All language subtitles for How the Universe Works s09e02 Mission to a Comet.eng

ROWE: Comets... messengers from

the dawn of the solar system.

For the first time,
we sent a spacecraft

to orbit and land on one.

OLUSEYI: To go outside of
the inner solar system

and catch a speeding comet

and then send a lander down
to land on its surface...

That is nuts.

ROWE:
The Rosetta mission answers

our most fundamental questions.

SUTTER:
Rosetta is really teaching us

how comets live in real time.

This is a game changer.

ROWE: Rosetta changes
our understanding

of the cosmos forever.

The secrets of life itself may
be wrapped up on Comet 67P.

??

[electricity buzzing]

??

ROWE:
530 million miles from the sun,

a tiny lump of ice and rock

starts its journey towards
the inner solar system.

THALLER: Scientifically,
comets are more precious

than pure gold...
They're actual time capsules.

They're preservations of what
the chemistry, the environment

was like when
our solar system formed,

and they hold clues as to how
our chemistry arrived

here on Earth...
Comets are some of

the most valuable scientific
treasures that exist.

ROWE: Finding a comet we can
study is not an easy task.

Finding the right comet
for a mission...

It's kind of like you're
auditioning them, right?

One comes in, and it's like,
oh, the other orbit's okay,

but it's not exactly
what we're looking for.

And, oh, this one is too big.

This one's too little.
This one's too active.

WALSH: What we really want to
see is that comet come from

outer parts of the solar
system and get heated up as it

gets close to the sun,

pass around the sun and head
back out to the outer part of

the solar system, so you need
to get way out in

the solar system and catch
a comet on its way in.

ROWE: Comet 67P fits the bill.

The special thing
about Comet 67P

is how accessible
it is here on Earth.

It actually orbits around
the sun once every 6.5 years,

and its orbit doesn't
take it all that far out,

only about this far out of
the planet Jupiter.

That gives us many chances
to actually

reach the comet and successfully
rendezvous with it.

[announcer
speaking indistinctly]

ROWE: 67P's moment in
the spotlight arrived.

[announcer
speaking indistinctly]

The European Space Agency
launched the groundbreaking

Rosetta probe.

PLAIT: For years,
we've studied comets from afar.

We had never seen one up close

with cutting-edge technology to

learn about how comets behave
as they orbit the sun.

ROWE: Missions to large,
planet-sized objects are hard.

The Hubble space telescope's
blurry images revealed

Rosetta's target
is just two miles wide,

and not just a small target,
a moving one.

Comet 67P
races through the solar system

at over 33,000 miles an hour.

The precision involved is
pretty incredible.

It's like making
that hole-in-one

golf shot from New York
to San Francisco.

ROWE: But catching Comet 67P
wasn't just a straight shot.

The spacecraft had to actually
get the same velocity

as the comet, and with current
propulsion systems,

we can't achieve that by
flying directly to the comet.

ROWE: Instead,
Rosetta performed a slingshot

maneuver that would take
10 years.

SHERIDAN: So what it's doing,
it's actually taking

some energy
away from the planet,

so it's slowing the planet
down a little bit and then

imparting that energy
into the probe.

ROWE: Rosetta flew past Jupiter

nearly 100 million miles
from the warmth of

the sun and entered the most
dangerous part of its journey,

hibernation.

RADEBAUGH: Even though Rosetta
had solar panels on it,

it still had to be put to sleep,
and the reason for that

is that it had to track 67P
a ways away from the sun,

and there just was not enough
collection area

and solar power to be able
to power the instruments.

SHERIDAN: It's normal to put
spacecraft into hibernation,

but the worrying thing at
the back of your mind is that it

had never been done
for this long before.

ROWE: 31 months of hibernation
gave Rosetta's team plenty

of time to worry about
what could go wrong.

WALSH: It's dark and cold
out there in space.

There's a lot
of things going on.

There's a lot of little
micro-meteorites out there,

and without the constant
communication with

that spacecraft,

just is a little nerve-racking
when the day comes

and it's time to flip
the switch and turn it back on.

ROWE: January 20th, 2014.

After almost 10 years in
space, it was finally time for

Rosetta to wake up,

reactivate its communication
system, and phone home.

RADEBAUGH: I mean, the tension
was just palpable.

It was just, I mean,
how long do we have to wait

before we're gonna
get the signal?

SHERIDAN: It's got to work.
It has to work.

Oh, gosh, maybe it's just not
gonna turn on it all.

Have we lost the spacecraft?

And then the peak
appeared on the graph.

[triumphant music]

It's like, yes, we've got
contact with the spacecraft.

[triumphant music]

??

ROWE: Rosetta began to send
back images of its target,

and after months of seeing
a small dot in the distance,

the comet slowly came
into focus.

THALLER: When I saw this comet,
crystal clear,

this mountain floating
in space of ice and rock,

my heart just dropped... they
are some of the most dramatic,

beautiful images
I have ever seen.

PLAIT: Then, as it got closer,
and we got to see more

and more details on it, yeah,

that's when things started
getting really strange.

WALSH:
Quite simply, arrival at 67P,

we expected to see something
shaped like a potato,

and we found something shaped
like a rubber duck.

ROWE: 67P is no ugly duckling,

but its strange shape created
a problem for Rosetta.

Orbiting the comet
was going to be

far more difficult
than anyone had imagined.

THALLER: A planet has
a lot of gravity,

so you can send the spacecraft
out there and then just slow it

down a little bit
with a rocket burn,

and it will drop into orbit.

In the case of Comet 67P,

you're dealing with
a very small little rock.

The spacecraft cannot feel
the gravity of that rock,

at least not until
it's right up against it.

SHERIDAN: Well, the engineers
had to actually plot

triangular orbits.

It was a very complicated
set of maneuvers.

ROWE: Once in orbit,
Rosetta could start work.

Its first task...

Figure out how 67P formed.

LANZA: So how did this comet
get this weird shape?

There's two main ideas.

One is just that it was eroded
somehow in the center.

And so it started off
as a more spherical thing

and became the shape
it is today through some

unknown process.

The other idea is that
it started

as two separate objects.

ROWE: Space rocks normally
hit each other hard.

They collide with
an average impact speed

of more than
11,000 miles an hour.

That's five times faster
than a rifle bullet.

Was 67P involved in a pile-up?

A clue came from the distinct
layers on the comet's surface.

The layers in Comet 67P

are a little like
the layers in an onion.

If you see them aligned,
that's a clue that perhaps

the object formed
as a single entity

and only eroded later
into its present form.

But if you see those layers
misaligned like we actually do

in the comet, that's a big clue

that it started out
as two separate

objects formed independently,

sticking together to form
the comet we see today.

ROWE: The layers prove
that 67P was

originally two separate
objects that fused together.

The process of potentially
putting Comet 67P

together from two
different pieces

is important, because it can
teach us about what was

happening in the early
solar system.

So as far as we can tell,

these two separate bodies
must have

been formed in the same area.

They're very similar
in composition,

but they are so light and
fluffy that they would have

destroyed each other
if they had hit fast.

They had a low speed collision

and basically stuck together
like two wet snowballs.

ROWE: With one mystery solved,
Rosetta began to

investigate the chemical
makeup of 67P.

The little comet
could answer one

of the biggest questions
in planetary science.

From where did our blue planet
get its water?

ROWE: The Rosetta Mission,
a four billion mile journey

to Comet 67P
and a 4.5 billion-year

trip back in time to
the birth of the solar system.

This comet is a remnant of

the formation of
the solar system itself.

So this is an opportunity
to open that time capsule

and get a view into
the ancient solar system.

ROWE: Comet 67P
could help us answer one

of the most important
questions about our planet.

One of the big mysteries
that we have

about Earth is where did
the water come from?

ROWE: Today, water covers
over two-thirds

of the Earth's surface.

But it wasn't always that way.

4.6 billion years ago,

the inner solar system formed
from a maelstrom

of rocky debris.

Temperatures were so hot,

any water on the early Earth
boiled away.

What the evidence suggests to
us is that Earth's water arrived

at Earth after the Earth formed,

and the primary mechanism that
we thought were responsible

were our comets.

CHRISTIANSEN: You have to
remember that comets are

basically just big balls
of ice and dirt.

One of the main goals of
the Rosetta mission was

to analyze the water
on this comet

and see if it matched
the water on Earth.

ROWE: Comets come from
two regions at

the outermost reaches
of our solar system...

The Oort Cloud

and the Kuiper Belt.

OLUSEYI: The Kuiper Belt is in
a plane of our solar system,

and the member that most
people are aware of is Pluto,

but beyond that,
there is a spherical

distribution of icy bodies,
the Oort Cloud.

WALSH: This is a repository of
comets that extend for

hundreds of thousands of times

further away from the sun
than we find the Earth,

and it's kind of a big,
spherical cloud around the sun

that's kind of deep freeze for
a bunch of big comets.

ROWE: Billions of icy objects

orbit safely beyond the chaos of

the inner solar system,

locking in primordial water.

WALSH: The life span of
a comet is fascinating.

They might do absolutely
nothing out there for over

four billion years
until the right

tweak of their orbit or tug
from a planet changes

everything, and all
of a sudden, they're

on a path in towards the sun
and that ice that's been in deep

freeze for billions of years
starts to heat up.

ROWE: Astronomers believe that
Comet 67P's former home turf

was the Kuiper Belt until
something sent it

ricocheting inwards.

SHERIDAN: The influence of
the gravity of Jupiter,

because Jupiter's such
a massive planet,

it actually attracted
the comet and pulled it into

the inner solar system.

DURDA: It slowly was perturbed

and migrated into
the inner solar system,

where we see it today...
It's traveled.

I mean, not just, you know,
billions of miles in linear

space, but, I mean, really,

when you think of all
the orbits that must have made,

just trillions upon trillions
of miles, that's a...

That's quite a journey for
such a little object.

ROWE: Now, 67P loops around
the sun in a 6.5-year orbit

that travels
as far out as Jupiter

and closer in than Mars.

As it gets nearer to the sun,
its frozen ice

begins to heat up,
leading to a famous effect,

the comet's tail.

Sunlight comes in and heats up
the comet's surface,

and it gets transferred to
the interior, where there's

a lot of ice,
and the ice gets heated up,

and it vaporizes and makes
its way to the surface.

And once it's sitting on
the surface, the solar wind is

bombarding that comet,

and it drags all of that
material out into

a long and beautiful tail.

ROWE: Rosetta flew
into the tail to

analyze the water vapor.

Its goal... confirm it's
the same type

of water as we have on Earth.

CHRISTIANSEN: Water does come
in different flavors,

and I don't mean saltwater
or freshwater.

I mean what the water's made
of, its molecules.

BYWATERS: We're talking about
the actual, actual chemical

makeup on the proton,

neutron, and electron level,

so you might have heard
of heavy water before.

And that just means that
there is an extra neutron.

WALSH: It's H20, but one of
the hydrogens

has an extra neutron,

so we call it deuterium.

And so that water actually
weighs more the normal water

because of that extra neutron
in the mix.

ROWE: On Earth, there are
roughly 160 molecules of

heavy water for every million
molecules of normal water.

As Rosetta moved
around the comet,

it measured the ratio of
hydrogen to deuterium in

the water vapor.

An exact match to Earth's
ratio would confirm that comets

were the source of our oceans,

but Rosetta discovered
something surprising.

What Rosetta found was that
the deuterium to hydrogen ratio

was three times that of
the water in Earth's ocean.

That means a comet like 67P

couldn't have been the source
of Earth's water.

SHERIDAN: If it was the same
as the measurement

of the ocean water,

then we could have said that
these comets could have been

a delivery mechanism for
the water that we see on Earth.

The fact it was three times
higher kind of indicates that

comets, like 67P,

were not the source of the water
that we see on Earth today.

ROWE: But although comets like
67P didn't deliver our water,

we can't rule out all comets.

OLUSEYI: We have to keep
this in perspective.

67P is but one comet,
and what we know about

objects like comets

is that they come in
families, and these families

have different compositions.

So it could be that some
have Earth-like water,

and those are the ones that
brought water to Earth,

and some don't.

BYWATERS: I don't see
wrinkles like this

as going back
to the drawing board.

Sure, we get more questions
sometimes than answers,

but that allows us to probe
farther and really expand

our knowledge into
how planets formed.

ROWE:
The composition of Comet 67P

rewrites our understanding of

the solar system, and the comet

has more secrets to reveal.

THALLER: The reason we went
all the way

out to Comet 67P,

and, in fact, the reason
we studied comets at all,

is we're looking for clues
about the origins of life.

ROWE: As Rosetta examined
the icy core of the comet,

it found a treasure trove
of molecules.

Could 67P carry the building
blocks of life itself?

ROWE: Rosetta's mission
to Comet 67P planned to answer

the biggest questions
in planetary science,

and none as bigger
than figuring out

the origins of life on Earth.

Life on Earth got started
not long after

our planet cooled, something
like 4.5 billion years ago.

But we don't know
the exact process.

Did all that stuff
come together here from

molecules on Earth and become
more and more complex?

Or was it brought from space?

[explosion blasts]

ROWE: Rosetta began to hunt for

the building blocks of life...
Basic organic compounds.

So we always thought
that comets probably

carry organic materials,
but we really couldn't

confirm that until Rosetta.

RADEBAUGH:
It detected aliphatic compounds,

which are organics rich in
carbon and hydrogen, and that is

the first time such a substance

has been detected at
the surface of a comet.

LANZA: These materials
are the raw building blocks

for making things like proteins,

which are required for all life.

This is really important,
because this says that

possibly these materials were
delivered to Earth by comets.

ROWE: The idea that comets
carried the building blocks for

life across the solar system
is known as

molecular panspermia.

You can kind of think of
comets is being like space

delivery trucks, right?

They're forming out there
in deep space.

All this stuff is being
formed with them,

and then they bring it to Earth.

Now, you know,
you don't want your

delivery truck smashing
through your front door,

but in the case of comets,
when they impact the Earth,

they can distribute those
molecules all over the place.

But in some way, we're kind of
skipping a step, right?

I mean, so the comet
may be bringing

the ingredients of life
to the Earth,

but where'd the comet get
those ingredients?

ROWE: The answer may be in
deep space,

in the matter
and radiation floating

between star systems
and a galaxy.

It's called
the interstellar medium.

DURDA:
The really interesting result

is that the overall
composition of Comet 67P

and some of the really
interesting ingredients

in that comet are very similar,

identical, in some cases,
two materials we find out there

in the interstellar medium,

floating out there in the gas
and dust between the stars.

DARTNELL: If this organic
chemistry is common on Earth,

it's likely to been common
across the entire galaxy.

This chemical kit for
making life is universal,

and therefore perhaps
life itself is common.

There could be plenty of
wet rocks out there in

the night sky teeming with life,

with this universal
chemistry kit.

ROWE: But scientists
still couldn't figure out

the origin of one vital element.

So phosphorus is one of
the atoms that's absolutely

essential for life,

but we don't really know
where it comes from.

ROWE:
Phosphorus is a vital part of

DNA, cell membranes,
and energy production.

But the problem is, it's quite
scarce in the universe,

and it's certainly scarce
on the surface of the Earth.

Any phosphorus that was around
would've been locked up in

the soluble rocks,
so that wouldn't have been

available for life
to use at that time.

ROWE: If rocks had locked in
all the mineralized phosphorus,

where did the phosphorus needed

for biological processes
come from?

67P held the key to the mystery.

What we found on the comet
is bioavailable forms

of phosphorus, not just
mineralized phosphorus.

ROWE: Bioavailable phosphorus
is a form that life can use,

but where did Comet 67P
get it from?

In January 2020,

astronomers combined data
from the Rosetta mission

with ALMA's observations of
the star forming region

AFGL 5142.

CHRISTIANSEN:
AFGL 5142 is a stellar nursery,

a gas cloud where large
and small stars are being born

simultaneously,
and it's very close to us,

which means we can study it
in great detail.

WALSH: The ALMA Observatory

takes really
high-resolution images

of different types of dust
and gas in very,

very close to a star that's
just formed so you can actually

see the process of
formation happening.

ROWE: The largest stars
live fast and die young,

exploding in supernovas.

SHERIDAN: So the phosphorous
itself, we believe,

is formed in massive stars.

So essentially, it's created
a star gets to the end of

its life, and when the star
goes supernova, it spews

this phosphorus out into
the interstellar medium.

ROWE: When midsized stars
burst into life,

they sent shockwaves
and radiation through the cloud,

transforming phosphorus
into a form

biology can use...
Phosphorus monoxide.

The phosphorus monoxide can
freeze out and get trapped on

the icy dust grains that
remain around the star.

These dust grains can come
together to form pebbles,

rocks, and eventually,

comets that become
the transporters of

phosphorus monoxide.

ROWE: Scientists traced
the cosmic trail of phosphorus

and organics from stars

to comets to planets
and even to life.

You could, in theory at least,
take the sort of chemistry

find preserved in a comet
and use it to construct a cell.

So often, people seem
to think of astronomy as

the study of things
that are very far away,

that has absolutely no bearing
on our day-to-day life.

Nothing is farther
from the truth.

We are looking for the origins
of ourselves out there,

and we are discovering them.

So a basic question like,

how did phosphorus end up in
our DNA when the only place

you seem to find it
is around young stars,

clouds thousands
of light-years away...

Now we know the mechanism,
because we asked the question,

and now we understand
ourselves that much better.

ROWE: The next stage
of the mission was

to get the lander
onto the comet's surface.

But there was a problem.

The craft's landing gear
was broken.

[beeping noises]

Would the team have to abort
the mission?

ROWE: Rosetta had discovered
the building blocks for life

on 67P.

Now the probe faced its
toughest challenge so far...

A historic first landing
on the surface of a comet.

One of the biggest challenges
about setting the Philae lander

down on the surface

is having no idea what
the surface is like

when you're designing
the lander.

Is it going to be like rock
or ice and be very stiff?

Is it gonna be brittle?

ROWE: Philae had harpoons to dig
into the soft surfaces

and top thrusters to stop it
from bouncing off the comet.

The lander team discovered
neither system was

working properly.

With the odds stacked
against them,

they decided to go
for landing anyway.

Philae made a slow,
12-mile, seven-hour descent.

[whirring noises]

Until finally...

...touchdown.

[cheering and applause]

So when we got the first
signal of touchdown,

we were relieved...
The lander was down,

and everything was good to go.

[applause and chatter]

But then we realized things
weren't quite going to plan.

ROWE: The data from the lander
was intermittent.

The only possible explanation?

It was tumbling end over end.

It could've bounced
and bounced right off

the comet and continued
all the way into space.

SHERIDAN: We really didn't know.

We just knew we were not on

the ground, and we were
rotating and tumbling.

ROWE: Then, 12 minutes later,
a ray of hope.

We saw a flash in one of
the computers, and we huddled

around it, and we realized

that the lander
was actually sending

data back as it was
programmed to do.

We were actually doing
science on

the comet and getting
the results back.

ROWE:
The lander stopped tumbling,

Philae's solar panels weren't
fully operational, and the tiny

craft's battery had only
60 hours of power left.

It was a race against time.

We had to completely rip up
all of our plans

of what we wanted to do,

and then we had to rewrite
everything to maximize

the amount of science return

in those 60 hours we had
available to us.

ROWE: Philae got to work.

Its first discovery...

The ground was stranger
than expected.

One of the things we found
about the surface

is the top 40 centimeters
were a very fluffy material.

DURDA: It is not dense
and rocky... it's pretty fluffy.

You could imagine something
on the order of cigar ash.

Imagine the rocks on Comet 67P,

but imagine them being as
light and fluffy as cigar ash.

PLAIT: It's so crumbly that
if you had in your hands,

you could break it apart
with almost no effort at all.

It has a fraction of
the strength of Styrofoam

ROWE: Philae's bounced landing

helped investigate the surface.

SHERIDAN: By bouncing,
we actually were able to sample

two sites, and interestingly,

both sites were different.

That kind of shows us that
the comet is not homogeneous.

It's not all the same material.

ROWE:
Philae's instruments revealed

Comet 67P is not just
two lobes fused together.

The comet formed from distinct

blocks pulled together
by gravity.

Philae's on board camera
also fed back vital

clues to the lander's
final resting place.

The images taken by the lander
were really amazing,

because from those, we were
actually able to work out

Philae was on its side.

ROWE:
Philae's location was unknown,

far from
the planned landing site.

With its battery dead,

it couldn't speak to Rosetta.

SHERIDAN: The problem with
not knowing exactly

where on the surface of
the comet we were was that some

of the instruments had needed
to know that exact location

to actually interpret
the data, and without that

information, the results
were meaningless.

OLUSEYI: Time is passing,
and still

no Philae lander is to be found.

Is there hope?

Is all lost?

HOWETT: When you make
or follow a space robot,

you kind of get invested
in that success

and the survival of that robot.

You want it to succeed against
the harsh reality of space.

You feel like you've...
You've lost a pioneer.

You've lost a fellow explorer.

ROWE:
As Rosetta scanned the comet,

searching for the lost lander,
it faced a new problem.

As this dirty snowball races
towards the sun, the radiation

from the sun heats it up,

and so it starts snapping
and popping and cracking,

transforming itself.

It's like a firework show.

ROWE: Huge jets blasted material
from the surface.

Would the outbursts

launch Philae into space?

ROWE: Rosetta's lander, Philae,
was lost somewhere

on the surface of Comet 67P,

its battery dead,
and its solar panels useless.

As Rosetta scanned the comet
for the missing probe,

it flew less than two miles
above the surface,

close enough to cast a shadow.

PLAIT: Seeing the shadow of
the spacecraft itself

on the comet

really brought home
the fact that

there was a piece of humanity,

one of our machines,
right there.

It makes the hair on the back
of my neck stand up.

It gives me chills.

ROWE:
The low passes allowed Rosetta

to smell the comet's surface.

The ROSINA instruments
allowed us to actually measure

the chemicals and the gases

coming off the comet, so we can
actually put those together

to make a comet perfume.

So what we've actually done is
impregnated those smells of

the comet into a card a bit

like a scratch and sniff card,
so you can actually

[sniffing noises] smell.

And that is the smell
of the comet.

And it's not very pleasant.

If you can imagine a mixture
of something like rotten eggs

and cat urine and, like,
bitter almond stuff,

it's like... it's not something
I would wanna

have to be able to smell.

To me, it kind of smells like,

um, baby's nappies.

And it's a bit of a pungent,
quite an unpleasant smell.

ROWE: Months passed
with no sign of Philae.

HOWETT:
The comet isn't terribly big.

You know,
where could it be hidden?

And if we understood
where it was hidden,

we would understand the images
that it had returned

so much better.

There was sort of that human
part of like,

where is my buddy?

And also that science part of,
what was it trying to tell us?

ROWE: Then, an eagle-eyed
researcher spotted

something in the corner of
an image.

SHERIDAN: The lander itself was
a tiny little speck,

and you really have to zoom in
to actually see it.

But there was Philae,
under a cliff.

OLUSEYI: Seeing the little
Philae lander there,

on its side, under a ledge,

in the shadows,
was just remarkable.

ROWE: Pinpointing
Philae's location on

the smaller lobe solved
one final mystery.

It explained data sent back
months earlier,

revealing this area of
the comet is solid.

HOWETT: There was a real
sense of closure,

and there was sort of
a Godspeed to it,

you know, like, we know
what's happened to you.

Thank you for the science.

And that was its final
resting place.

And it was... it was really...
It was really wonderful to see.

SHERIDAN: Philae was
an amazing success.

Despite all those problems,

we got the lander down...
Despite the bounce

and it coming to a rest
under the cliff face,

we got actually amazing images
on some fantastic measurements

of the gases and the structure
of the comet itself,

which we would never have had
had we not tried

to put a lander on a comet.

ROWE:
Rosetta's mission was not over.

As the comet approached
the sun, chaos erupted.

[whooshing noises]

RADEBAUGH: All of a sudden,
there were these jets that would

appear and eject a huge amount
of material from the interior.

ROWE: When sunlight warms
the comet's surface,

ice turns directly into gas.

The uneven surface of
the comet funnels the escaping

water vapor into narrow jets
of gas and dust.

But Rosetta also spotted
huge violent gas outbursts.

[whooshing noises]

We would expect to see
jets on comets that

kind of gradually rise and fall

with the solar illumination.

But these outbursts
were utterly different.

They would just sharply
increase and then stop.

These outbursts weren't timed
with the sun.

They came out at random times,

so there must be something
else going on.

ROWE: The outbursts exploded
in sudden, brief,

high speed events.

Because we only take pictures of

the surface every
5 to 30 minutes,

when we see an outburst
in only one image,

we know that the whole event
lasted less than that time,

which is very mysterious.

RADEBAUGH: So it was like
putting together a puzzle,

and you have 1,000 pieces,
and you're putting together,

and you realize,
oh, I only actually have 600

of the 1,000 pieces,
and I've got to

make sense of this,
and it's really frustrating.

ROWE: As the comet swung
closer to the sun, Rosetta's

cameras captured 34 outbursts
in just three months.

They were all huge.

In the span of only a few
minutes, a single jet can

release up to 260 tons
of material,

and one jet was spotted
releasing 40 pounds of

material every single second.

ROWE: The cause of the outbursts
was a complete mystery.

Then, in September 2014,
close-up images

revealed a 230-foot-long,

3-foot-wide fracture on
the cliff edge named Aswan.

Less than a year later,
Rosetta photographed

a large outburst from the same
region of the comet.

When the probe investigated

the area, it found
a devastated landscape.

The cliff had collapsed
in a huge landslide.

SUTTER: This is like
a crime scene.

We have the before pictures
and the after pictures,

and we have to solve
the mystery of how

this happened.

ROWE: What links the outburst
and the cliff collapse?

SUTTER: We think there's
a domino effect going on here,

where the sun heats up some of
the volatiles deep under

the surface, and they sublimate

and turn into a gas, and they
find any little crack

or crevice they can
to shoot out of the comet.

But that process weakens
part of the comet's face.

RADEBAUGH: This is gonna
release more material, and,

as it does that, it's going to
start forcing gas up through

kind of nozzle-like features,
having them act like jets.

And as they do that,
then that's gonna cause even

more weakening of the material

and maybe
a large-scale collapse,

which could actually cause
a giant outburst.

ROWE: We think escaping gas
weakens the surface,

which triggers a collapse
and a release of gas

in a giant outburst.

Like sticking dynamite
charges in the side of

the cliff and then setting
them all off.

??

[whooshing noises]

ROWE: Comet 67P revealed
the hidden geological

activity going on inside
these dirty snowballs,

and Rosetta spotted another
surprising phenomenon...

...strange shapes moving
across the surface...

...dunes.

Dunes are driven by wind...
Well on an airless comet,

how do you have that happen?

ROWE: Rosetta rewrote the book
on how comets form,

how they live,
and how to explore them.

When Rosetta flew close
to the surface,

it spotted
something unexpected...

...dunes.

DURDA: These are serious dunes,
They're about 6 feet high,

and they extend for hundreds
of feet in every direction.

So we've got to figure out
what this mechanism is that

it's causing this, you know,
pretty significant

geologic structure on the...
On the comet.

ROWE: Jani Radebaugh
is a planetary scientist.

She studies space dunes
on Earth.

One of the most fascinating
things that Rosetta saw on 67P

was a set of dune-like
features on an airless body.

These were found up by the neck

region in a set of
soft sediments.

They actually look very
similar to

these ripple-like landforms.

They were spaced
about 10 yards across,

and there were maybe 10 to 15
of them across the surface.

And we even saw them
change over time.

When we first saw these,

no one had any idea
what could have formed them.

In fact, people even doubted
that they were

dune-like landforms,

because how would they
possibly form on a body that

has no atmosphere,
let alone any wind?

ROWE: On Earth, land heated
unequally creates winds.

RADEBAUGH: The atmosphere on
Earth and the really transient

atmosphere on a comet

are very different
from each other.

On Earth, it's a stable,
thick, dense atmosphere.

It's got constant solar
heating that can pick up air

masses and move them across
the surface and generate wind.

ROWE: Comet 67P does have
an atmosphere, created when

the sun heats the surface,
releasing gases.

But this atmosphere
has a maximum

pressure 100,000 times
lower than Earth's.

Using Rosetta images,
researchers in France modeled

the movement of gases
on the surface of the comet,

and they think
they've found the answer.

Researchers found that there are
winds on the comet's surface.

These are brief, and they form

because gases escape
on the sunlit side,

and then they rush over to the
cold side across the surface,

and this movement of
air across the surface is

what picks up the sand
and forms the dunes.

ROWE:
The grains on the comet are

far bigger than sand grains
on Earth.

On the Earth,

the sand dunes are made of
grains about this size.

But on the comet, the sand
dune grains are this size.

Gravity also plays
a really important role

in moving grains
around on the comet.

The comet has such a low
density that if you were

walking around, you'd be able
to fly into orbit very easily,

and so a grain this big but
low density would easily be

lofted by the very thin
air there.

ROWE: After two years orbiting
the sun with Comet 67P,

Rosetta's mission
was almost over.

So the plan was
to lower the orbiter

down towards the surface
in a controlled way.

And this would allow very
high-resolution images to be

taken and also to get some
gas measurements right

close to the surface
of the comet.

ROWE: September 30th, 2016.

Rosetta spent 14 hours
slowly free-falling

towards the comet,
taking photos on the way down.

SHERIDAN: As Rosetta was
lowered, it touched the surface,

and then the link back
to Earth was broken,

and the mission was
effectively over.

The thing that's kind of
wonderful about it is

that there's
a little piece of us

on that comet
out there right now.

As that comet spins around out

to Jupiter and back every
6.5 years,

it will always have a little
bit of humanity on it.

ROWE: There was one final
twist to Rosetta's story.

DURDA: We thought we had
the last image back

from Rosetta before
the planned end of the mission.

But it turns out that actually,

just before touching down,
before the come were able to

finish transmitting back to

Earth, an incomplete packet of
information was sent back.

ROWE: The computer discarded
this incomplete data packet.

Recently, human experts
re-examined it.

The result...
Rosetta's closest photo of 67P.

And the image is amazing,
because when you actually blow

it up and look at it,
you can actually feel like

you could reach out
and touch the surface.

ROWE:
It's a fitting postscript to

the only mission to land on
a comet.

DURDA: It's an incredible chance
to learn about the geologic

structure at the size scale of

a human being exploring there...
It's a way to look at comets

that we've never had before.

ROWE: The Rosetta mission has
transformed our understanding

of comets.

WALSH: Between the strange shape
and the measurements of

heavy water, we're finding

that there's a big diversity
out there of comets,

and that every time
we go to visit a new one,

we're probably gonna be
surprised all over again.

ROWE: Rosetta may inspire
new missions.

THALLER: We may actually be
returning to Comet 67P itself.

There's a lot more there
to discover

about the origin of
our lives, ourselves,

here on Earth.

There's so much more to learn

and so many more mysteries
to uncover.

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