Knowledge EncyclopediaDK, Smithsonian
A bold new approach to family reference that uses 3-D rendered images to explore the wonders of the world in unprecedented detail-from such categories as space, the human body, and history.
Created in association with the Smithsonian Institution and using the latest CGI technology to illustrate concepts, Knowledge Encyclopedia is divided into six chapters-Space, Earth, Nature, Human Body, Science & Technology, and History & Culture. These chapters combine with a reference section in bringing a wide range of topics to life. Illustrated with fascinating facts, maps, timelines, and graphics, this reference book makes complex subjects easy to understand.
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ENCYCLOPEDIA smithsonian ENCYCLOPEDIA LONDON, NEW YORK, MELBOURNE, MUNICH, AND DELHI US Senior Editor Rebecca Warren US Editor Kate Johnsen Senior Editors Shaila Brown, Daniel Mills, Ben Morgan CONTENTS Senior Art Editors Vicky Short, Smiljka Surla Editors Lizzie Munsey, Sam Priddy, Alison Sturgeon Designers Daniela Boraschi, Tannishtha Chakraborty, Richard Horsford, Hedi Hunter, Fiona Macdonald SPACE Visualizer Peter Laws Illustrators Peter Bull, Rob Cook, FOREAL™, Mike Garland, Mark Garlick, Gary Hanna, Jason Harding, Arran Lewis, Maltings Partnership, Medi-Mation, Peter Minister, Gerson Mora and Anna Luiza Aragão/Maná e.d.i., Moonrunner Design, Ian Naylor, Alex Pang, Dean Wright and Agatha Gomes DK Picture Library Emma Shepherd, Rob Nunn Jacket Designer Laura Brim Producer, pre-production Francesca Wardell Producer Alice Sykes Managing Editors Julie Ferris, Paula Regan Managing Art Editor Owen Peyton Jones Publisher Sarah Larter Art Director Phil Ormerod Associate Publishing Director Liz Wheeler Publishing Director Jonathan Metcalf Contributors Kim Bryan, Robert Dinwiddie, Jolyon Goddard, Ian Graham, Reg G. Grant, Jacqueline Mitton, Darren Naish, Douglas Palmer, Philip Parker, Penny Preston, Sally Regan, David Rothery, Carole Stott, Paul Sutherland, Chris Woodford, John Woodward First American Edition, 2013 Published in the United States by DK Publishing, 345 Hudson Street New York, New York 10014 13 14 15 16 17 10 9 8 7 6 5 4 3 2 1 001—187527—Oct/13 Copyright © 2013 Dorling Kindersley Limited All rights reserved Without limiting the rights under copyright reserved above, no part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of both the copyright owner and the above publisher of this book. Published in Great Britain by Dorling Kindersley Limited. A catalog record for this book is available from the Library of Congress. ISBN: 97; 8-1-4654-1417-5 DK books are available at special discounts when purchased in bulk for sales promotions, premiums, fund-raising, or educational use. For details, contact: DK Publishing Special Markets, 345 Hudson Street, New York, New York 10014 or SpecialSales@dk.com. Printed and bound in Hong Kong by Hung Hing Printing Group. Discover more at www.dk.com THE UNIVERSE The Big Bang Galaxies Star birth Star death The Sun The Solar System Inner planets Outer planets The Moon 10 12 14 16 18 20 22 24 26 28 SPACE EXPLORATION Astronomy Mission to the Moon Exploring the planets 30 32 34 36 THE SMITHSONIAN Established in 1846, the Smithsonian—the world’s largest museum and research complex—includes 19 museums and galleries and the National Zoological Park. The total number of artifacts, works of art, and specimens in the Smithsonian’s collection is estimated at 137 million. The Smithsonian is a renowned research center, dedicated to public education, national service, and scholarship in the arts, sciences, and history. EARTH NATURE PLANET EARTH Inside the Earth Earth’s climate 40 42 44 TECTONIC EARTH Plate tectonics Volcanoes Earthquakes 46 48 50 52 EARTH’S RESOURCES Rocks and minerals 54 56 WEATHER Hurricanes The water cycle 58 60 62 SHAPING THE LAND Caves Glaciers 64 66 68 EARTH’S OCEANS The ocean ﬂoor 70 72 HOW LIFE BEGAN Timeline of life The dinosaurs Tyrannosaurus rex How fossils form 76 78 80 82 84 THE LIVING WORLD Plant life Green energy 86 88 90 INVERTEBRATES Insects Butterﬂy life cycle 92 94 96 VERTEBRATES Fish Great white shark Amphibians Frog life cycle Reptiles Crocodile Birds How birds ﬂy Mammals African elephant 98 100 102 104 106 108 110 112 114 116 118 SURVIVAL SECRETS Habitats American desert Amazon rainforest African savanna Coral reef Animal architects Predators and prey 120 122 124 126 128 130 132 134 HUMAN BODY SCIENCE BODY BASICS Building blocks The skeleton Muscle power The skin 138 140 142 144 146 FUELING THE BODY From mouth to stomach The intestines In the blood The heart Fighting germs Cleaning the blood Air supply 148 150 152 154 156 158 160 162 IN CONTROL Nervous system Brainpower How vision works Inside the ear Taste and smell Control chemicals 164 166 168 170 172 174 176 LIFE CYCLE A new life Life in the womb Growing up Genes and DNA 178 180 182 184 186 MATTER Atoms and molecules Atom smasher Solids, liquids, and gases The elements Chemical reactions Material world 190 192 194 196 198 200 202 FORCES Laws of motion Engines Simple machines Flotation Magnetism Gravity Flight 204 206 208 210 212 214 216 218 ENERGY Electromagnetic spectrum Signals from space Light Telescopes Sound Heat Electricity Power network Radioactivity 220 222 224 226 228 230 232 234 236 238 ELECTRONICS Digital world Robotics 240 242 244 HISTORY THE ANCIENT WORLD The ﬁrst humans The ﬁrst towns Early empires Ancient Egypt The pharaohs Ancient Greece Ancient Athens The Roman Empire Roman society 248 250 252 254 256 258 260 262 264 266 THE MEDIEVAL WORLD Viking raiders Fortresses Wars of faith World religions The Ottoman Empire The Silk Road Samurai warriors 268 270 272 274 276 278 280 282 THE AGE OF DISCOVERY Voyage to the Americas Ancient Americas The Renaissance Shakespeare’s theater 284 286 288 290 292 Imperial China Rulers of India THE MODERN WORLD The slave trade The Enlightenment The American Revolutionary War The French Revolution The Industrial Revolution The Age of Steam The Civil War World War I Trench warfare World War II Modern warfare The Cold War The 1960s The 21st century REFERENCE GLOSSARY INDEX ACKNOWLEDGMENTS 294 296 298 300 302 304 306 308 310 312 314 316 318 320 322 324 326 328 346 350 359 SPACE When you look into the blackness of the night sky, you are peering into the fathomless depths of the Universe. Stars, planets, and galaxies stretch into space, not just farther than you can see, but farther than you can imagine. 10 THE UNIVERSE The Universe is the whole of existence—all of space, matter, energy, and time. The Universe is so vast that it seems unimaginable, but we do know that it has been steadily expanding following its beginning 13.8 billion years ago in an explosive event called the Big Bang. UNDERSTANDING THE UNIVERSE People used to think of the Universe as a giant sphere, but we now know that things are not so simple. The Universe probably has no center or outer edge. Only a fraction of it—the observable Universe—is visible to us. The whole Universe may be vastly bigger than this, perhaps inﬁnitely so. CELESTIAL BODIES The Universe is at least 99.999999999999 percent empty space. Floating in this vast, dark void are all sorts of different objects, which astronomers call celestial bodies. They range from grains of dust to planets, stars, and galaxies. Our Solar System includes a star (the Sun) and a large family of planets and moons that formed from the same cloud of gas that gave birth to the Sun. In recent years, planets have been seen around hundreds of other stars, showing that our Solar System may be one of billions in our galaxy. Asteroid Rocky lumps left over from the formation of the Solar System are called asteroids. They range in size from boulders to bodies close to the size of a dwarf planet. Looking back in time Closed A dense Universe would bend itself into a closed shape. Traveling in a straight line would bring you back to your starting point. Because light takes time to travel, when we look into space we are looking back in time. The most distant objects visible are galaxies photographed by the Hubble Telescope. We see them as they were 13 billion years ago. The Universe extends far beyond these, but it’s impossible to see objects much further because their light hasn’t had time to reach us. Furthest objects The light from the faintest galaxies in this photo from the Hubble Space Telescope took 13 billion years to reach Earth. The shape of space The three dimensions of space are bent by the force of gravity from matter in the Universe into a fourth dimension that we can’t see. This is hard to visualize, so scientists use the metaphor of a two-dimensional rubber sheet to explain the idea. The mass of the Universe could bend this rubber sheet in one of three ways, depending on how densely packed with matter the Universe is. Most scientists now think the shape of the Universe is ﬂat. Open If the Universe isn’t dense enough, it might stretch into an open shape, making it inﬁnite in size with no outer edge. Flat Just the right amount of matter would give the Universe a ﬂat shape. This would also be inﬁnite in size with no outer edge. What’s the matter? The elements hydrogen and helium make up 98 percent of the matter we can see in the Universe. But there doesn’t seem to be enough matter to account for the way stars and galaxies are pulled by gravity. As a result, astronomers think galaxies contain dark matter, which we cannot see. There is also an unknown force making the Universe expand, known as dark energy. 27% dark matter 23% helium 75% hydrogen 68% dark energy 5% matter 2% other elements MAKEUP OF THE UNIVERSE THE SCALE OF SPACE The Universe is so vast that we cannot appreciate its size without making leaps of scale. In this series of pictures, each stage represents a microscopic speck of the image to its right. When dealing with the vast distances in space, miles aren’t big enough. Instead, astronomers use the speed of light as a yardstick. Light is so fast it can travel around the Earth 7.5 times in a second. One light year is the distance light travels in a year: nearly 6 trillion miles (10 trillion km). Earth and Moon Earth is 7,926 miles (12,756 km) wide. Our nearest neighbor in space—the Moon— orbits Earth at a distance of 238,855 miles (384,400 km). If Earth were the size of a soccer ball, the Moon would be the size of a cantaloupe about 69 ft (21 meters) away. Solar System The Sun’s family of eight planets occupy a region of space 5.6 billion miles (9 billion km) wide. If Earth were a soccer ball, it would take ﬁve days to walk across this part of the Solar System. The nearest star would be a 58-year walk away. Stellar neighborhood The nearest star to the Sun is Proxima Centauri, which is just over four light years away. There are around 2,000 stars within 50 light years of the Sun. These make up our stellar neighborhood, which is a tiny fraction of the Milky Way galaxy. 11 Comet Comets are chunks of ice from the outer reaches of the Solar System. Some grow long tails of gas and dust as they approach the Sun and are warmed by it. Moon Also called a natural satellite, a moon is a body that orbits a planet. Earth has only one moon but the planet Jupiter has 67, including Io (above). IS THERE ANYBODY OUT THERE? One of the biggest questions in science is whether life is unique to Earth or has arisen on other worlds. And if life has appeared elsewhere, could intelligent beings have evolved? Scientists have set up projects to watch and listen for signals from extraterrestrials, and messages have been sent to the stars to inform any aliens out there of our existence. Dwarf planet Dwarf planets are larger than asteroids but smaller than planets. Like planets, they are round in shape. Pluto (above) is the best known dwarf planet. Arecibo message In 1974, scientists used the giant Arecibo radio telescope in Puerto Rico to broadcast a radio message toward the star cluster M13. The message contains symbols (right) that represent human beings, our base-10 counting system, the DNA molecule, and the Solar System. More a publicity stunt than a serious attempt to contact aliens, the broadcast will take 25,000 years to reach M13, and a reply will take 25,000 years to return. Planet A planet is a large and nearly spherical object that orbits a star and has swept its orbital path clear of debris. The Solar System has eight planets. Stars These luminous balls of gas, such as the Sun, shine by generating their own nuclear power. Stars come in a wide range of types, temperatures, and sizes. Numbers 1 to 10 in binary, reading left to right Chemical formula of DNA (the molecule that carries the blueprint of life) Nebula A glowing cloud of gas and dust in space is known as a nebula. Some nebulae are clouds of wreckage created by dying stars. Others give birth to new stars. Pioneer plaque The robotic spacecraft Pioneer 10 and Pioneer 11 visited the planets Jupiter and Saturn in 1973–74 and then ﬂew off into deep space. If aliens ever discover either craft drifting through interstellar space, they will ﬁnd a gold-plated plaque engraved with a message from Earth. Hydrogen atom Man and woman in front of Pioneer craft Shape of DNA molecule SETI Astronomers involved in the SETI (search for extraterrestrial intelligence) project use powerful radio telescopes to scan the skies in search of artiﬁcial radio signals broadcast by alien civilizations. The SETI project has been running since 1960, but it has so far found no conclusive evidence of alien signals, despite some false alarms. Milky Way galaxy The Milky Way is a vast cloud of 200 billion stars. Its shape resembles a pair of fried eggs held back to back, with a central bulge surrounded by a ﬂat disk. It measures 100,000 light years across the disk and 2,000 light years deep through the bulge. Human ﬁgure and population of Earth in 1974 Earth’s position in Solar System RADIO TELESCOPE Local Group of galaxies The Milky Way is just one of perhaps seven trillion galaxies in the observable Universe. Galaxies exist in groups called clusters, held together by gravity. The Milky Way is part of a cluster known as the Local Group, which is about 10 million light years wide. Arecibo Telescope, which sent message Supercluster Clusters of galaxies exist in even larger groupings called superclusters. We live in the Virgo Supercluster, which is one of millions of superclusters in the known Universe. Between these are immense empty areas called cosmic voids. Position of Sun in Milky Way galaxy Plan of Solar System, with Pioneer’s ﬂight path Universe Superclusters are thought to form a vast web of ﬁlaments riddled with enormous voids containing no galaxies. The true size of the Universe is a mystery, and only a fraction of it is visible to us. The Universe may even be inﬁnite in size. 12 space 570 THE UNIVERSE The Big Bang thousand million million million miles (1 million million million million kilometers)—the diameter of the observable Universe. The expanding Universe About 14 billion years ago, the Universe materialized out of nothing for unknown reasons. Inﬁnitely smaller than an atom to begin with, the Universe expanded to billions of miles across in under a second—an event called the Big Bang. Time came into existence when the Universe began, so the question “What happened before?” has no meaning. Space also came into existence. The Big Bang was not an explosion of matter through space—it was an expansion of space itself. At ﬁrst the Universe consisted of pure energy, but within a trillionth of a second some of this energy turned into matter, forming a vast soup of subatomic particles (particles smaller than atoms). It took nearly 400,000 years for the particles to cool down enough to form atoms, and then another 300 million years before the atoms formed planets, stars, and galaxies. The expansion that began in the Big Bang continues to this day, and most scientists think it will carry on forever. The illustration below does not show the shape of the Universe, which is unknown. Instead, it is a timeline that shows how the Universe has expanded and changed since the Big Bang. We know the Universe is expanding because the most distant galaxies are rushing apart at rapid speeds. By running the clock backward, astronomers ﬁgured out that the expansion began 13.8 billion years ago at a single point: the Big Bang. Rate of expansion increases First galaxies form Stars form The Universe began as something called a singularity: a point of zero size but infinite density. 9 Atoms form Protons and neutrons form Energy turns into particles 8 The Universe begins 7 6 5 4 2 Within a tiny fraction of a second, the Universe balloons in size from trillions of times smaller than an atom to the size of a city. The rate of expansion then slows. 3 2 1 1 The Universe appears out of nowhere. At the start, the Universe consists purely of energy and is inﬁnitely dense and unimaginably hot—18 billion trillion trillion °F (10 billion trillion trillion °C). 3 The intense energy of the newborn Universe creates matter. At ﬁrst, the matter is a soup of particles and antiparticles. These crash into and cancel each other out, turning back into energy. But some of the matter is left over— this will eventually turn into atoms and later stars and galaxies. 13 11 Discovery of the Big Bang The ﬁrst scientiﬁc evidence for the Big Bang was found in 1929, when astronomers discovered that light from distant galaxies is reddened. This color change happens when objects are moving away from us, making lightwaves stretch out and change color. The more distant the galaxies are, the faster they are rushing away. This shows that the whole Universe is expanding. STATIC UNIVERSE No change in starlight 10 EXPANDING UNIVERSE Light waves stretched Big Bang afterglow More evidence of the Big Bang came in the 1960s, when astronomers detected faint microwave radiation coming from every point in the sky. This mysterious energy is the faded remains of the intense burst of energy released in the Big Bang. MICROWAVE MAP OF WHOLE SKY Solar System forms 24% helium Changing elements At 300 million years, stars appear. Stars form when great clouds of gas are pulled into tight knots by gravity. The pressure and heat become so intense in the dense pockets of gas that nuclear reactions begin, igniting the star. 7 4 The Universe is now about 1 microsecond old and 60 billion miles (100 billion km) wide. The leftover particles begin to form protons and neutrons—the particles that today make up the nuclei of atoms. But the Universe is too hot for atoms to form yet. Light cannot pass through the sea of particles, so the young Universe resembles a dense fog. 5 After 379,000 years, the Universe cools enough for atoms to form. The Universe is now an enormous cloud of hydrogen and helium. Light can pass through space more easily, and the Universe becomes transparent. 6 Half a million years after the Big Bang, matter is spread out almost evenly in the Universe, but tiny ripples exist. Working on these denser patches, gravity begins pulling the matter into clumps. 8 At 500 million years, the ﬁrst galaxies are forming. Galaxies are enormous clouds of stars, held together by gravity. 9 Now 5 billion years old, the Universe consists of vast clusters of galaxies arranged in threads, with gigantic voids between them. The voids get ever bigger as space continues to expand. At 8 billion years, the expansion of the Universe begins to accelerate. 10 Our Solar System forms at 9 billion years. When the Universe is 20 billion years old, the Sun will expand in size and destroy Earth. 11 The Universe will carry on expanding forever, becoming cold and dark everywhere. For hundreds of millions of years, the Universe consisted almost entirely of hydrogen and helium —the very simplest chemical elements. After stars appeared, new elements began to be made in the cores of dying stars. All the complex elements in our bodies were forged in dying stars this way. 76% hydrogen EARLY UNIVERSE 75% hydrogen 23% helium 1% oxygen 0.4% carbon 0.4% neon 0.1% iron 0.1% nitrogen + traces of other UNIVERSE TODAY Big Bounce theory What caused the Big Bang? We may never know for sure, but some scientists have suggested that there may have been lots of big bangs, with the Universe expanding after each one and then shrinking again. This theory is called the Big Bounce because the process repeats itself. Universe expands Big Bang Universe shrinks TIME 14 space THE UNIVERSE Galaxies Our Sun belongs to a giant whirlpool of stars called the Milky Way. Huge collections of stars are called galaxies, and like all galaxies the Milky Way is unimaginably vast. Galaxies come in many shapes and sizes. Some are spirals like our own galaxy, but others are fuzzy balls or shapeless clouds. The smallest have just a few million stars. The largest contain trillions. Although they look packed with stars, galaxies are mostly empty space. If you made a scale model of the Milky Way with a grain of sand for each star, the nearest star to the Sun would be 4 miles (6 km) away. The furthest would be 80,000 miles (130,000 km) away. The stars in a galaxy are held together by gravity and travel slowly around the galactic heart. In many galaxies, including ours, a supermassive black hole lies hidden in the center. Stars and other material are sucked into this cosmic plughole by gravity and disappear forever. 200 23 billion—the approximate number of stars in the Milky Way galaxy. —the number of times our Solar System has orbited the Milky Way. The Milky Way Orion Arm Our Solar System lies in this small arm. Many of the stars we see in the night sky are in the Orion Arm. If you could look down on the Milky Way galaxy from above, the view would be like ﬂying over a glittering city at night. Most of the galaxy’s 200 billion stars are in the central bulge. Curving around this are two vast spiral arms and several smaller arms. The Milky Way is thought to be a barred spiral (see panel), but we can’t see its shape clearly from Earth since we view it from the inside. In the night sky, the Milky Way appears only as a milky band of light. MILKY WAY OVERHEAD VIEW U SC T UM CE RUS U A NT 10 ,00 0 1 Galactic center This photo from an infrared (heat-sensitive) telescope shows stars and gas clouds packing the center of the Milky Way. A supermassive black hole lies hidden somewhere in this area. 20 ,00 NO 2 Solar System Our Solar System is in a minor spiral arm called the Orion Arm. We orbit the center of the galaxy once every 200 million years, traveling at about 120 miles (200 km) a second. 3 Crab Nebula Clouds of gas and dust occur throughout the Milky Way, especially in the spiral arms. The Crab Nebula is a cloud of wreckage left behind by a dying star that exploded. 30 40 ,0 00 lig , 00 0 li ght-y ears ht yea rs MILKY WAY SIDE VIEW 4 Globular cluster Not all the Milky Way’s stars are in the main disc of the galaxy. Many are in globular clusters—tightly packed balls of ancient stars ﬂoating above and below the galaxy in a spherical region called the halo. Main disc containing arms 0 ligh t-years RM AA RM AR M light-years 7 6,000 trillion—the approximate number of galaxies in the observable Universe. years—the length of time it would take to count the Milky Way’s stars at a rate of one a second. Perseus Arm This is one of the Milky Way’s two major spiral arms. We see stars in this arm when we look toward the constellation Perseus. 15 Galaxy shapes Astronomers classify galaxies into just a few main types, depending on the shape we observe from Earth. Circles show distance from the center of the galaxy. 3 2 YOU ARE HERE SAG IT Spiral A central hub of stars is surrounded by spiral arms curving out. Barred spiral A straight bar runs across the center, connecting spiral arms. Elliptical More than half of all galaxies are simple ball shapes. Irregular Galaxies with no clear shape are classified as irregular. TA R SA IU RM CROSS SECTION THROUGH MAIN DISC OF GALAXY Bulge formed by spiral arm Central bar 1 How spiral arms form P E RS EU R SA The stars in a galaxy orbit the center, taking millions of years to make one circuit. Spiral arms appear where stars pass in and out of crowded areas, like cars passing temporarily through a traffic jam. One theory is that these traffic jams happen because the orbits of different stars don’t line up neatly. M If stars all had neat, parallel orbits, the galaxy would have no spiral arms. Gas cloud If stars’ orbits don’t line up neatly, crowded zones form, giving the galaxy spiral arms. Colliding galaxies Dark lane formed by dust Sometimes galaxies crash and tear each other apart. Individual stars don’t collide, but gas clouds do, and gravity pulls the colliding galaxies into new shapes. Scutum-Centaurus Arm This is the second of the two main spiral arms. The area where it joins the central bar is rich in star-forming clouds. Central bulge 4 Globular cluster End of the Milky Way In 4 billion years our galaxy will collide with the Andromeda galaxy. This artist’s impression shows what the sky might look like as they merge. 16 space Star birth The clouds that give birth to the stars are cold and dense and consist mainly of hydrogen gas. The newly formed stars are huge spinning globes of hot, glowing gas—mainly hydrogen, with helium and small amounts of other elements. Much of this material is packed tightly into the stars’ cores, and it is here that nuclear reactions release energy in the form of heat and light. 1 Interstellar cloud Stars are born within enormous, cold, dense clouds of gas and dust. The process of star formation may be triggered if something disturbs the cloud, such as a collision with another cloud or a shockwave from a supernova explosion. Starbirth nebulas Clouds of gas and dust in space are called nebulas. Much of the gas and dust in a nebula is debris from old stars that exploded when they ran out of fuel. Over millions of years, this material is recycled to make new stars. Starbirth nebulas are among the most beautiful objects in space, their colorful clouds illuminated from within by the blue light of newborn stars. Orion Nebula The Orion Nebula is one of the closest star-forming regions to Earth. In the night sky it looks like a fuzzy star in the sword of Orion. In reality, it is a vast cloud of gas and dust thousands of times bigger than the Solar System. billion years—the age of the oldest known star. The largest stars in the night sky are big enough to swallow our Sun a billion times. Stars have been forming throughout the Universe for most of its life. They take shape in vast clouds where thousands of stars are born at a time. Interstellar gas cloud 13 The brightest stars emit 6 million times more light than the Sun. THE UNIVERSE How new stars form The star-forming process begins when the cloud becomes unstable and breaks up into fragments. Gravity pulls the material in a fragment into an ever-tighter clump, and the clump slowly forms a sphere as it shrinks. Now a protostar, this star-to-be keeps on shrinking, its core getting denser and hotter. Eventually the pressure and temperature are so high that nuclear reactions begin, and the star starts to shine. Cloud breaks up into fragments 2 Fragments form Now unstable, the cloud breaks up into fragments of different size and mass. The most massive and dense of these fragments are gradually pulled by their own gravity into tighter clumps. These shrinking fragments will eventually turn into protostars. Protostar 3 Protostar A protostar forms. Gravity pulls material into its core, where the density, pressure, and temperature build up. The more matter the original cloud fragment contained, the greater the temperature and pressure rise as the protostar develops. Rotating disc 17 Jets of gas 4 Spinning disc The growing mass at the center creates a gravitational pull, drawing ever more gas and dust inward. A little like water going down a drain, the material being pulled in starts to spin around. Powerful winds develop, blowing jets of gas out from the center. Star clusters Stars are not formed singly—they are born in clusters from the same cloud of material at roughly the same time. Eventually, the stars of a cluster will drift apart and exist alone in space, or with a close companion or two. Our Sun, like about half of the stars nearest to us, is alone. About a third of the stars in the night sky are in pairs, bound together by gravity. Pleiades cluster A handful of the 5,000 or so stars that make up the Pleiades cluster can be seen with the naked eye. In about 250 million years time, the stars will have dispersed and the cluster will no longer exist. Supergiant MA Blue supergiants LUMINOSITY IN SEQ UE NC E S TA R S Red giants Sun White dwarfs Dimmer A star begins to shine when nuclear reactions in its core convert hydrogen into helium and release energy. It is then called a main sequence star. Not all main sequence stars are the same—they differ in size, temperature, color, brightness, and the amount of matter they contain. When stars begin to run out of fuel and near the end of their lives, they stop being main sequence stars and may swell up and turn into red giants or shrink to become white dwarfs. Brighter Types of star Hotter TEMPERATURE Cooler Leftover material may form planets Star begins to shine 5 A star is born Squeezed by the force of gravity, the protostar’s core becomes so hot and dense that nuclear reactions occur, and the star begins to shine. The glowing core produces an outward pressure that balances the inward pull of gravity, making the star stable. It is now a main sequence star. Classifying stars The Hertzsprung–Russell diagram is a famous graph that astronomers use to classify stars. The graph plots brightness against temperature and reveals that there are distinct groupings of stars, such as red giants (dying stars) and main sequence stars (ordinary stars). Astronomers also classify stars by color, which is linked to their temperature: hot stars are blue; cooler stars are orange or red. 6 Planets form Not all the material from the gas cloud has been used to make the star. The leftovers form a spinning disc of gas and dust around the star. This debris may be lost into space, or it may clump together to form planets, moons, comets, and asteroids. 18 space THE UNIVERSE Supergiant stars can grow to 1 billion times the volume of our Sun. Four ways to die Stars can die in four different ways, all of which are shown on these pages. Our Sun, a typical star, will follow the central path, but not yet—it has enough fuel to keep shining for 5 billion years. When larger stars die, they turn hydrogen into heavier chemical elements such as carbon and oxygen, which are later recycled to form new stars and planets. All the atoms in your body were created this way. 5 billion tons—the weight of one teaspoonful of material from the core of a neutron star. Stable star Every young star goes through a stable phase in which it shines steadily. Star death All stars eventually run out of fuel and die. Most fade away quietly, but the most massive stars self-destruct in a huge explosion that can outshine an entire galaxy. Like Earth, stars generate the force of gravity, which squeezes their hot cores. The more matter a star has, the greater the force of gravity and the hotter and denser the core becomes. The way a star dies depends on how much matter it contains (its mass) and how powerfully its core is squeezed by gravity. Stars make heat and light by the process of nuclear fusion: hydrogen atoms in the core crash together to form helium, releasing energy. In small stars, when hydrogen in the core runs out, the star’s light slowly fades. But in more massive stars, the core is so hot and dense that fusion can spread beyond it, changing the star’s appearance. The most massive stars are eventually overwhelmed by their own gravity, which crushes them so violently that they collapse into a pinprick to create a black hole. Small stars Stars with less than half the mass of the Sun fade away very slowly. Once the hydrogen in the core is used up, the star begins to feed off hydrogen in its atmosphere. But it doesn’t generate enough gravity to use other elements as fuel, so it slowly shrinks to become a black dwarf. This will take far longer than the age of the Universe—up to a trillion years. Medium stars When a Sunlike star has used up the hydrogen in its core, nuclear fusion spreads outside the core, making the star expand into a red giant. The core collapses until it is hot and dense enough to fuse helium, but eventually it runs out of helium too. Finally, it becomes a white dwarf, and its outer layers spread into space as a cloud of debris. Massive stars Stars over eight times more massive than our Sun end their lives in strange and violent ways. The heat and pressure inside the core become so great that nuclear fusion can not only fuse hydrogen atoms together to form helium but can fuse helium and larger atoms to create elements such as carbon or oxygen. As this takes place, the star swells into the largest star of all: a supergiant. Star begins to shrink Star expands Star expands 1 1,600 teaspoonful of material from a red giant weighs less than a grain of salt. Light intensity fades as fuel runs out Star continues to shrink and fade Red giant Nuclear fusion spreads to the layer around the core, heating it up and making the star expand. Nearby planets may be swallowed up by the growing giant. Outer layer light years—the distance from Earth to the nearest black hole. Light becomes increasingly dim 19 Black dwarf Finally, its fuel used up and its light extinguished, the star becomes a black dwarf— an Earth-sized cinder. Planetary nebula The star’s outer layers disperse into space as a glowing cloud of wreckage —a planetary nebula. The material in this cloud will eventually be recycled to form new stars. White dwarf All that remains is the dying core—a white dwarf. This Earth-sized star will slowly fade and become a cold, dead black dwarf. Core Neutron star Up to three times heavier than the Sun, yet just a few miles wide, neutron stars are unimaginably dense, fast-spinning stars. Red supergiant The star has grown into a supergiant. Nuclear fusion carries on inside the core, forcing atoms together to form heavier and heavier elements, until the star’s core turns into iron. When this happens, the core no longer generates enough outward pressure to resist the crushing force of gravity, and the whole star suddenly collapses, causing a catastrophic explosion—a supernova. Supernova The star self-destructs in an explosion brighter than a billion suns. Its outer layers are blasted into space, but its massive core continues to collapse in on itself. What happens next depends on how massive the core is. A smaller core becomes a neutron star, but a massive core never stops collapsing. It shrinks until it’s billions of times smaller than an atom and becomes a black hole. Black hole The force of gravity close to a black hole is so intense that nothing can escape from it—not even light. Anything falling inside is torn apart by gravity and then crushed into a point of inﬁnite density. 20 space THE UNIVERSE Loop prominence Gigantic loops of glowing gas extend high above the Sun’s surface, anchored to the star’s tangled magnetic ﬁeld. Called loop prominences, these gas eruptions can last for months. Spikes of gas Jets of hot gas rise all the time from the Sun’s surface, forming towering spikes that last just a few minutes before collapsing. Called spicules, these formations can reach thousands of miles in height. Seen from above (right), they form shimmering, hairlike patterns around a sunspot. The Sun The Sun is a nearly perfect sphere of hot, glowing gas. Its source of power lies buried deep in the central core, where a nuclear furnace rages nonstop, turning matter into pure heat and light. Slightly bigger than a typical star, the Sun is large enough by volume to swallow 1.3 million Earths. It contains 99.8 percent of all the matter in the Solar System, and the force of gravity generated by this enormous mass keeps the planets trapped in orbit around it. Seen from Earth, the Sun is a life-sustaining source of light and warmth that shines steadily on us. Closer views, however, reveal a world of astonishing violence, its seething surface bursting with vast eruptions that hurl ﬁery gases into space. Core Inside the hot, dense core, the process of nuclear fusion releases energy. Every second, 683 million tons of hydrogen are fused into helium in the core. Radiative zone Outside the core is the radiative zone, which is not dense enough for nuclear fusion to take place. Energy from the core seeps very slowly out through this layer. Convective zone In the convective zone, vast bubbles of hot gas rise to the surface, cool, and then fall, transferring energy from the Sun’s heart to the exterior. Inside the Sun Scientists divide the Sun’s interior into three distinct layers: the core, the radiative zone, and the convective zone. All three are made solely of gas, but the gas gets hotter and denser toward the center. In the core, the temperature soars to 27 million °F (15 million °C) and the gas is 150 times more dense than water. Photosphere The Sun’s apparent surface is called the photosphere. Energy escapes into space from here as light. 385 million billion gigawatts—the amount of energy output from the Sun each second. SIZE OF EARTH COMPARED TO THE SUN 21 Sun statistics Diameter .......... 865,374 miles (1,393,684 km) Distance from Earth ................. 93 million miles (150 million km) Mass (Earth = 1) ........................................ 333,000 Surface temperature ....... 10,000 °F (5,500 °C) Core temperature.. 27 million °F (15 million °C) Energy release Solar ﬂare A sudden burst of energy from the Sun’s surface is called a solar ﬂare. Flares are often followed by a coronal mass ejection (see panel). It takes only eight minutes for light from the Sun to reach Earth, but it can take 100,000 years for energy released in the Sun’s core to travel to the surface and emerge as light. The journey is slow because the energy is absorbed and reemitted by trillions of atoms as it passes through the dense radiative zone. 100,000 years Sunspots Cooler, darker patches on the Sun are called sunspots. The number of sunspots rises and falls over an 11-year cycle. 8 minutes EARTH SUN Rotation Like all objects in space, the Sun rotates. Unlike the Earth, which rotates as a solid object, the Sun is a ball of gas and turns at different speeds in different places. The equator takes 25 Earth days to rotate once, but the polar regions take 34 days. 34 days 25 days 24 hours EARTH 34 days SUN Mass ejections Vast bubbles of superhot gas (plasma), each with a mass of around 1.1 billion tons, erupt from the Sun up to three times a day. Called coronal mass ejections, these bubbles grow millions of miles wide in a few hours and then burst, sending a blast of charged particles hurtling across the Solar System. The blast waves sometimes collide with the Earth, lighting up the polar skies with unusually brilliant auroras. Grainy surface The bubbles of hot gas that rise up inside the Sun make its surface look grainy. There are some 4 million granules on the Sun’s face, each about 600 miles (1,000 km) wide and lasting for around eight minutes. 3:23 PM 6:09 PM 6:25 PM space 22 THE UNIVERSE If the Sun were the size of a basketball, Earth would be a pea 270 ft (83 m) away. If the Sun were the size of a basketball, Neptune would be a strawberry 170 miles (270 km) away. Asteroid Asteroids are giant space rocks that drift around the inner Solar System. Most lie in a belt between Mars and Jupiter, but some occasionally come dangerously close to the Earth. The smallest are the size of houses, while the largest are big enough to be classiﬁed as dwarf planets. Scientists think asteroids are leftovers from the material that formed the planets. All of them together amount to less than a twentieth of the Moon’s mass. Saturn The second biggest planet is striking for the dazzling system of bright rings that encircle it. It has 62 moons and dozens more moonlets. Sun The Sun is like a vast nuclear power station that produces energy by converting hydrogen into helium. It is the only star we can study close up. Earth Our home planet is the only place known to support life, thanks to the liquid water on its surface. Mercury The closest planet to the Sun, Mercury is also the smallest planet. Its surface is scarred by ancient craters. Mars Mars is a bitterly cold, desert world. Like Earth, it has mountains, canyons, and icy poles. Venus Though similar in size to Earth, Venus is a hellish world where any visiting astronaut would be crushed and boiled alive. Mars Earth Venus Sun Mercury Orbital distance The scale bar below shows the relative distances of the planets from the Sun. The distance between one planet and the next increases greatly as we move out through the Solar System. Jupiter Saturn 500 million miles (805 million km) 1,000 million miles (1,609 million km) 26 The number of asteroids known to be more than 124 miles (200 km) wide. Comet These small, icy bodies can develop spectacular tails of gas and dust when they approach the Sun. Neptune The most distant planet, Neptune is a blue giant with 13 known moons. It takes Neptune nearly 164 years to orbit the Sun once. Kuiper Belt Thousands, if not millions, of small icy bodies occupy the region beyond the planets. The Kuiper Belt is home to dwarf planet Pluto and is a likely source of comets. Uranus Blue giant Uranus orbits the Sun tipped over on its side, perhaps because of a crash with a smaller planet. It has 27 moons. The Solar System The force of gravity generated by the Sun’s vast mass keeps a family of planets and other bodies trapped in orbit around it. Together, the Sun and all these bodies make up our Solar System. Jupiter The largest planet in the Solar System, Jupiter is more massive than all the other planets put together. It has its own family too, with at least 67 moons, some as big as planets. The Solar System The Solar System’s planets form two groups. There are four small, inner planets made of rock and metal, and four giant, outer planets made of gas and liquid. Between the two is a belt of rocky bodies called asteroids, and beyond the planets is a zone of icy bodies including dwarf planets and comets. Even farther out is a vast, spherical cloud of more comets—the Oort Cloud. The Solar System has no certain outer boundary. A slow cannonball falls to the ground. Orbits Every major body in the Solar System orbits the Sun counterclockwise. The planets are on near-circular orbits in the same plane as the disc of gas and dust from which they formed. Many smaller objects, such as dwarf planets Pluto and Eris, have stretched orbits tilted to this plane. Comets arrive from all directions. Our Sun formed from a great cloud of dust and gas around 4.6 billion years ago. Vast amounts of matter were drawn in by the developing star, but not all of it was fully absorbed. A tiny fraction of leftover material—a mere 0.14 percent of the Solar System’s mass—formed a disc of gas and dust encircling the newborn star. Over millions of years, the grains of dust in this disc clumped together, growing into ever larger bodies until they grew to the size of planets, pulled into spheres by their own gravity. In the inner Solar System, where the Sun’s heat was too intense for gases to condense, planets formed from rock and metal. In the outer Solar System, gases condensed to form much bigger planets. Today the Solar System has eight planets, more than 100 moons, an unknown number of dwarf planets, and countless millions of comets and asteroids. A very fast cannonball escapes Earth’s gravity. At just the right speed, a cannonball keeps falling but never lands. Dwarf planets Dwarf planets are round in shape but smaller than true planets, and their gravity is not strong enough to sweep their region of space clear of smaller debris. The most famous dwarf planet is Pluto, which was classiﬁed as a true planet until 2006. Ceres Uranus Neptune Pluto Saturn Sun Eris Haumea How orbits work English scientist Isaac Newton was the ﬁrst person to realize why moons and planets travel in orbits: because they are trapped by gravity. To explain his theory, he drew a giant cannon ﬁring cannonballs off Earth. If a cannonball moved fast enough, the curve of its path as it fell back would be gentler than the curve of Earth’s shape, and it would never land—it would stay in orbit. Makemake Earth Pluto Eris SIZE COMPARED TO EARTH Uranus 1,500 million miles (2,414 million km) Neptune 2,000 million miles (3,219 million km) 2,500 million miles (4,023 million km) 24 space Mercury’s iron core accounts for two-thirds of the planet’s mass. THE UNIVERSE Inner planets This view of Venus’s surface was made using radar to see through the planet’s thick clouds Maat Mons is Venus’s largest volcano Mercury, Venus, Earth, and Mars are the Solar System’s inner planets. On the face of it, they are worlds apart—but underneath the surface, it is a different story. The inner planets all formed from the same material about 4.6 billion years ago. All are a mix of rock and metal, with interiors that are roughly divided into layers. The heavier metals are concentrated toward the center, while the lighter rock is on top. Each of these planets was bombarded by asteroids and comets early in the Solar System’s history, and each has been affected by volcanic activity too. Mercury’s heavily cratered face still bears the scars of the early bombardment, but the surfaces of the other three worlds have changed over time. Smooth plains formed from lava ﬂows in distant past Brightest craters are the youngest MERCURY VENUS In the Sun’s glare Mercury proﬁle Lava land Venus proﬁle Mercury is the smallest of the Solar System’s planets and lies closest to the Sun. It is a lifeless world that has hardly changed in 3 billion years. The planet’s entire surface is pitted with craters formed when asteroids crashed into it while Mercury was young. The craters range from small, bowl-shaped ones to the huge Caloris Basin, which is nearly one-third the width of the planet. Mercury orbits the Sun more quickly than any other planet, but it rotates slowly: for every two orbits, it spins around just three times. So a “day” on Mercury (sunrise to sunrise) takes 176 Earth days. Such long days and nights, coupled with a very thin atmosphere, give Mercury the greatest surface temperature range of all the planets. In the daytime, the surface is hot enough to melt lead, but at night it’s cold enough to liquefy air. Diameter .... 3,032 miles (4,879 km) Average surface temperature .................333°F (167°C) One spin on axis .... 58.6 Earth days One orbit of Sun......... 88 Earth days Number of moons ..............................0 Venus is sometimes described as Earth’s twin because it’s almost the same size as our planet and has a similar internal structure. But the two worlds are very different. Any astronaut who tried to walk on Venus would be killed in seconds. The surface is as hot as the inside of a pizza oven, and the crushing air pressure is 90 times greater than that on Earth. Venus’s deadly surface is hidden from our view by thick cloud cover, but orbiting spacecraft have used radar to see through the gloom, and landers have touched down to take photos. Venus is a world of volcanoes, many thought to be active, and its surface is littered with broken rock from solidiﬁed lava. It is permanently overcast, with a sickly yellowish light ﬁltering through the cloud. Venus spins more slowly than any other planet. It also spins in the opposite direction (clockwise) to every planet apart from Uranus. Diameter ..7,521 miles (12,104 km) Average surface temperature .................867°F (464°C) One spin on axis ..... 243 Earth days One orbit of Sun...224.7 Earth days Number of moons ..............................0 Hot spots The colors on this heat map of Mercury show the planet’s surface temperature. The red region, which is on the equator, faces the Sun and is hottest. Next warmest are yellow areas, then green. The planet’s polar regions (blue) are coolest. Greenhouse effect Venus is hot because of a process called the greenhouse effect. The Sun’s heat passes through the atmosphere and warms the ground, which then reemits warmth. The reemitted warmth is trapped by the atmosphere, much as glass traps heat in a greenhouse. Sunlight warms the ground Heat from ground is trapped by atmosphere Atmosphere is about 50 miles (80 km) deep 2,500 miles (4,000 km)—the length of Mars’s Valles Marineris canyon. Mars’s moons Phobos and Deimos were once asteroids. Liquid water covers most of Earth’s surface 14 miles (22 km)—the height of the Olympus Mons volcano on Mars. Ice cap at north pole 25 Like Earth, Mars has a crust made of solid rock Like all the inner planets, Mars has a core made of red-hot iron The reddish color comes from iron oxide (rust) in the soil EARTH MARS Living world Earth profile The red planet Mars profile Third out from the Sun, Earth is the largest of the inner planets. It’s the only planet with liquid water flowing freely on the surface, and it’s the only planet in the Universe known to sustain life. Earth’s surface consists of vast oceans (71 percent), continents of land, and two polar ice caps—all supported by a thin, rocky crust. The crust is broken into seven huge segments and many smaller ones. Called tectonic plates, these giant slabs of rock creep slowly over Earth’s surface, pushed by churning movements in the softer, hot rock that fills most of Earth’s interior. As tectonic plates move, they bump into each other and grind past one another, generating immense forces that thrust up mountain ranges, unleash volcanic eruptions, and trigger earthquakes. These powerful forces continually change Earth’s appearance, as do the actions of wind and water— and the planet’s 7 billion human inhabitants. Diameter ..7,926 miles (12,756 km) Average surface temperature ......................59°F (15°C) One spin on axis .............. 23.9 hours One orbit of Sun...............365.3 days Number of moons ..............................1 The second smallest planet in the Solar System, Mars is half the size of Earth. It’s sometimes called the red planet because of its rusty coloring. A vast canyon called Valles Marineris stretches a quarter of the way around this frozen desert world. It formed long ago when the crust of the young planet split open. Elsewhere are dusty plains strewn with boulders and giant, extinct volcanoes, including Olympus Mons—the Solar System’s largest volcano. Diameter .... 4,220 miles (6,792 km) Average surface temperature ..................–81°F (–63°C) One spin on axis .............. 24.6 hours One orbit of Sun...... 687 Earth days Number of moons ..............................2 NORTH POLE 23°.5 Axis Northern hemisphere Equator Southern hemisphere SOUTH POLE Tilted planet Earth spins around once a day, but it isn’t perfectly upright. Its axis— the imaginary line from pole to pole around which it spins—is tilted by 23.5°. So as Earth travels around the Sun, one hemisphere and then the other is tilted toward the Sun. This is what causes the seasons. Rocky ﬂoodplain Mars hasn’t always been a desert. Dry river beds show that water flowed here long ago. Floods swept rocks across the land and dumped them on floodplains like the one below. Mars may even have been warm and wet enough for life to flourish. space 26 1,300 THE UNIVERSE The number of times Earth could ﬁt inside Jupiter’s volume. Size of Earth relative to planets 1665 The year Jupiter’s Great Red Spot was ﬁrst seen. Gas atmosphere Jupiter’s swirling outer atmosphere is 620 miles (1,000 km) thick and consists mainly of hydrogen gas. Liquid layer The outer atmosphere merges gradually into a deep layer of liquid hydrogen and helium. Blue color caused by methane in atmosphere Great Red Spot This giant storm is bigger than Earth and has been raging for over 300 years. JUPITER Solid core Jupiter’s rocky core is hotter than the surface of the Sun. Liquid metal layer Deep inside Jupiter, intense pressure turns hydrogen into a liquid metal. NEPTUNE King of the planets Jupiter profile Blue planet Neptune profile Mighty Jupiter is the fifth planet from the Sun and the largest in the Solar System—so big, in fact, that it’s 2.5 times more massive than all the other planets put together. Its strong gravitational pull greatly affects the orbits of other bodies in the Solar System. Jupiter’s fast rate of spin has stretched its surface clouds into bands, with spots (storms) and ripples where neighboring bands swirl together. Several craft have visited Jupiter, including Galileo, which orbited from 1995 to 2003. Diameter ......................... 88,846 miles (142,984 km) Average surface temperature .............–186°F (–121°C) One spin on axis ................. 9.9 hours One orbit of Sun ......11.9 Earth years Number of moons ........................... 67 Neptune, the eighth and furthest of the planets from the Sun, was discovered in 1846. Astronomers had noticed Uranus wasn’t following its expected path—there seemed to be an unseen body, perhaps an undiscovered planet, pulling on it. Two mathematicians— John Couch Adams in England and Urbain Le Verrier in France— calculated where in the sky the undiscovered planet must be. Within days, Neptune was spotted from an observatory in Germany. Neptune is slightly smaller than Uranus and looks bluer because its atmosphere contains more methane. It has a deep, fluid mantle that is hot and dense and contains water, ammonia, and methane. Neptune also has a barely visible system of rings. Its biggest moon, Triton, resembles Pluto and was likely captured by Neptune’s gravity in an encounter billions of years ago. Diameter ........................ 30,775 miles (49,528 km) Average surface temperature .............–330°F (–201°C) One spin on axis ...............16.1 hours One orbit of Sun 163.7 Earth years Number of moons ........................... 13 Metis The Jupiter system Like a king surrounded by his courtiers, Jupiter is circled by a great number of moons. The inner moons, including the four largest, are shown below. Ganymede, the largest, is bigger than the planet Mercury. Most of Jupiter’s other moons are probably asteroids captured by the planet’s gravity. Thebe Io Ganymede Europa Adrastea Amalthea Callisto Fastest known winds When Voyager 2 flew past Neptune in 1989, it photographed white clouds blown into streaks by winds of up to 1,300 mph (2,100 kph)—the fastest sustained winds in the Solar System. This violent weather is thought to be powered by heat from inside Neptune since the planet is too far from the Sun to absorb much of its warmth. 43,000 °F (24,000 °C)—the estimated temperature of Jupiter’s core. Cloud bands Saturn’s clouds form bands around the planet, like those on Jupiter. Outer atmosphere Although it looks calm, ﬁerce winds whip through Saturn’s atmosphere at 1,800 kph (1,120 mph). 150 The number of moons and moonlets observed orbiting Saturn. 27 Outer planets Four gigantic planets dominate the outer Solar System. Very different from the rocky, inner planets, these strange worlds are huge globes of gas and liquid, with no solid surface and hundreds of moons. After the Sun ﬁrst formed, its heat drove gases out of the inner Solar System, leaving behind heavier compounds such as rock and metal. The rock and metal formed the solid inner planets, while the gases formed the outer planets. Astronomers call the outer planets gas giants, though they consist mostly of liquid and they have solid cores. These four worlds have much in common. All have numerous moons, a deep, stormy atmosphere, and a set of rings made of ﬂecks of rock or ice. On a roll Uranus leans so far to one side that it appears to roll along as it travels around the Sun. Gaps Gaps in the rings are areas swept clear of ice by the gravity of Saturn’s moons. Main rings The rings consist of billions of sparkling fragments of ice, varying in size from microscopic to as big as a house. SATURN Rings around Uranus Thirteen rings are known to circle the planet. URANUS Lord of the rings Saturn proﬁle Topsy-turvy world Uranus proﬁle The second-largest planet and the sixth farthest from the Sun, Saturn shines like a bright yellow star. Even a small telescope will reveal its most famous feature: a magniﬁcent ring system. Despite Saturn’s size, it is only half as dense as Jupiter. Its clouds form less obvious bands than Jupiter’s, but ﬁerce storms blow up every 30 years or so, creating giant white spots. Saturn’s largest moon, Titan, has a dense atmosphere and a rocky surface with seas of liquid methane. The Cassini spacecraft has been orbiting Saturn since 2004. It released a probe, Huygens, that landed on Titan in 2005. Diameter ........................ 74,898 miles (120,536 km) Average surface temperature ........... –292°F (–180°C) One spin on axis ...............10.7 hours One orbit of Sun.... 29.5 Earth years Number of moons ........................... 62 Uranus, the seventh planet from the Sun, was unknown to ancient astronomers, even though it is just visible with the naked eye in perfectly clear and dark skies. It was discovered by musician William Herschel from his back garden in Bath, England, in 1781. Uranus is similar to Neptune but has a paler blue, almost featureless face. It is the coldest of all the planets and generates very little heat from within. It orbits on its side—perhaps because it was knocked over by a collision with another planet early in its history. Its extreme tilt gives it very long seasons. Uranus has a faint set of rings, which were discovered in 1977. The planet’s moons are all named after characters in works by William Shakespeare or the English poet Alexander Pope. Diameter ................... 36,763 miles (51,118 km) Average surface temperature ........–315°F (–193°C) One spin on axis ..........17.2 hours One orbit of Sun...84 Earth years Number of moons ...................... 27 Ring system Saturn’s main rings are 220,000 miles (360,000 km) wide, yet they are only 30 ft (10 m) thick. A scale model of the rings made with a sheet of paper would be 2 miles (3 km) wide. Beyond the main rings are hazy outer rings, photographed by Cassini while the Sun was behind Saturn (below). Outer atmosphere, the cloud layer Atmosphere (hydrogen, helium, and methane gases) Core of silicate rock Hot, liquid mantle Ice giant Uranus’s pale blue color is due to the methane in its atmosphere. Water and ammonia have also been detected in the clouds. The planet contains less hydrogen and helium than Jupiter and Saturn but has more rock and water. space 28 THE UNIVERSE Moon proﬁle Diameter ........................ 2,159 miles (3,474 km) Average surface temperature .....–63°F (–53°C) Length of lunar day ...................... 27 Earth days Time to orbit Earth ....................... 27 Earth days Gravity (Earth = 1) ...........................................0.17 How the Moon formed Scientists think the Moon formed as a result of a collision between Earth and a planet 4.5 billion years ago. The debris was pulled together by gravity and became the Moon. Impact A planet smashes into Earth and blasts molten rock into space. Moon formation A disc of debris forms. The particles slowly join to form a Moon. 18 days—the time it would take to ﬂy to the Moon at the speed of a jumbo jet. The Moon The Moon is Earth’s closest neighbor in space and looms larger than any other object in the night sky. Its cratered surface may be cold and lifeless, but deep inside the Moon is a gigantic ball of white-hot iron. Earth and Moon have existed together in space ever since the Moon formed as the result of a cosmic collision. It orbits around our planet, keeping the same face toward us at all times. As we gaze on its sunlit surface, we look at a landscape that has barely changed since 3.5 billion years ago. Back then, the young Moon was bombarded by asteroids. For millions of years they blasted out surface material and formed craters. The largest of these were then ﬂooded with volcanic lava, creating dark, ﬂat plains that look like seas. 1 SEA OF SHOWERS (MARE IMBRIUM) Lunar maria Dark, ﬂat areas known as maria, or seas, are huge plains of solidiﬁed lava. Phases of the Moon As the Moon orbits the Earth, a changing amount of its face is bathed in sunlight. The different shapes we see are the Moon’s phases. One cycle of phases lasts 29.5 days. NEW MOON WAXING CRESCENT FIRST QUARTER WAXING GIBBOUS FULL MOON WANING GIBBOUS LAST QUARTER WANING CRESCENT NEW MOON The far side The Moon keeps the same face toward the Earth all the time. The face we never see—the far side—can only be viewed by spacecraft. Its crust is thicker and more heavily cratered than the near side. Elevation (height) maps reveal high and low areas of land. Highlands Gray surface The Moon’s surface is covered in a layer of ﬁne gray dust an inch or so deep. Lunar craters Craters exist all over the Moon. They range from small bowl-shaped hollows a few miles wide to the vast South Pole–Aitken Basin, which is 1,600 miles (2,500 km) in diameter. Many craters, like Eratosthenes (above), have central hills that formed as the ground rebounded after the asteroid struck. Man on the Moon Astronauts landed on the Moon six times during NASA’s Apollo program. They found a world of gray, dusty plains and rolling hills under an inky black sky. Below, an astronaut heads back to his rover vehicle parked near Camelot Crater (left), where he had been collecting samples. The large boulders were ﬂung out of the crater when it formed. Lunar layers Like Earth, the Moon is made of different layers that separated out long ago, when its whole interior was molten. Lightweight minerals rose to the top, and heavier metals sank to the center. The outermost layer is a thin crust of rock like the rock on Earth. Under this is the mantle—a deep layer of rock that gets hotter toward the center. The bottom part of the mantle is partly molten. In the Moon’s center is an iron core heated to about 2,600°F (1,400°C) by energy from radioactive elements. Scientists think the outer core is molten but the inner core is squeezed solid by the pressure of the rock around it. Height of surface High Low South Pole–Aitken Basin 2 Every year, the Moon drifts 1.48 in (3.78 cm) further away from Earth. 12 people have walked on the Moon. Highlands All the Moon’s hills and mountains are the rims of craters or central peaks in craters. 29 3 2 SEA OF SERENITY (MARE SERENITATIS) 3 SEA OF TRANQUILITY (MARE TRANQUILLITATIS) The mantle gets hotter toward the center. Hadley Rille A deep gorge named Hadley Rille cuts through ﬂat plains at the edge of the Moon’s Sea of Showers, winding for more than 60 miles (100 km). How it formed is a mystery, but it might be an ancient lava channel. In July 1971, Apollo astronauts drove their rover to the edge of Hadley Rille to take photographs and study it. Inner core A hot, 300-mile- (480-km-) wide ball of solid iron forms the inner core. 1 Outer core The outer part of the Moon’s iron core is molten or partially molten iron. Inner mantle Heat from radioactive elements has partially melted the inner mantle. Mantle The mantle is mainly solid rock and is rich in silicate minerals, which are common on Earth. Crust Made of granitelike rock, the Moon’s crust is about 30 miles (48 km) thick on the near side and 46 miles (74 km) thick on the far side. Far side The Moon’s far side, which is not visible from Earth, is covered in craters and has no large maria (seas). Craters in craters On many parts of the Moon the craters overlap each other, the newer ones lying on top of older craters. 30 SPACE EXPLORATION Robot explorers EXPLORING THE PLANETS Stars and planets have fascinated people since ancient times, but it wasn’t until the 20th century that exploring space became possible. In recent decades we have sent astronauts to the Moon, robotic spacecraft to the outer reaches of the Solar System, and used huge telescopes to peer across the vastness of the Universe. Robotic spacecraft can visit places too far or dangerous for human beings. Launched into space by rocket, they travel vast distances across space and may take years to reach their target. There are various types of spacecraft, each suited to a particular mission. The planets are too far for manned missions, so robotic spacecraft are sent instead. The ﬁrst to visit another planet was Mariner 2, a US craft that ﬂew past Venus in 1962. Since then, and despite a number of early failures, hundreds of spacecraft have visited the Solar System’s planets, moons, asteroids, and comets. Most spacecraft either ﬂy past or orbit their target, but some also release landers that touch down on the surface. FLYBY SPACECRAFT Some spacecraft observe a target as they fly past. NASA’s famous Voyager 1 and Voyager 2 flew past several planets. ORBITER An orbiter flies around a planet repeatedly, giving it plenty of time to study its target. Orbiters have visited the Moon and all the planets except Uranus and Neptune. OBSERVING THE SKIES Capturing light LAUNCH VEHICLES For centuries, astronomers have observed the heavens with their eyes alone or used simple telescopes that magnify the view. But the visible light we see is just one part of a much bigger spectrum of electromagnetic rays that reaches Earth from space. Stars and other objects also emit invisible radio waves, X-rays, infrared, and ultraviolet rays. Modern telescopes can see all of these, and each type of radiation reveals something different. Telescopes come in many different styles and designs, but basically all do the same thing: collect electromagnetic radiation from space and focus it to create an image. Earth’s atmosphere can block or blur the image, so some telescopes are located on high mountaintops or even launched into space. Space is only 60 miles (100 km) above the Earth’s surface and takes less than 10 minutes to reach in a rocket. Although the journey is short, it takes tremendous power to escape the pull of Earth’s gravity. Launch vehicles are built to make the journey only one time, and most of their weight is fuel. Mapping the stars Because Earth is surrounded by space, when we look at the night sky it seems as though all the stars are pinned to the inside of a giant sphere. Astronomers call this the celestial sphere and use it to map the positions of stars and planets. Vertical and horizontal lines are used to divide the celestial sphere into a grid, just like the grid of longitude and latitude lines used to map Earth’s surface. Celestial north pole This is the point directly above Earth’s North Pole. Celestial south pole This is the point directly above Earth’s South Pole. X-ray telescopes These telescopes capture high energy rays from extremely hot objects. X-ray telescopes only work in space. Declination lines These split the sky into north–south segments. Right ascension lines These divide the sky into east–west segments. Saturn V, which sent astronauts to the Moon, was the largest rocket ever built. Its Soviet rival, the N1, was launched four times but each attempt ended in disaster. SATURN V (US)—364 ft (111 m) tall Ultraviolet telescopes Astronomers use ultraviolet telescopes to examine radiation from the Sun, stars, and galaxies. X-RAY N1 (USSR)—344 ft (105 m) tall Optical telescopes Using large lenses or mirrors, optical telescopes gather faint visible light and can see much further than the human eye. ULTRAVIOLET DELTA IV HEAVY (US)—236 ft (72 m) tall Infrared telescopes These instruments, some of which are sent into space, detect the heat from objects such as clouds of gas and dust. VISIBLE LONG MARCH 2F (CHINA)—203 ft (62 m) tall Radio telescopes Huge, curved dishes are used to focus radio waves given out by sources such as galaxies, pulsars, and black holes. INFRARED ARIANE 4 (EUROPE)—193 ft (59 m) tall RADIO World’s largest rockets Launch sites Many countries have spaceflight launch sites. Sites closer to the equator can launch heavier cargo, because rockets there are given a boost by the speed of Earth’s spin. Baikonur, Kazakhstan Cape Canaveral, FL Celestial equator This imaginary line over Earth’s equator divides the sky into north and south hemispheres. MAJOR LAUNCH SITES Xichang, China 31 12 Most visited This chart shows the number of missions to the major bodies in the Solar System. SATELLITES LIVING IN SPACE About 1,000 operational satellites orbit the Earth, carrying out tasks such as beaming TV signals around the world, gathering data for weather forecasters, and spying for the military. Many thousands more pieces of space junk—old satellites, discarded rocket parts, and debris from collisions—also circle our planet. The growing cloud of space debris is a hazard to spacecraft. Astronauts must adapt to a zero-gravity environment when living in space. Although ﬂoating weightlessly can be fun, it can also cause medical problems. Space stations are cramped places with few luxuries. Astronauts eat ready-made meals that are either freeze-dried or served in pouches. All water is recycled, including the water vapor from human breath. Astronauts clean themselves with special shampoos and soaps that don’t need water, and they use space toilets that suck away waste rather than ﬂushing with water. Effects on the body Over 500,000 objects, including satellites and space junk, orbit Earth. Satellite orbits Some satellites are a few hundred miles above Earth’s surface, but others are much further. Some of the highest ones, such as weather, TV, and phone satellites, have geostationary orbits, which means they stay over a ﬁxed point on Earth. Satellites with lower orbits change position all the time. When the human body spends a long time in space, it changes. Without gravity pulling on the spine, the body gets about 2 in (5 cm) taller. Body ﬂuids that ﬂow downward on Earth build up in the head. This gives astronauts swollen faces and blocked noses, making food seem tasteless. When astronauts come back to Earth, the return of full gravity can make them feel extremely weak. 30 20 10 MERCURY Pioneer spacecraft Pioneers 10 and 11 made ﬂybys of Jupiter and Saturn. They are now heading out of the Solar System into deep space. 40 years and 43 days – the time it took the spacecraft Voyager 2 to reach Neptune from Earth. SUN Generator PLUTO 50 Asteroid detector NEPTUNE 60 URANUS 70 JUPITER 80 SATURN PENETRATOR A penetrator is designed to hit its target at high speed and bury itself. In 2005, Deep Impact penetrated the surface of a comet. In little more than 50 years, around 200 spacecraft have left Earth’s orbit and headed off to explore the Solar System. More than half the missions have been to Earth’s nearest neighbors in space: the Moon and the planets Mars and Venus. ASTEROIDS Communication dish LANDER Some craft can touch down on the surface of another world. In 1976, Viking 1 became the ﬁrst craft to successfully land on Mars. Solar System missions MARS Infrared sensor ROVER A rover is a robotic lander with wheels that can drive around. Rovers sent to Mars have studied its rocks for signs of ancient life. MOON Ultraviolet sensor ATMOSPHERIC PROBE This type of craft enters a planet’s atmosphere. The Galileo probe dove into Jupiter’s stormy atmosphere in 2005. VENUS Magnetometer 0 Brain and balance Without gravity, the inner ear’s balance system no longer works, which can make astronauts feel sick. Heart Blood ﬂows more easily, so the heart doesn’t need to work as hard. Muscles Movement is easy when you’re weightless, and muscles waste away if not used. Workouts in the onboard gym help to slow this process. Bones Bones weaken and become less dense. Regular exercise is essential to keep them strong. ORBITAL PERIOD 20 HOURS Geostationary orbit (communications satellites) 15 HOURS 10 HOURS Medium-Earth orbit (GPS satellites) 5 HOURS Low-Earth orbit (International Space Station) Space stations A space station is a crewed satellite— a kind of orbiting laboratory in which astronauts and scientists live and work. The USSR launched the ﬁrst station, Salyut 1, in 1971. The US soon followed with Skylab, in 1973. Russia’s Mir, in use from 1986 to 2001, was the most successful station until the US, Russia, and more than 10 other countries joined forces to build the International Space Station, in orbit since 1998. China’s own space station prototype, Tiangong-1, was launched in 2011. SKYLAB (US) SALYUT 1 (USSR) MIR (USSR) TIANGONG-1 (CHINA) INTERNATIONAL SPACE STATION d ar ph Al I IN or GEM Pollux o S NI R Pr CAINO M luster hive C R CE CAN M B SOUTH A DR HY A ERID S NU AR I ES AX Mi ra IX N OE PH CE TU S SC U Measuring brightness The size of the white spots on the chart show how bright the stars are. Astronomers call this magnitude and measure it on a scale that runs backward—the smaller the number, the brighter the star. -1 0 Ecliptic RN FO Hamal D en Star magnitudes 6 M3 Elnath 3000 bce 150 ce 1543 s cy on O LEPUS Arneb CAELUM n et ara el Aldeb ge 22 u s e trix 4 4 Bella ORION a A tak Alnlnilam Min itak M42 l Rige N OC ER OS Siriu s Adh ara 1610 H or iz on MACAN JO IS R PU PP IS COLUMBA Orion the hunter One of the best-known and brightest constellations is Orion the hunter, which is visible the world over. Orion includes the red giant star Betelgeuse and the blue-white supergiant Rigel—two of the brightest stars in the night sky. H M3 8 ya de s Alcy TA o ne UR Ple US iades IUM Today, space telescopes such as Hubble, which was launched in 1990, give us breathtaking views of distant objects in space, including the furthest galaxies ever seen. PY XI S VE LA G OLO Modern astronomy H HOR Isaac Newton English scientist Isaac Newton worked out the laws of gravity—the force that makes objects fall to the ground. He discovered that gravity keeps the Moon in orbit around Earth and keeps the planets in orbit around the Sun. Canopus DORADO 1687 Colored circles on the star chart show the night sky you can see from the locations highlighted in color above. PICTOR Betelgeuse Rigel 1990 LATITUDE 60°N 40°N 20°N M RETICULU Italian scientist Galileo Galilei built a telescope and used it to study the night sky. He saw spots on the Sun, mountains on the Moon, and four moons orbiting the planet Jupiter. Ancient stargazers saw patterns in the stars and named groups of stars after mythical beings and animals. These star patterns, called constellations, look little like the objects they are meant to represent, but we still use the old names. Today, astronomers divide the whole sky into 88 segments, each one named after the constellation within it. Star charts like the one here show which constellations are visible at a particular time and place. This chart shows the stars you can see at midnight in January from the northern hemisphere. A Galileo Galilei The sky at night CA RIN Earth Polish astronomer Nicolaus Copernicus proposed that the Sun, not Earth, is the center of the Solar System. It was a shocking idea since it meant Earth must be ﬂying through space, spinning around. Copernicus Professional astronomers investigate not only stars but everything to do with space—from the meteors that burn up spectacularly as shooting stars in Earth’s atmosphere and the planets of the Solar System to distant galaxies billions of light years away. Astronomy makes a rewarding hobby too, and many amateur stargazers enjoy observing the night sky with backyard telescopes or binoculars. Whenever astronomers observe the sky, they are looking back in time. This is because light takes such a long time to reach us from distant objects in space. We see the Moon as it was one and a quarter seconds ago and the stars as they were hundreds of years ago. Regulu The Greek astronomer Ptolemy cataloged 1,022 stars in 48 constellations. He believed that Earth was the center of the Solar System and Universe, orbited by the Sun, Moon, planets, and stars. People have been looking up at the night sky and marveling at its beauty and mystery for thousands of years. Today, a whole branch of science—astronomy—is devoted to studying stars. IA TL AN Ptolemy Many monuments built by ancient peoples, such as Stonehenge in the UK, align with the Sun. These monuments may have been used as calendars so that farmers knew when to sow crops. Astronomy n izo Many ancient cultures followed the Sun and stars in order to keep track of the time of year, and by Ancient Greek times, astronomers had already worked out that Earth is round. Today, powerful telescopes allow us to peer so far into space that we can look back in time almost to the birth of the Universe. trillion miles (40 trillion km)—the distance to the nearest star, Proxima Centauri. ANS SEXT A brief history of astronomy Astronomical calendars 25 SPACE EXPLORATION r Ho space 0ºN n2 izo Hor 32 1 PISCES eb Ka ito s LP TOR 2 3 4 5 15 The number of stars visible to the naked eye. 33 How telescopes work The invention of the telescope revolutionized astronomy. A telescope collects more light from an object than a human eye can. It uses this light to form a magniﬁed image. There are two basic types of telescopes: refracting and reﬂecting. The refracting telescope has a large convex (outward-curving) lens that gathers and focuses the light. The reﬂecting telescope uses a curved mirror instead. Eyepiece Lens Eyepiece lens magniﬁes image Focal point Light from star Light from star Smaller mirror Light rays converge Focal point Refracting telescope A convex lens bends light entering the telescope to focus it, forming an image. At the other end of the telescope, a smaller lens called the eyepiece magniﬁes the image. Reﬂecting telescope A concave (inward-curving) main mirror reﬂects light on to a smaller, ﬂat mirror. The resulting image is magniﬁed by an eyepiece lens. Main mirror EAST The dotted white line running down the chart is called the ecliptic and shows the path of the Sun. The planets always stay close to the ecliptic. NA RO CO A r lco Horizon Professional astronomers don’t just use visible light to see the night sky. Their telescopes can also create images from wavelengths of light that our eyes cannot see, such as X-rays, radio waves, and infrared rays. The images below all show Kepler’s Supernova—the wreckage left by a giant star that exploded in 1604. X-ray image This image of Kepler’s Supernova is from the orbiting Chandra X-ray Observatory. It shows a cloud of incredibly hot gas that emits high-energy X-rays. Infrared image Taken by the Spitzer Space Telescope, this infrared image shows dust clouds that were heated by a shock wave from the exploding star. Cy gn us Ri ft CY GN US DRACO Horizon Horizo 20ºN Ca ph LA CE RT A De ne b Polaris PE IA CA ME L OPARDALIS URSA MINOR Cas X AU R I GA AS SIO Al Delt de a ram Cep C hei EPHEUS in LY N fa k Seeing the invisible Visible light image Very little of the object can be seen in visible light, even in this image from the Hubble Space Telescope. The bright areas are clumps of gas. Alb ire o Be h e bh oug Du Pl M U Me M RS rak AJ A OR S North star BO ar iz r Co C LE MINO OR ap ella Eps ilon Vega Lyrae HERCULES LYRA Algieba A ED ROM A ND z t a Alpher Pointers ºN n 40 LEO ar ol i C V AN E NA ES TIC I C EU RS h ac Alm M LU NGU T RI A h c Mira ab Koch an Thub ola PE E ÖT BO IS AL RE Deneb ir M Algol S C BEOMA RE NI C ES C Epsilon Aurigae The Great Bear Ursa Major is named after a bear. The seven bright stars running from its tail form a famous group of stars known as the Big Dipper or Plow. The last two of these point to the North Star, which is always due north. In ancient times, sailors used this star to ﬁnd the way. NORTH Haedi 8 2,000 million trillion miles (24 million trillion km)—the distance to the Andromeda galaxy, the furthest object visible to the naked eye. S ASU PEG Markab On a clear, dark night you can see the Milky Way— the galaxy to which our Solar System belongs. WEST GALAXY GLOBULAR CLUSTER OPEN CLUSTER Star clusters are clouds of stars. Cassiopeia This constellation is named after a vain queen in Greek mythology. It’s very easy to spot in northern skies as it looks like a funky letter W. Combined image Combining all three sources produces a complete image: a shell of supernova debris expanding into space at 1,240 miles (2,000 km) per second. 34 space SPACE EXPLORATION 1 million people traveled to Florida to watch the launch of Apollo 11 on July 16 1969. The Saturn V rocket was taller than the Statue of Liberty in New York City. Fuel tanks Tanks within the Service Module supplied fuel to the main engine. Escape rocket (for emergencies during launch) Command Module Astronauts stayed in here during launch. Astronauts The crew of three stayed in the Command Module for most of their journey to and from the Moon. Service Module This module powered the Apollo spacecraft. Lunar Module The Lunar Module was housed in an aluminum cone. Instrument unit Third stage This stage reached low-Earth orbit and then put Apollo on course for the Moon. Single third-stage engine Interstage adapter Covering the thirdstage engine, this section linked the rocket’s second and third stages. Second stage The second stage held a tank of liquid hydrogen fuel and a tank of liquid oxygen. Second-stage engines Interstage adapter This section linked the rocket’s ﬁrst two stages and also covered the secondstage engines. First stage fuel The ﬁrst stage had a tank of kerosene fuel and a tank of liquid oxygen to burn it. The ﬁve engines burned 16 tons of fuel per second during the launch. Five ﬁrst-stage engines Engine nozzle Nozzle for the main engine, which propelled the Apollo craft through space. Service Module This module provided lifesupport systems and power for the crew, and housed the Apollo craft’s main engine. Mission to the Moon Humans have set foot on only one world beyond Earth: the Moon. Just 27 daredevil astronauts have traveled there, of whom 12 walked on its cratered, lifeless surface. Eight space missions visited the Moon between 1968 and 1972 as part of NASA’s Apollo program. Each mission carried three American astronauts inside an Apollo spacecraft, which was launched by a Saturn V rocket. Apollo 8 tested the craft as it orbited the Moon. Then, in a dress rehearsal prior to landing, Apollo 10 ﬂew close to the lunar surface. The ﬁrst of the six missions that successfully landed on the Moon was Apollo 11 in 1969. Astronauts Neil Armstrong and Buzz Aldrin touched down on the surface in July of that year. As Armstrong took the ﬁrst historic step, he said, “That’s one small step for man, one giant leap for mankind.” Thrusters Small thrusters made ﬁne adjustments to the Apollo spacecraft’s movements. Command Module The Command Module was the only part of the Apollo craft to return to Earth. Its conical shape helped it withstand the heat of reentry into Earth’s atmosphere. Apollo spacecraft The Apollo spacecraft had three parts: the Command, Service, and Lunar Modules. These were all linked together for the 250,000-mile (400,000-km) trip to the Moon. Once there, the Lunar Module took two astronauts down to the Moon’s surface, while the third crew member remained in lunar orbit in the combined Command and Service Module (CSM). The top half of the Lunar Module, known as the ascent stage, later returned the two astronauts to the CSM for the journey back to Earth. Saturn V rocket The Apollo astronauts were blasted into space inside the nose cone of the largest rocket ever built: Saturn V. Standing nearly 364 ft (111 m) tall, the Saturn V was as tall as a 30-story building. This giant launch vehicle consisted of three rockets in one. The ﬁrst two parts, or stages, lifted the Apollo craft into space, and the third stage set the spacecraft on course for the Moon. 21 hours—the length of time Apollo 11 astronauts Neil Armstrong and Buzz Aldrin spent on the Moon. Apollo 17 astronauts spent three days on the lunar surface. 300 The total number of hours that astronauts have spent on the Moon. There and back Each of the six Apollo missions that landed men on the Moon took the same route, taking off from Florida, and ending with the astronauts splashing down in the Paciﬁc Ocean. Saturn V rocket carrying Apollo craft blasts off and positions craft in Earth’s orbit. 2 The rocket’s third stage and Apollo craft leave Earth’s orbit and head toward the Moon. 3 Combined Command and Service Module (CSM) separates from the rocket. 4 CSM turns and docks with 1 Docking tunnel Astronauts used this tunnel to move between the Command and Lunar Modules. 842 Beef, potatoes, and grape juice—the ﬁrst meal eaten by the Apollo 11 astronauts in space. lb (382 kg) of lunar rock and soil were brought back by the Apollo astronauts. The trip from the Earth to the Moon took about three days. 11 10 1 35 Landing sites The Apollo landing sites were on the side of the Moon that faces Earth. 12 5 2 7 3 4 Lunar Module. Third rocket stage is now discarded. 5 Apollo craft adjusts its course to go into lunar orbit. 6 Lunar Module transports two astronauts to lunar surface. 7 Third crew member continues to orbit the Moon in CSM. 8 Ascent stage of Lunar Module takes astronauts back to CSM, 8 9 6 after which it is discarded. 9 CSM adjusts its course and heads back to Earth. 10 Service Module is jettisoned. 11 Command Module enters Earth’s atmosphere. 12 Command Module makes a parachute landing in the sea. Ascent stage The ascent stage of the Lunar Module was the astronauts’ home while they explored the Moon. Legs and pads Flexible legs with wide pads on the bottom cushioned the Module’s landing and kept it stable on the surface. Descent stage This bottom half of the Lunar Module acted as the launch platform when the top half blasted off back into space. The descent stage stayed on the Moon. Hatch The astronauts climbed through a hatch to go outside. Descent engine This engine was used to slow down the Lunar Module’s descent during landing. Gas tanks The larger tank contained helium; oxygen was held in the adjacent smaller tank. Leg ladder Astronauts used the ladder to climb down to the lunar surface. Fuel tank This tank contained fuel for the Lunar Module’s descent engine. Man on the Moon The Lunar Module was the only part of the Apollo craft to reach the Moon’s surface. Preprogrammed controls maneuvered it into position above the landing site, then an astronaut steered the craft to touchdown. Scientiﬁc equipment, a TV camera, and tools and storage boxes for rock collecting were all stored in the bottom half. Sensing probe Probes on the legs touched the ground ﬁrst during landing and sent signals to shut down the engine. space 36 SPACE EXPLORATION 4 spacecraft have visited the planet Saturn. Cassini-Huygens spacecraft Path to the planets The paths of spacecraft are often carefully planned to take them close to one or more planets on the way to their ﬁnal destination. Using the pull of gravity of each planet boosts their speed and saves fuel. Cassini-Huygens ﬂew past Venus, Earth, and Jupiter on its way to Saturn. 6. Arrival at Saturn 2. First Venus ﬂy-by Cassini-Huygens is the largest spacecraft to visit another planet. It was launched in 1997 and arrived at Saturn in 2004. It had two parts: the Cassini orbiter, designed to orbit Saturn until 2017, and a probe called Huygens, which touched down on Saturn’s large moon Titan. The main aim of the mission was to discover more about Titan—the only world in the Solar System other than Earth that has a dense, nitrogen-rich atmosphere. 3. Second Venus ﬂy-by Jupiter’s orbit Boom A 36-foot (11meter) boom carries instruments to measure Saturn’s magnetic ﬁeld. The long boom reduces interference from other instruments on Cassini. CASSINI 1. Launch 5. Jupiter ﬂy-by 4. Earth ﬂy-by Landmark missions Since the ﬁrst spacecraft to visit a planet was launched in 1962, about 200 craft have explored the Solar System. Some of the most famous missions are shown here. Descent capsule Venera 7 The ﬁrst craft to touch down on another planet, Venera 7 landed on Venus in 1970. It lasted 23 minutes before the searing heat destroyed it. Solar panel Lunokhod 1 Russian-built Lunokhod 1 was the ﬁrst lunar rover. It landed on the Moon in 1970 and spent 322 days exploring, traveling a total of 6.5 miles (10.5 km). Radio dish Spectrometer Voyager 1 Launched in 1977 and still operational, Voyager 1 is the furthest manmade object from Earth. It visited Jupiter in 1979 and Saturn in 1980. Rovers A rover is a robotic vehicle built to explore the surface of a planet or moon. Four rovers have landed successfully on Mars. They receive radio commands from Earth but ﬁnd their way around and carry out tasks independently. Curiosity lands The Curiosity rover was lowered on to Mars in 2012 by a rocket-powered craft. Sojourner rover The ﬁrst rover to explore another planet was Sojourner. It reached Mars in 1997 and spent 12 weeks studying the soil and taking photos. Small radio antenna This is one of two small radio dishes that serve as backups in case the main dish breaks down. CASSINI ORBITER (GREY) HUYGENS LANDER (RED) Large radio dish The large radio dish communicates with Earth. It is also used to map Titan, which it does by bouncing radio waves off the surface. 98,346 mph (158,273 kph)—the top recorded speed of Cassini-Huygens. 53 spacecraft have attempted to reach the planet Mars. 37 While manned spacecraft have ventured no further than the Earth’s Moon, robotic craft have visited all the planets in the Solar System—and more than 100 moons. Plutonium power supply (one of three) Main engine Robotic spacecraft can visit places that would prove lethal to astronauts, such as the scalding surface of Venus or the deadly radiation belts around Jupiter. Packed with scientiﬁc instruments, telescopes, and cameras, they carry out dozens of experiments during their missions and capture thousands of images, which are sent back to Earth by radio. Helium tank Helium gas from this tank pushes fuel from the fuel tanks to the engine. Fuel tanks The fuel tanks carry two different liquids that burst into ﬂame when mixed. Cosmic dust analyzer This device measures the size and speed of cosmic dust particles in space. Front case of Huygens probe Fuel for thrusters Back cover of Huygens probe Inner body case of Huygens probe Experiment platform Huygens carried a range of scientiﬁc instruments to study conditions on Saturn’s moon Titan. Plasma spectrometer This device measures charged particles trapped by Saturn’s powerful magnetic ﬁeld. missions to Mars have ended in failure. Exploring the planets Thruster Cassini has four thrusters. These small engines adjust the craft’s ﬂight path precisely. Radio antennas The three long antennas of the radio and plasma wave science (RPWS) instrument detect radio waves generated by Saturn’s outer atmosphere. 27 Heat shield Without a heat shield, the Huygens probe would have burned up like a shooting star when it entered the atmosphere of Saturn’s moon Titan. The shield was made of silica ﬁber tiles able to withstand temperatures up to 2,700°F (1,500°C). Triple parachute Packed under Huygens’s back cover were three parachutes that opened in turn to slow the lander’s descent onto Titan. Huygens discovered a world of freezing, orange-brown plains littered with pebbles of ice. EARTH Oceans of water, an oxygen-rich atmosphere, and the existence of life make Earth a unique planet. Its surface is continually changing as plates slowly shift and the relentless force of erosion reshapes the land. 40 Earth is the only place in the Universe known to support life. It is thought that life developed after water began to collect on the Earth’s surface. Eventually, tiny life forms evolved that could survive on water, sunlight, and chemicals in the water. These microbes added oxygen to the atmosphere—an essential step for the development of plants and animals. Earth’s interior has layers. Scientists discovered this by studying the paths by which earthquake waves pass through the planet. Earth’s atmosphere What’s in a layer? The atmosphere of Earth is made up of several different gases. Earth’s crust and mantle are mostly made of minerals called silicates, which are a combination of silicon dioxide and metal oxides. The mantle is rich in magnesium-containing silicates, while the two different types of crust have less magnesium and more aluminum and calcium. The core is dominated by metallic iron. No part of it has ever been brought to the surface, but its composition has been worked out by scientiﬁc methods such as studying earthquake waves. 0.9% ARGON 0.1% OTHER 21% OXYGEN 78% NITROGEN NITROGEN – 78% A gas that can be ﬁxed in the soil as well as loose in the atmosphere. Plants need nitrogen from the soil to survive. OXYGEN – 21% Essential for animals to breathe, oxygen was absent until microbes evolved that could use sunlight to turn carbon dioxide and water into carbohydrates, releasing oxygen. ARGON – 0.9% An inert gas (one that doesn’t react with other substances). OTHER – 0.1% These include carbon dioxide (C0₂), which was once abundant, but is now mostly incorporated into materials such as limestone rock. JOINED TOGETHER, FORMING AN ENORMOUS SUPERCONTINENT KNOWN AS PANGAEA. Thickness 3.7–56 miles (6–90 km) 1,790 miles (2,880 km) 1,400 miles (2,255 km) 755 miles (1,215 km) CRUST Different types of crust make up Earth’s surface and its ocean ﬂoor. The crust under the surface is thicker and contains more rock types. OUTER CORE The only liquid layer, the outer core is mainly iron but also contains some nickel and small amounts of other substances. MANTLE This rocky layer is denser than the crust. It is mostly solid, although it can very slowly deform and ﬂow. INNER CORE This is solid, and is mostly iron with some nickel. Its temperature is very hot—about 9,900°F (5,400°C). Key Silicon dioxide Aluminum oxide Iron and iron oxides Calcium oxide Magnesium oxide Nickel Other The oceans Earth’s surface and atmosphere contain the equivalent of 333 million miles³ (1.39 billion km³) of water. There are regions of deep ocean as well as shallow seas that cover areas around the edges of the continents—these are called continental shelves. Earth’s surface has not always been as dominated by liquid water. In the past, during ice ages when the polar ice caps were much thicker and more extensive, so much water became locked up in them that sea level was at least 400 ft (120 m) lower than it is today, exposing the continental shelves as dry land. 25% LAND 75% WATER Water world Almost three-quarters of Earth’s surface is water. Over 97 percent of Earth’s water is found in the oceans. CORE Inside our planet EARTH’S CONTINENTS ALL MANTLE UNIQUE PLANET 250 MILLION YEARS AGO, OCEANIC CRUST Earth formed about 4.5 billion years ago, but it was a very different place then. Its surface was a hot inferno of mostly molten rock, with little or no liquid water and no oxygen in the atmosphere. Since then Earth has developed oceans, continents, an oxygen-rich atmosphere—and life. Look at a map and it will show you the position of the continents, but in fact our world is always changing. Earth’s surface is split up into large slabs called tectonic plates. The plates steadily shift around, carrying continents and oceans with them. When they collide, new mountain ranges are pushed up. Afterward, over millions of years, wind, water, and ice gradually wear the mountains down. CONTINENTAL CRUST PLANET EARTH THE CHANGING EARTH 41 Continental drift Plate movement Over millions of years, tectonic plates have moved, shifting around the continents on Earth’s surface. Chunks of continents split away and push North America, Europe, and parts of Asia are one landmass India moves north into each other, creating new land masses and moving the oceans in a process called “continental drift.” Australia is joined to Antarctica South America separates from Africa Australia moves into the Paciﬁc Ocean The continents get rearranged because they are carried along as parts of moving plates. This process has been going on for billions of years, and is thought to be caused by slow, heatdriven movements in Earth’s mantle. 0 200 MILLION YEARS AGO The supercontinent Pangaea has just begun to break into two main landmasses. 130 MILLION YEARS AGO India has escaped from the southern landmass, and is slowing moving north, toward Asia. 70 MILLION YEARS AGO South America has split from Africa, while in the north, North America is splitting from Europe. TODAY Australia has separated from Antarctica, and India has collided with Asia, forming the Himalayas. LOOKING AT EARTH A spinning planet Magnetic Earth Our planet is far from smooth— its continents and ocean ﬂoors are scarred and pitted with marks caused by movement of plates. Earth’s place in space also affects its shape, as constantly spinning makes it bulge out around the middle so it is not a perfect sphere. Spinning also creates a magnetic safety ﬁeld around the planet. Earth’s gravity would pull it into the shape of a sphere, but its rotation makes it bulge slightly. This means its diameter at the equator is 25 miles (41 km) more than the distance between its poles. Because Earth’s outer core is liquid, the planet’s rotation stirs it into motion. This motion causes electric currents to develop in the liquid iron itself. Any pattern of electric currents creates a magnetic ﬁeld, and in Earth’s case, the ﬁeld is similar to what would be produced by a large bar magnet inside the planet. The ﬁeld protects Earth from damage by harmful, energetic particles that come from the Sun. MOUNTAINS Direction of rotation ONE-FIFTH Not quite round At the moment, scientists think that Earth’s equatorial bulge is growing at a rate of 0.3 in (7 mm) every 10 years. MAKE UP ABOUT OF THE EARTH’S LANDSCAPE. Bulge at middle The magnetic ﬁeld The magnetic poles do not coincide exactly with the geographic (rotational) poles, and they gradually change position over time. Earth’s surface The solid surface of the Earth ranges from about 35,750 ft (10,900 m) below sea level in the Challenger Deep (part of the Paciﬁc’s Mariana trench) to 29,029 ft (8,848 m) above sea level at the summit of Everest, which may be rising at about 0.16 in (4 mm) per year. The surface of most land areas is less than 1,650 ft (500 m) above sea level. Elevation Over 13,125 ft (4,000 m) 6,500–13,125 ft (2,000–4,000 m) 3,300–6,500 ft (1,000–2,000 m) 1,600–3,300 ft (500–1,000 m) 800–1,600 ft (250–500 m) 300–800 ft (100–250 m) 0-300 ft (0–100 m) Sea depth 0–800 ft (0–250 m) 800–6,500 ft (250–2,000 m) 6,500–13,000 ft (2,000–4,000 m) Below 13,000 ft (4,000 m) Mountains and trenches Earth’s solid surface is far from ﬂat. This map shows its elevations and depths—from the highest mountain peaks to the deepest ocean trenches. 1 2 Yearly shift Plates typically move at a rate of about 1 in (2.5 cm) in a year. That’s about as fast as your ﬁngernails grow. Some move faster—up to 4 in (10 cm) a year. Geographic North Pole Magnetic south pole Magnetic north pole Magnetic force Geographic South Pole 42 earth 4.5 PLANET EARTH Inside the Earth Continental crust Thicker and less dense than the oceanic crust. We can’t explore much of the Earth—our deepest mines only travel about a mile into the crust. However, there are scientiﬁc ways to ﬁnd out what it is like inside. Geologists are able to study rocks from all depths within the Earth’s crust, because collisions between continents push up rock that used to be below the surface, forming mountains. In some areas, collisions have even unearthed vast swathes of the mantle. Volcanoes also sometimes erupt lumps of rock from the mantle. Under the mantle is the core, which has never been seen at the surface. However, scientists have used the waves from earthquakes to work out that the core is split into two layers—a liquid outer core and a solid inner core. Volcanoes in Hawaii The Hawaiian islands in the mid-Paciﬁc have been built by volcanic eruptions. The rock that formed them was pushed to the Earth’s surface by hot rock moving upward in the mantle. Rifting Two tectonic plates pull away from each other, and new land is created between them. Oceanic crust Thinner and denser than the continental crust. 9,900°F (5,400°C) – the approximate temperature of Earth’s inner core. Layered Earth Earth is made up of many rocky layers. The top layer is the crust. Below that, uniform and slightly denser rock forms the mantle. The crust and the top of the mantle form a single rigid layer together, which is called the lithosphere. This is broken into sections called tectonic plates. Below the lithosphere is the asthenosphere. Only tiny parts of the asthenosphere are liquid, but it is soft enough to move, pushing around the plates above. Under the mantle lies the core. The outer core is a liquid mix of iron and sulfur, while the inner core is solid iron and nickel. billion years—the approximate age of Earth. Subduction When two plates meet, one can be pushed underneath the other. Paciﬁc Ocean There are four main layers inside the Earth. From the outside in, they are the crust, mantle, outer core, and inner core. Lithosphere The rigid outer shell, made of the crust and the top layer of the mantle. 43 The atmosphere Mantle A solid layer that is Earth’s thickest. Outer core Molten iron and sulfur. Currents in this liquid generate Earth’s magnetic ﬁeld. Earth’s atmosphere is made up of gases, which are held in place by gravity. There is no clear boundary to the outer edge of the atmosphere—it just fades into space. Ou