Transcription complète de Falling Into Jupiter, Slowly Explained for Sleep

discovered by someone looking with their own eye, rather than through a camera. The other three, Metis, Adrastea and Thebe, were first seen in pictures sent home by the Voyager spacecraft. And these little moons have a quiet task. Very slowly, they are making a ring. Jupiter has a ring. Most people never know it is there. It is nothing like the broad, bright rings of Saturn. It is thin, and dark, and very faint, and it was not discovered until nineteen seventy-nine, when Voyager one caught it in a single long-exposure picture. The ring is made of dust, grains as fine as the particles in smoke. Whenever a tiny meteorite strikes Metis, or Adrastea, or Amalthea, or Thebe, a small puff of dust is thrown out into space, and drifts away to join it. The main ring is about six thousand kilometres wide, with a soft halo of dust inside it, and outside, two very faint, gauzy bands called the gossamer rings. From most angles, they can barely be seen. But with the Sun behind them, the fine dust lights up, like motes shining in a beam of afternoon sunlight. Around Jupiter, there is something vast that we cannot see at all. Jupiter has a magnetic field, like the Earth's, but far stronger. It pushes back against the thin wind of particles streaming out from the Sun, and carves a huge bubble in space, which astronomers call the magnetosphere. On the side facing the Sun, it reaches out several million kilometres. On the far side, it trails away in a long tail that stretches past the orbit of Saturn. If our eyes could see it, it would hang in our night sky larger than the full Moon. Inside it, gas from the moon Io is gathered into a faint, glowing ring around the planet, called the Io torus, and swept around by the spinning field, once every ten hours. We have been inside this bubble for some time already, drifting through it, without feeling anything at all. Silent, invisible, and enormous. Closer in, the magnetic field holds something else: belts of tiny particles, racing around the planet at enormous speeds. The Earth has belts like these too, high above us, called the Van Allen belts. But Jupiter's are the most intense of their kind around any planet in the solar system. For a spacecraft, they are a place to pass through quickly and carefully. Juno carries its most delicate parts inside a small vault with walls of titanium, and its path dives in close beneath the belts, through a narrow, calmer gap just above the clouds. Even there, Jupiter had a surprise: in twenty seventeen, Juno found another belt, tucked just above the atmosphere near the equator. But none of it can touch us. We are travelling only in thought, and the racing particles pass through our imagination like light through clear glass. We drift on, untouched. As we come closer, we see a glow around Jupiter's poles. On Earth, the northern lights are lit mostly by the wind from the Sun. Jupiter's are different. Most of their power seems to come from the planet itself: from its fast spin, turning the

gas around it, and pouring energy down along the magnetic field onto the poles. Much of their glow is in colours our eyes cannot see. In infrared light, they shine with an unusual form of hydrogen, a tiny cluster of three hydrogen atoms sharing two electrons, first seen glowing in Jupiter's auroras in nineteen eighty-eight. They give off warmth, too. Jupiter's highest air is hundreds of degrees warmer than the faint sunlight could make it. In twenty twenty-one, a team using the Keck telescopes in Hawaii mapped that warmth, and found what looked like heat spreading from the auroras toward the equator, like the warmth of a fireplace slowly filling a cold house. Beneath the northern lights, there is a strange patch of cold. In twenty seventeen, astronomers using the Very Large Telescope in Chile found it: a region high above the clouds, about two hundred degrees cooler than the air around it, and around twenty-four thousand kilometres from end to end, wide enough to lay nearly two Earths across. They named it the Great Cold Spot, a gentle echo of the Great Red Spot far below. It drifts and changes shape, but it stays in roughly the same place, beneath the aurora. It has been there for at least fifteen years, and may be thousands of years old. A patch of weather made of cold, floating quietly where almost no one had thought to look. Now we come close enough to see the clouds clearly. And the whole planet is wrapped in stripes. There are bright, creamy bands, and darker bands of tan and brown and rust, each one running all the way around the world. The pale bands are called zones, and the darker ones, belts. Astronomers have long thought that in the zones, the air is gently rising, carrying fresh white clouds of ammonia ice up into the sunlight, while in the belts it sinks, and the high clouds thin, so that we see down to darker layers below. Around the middle runs the Equatorial Zone. On either side lie the North and South Equatorial Belts, the two broad dark stripes you can see even in a small telescope. Beyond them, band after band, the stripes continue toward the poles. Seen from above, it looks like a great painted bowl, turning slowly on a potter's wheel. Between the bands, the wind runs. Jupiter is wrapped in jet streams, great rivers of air blowing east, then west, then east again. Each stripe slides past its neighbours the other way. The fastest jet blows in the northern half of the planet, about a quarter of the way from the equator to the pole, at more than five hundred kilometres an hour. The winds are remarkably faithful. They have kept the same places, and much the same speeds, for well over a century. And they go deep. By measuring tiny ripples in Jupiter's gravity, Juno found that the jet streams reach about three thousand kilometres down. Below that, the winds seem to fade, and the whole interior turns together, almost as one smooth, solid ball. Where do Jupiter's colours come from? The honest answer is that no one is entirely sure. The white is the easy part: fresh clouds of ammonia ice, as pale as a cloud on Earth. But hydrogen and helium, the gases that make up most of Jupiter, have no colour at all. So something else, mixed in tiny amounts, must be doing the painting. Scientists call these unknown ingredients chromophores, which simply means colour-bearers. They may be compounds of sulphur, or of phosphorus, or new molecules made when sunlight breaks apart gases high in the clouds. In one experiment, scientists shone ultraviolet light on ammonia mixed with a gas called acetylene, and made a reddish material very close to the colour of the Great Red Spot. Whatever they are, they are only the faintest dusting. The richest colours on Jupiter come from the smallest of traces. Now we sink down to the cloud tops. The highest clouds are made of tiny crystals of frozen ammonia. On Earth, ammonia is a gas with a sharp, clean smell. Here, well below minus a hundred degrees, it freezes into fine white flakes, like frost. A little way beneath these clouds, the air presses down just as hard as our own air does at sea level. Astronomers use that depth as a starting line, measuring the width of Jupiter, and the depth of its storms, from the place where the air pushes just as it does on a beach on Earth. It is the nearest thing Jupiter has to a ground level. But there is no ground here. Only a soft white haze of ice crystals, drifting in a thin golden light, and beneath it, more cloud. Jupiter's stripes are not fixed in place. Sometimes, one of them simply fades away. The South Equatorial Belt is usually one of the darkest, broadest bands on the planet. But every so often, it grows pale over a few weeks or months, and then vanishes, until Jupiter looks strangely bare, with only one dark stripe left. It last happened in twenty ten. Amateur astronomers around the world watched the belt grow fainter through the spring, until it was gone. Most astronomers think it was only hidden, under a high layer of pale ammonia cloud, like fresh snow settling over a dark field. Then, late that year, a small bright spot appeared, and a great stirring spread out from it. Within a few months, the dark belt had returned, as if nothing had happened. The planet is always changing, slowly, in its own time. And now, ahead of us, in the southern half of the planet, a great oval of rust-red cloud comes into view. On Jupiter, there is a storm wider than the whole Earth. It is called the Great Red Spot: a vast oval of rust-coloured cloud. If you could lay our world across it, the Earth would not reach from one side to the other. The storm turns about once every six days, with winds around its edge of more than four hundred kilometres an hour. And it is old. People have been watching it through telescopes for almost two hundred years. On Earth, a storm lasts days, or weeks. This one has outlived everyone who first saw it, and it is still turning. Its clouds stand several kilometres higher than those around it, like

a great soft dome. It is not a hollow of low pressure, like the hurricanes of Earth, but a vast, slow mountain of air, spreading gently outward from its top. In sixteen sixty-five, the Italian astronomer Giovanni Domenico Cassini saw a dark spot in the southern half of Jupiter. He followed it, on and off, for years, and it came to be called the permanent spot. Then, after seventeen thirteen, no one recorded it again for more than a century. The storm we see today was first drawn clearly in eighteen thirty-one, by the German astronomer Heinrich Schwabe, and it has been watched ever since. Were they the same storm? In twenty twenty-four, a team of scientists gathered all the old drawings and records, and concluded that they probably were not. Cassini's spot, they think, faded away, and the Great Red Spot formed later, perhaps from a long stretch of disturbed wind that slowly wound itself up into an oval. So the storm may be younger than we once believed, though it has been turning for at least two centuries. How deep does the Great Red Spot go? For centuries, nobody knew. Then, in twenty seventeen, Juno passed directly overhead, and listened to the faint natural glow of microwaves rising from beneath the clouds. Putting that together with tiny changes in Jupiter's gravity, scientists found that the storm's roots reach somewhere between three hundred and five hundred kilometres down. That is far deeper than any storm on Earth, where even the tallest thunderclouds rarely rise more than about fifteen kilometres. And yet, compared with its width, the Great Red Spot is remarkably thin. Shrink it to the width of a dinner plate, and it would be less than a centimetre deep. A broad, slow pancake of turning air, resting on the deeper atmosphere beneath. It even seems warmer at its base than at its top, as if it draws a little of the planet's deep warmth upward. The Great Red Spot is not the only red storm on Jupiter. In nineteen thirty-nine and nineteen forty, three white oval storms formed a little to the south of it. For almost sixty years, they drifted around the planet side by side, like three pale stones in a stream. Then, in nineteen ninety-eight, two of them merged, and in the year two thousand, that one merged with the third. The new storm was given a plain name: Oval B A. Around the end of two thousand and five, it slowly began to turn red. An amateur astronomer in the Philippines named Christopher Go noticed the change in his own photographs, and soon people everywhere were calling it Red Spot Junior. It is roughly as wide as the Earth. Every couple of years, it drifts quietly past its larger cousin, the two red storms sliding by, never quite touching. Jupiter is full of smaller storms, too. Far to the south, there is a row of white oval storms, evenly spaced, which astronomers call the string of pearls. Juno has photographed eight of them, side by side. In the north, there are long, dark shapes called brown barges, quiet places where the clouds seem to open, and we glimpse darker layers below. And now and then, a new storm appears. In twenty twenty, an amateur astronomer named Clyde Foster was photographing Jupiter from his home in South Africa when he noticed a small bright spot that had not been there before. Two days later, by good fortune, Juno passed overhead and photographed it up close. It is still known as Clyde's Spot. A new storm, first noticed by someone looking up from home. As we drift toward the top of the planet, Jupiter begins to look like a different world. From Earth, we always see Jupiter almost exactly side-on, so its poles are tipped away and hard to see. It was only when Juno began swinging over the top that anyone had a clear view straight down. Near the poles, the tidy stripes fade away. Instead, there is a mottled, bluish grey, scattered with swirls and curls of cloud, like a stormy sea seen from very high above. At each pole, there is a cluster of great cyclones: at the north, eight of them circling a single central one, and at the south, a smaller ring. Each is several thousand kilometres across, and yet they stay apart, year after year. Scientists think each may be wrapped in a thin ring of wind turning the opposite way, a soft cushion between neighbours. Deep in Jupiter's clouds, there are thunderstorms, and in the thunderstorms, there is lightning. It was first seen in nineteen seventy-nine, when Voyager one looked across the dark night side of the planet, and saw bright flashes flickering beneath the clouds. But Juno found something unexpected. Listening for the crackle that lightning makes in radio waves, it heard most of the flashes near the poles, and very few near the equator: the opposite of the Earth, where lightning gathers in the warm tropics. The reason may be sunlight. Near Jupiter's equator, the Sun warms the upper air and keeps it settled, so towering storms struggle to grow. Near the poles, warmth from deep inside can rise more freely. Juno has even caught flashes high in the cold upper clouds, where scientists think slushy hailstones of ammonia and water may form, and tumble back into the deep. Below the white ammonia clouds, there is a second layer, darker and a little warmer. Scientists think it is made of ammonium hydrosulphide, a compound of ammonia and hydrogen sulphide, the gas that gives old eggs their smell. These clouds are thought to be tinted tan and brown, and seen through gaps above, they may help give the dark belts their colour. Here, the air presses about twice as hard as at sea level on Earth, and the temperature has climbed to around seventy degrees below zero. The light is dimmer now. Overhead, the white ceiling of ammonia ice filters the sunlight into a soft, pale glow, like daylight beneath a thick, grey blanket of cloud. Deeper still, we reach the third and lowest layer of cloud. And it is made of something familiar. Water. Here, at about five times the pressure of our air at sea level, it is warm enough for water to

gather into clouds, much as it does in our own sky, perhaps around the temperature of a cool day on Earth. These clouds feed the great thunderstorms, rising through the layers above like towers. Water helps to tell the story of how Jupiter was made. In twenty twenty, Juno measured how much there is in the air around the equator, beneath the clouds, and found that about one in every four hundred molecules is water: a few times more, in proportion, than in the Sun. So somewhere beneath the coloured stripes, in the dim, damp warmth, rain may be falling softly, drop by drop, on a world with no ground to catch it. Once, a small visitor from Earth came drifting down through these clouds. In July nineteen ninety-five, the Galileo spacecraft, still on its way to Jupiter, set free a small probe. For five months, it travelled on alone, in silence, toward the planet. On the seventh of December, it arrived, meeting the air of Jupiter at about forty-seven kilometres every second, one of the fastest arrivals of anything ever made by people. The thin upper air slowed it, wrapping it in a glow hotter than the surface of the Sun. Within minutes, that great speed had melted away, and the glow faded. High above the clouds, all was quiet again. In nineteen ninety-five, a small probe from Earth fell into Jupiter, and spoke to us for fifty-eight minutes. Carried there by the Galileo spacecraft, it drifted down beneath a parachute, measuring everything around it. It felt the winds grow stronger as it sank. It found the air drier, and the clouds thinner, than anyone had expected. Far away, it heard the faint radio crackle of lightning. The deeper it went, the warmer and heavier the air became. Then, about a hundred and fifty kilometres down, its voice fell silent. It is still the only probe ever to send word from inside Jupiter. It had fallen, by chance, into one of the clearest, driest places on the planet, a gap in the clouds called a hot spot: a little like a traveller arriving in a desert, and deciding that the whole Earth must be dry. After its signal stopped, it went on sinking into warmer air, until it slowly melted away, and became a part of Jupiter. Eight years later, Galileo itself followed it, by choice, to keep the icy moons untouched. Below the water clouds, the sky changes. There are no more clouds now, only a deep, clear, thickening haze of hydrogen and helium. The sunlight fades, little by little, and we sink into the dark. But it is not a cold darkness. With every kilometre we fall, the air grows warmer. Where the probe went quiet, it was already about a hundred and fifty degrees, hotter than boiling water. A little further down, it is hotter than any kitchen oven. Deeper still, it climbs past a thousand degrees, and keeps climbing. The air grows heavier, too, until it begins to feel less like air, and more like something you could swim through. If our eyes could see it, the deep air would begin to glow faintly here, a dull, warm red, like embers late at night. Jupiter's own warmth, quietly lighting its own darkness. Jupiter has no surface. There is no ground to stand on. If you could fall into it, you would drift down through the clouds, then through clear, darkening air, growing warmer, and thicker, and heavier the deeper you go. Slowly, the gas begins to behave more like a liquid. But you would never splash into an ocean. There is no shoreline, and no edge. The sky simply becomes a sea, so gradually that no one could say where one ends and the other begins. Thousands of kilometres of it, with no solid place to rest. A whole world, made of sky, all the way down. Physicists call this in-between state supercritical. On Earth, there is a clear line between a gas and a liquid: the surface of a lake, the edge of a raindrop. But above a certain temperature, that line disappears. A substance can no longer boil, or condense. It simply grows denser, smoothly, without ever changing its nature. For hydrogen, that temperature is very low, about two hundred and forty degrees below zero, and everywhere inside Jupiter is far warmer. So the hydrogen can never form a shore. It just thickens, kilometre after kilometre, from a thin sky into a deep, dark, warm fluid. We are drifting now through the largest ocean in the solar system, an ocean of hydrogen, tens of thousands of kilometres deep. No waves, no tides, only the slow rise of warmth from below. Deep inside Jupiter, it may be raining helium. Most of the planet is hydrogen, mixed with a little helium, the gas that fills party balloons. Near the top, the two blend easily, like sugar stirred into warm tea. But thousands of kilometres down, where the heat and pressure are immense, scientists think the helium may stop mixing. It gathers into droplets, and the droplets, heavier than the hydrogen, begin to fall. No one has ever seen it. But the Galileo probe found less helium near the top than expected. It may have been falling for billions of years. A soft, endless rain, deep in the dark. The idea was first suggested in the nineteen seventies, by the physicists David Stevenson and Edwin Salpeter, and in twenty twenty-one, experiments with powerful lasers showed signs that the two really can separate. The rain may carry something else with it. The probe found only about a tenth as much neon as expected, and neon dissolves easily into helium. So the falling drops may gather up the neon as they sink, like raindrops washing dust from the air. Deeper still, perhaps ten thousand kilometres or more below the clouds, something remarkable happens to the hydrogen. Here the pressure is a couple of million times greater than the air at sea level. Under that weight, the simplest atom of all begins to change. Its single electron is pressed so close to its neighbours that it can no longer stay at home. It wanders freely from atom to atom, as electrons do inside a copper wire. The hydrogen has become a metal: a liquid metal, shining and flowing, like a vast, dark sea of

mercury. The idea was worked out in nineteen thirty-five, by the physicists Eugene Wigner and Hillard Huntington. In nineteen ninety-six, William Nellis and his colleagues at the Lawrence Livermore National Laboratory, in California, squeezed a thin layer of hydrogen for less than a millionth of a second, and watched it carry electricity. On Jupiter, it may be most of the planet. That ocean of liquid metal is where Jupiter's magnetic field is made. Heat rising from within keeps the metal slowly churning, and the fast spin twists that churning into spirals. Moving metal carries electric currents, and the currents make a magnetic field. The Earth makes its field in much the same way, in its core of molten iron. But at Jupiter's cloud tops, the field is more than ten times as strong as at the Earth's surface. In twenty eighteen, Juno mapped it more closely than ever before, and found it lumpy and uneven, quite unlike ours. Near the equator lies a small, intense patch that scientists nicknamed the Great Blue Spot, after its colour on their maps. Compared with measurements made decades earlier, the field has shifted. The Great Blue Spot is drifting slowly eastward, as if carried by deep, slow currents in the metal sea. Below the metal sea, we come toward the heart of Jupiter. Most scientists think the planet began, four and a half billion years ago, as a seed of rock and ice, several times heavier than the whole Earth. Once the seed was large enough, its gravity began to pull in the gas around it, faster and faster, until it grew into the giant we see today. People long expected to find that seed still sitting at the centre, small and hard, like a marble buried in sand. But Juno's gravity measurements suggest something softer. The heavy material seems to blend gradually into the hydrogen around it, like a drop of ink in still water, slowly spreading outward until there is no clear edge. The first seed of the largest planet may still be down here. Only now, it is a gentle cloudiness, spread through the deep. And now we come to the very centre of Jupiter, about seventy thousand kilometres below the tops of the clouds. The pressure here may be around forty million times that of our air, and the temperature perhaps twenty thousand degrees, several times hotter than the surface of the Sun. No light from the sky has ever reached this place. And yet it is perfectly still. No wind, no weather. Only warmth, and darkness, and the weight of a whole world, resting evenly from every side. And here is a lovely thing. At the very centre of a planet, gravity pulls equally in every direction at once. So if you could float here, you would feel weightless, held gently in place by the whole of Jupiter around you, pulling you nowhere at all. Nothing here can reach you. We are only visiting, in imagination. Rest here for a while, at the quiet centre of the largest world. Jupiter has a secret, first noticed in the nineteen sixties by astronomers measuring its warmth in infrared light. It gives off more heat than it receives from the Sun. By some measurements, nearly twice as much. Some of that warmth is left over from its birth. Some comes from a slow settling: Jupiter is very gradually shrinking, by perhaps a couple of centimetres a year, and as it settles, its own weight squeezes out a little more heat. The helium rain may add a little warmth of its own. In infrared pictures, Jupiter glows like a lantern, its heat shining up through the gaps between the clouds, brightest of all in the clear, dry hot spots, like the one where the probe once fell. Billions of years after its birth, it is still quietly warm, slowly cooling, like a stone that has rested all day in the sun. Jupiter is sometimes called a failed star. But it was never really close. To shine like the Sun, a ball of gas must squeeze its heart so hard that hydrogen begins to fuse, giving off light. For that, Jupiter would need to be about eighty times heavier than it is. And there is something curious about its size. Add more gas, and it would hardly grow any wider. The extra weight would simply squeeze it tighter, making it denser rather than larger. Jupiter is already close to the largest size a cold ball of hydrogen can be. Between the planets and the smallest stars, there are in-between worlds called brown dwarfs, glowing faintly with their own warmth. Many are almost exactly as wide as Jupiter. So Jupiter is not a failed star. It is something just as wonderful: very nearly as large as a planet can be. Jupiter may be the oldest planet of all. When the Sun was young, it was surrounded by a wide, flat disc of gas and dust, and the planets grew slowly within it. Jupiter, it seems, grew first, and quickly. The clue comes from meteorites, small stones that fall to Earth from space. In twenty seventeen, a team led by the scientist Thomas Kruijer found that they come in two great families, which seem to have been kept apart, very early on, for millions of years. Something had divided the young disc in two. The best explanation, they suggested, was Jupiter itself: already more than ten times heavier than the Earth within perhaps the first million years, holding the inner and outer parts of the disc apart, like a long, slow wall of gravity. If so, Jupiter was here before the Earth. Jupiter may not always have been where it is now. In twenty eleven, a team of astronomers led by Kevin Walsh proposed a sweeping idea. Early on, the young Jupiter may have drifted inward, toward the Sun, perhaps as close as the place where Mars circles today. Then Saturn, growing further out, followed it in, until the two giants fell into step, turned around together, and drifted slowly back out to where they are now. The idea is called the Grand Tack, after the way a sailing boat tacks, turning against the wind in a long, graceful curve. If it happened, Jupiter would have swept away much of the material where Mars was forming, which might explain why Mars is so small. It is only

one idea, still being tested. But it is a lovely picture: two great worlds, sailing in and out across the young solar system. Jupiter does not really go around the centre of the Sun. When two things circle each other, they both turn around a shared balance point between them, like two dancers holding hands, spinning around the space in between. For most planets, that point lies deep inside the Sun. But Jupiter is so heavy that its balance point with the Sun lies just outside the Sun's surface. So the Sun itself moves in a small, slow circle, once every twelve years, swaying gently around an empty point in space. And far away, astronomers have found new planets by watching for that same gentle sway in other stars. Jupiter's gravity reaches far across the solar system. In the belt of asteroids, there are empty lanes, first noticed in eighteen sixty-six by the American astronomer Daniel Kirkwood. An asteroid in one of those lanes would meet Jupiter at the same part of its path, again and again, and each small tug would nudge it a little further, until it moved away. Along Jupiter's own path, there are two great swarms of asteroids, one travelling ahead of the planet, one behind. They are called the Trojans, after the heroes of the Trojan War. The first, Achilles, was found in nineteen oh six by the German astronomer Max Wolf. Many thousands are known today, and a spacecraft named Lucy is on its way to visit a few. People sometimes say Jupiter shields the Earth from comets. The truth seems more mixed: it turns some aside, and sends some inward. Less a shield, perhaps, than a shepherd. In March nineteen ninety-three, three astronomers at the Palomar Observatory in California, Carolyn Shoemaker, Eugene Shoemaker and David Levy, found a strange comet on one of their photographs, shaped like a long row of glowing beads. A year earlier, it had passed so close to Jupiter that the planet's pull had gently drawn it apart, into more than twenty pieces, travelling one behind another. It was named Shoemaker-Levy nine. And it was coming back. Over six days in July nineteen ninety-four, the pieces met Jupiter, one after another, just out of our sight, and Jupiter's quick spin soon carried those places into view. Dark marks bloomed in the southern clouds, some larger than the Earth, clear enough to see in small telescopes. Over the following months, they softened, and faded, and the clouds closed over them. In July two thousand and nine, an amateur astronomer in Australia named Anthony Wesley noticed a fresh dark mark near Jupiter's south pole. Another small visitor had been quietly taken in. Now it is time to go home. In our minds, we rise slowly from the still centre of the planet. Up through the blurred seed and the shining sea of metal. Up through the soft helium rain and the warm, dark ocean of hydrogen, thinning around us, little by little, into sky. We rise through the water clouds, and the brown middle clouds, and out through the white frost of ammonia, into the pale golden light of the distant Sun. Below us, the bands spread out, cream and tan and rust. The Great Red Spot turns in its place, and Red Spot Junior drifts by. We rise past the glow of the auroras and the faint, dusty ring. The four great moons sail by, each in its turn. And slowly, the vast striped world behind us becomes a golden ball, then a small round disc, then a single point of light. And here we are, back home, on a quiet night, looking up. There it is again, that bright, steady, golden light. After the Moon and Venus, Jupiter is usually the brightest thing in the night sky. And because it is a tiny disc, rather than a single point like a star, it barely twinkles. It simply shines, while the stars flicker around it. It takes Jupiter almost twelve years to circle the Sun, so each year it moves on by about one constellation. Astronomers in ancient China called it the Year Star, and used it to keep track of time. With a pair of binoculars, held steady, you can see three or four tiny points of light beside it, the same moons we passed tonight, changing places from one night to the next. And the light reaching your eyes left Jupiter only about forty minutes ago, brushing over those bands and storms, just to arrive here, quietly, in the dark. Let us come home now, slowly, from all of it. The pale golden light in the evening sky. The four moons passing. The faint ring of dust, lit from behind by the distant Sun. The white frost of ammonia. The bands, sliding gently past each other. The great red storm, turning in its lane, as it has turned for two hundred years. The small probe, drifting down beneath its parachute, listening, until it fell silent. The sky that slowly became a sea. The soft rain of helium, falling through the deep. The still, warm centre, where nothing pulls at all. All of it is very far away now, turning quietly in the dark, while its moons circle and its storms drift, as they have for billions of years. And here, the night is soft and still. You are warm, and safe, and it is time to sleep. Sleep well.