How the Universe Works - S09E02 - Mission to a Comet
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00:00Comets, messengers from the dawn of the solar system.
00:11For the first time, we sent a spacecraft to orbit and land on one.
00:21To go outside of the inner solar system and catch a speeding comet and then send a lander
00:28down to land on its surface, that is nuts.
00:33The Rosetta mission answers our most fundamental questions.
00:38Rosetta is really teaching us how comets live in real time.
00:43This is a game changer.
00:45Rosetta changes our understanding of the cosmos forever.
00:49The secrets of life itself may be wrapped up on Comet 67B.
01:06Five hundred and thirty million miles from the sun, a tiny lump of ice and rock starts
01:13its journey towards the inner solar system.
01:17Scientifically, comets are more precious than pure gold.
01:21They are actual time capsules.
01:23They are preservations of what the chemistry, the environment was like when our solar system
01:28formed.
01:29And they hold clues as to how our chemistry arrived here on Earth.
01:34Comets are some of the most valuable scientific treasures that exist.
01:38Finding a comet we can study is not an easy task.
01:43Finding the right comet for a mission, it's kind of like you're auditioning them, right?
01:47One comes in and it's like, oh, that orbit's okay, but it's not exactly what we're looking
01:51for.
01:52Oh, this one's too big, this one's too little, this one's too active.
01:57What we really want to see is that comet come from outer parts of the solar system and get
02:02heated up as it gets close to the sun, pass around the sun and head back out to the outer
02:06part of the solar system.
02:08So you need to get way out in the solar system and catch a comet on its way in.
02:13Comet 67P fits the bill.
02:17The special thing about Comet 67P is how accessible it is here on Earth.
02:22It actually orbits around the sun once every six and a half years, and its orbit doesn't
02:26take it all that far out, only about as far out as the planet Jupiter.
02:29That gives us many chances to actually reach the comet and successfully rendezvous with it.
02:3867P's moment in the spotlight arrived.
02:43The European Space Agency launched the groundbreaking Rosetta probe.
02:49For years, we've studied comets from afar.
02:52We had never seen one up close with cutting-edge technology to learn about how comets behave
02:57as they orbit the sun.
03:03Missions to large, planet-sized objects are hard.
03:07The Hubble Space Telescope's blurry images revealed Rosetta's target is just two miles
03:13wide and not just a small target, a moving one.
03:20Comet 67P races through the solar system at over 33,000 miles an hour.
03:27The precision involved is pretty incredible.
03:30It's like making that hole-in-one golf shot from New York to San Francisco.
03:34But catching comet 67P wasn't just a straight shot.
03:40The spacecraft had to actually get the same sort of velocity as the comet, and with current
03:45propulsion systems, we can't achieve that by flying directly to the comet.
03:51Instead, Rosetta performed a slingshot maneuver that would take ten years.
03:59So what it's doing, it's actually taking some energy away from the planet, so it's slowing
04:05the planet down a little bit and then imparting that energy into the probe.
04:12Rosetta flew past Jupiter, nearly 100 million miles from the warmth of the sun, and entered
04:20the most dangerous part of its journey, hibernation.
04:26Even though Rosetta had solar panels on it, it still had to be put to sleep.
04:29And the reason for that is that it had to track 67P a ways away from the sun, and there
04:34just was not enough collection area and solar power to be able to power the instruments.
04:40It's normal to put spacecraft into hibernation, but the worrying thing at the back of your
04:44mind is that it had never been done for this long before.
04:48Thirty-one months of hibernation gave Rosetta's team plenty of time to worry about what could
04:53go wrong.
04:55It's dark and cold out there in space, there's a lot of things going on, there's a lot of
04:59little micrometeorites out there, and without the constant communication with that spacecraft,
05:06just is a little nerve-wracking when the day comes and it's time to flip the switch and
05:10turn it back on.
05:12January 20, 2014.
05:16After almost ten years in space, it was finally time for Rosetta to wake up, reactivate its
05:22communications system, and phone home.
05:28I mean the tension was just palpable, it was just, I mean, how long do we have to wait
05:32before we're going to get the signal?
05:34It's got to work, it has to work.
05:36Oh gosh, maybe it's just not going to turn on at all.
05:39Have we lost the spacecraft?
05:41And then, the peak appeared on the graph.
05:44It's like, yes, we've got contact with the spacecraft.
06:04Rosetta began to send back images of its target, and after months of seeing a small dot in
06:09the distance, the comet slowly came into focus.
06:15When I saw this comet, crystal clear, this mountain floating in space of ice and rock,
06:22my heart just dropped.
06:23They are some of the most dramatic, beautiful images I have ever seen.
06:27And then as it got closer, and we got to see more and more details on it, yeah, that's
06:33when things started getting really strange.
06:38Simply, arrival at 67P, we expected to see something shaped like a potato, and we found
06:44something shaped like a rubber duck.
06:4767P is no ugly duckling, but its strange shape created a problem for Rosetta.
06:56Orbiting the comet was going to be far more difficult than anyone had imagined.
07:02A planet has a lot of gravity, so you can send the spacecraft out there and then just
07:07slow it down a little bit with a rocket burn, and it'll drop into orbit.
07:11In the case of comet 67P, you're dealing with a very small little rock.
07:15The spacecraft cannot feel the gravity of that rock, at least not until it's right up
07:19against it.
07:21The engineers had to actually plot triangular orbits.
07:25It was a very complicated set of maneuvers.
07:29Once in orbit, Rosetta could start work.
07:33Rosetta's first task?
07:34Figure out how 67P formed.
07:38So how did this comet get this weird shape?
07:41There's two main ideas.
07:44One is just that it was eroded somehow in the center, and so it started off as a more
07:50spherical thing and became the shape it is today through some unknown process.
07:55The other idea is that it started as two separate objects.
08:00These rocks normally hit each other hard.
08:05They collide with an average impact speed of more than 11,000 miles an hour.
08:11That's five times faster than a rifle bullet.
08:15Was 67P involved in a pileup?
08:18A clue came from the distinct layers on the comet's surface.
08:23The layers in comet 67P are a little like the layers in an onion.
08:29If you see them aligned, that's a clue that perhaps the object formed as a single
08:34entity and only eroded later into its present form.
08:37But if you see those layers misaligned, like we actually do in the comet, that's a big
08:41clue that it started out as two separate objects formed independently, sticking together to
08:47form the comet we see today.
08:49The layers proved that 67P was originally two separate objects that fused together.
08:56The process of potentially putting comet 67P together from two different pieces is
09:01important because it can teach us about what was happening in the early solar system.
09:06So as far as we can tell, these two separate bodies must have been formed in the same area.
09:12They're very similar in composition, but they are so light and fluffy that they would have
09:16destroyed each other if they had hit fast.
09:20They had a low-speed collision and basically stuck together like two wet snowballs.
09:26With one mystery solved, Rosetta began to investigate the chemical makeup of 67P.
09:32The little comet could answer one of the biggest questions in planetary science.
09:37From where did our blue planet get its water?
09:46The Rosetta mission, a 4 billion mile journey to comet 67P, and a 4.5 billion year trip
09:54back in time to the birth of the solar system.
09:59This comet is a remnant of the formation of the solar system itself.
10:05So this is an opportunity to open that time capsule and get a view into the ancient solar
10:12system.
10:15Comet 67P could help us answer one of the most important questions about our planet.
10:21One of the big mysteries that we have about Earth is, where did the water come from?
10:28Today water covers over two-thirds of the Earth's surface, but it wasn't always that
10:35way.
10:364.6 billion years ago, the inner solar system formed from a maelstrom of rocky debris.
10:45Temperatures were so hot, any water on the early Earth boiled away.
10:50What the evidence suggests to us is that Earth's water arrived at Earth after the
10:55Earth formed, and the primary mechanism that we thought were responsible were our comets.
11:03You have to remember that comets are basically just big balls of ice and dirt.
11:07One of the main goals of the Rosetta mission was to analyze the water on this comet and
11:11see if it matched the water on Earth.
11:15Comets come from two regions at the outermost reaches of our solar system.
11:21The Oort Cloud and the Kuiper Belt.
11:25The Kuiper Belt is in the plane of our solar system, and the member that most people are
11:29aware of is Pluto.
11:31But beyond that, there is a spherical distribution of icy bodies, the Oort Cloud.
11:38This is a repository of comets that extend for hundreds of thousands of times further
11:43away from the sun than we find the Earth.
11:45And it's kind of a big spherical cloud around the sun that's a kind of deep freeze for a
11:50bunch of big comets.
11:52Billions of icy objects orbit safely beyond the chaos of the inner solar system, locking
11:59in primordial water.
12:02The lifespan of a comet is fascinating.
12:04They might do absolutely nothing out there for over 4 billion years until the right tweak
12:11of their orbit or tug from a planet changes everything.
12:15And all of a sudden they're on a path in towards the sun and that ice that's been in deep freeze
12:19for billions of years starts to heat up.
12:24Astronomers believe that Comet 67P's former home turf was the Kuiper Belt, until something
12:31sent it ricocheting inwards.
12:35The influence of the gravity of Jupiter, because Jupiter is such a massive planet, it actually
12:39attracted the comet and pulled it into the inner solar system.
12:42It slowly was perturbed and migrated into the inner solar system where we see it today.
12:46It's traveled, I mean not just billions of miles in linear space, but I mean really when
12:51you think of all the orbits it must have made, just trillions upon trillions of miles.
12:56That's quite a journey for such a little object.
12:59Now 67P loops around the sun in a six and a half year orbit that travels as far out
13:06as Jupiter and closer in than Mars.
13:11As it gets nearer to the sun, it's frozen ice begins to heat up, leading to a famous
13:16effect, the comet's tail.
13:20Sunlight comes in and heats up the comet's surface and it gets transferred to the interior
13:24where there's a lot of ice and the ice gets heated up and it vaporizes and makes its way
13:28to the surface.
13:29And once it's sitting on the surface, the solar wind is bombarding that comet and it
13:33drags all of that material out into a long beautiful tail.
13:39Rosetta flew into the tail to analyze the water vapor.
13:44Its goal?
13:45Confirm it's the same type of water as we have on Earth.
13:50Water does come in different flavors and I don't mean salt water or fresh water.
13:54I mean what the water's made of, its molecules.
13:57We're talking about the actual chemical makeup on the proton, neutron and electron level.
14:05So you might have heard of heavy water before and that just means that there is an extra
14:10neutron.
14:11It's H2O, but one of the hydrogens has an extra neutron.
14:17So we call it deuterium.
14:18And so that water actually weighs more than normal water because of that extra neutron
14:24in the mix.
14:27On Earth, there are roughly 160 molecules of heavy water for every million molecules
14:34of normal water.
14:38As Rosetta moved around the comet, it measured the ratio of hydrogen to deuterium in the
14:44water vapor.
14:46An exact match to Earth's ratio would confirm that comets were the source of our oceans.
14:53But Rosetta discovered something surprising.
14:57What Rosetta found was that the deuterium to hydrogen ratio was three times that of
15:02the water in Earth's ocean.
15:04That means a comet like 67P couldn't have been the source of Earth's water.
15:08If it was the same as the measurement of the ocean water, then we could have said that
15:12these comets could have been a delivery mechanism for the water that we see on Earth.
15:17The fact it was three times higher kind of indicates that comets like 67P were not the
15:23source of the water that we see on Earth today.
15:29But although comets like 67P didn't deliver our water, we can't rule out all comets.
15:38We have to keep this in perspective.
15:4167P is but one comet.
15:43And what we know about objects like comets is that they come in families.
15:48And these families have different compositions.
15:51So it could be that some have Earth-like water, and those are the ones that brought
15:54water to Earth, and some don't.
15:58I don't see wrinkles like this as going back to the drawing board.
16:02Sure, we get more questions sometimes than answers, but that allows us to probe farther
16:08and really expand our knowledge into how planets formed.
16:15NASA's mission to Comet 67P planned to answer the biggest questions in planetary science.
16:24And none is bigger than figuring out the origins of life on Earth.
16:29Life on Earth got started not long after our planet cooled, something like 4.5 billion
16:34years ago.
16:35But we don't know the exact process.
16:38Did all that stuff come together here from molecules on Earth and become more and more
16:42complex, or was it brought from space?
16:48Rosetta began to hunt for the building blocks of life, basic organic compounds.
16:55So we always thought that comets probably carry organic materials, but we really couldn't
17:00confirm that until Rosetta.
17:03It detected aliphatic compounds, which are organics rich in carbon and hydrogen.
17:08And that is the first time such a substance has been detected at the surface of a comet.
17:13These materials are the raw building blocks for making things like proteins, which are
17:18required for all life.
17:20This is really important because this says that possibly these materials were delivered
17:24to Earth by comets.
17:27The idea that comets carry the building blocks for life across the solar system is known
17:32as molecular panspermia.
17:36You can kind of think of comets as being like space delivery trucks, right?
17:40They're forming out there in deep space.
17:42All this stuff is being formed with them, and then they bring it to Earth.
17:46Now, you know, you don't want your delivery truck smashing through your front door.
17:50But in the case of comets, when they impact the Earth, they can distribute those molecules
17:54all over the place.
17:58But in some way, we're kind of skipping a step, right?
18:00I mean, so the comet may be bringing the ingredients of life to the Earth, but where'd
18:06the comet get those ingredients?
18:09The answer may be in deep space, in the matter and radiation floating between star systems
18:15in a galaxy.
18:17It's called the interstellar medium.
18:21The really interesting result is that the overall composition of Comet 67P and some
18:25of the really interesting ingredients in that comet are very similar, identical in
18:30some cases, to materials we find out there in the interstellar medium, floating out there
18:35in the gas and dust between the stars.
18:39If this organic chemistry is common on Earth, it's likely to be common across the entire
18:45galaxy.
18:47This chemical kit for making life is universal, and therefore, perhaps life itself is common.
18:54There could be plenty of wet rocks out there in the night sky teeming with life with this
19:00universal chemistry kit.
19:04And as Rosetta further probed the surface of the comet, there was another chemical scientists
19:09were keen to find, nitrogen.
19:13Comets formed in the very early solar system billions of years ago.
19:16Their composition should be very similar to the disk that formed our planets, to the interstellar
19:21medium, but there was always this one kind of nagging difference.
19:25They were very much lacking the element nitrogen.
19:28So that seems odd.
19:29Why was all the nitrogen disappearing from comets?
19:34Was nitrogen too volatile to condense into ice when the comet formed?
19:40Or was it lost over the lifetime of the solar system?
19:44Well finally, Rosetta answered that.
19:46We realized there was something called ammonium salt, which has lots of nitrogen in it.
19:51While comets couldn't transport reactive nitrogen, they could carry nitrogen-rich ammonium salts.
19:59Nitrogen was there, it was just in a slightly different form.
20:02And nitrogen itself is a crucial component of life.
20:07You have a lot of nitrogen in amino acids that make up proteins, and indeed, we found
20:13glycine, the simplest amino acid, on this comet.
20:19But scientists still couldn't figure out the origin of one vital element.
20:24So phosphorus as one of the atoms is absolutely essential for life, but we don't really know
20:30where it comes from.
20:33Phosphorus is a vital part of DNA, cell membranes, and energy production.
20:38The problem is, it's quite scarce in the universe, and it's certainly scarce on the
20:43surface of the earth.
20:44Any phosphor that was around would have been locked up in the soluble rock, so that wouldn't
20:49have been available for life to use at that time.
20:53If rocks had locked in all the mineralized phosphorus, where did the phosphorus needed
20:58for biological processes come from?
21:0267P held the key to the mystery.
21:07What we found on the comet is bioavailable forms of phosphorus, not just mineralized
21:14phosphorus.
21:17Bioavailable phosphorus is a form that life can use.
21:22But where did comet 67P get it from?
21:27In January 2020, astronomers combined data from the Rosetta mission with ALMA's observations
21:34of the star-forming region AFGL 5142.
21:38AFGL 5142 is a stellar nursery, a gas cloud where large and small stars are being born
21:45simultaneously, and it's very close to us, which means we can study it in great detail.
21:50The ALMA observatory takes really high-resolution images of different types of dust and gas
21:57in very, very close to a star that's just formed, so you can actually see the process
22:01of formation happening.
22:04The largest stars live fast and die young, exploding in supernovas.
22:12So the phosphorus itself, we believe, is formed in massive stars, so essentially it's created
22:17as the star gets to the end of its life, and when the star goes supernova, it spews this
22:22phosphorus out into the interstellar medium.
22:25When mid-sized stars burst into life, they sent shock waves and radiation through the
22:30cloud, transforming phosphorus into a form biology can use, phosphorus monoxide.
22:38The phosphorus monoxide can freeze out and get trapped on the icy dust grains that remain
22:42around the star.
22:45These dust grains can come together to form pebbles, rocks, and eventually comets that
22:49become the transporters of phosphorus monoxide.
22:53Astronomers trace the cosmic trail of phosphorus and organics from stars to comets to planets
23:00and even to life.
23:04You could, in theory at least, take the sort of chemistry you find preserved in a comet
23:09and use it to construct a cell.
23:13So often people seem to think of astronomy as the study of things that are very far away,
23:17that have absolutely no bearing on our day-to-day life.
23:21Nothing is farther from the truth.
23:22We are looking for the origins of ourselves out there, and we are discovering them.
23:27So a basic question like, how did phosphorus end up in our DNA, when the only place you
23:32seem to find it is in around young stars, clouds, thousands of light years away?
23:37Now we know the mechanism because we asked the question, and now we understand ourselves
23:41that much better.
23:44The next stage of the mission was to get the lander onto the comet's surface.
23:50But there was a problem.
23:52The craft's landing gear was broken.
23:56Would the team have to abort the mission?
24:05Rosetta had discovered the building blocks for life on 67P.
24:12Now the probe faced its toughest challenge so far, a historic first landing on the surface
24:19of a comet.
24:20One of the biggest challenges about setting the Philae lander down on the surface is having
24:25no idea what the surface is like when you're designing the lander.
24:29Is it going to be like rock or ice and be very stiff?
24:32Is it going to be brittle?
24:36Philae had harpoons to dig into the soft surfaces and top thrusters to stop it from bouncing
24:41off the comet.
24:44The lander team discovered neither system was working properly.
24:48With the odds stacked against them, they decided to go for a landing anyway.
24:57Philae made a slow, 12-mile, 7-hour descent.
25:10Until finally, touchdown.
25:13So when we got the first signal of touchdown, we were relieved.
25:20The lander was down and everything was good to go.
25:24But then we realized things weren't quite going to plan.
25:28The data from the lander was intermittent.
25:32The only possible explanation?
25:34It was tumbling end over end.
25:38It could have bounced and bounced right off the comet and continued all the way into space.
25:43We really didn't know.
25:44We just knew we were not on the ground and we were rotating and tumbling.
25:49Then 12 minutes later, a ray of hope.
25:53We saw a flash in one of the computers and we huddled around it and we realized that
25:57the lander was actually sending data back as it was programmed to do.
26:00We were actually doing science on the comet and getting the results back.
26:05The lander stopped tumbling.
26:08Philae's solar panels weren't fully operational and the tiny craft's battery had only 60
26:13hours of power left.
26:15It was a race against time.
26:18We had to completely rip up all of our plans of what we wanted to do and then we had to
26:21rewrite everything to maximize the amount of science return in those 60 hours we had
26:26available to us.
26:28Philae got to work.
26:32Its first discovery.
26:33The ground was stranger than expected.
26:38One of the things we found about the surface is the top 40 cm were a very fluffy material.
26:45It's not dense and rocky, it's pretty fluffy.
26:49You could imagine something on the order of cigar ash.
26:52Imagine the rocks on comet 67P, but imagine them being as light and fluffy as cigar ash.
27:00It's so crumbly that if you had it in your hands, you could break it apart with almost
27:03no effort at all.
27:04It has a fraction of the strength of styrofoam.
27:08Philae's bounce landing helped investigate the surface.
27:13By bouncing, we actually were able to sample two sites and interestingly, both sites were
27:17different.
27:19That kind of shows us that the comet is not homogeneous, it's not all the same material.
27:26Philae's instruments revealed comet 67P is not just two lobes fused together.
27:32The comet formed from distinct blocks pulled together by gravity.
27:39Philae's onboard camera also fed back vital clues to the lander's final resting place.
27:45The images taken by the lander were really amazing because from those, we were actually
27:49able to work out that Philae was on its side.
27:53Philae's location was unknown, far from the planned landing site.
27:58With its battery dead, it couldn't speak to Rosetta.
28:02The problem with not knowing exactly where on the surface of the comet we were was that
28:06some of the instruments had needed to know that exact location to actually interpret
28:11the data and without that information, the results were meaningless.
28:16Time is passing and still no Philae lander is to be found.
28:21Is there hope?
28:22It's all lost.
28:25When you make or follow a space robot, you kind of get invested in that success and the
28:30survival of that robot.
28:32You want it to succeed against the harsh reality of space.
28:35You feel like you've lost a pioneer, you've lost a fellow explorer.
28:42After seven months of hibernation, Philae woke up and spoke with mission control.
28:48A fleeting call from a lost lander.
28:52We got very excited because we knew it was waking up, it was charging up, it was seeing
28:55sunlight and it was doing everything it was supposed to do.
29:00But the calls faded away and in March 2016, the little lander was declared lost.
29:08Declaring Philae lost was a very, very sad moment because it was almost like losing one
29:13of the team and at that stage, the lander element of the mission was over and as was
29:18our role in the mission.
29:26As Rosetta scanned the comet, searching for the lost lander, it faced a new problem.
29:33As this dirty snowball races towards the sun, the radiation from the sun heats it up and
29:40so it starts snapping and popping and cracking, transforming itself.
29:45It's like a firework show.
29:48Large jets blasted material from the surface.
29:51Would the outbursts launch Philae into space?
29:57As Rosetta scanned the comet for the missing probe, it flew less than two miles above the
30:02surface, close enough to cast a shadow.
30:08Seeing the shadow of the spacecraft itself on the comet really brought home the fact
30:14that there was a piece of humanity, one of our machines right there.
30:21It makes the hair on the back of my neck stand up.
30:23It gives me chills.
30:28The low passes allowed Rosetta to smell the comet's surface.
30:33The Rosina instrument has allowed us to actually measure the chemicals and the gases coming
30:37off the comet.
30:38So we can actually put those together to make a comet perfume.
30:43So what we've actually done is impregnated those smells of the comet into a card, a bit
30:47like a scratch and sniff card.
30:49So you can actually smell and that is the smell of the comet.
30:54And it's not very pleasant.
30:56If you can imagine a mixture of something like rotten eggs and cat urine and like bitter
31:03almond stuff, it's like, it's not something I would want to have to be able to smell.
31:09To me, it kind of smells like baby's nappies and it's a bit of a pungent, quite an unpleasant
31:17smell.
31:19Months passed with no sign of Philae.
31:22The comet isn't terribly big, you know, where could it be hidden?
31:25And if we understood where it was hidden, we would understand the images that it had
31:29returned so much better.
31:30There was sort of that human part of like, where is my buddy?
31:34And also that science part of what was it trying to tell us?
31:38An eagle-eyed researcher spotted something in the corner of an image.
31:43The lander itself was a tiny little speck and you really have to zoom in to actually
31:47see it.
31:48But there was Philae, under a cliff.
31:52Seeing a little Philae lander there, on its side, under a ledge, in the shadows, was just
32:00remarkable.
32:01Pinpointing Philae's location on the smaller lobe solved one final mystery.
32:07It explained data sent back months earlier, revealing this area of the comet is solid.
32:14There was a real sense of closure and there was sort of a Godspeed to it, you know, like
32:19we know what's happened to you, thank you for the science.
32:22And that was its final resting place and it was really wonderful to see.
32:27Philae was an amazing success.
32:30Despite all those problems, we got the lander down, despite the bounce and it coming to
32:34a rest under the cliff phase, we got some absolutely amazing images and some fantastic
32:39measurements of the gases and the structure of the comet itself, which we would never
32:43have had had we not tried to put a lander on a comet.
32:49Rosetta's mission was not over.
32:52As the comet approached the sun, chaos erupted.
33:01All of a sudden, there were these jets that would appear and eject a huge amount of material
33:06from the interior.
33:08When sunlight warms the comet's surface, ice turns directly into gas.
33:14The uneven surface of the comet funnels the escaping water vapor into narrow jets of gas
33:20and dust.
33:24Rosetta also spotted huge, violent gas outbursts.
33:32We would expect to see jets on comets that kind of gradually rise and fall with the solar
33:36illumination, but these outbursts were utterly different.
33:40They would just sharply increase and then stop.
33:43These outbursts weren't timed with the sun.
33:46They came out at random times, so there must be something else going on.
33:52The outbursts exploded in sudden, brief, high-speed events.
33:58Because we only take pictures of the surface every five to thirty minutes, when we see
34:04an outburst in only one image, we know that the whole event lasted less than that time,
34:10which is very mysterious.
34:13So it was like putting together a puzzle, and you have a thousand pieces and you're
34:18putting them together and you realize, oh, I only actually have 600 of the thousand pieces
34:21and I've got to make sense of this, and it's really frustrating.
34:25As the comet swung closer to the sun, Rosetta's cameras captured 34 outbursts in just three
34:32months.
34:33They were all huge.
34:35In the span of only a few minutes, a single jet can release up to 260 tons of material.
34:42And one jet was spotted releasing 40 pounds of material every single second.
34:49The cause of the outbursts was a complete mystery.
34:53Then, in September 2014, close-up images revealed a 230-foot-long, three-foot-wide fracture on
35:03the cliff edge named Aswan.
35:06Less than a year later, Rosetta photographed a large outburst from the same region of the
35:11comet.
35:15When the probe investigated the area, it found a devastated landscape.
35:20The cliff had collapsed in a huge landslide.
35:25This is like a crime scene.
35:27We have the before pictures and the after pictures, and we have to solve the mystery
35:32of how this happened.
35:35What links the outburst and the cliff collapse?
35:39We think there's a domino effect going on here, where the sun heats up some of the volatiles
35:44deep under the surface, and they sublimate and turn into a gas, and they find any little
35:50crack or crevice they can to shoot out of the comet.
35:54But that process weakens part of the comet's face.
36:02This is going to release more material, and as it does that, it's going to start forcing
36:06gas up through kind of nozzle-like features, having them act like jets.
36:11And as they do that, then that's going to cause even more weakening of the material,
36:15and maybe a large-scale collapse, which could actually cause a giant outburst.
36:20We think escaping gas weakens the surface, which triggers a collapse and a release of
36:26gas in a giant outburst.
36:29Like sticking dynamite charges in the side of the cliff and then setting them all off.
36:42Comet 67P revealed the hidden geological activity going on inside these dirty snowballs.
36:50And Rosetta spotted another surprising phenomenon.
36:55Strange shapes moving across the surface.
36:59Dunes.
37:00Dunes aren't driven by wind.
37:03Well, on an airless comet, how do you have that happen?
37:12Rosetta rewrote the book on how comets form, how they live, and how to explore them.
37:20When Rosetta flew close to the surface, it spotted something unexpected.
37:27Dunes.
37:30These are serious dunes.
37:31They're about six feet high, and they extend for hundreds of feet in every direction.
37:36So we've got to figure out what this mechanism is that it's causing this pretty significant
37:40geologic structure on the comet.
37:46Janie Radovop is a planetary scientist.
37:50She studies space dunes on Earth.
37:54One of the most fascinating things that Rosetta saw on 67P was a set of dune-like features
37:59on an airless body.
38:02These were found up by the Neck region in a set of soft sediments, and they actually
38:06looked very similar to these ripple-like landforms.
38:11They were spaced about 10 yards across, and there were maybe 10 to 15 of them across the
38:16surface, and we even saw them change over time.
38:21When we first saw these, no one had any idea what could have formed them.
38:24In fact, people even doubted that they were dune-like landforms, because how would they
38:28possibly form on a body that has no atmosphere, let alone any wind?
38:33On Earth, land heated unequally creates winds.
38:38The atmosphere on Earth and the really transient atmosphere on a comet are very different from
38:42each other.
38:43On Earth, it's a stable, thick, dense atmosphere.
38:46It's got constant solar heating that can pick up air masses and move them across the surface
38:50and generate wind.
38:53Comet 67P does have an atmosphere created when the sun heats the surface, releasing
38:59gases.
39:02This atmosphere has a maximum pressure 100,000 times lower than Earth's.
39:09Using Rosetta images, researchers in France modeled the movement of gases on the surface
39:14of the comet, and they think they've found the answer.
39:20Researchers found that there are winds on the comet's surface.
39:23These are brief, and they form because gases escape on the sunlit side, and then they rush
39:27over to the cold side across the surface, and this movement of air across the surface
39:32is what picks up the sand and forms the dunes.
39:36The grains on the comet are far bigger than sand grains on Earth.
39:42On the Earth, the sand dunes are made of grains about this size, but on the comet, the sand
39:46dune grains are this size.
39:49Gravity also plays a really important role in moving grains around on the comet.
39:53The comet has such a low density that if you were walking around, you'd be able to fly
39:57into orbit very easily, and so a grain this big, but low density, would easily be lofted
40:02by the very thin air there.
40:07After two years orbiting the sun with Comet 67P, Rosetta's mission was almost over.
40:14So the plan was to lower the orbiter down towards the surface in a controlled way, and
40:19this would allow very high resolution images to be taken, and also to get some gas measurements
40:24right close to the surface of the comet.
40:29September 30th, 2016, Rosetta spent 14 hours slowly free-falling towards the comet, taking
40:38photos on the way down.
40:44As Rosetta was lowered, it touched the surface, and then the link back to Earth was broken,
40:51and the mission was effectively over.
40:55The thing that's kind of wonderful about it is that there's a little piece of us on that
40:58comet out there right now.
41:01As that comet spins around, out to Jupiter and back every six and a half years, it will
41:05always have a little bit of humanity on it.
41:11There was one final twist to Rosetta's story.
41:15We thought we had the last image back from Rosetta before the planned end of the mission,
41:20but it turns out that actually just before touching down, before the comms were able
41:25to finish transmitting back to Earth, an incomplete packet of information was sent back.
41:31The computer discarded this incomplete data packet.
41:36Recently, human experts re-examined it.
41:41The result?
41:43Rosetta's closest photo of 67P.
41:47And the image is amazing because when you actually blow it up and look at it, you can
41:50actually feel like you could reach out and touch the surface.
41:56It's a fitting postscript to the only mission to land on a comet.
42:02It's an incredible chance to learn about the geologic structure at the size scale of a
42:08human being exploring there.
42:10It's a way to look at comets that we've never had before.
42:16And years after the mission, Rosetta is still providing surprises.
42:20In 2020, scientists combined data from different instruments and 4,000 observations, finally
42:29revealing comet 67P in color.
42:34At the beginning of the Rosetta mission, when 67P was relatively far from the sun, the comet
42:40had a reddish color.
42:42And that's because of dust grains trapped on the surface that had been hit by ultraviolet
42:47light gave it this distinct reddish hue.
42:54As the comet moved towards the sun, it started to be a lot more active.
42:58Literally, water is coming off the comet and bringing dust with it.
43:01And so the under layers of the comet are being exposed, which are more pristine water.
43:06So it's less red, more blue in color.
43:11And as the comet gets further away, some of that dust freezes back down on the surface
43:16and your comet is red again.
43:18So it goes from red to blue to red.
43:20It's kind of a chameleon changing color throughout its orbit.
43:27The Rosetta mission has transformed our understanding of comets.
43:32Between the strange shape and the measurements of heavy water, we're finding that there's
43:35a big diversity out there of comets and that every time we go to visit a new one, we're
43:39probably going to be surprised all over again.
43:43Rosetta may inspire new missions.
43:46We may actually be returning to Comet 67P itself.
43:49There's a lot more there to discover about the origin of our lives, ourselves, here on
43:54Earth.
43:55There's so much more to learn and so many more mysteries to uncover.