The International Space Station, 250 miles above Earth.
While conducting a simple science experiment
for television viewers,
NASA astronaut Don Pettit makes a major
and unexpected discovery.
PYLE: In 2003,
NASA started a Saturday morning program
by which the astronauts would do citizen science for kids.
Don Pettit wanted to see what would happen
with small grains of salt if they were shaken up in zero-g.
So he thought this would be an interesting experiment.
LAU: By taking a little plastic bag of salt crystals
and shaking them up,
salt crystals started clumping together
and making larger gloms inside of this bag, basically,
showing off some of the things that happen in microgravity,
this experience of weightlessness
that the astronauts feel.
SHATNER: At first, when the grains of salt floated together
in zero gravity,
Don Pettit thought it was merely a strange space anomaly.
But when he repeated the experiment with other materials,
including sugar...
...he realized that his Saturday morning science presentation
had revealed the process
of how Earth's very first particles began to bind together
billions of years ago.
LAU: For a long time,
we weren't really sure
how all of the dust and the gas
around our Sun
coalesced together and formed our Earth.
And so, these experiments where particles
start to aggregate together
and-and grow larger clumps actually helps us to solve
a really old problem for understanding
the early evolution of our planet.
SHATNER: If, 4.5 billion years ago,
particles of space dust came together
and started to build the foundation of our planet,
then what happened next?
How did Earth become the enormous celestial body
that it is today?
On the surface, this remote countryside
may appear to be a simple valley dotted by rolling hills.
But, curiously, in July of 2005,
the area was designated as a UNESCO World Heritage Site.
So what's special about this rural landscape?
As it turns out,
Vredefort is home to an important clue
about Earth's origins that can only be seen
from space.
LAU: When we look at Vredefort from space,
what we see is an asteroid impact crater
made by an extremely large impactor.
That impactor might have been 20 to 25 kilometers in diameter.
So this giant structure
striking the Earth...
...with so much force that, if it happened today,
it could have obliterated all of the life on Earth.
But the Vredefort impact
was slightly over two billion years ago...
...a much different time in Earth's history.
The very early Earth would have been a molten ball of rock.
It would have looked hot from space.
You would have seen all this energy coming off.
And so, our world was a much different place back then.
SHATNER: Scientists believe that the Vredefort Crater
is just one of many asteroid impacts
that occurred in Earth's early history.
And while not every one of these giant space rocks
made a mark that is still visible...
...they all contributed
to Earth's growing into the massive planet it is today.
KAKU: If you take a look at all the craters on the planet Earth,
they are reminders of the fact that we live
in the middle of a cosmic shooting gallery.
Given the fact that the Earth is 8,000 miles across,
it means that there must have been hundreds
of other cataclysmic impacts
that eventually gave rise to the planet Earth.
And it was created by the slow accumulation
of tremendous amounts of debris, rocks and asteroids
from all these collisions.
GÜLCHER: There are hints everywhere
across the surface of the Earth
that give us clues about how it has evolved over time.
We generally agree that,
ultimately, all these collisions
of bigger and bigger material
is really key for how the Earth has been formed
to what we already know today.