Original subtitles

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.