Solar Dark Worlds
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00:00There's something out there in the darkness, a world with no name.
00:26Its surface is ancient, and probably pink.
00:34We don't know where it came from, or how long it's been there, but we do know it's not alone.
00:44It's a dwarf planet, only discovered in 2018.
00:57Now you might reasonably ask, how could thousands of astronomers, both amateur and professional,
01:03miss a world that is pink and 400 kilometres across?
01:07Well, the answer might lie in its nickname, which is Far, Far Out, because it really is far, far out.
01:20You might have thought of the solar system as the sun, and then all the planets lined up all the way out to Pluto.
01:26But we now know that that's just the tip of the iceberg.
01:36This is a journey to the least explored regions of our solar system,
01:44as we probe the mysteries of the asteroid belt,
01:52visit frozen worlds that we're discovering beyond Pluto,
01:58and reveal a vast hidden kingdom that even our most powerful telescopes can't see.
02:06Lost in the dark.
02:36It was the sound that most people noticed.
02:41A sonic boom in the night.
02:51Travelling at almost 50,000 kilometres per hour,
03:01a rock tore through the atmosphere,
03:06broke apart,
03:11and rained down across Winchcombe.
03:18A visitor from a distant realm.
03:25A visitor from a distant realm.
03:45On the 28th of February 2021, a rock from space landed there on this driveway,
03:53and it shattered into hundreds of pieces.
03:56Most of it went onto the lawn, bits of it went onto the neighbour's driveway,
04:00and the indentation in the driveway that was here is now in the Natural History Museum.
04:06And here is a piece of that rock.
04:10And you see that it's very dark, it's almost black,
04:15very different to the rocks that you find naturally around here.
04:19So the question is, other than the rather nonspecific,
04:23it came from space, what exactly is this?
04:27And where exactly did it come from?
04:35The cows weren't the only ones watching that night.
04:41Doorbell and CCTV cameras never sleep.
04:47A network of specialist meteor cameras also captured it.
04:57And that meant it was possible to calculate its trajectory.
05:06Back over the skies of England.
05:11Out of Earth's atmosphere.
05:17Past Mars.
05:25And into the darkness.
05:31In between Mars and Jupiter lies a realm of rocky worlds.
05:39This is where that rock came from.
05:43The asteroid belt.
05:49Asteroids are rubble left over from the formation of the solar system.
05:55The remnants of planets that never were.
05:59It's thought that there are at least a million out here.
06:03So dark, they're incredibly difficult to see.
06:09But asteroids don't always stay in the asteroid belt.
06:19Every now and then, two asteroids collide.
06:25Causing fragments, big and small, to be lost from the belt.
06:31This can create havoc for neighbouring planets.
06:47Including our own.
06:51We've flown several spacecraft straight through the asteroid belt.
06:57But only one has stayed to explore the region itself.
07:07Most asteroids have been lost for centuries.
07:13But one stands out from the rest.
07:21Much bigger than the others.
07:25And almost perfectly spherical.
07:31Dawns, tides, and storms.
07:35And almost perfectly spherical.
07:41Dawn's target world.
07:49At first, Ceres appears dark and heavily cratered,
07:53like its fellow asteroids.
07:57But Dawn has discovered that it's different.
08:01Its surface is peppered with bright white crystals.
08:11So what are they?
08:22Now, there is another world in the solar system
08:26where white crystals form on the surface.
08:30It's Earth.
08:32This is a mountain composed almost entirely of salt.
08:36It's mainly sodium chloride, actually table salt.
08:40There's a bit of magnesium and potassium in there,
08:44which gives it those pinky colours.
08:48This is enormous.
08:50It extends hundreds of metres down into the ground.
08:54It formed because, around 40 million years ago or so,
08:58Ceres created a way and left the salt behind.
09:02And then plate tectonics raised the ground up
09:06to form salt mountains.
09:28Now, salt crystals like this
09:32only form in the presence of liquid water.
09:38In this case, it was the Atlantic Ocean.
09:42That raises an intriguing question.
09:46Are those crystal deposits on the surface of Ceres salt?
09:50And if so, does that imply there was once an ocean on Ceres?
09:58Dawn entered into a close orbit.
10:08And by measuring sunlight reflected off the crystal deposits,
10:12it could determine what they're made of.
10:16Sodium carbonate.
10:20A common type of salt.
10:24A tantalising sign
10:28that Ceres had an ocean in its past.
10:40Orbiting just 35km from the surface,
10:44Dawn found another clue that suggests an ocean
10:48may still be there today.
10:54It detected a different type of salt crystal
10:58that's only been found in one other place in the solar system.
11:10When salt water evaporates,
11:14then it leaves these things behind, salt crystals.
11:18Essentially, sodium and chlorine ions bonded together
11:23to form a crystal lattice.
11:27But under the right conditions, when it's very cold,
11:31then a different sort of crystal can form when seawater freezes.
11:35It's called a hydrohalite.
11:39Essentially, water molecules are incorporated into the crystal lattice.
11:43Now, we see hydrohalites here on Earth,
11:47but the fact that we also see them on Ceres presents a mystery.
11:51It's been calculated they should only last around 100 years
11:55on the surface of Ceres before the water escapes out into space.
11:59So the fact that we find them
12:03implies that they must be constantly forming.
12:07And that, in turn, must imply that below the surface of Ceres
12:11there is a reservoir of salt water today.
12:21Ceres is a world of water.
12:25It's thought salty subterranean lakes 40 kilometres deep
12:29exist beneath the surface today.
12:33But how did the salt end up on the surface?
12:39The answer lies in Ceres' location.
12:43Incoming asteroids
12:47can punch straight through to the lakes below.
12:51Exposed to the vacuum of space,
12:55the water vaporises instantly...
13:03..leaving its cargo of salt behind
13:07and peppering Ceres with bright white spots.
13:13The discovery of water
13:17beneath the surface
13:21means this asteroid is now a candidate
13:25in the search for life.
13:29The more we've explored
13:33the asteroid belt, the more we've come to realise
13:37that it's not just a band of rubble.
13:41The dark worlds hiding here
13:45are worthy of exploration in their own right.
13:49And the asteroids that leave the belt
13:53can transform the planets nearby.
14:03Mars orbits closer to the asteroid belt
14:07than any other planet.
14:15And around 100 fresh impact sites appear on Mars every year.
14:27NASA's rovers have found some spectacular meteorites
14:31scattered across its surface.
14:37And one even spotted
14:41what looks like an asteroid in the sky.
14:51Floating in front of the sun.
15:01But it's actually one of Mars' two moons.
15:07It's a comet.
15:15And it's a comet.
15:23And it's a comet.
15:31And it's a comet.
15:37Phobos isn't spherical like our moon.
15:41It's kind of a strange, irregular shape.
15:45So there's a theory, a simple theory.
15:49Maybe Phobos is an asteroid that came in from the asteroid belt
15:53and was captured by Mars' gravity.
15:57Well, actually, wonderfully. Turns out things aren't that simple.
16:01The European Space Agency
16:05has a probe in orbit, busily surveying
16:09the red planet.
16:13Mars Express orbits Mars three times
16:17every day, regularly passing beneath Phobos.
16:25Its high-resolution cameras captured the most detailed images
16:29of the moon ever seen.
16:37Showing a surface covered in strange grooves.
16:45Phobos looks like someone's ridden a bike all over it.
16:53But perhaps more intriguing were the measurements
16:57of the mass.
17:03It's pretty hard, actually, to measure the mass of a small moon.
17:07But the way it was done was to measure how the path of the Mars Express
17:11spacecraft was deflected by the weak pull of Phobos' gravity.
17:15Now, once you have the mass and we have a reasonable idea of the volume,
17:19we calculate the density. And that came as a surprise
17:23because it turned out that Phobos is more like
17:27a kind of a gravel pile, weakly held together.
17:31It's not very dense at all. And that does support the theory
17:35that maybe Phobos was formed from the debris of some
17:39kind of collision.
17:47So it's possible that Phobos is not a captured asteroid
17:51after all.
17:55But was, in fact, formed after a massive asteroid hit Mars.
18:03But there's still something about Phobos that neither theory can explain.
18:09Its strange grooves.
18:15This landscape was sculpted by the elements, by the wind and the rain.
18:19But on Phobos, there is no wind and rain.
18:23And so the landscape is sculpted by something else.
18:27It's actually sculpted by gravity.
18:31When you've got a planet and a moon orbiting around each other and spinning on their axes,
18:35very complex gravitational forces come into play.
18:39They're called tidal forces. And they act to deform the moon
18:43and also actually slightly deform the planet.
18:47They change the orbits.
18:55In the case of Mars and Phobos, what's changing is that Phobos
18:59is descending towards Mars in its orbits.
19:03And it's only about 6,000km above the surface now.
19:07But ultimately, over time, Phobos gets closer to Mars
19:11and that means the tidal forces get stronger and stronger.
19:17The grooves on the surface
19:21are stretch marks and fractures.
19:29Mars's gravity is tearing Phobos apart.
19:37Eventually, Phobos will drift so close
19:41that Mars's gravity will destroy it.
19:47MUSIC
19:55But from this destruction,
19:59something beautiful will emerge.
20:07As Phobos begins to break apart,
20:11most of the debris will fall to the surface of Mars.
20:17MUSIC
20:21But the rest will remain in close orbit,
20:25spreading out to encircle the entire planet...
20:31..leaving Mars with a spectacular set of rings.
20:35MUSIC
20:45So what is Phobos?
20:49The truth is, and I think this is wonderful,
20:53we still don't know the full story of the origin of Phobos.
20:59But what we can say is that this little moon
21:03is going to make a difference.
21:07Because in the future, it's going to turn the red planet
21:11into a ringed world.
21:15MUSIC
21:19MUSIC
21:31Leaving the asteroid belt behind,
21:35we travel out, further into the darkness.
21:39MUSIC
21:47Beyond Jupiter, the gaps between the planets get ever wider.
21:55And the temperature plummets.
22:03MUSIC
22:09We continue on through millions of kilometres of empty space...
22:17..until we reach the furthest planet from the sun.
22:23MUSIC
22:27MUSIC
22:36Neptune is wrapped in a dense blue blanket.
22:42Clouds of methane and ammonia thousands of kilometres thick,
22:46beneath which there is no detectable surface.
22:52But there are worlds out here that you could stand on.
22:57Neptune has at least 16 moons.
23:01And one of them is very unusual.
23:14Triton's surface is coated with pale nitrogen ice.
23:20But unlike the other moons, which are frozen to the core,
23:24Triton is an active world.
23:31Geyser-like plumes of gas and dust eight kilometres high
23:35stretch into Triton's upper atmosphere,
23:39which flattens them abruptly at 90 degrees,
23:43creating a vista so strange...
23:49..it's hard to believe it's real.
23:54MUSIC
23:56How did a moon 4.5 billion kilometres from the sun...
24:02..become so active?
24:07MUSIC
24:24In the 10th century, monks built a chapel on this hillside.
24:28It's an astonishing achievement.
24:30They carried all the stones up by hand.
24:33And when they'd finished, a solitary monk, a hermit,
24:37lived here, contemplating the great mysteries of existence.
24:44And every morning, he would have seen the sun rise in the east.
24:49Now we know the reason for that.
24:51It's because the Earth's spinning on its axis.
24:53And, in fact, everything spins in the solar system,
24:57and almost everything spins in the same direction.
25:03Now, it's a fundamental property of nature,
25:05that once things are spinning, they continue to spin,
25:09and continue to spin in the same direction,
25:12unless something happens.
25:14Triton doesn't spin in the same direction
25:17as almost everything else.
25:19It goes in the opposite direction in its orbit around Neptune.
25:23So there must be a reason for that.
25:37Triton is a very special object.
25:41Triton's unusual orbit,
25:43which takes it around Neptune
25:45in the opposite direction to the inner moons,
25:49suggests that Triton didn't form alongside Neptune.
25:59It came from somewhere else.
26:11Sometimes in science, things are quite simple.
26:14There are sort of rules of thumb that apply and are useful.
26:19And one of them, in the solar system,
26:21is that planets and moons
26:23that form in the same region around the sun,
26:26the same distance from the star,
26:28are made of the same stuff.
26:30So, for example, here's Neptune,
26:33which is mainly made of hydrogen, helium, a bit of methane.
26:37And there is Uranus,
26:39which is made of hydrogen, helium, and a bit of methane,
26:42because they formed in the same icy region far from the sun.
26:46Here's Mars, formed much closer in,
26:49and it's silicates and iron oxides,
26:52aluminium, magnesium, things like that.
26:55And there's the Earth, roughly the same place,
26:58same stuff again.
27:00Here is Pluto,
27:03made of water ice, nitrogen ices,
27:06methane ices, carbon monoxide.
27:09And here is Triton.
27:12Same.
27:14So, we have a hypothesis,
27:17which is a guess, I suppose, that we could test.
27:20The hypothesis is that maybe Triton and Pluto
27:25formed in the same place.
27:30To understand Triton,
27:33we have to look deeper into the darkness.
27:38Neptune might be the furthest planet from the sun,
27:42but it's not the edge of the solar system.
27:45Not even close.
27:49Over a billion kilometres further out lies Pluto.
27:55And it's not alone.
27:58There are hundreds of thousands of other worlds out here.
28:03This is the Kuiper Belt.
28:09A vast frozen realm,
28:12billions of kilometres wide.
28:17It's home to an exotic collection of icy worlds,
28:23including some 200 dwarf planets.
28:28Some have rings and moons,
28:31others are bizarre and misshapen.
28:35But they all share one thing in common.
28:38They're made of similar material to Triton.
28:45It seems, then, that the Kuiper Belt
28:48is where Triton belongs.
28:54So how did it end up in orbit around Pluto?
28:58So how did it end up in orbit around Neptune?
29:06The answer might lie in this strange geography
29:09of the Kuiper Belt itself.
29:12In one region, everything orbits the sun
29:15in pretty much circular orbits,
29:17like almost everything else in the solar system.
29:20But there's another region where things orbit in ellipses.
29:24It's almost as if something flattens out
29:27and something plouged really the Kuiper Belt and disturbed it.
29:36There is one prime suspect.
29:41Neptune.
29:45It's thought Neptune formed much closer to the sun,
29:50then slowly drifted out.
29:53Its gravity disrupted the Kuiper Belt
29:56and scattered the icy worlds.
30:01But Triton was unable to escape.
30:07To this day, Triton is trapped
30:10in a backwards orbit around Neptune.
30:16And being this close to a giant has consequences.
30:20Just as our moon raises tides on the Earth,
30:24Neptune raises tides on Triton.
30:28Stretching and squashing it.
30:38Heating up its rock and ice.
30:44It's thought that this is what melts
30:47It's thought that this is what melts its frozen interior
30:54and powers the spectacular plumes.
31:10So here is the wonderful hypothesis
31:13of what might have happened to explain what we see on Triton.
31:17So as Neptune ploughed through the Kuiper Belt,
31:20in the ensuing gravitational chaos,
31:23Triton got captured into its reversed orbit.
31:26And even to this day, that orbit is ejecting energy
31:30into the moon where the tide is raised upon it
31:33by Neptune's gravity, and that's driving the geology.
31:36And isn't that a wonderful example
31:39of how you build a hypothesis in science?
31:42You put together the clues.
31:44So we have the composition of Triton being the same as Pluto.
31:48We have the strange geography of the Kuiper Belt
31:51and we know that gas giants can change their orbits.
31:55Put those together and we have a hypothesis
31:58of why Triton is the strange, magical, twilight world
32:03that we see today.
32:12Leaving Triton behind, we travel into the Kuiper Belt.
32:20We know very little about the worlds out here.
32:26They're so far away, we can't make them out in much detail.
32:33But we have sent one spacecraft to explore this region.
32:38It took over nine years for New Horizons to get here
32:43and hone in on its primary target.
32:46The spacecraft captured the first close-up images
32:49of a Kuiper Belt world ever seen.
32:54It was the first of its kind.
32:58It was the first of its kind.
33:02It was the first of its kind.
33:06It was the first of its kind.
33:10It was the first of its kind.
33:12This is the closest image of a Kuiper Belt world ever seen.
33:20Revealing mountains on Pluto as high as the Alps,
33:25made from water, frozen, hard as granite.
33:30But there's something else here.
33:34Another world,
33:37hanging uncomfortably close in the sky.
33:50Sharon doesn't rise or set,
33:53but she's still looking for answers.
33:56Sharon doesn't rise or set,
33:59and it's only visible from one side of Pluto.
34:10Understanding why these two are so close together
34:15reveals something unique about this dark and distant realm.
34:20When Sharon was first discovered in 1978,
34:23we just assumed that it was a moon.
34:25Pluto's moon.
34:27But the more we've learnt about Sharon,
34:29and crucially, the more we've learnt about the Pluto-Sharon system,
34:33the more we've come to realise that there's more to it.
34:40All large moons in the solar system,
34:42including our own,
34:44have been discovered by humans.
34:46All large moons in the solar system,
34:48including our own,
34:50have something in common.
34:53They only show one face to their parent planet.
34:59And the fact that all moons do this
35:01is not a coincidence.
35:05It's to do with the complexity of orbits.
35:07So let's say this is the Earth and I'm the moon,
35:09and we tend to think of the moon just orbiting around the Earth
35:12and the Earth sitting still.
35:14That's not what happens.
35:15Actually, both Moon and Earth
35:17orbit around what's called the common centre of mass
35:20of the Earth-Moon system.
35:22It's just that because the Earth is way more massive than the Moon,
35:25the centre of mass is actually inside the Earth,
35:27so the Earth is kind of doing that.
35:34Let me show you what I mean.
35:36Bueno.
35:38So here's the Earth spinning on its axis once every 24 hours.
35:42That's one day.
35:43And there is the Moon orbiting around the Earth
35:46once every 27 and a bit days.
35:51The gravitational interactions between Earth and Moon
35:55cause their orbits to synchronise.
35:58The Moon rotates on its axis exactly once
36:02for each lap it makes around the Earth.
36:07With the result that the same face of the Moon
36:11always points to the Earth.
36:16But in the case of Pluto and Charon,
36:18the same face of the Moon always points to the Earth.
36:22But in the case of Pluto and Charon,
36:24the centre of mass of the system is outside of both bodies.
36:28And so they both orbit around the centre of mass,
36:32which is somewhere in space.
36:34It's where I'm standing.
36:36Now, Pluto orbits once every 6.5 Earth days or so
36:40and spins on its axis once every 6.5 days or so.
36:45And Charon also orbits once every 6.5 days or so.
36:50And Pluto orbits once every 6.5 days or so
36:53and spins on its axis once every 6.5 days or so.
36:57And the result of all that is that Charon
37:01always presents the same face to Pluto.
37:06And Pluto always presents the same face to Charon.
37:15Both worlds face each other in a synchronous dance.
37:20Now, this might seem really strange and unusual.
37:24It isn't.
37:25It's a consequence of all those forces acting.
37:28This is what would happen to our Earth and Moon,
37:31given enough time,
37:32but enough time would be many times
37:34the current age of the universe.
37:36The difference here is that this is not a Moon
37:40orbiting around a planet.
37:42It really should be thought of as two objects
37:45of similar mass orbiting around each other.
37:50Thank you, thank you.
37:55Pluto and Charon are a double planetary system,
38:00also known as a binary pair.
38:04Just 20,000 kilometres apart,
38:07they gaze at each other without ever turning away.
38:12This is the first double planetary system we've observed,
38:16but we think there could be many more.
38:21New Horizons is still out there today,
38:24searching for more Kuiper Belt worlds.
38:29So far, it's encountered just one.
38:36Arrokoth is formed from another pair,
38:40now so close they're touching.
38:46So why do the worlds out here in the Kuiper Belt
38:50form these rare partnerships?
38:57The further from the Sun planets are,
39:00the slower they move.
39:04And way out here, they move very slowly indeed.
39:10But occasionally, collisions do occur.
39:14It's thought that in the past,
39:18Pluto and Charon crossed paths.
39:24But their glacial movement
39:26meant that this was less of an impact
39:30and more of an embrace.
39:35Instead of destruction,
39:37the pair became partners for life.
39:41Now, we've only actually visited
39:43three worlds in the Kuiper Belt,
39:45Pluto, Charon and Arrokoth.
39:47But we have seen more using the Hubble Space Telescope.
39:51And what's interesting is that when we zoom in
39:54on those what look like single pixels of light,
39:57many of them aren't single pixels.
39:59They're binaries, just like Pluto and Charon.
40:02And what's interesting is that
40:04when we zoom in on those what look like
40:06single pixels of light,
40:08many of them aren't single pixels.
40:10They're binaries, just like Pluto and Charon.
40:12So this is the pair of worlds
40:14called Typhon and Echidna.
40:16And it's a binary.
40:18And this is another pair of worlds,
40:20Banff and Orcas.
40:22And again, you can see them orbiting around each other,
40:25orbiting around their common centre of mass.
40:28And this complex gravitational dance far from the Sun
40:32means that the Kuiper Belt has a very complex structure
40:35which we don't really understand.
40:37The Kuiper Belt does, in a very real sense,
40:40represent the frontier
40:42of our knowledge about the solar system.
40:45Almost.
40:50Past the Kuiper Belt,
40:52we enter a realm of true darkness.
40:56All we see are the bright stars of the Milky Way.
41:01Even our most powerful telescopes
41:04are unable to see anything in the solar system out here.
41:11But every now and then,
41:14things do show up in the darkness.
41:26In 2018, a faint point of light was detected.
41:3110 billion kilometres beyond the Kuiper Belt.
41:41It's a dwarf planet.
41:46Nicknamed Far, Far Out.
41:51But we think there's more out there.
41:56A lot more.
41:58There's another realm of our solar system,
42:01far beyond the reaches
42:03of any spacecraft or telescope we've ever built.
42:09So vast, so strange,
42:12and so deep into the darkness...
42:18..it's almost impossible to imagine.
42:21Imagine that pinnacle is the Sun,
42:24and I'm standing somewhere around the orbit of Neptune,
42:27and let's head out to the Kuiper Belt.
42:30Now, Neptune is about 30 metres away from the Sun,
42:34so on our scale,
42:36that means that the Earth would be one metre away.
42:39One metre.
42:41That's called one astronomical metre.
42:44So we're going to have to find out
42:46how close we are to Neptune.
42:49That's called one astronomical unit,
42:51the distance of the Earth from the Sun.
42:53It's about 150 million kilometres.
42:56So now I'm inside the Kuiper Belt,
42:59traversing through that realm of icy worlds,
43:02and I reach Pluto,
43:04the furthest extent of its orbit
43:06at the edge of the Kuiper Belt.
43:0850 astronomical units.
43:10About 50 metres away.
43:12And then, to the most distant object we can see...
43:15Well...
43:17We have to get a move on,
43:19because it's a long way away.
43:21It is, of course,
43:23far, far out.
43:25That is the most distant object
43:28we have ever seen,
43:30and it is, well,
43:32130 astronomical units away.
43:35130 metres.
43:38But remarkably,
43:40we strongly believe
43:43that there's another region,
43:45another realm to the Solar System.
43:47But to go to the edge of that,
43:50we have to go far, far, far away.
44:13♪
44:43♪
45:13♪
45:33♪
45:43♪
45:48So, here we are.
45:50Two kilometres away from the Sun.
45:53That's 2,000 astronomical units.
45:55And this is where we think another realm begins.
45:59And astonishingly to me,
46:01we think it extends
46:03100 kilometres further.
46:07100,000 astronomical units.
46:09That's over a light-year.
46:11Just imagine
46:13how many strange worlds
46:15that contains.
46:22This distant realm
46:24is known as the Oort Cloud.
46:26A mighty kingdom,
46:28at least a trillion strong.
46:33Hidden in the dark.
46:37Most are thought to be icy objects,
46:40the size of mountains.
46:42But it's likely some
46:44are made of rock.
46:46And if we were able
46:48to go there and visit them,
46:50we'd discover that they are pristine.
46:55Barely changed
46:57since the dawn of the Solar System.
47:06Stretching out into interstellar space,
47:09and held in place
47:11by the Sun's gravity.
47:13The Oort Cloud
47:15is our Solar System's largest
47:17and most mysterious realm.
47:22But here's the thing.
47:24No telescope has ever seen anything
47:26in the Oort Cloud.
47:28And the most distant spacecraft from Earth,
47:30Voyager 1,
47:32is all the way back there.
47:34It's just beyond, far, far out.
47:36It's travelling
47:38a million miles a day,
47:40and it will take 300 years
47:42to get here,
47:44and 30,000 years
47:46to cross the Oort Cloud.
47:48But science doesn't
47:50deal in fairy stories.
47:52It's not as if somebody just said,
47:54well that'd be a good idea, let's invent it.
47:56So why are we so sure that it's there?
47:58For as long as we've been looking
48:00into the night sky,
48:02there have been signs
48:04that the Oort Cloud is real.
48:09And this is one of them.
48:19It's existed for billions of years.
48:23And it's been there for thousands of years.
48:28And for most of that time,
48:30it's been locked in a deep,
48:32frozen slumber.
48:37But now,
48:39it stirs.
48:45As it nears the sun,
48:47warm rays
48:49bathe its surface.
48:51And it begins to thaw.
49:21So much material
49:23is torn from the surface.
49:27It stretches out
49:29to form a tail.
49:3520 million kilometres
49:37long.
49:40It's a comet.
49:51Comet Nishimura
49:53was visible from Earth
49:55for just a few weeks.
49:57And it's by no means
49:59the first of its kind.
50:05Hale-Bopp
50:10and Neowise
50:12lit up our skies
50:14for weeks on end.
50:21And when we followed
50:23their trajectories back,
50:25we discovered that they all came
50:27from the same place.
50:30300 billion kilometres
50:32from the sun.
50:38Countless comets
50:40throughout recent history
50:42have orbits that started
50:44in the Oort Cloud.
50:46And that's doubly surprising
50:48because comets have a very short
50:50lifetime. Many of them only
50:52last a few orbits around
50:54the sun. So that
50:56implies that there must be a vast
50:58reservoir of comets
51:00out here.
51:02And that is a central
51:04piece of evidence for the existence
51:06of the Oort Cloud.
51:11What does the Oort Cloud look like?
51:13Well, here's a fascinating thing.
51:15The comets all come from
51:17different directions.
51:19And over there and down and across
51:21from everywhere.
51:23Now if the Oort Cloud was
51:25flat, like the Kuiper Belts,
51:27we'd expect them to come
51:29from similar directions.
51:32Which implies that the
51:34Oort Cloud is not a flat
51:36disk.
51:39But some kind of giant sphere
51:41surrounding the sun.
51:45Now just imagine the scale
51:47of the Oort Cloud.
51:49A giant sphere enveloping
51:51the entire solar system
51:53stretching more than a
51:55light year
51:57every direction.
52:05We tend to think of the solar system
52:07as the sun
52:09and all the planets and moons.
52:17But as
52:19technology's improved,
52:21we've discovered more.
52:23And more.
52:27Our exploration of the solar
52:29system has only
52:31just begun.
52:37We know that we've only
52:39explored the tip of the iceberg.
52:41A fraction of what's out there.
52:43Most of the solar system
52:45lies, at least for now,
52:47way beyond our reach.
52:49But by just making some
52:51observations of a few comets
52:53and asteroids
52:55and even captured moons
52:57that have ventured inwards
52:59from the outer solar system
53:01and by doing science,
53:03we've been able to build a picture
53:05to tell a story
53:07of our solar system.
53:09And we've discovered that
53:11it isn't just a few planets orbiting
53:13close to a lonely star.
53:15It is a vast structure
53:17that stretches outwards
53:19maybe halfway
53:21to the nearest star.
53:43There's a real mystery
53:45surrounding Phobos,
53:47which is we don't really know
53:49what it is.
53:51It looks exactly like an asteroid
53:53but it orbits
53:55Mars in a very circular
53:57orbit around its equator,
53:59which is not what you would expect.
54:01Phobos looks like an asteroid
54:03but it behaves like a moon.
54:05So which one is it?
54:09We may soon discover
54:11We may soon
54:13have an answer.
54:15The Japanese
54:17Space Agency is planning
54:19an audacious mission to Phobos
54:21in 2026.
54:23The MMX mission,
54:25the Martian Moons
54:27Exploration Mission,
54:29aims at going to Mars' moon
54:31and bringing some samples
54:33of Phobos back to the Earth.
54:37I'm really confident that as soon
54:39as we have a sample of it on Earth
54:41we'll know almost straight away
54:43what Phobos is.
54:45But landing on Mars'
54:47tiny moon with its low gravity,
54:49drilling into its
54:51surface and getting the samples
54:53all the way back to Earth
54:55in a single round trip
54:57is no mean feat.
55:03People have wanted to explore
55:05the Martian moons for a long time,
55:07to send missions there,
55:09but they've failed for a number of reasons.
55:17Unfortunately, Phobos 1
55:19didn't work on its way
55:21to the Mars system.
55:25And then Phobos 2 got there
55:27and sent back a little bit of data
55:29and then was lost contact with.
55:31In 2011,
55:33the Russian Space Agency,
55:35made a third attempt.
55:43Phobos 1's probe
55:45blasted off from Baikonur Cosmodrome.
55:47But the Russian
55:49spacecraft failed to fire its own engines
55:51to set it on its path to Mars.
55:53When a mission fails
55:55it's devastating.
55:57It's disappointing.
55:59But we've got to learn from mistakes.
56:01We need those
56:03failures to help us develop
56:05our instrument, to develop
56:07our mission in order
56:09to make it a success.
56:11If all goes to plan,
56:13the Japanese mission will
56:15transform our understanding
56:17of what Phobos is.
56:19Giving new insight
56:21into how the moons and planets
56:23of our solar system formed.
56:27The MMX spacecraft is amazingly capable.
56:29It's got cameras and spectrometers
56:31and a rover and two sampling
56:33mechanisms.
56:35But it's also doing something that's
56:37never been done before, which makes
56:39it hard.
56:41And so to be ready for that
56:43you have to put things through a whole array
56:45of tests.
56:47They'll vibrate everything the way it will
56:49when it launches. They'll put it through
56:51hot and cold temperatures like you'll
56:53experience from direct sun
56:55or from being in the cold of space.
56:57If the mission succeeds,
56:59the returned spacecraft is due home
57:01with its samples
57:03in 2031.
57:05When the sample
57:07gets back to Earth, we'll analyse it
57:09in our laboratories to find out
57:11what it's made of and therefore
57:13what Phobos is.
57:15Understanding the
57:17origin of Mars moon Phobos
57:19will help us
57:21to take a step forward
57:23to understand
57:25the evolution
57:27and the formation of our solar system.
57:35Next time
57:37our solar system's most violent
57:39worlds
57:41where we encounter
57:4320km tall whirlwinds
57:47lightning 10 times more
57:49powerful than that on our planet
57:51and slow motion
57:54rainstorms
57:56on a distant moon.
57:58These
58:00are the storm worlds.
58:24Music
58:28Music
58:32Music