The Universe

Can something in space

cause animals to be born with two heads?

Why were the most fearsome warriors

our planet has ever known

stopped by a flaming light in the sky?

How did Columbus use secret knowledge of the heavens

to cheat death in the New World.

Our ancestors saw a universe filled with bad omens.

But what were they really seeing?

And how did fear of these objects in the sky

change history?

Ancient mysteries,

shrouded in the shadows of time...

Now, can they finally be solved

by looking to the heavens?

The truth is up there,

hidden among the stars

in a place we call...

For our ancestors,

the skies were filled with equal parts wonder...

And terror.

Bad omens from above changed human history...

again and again.

Four centuries before the birth of Jesus,

it is a time of war...

Athens versus Sparta.

And this time it's for keeps.

Control of the ancient world hangs in the balance.

Thousands of brave warriors,

hundreds of ships at sea...

all poised for action,

waiting for the command from their leaders

to unleash hell on Earth.

And then...

An omen...

a strange light in the sky...

a fiery object said to be visible

for 75 days...

as both sides nervously watch and wait.

It's an omen.

But what does it mean, and what is the object?

The Spartans devise a strategy.

They send an envoy to the Greeks,

saying that with such a bad omen overhead,

battle would have to wait.

As days stretch into weeks,

the Greeks grow confident no attack is coming.

So that is when the Spartans strike--

a sneak attack...

decimating the Greeks

and marking the beginning of the end

for the nearly 30-year-long Peloponnesian War.

The final toll is staggering--

more than 3,000 Greek men captured and killed on the spot.

It couldn't be any clearer.

To the Greeks, the defeat was definitive proof

the Greek Gods had sent the sky object as a bad omen.

If you're in the middle of a war

and you see an omen in the skies,

that's gonna mean something bad.

It's something to be concerned about.

But what was the object in the sky

that brought an end to nearly 30 years of war?

One account of the object describes,

"A fiery body of vast size,

"as if it had been a flaming cloud,

"not resting in one place, but moving along

with intricate and irregular motions."

Could the ancient Greeks have been describing a meteor?

Also known as shooting stars,

meteors make a fiery display as they streak across the sky.

But since they're actually small rocks and bits of dust

burning up as they fall through the atmosphere,

meteors don't last very long.

They're visible for a matter of seconds,

not days or months, as the Greeks described.

Asteroids take longer to pass through the sky,

but because they're dark,

almost none of them are visible to the naked eye.

What the ancient Greeks could have seen, however,

was a comet.

Mythmakers fear comets, because they linger in the sky

for weeks, even months at a time.

The idea of seeing a comet for 75 days, I could buy that.

On average, only one bright comet is visible

to the naked eye each decade,

meaning the appearance of one in the sky

would've been a rare and remarkable event

to the ancients.

Comets are made of materials

that we find readily here on Earth.

They're largely made up of dry ice,

frozen carbon dioxide like we exhale,

as well as water, a little bit of organic material,

ammonia, not too different than what's in cleaning materials,

and even silicates like in sand from the beach.

When you mix all of these ingredients together,

you have the recipe for a comet.

When the ancient Greeks

described a flaming cloud in the sky,

is this what they were seeing--

a comet, changing its position in the sky

almost nightly,

faster than anything they had ever seen?

And what gives these icy objects the fiery appearance

that spooked the ancients?

As a gross overview, a comet has two parts.

It has the solid part

and the long tail.

When people talk about a comet's tail,

they're really talking about two tails.

You see, a comet has a dust tail--

the material, the solid bits,

the icy bits that come off the comet--

and an ion tail--

ionized material from the comet pushed away by solar wind.

Comet tails come in all shapes and sizes.

But they also have one thing in common--

the tail points away from the sun,

no matter what part of its orbit the comet is in.

As comets streak

into the inner part of the solar system,

they slowly heat up from the sun's heat.

As they heat up, they grow an amazing tail

that doesn't streak out behind them,

the way a lot of people think,

but rather it's pushed by the sun's radiation.

How we're able to see that tail

depends on where we are relative to the sun

and the comet.

And sometimes that tail's pushed out

such that as the comet flies away from the sun,

it flies into its own tail.

Comets may hold the answer

to one of the most fundamental questions

about our planet.

On Earth, where there is water,

there is life.

But just where did that water come from?

Many have proposed the water in Earth's oceans

was delivered by comets

crashing into the planet.

Others believe the water hitched a ride

inside rocky asteroids.

Which theory is correct?

Enter Rosetta--

a mission designed to survey then land a space probe

on a comet.

Rosetta and its lander, called Philae,

tracked down a comet known as 67P

after a ten-year journey through space.

So how do you get to a comet?

Loop around our solar system multiple times,

including a daring low-altitude skim

less than 200 miles above the surface of Mars.

Then jam on the brakes.

After establishing a stable orbit of the comet,

the Philae lander was deployed,

and the world watched and waited.

The gravity on a comet is so little

that the pull on that spacecraft was no different

than the weight of a piece of paper on your hand.

The lander had to be equipped with harpoons

that would help attach it to the surface of the comet.

When those didn't deploy,

the lander actually bounced,

making it not just a day for the first comet landing,

but also the second.

Unfortunately, when Philae finally came to est,

it was partially in the shade of a cliff.

That meant that the solar panels were only receiving

about an hour and a half of sunlight,

instead of the six hours that we were anticipating

that they would.

However, the Rosetta team deployed all

of its instruments at once,

trying to get as much data as they possibly could

before the lander ran out of batteries.

Despite the bad luck,

the Rosetta mission was able to make a significant discovery.

It appears unlikely a comet like this one

brought water to our planet.

Its vapor has a different chemical mix

than we see on Earth.

That leaves asteroids as the most likely source

of our water...

an important finding that could tilt

decades of debate.

Even if comets didn't bring water here,

they do carry with them a set of beliefs,

superstitions, and omens

unlike anything else in the night sky.

Were the ancients right

that there's one comet in the solar system

with the power to cause the birth of two-headed animals?

And could that same comet end all life on Earth?

Normandy, France,

the year 1066...

a grand army prepares for an invasion

that could change the course of history.