Slow Orbit · Episode 5 · 33 min

Jupiter, the King of the Planets

Galilean moons: from Galileo Galilei, who first saw them in January 1610 and took them for four little stars. A storm wider than the Earth that has been turning for nearly two hundred years, a world with nowhere to land, four moons that changed what people believed about the heavens, and a comet that fell in pieces while the whole world watched... Inside: the speed of light measured by a small moon, a flower of storms at the pole, a moon of volcanoes, an ocean under the ice, three moons keeping time like dancers. Ole Rømer will be there too. Chapters: The wandering star · Four little stars · The face of the giant · The Great Red Spot · Inside · The visitors · The day the comet fell · A family of worlds · On the way · The king. Sources: NASA (Pioneer, Voyager, Galileo, Juno), ESA (Juice), Galileo's Sidereus Nuncius. Read by a synthetic voice; script written and fact-checked by the show's team.

The script

There is a storm on Jupiter that is older than anyone now alive.

It is red, and it is wider than the whole Earth. It turns slowly, counterclockwise, in the southern clouds of the planet. People on Earth have been watching it, through their telescopes, for nearly two hundred years. And in all that time, it has never once stopped.

Slow Orbit. Lecture five... Jupiter, the king of the planets.

This lecture is read by a synthetic voice. If these lectures keep you company at night, follow the show... it helps others find it.

Chapter one. The wandering star.

Long before anyone knew what a planet was, people knew Jupiter.

On most nights of the year, it is there... a steady, bright, cream-coloured light that does not twinkle the way the stars do. Only the Moon and Venus are usually brighter. And unlike the stars, it moves. Night after night, slowly, it shifts its place against the background of the constellations. It takes almost twelve years to travel all the way around the sky and come back to where it started.

The ancient Babylonians watched it carefully and wrote its movements on clay tablets. The Greeks called the planets the wanderers... planetes... and they gave this one the name of their highest god, Zeus. The Romans gave it the name of theirs. Jupiter... king of the gods, god of the sky and of thunder.

They could not have known how well the name would fit.

Jupiter is the largest planet of the Sun. It is so large that all the other planets together... Mercury, Venus, Earth, Mars, Saturn, Uranus and Neptune, all of them... would weigh less than half of it.

Eleven Earths, side by side, would just about reach across its face. More than a thousand Earths could fit inside it.

And yet it is not made of rock. If you could fly down to Jupiter, there would be nowhere to land.

Chapter two. Four little stars.

In January of the year 1610, in the city of Padua, in Italy, a professor of mathematics named Galileo Galilei pointed a telescope at Jupiter.

It was not a large telescope. It was a tube of wood and leather, a little longer than a man's arm, with a lens at each end, which he had ground himself. It made things look about twenty times larger. Through it, Jupiter was no longer a point of light. It was a small, round disk.

And beside the disk, in a straight line, there were three tiny stars.

The next night, the three little stars had moved. A few nights later, there were four. Night after night, Galileo drew them in his notebook... small dots on either side of the planet, changing their places, sometimes hiding behind it, then coming out again on the other side.

They were not stars at all. They were moons. And they were going around Jupiter.

It is hard, now, to feel how strange that was. At the time, most learned people believed that everything in the heavens went around the Earth. And here, in a little tube of wood, were four worlds that plainly did not. They went around something else.

Galileo wrote about them in a short book that same spring. He called it The Starry Messenger. Within months, it was being read across Europe.

Galileo wanted to name the moons after his patrons, the Medici family of Florence. That name did not last. The names we use today were proposed by a German astronomer, Simon Marius, who saw the moons at almost the same time.

He named them after the companions of Zeus in the old Greek stories. Io. Europa. Ganymede. Callisto.

We still call them the Galilean moons. And they are still there, every night, for anyone with a pair of good binoculars and a steady hand... four small sparks in a line beside the planet.

Some sixty years later, the moons of Jupiter told us something else.

A Danish astronomer named Ole Rømer, working in Paris, was timing the moments when Io slipped into Jupiter's shadow. It should have been as regular as a clock. But it was not. When the Earth was moving away from Jupiter, the eclipses came a little late. When the Earth was moving toward it, they came a little early.

Rømer understood why. The light from Io was taking time to cross the space between the planets. When the Earth was farther away, the light simply took longer to arrive.

Until then, many people believed light travelled instantly. In 1676, because of a small moon going around Jupiter, we learned that it does not. Light has a speed. A great speed... but a speed.

Chapter three. The face of the giant.

Look at Jupiter through a larger telescope, and the little disk becomes a face.

It is striped. Pale bands of cream and white lie across it, one above the other, and between them darker bands, brown and orange and rust. Astronomers call the pale ones zones, and the dark ones belts. They run parallel to the planet's equator, all the way around.

The bands are clouds. And the clouds are moving... fast.

Jupiter spins faster than any other planet. One day on Jupiter... one full turn... takes less than ten hours. For a world this size, that is an enormous speed. At its equator, the cloud tops are carried around at more than forty thousand kilometres an hour.

The spin is so fast that it changes the planet's shape. Jupiter is not quite round. It bulges at the middle and is flattened at the poles. Even in a small telescope, you can see that it is a little wider than it is tall.

The winds run between the bands in long streams, some flowing east, some flowing west, side by side. Where they meet, they curl into swirls and loops and long, feathered edges. White ovals drift along the belts like small boats. Dark knots appear and fade.

No two pictures of Jupiter are ever quite the same. Its face changes from week to week. Sometimes a whole belt fades almost to white for a year or two, and then, slowly, it darkens again.

And there, in the southern half, sits the largest knot of all.

Chapter four. The Great Red Spot.

The Great Red Spot is a storm.

It is a vast, high, spinning mass of cloud, turning once every six days or so. Around its edge, the winds blow faster than in any hurricane on Earth. Its clouds rise higher than the clouds around it, like the top of a giant tower.

Astronomers have been watching it without a break since the eighteen-thirties. There are older drawings, too, from the sixteen-sixties, that show a spot in about the same place. Whether that was the same storm, nobody can say for certain. It may have faded, and a new one may have grown in its place.

But for nearly two hundred years at least, the Red Spot has been there.

Over those years, it has changed.

In the late eighteen-hundreds, it was long and stretched out... about three Earths wide. Today it is rounder, and smaller. It is now only a little wider than one Earth. Year by year, it has been slowly shrinking.

Its colour changes too. Some years it is a deep brick red. Some years it is pale salmon, almost pink. Nobody yet knows exactly what gives it its colour. It may be sunlight acting on chemicals carried up from deep inside the planet, the way sunlight slowly yellows a page.

Will it one day disappear? It may. A storm on Earth fades when it moves over land, where there is no warm water to feed it. On Jupiter, there is no land. Nothing to slow it down. That may be why the storm has lasted so long.

It may last centuries more. Or it may fade within the lifetime of the people watching it now. Jupiter does not say.

Chapter five. Inside.

What is under all those clouds?

Jupiter is made, almost entirely, of the two lightest substances there are: hydrogen and helium. It is made of nearly the same mixture as the Sun itself. If it had formed with much more of it... about eighty times more... the weight pressing on its centre would have started the fires of a star.

It did not. It stayed a planet. But it is still, in a way, a little like an unfinished star.

The clouds we see are only the very top. They are made of tiny crystals of ammonia ice, white and cold, and below them, darker clouds of other chemicals, and below those, clouds of water.

Go deeper, and the air becomes thick and hot. There is no moment where you would reach the surface. The gas simply grows denser and denser, until, without any clear edge, it behaves more like a liquid than a gas.

Deeper still, thousands of kilometres down, the pressure becomes so great that hydrogen changes into something we almost never see on Earth. It becomes a kind of liquid metal. It flows like water, but it carries electricity like copper.

And in the very centre, there may be a heavy core... rock and ice and metal, mixed and smeared out over a wide region, a little like a cloud of sand in a jar of honey.

That ocean of metallic hydrogen, turning fast with the planet, makes Jupiter's magnetic field. It is the strongest of any planet... thousands of times stronger than the Earth's.

It reaches far out into space, in a vast invisible bubble. If you could see it from the Earth, it would look larger in the sky than the full Moon.

Inside that bubble, the space around Jupiter is filled with radiation, charged particles moving close to the speed of light. It is one of the harshest places in the solar system for anything built by people. Every spacecraft that has gone there has had to be shielded, and every one of them has been slowly damaged by it.

And at the poles, where that field touches the planet, there are auroras... vast rings of light, larger than the whole Earth, glowing above the clouds. They never stop.

There is one more strange thing about Jupiter.

It gives out more heat than it receives from the Sun. It is still releasing the heat of its own birth, four and a half billion years ago, and it is still, very slowly, settling and shrinking as it cools. A few centimetres a year.

The giant is still warm from the day it was made.

Chapter six. The visitors.

For more than three hundred years, we could only look at Jupiter from far away. Then, in the 1970s, we went.

In December 1973, a small spacecraft called Pioneer 10 flew past the planet. It was the first object made by people ever to cross the belt of asteroids beyond Mars and reach the outer solar system. It took pictures... soft, orange pictures, built up line by line... of a world no one had ever seen up close. Then it flew on, out of the solar system, never to return.

A year later, Pioneer 11 followed.

In 1979, two more visitors arrived: the twin spacecraft Voyager 1 and Voyager 2.

Their pictures were sharp and full of colour, and they changed what we knew. They showed the swirling storms in detail. They showed lightning flashing in the clouds on the night side of the planet. And they showed something nobody had expected: Jupiter has rings.

They are nothing like the bright, broad rings of Saturn. They are faint and thin and dark, made of fine dust, knocked off the small inner moons by tiny meteorites. You could look at Jupiter all your life through a telescope and never see them. But they are there.

In 1995, after a journey of six years, a spacecraft named after Galileo himself went into orbit around Jupiter.

It was the first to stay. It circled the planet for nearly eight years. It dropped a small probe into the atmosphere, which fell through the clouds on a parachute, measuring the winds and the heat, sending back its findings for about an hour, until the pressure and the heat silenced it.

At the end, its fuel nearly gone, the Galileo spacecraft was steered on purpose into Jupiter, where it burned up in the clouds. It was done with care. Its controllers did not want it ever to crash, by accident, on one of the moons... in case one of them was a place where life might someday be found.

That moon has a name. We will come to it.

And in 2016, on the fourth of July, a spacecraft named Juno arrived.

In the old Roman stories, Juno was the wife of Jupiter. She could see through the clouds he drew around himself to hide his mischief. The spacecraft was given the same task... to look through the clouds.

Juno flies on long, looping paths over the poles of the planet, swinging in close, then far out again. And over the poles, it saw something no one had seen before.

At the north pole of Jupiter, there is a ring of eight huge storms, each larger than a continent, circling around one storm in the middle, like the petals of a flower around its centre. At the south pole, there is another such flower, with five, and later six, around its middle.

They hold their pattern, month after month. They do not merge. They do not drift apart. Nobody is yet sure why.

Chapter seven. The day the comet fell.

In the spring of 1993, astronomers in California found a comet that looked strange. It was not one comet, but a string of them, lined up like pearls on a necklace.

They named it after its discoverers... Comet Shoemaker-Levy 9.

It turned out that, decades earlier, the comet had been captured by Jupiter's gravity. In 1992, it had passed so close to the planet that it had been torn into pieces. And now the pieces were on their way back.

They were going to hit Jupiter.

For the first time in history, people knew a collision between two worlds was coming, and when.

In July of 1994, over six days, more than twenty pieces of the comet fell into Jupiter, one after another, each striking at about sixty kilometres a second.

The impacts happened on the far side of the planet, just out of sight from the Earth. But Jupiter turns fast. Within minutes, the places where the pieces had struck rotated into view.

And there they were: great dark marks on the clouds, like bruises. Some of them were larger than the Earth. They could be seen with small telescopes, from back gardens, by people all over the world. They lasted for months before the winds slowly smoothed them away.

Watching it, people felt something that is easy to forget.

Things still collide in the solar system. And Jupiter, with its enormous gravity, pulls many of the wandering comets and rocks toward itself, or flings them away. Some astronomers think that, over billions of years, this has spared the Earth from more than a few impacts. Others think it has sometimes sent them our way. The question is still open.

But on those six days in July, Jupiter took the blows.

Chapter eight. A family of worlds.

Jupiter is not alone. It has nearly a hundred known moons, and more are found every few years. Most of them are small... rocks a few kilometres wide, many of them probably captured long ago, going around the planet backwards on distant, tilted paths.

But the four that Galileo saw are worlds of their own. And each one is different.

The closest is Io.

Io is the most volcanic place in the solar system. More than four hundred volcanoes are active on it. Some of them throw plumes of gas and dust hundreds of kilometres up into space, where they rise in slow, glittering umbrellas and fall back down on the surface. Its face is painted in yellow, orange, white and black, from sulphur and from fresh lava. It never looks the same twice.

Why is a small moon so hot inside? Because it is pulled. Io's path around Jupiter is not quite a circle, and the other moons keep tugging it, so that it can never settle. As it moves closer to Jupiter and farther away, the giant's gravity squeezes and stretches it, again and again. Its rock flexes, the way a ball of clay grows warm when you knead it in your hands.

The volcanoes of Io were first seen in pictures taken by Voyager 1, in 1979. An engineer named Linda Morabito was studying one of the pictures, to help steer the spacecraft by the stars, when she noticed a strange cloud rising from the edge of the moon. It was the first active volcano ever seen anywhere other than on the Earth.

Next comes Europa.

Europa is a little smaller than our own Moon. It is covered in ice... smooth, white, bright ice, crossed by long reddish-brown lines and cracks, like the lines on a cracked eggshell. There are very few craters. Its surface is young. Something keeps renewing it.

Beneath that ice, scientists are confident, there is an ocean of liquid water. A real ocean, dark and salty, perhaps a hundred kilometres deep... going all the way around the moon. It may hold about twice as much water as all the oceans of the Earth together.

It is kept from freezing by the same squeezing and stretching that heats Io.

And where there is liquid water, warmth and the right chemicals, there might be... might be... life. That is why the Galileo spacecraft was steered into Jupiter and not left to drift. That is why Europa is one of the places scientists most want to visit.

Then Ganymede.

Ganymede is the largest moon in the whole solar system. It is larger than the planet Mercury. It is made of rock and ice, with dark old regions covered in craters, and lighter regions crossed by long grooves and ridges.

It is the only moon we know of that has its own magnetic field, and so its own small auroras, glowing in the thin air above its poles. Deep inside, under a thick shell of ice, it too seems to hide a layer of salty water.

And the farthest of the four is Callisto.

Callisto is dark, quiet and old. Its surface is the most heavily cratered of any world we know. Craters on top of craters, so close together that there is hardly a flat place between them. Among them is one great scar, called Valhalla, made by an impact so long ago that its rings spread out across a large part of the moon, like ripples frozen in a pond.

Callisto is far enough from Jupiter that the squeezing barely reaches it. Little has changed on its surface for billions of years. If you want to see what the young solar system was like, they say, look at Callisto.

Four moons, going around one giant. A moon of fire. A moon of ice with an ocean underneath. A moon larger than a planet. And a moon so old that it remembers the beginning.

Io, Europa and Ganymede are bound together in a rhythm. For every single trip Ganymede makes around Jupiter, Europa makes exactly two, and Io makes exactly four. One, two, four. Over and over, for billions of years, like three dancers keeping time.

Chapter nine. On the way.

Right now, as these words are read, two spacecraft are crossing the space between the Earth and Jupiter.

One was sent by Europe. It is called Juice... for Jupiter Icy Moons Explorer. It left the Earth in April 2023. It will spend years among the moons and, at the end, settle into orbit around Ganymede. It will be the first spacecraft ever to orbit a moon of another planet.

The other was sent by America. It is called Europa Clipper, and it left in October 2024. It will not land. It will fly past Europa again and again, dozens of times, each time looking down through the ice from a different angle, measuring it, listening to it, tasting the thin gas above it.

Both are expected to arrive at the start of the 2030s.

What will they find under the ice? Perhaps only cold dark water. Perhaps something more. Nobody can say. That is why they are going.

Chapter ten. The king.

Seen from the Earth, Jupiter is just a bright point of light.

It moves so slowly across the sky that from one month to the next you would hardly notice. It takes nearly twelve of our years to go once around the Sun. A child born in the year it enters one constellation will be twelve before it returns there.

And yet, on that point of light, the clouds are racing around a whole world in less than ten hours. A storm larger than the Earth is turning. Rings of auroras are glowing at the poles. On one small moon, volcanoes are erupting. On another, an ocean is moving in the dark, under the ice.

All of it is happening now, tonight, while that quiet point of light hangs in the sky.

If you could stand somewhere high above its clouds, Jupiter would fill the whole sky. The bands would stretch from one side to the other, slowly sliding past one another. The Sun would look small and far away... five times farther than from the Earth, and so it would be much dimmer... a bright, cold star. And around you, the moons would rise and set: Io, glowing faintly orange, Europa, white as a pearl, Ganymede and Callisto, large and grey.

The Romans called it the king of the gods.

They knew only a light that wandered among the stars. They did not know about the storm, or the moons, or the ocean under the ice. But somehow they picked the right name.

There is a storm on Jupiter that is older than anyone now alive.

It is red, and it is wider than the whole Earth. It turns slowly in the southern clouds, as it turned when Galileo's telescope was new, and when the Voyagers flew past, and when the comet fell.

And tonight, somewhere above us, out beyond the edge of the daylight, it is turning still.

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