --> 00:20:45,400
of the race with Americans.

243
00:20:45,600 --> 00:20:47,960
And he wanted to be first,

244
00:20:48,040 --> 00:20:50,960
he wanted to be 
ahead of the Americans,

245
00:20:51,040 --> 00:20:52,240
like all of us.

246
00:20:52,320 --> 00:20:54,280
And I think he wanted 
to do this

247
00:20:54,360 --> 00:20:57,840
maybe 100 times 
more than any others.

248
00:21:00,080 --> 00:21:02,320
Then he called my father 
and told (him),

249
00:21:02,400 --> 00:21:05,280
"I want to launch 
this first satellite."

250
00:21:05,360 --> 00:21:10,760
"Let's do this before the 
Americans, as soon as we can."

251
00:21:12,400 --> 00:21:14,400
It would be a huge gamble,

252
00:21:14,480 --> 00:21:17,360
but finally Khrushchev 
agreed to let him try.

253
00:21:27,200 --> 00:21:32,680
Now Korolev had to convince his engineers 
that they could do it too.

254
00:22:34,520 --> 00:22:37,720
On October 4th, 1957,

255
00:22:37,800 --> 00:22:41,080
while the Americans were still 
finalising their plans,

256
00:22:41,360 --> 00:22:44,000
Sputnik was launched.

257
00:23:52,000 --> 00:23:53,200
40 years on,

258
00:23:53,280 --> 00:23:56,440
and Korolev's achievement 
is still celebrated in Russia.

259
00:24:00,520 --> 00:24:03,280
That evening, 
he was very proud,

260
00:24:03,360 --> 00:24:06,800
he realised that 
it was a great achievement.

261
00:24:07,840 --> 00:24:12,400
And next day, he understood that 
the reaction of the outside world

262
00:24:12,480 --> 00:24:15,520
was much stronger 
than it was in our country,

263
00:24:15,760 --> 00:24:21,600
and really the feeling was much 
stronger than even his feeling,

264
00:24:21,680 --> 00:24:23,880
specially in the United States.

265
00:24:29,600 --> 00:24:33,480
Korolev's rockets 
had opened the door to space.

266
00:24:33,560 --> 00:24:36,160
The planets were beckoning.

267
00:24:37,960 --> 00:24:41,760
Bruce Murray is a veteran 
of the US space programme.

268
00:24:43,240 --> 00:24:45,440
When his career started,

269
00:24:45,520 --> 00:24:48,640
the planets seemed 
a very long way away.

270
00:24:50,320 --> 00:24:52,000
He still remembers 
the first time

271
00:24:52,080 --> 00:24:54,160
he saw Mars through a telescope.

272
00:24:54,400 --> 00:24:56,280
And it just blew me away.

273
00:24:56,360 --> 00:24:59,640
I was so taken with the fact

274
00:24:59,720 --> 00:25:01,640
that here was a real object,

275
00:25:01,720 --> 00:25:03,880
it was three-dimensional, 
or seemed to be three-dimensional.

276
00:25:03,960 --> 00:25:08,800
It was colourful, glowed, 
and really drove home to me

277
00:25:08,880 --> 00:25:10,800
there's another place out there, 
a real place,

278
00:25:10,880 --> 00:25:13,520
not just something 
I studied in school or somewhere.

279
00:25:15,720 --> 00:25:20,480
As a young man, Bruce Murray was taken 
under the wing of physicist Bob Leighton,

280
00:25:20,560 --> 00:25:24,400
who had developed a way to make 
time-lapsed films of the planets.

281
00:25:25,720 --> 00:25:28,880
The images were extraordinary 
because they could show

282
00:25:28,960 --> 00:25:31,680
the planet rotating, you could 
time-lapse it, take one frame,

283
00:25:31,760 --> 00:25:35,320
wait a minute, take another frame, 
and so forth and make this time-lapse.

284
00:25:35,400 --> 00:25:38,400
And it brought to everybody 
the image of Mars

285
00:25:38,480 --> 00:25:41,760
that the most dedicated 
astronomers only infer,

286
00:25:41,840 --> 00:25:45,280
because they don't see it that way either, 
they have to remember all those frames.

287
00:25:45,360 --> 00:25:46,800
It was an extraordinary 
achievement.

288
00:25:46,880 --> 00:25:48,720
And he did it for fun.

289
00:25:52,320 --> 00:25:55,760
Leighton's films 
brought the planets to life.

290
00:26:06,120 --> 00:26:08,080
For the first time,

291
00:26:08,160 --> 00:26:10,840
astronomers could see 
one of the moons of Jupiter

292
00:26:10,920 --> 00:26:13,320
orbiting its giant parent.

293
00:26:15,160 --> 00:26:18,320
The outer planets,

294
00:26:18,400 --> 00:26:21,960
the ones that are 
huge masses of gas,

295
00:26:22,040 --> 00:26:23,840
in the case 
of Jupiter and Saturn,

296
00:26:23,920 --> 00:26:26,000
you could actually 
see some beautiful structure.

297
00:26:26,080 --> 00:26:30,600
The first thing that strikes one, 
as in the inner solar system,

298
00:26:30,680 --> 00:26:34,000
is diversity - "My Lord, 
everything is different."

299
00:26:34,080 --> 00:26:37,440
But Mars, the Earth's 
smaller cousin,

300
00:26:37,520 --> 00:26:40,080
was always 
the most tantalising.

301
00:26:40,160 --> 00:26:42,800
Leighton could see 
mysterious dark patches

302
00:26:42,880 --> 00:26:45,080
rotating with the planet.

303
00:26:45,160 --> 00:26:48,760
But what would a close encounter 
with the surface reveal?

304
00:26:57,080 --> 00:27:01,720
In 1963, the American
probe Mariner 4

305
00:27:01,800 --> 00:27:05,760
set off to send back the first 
pictures from another planet.

306
00:27:21,000 --> 00:27:24,560
Bob Leighton was charged 
with bringing back the images.

307
00:27:24,640 --> 00:27:28,560
..and blue clouds, oh, yes...

308
00:27:28,640 --> 00:27:31,600
He asked Bruce Murray
to join him.

309
00:27:31,680 --> 00:27:35,640
I was dragged or sucked along, 
however you want to look at it,

310
00:27:35,720 --> 00:27:37,280
into this wonderful 
experience,

311
00:27:37,360 --> 00:27:41,480
of becoming the first 
experimenters to look at Mars

312
00:27:41,560 --> 00:27:43,000
through a close-up camera.

313
00:27:43,080 --> 00:27:45,360
This is Mariner Control  
Center at JPL.

314
00:27:45,440 --> 00:27:51,400
The spacecraft is 134.217  
million miles from Earth

315
00:27:51,480 --> 00:27:54,680
and 50,142 miles from Mars.

316
00:27:58,400 --> 00:28:01,040
After a journey of eight months,

317
00:28:01,120 --> 00:28:04,800
Mariner 4 was homing 
in on its target.

318
00:28:04,880 --> 00:28:07,480
The first picture will 
cover an area of approximately

319
00:28:07,560 --> 00:28:11,040
176 miles square on 
the sunlit bit of the planet.

320
00:28:11,120 --> 00:28:12,280
I wish I was as sure as he is!

321
00:28:12,360 --> 00:28:15,400
About four minutes from now, 
we should be able to determine

322
00:28:15,480 --> 00:28:18,280
the camera shutter 
is operating

323
00:28:18,360 --> 00:28:20,240
and that the recorder 
is running.

324
00:28:20,320 --> 00:28:22,920
The anticipation of not 
just the scientists,

325
00:28:23,000 --> 00:28:26,000
but the public and news media, 
was incredible,

326
00:28:26,080 --> 00:28:30,560
because Mars was thought to have 
life, and in the popular mind,

327
00:28:30,640 --> 00:28:32,880
maybe it had Martians, 
as far as we were concerned.

328
00:28:33,560 --> 00:28:36,120
Mariner 4 was a fly-by.

329
00:28:36,200 --> 00:28:39,360
It would get only one chance 
at the pictures.

330
00:28:39,800 --> 00:28:45,600
..the scan position for 5605 
is 323. Congratulations.

331
00:28:50,480 --> 00:28:53,480
323! Exactly where 
they wanted her!

332
00:28:56,400 --> 00:28:59,120
10,000 miles from the surface,

333
00:28:59,200 --> 00:29:02,280
Mariner 4's cameras 
whirred into life.

334
00:29:08,040 --> 00:29:14,080
These signals came back - if you 
think of one element, one picture of element,

335
00:29:14,160 --> 00:29:20,280
one sample of light - the rate 
at which these came in, were from Mars,

336
00:29:20,360 --> 00:29:23,720
was one of these per second.

337
00:29:25,560 --> 00:29:29,600
- Hey! Here we go. 
- There she goes. That's data!

338
00:29:29,880 --> 00:29:33,840
And so it took three weeks for 
our 20 pictures to come back.

339
00:29:47,880 --> 00:29:50,360
Give me Bruce Murray's 
phone number.

340
00:29:52,160 --> 00:29:55,560
Where the devil are 
the Mars picture interpreters?

341
00:29:57,800 --> 00:30:01,920
Yeah, data's coming in, boy. 
What are you doing in bed?

342
00:30:03,200 --> 00:30:04,440
There we go.

343
00:30:05,440 --> 00:30:07,080
We got some pictures.

344
00:30:13,800 --> 00:30:16,360
The planet was not 
what they had expected.

345
00:30:18,520 --> 00:30:23,400
There was no sign 
of life here. No vegetation.

346
00:30:23,480 --> 00:30:27,440
Just picture after picture 
of a dull, flat landscape.

347
00:30:31,560 --> 00:30:33,720
It wasn't until frame 12

348
00:30:33,800 --> 00:30:36,600
that the first features 
became visible.

349
00:30:37,880 --> 00:30:41,360
What we could see 
were these huge craters.

350
00:30:41,440 --> 00:30:47,800
300 kilometres, 200-mile 
craters across, on Mars.

351
00:30:47,880 --> 00:30:50,120
Impact craters -

352
00:30:50,200 --> 00:30:56,240
and that meant that Mars 
was preserving a signature

353
00:30:56,320 --> 00:31:00,320
from its earliest times, 
3 or 4 billion years ago.

354
00:31:00,400 --> 00:31:03,880
And so we had a major conclusion, 
stunning everybody,

355
00:31:03,960 --> 00:31:07,880
from these very few 
pictures we got.

356
00:31:11,280 --> 00:31:13,760
When the news filtered 
through to the Soviet Union,

357
00:31:13,840 --> 00:31:18,200
one man wasn't as surprised 
as his Western rivals.

358
00:31:18,760 --> 00:31:22,880
Craters were exactly 
what Victor Safronov expected.

359
00:31:28,200 --> 00:31:31,400
Soon, Safronov's ideas were 
being discussed in the West,

360
00:31:31,480 --> 00:31:34,200
where superior technology 
allowed George Wetherill

361
00:31:34,280 --> 00:31:36,400
to take the accretion 
theory further.

362
00:31:36,480 --> 00:31:39,000
I'd called it 
the planetesimal problem.

363
00:31:39,080 --> 00:31:43,800
And it simply says that you've got 
a lot of objects, small planets,

364
00:31:43,880 --> 00:31:45,720
moving around the sun 
in orbits.

365
00:31:45,800 --> 00:31:49,600
And what you'd like to understand 
is how they accumulate together

366
00:31:49,680 --> 00:31:50,840
to form large planets.

367
00:31:52,640 --> 00:31:54,120
Wetherill's computers uncovered

368
00:31:54,200 --> 00:31:57,560
a terrifying period 
of planet formation.

369
00:31:58,200 --> 00:32:02,480
What you actually find if you do 
the problem with the computer

370
00:32:02,560 --> 00:32:08,040
is that, as they grow, they 
start to perturb one another

371
00:32:08,120 --> 00:32:12,480
into orbits which cross 
the orbit of another planet.

372
00:32:13,200 --> 00:32:15,920
Soon, the neat orbits 
of Safronov's army of planets

373
00:32:16,000 --> 00:32:17,960
became fatally disrupted.

374
00:32:18,840 --> 00:32:21,080
As they started 
tugging each other off course,

375
00:32:21,160 --> 00:32:23,680
the solar system 
was brimming with loose cannon.

376
00:32:23,760 --> 00:32:26,520
World-shattering collisions 
were inevitable.

377
00:32:35,360 --> 00:32:40,160
George realised that 
it was sort of like a wild frat party.

378
00:32:40,240 --> 00:32:43,720
All sort of hell breaks loose 
in the inner part of the solar system.

379
00:32:43,800 --> 00:32:44,960
Things are swung around,

380
00:32:45,040 --> 00:32:46,800
half the stuff is either...

381
00:32:46,880 --> 00:32:48,720
hits the sun or gets 
thrown out to Jupiter,

382
00:32:48,800 --> 00:32:50,960
which can then knock it 
out of the solar system.

383
00:32:51,040 --> 00:32:53,800
It's a very violent, 
happening party.

384
00:33:05,400 --> 00:33:07,360
If Wetherill was right,

385
00:33:07,440 --> 00:33:09,680
then during this period, 
the inner solar system

386
00:33:09,760 --> 00:33:14,120
must have been strewn 
with planetary debris.

387
00:33:16,000 --> 00:33:18,480
The four surviving planets 
would have had to endure

388
00:33:18,560 --> 00:33:22,080
a final stage 
of intense bombardment.

389
00:33:27,960 --> 00:33:33,920
In 1973, George Wetherill 
got the chance to test his work.

390
00:33:34,200 --> 00:33:36,720
Mariner 10 was on 
its way to Mercury.

391
00:33:45,120 --> 00:33:47,960
78 million kilometres 
from Earth,

392
00:33:48,000 --> 00:33:50,920
and way beyond the scope 
of even the most powerful telescopes,

393
00:33:51,000 --> 00:33:54,480
the surface of this planet 
was a total mystery.

394
00:33:54,640 --> 00:33:57,120
Just a few months 
before the Mercury mission,

395
00:33:57,200 --> 00:34:00,160
I was in a meeting 
where people discussed

396
00:34:00,240 --> 00:34:02,680
what we might find on Mercury 
in a sort of...

397
00:34:02,760 --> 00:34:05,920
to get our minds active 
for the thinking about Mercury.

398
00:34:06,000 --> 00:34:09,160
And a very distinguished 
planetary astronomer

399
00:34:09,240 --> 00:34:13,280
in answer to a question, 
proclaimed that Mercury

400
00:34:13,360 --> 00:34:15,800
would have no craters on, 
 or very few craters.

401
00:34:15,960 --> 00:34:18,840
The curious thing is 
that the craters on Mars

402
00:34:18,920 --> 00:34:22,520
were also a surprise 
to most planetary astronomers.

403
00:34:24,200 --> 00:34:26,920
After a journey that took 
in a fly-by of Venus,

404
00:34:27,000 --> 00:34:30,400
by February, Mariner 10 
was nearing its target.

405
00:34:33,120 --> 00:34:38,520
Subsequently (though) however, 
I had the opportunity to be invited to JPL

406
00:34:38,600 --> 00:34:41,760
and sit in the little room 
up above a mission control

407
00:34:41,840 --> 00:34:44,640
and see the pictures 
of Mercury as they came in.

408
00:34:46,960 --> 00:34:49,320
The first pictures 
of Mercury started coming

409
00:34:49,400 --> 00:34:51,080
and first that we saw 
was a fuzzy ball.

410
00:34:51,160 --> 00:34:53,520
And you could (sort of) imagine 
you saw craters,

411
00:34:53,600 --> 00:34:55,600
but after a while but 
it got closer and closer.

412
00:34:55,720 --> 00:34:57,840
Pretty soon, it started 
to look just like the Moon.

413
00:34:58,400 --> 00:35:03,320
Mercury turned out to be the most 
heavily cratered planet in the solar system.

414
00:35:03,600 --> 00:35:05,320
One impact was so great

415
00:35:05,400 --> 00:35:07,320
that it left shock waves 
set in stone

416
00:35:07,400 --> 00:35:09,160
on the other side 
of the planet.

417
00:35:12,000 --> 00:35:15,480
It was proof of the final stage 
of the accretion theory.

418
00:35:16,800 --> 00:35:21,760
I was just thrilled by this. 
I knew they were there,

419
00:35:21,840 --> 00:35:25,680
but actually seeing them, 
that I'd been thinking about

420
00:35:25,760 --> 00:35:30,080
all these years, and now here they are 
for me to look at them.

421
00:35:30,160 --> 00:35:31,400
It made me very excited.

422
00:35:31,480 --> 00:35:34,480
And also excited all these 
military men around - they said,

423
00:35:34,560 --> 00:35:38,720
They kept saying, "Isn't that beautiful? It's 
just like a 52 drop in 'Nam."

424
00:35:50,840 --> 00:35:53,640
Here, then, are 
the inner planets.

425
00:35:54,320 --> 00:35:57,040
The survivors 
of a life-or-death struggle.

426
00:35:58,960 --> 00:36:01,920
Mercury, Venus and Mars.

427
00:36:02,280 --> 00:36:05,440
Each bearing 
the scars of creation.

428
00:36:10,760 --> 00:36:12,680
But what of the Earth?

429
00:36:13,000 --> 00:36:16,760
Surely our planet could not 
have survived unscathed?

430
00:36:36,480 --> 00:36:41,480
In the field, Hal Levison gets 
a real sense of the violence

431
00:36:41,560 --> 00:36:45,680
that rained down on the planets, 
including our own.

432
00:37:06,440 --> 00:37:11,200
This hole in the ground 
was made in a matter of seconds.

433
00:37:13,440 --> 00:37:15,880
Despite being 
a very awesome sight,

434
00:37:15,960 --> 00:37:21,600
something that tells us that 
the solar system is still active

435
00:37:21,680 --> 00:37:23,960
and that things are still 
running into each other,

436
00:37:24,040 --> 00:37:28,320
It's a relatively small 
insignificant hole in the ground.

437
00:37:34,040 --> 00:37:39,200
50,000 years ago, a 50-metre 
fragment of a world blown apart

438
00:37:39,280 --> 00:37:42,360
billions of years earlier 
careered into the Earth

439
00:37:42,440 --> 00:37:44,400
in what is now Arizona.

440
00:37:45,680 --> 00:37:48,840
Here is evidence of 
the final stages of accretion.

441
00:37:53,600 --> 00:37:56,480
But what of the worlds 
that dwarf the inner planets?

442
00:37:57,360 --> 00:38:00,680
How does the accretion theory 
account for the gassy giants

443
00:38:00,760 --> 00:38:04,720
that rule the distant regions 
of the solar system?

444
00:38:07,520 --> 00:38:11,760
We have very different planets, 
types of planets, as we get farther from the sun.

445
00:38:11,840 --> 00:38:15,840
That's because as you get farther from 
the sun, the temperatures drop.

446
00:38:15,920 --> 00:38:20,080
And particularly about four times 
more distant from the sun than the Earth is,

447
00:38:20,160 --> 00:38:23,840
we hit a point where water would 
condense and become a solid.

448
00:38:24,600 --> 00:38:28,320
With water turning to ice, 
the amount of material available

449
00:38:28,400 --> 00:38:31,320
to form the outer planets 
was far greater.

450
00:38:32,000 --> 00:38:35,840
Jupiter and Saturn grew so large 
that they started sucking in

451
00:38:35,920 --> 00:38:38,720
the primordial gases 
from the original dust cloud,

452
00:38:38,800 --> 00:38:42,280
swelling them to hundreds 
of times the mass of the Earth.

453
00:38:45,960 --> 00:38:50,520
This region was populated with 
many more worlds than exist today.

454
00:38:50,600 --> 00:38:53,360
Their orbits 
were also disrupted.

455
00:38:53,640 --> 00:38:57,000
We can find no traces of impacts 
in their gassy atmospheres,

456
00:38:57,080 --> 00:38:59,840
but evidence can be seen 
in their rotation.

457
00:38:59,920 --> 00:39:02,600
It is believed that a world 
the size of the Earth

458
00:39:02,680 --> 00:39:04,680
collided with Uranus.

459
00:39:08,400 --> 00:39:13,240
Today, Uranus still rolls 
around the sun on its back.

460
00:39:28,320 --> 00:39:31,800
When did these planet-building 
impacts come to an end?

461
00:39:34,280 --> 00:39:35,760
I've found a lot of comets.

462
00:39:35,840 --> 00:39:38,600
And I've been involved now 
in the discovery of 21 of them.

463
00:39:38,680 --> 00:39:41,040
There is nothing like the night

464
00:39:41,120 --> 00:39:42,600
we found the Shoemaker-Levy 9.

465
00:39:42,680 --> 00:39:46,440
We had no idea how important 
that discovery was going to be.

466
00:39:46,520 --> 00:39:49,560
We had made page 23 
in the London Times,

467
00:39:49,640 --> 00:39:53,360
that Carolyn and Jean Shoemaker 
and I discovered this comet.

468
00:39:55,280 --> 00:39:58,000
Interest increased 
several months later,

469
00:39:58,080 --> 00:40:00,760
when it was announced 
that Shoemaker-Levy 9

470
00:40:00,840 --> 00:40:03,120
was on a collision course 
with Jupiter.

471
00:40:03,200 --> 00:40:08,960
This was not page 23 of the 
London Times, this was now page 1.

472
00:40:09,040 --> 00:40:10,640
This was a different story.

473
00:40:10,720 --> 00:40:15,080
Shoemaker-Levy 9 was going 
to show us what it's all about.

474
00:40:17,040 --> 00:40:19,720
In all of civilisations,

475
00:40:19,800 --> 00:40:23,200
since Galileo first looked 
through a telescope, in 1609,

476
00:40:23,280 --> 00:40:26,360
and since he first 
looked at Jupiter, in 1610,

477
00:40:26,440 --> 00:40:31,560
this is the first time that we all ever 
have seen a comet strike a planet.

478
00:40:32,080 --> 00:40:36,240
July 16th, 1994. 
Impact day.

479
00:40:36,720 --> 00:40:39,880
And every available telescope 
is trained on Jupiter.

480
00:40:40,360 --> 00:40:45,200
Oh, look! Oh, my God! 
Look at that!

481
00:40:58,040 --> 00:41:00,720
This is how the solar 
system was built,

482
00:41:00,800 --> 00:41:04,040
comets hitting planets. 
Comets first hitting each other.

483
00:41:04,120 --> 00:41:08,120
Very slowly, it's kind of almost an embrace 
rather than a collision,

484
00:41:08,200 --> 00:41:11,040
and then these objects get bigger 
and their gravity gets bigger,

485
00:41:11,120 --> 00:41:15,400
the collisions get faster, and the speed gets higher, 
and it gets more violent.

486
00:41:15,480 --> 00:41:20,400
As the solar system reaches its teenage years 
it's become a little bit dysfunctional.

487
00:41:20,760 --> 00:41:24,160
And finally, 
when does it end?

488
00:41:24,240 --> 00:41:27,920
What Shoemaker-Levy 9 taught us 
is it hasn't happened yet.

489
00:41:28,240 --> 00:41:32,400
Right then, 
in the summer of 1994,

490
00:41:32,480 --> 00:41:36,520
around Jupiter, there's 
a big yellow police fence,

491
00:41:36,600 --> 00:41:40,480
that says danger, keep out, 
solar system under construction.

492
00:41:40,600 --> 00:41:42,200
It's still happening.

493
00:41:42,320 --> 00:41:47,520
Jupiter grew a little bit during 
the week of July 16, 1994.

494
00:41:47,600 --> 00:41:49,640
Water was dumped on Jupiter.

495
00:41:49,720 --> 00:41:52,600
It had more carbon sulphide 
down there it happened.

496
00:41:52,760 --> 00:41:54,800
It was as if nature 
had said,

497
00:41:54,880 --> 00:41:58,360
"OK, guys, I'm going 
to show you how it works,"

498
00:41:59,200 --> 00:42:02,240
"and all you have 
to do is watch it."

499
00:42:30,400 --> 00:42:33,800
Here, then, are 
the gas giants.

500
00:42:35,080 --> 00:42:37,600
Jupiter and Saturn 
mark the current limit

501
00:42:37,680 --> 00:42:39,560
of the planet builders' theories.

502
00:42:44,000 --> 00:42:48,000
Far beyond these gargantuan 
worlds lie the ice giants,

503
00:42:48,080 --> 00:42:50,120
Uranus and Neptune.

504
00:42:53,120 --> 00:42:57,920
But out here, the accretion 
theory runs into trouble.

505
00:42:59,520 --> 00:43:01,280
The formation 
of Uranus and Neptune

506
00:43:01,360 --> 00:43:05,520
are the greatest mysteries 
in the formation of the solar system,

507
00:43:05,600 --> 00:43:09,200
because everything goes more 
slowly at greater distances from the sun,

508
00:43:09,280 --> 00:43:11,320
so all these processes 
slow down.

509
00:43:11,400 --> 00:43:15,120
When we try to run the same 
computer programs out there

510
00:43:15,200 --> 00:43:17,600
that we did in the terrestrial 
planet zone,

511
00:43:17,680 --> 00:43:20,080
we don't get planets forming.

512
00:43:20,160 --> 00:43:24,000
No matter what we do, 
we can't form Uranus and Neptune

513
00:43:24,080 --> 00:43:25,520
using those kind of models.

514
00:43:26,200 --> 00:43:30,840
No matter how hard I try 
I can't make Uranus and Neptune go away.

515
00:43:30,920 --> 00:43:33,960
They're there, and our models 
can't make them.

516
00:43:34,040 --> 00:43:37,320
So we do indeed 
have a long way to go

517
00:43:37,400 --> 00:43:39,200
before we really 
figure all this out.

518
00:43:40,440 --> 00:43:43,560
How these worlds formed 
so quickly is a puzzle.

519
00:43:43,960 --> 00:43:46,600
Scientists don't know enough 
about early conditions

520
00:43:46,680 --> 00:43:48,360
this far from the sun.

521
00:43:48,840 --> 00:43:51,320
What kinds of worlds 
went into the formation

522
00:43:51,400 --> 00:43:53,520
of Uranus and Neptune?

523
00:43:56,120 --> 00:44:00,240
In 1992, two astronomers 
were surveying the space

524
00:44:00,320 --> 00:44:03,520
beyond Neptune when they found 
a substantial chunk of ice.

525
00:44:04,280 --> 00:44:07,240
Since then, they have 
found many more.

526
00:44:07,320 --> 00:44:10,040
Called the Kuiper Belt, 
it is now thought that they are

527
00:44:10,120 --> 00:44:13,600
the building blocks 
of ice giants that never were.

528
00:44:15,000 --> 00:44:21,120
The Kuiper Belt is a region 
where the small ice mountains

529
00:44:21,200 --> 00:44:22,880
that we've talked about 
actually started accreting

530
00:44:22,960 --> 00:44:24,640
and building into 
larger things.

531
00:44:24,720 --> 00:44:27,520
That's really to me the region 
we need to look at,

532
00:44:27,600 --> 00:44:31,040
because what happened there is 
planet formation started there,

533
00:44:31,160 --> 00:44:32,280
and it was frozen in

534
00:44:32,360 --> 00:44:34,320
at some intermediate state.

535
00:44:34,400 --> 00:44:37,800
And trying to understand 
that will let us know

536
00:44:37,880 --> 00:44:41,520
in detail how the accretion 
process started,

537
00:44:41,600 --> 00:44:43,800
but what shut it off 
is also going to be interesting,

538
00:44:43,880 --> 00:44:46,680
and will tell us something 
about the process as well.

539
00:44:46,760 --> 00:44:50,960
So to me, the future really lies 
in the outer part of the solar system.

540
00:44:51,200 --> 00:44:52,600
But there is a planet

541
00:44:52,680 --> 00:44:55,600
that lies at the inner edge 
of the Kuiper Belt.

542
00:44:55,920 --> 00:45:01,040
70 years after its discovery, 
the strange, tiny world of Pluto

543
00:45:01,120 --> 00:45:03,440
may at last be making sense.

544
00:45:04,320 --> 00:45:07,200
Pluto was discovered in 1930,

545
00:45:07,280 --> 00:45:10,040
and it was the oddball 
of the solar system.

546
00:45:10,120 --> 00:45:13,200
Most of the planets are in nice 
circular orbits. Not Pluto.

547
00:45:13,280 --> 00:45:18,360
Most of the planets are set in this plane 
that represents the accretion disc.

548
00:45:18,600 --> 00:45:19,600
Not Pluto.

549
00:45:19,800 --> 00:45:24,200
And it was just an oddball, 
it was small and icy,

550
00:45:24,360 --> 00:45:27,640
and had no similarity to anything 
else that we really knew about.

551
00:45:28,520 --> 00:45:32,720
Could this small, icy world 
be one of the survivors of accretion?

552
00:45:33,200 --> 00:45:35,520
A world that somehow 
escaped being swallowed up

553
00:45:35,600 --> 00:45:39,160
by the growing Neptune, or being 
hurled out of the solar system?

554
00:45:39,880 --> 00:45:42,080
Could Pluto be the 
missing link

555
00:45:42,160 --> 00:45:44,800
in the formation 
of the ice giants?

556
00:45:45,600 --> 00:45:48,440
Turns out Pluto was just 
the largest known member

557
00:45:48,520 --> 00:45:49,800
of this population.

558
00:45:49,880 --> 00:45:55,600
So it went from being 
this lonely remote oddball,

559
00:45:55,680 --> 00:45:59,720
to being essentially 
the grandfather of a population.

560
00:45:59,800 --> 00:46:03,480
And we were talking about the Kuiper Belt 
probably has more objects in it

561
00:46:03,560 --> 00:46:05,360
than any other region 
in the solar system.

562
00:46:05,520 --> 00:46:07,840
So it's the most populous 
region in the solar system

563
00:46:07,920 --> 00:46:10,000
and yet we didn't know 
about it 10 years ago.

564
00:46:11,240 --> 00:46:15,200
In the 40 years since Mechta 
broke free from the Earth's gravity,

565
00:46:15,280 --> 00:46:17,560
we've sent probes 
to all the planets.

566
00:46:18,400 --> 00:46:21,120
We've sampled the corrosive 
clouds of Venus,

567
00:46:21,200 --> 00:46:25,160
and recorded planet-wide 
thunderstorms on its surface.

568
00:46:26,200 --> 00:46:28,920
We've survived 
dust storms on Mars,

569
00:46:29,000 --> 00:46:32,600
and seen canyons that 
could swallow countries.

570
00:46:32,680 --> 00:46:35,720
We've mapped the icy 
moons of Jupiter,

571
00:46:35,800 --> 00:46:38,080
and plunged into 
its atmosphere.

572
00:46:39,680 --> 00:46:42,240
We've skimmed 
the rings of Saturn.

573
00:46:45,200 --> 00:46:46,560
We've seen active geysers

574
00:46:46,640 --> 00:46:49,720
on the most distant and freezing 
moon in the solar system.

575
00:46:52,640 --> 00:46:55,040
But just as the first 
stage of our reconnaissance

576
00:46:55,120 --> 00:46:58,000
of the planets draws 
to a close,

577
00:46:58,120 --> 00:47:01,440
we have a new region 
to explore.

578
00:47:03,920 --> 00:47:07,800
In 1992, Clyde Tombaugh 
got a request from NASA -

579
00:47:07,880 --> 00:47:10,520
permission to visit his planet.

580
00:47:11,000 --> 00:47:14,800
Clyde was melted. He melted 
when he got that letter.

581
00:47:14,880 --> 00:47:20,480
He felt that all of his life's effort, 
 all of his life's work with Pluto,

582
00:47:20,560 --> 00:47:23,520
with his work at White Sands,
was coming to a head.

583
00:47:23,600 --> 00:47:28,080
He felt that that letter 
was really a sign that NASA,

584
00:47:28,160 --> 00:47:29,880
through their mission 
to Pluto,

585
00:47:29,960 --> 00:47:35,240
was finally acknowledging him 
as the man that he really was.

586
00:47:35,320 --> 00:47:37,960
Clyde Tombaugh died in 1997.

587
00:47:38,440 --> 00:47:42,040
Pluto Express is planned 
to launch in 2003.

588
00:47:42,520 --> 00:47:45,160
It will take 12 years 
to reach its goal.

589
00:47:45,240 --> 00:47:48,120
After analysing 
Pluto's composition,

590
00:47:48,200 --> 00:47:51,240
it will head out in search 
of a Kuiper Belt object.

591
00:47:51,320 --> 00:47:54,480
Perhaps something in their 
cratering record or their chemistry

592
00:47:54,560 --> 00:47:58,120
will provide the final piece 
of the creation jigsaw.

593
00:47:58,200 --> 00:47:59,880
Whatever the craft finds,

594
00:47:59,960 --> 00:48:04,600
Pluto's importance in the grand order 
of the solar system is assured.

595
00:48:04,760 --> 00:48:08,160
It will be a manned mission 
to Pluto in a very special sense.

596
00:48:08,240 --> 00:48:10,680
It's not going to have 
a real living person,

597
00:48:10,760 --> 00:48:14,720
but you can bet it's going 
to have Clyde's spirit on board

598
00:48:14,800 --> 00:48:18,440
on its way to Pluto, to see 
really what kind of a planet

599
00:48:18,520 --> 00:48:20,040
that little guy really is.

600
00:48:21,840 --> 00:48:25,840
subtitles corrected and synchronised by m06166