# I was strolling on the Moon one day... #
[Narrator] When astronaut Gene Cernan
stepped off the lunar surface for the last time,
it was no giant leap for mankind,
but the last stumble of a dying era.
NASA cancelled the next three Moon missions
and quietly drew the Apollo programme to a close.
Cernan was the last human being ever to walk on the Moon.
To this day, no one has returned.
The love affair was over.
But although the public's relationship with the Moon had gone sour,
for a small band of dedicated scientists,
the romance was just beginning.
They now had actual pieces of the Moon to study –
nearly 400kg of lunar rock had been brought back by the astronauts.
They hoped that these rocks would unlock the unanswered mysteries of the Moon,
because despite the Moon landings,
scientists still didn't know the answer to the big questions.
Where had the Moon come from?
And how had it formed?
One of those starry-eyed young scientists was Gary Lofgren,
a geologist working for NASA.
He was given the job of cutting up each sample, ready for study.
[Gary] You just had no idea what you were going to see.
Looking at these really strange-looking rocks
that were just jumbles of debris.
I mean, it was a chance to really start to look at them closely,
to not actually touch them, but come very close.
And we realised we'd never seen anything quite like that on Earth,
or at least, we had never recognised it on Earth.
[Narrator] Most scientists had assumed
that the Moon would be similar to Earth.
There'd be a mixture of young and old rocks,
formed in many different ways.
They were in for a surprise.
[Gary] It turned out that our thinking about the Moon was really wrong.
The science had not done a very good job
of guessing what the Moon was going to be like.
People did think it was probably fairly old,
but they didn't realise it was as old as it turned out to be.
We found rocks that are almost 4.5 billion years old,
almost the age of our solar system.
You know, some of these rocks formed just 1,500 million years after the beginning of the planet.
We just don't find rocks that old on Earth.
[Narrator] The Moon was an ancient, fossilised world.
Its rocks hadn't changed for billions of years.
Scientists were thrilled.
[Gary] Basically, the surface of the Moon
kind of froze about, roughly, 3 billion years ago
and preserved, really, the first
1.5 billion years of its history.
The Moon tells us very much about the early history of our solar system.
It's probably one of the best recorders
of the early history of our solar system.
[Narrator] This ancient fossil was a scientific goldmine.
Because the Moon was so well-preserved,
it meant scientists could finally answer the question
that had come to obsess them.
How was the Moon formed?
At the time, there were two competing theories.
The first was that the Moon and the Earth were formed at the same time,
from the same cloud of dust and gas.
The other theory
was that the Moon was originally nothing to do with the Earth,
but was wandering alone in space
until the Earth sucked it in with the power of its gravity.
But the rocks themselves didn't seem to support either theory.
They were different enough from rocks on Earth
to make it unlikely that they were all formed at the same time.
But they had enough similarities to make it equally unlikely
that the Moon was a completely foreign body.
Eventually, scientists came up with a new theory that explained these strange rocks.
It was a brutal tale.
It takes us back 4 billion years,
to when the Solar System was a young and volatile place.
There were many planets and asteroids circling the Sun.
One of these was a young Earth,
but there was also another young planet a bit smaller.
The two were on a collision course.
Eventually, they crashed together.
It was the biggest bang the Solar System had ever seen.
The impact was so massive
that it spewed out millions of tonnes of molten rock and gases.
As this debris circled the Earth,
it came together, forming a separate body –
our moon.
When it first formed,
the Moon was 10 times closer to the Earth than it is today,
so it appeared much bigger in the sky
and its gravitational pull was much stronger.
But over time,
it slowly drifted away from the Earth to its present position,
about a quarter of a million miles away.
And there, its orbit seemed to have stabilised,
its distance from Earth fixed for all time.