The Planets (2019) s01e05 Episode Script
Into the Darkness
1 Lift off of Messenger on Nasa's mission to Mercury.
They'll say No-one can see us That we're estranged And all alone They believe Nothing can reach us And pull us out of The boundless gloom They're wrong They're wrong They're wrong.
Far, far away beyond Mars past the storms of Jupiter, and the rings of Saturn conditions become very different.
Temperatures plummet, and distances between worlds are measured not in millions, but in billions of kilometres.
Here lie the most mysterious planets of them all.
Uranus.
A pale blue marble hanging in the dark, frozen depths of space.
And, further out the solar system's final true planet Neptune.
Beyond, we thought we'd only ever find tiny, lifeless worlds frozen to the core.
How wrong we were.
We live at a time when our civilisation has unprecedented reach, and we have a near-permanent presence at Mars, both in orbit and on the surface, and we've had spacecraft orbiting Mercury, Venus, Jupiter and Saturn for extended periods.
But, once we get beyond Saturn, things become much more difficult.
The distances are vast.
Travel times are measured not in one months, but in years, decades, even.
And that means that we've only visited the outer planets once.
Each visit is a voyage into the unknown, as we strive to reach the most distant planetary systems ever visited.
And, when we do get there, our spacecraft are travelling so fast that they fly straight through, only spending a few hours in the system.
But those few hours of precious data have allowed us to begin to understand the mysteries that lie there on the outer edges of the solar system.
We have ignition and we have liftoff.
Reports coming back indicate those twin, large, solid motors are functioning perfectly.
Our first mission into the dark was only possible thanks to some help from the planets themselves.
Each planet orbits at a different speed.
And, for most of the time, they're scattered around the Sun.
But once every 175 years something rather wonderful happens.
The outer planets align creating a path that leads from Earth past Jupiter and Saturn all the way to Uranus and Neptune.
Voyager 2 had been launched at precisely the right moment to take advantage of this rare planetary alignment.
It arrived at Jupiter in a little under two years.
Two and a half times more massive than all the other planets combined Jupiter gave Voyager a gravitational kick.
Another two years on, and Voyager witnessed one of the most beautiful sights in the solar system.
The icy rings of another gas giant Saturn.
And, then, on and on further into the darkness.
Almost nine years after leaving Earth, Voyager approached an entirely new class of planet.
Just like Jupiter and Saturn, the planet's upper atmosphere is composed mostly of swirling hydrogen and helium gas.
And hidden beneath lies an exotic, icy mix of methane, ammonia and water.
But, unlike the other gas giants, Uranus is almost featureless.
For all the time that Voyager stared at the planet, it saw just ten cloud formations.
And we soon discovered why.
Uranus, at -224 degrees Celsius, is the coldest planet in the solar system.
The first of the ice giants, in a permanent state of deep freeze.
Voyager spent just six hours with Uranus and, as its gaze widened, it took in the entire system.
Just like Saturn Uranus has rings.
The rings are so dark, so faint, that they're very difficult to see from Earth.
In fact, we didn't discover them until 1977.
They must be made of some kind of material that doesn't reflect a lot of sunlight back, and we don't know what that is.
It can't be water ice like the rings of Saturn, because they would be much brighter.
They're also extremely delicate and thin, and, again, that's a great mystery, cos you'd expect the particles to collide and the rings to spread out over time.
That doesn't happen.
So, something must be holding those rings in place, and the answer can be seen in this photograph taken by Voyager 2.
You can see a bright, thin ring, which is known as the epsilon ring, and, above it and below it, you can see two moons.
The inner one is called Cordelia, and the outer one is called Ophelia.
Now, the thing you need to know about orbits, a moon or a ring particle that's orbiting closer to a planet is moving more slowly than a moon or ring particle that's orbiting further away.
And, so, if there's a particle in the epsilon ring that maybe has a collision, loses a bit of energy and starts to drop down, then Cordelia tends to accelerate that particle back up into the ring again to speed it up by its gravitational influence.
Whereas, if it's a particle on the outer edge of the epsilon ring, it gets a bit more energy and raises up in altitude, then Ophelia tends to slow it down, and, so, it falls back into the ring again.
And, by that mechanism, these moons keep the ring nice and neat around Uranus, and, for that reason, these moons have become known as shepherd moons.
Now, we haven't discovered any more shepherd moons.
Voyager was only in the system for a few hours.
But we assume that they're there, orbiting around Uranus, keeping the other rings precisely in place.
The missing moons are not the only mysterious thing about the planet.
Uranus orbits the Sun in a unique way, a consequence of events that happened long ago.
Uranus was born from the vast cloud of material that surrounded the young Sun.
Over time, this material grew together under the influence of gravity forming each of the planets all orbiting in the same anticlockwise direction as the primordial cloud of gas and dust.
This rotation remains to this day.
Almost all the planets spin on their axis in the same anticlockwise direction.
But, for reasons we don't yet fully understand, Venus and Uranus spin on their axes in the opposite direction.
And Uranus is even more curious.
The entire planet is on its side.
It's not known for certain why Uranus spins in this way.
But it seems likely that, at some point, probably in the distant past, it was hit by another planet, possibly the size of Earth or even larger, which knocked the planet over, and, indeed, modern computer simulations suggest that when you do that to a planet, all the moons follow, and you end up with the planet and its system of moons, and now its rings, sort of corkscrewing around the Sun on its side.
Voyager had taken us to the most distant planets we had ever visited.
And, now, an almost unthinkable distance of 1.
6 billion kilometres of icy cold space lay between the spacecraft and its next encounter.
The size and scale of the solar system is impossible to imagine.
But, when we scale everything down, then it becomes easier to visualise.
The Sun is 1.
4 million kilometres in diameter.
But scale that down by a factor of around 600 million, and it becomes the size of that light.
And the Earth, Mars and the rocky planets are about that big on this scale.
And the distances are, I think, even more remarkable.
So, Mercury, the closest planet to the Sun, would be all the way over there by the sea.
So, this would be where Mercury orbits.
And Mercury is 96 metres away.
It's about just under a centimetre in diameter.
And the next planet, Venus, is somewhere over there.
We'd fly past Venus about 180 metres away from the Sun.
And then head onwards and outwards towards the Earth.
Here, at the Earth's orbit, the true scale of our fragility and isolation becomes apparent.
The Sun there, 250 metres away with the twinkling stars of the galaxy beyond, and here's the Earth.
Two centimetres across.
On that pebble, all of human history play out.
At around 380 metres, we pass by Mars, the last of the rocky planets, and then onwards into the darkness.
Crossing the harbour takes us to the fringes of the outer solar system.
And, now, the distances and the sizes are beginning to get really big.
Jupiter, the largest planet in the solar system, is over ten times the diameter of our Earth, and there's the Sun, 1.
3km across the harbour.
At almost twice the distance from the Sun to Jupiter, we reach Saturn.
2.
4km on our scale.
And we really are now entering the twilight of the solar system.
Light levels are like early morning or late evening here on Earth, but we can still see Saturn in the night sky, because the planet and its rings are highly reflective.
As far again as all the rocky planets, and Jupiter and Saturn, we reach Uranus.
3 billion kilometres from the Sun, and we begin to fully appreciate the distances Voyager had travelled on its journey to the farthest reaches of the solar system.
Journey's end, Neptune, a ball of gas about 7.
5 centimetres across.
7.
4km away from the Sun on this scale.
And we are still down there on the island in Reykjavik harbour.
Little, microscopic ants crawling around on the surface of a pebble.
It had taken 12 years, but Voyager had finally crossed the great expanse of space between Earth and the most distant planetary system.
Neptune.
17 times the mass of the Earth, and even more massive than Uranus.
But, in contrast to its sister planet, Neptune's atmosphere is bursting with activity.
What we found was a planet of extreme weather, where high altitude winds whip white methane clouds around at speeds of over 2,000km/h.
Those are the highest wind speeds anywhere in the solar system.
And Voyager saw a great, dark spot, not unlike Jupiter's great, red spot.
A storm system the size of Earth, that, when we looked only four or five years later, we found had vanished.
But, over the years, we've observed more storms raging over the planet.
It's one of the great mysteries of planetary exploration, why a planet so far from the Sun, with so little energy falling into its atmosphere from sunlight, can have the most extreme winds in the solar system.
And Voyager made yet another puzzling discovery.
Although further from the Sun, the planet is warmer than Uranus.
The source of this extra heat remains a mystery.
But, for some reason, Neptune emits over two and a half times the heat it receives from the Sun.
And this internal heat helps to explain the ferocious storms.
As the heat makes its way from the core of the planet and out into space it churns up the entire atmosphere creating winds unlike anything seen elsewhere in the solar system.
The probable reason that wind speeds can be so high is because there's no solid surface on a planet like Neptune.
It's exotic liquids further down, and gases further up.
But there's no rocky surface.
There are no mountains and continents to break up the flow of atmospheric gases, and, so, the winds can just whip around the planet at supersonic speeds.
Voyager 2 had almost completed its grand tour of the solar system.
But before it began its lonely journey out into interstellar space it had one, last world to visit.
Triton, a vast moon covered in a sheen of frozen nitrogen.
We expected it to be a silent, still world, but Voyager was in for one, last surprise.
But, when Voyager arrived, we saw geological activity on that frozen world.
We saw geysers erupting up into space 8km high, and then carrying dark material 100km downwind.
Now, the key to understanding what process caused those eruptions was that we noticed that the geysers tended to erupt on a place on Triton's surface below the faint Sun, even though it's so many billions of miles away.
What's happening is the sunlight is falling on the thin layer of nitrogen ice, going through, and heating up a layer of darker particles a metre below the surface.
A difference of just four degrees Celsius is enough for the heat to radiate up and vaporise the frozen nitrogen, creating gas.
That gas is under pressure, and, ultimately, it punches through the nitrogen ice carrying those dark particles up and making the geysers of Triton.
In the furthest reaches of the solar system, the faint light of the Sun is still enough to power the most distant of geological wonders.
But this can't explain the rest of Triton's mysterious features.
Terrain on this scale could only be created by a much more powerful force.
And a clue seems to be lurking in Triton's odd orbit.
Unlike every other large moon in the solar system, Triton orbits in the opposite direction to the spin of the planet.
And that means it's highly unlikely that Triton and Neptune formed at the same time.
If you formed a planet and a moon out of the same collapsing cloud of gas and dust, then they tend to spin and orbit in the same direction as the spin of the initial dust cloud, so, the most likely explanation for that is that Triton is a visitor to the Neptunian system, that, unlike Voyager, never left.
One theory is that, billions of years ago, Triton was not a moon at all.
It grew up in a region of space way beyond Neptune the Kuiper belt.
Here, trillions of frozen lumps of water, ammonia and methane circle the Sun.
The frozen leftovers from the formation of the solar system.
Perhaps Triton ended up on the inner edge of this region and ventured close enough to Neptune to be drawn in by its gravity until, eventually, it was plucked from the Kuiper belt, and forever trapped in orbit around the distant, blue planet.
Today, Triton orbits in a nice, regular circle around Neptune, but it wouldn't have started out that way.
When it was first captured, it would have glanced past the planet and ended up in a wide, elliptical orbit, sometimes being far away from the planet, and sometimes close.
That means that, as Triton orbited around Neptune, the gravitational pull was constantly changing, and that stretches and squashes the moon, and heats up the interior by friction.
It's a process called tidal heating.
The molten interior would have exploded up through cracks and faults in Triton's crust creating the ragged surface we see today.
And what you're left with is a cooling moon, orbiting in a circle the wrong way around the planet, the dramas of its past hidden from view.
All that remains are the geysers, powered by the faint light of the Sun, painting dark streaks across the now-quiescent surface.
Voyager had fulfilled its mission, and now headed onwards, out of the solar system where it would never encounter another planet.
But, elsewhere, another world awaited our arrival far out in the darkness.
Mission And lift off of Nasa's New Horizons spacecraft on a decade-long voyage to visit the planet Pluto and then beyond.
The New Horizons spacecraft, on its way to the very edge of our solar system.
Our ambition to learn more about a small, intriguing world depended on the success of New Horizons.
As the tiny probe, less than three metres in length, was sent out into the abyss most of its electronics and all nonessential systems were shut down to save power.
It would hibernate for almost its entire 4.
8 billion kilometre trek.
Whilst New Horizons continued its voyage to Pluto, a controversy arose here on Earth about the definition of Pluto itself.
See, we always knew that Pluto is small, smaller than Neptune's moon Triton, but it was discoveries made in the vicinity of Pluto by the Hubble Space Telescope that really challenged its status as a planet.
From Earth, Pluto, far away on the inner edge of the Kuiper belt, is so distant that, even with Hubble, it appears as nothing more than a fuzzy image.
But Pluto was not alone.
We began to detect other distant objects.
Then, Hubble discovered that another world is almost the same size as Pluto.
The discovery of these new worlds has forced us to reconsider what we mean by the word "planet".
The International Astronomical Union has a three-part definition of a planet.
Firstly, it has to orbit the Sun, and Pluto certainly does that.
It goes round once every 248 years.
Secondly, it has to be massive enough for gravity to sculpt it almost into a sphere, and Pluto indeed is almost spherical.
But, thirdly, it has to clear its own orbit around the Sun, and that's essentially a mass constraint as well.
It has to have enough gravity to throw things out from its orbital path.
And that's where Pluto falls down.
Now, why that extra idea? Well, ultimately, it's because if Pluto is big enough to be a planet, then Eris is big enough to be a planet, and Makemake is big enough to be a planet, and tens or even potentially hundreds of objects out there are big enough to be planets.
Now, does it really matter? Well, I don't think so.
Ultimately, Pluto is a world, and, therefore, we should explore it.
And, in July 2015, that's exactly what we did.
After nine years of flight, New Horizons awoke.
Copy that.
We are locked by telemetry with the phase path.
We had our first glimpse of the most distant world ever visited.
Pluto is beautiful.
And, far from being a frigid, featureless world, it turns out that it has all the characteristics of a dynamic, living planet.
And, yes, it is cold on the surface, about -230 degrees Celsius.
I'd be walking over solid nitrogen.
But the blotches that could be made out in the grainy Hubble Space Telescope images turned out to be a tremendous variety of geological form and structure.
But by far the most recognisable feature is the region known as Tombaugh Regio, or to give it its more popular name, Pluto's Heart.
The western lobe of the heart is called Sputnik Planitia.
A giant plain of frozen nitrogen, methane and carbon monoxide that stretches for a million square kilometres.
And, at its edge, lies a range of mountains made of pure, frozen water ice that rise up to 6km above the plain.
But there's something very strange about the region, something that sets it apart from the rest of this dwarf planet.
The surface of Pluto is covered in craters, the scars of impacts that have taken place over many billions of years, just like the surface of our moon.
Except, if you look on Sputnik Planitia, it is absolutely smooth.
There are no craters there at all, not a single one.
From space, the lack of craters is striking.
And, the closer you look, the stranger the mystery gets.
Detailed imagery beamed back by New Horizons revealed a network of hexagon and pentagon shapes that crisscross the frozen nitrogen surface.
A clue to what might be happening can be seen in these images.
Those kind of patterns are found elsewhere in nature.
Here, for example, on the surface of the Sun.
Or here, in a liquid that's been heated from below.
These patterns are characteristic of convection.
There's a heat source which is causing material to rise up, and then it cools and falls back again, and in those circulating convection currents, you tend to get patterns like this.
So, what could be happening below the surface of Sputnik Planitia is that there is a heat source deep down which is melting the nitrogen ice and causing it to rise, cool and then fall again, and those convection currents are constantly resurfacing the area.
That a small world like Pluto is still active was a huge surprise.
Our best theory of how this could be is that somewhere deep in its interior there are radioactive elements that generate heat as they decay.
This heat warms a sunless, half-frozen ocean of water that has existed for billions of years beneath the surface of Pluto.
Now, the evidence from Sputnik Planitia that there is still an internal heat source, and studies of other geological features on Pluto, have led some scientists to suggest that that ocean of water might still be there.
And the relatively warm ocean could explain the lack of craters on Sputnik Planitia.
The entire area is constantly being repaved as the nitrogen surface is slowly turned over.
But why all this activity here and nowhere else? Perhaps Pluto's heart is the site of a huge impact that, long ago, punched a large hole in the surface, almost down to the vast ocean beneath.
A hole that's slowly filled with soft nitrogen ice that now gently churns just above a warmer ocean.
Imagine that.
An ocean billions of miles away from the Sun, on the frozen frontier of the solar system.
On this most surprising of worlds, there may still be an ocean of water.
New Horizons' closest encounter with Pluto lasted just hours during which only one hemisphere was visible leaving the other side a mystery.
Pluto held a final surprise.
As New Horizons turned its camera back for one, last look it captured an image of Pluto's atmosphere, glowing in the dark.
A thin, blue sky over a hidden ocean of water 4.
8 billion kilometres from Earth.
We've come a long way.
After just half a century of space exploration we've reached every one of the planets in the solar system and begun to tell their story.
What's emerging is a tale of never-ending change of dramatic origins moments of hope and loss.
Where just eight motes of rock, ice and gas set against the dark backdrop of space have conspired together to produce life on at least one world.
"Why do we explore?" some people ask.
Shouldn't we deal with the problems here on Earth before committing time and energy and resources to exploring the stars? Well, I think focusing entirely on our mote of dust would be a profound mistake.
It would mean that we've taken the decision to sit huddled in a tiny corner of the solar system, wondering what we're doing here.
It would mean that we've taken the decision to fight amongst ourselves for ever-more-precious resources, confined below a thin shell of air on a small rock, rather than following the three-dimensional path marked out by the lights in the night.
We live in a solar system of wonders, of planets of storms and moons of ice and landscapes and vistas that stir the imagination and enrich the soul, a system of limitless resources, limitless beauty and limitless potential.
A system that we've only just begun to explore in a journey that has already rewarded us with so much.
Pluto was the most distant world that Nasa had ever planned to visit.
Ten, nine, eight, seven The launch of New Horizons was incredibly dramatic.
Mission And liftoff.
And it just shoots up so fast.
It's the fastest launch ever from Earth.
About ten miles per second.
In order to ensure that it was going to last for the decade across space and actually work there, they built in a lot of redundancy.
There were sort of two of everything, computers and the guidance systems, and they figured out how to hibernate.
The first spacecraft that hibernated.
To put the spacecraft largely to sleep let the spacecraft just silently coast, then we could extend the life of the electronics.
No spacecraft mission had ever really used hibernation as a day-to-day way to cross the solar system.
As the spacecraft slept, it sped away from Earth towards its destination to a world that had lain hidden amongst the stars for almost all of human history.
The discovery of Pluto was in 1930, when Clyde Tombaugh, a Kansas farm boy, was using the telescope at the Lowell Observatory in Arizona.
Every clear night, Clyde would go out and take images of the sky on photographic plates, and when it was cloudy or during the day, he would compare those plates.
And Clyde Tombaugh noticed that there was this little speck moving across the stars.
And, then, in 1930, Clyde Tombaugh discovered Pluto.
But Tombaugh would never see Pluto up close.
He died nine years before the launch of New Horizons, at the age of 90.
He had asked if a mission ever did get launched some of his ashes could be sent on the journey.
And to think that Clyde's ashes, you know, flew by Pluto, and, you know, he was finally visiting the place that he discovered, that was pretty cool.
Wow! Now, I mean, Pluto was a shock and a revelation in so many ways.
Our jaws just dropped to the floor.
This amazing world that we would never have known about, unless we sent a spacecraft out there.
But New Horizons didn't stop after its flyby of Pluto.
It continued on for three more years, ever deeper into the Kuiper belt.
The temperatures out there are almost absolute zero, so everything is so well preserved.
The Kuiper belt completely changed everything.
It's an archaeological dig, if you will, into the early history of our solar system, so it's a scientific wonderland.
On New Year's Day, 2019, New Horizons had reached the centre of the region and was about to encounter the most distant object ever visited.
This object may be the most primitive object ever encountered by a spacecraft.
By looking at Ultima Thule today, we think we're looking back in time to the origin of the solar system.
Ultima Thule's formation was perhaps just one of the countless collisions that formed the planets themselves.
But, somehow, it escaped and was flung far out into the Kuiper belt.
The worlds out there, which are billions of miles from the Sun, there are so many mysteries kept in these objects, and we can really go out and explore them, and start to understand these mysteries.
Like every mission before it, New Horizons has helped write the story of the planets.
A story that we're only just beginning to tell.
The planets of our solar system, they're in our back yard.
We have the technology to be able to go and stand on these surfaces.
To be able to go out there and be able to continue doing something that is very natural and intrinsic to all of us, and that is answering the big questions and exploring the unknown.
I feel as though we're absolutely just at the beginning of planetary exploration.
There's a lot of exploration left for us.
Journey through our solar system with this free poster produced by the Open University, and discover more about its planets and moons.
Order your free copy by calling Or go to And follow the links to the Open University.
They'll say No-one can see us That we're estranged And all alone They believe Nothing can reach us And pull us out of The boundless gloom They're wrong They're wrong They're wrong.
Far, far away beyond Mars past the storms of Jupiter, and the rings of Saturn conditions become very different.
Temperatures plummet, and distances between worlds are measured not in millions, but in billions of kilometres.
Here lie the most mysterious planets of them all.
Uranus.
A pale blue marble hanging in the dark, frozen depths of space.
And, further out the solar system's final true planet Neptune.
Beyond, we thought we'd only ever find tiny, lifeless worlds frozen to the core.
How wrong we were.
We live at a time when our civilisation has unprecedented reach, and we have a near-permanent presence at Mars, both in orbit and on the surface, and we've had spacecraft orbiting Mercury, Venus, Jupiter and Saturn for extended periods.
But, once we get beyond Saturn, things become much more difficult.
The distances are vast.
Travel times are measured not in one months, but in years, decades, even.
And that means that we've only visited the outer planets once.
Each visit is a voyage into the unknown, as we strive to reach the most distant planetary systems ever visited.
And, when we do get there, our spacecraft are travelling so fast that they fly straight through, only spending a few hours in the system.
But those few hours of precious data have allowed us to begin to understand the mysteries that lie there on the outer edges of the solar system.
We have ignition and we have liftoff.
Reports coming back indicate those twin, large, solid motors are functioning perfectly.
Our first mission into the dark was only possible thanks to some help from the planets themselves.
Each planet orbits at a different speed.
And, for most of the time, they're scattered around the Sun.
But once every 175 years something rather wonderful happens.
The outer planets align creating a path that leads from Earth past Jupiter and Saturn all the way to Uranus and Neptune.
Voyager 2 had been launched at precisely the right moment to take advantage of this rare planetary alignment.
It arrived at Jupiter in a little under two years.
Two and a half times more massive than all the other planets combined Jupiter gave Voyager a gravitational kick.
Another two years on, and Voyager witnessed one of the most beautiful sights in the solar system.
The icy rings of another gas giant Saturn.
And, then, on and on further into the darkness.
Almost nine years after leaving Earth, Voyager approached an entirely new class of planet.
Just like Jupiter and Saturn, the planet's upper atmosphere is composed mostly of swirling hydrogen and helium gas.
And hidden beneath lies an exotic, icy mix of methane, ammonia and water.
But, unlike the other gas giants, Uranus is almost featureless.
For all the time that Voyager stared at the planet, it saw just ten cloud formations.
And we soon discovered why.
Uranus, at -224 degrees Celsius, is the coldest planet in the solar system.
The first of the ice giants, in a permanent state of deep freeze.
Voyager spent just six hours with Uranus and, as its gaze widened, it took in the entire system.
Just like Saturn Uranus has rings.
The rings are so dark, so faint, that they're very difficult to see from Earth.
In fact, we didn't discover them until 1977.
They must be made of some kind of material that doesn't reflect a lot of sunlight back, and we don't know what that is.
It can't be water ice like the rings of Saturn, because they would be much brighter.
They're also extremely delicate and thin, and, again, that's a great mystery, cos you'd expect the particles to collide and the rings to spread out over time.
That doesn't happen.
So, something must be holding those rings in place, and the answer can be seen in this photograph taken by Voyager 2.
You can see a bright, thin ring, which is known as the epsilon ring, and, above it and below it, you can see two moons.
The inner one is called Cordelia, and the outer one is called Ophelia.
Now, the thing you need to know about orbits, a moon or a ring particle that's orbiting closer to a planet is moving more slowly than a moon or ring particle that's orbiting further away.
And, so, if there's a particle in the epsilon ring that maybe has a collision, loses a bit of energy and starts to drop down, then Cordelia tends to accelerate that particle back up into the ring again to speed it up by its gravitational influence.
Whereas, if it's a particle on the outer edge of the epsilon ring, it gets a bit more energy and raises up in altitude, then Ophelia tends to slow it down, and, so, it falls back into the ring again.
And, by that mechanism, these moons keep the ring nice and neat around Uranus, and, for that reason, these moons have become known as shepherd moons.
Now, we haven't discovered any more shepherd moons.
Voyager was only in the system for a few hours.
But we assume that they're there, orbiting around Uranus, keeping the other rings precisely in place.
The missing moons are not the only mysterious thing about the planet.
Uranus orbits the Sun in a unique way, a consequence of events that happened long ago.
Uranus was born from the vast cloud of material that surrounded the young Sun.
Over time, this material grew together under the influence of gravity forming each of the planets all orbiting in the same anticlockwise direction as the primordial cloud of gas and dust.
This rotation remains to this day.
Almost all the planets spin on their axis in the same anticlockwise direction.
But, for reasons we don't yet fully understand, Venus and Uranus spin on their axes in the opposite direction.
And Uranus is even more curious.
The entire planet is on its side.
It's not known for certain why Uranus spins in this way.
But it seems likely that, at some point, probably in the distant past, it was hit by another planet, possibly the size of Earth or even larger, which knocked the planet over, and, indeed, modern computer simulations suggest that when you do that to a planet, all the moons follow, and you end up with the planet and its system of moons, and now its rings, sort of corkscrewing around the Sun on its side.
Voyager had taken us to the most distant planets we had ever visited.
And, now, an almost unthinkable distance of 1.
6 billion kilometres of icy cold space lay between the spacecraft and its next encounter.
The size and scale of the solar system is impossible to imagine.
But, when we scale everything down, then it becomes easier to visualise.
The Sun is 1.
4 million kilometres in diameter.
But scale that down by a factor of around 600 million, and it becomes the size of that light.
And the Earth, Mars and the rocky planets are about that big on this scale.
And the distances are, I think, even more remarkable.
So, Mercury, the closest planet to the Sun, would be all the way over there by the sea.
So, this would be where Mercury orbits.
And Mercury is 96 metres away.
It's about just under a centimetre in diameter.
And the next planet, Venus, is somewhere over there.
We'd fly past Venus about 180 metres away from the Sun.
And then head onwards and outwards towards the Earth.
Here, at the Earth's orbit, the true scale of our fragility and isolation becomes apparent.
The Sun there, 250 metres away with the twinkling stars of the galaxy beyond, and here's the Earth.
Two centimetres across.
On that pebble, all of human history play out.
At around 380 metres, we pass by Mars, the last of the rocky planets, and then onwards into the darkness.
Crossing the harbour takes us to the fringes of the outer solar system.
And, now, the distances and the sizes are beginning to get really big.
Jupiter, the largest planet in the solar system, is over ten times the diameter of our Earth, and there's the Sun, 1.
3km across the harbour.
At almost twice the distance from the Sun to Jupiter, we reach Saturn.
2.
4km on our scale.
And we really are now entering the twilight of the solar system.
Light levels are like early morning or late evening here on Earth, but we can still see Saturn in the night sky, because the planet and its rings are highly reflective.
As far again as all the rocky planets, and Jupiter and Saturn, we reach Uranus.
3 billion kilometres from the Sun, and we begin to fully appreciate the distances Voyager had travelled on its journey to the farthest reaches of the solar system.
Journey's end, Neptune, a ball of gas about 7.
5 centimetres across.
7.
4km away from the Sun on this scale.
And we are still down there on the island in Reykjavik harbour.
Little, microscopic ants crawling around on the surface of a pebble.
It had taken 12 years, but Voyager had finally crossed the great expanse of space between Earth and the most distant planetary system.
Neptune.
17 times the mass of the Earth, and even more massive than Uranus.
But, in contrast to its sister planet, Neptune's atmosphere is bursting with activity.
What we found was a planet of extreme weather, where high altitude winds whip white methane clouds around at speeds of over 2,000km/h.
Those are the highest wind speeds anywhere in the solar system.
And Voyager saw a great, dark spot, not unlike Jupiter's great, red spot.
A storm system the size of Earth, that, when we looked only four or five years later, we found had vanished.
But, over the years, we've observed more storms raging over the planet.
It's one of the great mysteries of planetary exploration, why a planet so far from the Sun, with so little energy falling into its atmosphere from sunlight, can have the most extreme winds in the solar system.
And Voyager made yet another puzzling discovery.
Although further from the Sun, the planet is warmer than Uranus.
The source of this extra heat remains a mystery.
But, for some reason, Neptune emits over two and a half times the heat it receives from the Sun.
And this internal heat helps to explain the ferocious storms.
As the heat makes its way from the core of the planet and out into space it churns up the entire atmosphere creating winds unlike anything seen elsewhere in the solar system.
The probable reason that wind speeds can be so high is because there's no solid surface on a planet like Neptune.
It's exotic liquids further down, and gases further up.
But there's no rocky surface.
There are no mountains and continents to break up the flow of atmospheric gases, and, so, the winds can just whip around the planet at supersonic speeds.
Voyager 2 had almost completed its grand tour of the solar system.
But before it began its lonely journey out into interstellar space it had one, last world to visit.
Triton, a vast moon covered in a sheen of frozen nitrogen.
We expected it to be a silent, still world, but Voyager was in for one, last surprise.
But, when Voyager arrived, we saw geological activity on that frozen world.
We saw geysers erupting up into space 8km high, and then carrying dark material 100km downwind.
Now, the key to understanding what process caused those eruptions was that we noticed that the geysers tended to erupt on a place on Triton's surface below the faint Sun, even though it's so many billions of miles away.
What's happening is the sunlight is falling on the thin layer of nitrogen ice, going through, and heating up a layer of darker particles a metre below the surface.
A difference of just four degrees Celsius is enough for the heat to radiate up and vaporise the frozen nitrogen, creating gas.
That gas is under pressure, and, ultimately, it punches through the nitrogen ice carrying those dark particles up and making the geysers of Triton.
In the furthest reaches of the solar system, the faint light of the Sun is still enough to power the most distant of geological wonders.
But this can't explain the rest of Triton's mysterious features.
Terrain on this scale could only be created by a much more powerful force.
And a clue seems to be lurking in Triton's odd orbit.
Unlike every other large moon in the solar system, Triton orbits in the opposite direction to the spin of the planet.
And that means it's highly unlikely that Triton and Neptune formed at the same time.
If you formed a planet and a moon out of the same collapsing cloud of gas and dust, then they tend to spin and orbit in the same direction as the spin of the initial dust cloud, so, the most likely explanation for that is that Triton is a visitor to the Neptunian system, that, unlike Voyager, never left.
One theory is that, billions of years ago, Triton was not a moon at all.
It grew up in a region of space way beyond Neptune the Kuiper belt.
Here, trillions of frozen lumps of water, ammonia and methane circle the Sun.
The frozen leftovers from the formation of the solar system.
Perhaps Triton ended up on the inner edge of this region and ventured close enough to Neptune to be drawn in by its gravity until, eventually, it was plucked from the Kuiper belt, and forever trapped in orbit around the distant, blue planet.
Today, Triton orbits in a nice, regular circle around Neptune, but it wouldn't have started out that way.
When it was first captured, it would have glanced past the planet and ended up in a wide, elliptical orbit, sometimes being far away from the planet, and sometimes close.
That means that, as Triton orbited around Neptune, the gravitational pull was constantly changing, and that stretches and squashes the moon, and heats up the interior by friction.
It's a process called tidal heating.
The molten interior would have exploded up through cracks and faults in Triton's crust creating the ragged surface we see today.
And what you're left with is a cooling moon, orbiting in a circle the wrong way around the planet, the dramas of its past hidden from view.
All that remains are the geysers, powered by the faint light of the Sun, painting dark streaks across the now-quiescent surface.
Voyager had fulfilled its mission, and now headed onwards, out of the solar system where it would never encounter another planet.
But, elsewhere, another world awaited our arrival far out in the darkness.
Mission And lift off of Nasa's New Horizons spacecraft on a decade-long voyage to visit the planet Pluto and then beyond.
The New Horizons spacecraft, on its way to the very edge of our solar system.
Our ambition to learn more about a small, intriguing world depended on the success of New Horizons.
As the tiny probe, less than three metres in length, was sent out into the abyss most of its electronics and all nonessential systems were shut down to save power.
It would hibernate for almost its entire 4.
8 billion kilometre trek.
Whilst New Horizons continued its voyage to Pluto, a controversy arose here on Earth about the definition of Pluto itself.
See, we always knew that Pluto is small, smaller than Neptune's moon Triton, but it was discoveries made in the vicinity of Pluto by the Hubble Space Telescope that really challenged its status as a planet.
From Earth, Pluto, far away on the inner edge of the Kuiper belt, is so distant that, even with Hubble, it appears as nothing more than a fuzzy image.
But Pluto was not alone.
We began to detect other distant objects.
Then, Hubble discovered that another world is almost the same size as Pluto.
The discovery of these new worlds has forced us to reconsider what we mean by the word "planet".
The International Astronomical Union has a three-part definition of a planet.
Firstly, it has to orbit the Sun, and Pluto certainly does that.
It goes round once every 248 years.
Secondly, it has to be massive enough for gravity to sculpt it almost into a sphere, and Pluto indeed is almost spherical.
But, thirdly, it has to clear its own orbit around the Sun, and that's essentially a mass constraint as well.
It has to have enough gravity to throw things out from its orbital path.
And that's where Pluto falls down.
Now, why that extra idea? Well, ultimately, it's because if Pluto is big enough to be a planet, then Eris is big enough to be a planet, and Makemake is big enough to be a planet, and tens or even potentially hundreds of objects out there are big enough to be planets.
Now, does it really matter? Well, I don't think so.
Ultimately, Pluto is a world, and, therefore, we should explore it.
And, in July 2015, that's exactly what we did.
After nine years of flight, New Horizons awoke.
Copy that.
We are locked by telemetry with the phase path.
We had our first glimpse of the most distant world ever visited.
Pluto is beautiful.
And, far from being a frigid, featureless world, it turns out that it has all the characteristics of a dynamic, living planet.
And, yes, it is cold on the surface, about -230 degrees Celsius.
I'd be walking over solid nitrogen.
But the blotches that could be made out in the grainy Hubble Space Telescope images turned out to be a tremendous variety of geological form and structure.
But by far the most recognisable feature is the region known as Tombaugh Regio, or to give it its more popular name, Pluto's Heart.
The western lobe of the heart is called Sputnik Planitia.
A giant plain of frozen nitrogen, methane and carbon monoxide that stretches for a million square kilometres.
And, at its edge, lies a range of mountains made of pure, frozen water ice that rise up to 6km above the plain.
But there's something very strange about the region, something that sets it apart from the rest of this dwarf planet.
The surface of Pluto is covered in craters, the scars of impacts that have taken place over many billions of years, just like the surface of our moon.
Except, if you look on Sputnik Planitia, it is absolutely smooth.
There are no craters there at all, not a single one.
From space, the lack of craters is striking.
And, the closer you look, the stranger the mystery gets.
Detailed imagery beamed back by New Horizons revealed a network of hexagon and pentagon shapes that crisscross the frozen nitrogen surface.
A clue to what might be happening can be seen in these images.
Those kind of patterns are found elsewhere in nature.
Here, for example, on the surface of the Sun.
Or here, in a liquid that's been heated from below.
These patterns are characteristic of convection.
There's a heat source which is causing material to rise up, and then it cools and falls back again, and in those circulating convection currents, you tend to get patterns like this.
So, what could be happening below the surface of Sputnik Planitia is that there is a heat source deep down which is melting the nitrogen ice and causing it to rise, cool and then fall again, and those convection currents are constantly resurfacing the area.
That a small world like Pluto is still active was a huge surprise.
Our best theory of how this could be is that somewhere deep in its interior there are radioactive elements that generate heat as they decay.
This heat warms a sunless, half-frozen ocean of water that has existed for billions of years beneath the surface of Pluto.
Now, the evidence from Sputnik Planitia that there is still an internal heat source, and studies of other geological features on Pluto, have led some scientists to suggest that that ocean of water might still be there.
And the relatively warm ocean could explain the lack of craters on Sputnik Planitia.
The entire area is constantly being repaved as the nitrogen surface is slowly turned over.
But why all this activity here and nowhere else? Perhaps Pluto's heart is the site of a huge impact that, long ago, punched a large hole in the surface, almost down to the vast ocean beneath.
A hole that's slowly filled with soft nitrogen ice that now gently churns just above a warmer ocean.
Imagine that.
An ocean billions of miles away from the Sun, on the frozen frontier of the solar system.
On this most surprising of worlds, there may still be an ocean of water.
New Horizons' closest encounter with Pluto lasted just hours during which only one hemisphere was visible leaving the other side a mystery.
Pluto held a final surprise.
As New Horizons turned its camera back for one, last look it captured an image of Pluto's atmosphere, glowing in the dark.
A thin, blue sky over a hidden ocean of water 4.
8 billion kilometres from Earth.
We've come a long way.
After just half a century of space exploration we've reached every one of the planets in the solar system and begun to tell their story.
What's emerging is a tale of never-ending change of dramatic origins moments of hope and loss.
Where just eight motes of rock, ice and gas set against the dark backdrop of space have conspired together to produce life on at least one world.
"Why do we explore?" some people ask.
Shouldn't we deal with the problems here on Earth before committing time and energy and resources to exploring the stars? Well, I think focusing entirely on our mote of dust would be a profound mistake.
It would mean that we've taken the decision to sit huddled in a tiny corner of the solar system, wondering what we're doing here.
It would mean that we've taken the decision to fight amongst ourselves for ever-more-precious resources, confined below a thin shell of air on a small rock, rather than following the three-dimensional path marked out by the lights in the night.
We live in a solar system of wonders, of planets of storms and moons of ice and landscapes and vistas that stir the imagination and enrich the soul, a system of limitless resources, limitless beauty and limitless potential.
A system that we've only just begun to explore in a journey that has already rewarded us with so much.
Pluto was the most distant world that Nasa had ever planned to visit.
Ten, nine, eight, seven The launch of New Horizons was incredibly dramatic.
Mission And liftoff.
And it just shoots up so fast.
It's the fastest launch ever from Earth.
About ten miles per second.
In order to ensure that it was going to last for the decade across space and actually work there, they built in a lot of redundancy.
There were sort of two of everything, computers and the guidance systems, and they figured out how to hibernate.
The first spacecraft that hibernated.
To put the spacecraft largely to sleep let the spacecraft just silently coast, then we could extend the life of the electronics.
No spacecraft mission had ever really used hibernation as a day-to-day way to cross the solar system.
As the spacecraft slept, it sped away from Earth towards its destination to a world that had lain hidden amongst the stars for almost all of human history.
The discovery of Pluto was in 1930, when Clyde Tombaugh, a Kansas farm boy, was using the telescope at the Lowell Observatory in Arizona.
Every clear night, Clyde would go out and take images of the sky on photographic plates, and when it was cloudy or during the day, he would compare those plates.
And Clyde Tombaugh noticed that there was this little speck moving across the stars.
And, then, in 1930, Clyde Tombaugh discovered Pluto.
But Tombaugh would never see Pluto up close.
He died nine years before the launch of New Horizons, at the age of 90.
He had asked if a mission ever did get launched some of his ashes could be sent on the journey.
And to think that Clyde's ashes, you know, flew by Pluto, and, you know, he was finally visiting the place that he discovered, that was pretty cool.
Wow! Now, I mean, Pluto was a shock and a revelation in so many ways.
Our jaws just dropped to the floor.
This amazing world that we would never have known about, unless we sent a spacecraft out there.
But New Horizons didn't stop after its flyby of Pluto.
It continued on for three more years, ever deeper into the Kuiper belt.
The temperatures out there are almost absolute zero, so everything is so well preserved.
The Kuiper belt completely changed everything.
It's an archaeological dig, if you will, into the early history of our solar system, so it's a scientific wonderland.
On New Year's Day, 2019, New Horizons had reached the centre of the region and was about to encounter the most distant object ever visited.
This object may be the most primitive object ever encountered by a spacecraft.
By looking at Ultima Thule today, we think we're looking back in time to the origin of the solar system.
Ultima Thule's formation was perhaps just one of the countless collisions that formed the planets themselves.
But, somehow, it escaped and was flung far out into the Kuiper belt.
The worlds out there, which are billions of miles from the Sun, there are so many mysteries kept in these objects, and we can really go out and explore them, and start to understand these mysteries.
Like every mission before it, New Horizons has helped write the story of the planets.
A story that we're only just beginning to tell.
The planets of our solar system, they're in our back yard.
We have the technology to be able to go and stand on these surfaces.
To be able to go out there and be able to continue doing something that is very natural and intrinsic to all of us, and that is answering the big questions and exploring the unknown.
I feel as though we're absolutely just at the beginning of planetary exploration.
There's a lot of exploration left for us.
Journey through our solar system with this free poster produced by the Open University, and discover more about its planets and moons.
Order your free copy by calling Or go to And follow the links to the Open University.