All language subtitles for The universe S07E06 Ride the Comet

Male narrator: In the beginning,
there was darkness,

and then, bang,

giving birth to an endless
expanding existence

of time, space, and matter.

Every day, new discoveries
are unlocking the mysterious,

the mind-blowing,
the deadly secrets

of a place we call
The Universe.

Chart a daring course
with comets,

the cosmic time travelers
of the skies.

- These mysterious objects
have been agents of change.

They've been agents
of destruction.

Narrator: Discover why
these celestial bodies

carry secrets from our past

and cast doom
over our future.

- If it's getting into
our on-ramp,

we need to worry about it.

- There's very little
we can do

other than sending
a giant bomb toward them

and trying to deflect them.

Narrator. Brace yourself
as we rendezvous

with the frozen intruders
from the great beyond

on an epic journey
to "Ride the Comet."

They appear out of nowhere,

gliding slowly
across the nighttime sky

and then disappearing
from sight.

Throughout human history,
ancient cultures

believed these
celestial intruders

carried special powers
from the gods,

mystical messengers
of life and death.

- Scientists like to think

that comets are fairly
mysterious objects,

because we know
that they're very old.

They're like frozen
time capsules

from when our solar system
was first formed.

- Comets are a bit
of show-offs.

They come into the night sky
unannounced, unpredicted.

In the Middle Ages, they were
thought to be fireballs

thrown at a sinful Earth

from the right hand
of an avenging god.

And so they've always been
feared or seen as mysterious.

Narrator: We're going
to hitch a ride

on a bold, uncharted journey
through the secret life

of our solar system's
comets.

Our daring quest begins
4.5 billion years ago,

as our futuristic spacecraft
time-travels

back to the birth
of our solar system.

We zigzag through
a maelstrom of debris

to catch our first glimpse
of comets,

the wayward rocks that
didn't meld into planets.

- Comets are basically
dirty snowballs

or icy dirtballs.

They consist of rocks
and dust and dirt

held together by ices—

Water ice, ammonia, methane,
carbon dioxide, dry ice.

That's sort of like the glue

that holds
all these little pebbles

and particles of dust
together.

Narrator: The heart of these
cosmic icebergs is the nucleus,

or central core,
which averages in size

from a half mile wide
to as big as a modern city.

- One of the things
we've learned

by flying close by and observing
the nuclei of comets

is that they're these strange

sort of compacted conglomerates
of ice and rock.

They're not uniform.

They're not perfectly mixed.

They're quite chunky
and oddly shaped.

- The nuclei of comets are...

very fragile entities.

If you had a chunk,

you could probably break it
easily in your hands.

Narrator: Up close,

comets look like bland chunks
of charcoal

rather than supersized
snowballs.

They don't even remotely
resemble

the glowing orbs we see
in our pitch-black skies.

- We think that most comets
are extremely dark

on their surfaces,
that the surfaces are covered

with something
kind of like barbecue soot,

this really dark,
black material

that makes them very difficult
to see.

Narrator:
While their asteroid cousins

share similar properties,
such as rock and metal,

scientists suspect that comets
were born farther out,

near the gas giant planets,

where it's much colder,

so they possess more ices
and volatile elements.

About 3.8 billion years ago,

many of these icy bodies
were whisked away

to the Kuiper belt,
a frigid neighborhood

just beyond the outermost
planet, Neptune.

- Icy objects in the Kuiper belt
probably formed

closer in toward the Sun,
where the giant planets are,

but then gravitational
interactions

with those giant planets
made those icy objects

migrate outward.

Narrator:
Our spacecraft navigates

through this dark,
disk like region,

which is almost
2 billion miles wide

and is estimated to hold
over 6 billion comets.

We eventually touch down

on the surface of one
of these icy boulders.

- Landing on a comet would be
a pretty tricky affair.

[engine whirring]

So you could kind of imagine
dirty snow

that's covered
with exhaust,

and if you step on it,

in places,
you can punch through

to places where there's more
fresh material underneath it.

Narrator:
The Kuiper belt objects

appear to have fairly stable
orbits,

so they won't be that much fun
to ride.

We lift off
in search of a comet

that will cut
a more exciting path

through our solar system.

We navigate farther out

to a sparsely populated region
called the scattered disc,

which is home to icy bodies
with long, elliptical orbits.

The slightest disturbance
can send these unstable objects

careening into
the inner solar system,

where they become known
as short-period comets,

those which orbit the Sun
every 20 to 200 years.

- The scattered disc region

is thought to be
the primary source

of the short-period comets.

They are kicked into
the inner solar system

either by interacting
with Neptune

or possibly passing stars.

Narrator:
Our spacecraft witnesses

Neptune's gravitational
influence at work,

dislodging a comet
out of the scattered disc.

This comet would be
too dangerous to land on,

so we'll track alongside it

on its transformational flight
through our solar system.

- So what would happen if it
was on a journey toward the Sun?

Well, it would start off
looking like a dark asteroid.

Nothing much is happening.

It doesn't have an atmosphere
around it.

- As you start to travel toward
the inner solar system,

get up to Jupiter
or even closer to the Sun,

the comet would start
to become active.

- The warmth from the Sun
starts to heat up the comet,

and all the frozen material
that's trapped on the inside

starts to convert from ice
into gaseous material,

and so these gas jets
can actually punch through

the crust of the comet
and form jets.

As these jets flow away
from the surface,

they can start to pull
dust and rocks with them,

and this forms
a kind of hazy atmosphere

around the comet's nucleus.

We call this the coma.

Narrator:
As it zooms by Earth,

the coma of a comet
can grow to be larger

than the diameter
of our Sun.

Solar radiation eventually
pushes this cosmic halo

behind the nucleus,
forming two bright tails:

one of dust
and the other of gas.

- There will be a tail growing
and growing and growing,

and you would eventually
become engulfed

in just a really rocky, icy,
particle-filled environment.

It would be very difficult
on your spacecraft

because it'd be like flying
through a blizzard.

Narrator: Some tails can stretch
for 100 million miles,

more than the distance between
the Sun and the Earth,

as it travels
towards our home star.

It's one of the most spectacular
spectacles of our solar system.

- Today we're going to see
what happens

when we take a comet

that's been in the cold, dark,
icy outer reaches

of the solar system,

and we're going to send it
inward towards its perihelion,

or the point of closest approach
to the Sun.

Now, dry ice
is frozen carbon dioxide,

and this is a very common
constituent of comets.

Let's see what happens.

When we take
this piece of dry ice,

and we drop it
into the warm water,

the dry ice starts
to sublimate.

That means it converts directly
from a solid, frozen state

into a gaseous state.

And as the water
starts to warm up

certain parts
of the dry ice first,

you can start to see that
little jets kind of form on it,

and this forms a hazy atmosphere
called a coma.

You can see that
the little chips of dry ice

are constantly changing
as they sublimate away.

Sometimes they speed up.

Sometimes they slow down.

So if this was a comet,

you can get the idea that,
as the comet approaches the Sun,

and the volatiles
start to vaporize,

this can actually change
the comet's rotational state

and even its orbit.

Narrator. Comets
are brightest and fastest

when they're closest
to the Sun.

The searing heat and immense
gravity of our mother star

can accelerate
a comet's speed

to nearly 2 million miles
per hour.

- The speed increases a lot,
and it whips around the Sun,

so it goes zoom,
like that,

like being propelled

at the bottom
of a giant roller coaster.

A roller coaster
starts out high, very slowly,

picks up speed
due to the gravity,

goes zooming past the bottom,

and then slows down again
on its outward journey.

- Eventually, as our comet
leaves the vicinity of the Sun,

it's going to travel
back out

into the far reaches
of the solar system.

Everything's going
to quiet down.

All of the gas and dust
is going to settle

back onto its surface.

And eventually, it'll cruise
away into the quiet dark.

Narrator: Back in the cold
region of the scattered disc,

a comet transforms

back into a dull, black lump
of rock and ice

until it makes its journey
around the Sun once again.

Haley, the most famous
short-period comet,

was first recorded
in 240 B.C.

and has passed by Earth
every 76 years.

Estimated to be the size
of Manhattan,

this superstar comet
is sometimes so bright,

it has been seen
in broad daylight.

Throughout history,

the appearance of Haley
has inspired awe and dread

among earthly observers.

-In 1910, it was feared,

because spectroscopy
had just been introduced,

and they had determined
that comets contain cyanogen,

which is an odorless
and poisonous gas,

and so astronomers predicted
that there'd be problems

because of this poisonous gas
sweeping the Earth,

and so there were actually
comet pills manufactured.

Life insurance policies
on the comet were taken out.

Narrator: While comets
have been thought to be

extraterrestrial
harbingers of doom,

astronomers have long wondered
if these glowing rocks

contain precious information
about our past.

By the 21st century,
NASA was ready to ignite

a bold offensive strike,
one that would hopefully expose

one of the holy grails
of planetary science.

Could these
interplanetary vagabonds

have been the cosmic
transporters of life?

So far, we've traveled
with comets

from the frozen hinterlands
of the Kuiper belt

and scattered disc

to an exhilarating orbit
around the Sun.

We've watched these dark bodies
come alive

as they formed brilliant comas
and tails.

But scientists
are still puzzled

about what's really inside
these cosmic interlopers.

- Man, I wish I knew

what the interior of a comet
looked like.

You would love to be able

to look inside
the nucleus of a comet,

to see if it's clumpy,
if it's uniform.

Is there crystalline ice?

Is there glassy ice?

Narrator:
Our protective spacecraft

time-travels back to 2005

and meets up with NASA's
Deep Impact space probe

on a comet-hunting mission.

It zeroes in on a short-period
comet named Tempel 1

with an aggressive
plan of attack.

We hover at a safe distance
as Deep Impact

launches an 800-pound projectile
into the passing comet.

Debris sprays out
for thousands of miles

as the impact or
excavates a crater

and sends tons of blinding gas,
rocks, and dust into space.

For the first time
in history,

scientists are able
to peer inside

the icy, muddy interior
of a comet.

- Right after the impact,

you could see silicate grains
fluorescing and glowing.

This tells us that comets
are very primitive material

and that they pretty much
do preserve in pristine state

the materials that we find

in the youngest
star-forming systems.

So indeed,
they really are time capsules

that let us look back
on our own solar system

at the very beginning.

Narrator: While Deep Impact

captures a rare glimpse
of a comet's interior,

it is unable to snap
clear images of the crater,

which was obscured by
an enormous plume of debris.

In February 2011,
another spacecraft,

named Stardust-NExT,

catches up to Tempel 1

as it completes its five-year
orbit around the Sun.

To everyone's surprise,

the impact crater is much
smaller than had been expected

from the historic smash-up that
occurred five years earlier.

- Amazing...

But the crater looked like
it was small.

So this now creates
this issue:

Was the crater always small?

We know the crater
isn't what we expected.

So we're going
to do some experiments

to see if we can explain why
the crater looks like it does.

- Sunlamp's coming on.

- Can we move it up a bit?

Narrator. At NASA's
Ames Vertical Gun Range,

Stardust-NExT
co-investigator Pete Schultz

is conducting high-velocity
impact experiments

to find out why Deep Impact
produced such a tiny crater.

He will use a massive

.30-caliber high-velocity
gas gun

to fire tiny projectile beads
at various targets

located inside
a vacuum chamber.

- We're trying to simulate

what the nature of the surface
of comet 9P/Tempel 1 is like,

so we're putting in per lite,

which is this low-density
material

that you find in gardens,

then we're adding in
hollow micro spheres,

and this also resemble what
the nature of the comet is like.

The purpose of the experiment
is to see what happens

to the crater
after formation.

Does it stay there?
Does it collapse?

With this experiment,
we should be able to find out.

Okay, I'm out of the tank.

Lock and load.

Narrator: High-speed cameras

mounted around
the vacuum chamber

will document the impact.

- We've got this one covered.

We just have to hit
the target.

Okay.

[claps hands]

Let's shoot this puppy.

J.P., are you ready?

- Yeah.

Narrator: As the crew anxiously
awaits in the control room,

Pete mixes science
with a little superstition.

- There's always a risk.

Sometimes you miss.
Sometimes you hit.

- Whoa!

- Oh, sweet.

The whole thing sort of
collapses in on itself.

It's getting smaller now.

It was bigger.
Now it's getting smaller.

So what we see here is that
there's a lot of dust and ejecta

sent upwards,
and that blocked the view.

We couldn't see
the crater forming.

So what started off
being a nice-looking crater,

it just doesn't stay there.

It just simply heals itself.

So what may have happened
for 9P/Tempel 1,

the nucleus, is that
that crater healed itself.

The nucleus healed itself
from the scar created

by us with Deep Impact.

Let's see what we did.

[groans]

So the results
were fantastic.

We got to see
just what we planned to.

There may be more
to the story,

so we're going to try
a different experiment as well.

Narrator: Pete has a hunch

there's a missing piece
to the Deep Impact mission.

He now wants to see
if the space probe

created a different type
of crater on Tempel 1,

due to the makeup
of the comet's nucleus.

- So this time,
we're going to put

a denser layer
below that per lite,

so we want to find out
whether or not

we'll get a different type
of crater

if we have two different types
of material:

one very soft on top

and one denser
on the bottom.

Narrator: Pete and his crew
return to the control room

to see what happens
to the new target of per lite

poured over a heavier layer
of sand.

[small explosion]

- [laughs]

Now, that formed a big crater
in the per lite.

Boy, look at that.

Oh, oh, man,
that is gorgeous.

Okay.

The projectile went deep,

and now the stuff
is coming back out that hole,

while, on the surface,
it's excavating debris.

Pow!
Okay.

Narrator: The double-layered
target created not one

but two craters,

a small crater inside
a larger, shallower crater.

- This could be what happened
for 9P/Tempel 1.

It went so deep
into the nucleus,

and then it just simply
collapses away,

and we're left with just
this very, very faint rim

on the outside with
a small pit in the center.

Narrator:
These impact experiments

yield surprising clues about
the interior of Tempel 1

and perhaps all comets.

- Now we know that comets
have history.

We see layers.

I don't know if these layers
go all the way

through the nucleus,

or are they only
in one part?

Narrator:
Unraveling the secrets

of what's inside comets
will help scientists understand

what causes them to exhibit
some really bizarre behavior.

Our spaceship now shadows
NASA's EPOXI mission

as it encounters Hartley 2,

a fast-spinning comet
that tumbles through space

like a hyperactive
cosmic peanut.

- Hartley 2
is a smaller comet.

It's only about a mile or so
across,

and so it's kind of a surprise
that it's so active.

It's putting off huge amounts

of CO2 and, actually,
cyanide gas.

And the question is,

are all small comets
active like this

or only a few?

And what makes them
so active?

I've got a peanut-shaped
plastic bottle here

that's supposed to represent
comet Hartley 2.

It's full of dry ice
and warm water,

and as you can see,

as the warm water starts to make
the dry ice sublimate,

it shoots out these jets

through the holes that
we've got poked in the bottle.

If I drop it into the water,

instead of holding it
fixed in space,

you can see that the jets
actually start

pushing the bottle around,

causing the comet's
rotational state to change.

This is very similar
to what's going on

on the surface
of comet Hartley 2.

The CO2 jets are actually
changing the rotational state

of the comet, causing it
to speed up and slow down.

Narrator:
When tracking Hartley 2,

our spacecraft gets caught
in a cosmic blizzard

as the spastic comet
burps and belches out

a trail of frozen snowballs that
extend for millions of miles.

- One of the big surprises
about Hartley 2

is that it was surrounded
by this posse of mini comets.

This is simply ices
that are coming off,

about the size of a snowball
all the way up to a basketball.

Narrator: Scientists suspect
the snowball-sized debris

rains back down on Hartley 2,

producing its unusual landscape
of craters and towering spires.

- Because these comets
have very little gravities,

most of the gas
is blown out into space,

but a very small fraction
can actually redeposit

on the surface, generating
very smooth textures,

and on other locations,
generating these very bumpy

or spire-like texture.

Narrator: From Hartley 2,

we blast off to the extreme
outer edge of our solar system,

a vast, eerie place,

barely within the gravitational
grasp of our Sun

and impossible to see,
even with modern telescopes.

Here, we come upon the largest
and perhaps most elusive

icy bodies in
our galactic neighborhood.

Our journey through
the fascinating world of comets

has provided us a ringside seat
to some of the greatest

cosmic shows observed
from Earth and space.

We now change course
and travel

over 50,000 times the distance
between Earth and the Sun

to the outermost edge
of our solar system.

We arrive at the Oort cloud,

an even larger fraternity
of comets,

perhaps over a trillion
of them.

- If we'd actually
got in a spaceship

and tried to go out and, say,
visit the Oort cloud,

this would have been
a long journey.

There's billions and billions
of these objects,

and there's a lot of space
in between them.

Narrator: Like the Kuiper belt
and scattered disc,

the Oort cloud objects may
have formed closer to the Sun,

but about 800 million years

after the solar system
was formed,

the gravitational influence
of the gas giant planets

flung these comets

out to the frigid edge
of our solar system.

- Some of these objects
were flung

into very highly elliptical
orbits

out into the Oort cloud.

Passing stars could
circularize those orbits,

making somewhat stable orbits
for them.

Narrator: Most of
the Oort cloud bodies

have been
in frozen hibernation

since the birth
of our solar system.

They only become
long-period comets

when they get sucked
into orbits

that carry them inward towards
the planets and the Sun.

Scientists have never
actually captured

an image of the Oort cloud,

but they have good reason
to believe it's there.

- Even though scientists can't
directly observe the Oort cloud

because it's so far away
and so faint,

what they are able to do
is infer its existence

because we can look at

where all these long-period
comets come from on the sky,

and they seem to come
from all different directions.

There doesn't really seem to be
a preferred direction for them.

This suggests that
the Oort cloud,

if it exists,
is probably roughly spherical.

Narrator:
One of the big mysteries

is what knocks these icy bodies
off course

and sends them cruising
by our neighborhood of Earth.

- The comets in the Oort cloud
are very susceptible

to gravitational pulls
from other things

outside the solar system.

One thing is passing stars.

If a star happens to come
close to our Sun,

it can scatter comets
from the outer Oort cloud.

- Icy objects in the Oort cloud
can also be dislodged

by gravitational perturbations
that occur

when the solar system goes
through the plane of our galaxy.

So the solar system
is basically orbiting

around the center
of our galaxy,

but it's also oscillating
up and down,

and when it goes
through the plane,

then gravitational interactions
can perturb objects

from the Oort cloud
into the inner solar system.

Narrator:
Some long-period comets

take up to about
30 million years

to complete one round trip
around the Sun.

Our spacecraft
tracks a long-period comet

for hundreds of thousands
of miles

until we approach
planet Earth.

As its icy tails unfurl,

a layer of dust and ice
zips through

our planet's
thin atmosphere,

leaving visible trails
known as a meteor shower.

- I like to think of comets

as sort of the pigpens
of the solar system,

because, as they orbit
around the Sun,

they leave a big, messy trail
of debris.

But sometimes the Earth's orbit
can intersect

some of these dust bands,

and when it does, we can
sometimes see meteor showers.

Those beautiful bright streaks
of light

that you see coming through
the night sky

are actually produced
by particles

that are most often no bigger
than a sand grain.

Narrator: While long-
and short-period comets

make up the bulk of comets
that have been observed,

new evidence suggests
that not all icy bodies

come from the subzero suburbs
of our solar system.

Some have secretly
taken up residence

in a much warmer
neighborhood.

Scientists have now
discovered

there are comets masquerading
as asteroids

in the asteroid belt,

located between
Jupiter and Mars.

- The only reason
that we've discovered

these main-belt comets
is because, occasionally,

the warming rays of the Sun
get into the interior,

vaporize the ices,

and so the comet just
bursts forth for a while,

and then they go back to being
inactive objects or asteroids.

- Scientists used to think that
there were strong distinctions

between asteroids and comets

and that they were two totally
different types of objects,

but what we're
finding nowadays is,

there are some asteroids
that have comet-like properties,

and there are some comets
that eventually kind of look

like asteroids.

Narrator: Some scientists
think main-belt comets

may have delivered water
to early Earth

and the materials
to create life.

We blast back
to January 2004

and follow NASA's
Stardust spacecraft

on an unprecedented mission

to collect pure comet dust
in space.

Just beyond the planet Mars,

we meet up with
a comet named Wild 2.

Upon approaching
its enormous coma,

Stardust flips open
a paddle-shaped collector tray

filled with a durable foam-like
substance called aerogel.

Cometary particles no bigger
than specks of dust

fly into the aerogel

at six times the speed
of a rifle bullet.

The mission's task is to
preserve the precious particles

without damaging
or altering them.

But this is no easy feat,
as we show here on Earth.

- We're here at a firing range
with Sergeant Connacht Brewer,

a former army paratrooper,

who is going to demonstrate
for us what happens

when this birds hot strikes
a large block of modeling clay.

This is an excellent analogy
for what happened

when cometary particles
impacted the aerogel

on the Stardust spacecraft.

Except there,
the particles were moving

about 50 times faster
than this birds hot.

Ready to give it a go?

-Yes, I am.

Go ahead and put on
your eye protection

and your earplugs.

What we're using
for this today

is a 12-gauge Model 1100
Remington shotgun

at a distance
of about 35 yards.

Basically, I'm just gonna
load a 12-gauge shot shell

into the shotgun...

[gun clicks]

And we're hot.

[gunshot]

- Whoa.

It really did some damage
to that clay.

You want to go take a look?

- Yeah, we're clear.

- All right.

Wow, look at the damage
it did to the clay.

- Yeah.

- Each pellet
created one of these holes.

So what do you say
we cut this open

and try to find some pieces?

- Sounds good.
- All right.

So you see, the trail here
that's formed inside the clay

is going to lead you,
ultimately,

to where the birds hot pellets
are located.

You can imagine how difficult
it would have been

and how time-consuming

for the Stardust team to locate
these microscopic pieces

of cometary material
inside the aerogel.

Narrator: Upon Stardust's
return to Earth,

scientists recovered
over 10,000 cometary fragments

from the aerogel.

Chemical analysis revealed
that the particles

contained the organic compound
glycine,

a fundamental building block
of life

that had been preserved in ice
for over 4 billion years.

- The discovery of
organic compounds in comets

suggests that organic compounds
can form pretty easily.

Now, it doesn't mean that there
was ever anything truly alive

in those comets, but at least
the building blocks of life

could have been built
in comets.

Narrator: Comets appear to hold
invaluable information

about the origin
of our solar system

and perhaps life itself.

But astronomers
have discovered

that these icy bodies
are not immortal.

Now state-of-the-art satellites
have captured images

of a select group of comets
that will end their life

in a suicidal death dive.

While our journey
with the comets

has felt like an endless
cosmic roller coaster ride,

these icy objects will not orbit
our Sun forever.

Most will make the voyage
for several thousand years

before evaporating
into specks of dust.

Even massive comets,
like Haley,

only have 150,000 years left.

That's because, every second,
a comet loses tons of ice.

- Comets gradually wither away
or fade away,

because every time
they pass close to the Sun,

they lose some of their ices.

They evaporate away.

Eventually,
there's very little ice left,

and so the comet coma and tail
doesn't form.

It just looks like
an asteroid.

Or it may even break apart

into a whole bunch
of little objects

because the icy glue
is no longer there

or because tidal effects
actually break it apart.

Narrator: But not all comets
quietly fade into the sunset.

We're now in hot pursuit
of a group of comets

called sun grazers
that live fast and die young.

Inside our spacecraft,
we feel the heat

as the comets' extremely
elongated orbits

bring us very close
to the Sun...

within a few hundred thousand
miles.

As we enter this danger zone,

we see that some of these
kamikaze comets

occasionally plunge
right into our home star,

creating
a ferocious spectacle.

- Sun grazer comets start out
their life as normal comets.

They probably live
either in the Oort cloud,

or they're short-period
comets,

but they've had an unlucky
encounter with another planet,

probably Jupiter.

If they're really,
really lucky,

they might just escape
and be able to get away

with one close passage
by the Sun,

but if they're not lucky,

they just get swallowed up whole
by the Sun.

Narrator: The Solar and
Heliospheric Observatory,

or SOHO,
is a space satellite

that has observed 2,000 comets
on suicidal orbits.

The most famous sun grazers
are the Kreutz family,

which originated
from one giant comet

that broke up into
many smaller pieces.

- It's a kind of cool thing
to think of a comet

that's lived
in the outer solar system

just falling all the way
and smashing into the Sun,

but they do.

We see them all the time.

Narrator: And some comets
do more than self-destruct.

They become the messengers
of mass destruction.

It's estimated
that a large comet

may have struck Earth
roughly every 40 million years,

based on the amount of craters
still visible on our planet.

A comet may have
even been responsible

for the most famous
extinction event of all time.

- For the Cretaceous-Tertiary
extinction event,

the one that took out
the dinosaurs

65 million years ago,
the jury is still out.

That was thought to be
a 10-kilometer-sized object,

and there are no asteroids
in near-Earth space

that are that large
that could impact the Earth,

and there are a number of comets
that are that large.

And since it occurred
65 million years ago,

and you would expect
a cometary impact

every 40 million years or so,

it may well have been
a comet.

Narrator: While comets
likely slammed into Earth

countless times in the past,

it's been difficult
to determine

if an impact crater
was made by a comet or asteroid

because the two bodies
appear to be similar.

- When we study
the fossil records,

a lot of the material is gone,

and just-—
we simply can't find it,

and it turns out that
asteroids and comets

have a lot of materials
in common,

so even if you do find
extraterrestrial material,

it's really hard to tell

whether it came from an asteroid
or from a comet.

Narrator:
And to complicate matters,

a comet doesn't even
have to impact Earth's surface

to ignite a catastrophe.

In 1908, a fireball exploded
in the atmosphere

above the Tunguska wilderness
in Siberia.

The heat and energy
from the air burst

propelled downward
like a hot tornado.

It propagated
across the forest,

flattening over 800 square miles
of trees.

For over a century,
some scientists have wondered

if the cosmic intruder
was a comet or an asteroid.

- If it were caused
by a comet,

you would imagine you'd find
in the sediment

some record of unusual things,

like a ratio of helium 3
to helium 4.

That could be an indication
that it may have been a comet.

Now, another possibility
is if you find

some strange isotope
buried in a lake somewhere.

That's tough.
This is not easy.

Narrator: While it's
been difficult

to substantiate
cometary impacts on Earth,

physical proof
of their colossal power

exists on
the gas giant planets.

And these dramatic events
warn that far worse collisions

loom in the future,

with Earth as
the potential bull's-eye.

As we have followed the trail
of comets through space,

we have passed through
the chilliest

and warmest places
in our solar system.

We've also investigated
whether these icy bodies

have been deliverers
and destroyers of life.

And new observations prove
many of these ancient bodies

haven't quietly retired
to the frigid outer limits

of our solar system.

- We think there are several
possible end states for comets.

In one case, they can actually
get pulled right into the Sun.

In other cases, they can
actually get scattered

by one of the planets
and kind of get kicked out,

maybe back into the Oort cloud
and never seen again.

And finally they can also
actually impact a planet.

Narrator: We now shuttle back
to July 1994,

We follow a string of
21 comet fragments

called Shoemaker-Levy 9

as they're gravitationally
pulled toward the gas giant

planet Jupiter.

- If we were riding on

one of the fragments of
the comet Shoemaker-Levy 9,

it would have been
truly spectacular

because we're heading
toward Jupiter,

and this giant planet
is looming ever bigger,

and then-splat!

We crash into it.

We throw up a whole bunch
of material from the insides.

It would be really
an amazing journey.

The plumes of material
coming out of Jupiter

were super heated gas
heated by this collision

and also excavated from
the interior of Jupiter.

Narrator. Shoemaker-Levy 9
left impact scars

the size of Earth,

driving home
the scale of violence

that comets can produce
if they slam into our planet.

- If any of those fragments
had hit the Earth

rather than Jupiter, we'd have
been in serious trouble,

because they were large

and they were coming in
extremely rapidly.

But fortunately,
big brother Jupiter

took all the hits for us
and didn't seem to suffer much

in the way of damage.

Narrator: But even with
Jupiter acting

as a planetary shield,

comets still
sneak by the gas giants

and have close encounters
with Earth.

- The good news is,

asteroid impacts are far more
likely to occur

because asteroids
outnumber comets 100 to 1

in near-Earth space.

The bad news is,
if we do find a comet

on an Earth-impacting
trajectory,

we wouldn't realize it

till it got inside
the orbit of Jupiter,

when it started throwing off
gas and dust.

- In the very unlikely event
that a comet

could get close to the Earth,
you would worry about it

because they tend to have
high average velocities

relative to an asteroid,

as high as tens of miles
a second.

So in other words, they would
pack a bigger punch.

Narrator: While the frequency
of near-Earth asteroids

heightens
the risk of impacts,

the speed of comets
is equally troubling.

This sobering fact
has prompted

viewer Robin W. from Boulder,
Colorado, to...

- Robin,
that's an important question.

The asteroids
are more numerous,

so there's more of them
that could hit us.

But we can track
their trajectories

and maybe do something about
one that's gonna hit us,

deflect it for example.

The comets are more rare,

but we have very little warning
when they come in.

And also, they move much faster
than asteroids,

so there's more energy
impacting Earth.

So I would say comets are
the most hazardous objects.

Narrator: NASA is taking
the threat of comets seriously.

So far, tracking satellites

have tagged 84
near-Earth comets,

objects with orbits that
come within 28 million miles

of Earth's path
around the Sun.

- There are no periodic comets

that currently have Earth's name
written on them.

But Jupiter
and the other giant planets

occasionally perturb
the orbits of comets.

So there could be
a periodic comet in the future

that will collide
with Earth.

Narrator: Since the beginning
of recorded history,

over 4,200 comets
have been observed,

a mere fraction of the total
number of these icy bodies.

This means billions
of unknown comets

still lie in wait
in the outer reaches of space.

One day, a doomsday comet
could be nudged out

and sent on a collision course
with Earth.

- If we're on
that cosmic highway,

and it's getting
into our on-ramp,

we need to worry about it.

And we need to worry about it
with very little lead time.

- There is very little
we can do,

other than sending
a giant bomb toward them

and trying to deflect them.

But that's very difficult

when there's only
a few months' warning.

Narrator: Just as comets
have inspired awe and fear

among our ancestors,

they remain a force
to reckon with.

By continuing to track
their tails of dust,

we may uncover more clues
about these frozen artifacts

of our ancient past.

- It's pretty remarkable
if you think about it.

Just a few hundred years ago,
we had no idea what they were.

We had no idea
what they meant.

And now we've actually been
to the surfaces of comets.

We've seen up close and personal
what they're really like

and what they're made out of.

So these mysterious objects
have meant a lot to humans

throughout our history.

They've been agents of change.

They've been agents
of destruction sometimes.

They've maybe been agents
of creation.