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  #1  
Old Wednesday, September 09, 2009
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Default All about space

UNIVERSE

Look up at the sky on a clear, starry night. What you see is part of the universe. You could never see all of the universe. No one even knows whether the universe has an edge or if it goes on forever. The universe contains all matter and energy. The universe holds all space and time. You are a part of the universe. The universe includes everything there is.

WHAT MAKES UP THE UNIVERSE?

Earth and all the planets in our solar system make up just a tiny part of the universe. Billions of other stars like our Sun all form a group called the Milky Way Galaxy. With telescopes we can see billions of other galaxies. Galaxies are in turn clumped together in enormous groups called clusters and superclusters. There are at least 100 billion—that’s 100,000,000,000—galaxies in the universe. The universe is a big place!

The universe holds many strange things, such as exploding stars. It holds great clouds of gas and dust where new stars form. It also holds black holes. Black holes have a pulling force called gravity. Gravity is the force that holds you to the ground and makes things fall when you drop them. Black holes suck in all the matter around them. The gravity of black holes is so strong that nothing, not even light, can escape.

Astronomers think there is even more matter in the universe than we can see. The matter we see makes up gas, dust, galaxies, stars, and planets. Astronomers think there is a type of invisible matter that they call dark matter. They think there may be a lot more of this mysterious dark matter in the universe than there is regular matter that we can see.

Astronomers also think the universe holds a strange substance that they call dark energy. They don’t know much about dark energy except that it seems to make the universe expand.

WHEN DID THE UNIVERSE BEGIN?

Many astronomers think the universe began about 14 billion years ago. They think it suddenly exploded into being. They call this beginning the big bang. All space and time began with the big bang. With a special telescope, astronomers can see radiation left over from the big bang. They call this cosmic background radiation.

The big bang theory says that the universe was hotter than you can even imagine at the moment it began. The universe started to fly outward, or expand. As it expanded, it started to cool. Tiny particles that would make up matter started to form. All of this happened in just a few minutes.

WHAT HAPPENED AFTER THE BIG BANG?

When the universe was about 1 million years old, it had cooled to about 5900° Fahrenheit (3300° Celsius). This temperature was still scorching, but it was cool enough that tiny bits of matter called atoms started to form. All the objects around you—the computer, the floor, the air—are made of atoms. The hot atoms gave off rays of light. The universe was like a huge, hot fireball.

Over millions of years, the force of gravity pulled gas and dust together. Stars and galaxies formed from clumps of gas and dust. The stars began to shine. Our Sun and our solar system formed about 4.6 billions years ago.

Rays of light travel across the universe. Even though light travels extremely quickly, it takes light millions or billions of years to travel from galaxies far away. When astronomers see light from the most distant galaxies they know the light is “old.” They know they are seeing the galaxies as they were billions of years ago.

WILL THE UNIVERSE END?

Astronomers want to know what will happen to the universe. They think it is still expanding. In fact, the universe seems to be expanding faster and faster as time goes on. Many astronomers think the universe will go on expanding forever. If it keeps on expanding, everything in the universe will eventually grow cold. Billions of years in the future even the stars will stop shining.
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Default Galaxies

GALAXIES

You look at a map to find out where you are. A map can tell you what city you are in and what street you are on. But do you know where you are in the universe? For hundreds of years, astronomers—scientists who study things in space—have been trying to figure out where our solar system is in the universe. They have learned that our Sun is part of a huge group of stars called a galaxy. Our galaxy is called the Milky Way. The Milky Way is just one of billions of galaxies.

WHAT ARE GALAXIES MADE OF?

A galaxy is made up of millions or billions of stars. Big clouds of gas and dust swirl in space between the stars.

Astronomers believe that most galaxies have an enormous black hole in their centers. A black hole is an object with lots of matter packed into it. A black hole has a powerful pull of gravity. Gravity is the force that holds you to the ground and pulls a ball back down after you throw it up in the air. The gravity of a black hole is so strong that it sucks in and crushes anything that comes near. Not even light can escape from a black hole.

WHAT DO GALAXIES LOOK LIKE?

Galaxies come in different shapes. Some galaxies look like giant whirlpools or pinwheels. They have long arms made of gas and dust clouds and stars. These are called spiral galaxies because the arms spiral into the center. The Milky Way is a spiral galaxy.

The stars and clouds of gas and dust in a spiral galaxy move slowly in a circle. They orbit, or go around, the center of the galaxy. New stars form in the clouds of a spiral galaxy.

Some galaxies are oval or round in shape. These are called elliptical galaxies. Elliptical galaxies tend to have older stars. Few new stars form in elliptical galaxies.

Some galaxies do not have any particular shape. These are called irregular galaxies.

When galaxies come close to each other, their shapes can change. Sometimes galaxies collide with one another. Irregular galaxies may be galaxies whose original shapes were distorted by collisions.

HOW DO WE LEARN ABOUT GALAXIES?

Galaxies give off different kinds of light. Astronomers study the light to learn about the galaxies.

Galaxies give off rays of light our eyes can see. They also give off radio waves. Galaxies give off heat, or infrared rays, too. They also give off X rays and gamma rays. Even though our eyes can’t see them, radio waves, infrared rays, X rays, and gamma rays are all types of light. Astronomers have instruments that can detect each of these types. The different types of light provide clues about what galaxies are made of and how they form.

Astronomers figure out how far away a galaxy is by studying the light that comes from it. They have found galaxies that are very far out in the universe. The Hubble Space Telescope took pictures of galaxies that are 13 billion light-years away from Earth. A light-year is how far light travels in one year. Light travels fast—about 6 trillion miles (about 10 trillion kilometers) a year. Six trillion is a 6 with twelve zeroes after it: 6,000,000,000,000! These most distant galaxies are so far away that it took their light 13 billion years to reach Earth.

Astronomers looking through powerful telescopes have also found that galaxies clump together. They form big groups called clusters. The clusters also tend to be clumped. They form even bigger groups called superclusters. Our Milky Way is part of a cluster of about 30 galaxies called the Local Group. The Local Group is part of a supercluster called the Local Supercluster. Long strands of superclusters wind through the universe. These strands look something like a jumbled spiderweb, with galaxies strung along the threads and vast voids of empty space between the threads.
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Default Milky Way

MILKY WAY

When you look up at the sky on a clear night, you see thousands of tiny points of light. A couple of the brightest points might be planets in the solar system. The rest are all stars. If you look closely, you might see a thick glowing band of faint white light crossing the sky. To see it, you’ll need to be somewhere without many streetlights. Ancient Greeks named this band of light the Milky Way.

The Sun is a star. It’s a lot closer to Earth than any other star, so it appears a lot brighter. The white glow of the Milky Way is made by billions of stars so far away that our eyes can’t see them as individual points of light.

WHAT IS THE MILKY WAY?

The Milky Way is a huge group of stars called a galaxy. There are billions of stars in the Milky Way. The Sun and all nearby stars are part of the Milky Way Galaxy. There are also huge clouds of gas and dust in between the stars. New stars form in the clouds of gas and dust.

The Milky Way Galaxy is shaped like a thick disk turning in outer space. There is a big bulge at the center of the disk. Curved arms spiral into the central bulge. The whole thing looks like an enormous whirlpool or pinwheel. Our Sun and solar system are in one of the whirlpool’s arms out toward the edge of the disk. That’s why the Milky Way looks like a band of light in the night sky. We’re looking at the edge of the disk instead of the round face.

The Milky Way turns slowly. Everything in the Milky Way orbits (circles) the center of the galaxy. It takes about 250 million years for our solar system to go once around the center of the Milky Way.

Astronomers think there might be an enormous black hole at the center of the Milky Way. A black hole sucks in everything around it. A black hole is invisible. Not even light can escape from a black hole.

The Milky Way is not the only galaxy in the universe. With powerful telescopes, we can see billions of other galaxies. Many of them are shaped like our own Milky Way, but some look like giant balls or strands of trailing stars.

HOW BIG IS THE MILKY WAY?

The Milky Way is huge. The entire Milky Way is about 100,000 light-years across. Astronomers measure great distances in light-years. One light-year is how far light travels in one year. Light travels extremely fast. A flash of light goes almost 6 trillion miles (10 trillion kilometers) in one year. That’s a 6 with twelve zeroes after it: 6,000,000,000,000! Even at that blazing speed, it would take a flash of light 100,000 years to cross the Milky Way.

The bulge at the center of the Milky Way is about 10,000 light-years thick. Our solar system is about 25,000 light-years from the center of the galaxy.
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Default Star

STAR

Go outdoors at night and look up at the sky. There are twinkling points of light everywhere. You are seeing thousands of stars that are millions of miles away. The stars look tiny because they are so distant. But if you could see those stars up close, you would see huge balls of fire.

The closest star to you on Earth is the Sun. The Sun is a star at the center of our solar system. Our Sun is about 4.6 billion years old. There are stars that are older or younger than our Sun. There are stars that are much bigger. There are stars that have exploded and stars that are just being born.

WHAT IS A STAR?

A star is a big ball of hot, glowing gas. The gas is mostly hydrogen and helium. Stars give off heat, light, and other kinds of energy.

A star has several layers. The part at the center of a star is called its core. A star shines because of its core. The core is so hot and tightly packed that atoms crunch together. Atoms are tiny bits of matter much too small to see. Hydrogen atoms crunch together and become helium atoms. This is called nuclear fusion. Nuclear fusion gives off enough energy to make the stars shine.

GROUPS OF STARS

Stars are part of groups called galaxies. Our Sun is in the Milky Way Galaxy.

People in ancient times grouped stars by patterns they thought they saw in the sky. The patterns are called constellations. They thought the patterns looked like people, animals, or objects. The Big Dipper is a constellation of seven stars in the shape of a dipping ladle. Astronomers map where the stars are in the sky using the constellations.

WHAT COLOR ARE STARS?

Stars come in different colors. They can be deep red, orange, yellow, white, or even blue. The color of a star depends on how hot the star is. The coolest stars are reddish and the hottest stars are bluish.

It is hard to imagine how hot a star can be. The temperature at the surface of red stars is about 5400° Fahrenheit (about 3000° Celsius). Yellow stars have surface temperatures about 11,000° Fahrenheit (about 6000° Celsius). Our Sun is a yellow star. White stars are about 18,000° Fahrenheit (about 10,000° Celsius)!

A star looks as if it is just one color. Starlight, however, is made up of many colors. Light from our Sun has all the colors of the rainbow. Astronomers study the light of other stars. Patterns in the light can tell astronomers what the stars are made of and how hot they are.

HOW BRIGHT IS A STAR?

Some stars in the sky look brighter than others. Some stars really are brighter. Other stars just look brighter because they are closer.

Some stars are not nearly as bright as the Sun. Other stars are as much as 500,000 times brighter.

HOW BIG IS A STAR?

The Sun is huge compared to Earth. If the Sun were hollow, a million Earths could fit inside it!

Astronomers compare the size of other stars to the size of the Sun. For example, a star called Betelgeuse is about 1,000 times bigger than the Sun.

WHERE DO STARS COME FROM?

Stars are born from swirling clouds of gas and dust. Gravity pulls the gas and dust together. The gas and dust form a spinning ball. As it spins, it gets hotter. The gas and dust get tightly packed. Finally, nuclear fusion begins and the star starts to shine.

A STAR’S LIFE

There are different stages in a star’s life, just as there are different stages in the lives of people. Right after a star is born it starts to get smaller. After a million years of shrinking, the star enters the main sequence of its life.

After about 10 billion years, the star’s core runs out of fuel. The star grows many times larger than it was during the main sequence. At this stage the star is called a red giant. What happens next depends on the size of the star.

GIANTS AND DWARFS

Medium-sized stars like our Sun become white dwarfs. White dwarfs can explode. The outside gas layers blow off and make clouds called nebulas. The core keeps shrinking. A spoonful of white dwarf core could weigh more than a dump truck. After several billion years, the star loses all its energy and becomes a cold black dwarf.

Really big stars become supergiants. Supergiants become supernovas, which are big exploding stars. The explosion sends gas and dust into space to make new stars. The core gets packed tighter and tighter. Some cores then turn to iron and become neutron stars. Some supernova cores turn into black holes, which swallow everything around them in space. Not even light can escape from a black hole.

COULD ASTRONAUTS VISIT A STAR?

Stars beyond our solar system are too far away for a spacecraft to reach. The closest star is Proxima Centauri. It is more than 4 light-years from Earth. A light-year is the distance light travels in a year, about 6 trillion miles (about 9 trillion kilometers) Most stars are much farther away than Proxima Centauri. No spaceship can travel fast enough to reach even the nearest star during an astronaut’s lifetime. It takes billions of years for even light to reach the most distant stars.
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Default Sun

SUN

The Sun is very important to you. You play in sunshine. You see in daylight. The Sun keeps you warm. Even ancient people knew the Sun was very important. They thought the Sun was a god. The ancient Greeks thought the Sun god drove a chariot across the sky every day. The ancient Egyptians thought the Sun god sailed a boat across the sky. Today we know that the Sun is a star. The Sun is the star at the center of our solar system. Earth and all the other planets orbit, or go around, the Sun. The Sun is very important to all life on Earth.

THE SUN IS A STAR

The Sun is a star—a ball of hot, glowing gas. It does not have any solid parts. It is made up mostly of hydrogen gas and helium gas. The Sun is huge compared with Earth. If the Sun were hollow, a million Earths could fit inside it. The Sun looks small only because it is far away. The average distance from Earth to the Sun is 93 million miles (150 million kilometers). You would have to go around the world more than 3,700 times in order to travel that far on Earth.

The force of Earth’s gravity holds you on the ground. The Sun’s gravity holds Earth and the other planets in their orbits. It holds asteroids, comets, and dust in orbit.

The Sun is one of about 400 billion stars in the Milky Way Galaxy. A galaxy is a large group of stars. The Sun and all the other stars orbit the center of the Milky Way.

ENERGY FROM THE SUN

Almost all the energy on Earth comes from the Sun. Heat from the Sun makes Earth warm enough for life. Plants use the Sun’s energy to live and grow. Plants give off a gas called oxygen. Animals eat the plants and breathe the oxygen. Animals need plants in order to live, and plants need the Sun.

You use plants to make heat and energy. You can burn wood from trees. You can burn fossil fuels called coal, gas, and oil. Fossil fuels formed deep underground from plants and animals that died millions of years ago.

The Sun’s energy can also do harm. Too much sunlight can burn your skin, causing sunburn. Harmful rays from the Sun can also cause a disease called skin cancer. Looking right at the Sun can harm your eyes. You need to be careful of the Sun.

THE CORE OF THE SUN

The center of the Sun is called the core. The core is extremely hot. The heat sends tiny bits of matter called atoms crashing into each other. The crashing atoms set off atomic or nuclear reactions. All the energy of the Sun comes from these nuclear reactions in its core. It takes a long time for the energy from the core to reach the surface of the Sun—about 170,000 years!

THE SURFACE OF THE SUN

The photosphere is the outer part of the Sun that we can see. Like the rest of the Sun, the photosphere is made of hot hydrogen and helium gas. Heat and light from the photosphere reach Earth. The temperature of the photosphere is about 9950° Fahrenheit (about 5500° Celsius).

Fountains of red-hot gas shoot up thousands of miles from the photosphere into the Sun’s atmosphere. Cooler dark spots called sunspots form on the photosphere.

Far above the photosphere is the corona. The corona is the top layer of the Sun’s atmosphere. The corona is so faint that the only time you can see it is when the light from the rest of the Sun is blocked. Astronomers use discs to block the light so they can study the corona. The temperature of the corona goes up to 4 million degrees Fahrenheit (2 million degrees Celsius). The corona trails off into space. Gases that blow off the corona are called the solar wind. The solar wind reaches far beyond Earth.

THE SUN IS A MAGNET

The Sun is a huge magnet. The magnetism of the Sun causes strange things to happen. Bright explosions called solar flares flash in the corona. The flares send gases looping out into space. Sometimes there are huge explosions in the corona that send billions of tons of material into space. The flares and explosions can cause magnetic storms on Earth. These storms cause problems for satellites and cell phones.

HOW DID THE SUN FORM?

Astronomers believe our solar system formed about 4.6 billion years ago from a swirling cloud of dust and gas. The Sun formed first at the center of the cloud. Then the planets formed from dust and gas going around the Sun.

Someday the Sun will burn out. It will use up all the fuel in its core. You don’t need to worry. Astronomers say that the fuel will last several billion more years.
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Default Mercury

MERCURY

Not even the hottest day at the hottest place on Earth comes close to how hot it gets on Mercury. Mercury is one of the nine planets in our solar system. Mercury is the closest planet to the Sun.

HOW HOT DOES IT GET ON MERCURY?

Daytime temperatures on Mercury can soar to 810° Fahrenheit (430° Celsius). That is hot enough to melt some metals! Mercury gets so hot because it is so close to the Sun. Mercury is 36 million miles (58 million kilometers) from the Sun, on average. Its distance from the Sun changes somewhat throughout the year. Earth is about three times farther from the Sun.

It really cools off at night on Mercury. The temperature can drop as low as -290° Fahrenheit (-180° Celsius) after the Sun goes down. So Mercury doesn’t just get hotter than anywhere on Earth. It also gets colder! It gets so cold partly because Mercury has almost no atmosphere. An atmosphere is the gases around a planet. On Earth, the gases act like a blanket that holds in heat. Another reason it gets so cold is that Mercury turns slowly. On Mercury, night lasts much longer than it does on Earth, and all the heat leaks away into space.

TIME ON MERCURY

A day on Mercury is a long time. One Mercury day lasts almost 59 days on Earth. One day is the time it takes for a planet to spin once around its axis. An axis is an imaginary line going through the center of a planet. Mercury turns very slowly on its axis.

A year on Mercury, however, is pretty short. Mercury moves fast around the Sun. It takes Earth 365 days to go around the Sun, so a year on Earth is 365 days long. A year on Mercury is only 88 Earth days. That’s the time it takes Mercury to go around the Sun. Since Mercury’s year is so short and its day is so long, the planet only has 3 days for every 2 years!

WHAT IS MERCURY MADE OF?

Mercury has a surface made of rocks. Some places on Mercury are smooth. Some places have wrinkly ridges.

There are many big, round holes called craters on the surface of Mercury. Meteors and comets crash into Mercury and make the craters. Meteors are chunks of stone and metal that fall from space. Comets are balls of ice and rock. There is ice at the bottom of some deep craters near Mercury’s north and south poles. Scientists think the ice may have come from comets. Mercury has no liquid water.

Scientists think that there must be a lot of iron metal inside Mercury. They think this because Mercury is a giant magnet. Earth is also a giant magnet. Earth is a magnet because it has hot, liquid iron in its core. The liquid iron spins and makes Earth a magnet. Scientists think the same thing happens on Mercury. But Mercury is a weaker magnet than Earth.

HOW DO WE LEARN ABOUT MERCURY?

Scientists do not know as much about Mercury as they do about most of the other planets. It is hard to study Mercury, because Mercury is hard to see.

Mercury is the second smallest planet, after Pluto. Its diameter (the width of the planet) is 3,032 miles (4,879 kilometers). That is about the distance across the United States. Mercury also orbits close to the Sun. The Sun is so bright that its glare makes Mercury hard to see from Earth. Special telescopes are needed to study Mercury.

Only one spacecraft has visited Mercury. Mariner 10 flew past Mercury in 1974 and 1975. Mariner 10 took lots of pictures. It measured Mercury’s temperature. Most of what scientists know about Mercury came from Mariner 10.
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Default Venus

VENUS

Days on Venus seem endless. Venus is the second planet from the Sun. One day on Venus is as long as 243 days on Earth. A year on Venus is only 225 Earth days long. So a day on Venus is longer than a year!

WHY DOES VENUS HAVE LONG DAYS?

Venus orbits, or goes around, the Sun just as all the planets do. It takes Venus 225 days to go around the Sun. Earth goes around the Sun in 365 days. We call the time it takes Earth to go around the Sun a year.

Venus also spins around on its axis. An axis is an imaginary line going through a planet from top to bottom. Every planet spins on its axis. One day on Earth is the time it takes for Earth to turn completely around on its axis. Earth turns once every 24 hours. Venus turns much more slowly. It takes Venus 243 Earth days just to turn once.

Venus also spins backwards around its axis. It spins in the opposite direction that Earth spins. On Venus, the Sun rises in the west and sets in the east.

WHAT IS VENUS LIKE?

Venus is more like Earth in some ways than any other planet. It is almost the same size as Earth. It is a similar distance from the Sun compared to the other planets. It is made mostly of rock and has an atmosphere (gases that surround a planet).

Venus has flat plains and high places, just like Earth. It has huge extinct (dead) volcanoes and big craters. Meteorites crashing into the planet made the big craters. But Venus has no moon.

In other ways, Venus is not at all like Earth. It is not a place you would like to visit. The atmosphere is poisonous. It is made up mainly of a gas called carbon dioxide. The clouds are filled with drops of powerful acid “rain” that would eat through your clothes—and through you. The atmosphere is so thick that its weight would crush you. There is no water on Venus.

It is very hot on Venus. The temperature on the surface is about 864° Fahrenheit (462° Celsius). That’s more than four times as hot as boiling water. No plants or animals could live in a place that hot.

WHY IS VENUS SO HOT?

The thick atmosphere of Venus keeps the planet hot. The carbon dioxide in the atmosphere of Venus is called a greenhouse gas. It acts like the glass in a greenhouse. Greenhouse glass traps heat from the Sun. The heat keeps a greenhouse warm all year long.

Earth has just enough carbon dioxide in its atmosphere to keep our planet warm enough for life. Venus has too much carbon dioxide. Venus traps too much heat from the Sun. This makes Venus too hot for anything to live there.

HOW DO WE LEARN ABOUT VENUS?

People in ancient times knew about Venus. They could see Venus when they looked up at night. Venus is brighter than any object in the sky other than the Sun and the Moon. Sometimes Venus is the first object to appear in the sky as it gets dark. Sometimes Venus is the last object to fade in the morning light. The ancient Romans called Venus the morning star or the evening star. They named Venus for the ancient Roman goddess of love and beauty.

Venus was the first planet to be visited by spacecraft. A United States spacecraft named Mariner 2 flew by Venus in 1962. Many spacecraft have visited Venus since then. These spacecraft carried instruments to take measurements. Some of them carried radar to see through the thick clouds on Venus. Radar “sees” with radio waves. Some U.S. and Russian spacecraft landed on Venus.

The spacecraft that landed on Venus had to plunge through thick clouds filled with sulfuric acid. They had to survive lightning flashes and powerful winds. The weight of the planet’s atmosphere crushed the spacecraft, and the heat on Venus baked them. Some spacecraft sent back pictures and measurements before they stopped working.
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Default Earth

EARTH

Pretend you are an alien explorer from outer space looking for life on other planets. Your spaceship flies into a group of stars that looks like a gigantic whirlpool. The whirlpool is the Milky Way Galaxy.

You head for a star with nine planets in one arm of the Milky Way. The third planet from the star is a beautiful blue, white, and green ball. This planet looks like it has life. The name of this planet is Earth.

HOW DOES EARTH MOVE IN SPACE?

Earth spins like a top on its axis. Earth’s axis is an imaginary line that goes through Earth from the North Pole to the South Pole. Earth’s axis is slightly tipped, like a spinning top leaning to one side.

Earth travels around the Sun at about 67,000 miles per hour (about 107,000 kilometers per hour). One year is one trip around the Sun. Earth’s path around the Sun is slightly oval-shaped. This oval shape causes Earth’s distance from the Sun to change during the year.

WHAT MAKES DAY AND NIGHT?

The Sun seems to rise in the morning, cross the sky during the day, and set at night. However, the Sun does not actually move around Earth. Earth’s turning on its axis makes it look as if the Sun is moving.

Earth makes a complete turn on its axis every 24 hours. As Earth turns, half of the planet faces the Sun, and the other half faces away. It is daytime on the half facing the Sun. It is night on the half facing away from the Sun.

WHY ARE THERE SEASONS?

Earth has seasons because of the tilt of its axis. For part of the year, the top half of Earth is tipped toward the Sun. The top half of Earth is called the Northern Hemisphere. During another part of the year, the bottom half of Earth is tipped toward the Sun. The bottom half is called the Southern Hemisphere. It is summer in the half that is tipped toward the Sun. It is winter in the half tipped away. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere. During spring and fall the hemispheres are tipped neither toward nor away from the Sun.

The equator is an imaginary line around Earth’s middle. The farther you are from the equator, the greater the difference in temperature between seasons. The equator never tips far from the Sun. Near the equator it is warm enough to go swimming all year long. The average temperature barely changes from month to month. In Alaska, far from the equator, the average temperature in January can be more than 60 degrees colder than it is in July.

WHY IS THERE LIFE ON EARTH?

Earth has just the right conditions for life. It is not too hot or too cold. Earth has lots of liquid water and an atmosphere (gases) that can support life.

The first kinds of life may have appeared on Earth 3.8 billion (3,800,000,000) years ago. Several times during Earth’s history, almost all life went extinct, or disappeared. Each time, some life forms survived. The survivors spread all over the planet. Dinosaurs appeared about 230 million years ago. Dinosaurs went extinct about 65 million years ago. Scientists believe that modern humans appeared about 130,000 years ago.

WHAT IS THE INSIDE OF EARTH LIKE?

Earth is made of layers. The top layer is called the crust. It is made of hard rock and soil.

More than 70 percent of Earth’s crust is covered with water. Most of the water is salt water in the ocean. Pieces of dry land called continents rise above the ocean. The part of Earth’s crust under the ocean is called the seafloor.

Under the crust is a layer of partly melted rock called the mantle. Under the mantle is Earth’s core. The core is mostly iron. The outer part of the core is liquid metal. The inside of the core is solid metal. Scientists believe that the liquid metal makes Earth a giant magnet and creates Earth’s magnetic field.

Earth’s crust is made of gigantic slabs of rock called plates that move over the mantle. Plates crash together to make mountains. They pull apart and let red-hot rock ooze up from inside Earth to make new crust.

HOW DID EARTH FORM?

Scientists think that Earth and the rest of the solar system formed from a spinning cloud of gas and dust. Gravity pulled most of the gas and dust together to form the Sun. Some leftover gas and dust formed Earth and the other planets. Scientists think that Earth and the Moon formed about 4.6 billion years ago.
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Default Mars

MARS

Someday people will stand on the surface of another planet. That planet will probably be Mars. Maybe you’ll be one of the people to go!

Mars is one of the brightest objects in the night sky. It looks like a reddish star. The ancient Romans named Mars after their god of war because it was the color of blood. The red planet is the next planet out from the Sun after Earth. Of all the planets in the solar system, Mars is the most like Earth.

WHAT IS MARS LIKE?

Mars is a small, rocky planet much like Earth. Pictures of the surface of Mars sent back by spacecraft look like deserts on Earth.

Mars is about half the size of Earth, but its land area is about equal to Earth’s land area. That’s because Mars has no oceans. Mars does have water, however. Some of the water is frozen in icecaps at the planet’s north and south poles. Scientists also think there may be a lot of water frozen underground.

A day on Mars is just a bit longer than a day on Earth. Mars even has seasons! Although Mars is much colder than Earth, on the warmest summer days it gets up to about 60° Fahrenheit (15° Celsius). That would be a very pleasant day on Earth.

STANDING ON MARS

Rocks and red, dusty soil cover the Martian surface. You feel light—you can jump higher and throw a ball farther than you can on Earth. Gravity, the force that holds you to the ground, is weaker on Mars than it is on Earth.

There’s a pinkish sky overhead, perhaps with a wisp of cloud. At night you see two tiny moons above you. The moons are named Phobos and Deimos. There might be a breeze blowing, but you cannot breathe the air. The air on Mars is very thin. It does not have much oxygen, the gas you need to breathe.

GIANT-SIZED LANDSCAPE

Mars may be a small planet, but many of its natural features are bigger than similar features on Earth. That’s because the weaker gravity on Mars doesn’t make huge mountains collapse or pull steep canyon walls down. The red planet has enormous volcanoes. The biggest volcano in the solar system, Olympus Mons, is on Mars. It is 16 miles (26 kilometers) high. That’s more than twice as high as Mount Everest, the tallest mountain on Earth! The base of Olympus Mons would cover the state of Arizona.

Mars also has enormous canyons. Valles Marineris, a group of connected canyons, would stretch from New York State to California on Earth! Valles Marineris is more than 4 miles (7 kilometers) deep in places. The Grand Canyon is just a ditch compared to that!

There are also enormous channels on Mars that look like dried up riverbeds. Scientists think that Mars once had lakes and rivers. They think that once there were big floods on Mars. The floods carved the channels. Mars is very dry today, however. Scientists are trying to figure out what happened to the water.

Mars has dust storms that sometimes engulf the entire planet. The giant storms can blow for months. The planet’s lower gravity allows the dust to float up in the air for a long time. The dust makes the sky pink.

HOW DO WE LEARN ABOUT MARS?

Scientists called astronomers look at Mars through telescopes. The best pictures of Mars come from the Hubble Space Telescope. This telescope orbits in space high above Earth.

Several countries have sent spacecraft to visit Mars. Some of these spacecraft landed on the red planet. In 2004, the United States landed a pair of robot rovers named Spirit and Opportunity on Mars. Rovers have wheels. They can roll around and look at rocks. They send pictures and other information back to Earth. Spirit and Opportunity found strong evidence that Mars once had liquid water.

IS THERE LIFE ON MARS?

So far, scientists have not found anything alive on Mars. They think that the surface of Mars is too cold and poisonous for life. They wonder if there is life underground. Perhaps Martian life forms live in cracks in the rocks.

Scientists also wonder if there was life on Mars in the past. Mars was once warm enough to have liquid water in lakes and rivers. Living things need water.

Mars is cold now because its air is so thin. Earth is warm enough for life because its air traps heat from the Sun. Scientists wonder if the air around Mars was once thick enough to trap heat from the Sun.

COULD PEOPLE LIVE ON MARS?

If people ever live on other planets, Mars will probably be the first one they move to. Some scientists and government officials want to send astronauts to Mars.

Other scientists and officials do not think that sending people to Mars is a good idea yet. They think it is best to keep exploring the red planet with better robot rovers.
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Default Jupiter

Jupiter

Could Jupiter have been a star instead of a planet? Jupiter is the fifth planet from the Sun and the biggest planet in our solar system. More than 1,300 planets the size of Earth could fit inside Jupiter. If you could lump together all eight of the other planets in the solar system, the resulting planet would still be smaller than Jupiter! In some ways, Jupiter is more like a star than a planet.

A FAILED STAR?

Like the Sun and other stars, Jupiter is a huge ball of hydrogen gas and helium gas. It gives off more heat than it receives from the Sun. Many moons and several thin rings go around Jupiter. Jupiter’s moons and rings are like a little solar system.

Our solar system formed from a big cloud of gas and dust. The Sun formed at the center. The Sun sucked in gas and got bigger and bigger and hotter and hotter. Finally, it was big enough and hot enough to become a star and shine. Jupiter formed the same way. But Jupiter never got big and hot enough to become a star like our Sun. There was not enough gas in the solar system to make two stars. Jupiter is more like a star that failed.

WHAT’S IT LIKE ON JUPITER?

You could never land a spacecraft on Jupiter. Jupiter does not have a hard surface. Astronomers (scientists who study space) call Jupiter a gas giant because it’s made almost entirely of gas. From space, Jupiter looks striped. The stripes are actually bands of colored clouds that circle the planet.

The clouds near the top of the atmosphere are bathed in deadly radiation. Jupiter's highest clouds are also very cold. The clouds get hotter deeper down. Strong winds blow in opposite directions in each band of clouds. In one band, the winds blow toward the east. In the next band, they blow toward the west.

Big storms rage in the clouds around Jupiter. The biggest storm is called the Great Red Spot. Three planets the size of Earth could fit across the Great Red Spot. This storm may have lasted for hundreds of years.

Deeper into the planet, the gas gets thicker and heavier. It gets so heavy that some of it gets squeezed into a layer of liquid hydrogen. Astronomers think that there might be some rock and metal in the center, or core, of Jupiter.

JUPITER’S MOONS AND RINGS

Jupiter has more moons than any other planet in our solar system. About 400 years ago, Italian astronomer Galileo Galilei turned a telescope to the sky and discovered moons going around Jupiter. The four biggest moons are the ones that Galileo discovered. Astronomers call them the Galilean satellites. Their names are Io, Ganymede, Europa, and Callisto.

Io is covered with volcanoes. Many of the volcanoes on Io are active, which means they are erupting or might erupt again someday. Io has more volcanoes than any other planet or moon.

Ganymede is the biggest moon in our solar system. It is even bigger than the planets Mercury and Pluto!

Europa and Callisto are covered with ice. Astronomers think that enormous oceans of water may lie under the ice.

The rings around Jupiter are thin, dark, and hard to see. They are made of rock and tiny bits of dust.

HOW DO WE KNOW ABOUT JUPITER?

The ancient Romans named the planet after Jupiter, their chief god, because it shines so brightly in the night sky.

In the early 1600s, Galileo became the first person to study Jupiter with a telescope. He helped prove that Jupiter and all the planets go around the Sun. Before then, astronomers thought that the Sun, other stars, and all the planets went around Earth. Today, astronomers use much more powerful telescopes to look at Jupiter and other objects in space.

Several spacecraft have visited Jupiter. The first, in 1972, was Pioneer 10. Cameras on the spacecraft took pictures of Jupiter. Instruments on the spacecraft made measurements of the planet.

In 1994, astronomers watched a comet crash into Jupiter. Jupiter’s gravity tore the comet apart as the comet got close. Pieces of the comet smashed into Jupiter’s atmosphere. The impacts caused huge explosions. Powerful winds on Jupiter then blew the remains of the comet away.
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