UFO

You’re watching the sky after dark. It’s a clear night. Many stars are visible. Suddenly you see a streak of light. Perhaps it’s a shooting star. As you watch, it slows down. Now it’s a moving point of light. Maybe it’s a plane or a space shuttle. You see it slow to a stop.

Half a minute passes. Just when you think you’re looking at an ordinary star, it suddenly speeds away. In seconds, it disappears over the horizon.

Have you seen an unidentified flying object? Most certainly. Was it a spaceship from another planet? Almost certainly not.

WHAT UFOS ARE—AND ARE NOT

An unidentified flying object (UFO) is any light or object seen in the sky that can’t be explained. Hundreds of UFO sightings are reported every year. More than nine-tenths of them are later identified as ordinary objects. They might be planes, satellites, or planets. They might be balloons, kites, clouds, or birds. Some are deliberate fakes.

But some UFOs remain unidentified. So some people believe that they come from other planets. Some even claim to have met aliens and been brought aboard their spacecraft.

It is indeed possible that intelligent life might exist somewhere beyond the solar system. But no one has ever produced proof of visitors from outer space. No photograph of a spaceship has ever been accepted as genuine.

PROJECT BLUE BOOK

UFOs have been reported since ancient times. Modern reports of UFO sightings began in 1947. Some observers reported seeing saucer-like shapes in the sky. Since then, UFOs of any shape are often called flying saucers.

The United States Air Force took these reports seriously. Air Force officials did not necessarily believe in visitors from outer space. They wanted to be sure that UFOs were not a threat to our country. A UFO might, for example, be an enemy airplane.

Project Blue Book was the Air Force’s program for investigating UFOs. It ran from 1947 through 1969. The Air Force investigated 12,618 UFO sightings. All but 701 were identified.

The Air Force concluded that UFOs were no threat. All sightings could be explained by science. There was no evidence that UFOs came from other planets.

WHAT ARE YOU REALLY SEEING?

In 1997, the United States government released some information. It had been operating high-flying spy planes. These flights were secret. More than half the UFOs reported during the 1950s and 1960s were these planes.

Many UFOs appear in the sky opposite the Sun. Their shapes may appear strange. But they’re actually ordinary objects that reflect the Sun’s rays. They often prove to be planes or balloons. The vapor trails of jet planes can take on odd shapes. They can look like saucers. They may appear to hang motionless in the sky. Such appearances can fool even experienced scientists.

If you see a UFO, look at it carefully. Note what time it is. Where are you? In what direction does the object appear in the sky? Sketch the UFO’s shape. Does it change as it moves? Such careful observation can usually identify a UFO.

Gravity

Try to jump as high as you can. Bend your knees. Now jump! No matter how hard you try, or how high you jump, you always come back down again.

Something called gravity pulls you back down. Gravity holds you down on Earth. Without gravity, you would fly off into space. You would jump up and just keep on going. That might sound like fun, but you could not live very high up. For one thing, you need to breathe air. The higher you go, the less air there is. You need gravity to keep you down on Earth.

WHAT IS GRAVITY?

Gravity is a force that pulls two objects toward each other. The force is also called gravitation. The bigger an object is, the stronger its gravity. Small objects, and even cars and buildings, have so little gravity that you can’t feel their pull. Huge objects like Earth, however, are a different story.

Earth is so much bigger than you that its gravity makes you “stick” to the ground. Earth is bigger than the Moon. Earth’s gravity pulls on the Moon. That is why the Moon orbits, or goes around, Earth. The Sun is bigger than Earth. Its gravity makes Earth go around the Sun.

Earth’s gravity pulls toward the center of Earth. Gravity holds the oceans and the atmosphere on Earth. The Sun’s gravity pulls toward the center of the Sun. It holds the Sun together.

WHO DISCOVERED GRAVITY?

Gravity has always been around. In ancient times, people tried to explain why things fall toward Earth. An English scientist named Sir Isaac Newton came up with a great idea about gravity in 1687. He thought about how an apple falls and wondered how far gravity went. He came up with the idea that gravity does more than hold people on Earth.

Newton thought of gravity as a kind of mysterious force pulling objects together. He said that gravity holds the Moon in orbit around Earth. It holds the planets in orbit around the Sun. Newton’s ideas on gravitation explained many things about how apples fall and how stars and planets move.

A DENT IN SPACE-TIME

In the early 1900s, another scientist named Albert Einstein came up with a very different idea of what gravity is. Einstein had some pretty amazing ideas. He said there was something called space-time. He thought of gravity as a dent in space-time.

This is not as hard to imagine as it might seem. Think about placing a bowling ball on a soft mattress. The mattress is like space-time. The bowling ball is like a star. The bowling ball makes a dent in the mattress. This is sort of how a star dents space-time. If you try to roll a tennis ball past the bowling ball, it curves around the bowling ball as it passes through the dent.

The planets in our solar system roll around the dent in space-time made by the Sun. The dent keeps planets from going off into space. The dent in space-time keeps the planets going around the Sun.

Scientists are still not sure what causes gravity. Finding out more about gravity is an important problem for the future.

Astronauts

“That’s one small step for man, one giant leap for mankind.” Astronaut Neil Armstrong spoke these words on July 20, 1969, as he became the first person ever to step onto the Moon.

An astronaut is a space traveler. Astronauts fly into space on spaceships. They go where there is no air, higher than any airplane can fly. While they orbit (go around) Earth, they are weightless—they float and feel no gravity.

WHO GETS TO BE AN ASTRONAUT?

In Neil Armstrong’s day, all astronauts were military test pilots. Today, they’re as likely to be medical doctors, scientists, or engineers. Astronauts are still pretty unusual, however. Only a few hundred people have ever traveled into space.

Astronauts receive intense training. Then they may prepare a year or more for a specific mission. Pilots must be able to control all spacecraft systems and deal with emergencies. They may need to make course changes or dock with a space station. They must be able to land the spacecraft. Mission specialists are experts on particular experiments. Any astronaut may have to perform duties in a space suit outside the craft.

WHAT DO ASTRONAUTS DO IN SPACE?

Most space missions do not require human passengers. Robots and computers can do many jobs without needing people. But astronauts can perform some experiments in space that machines cannot. They can examine, for example, how flames burn or how crystals grow without gravity. Some experiments test the effects of spaceflight on human beings. Astronauts also launch and repair satellites, machines that orbit Earth. They also retrieve objects in space for return to Earth.

Astronauts may spend weeks or even months in space. Russian cosmonaut Valeriy Polyakov holds the record for the most consecutive days in space. He spent 438 days aboard the Mir Space Station in 1994 and 1995.

THE FIRST ASTRONAUTS

Russian astronauts are called cosmonauts. The first person in space was cosmonaut Yuri Gagarin. He made the trip in April 1961. The first American, a month later, was astronaut Alan Shepard. The first capsules that carried astronauts into space were barely large enough to hold their single passenger.

FLYING TO THE MOON

The Apollo program began during the 1960s. Apollo was an American project to send people to the Moon and back. There were three astronauts in each Apollo crew. Two of them explored the Moon while the third stayed aboard the main spacecraft.

Neil Armstrong, Buzz Aldrin, and Michael Collins were the crew of Apollo 11. This was the first mission to land on the Moon. Altogether, 12 astronauts walked on the Moon as part of the Apollo program. They performed experiments and brought back moon rocks for study. No one else has ever been to the Moon.

THE SPACE SHUTTLE

During the 1980s, the United States began to use space shuttles to send astronauts into space. Previously, spaceships could only fly once. Each trip required a new spaceship. Now, space shuttles can fly into space many times. They are launched from the top of a rocket, but they land like an airplane. Up to seven crew members can live aboard a shuttle.

The United States has begun research on a new type of reusable spaceship to replace the space shuttle. This new spaceship will use newer technology and will be cheaper to operate than the space shuttle. Someday astronauts may walk on the Moon again, or even travel to other planets!

Atmosphere

Every time you take a breath, you are inhaling Earth’s atmosphere. You cannot see, smell, or taste Earth’s atmosphere. It is the air all around you. Other planets also have an atmosphere. An atmosphere is a blanket of gases that wraps around a planet or any other object in space.

EARTH’S ATMOSPHERE IS AIR

Earth’s atmosphere is made up of a mix of gases called air. Air contains more nitrogen than any other gas. Nitrogen makes up 78 percent of the air. Oxygen, the gas that is most important for keeping you alive, makes up 21 percent. Earth is the only planet to have so much oxygen in its atmosphere. Water vapor and other gases are also present in small amounts in Earth’s atmosphere.

The pull of gravity holds the atmosphere in place. Without gravity, the air in Earth’s atmosphere would float off into space. Gravity is the force that also keeps you from floating away from Earth.

THE WEIGHT OF AIR

Air has weight. You cannot feel the weight of air, but all the air in the atmosphere presses downward. This weight is called atmospheric pressure. Atmospheric pressure depends on how much gas is in the atmosphere. The higher you go, the less air there is and the lower the atmospheric pressure gets. The atmosphere is heaviest and the atmospheric pressure highest close to Earth.

Differences in air temperature close to Earth form areas of high and low pressure. Warm air is light and rises upward. It makes low-pressure areas. Cold air is heavy and sinks. It makes high-pressure areas.

WEATHER AND THE ATMOSPHERE

Air in the atmosphere is always moving. You can feel air blowing on your face. You can see air scattering autumn leaves and making tree branches sway. Moving air is called wind. The wind blows where areas of high and low atmospheric pressure meet. As warm air rises, cold air rushes in to take its place.

Big areas of high and low atmospheric pressure cause storms. Thunderstorms often occur where big areas of high and low pressure come together. Huge thunderclouds form in these places.

Water vapor in the atmosphere makes clouds. Water vapor is a gas. As the gas cools, it turns to liquid water. The water falls to Earth as rain or snow.

A LAYER CAKE OF AIR

Earth’s atmosphere extends about 6,000 miles (9,600 kilometers) above the surface of Earth, where we live. You can think of the atmosphere as having several layers. Most of our weather comes from winds, temperature changes, and water vapor in the layer nearest Earth’s surface. This layer is called the troposphere. Most of the clouds you see in the sky are floating in the troposphere.

The stratosphere is the layer above the troposphere. Jet airplanes fly in the stratosphere because there are few clouds up so high and the ride is usually less bumpy. Earth’s ozone layer is in the stratosphere. The ozone layer absorbs, or soaks up, harmful rays from the Sun. These harmful rays would probably destroy life if they reached Earth’s surface.

The atmosphere gets thinner and thinner in the next layers up, the mesosphere and thermosphere. The top layer of Earth’s atmosphere is the exosphere. The atmosphere ends here, about 6,000 miles (9,600 kilometers) above Earth’s surface. The thin air here gradually merges with outer space.

ATMOSPHERES ON OTHER PLANETS

Any planet that has gas around it has an atmosphere. Mercury, the planet closest to the Sun has almost no atmosphere. Pluto, the planet farthest from the Sun, is so cold that sometimes its atmosphere freezes. The gases in Pluto’s atmosphere turn to ice.

There are colorful bands of clouds in the atmospheres of some planets. A gas called methane makes the atmospheres of Neptune and Uranus look smooth and blue. Jupiter has a swirling storm in its atmosphere called the Great Red Spot that may be like a giant hurricane.

The Solar System

*Sun:-

Sun is the hottest planet in the solar system It has a diameter of 1,391,940 Km It is 1,391.940 Km away from earth. Its mass is equivalent to 99.98 % of the mass of entire Solar system two gases, 75% Hydrogen and 23% Helium gas , with relatively small quantities of other elements.

The Sun has a Surface temperature of 5,880 K but it can be 56,000,000 K at its core. The yellow  surface we see is called Photosphere. The Sun rotates once every 25.4  days  but because it is not solid like other planets, the pole spin at a different rate, taking as much as 36 days to complete a single revolution.

*First on Moon:- Neil A  Armstrong was the first person went  to moon on 16-24 Jul 1969 from Apollo 11 at 2:32.

*Murcury:-

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.

*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.

*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.

*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.

*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.

*Saturn:-

Saturn is the second largest planet in our solar system, after Jupiter. It is the sixth planet from the Sun. Saturn is surrounded by spectacular rings. Italian astronomer Galileo was the first person to see the rings around Saturn. He thought the rings looked like handles. Galileo was looking at Saturn through one of the first telescopes, in 1610. In the 1650s, a Dutch astronomer named Christiaan Huygens was the first astronomer to see that the “handles” were really huge rings.

SATURN’S RINGS

Saturn has seven main rings. The rings are made of many smaller ringlets. They are by far the biggest and brightest rings of any planet in our solar system. Jupiter, Neptune, and Uranus also have rings. Saturn’s rings go out hundreds of thousands of miles from the center of the planet. You could just fit Saturn and its rings in the space between Earth and our Moon.

Saturn’s rings are pretty thin. In places they are only 16 feet (5 meters) thick. The rings are made of dust, pieces of rock, frozen gases, and ice.

WHAT IS SATURN LIKE?

Saturn is a huge ball of gas. Like the planets Jupiter, Neptune, and Uranus, Saturn is called a gas giant. None of the gas giants have a solid surface that you could land a spacecraft on.

The gases around Saturn are poisonous to people. You need to breathe oxygen in order to live. Saturn’s atmosphere is mostly hydrogen and helium gas. An atmosphere is the layer of gases around a planet.

There are thick clouds in Saturn’s atmosphere. The clouds look like colored stripes going around the planet. Strong winds and storms make Saturn’s clouds whirl and swirl.

Saturn’s atmosphere blends into the center of the planet. The gases get thicker and heavier the farther down you go. Finally, the gas turns to liquid.

Astronomers think that the center, or core, of Saturn is very hot. They think the temperature could be about 27,000° Fahrenheit (15,000° Celsius). There are probably rocks and possibly iron in the core of Saturn.

Saturn acts like a big magnet in space. Other planets, including Earth, also act like big magnets.

DOES SATURN HAVE MOONS?

Saturn has 47 moons. Some of the moons orbit, or go around, Saturn inside the rings. Others orbit farther out, beyond the rings.

The largest moon is Titan. Titan is larger than the planets Pluto and Mercury. It has an atmosphere made of nitrogen gas. Scientists think that Titan’s atmosphere may be like Earth’s atmosphere was billions of years ago.

Saturn’s other moons are smaller and icy cold. Enceladus is covered with bright ice, and it has geysers. It is the brightest moon in the solar system. Mimas has a huge crater that formed when a meteor hit. The enormous impact must have almost shattered the moon.

HOW DO WE LEARN ABOUT SATURN?

Ancient people could see Saturn without a telescope. But they could not see Saturn’s rings. They named Saturn after the Roman god of farming.

Astronomers study Saturn through telescopes. They send spacecraft to take pictures and measurements of Saturn and its moons and rings. The first spacecraft to visit Saturn was Pioneer 11 in 1979. Voyager 1 and Voyager 2 visited Saturn in the 1980s. The Cassini spacecraft arrived at Saturn in 2004.

*Uranus:-

There are no cars on the planet Uranus. Yet Uranus is covered with a kind of smog. Smog on Earth is pollution that comes mostly from cars that burn gasoline. Sunlight changes fumes from the cars to smog.

Smog on Uranus comes from gases called ethane and methane. Small amounts of these gases are part of the atmosphere of Uranus. An atmosphere is the layer of gases around a planet, like air on Earth. Sunlight shining on methane and ethane makes smog. The haze of smog around Uranus makes the planet look like a smooth, bluish-green beach ball.

WHO DISCOVERED URANUS?

Uranus was the first planet discovered with a telescope. William Herschel, an astronomer in England, found Uranus in 1781. Astronomers are scientists who study things in space. Astronomers had known about Uranus since 1690, but they did not know Uranus was a planet. At first, they thought it was a star or a comet. Astronomers named the planet Uranus for an ancient Greek god of the heavens.

WHAT IS IT LIKE ON URANUS?

Uranus is a huge planet. It is the third largest planet in the solar system, after Jupiter and Saturn. The diameter (width) of Uranus is more than four times bigger than the diameter of Earth.

You could not walk around on Uranus. Uranus does not have a solid surface. The planet is made up mostly of gas. It has clouds made of methane ice high in the atmosphere. The clouds get thicker and thicker the farther down you go.

The thick clouds blend into a liquid ocean. Astronomers think the ocean on Uranus is made of water and the chemicals ammonia and methane.

THE ATMOSPHERE OF URANUS

You could not breathe on Uranus. The atmosphere there is made mostly of hydrogen gas and helium gas. There are also small amounts of the gases ethane and methane.

There are thick blue clouds in the atmosphere. Uranus has some of the brightest clouds in the solar system. Strong winds blow the clouds around. The clouds make striped patterns under the smog.

URANUS LIES ON ITS SIDE

Uranus looks like it is spinning lying down. All planets spin. They spin around an imaginary line called an axis. The line goes from the north pole to the south pole. It goes from the top to the bottom of a planet.

Most planets are tilted a bit. Earth is tilted as it spins around its axis. But Uranus is tilted so much that it is lying on its side. Astronomers think that something big crashed into Uranus after it formed. The crash may have knocked Uranus onto its side.

There are weird seasons on Uranus. At times, the north pole or the south pole faces directly toward the Sun. Each season on Uranus is more than 20 years long!

WHAT ARE THE RINGS AND MOONS LIKE?

Astronomers have found 13 rings around Uranus. The rings go around the equator. An equator is an imaginary line around the middle of a planet. The rings are dark and faint. They may be made of ice and dust.

Uranus has many moons. Astronomers believe the planet has at least 27 moons. The 2 biggest and brightest moons are Oberon and Titania.

The next largest moons are Umbriel and Ariel. The surfaces of all 4 of these big moons are covered with holes called craters. Meteorites (rocks from space) probably made the craters. Astronomers think that these 4 moons are made of ice and rock.

Another smaller, icy moon named Miranda looks very strange. It has a jumble of ridges. It has three weird areas shaped like oval racetracks. There is nothing like Miranda anywhere else in the solar system. Miranda is a very mysterious little moon.

HOW DO WE LEARN ABOUT URANUS?

Astronomers use telescopes to learn how Uranus moves across the sky. Uranus is the seventh planet from the Sun. It orbits, or goes around, the Sun in an oval-shaped path. Its average distance from the Sun is a bit less than 2 billion miles (3 billion kilometers). Uranus is so far away that it takes 84 years for the planet to go around the Sun once!

Most of what we know about Uranus comes from a trip to the planet by the Voyager 2 spacecraft. Voyager 2 flew past Uranus in 1986. Voyager 2 took many pictures of the planet and its rings and moons.

Astronomers also study pictures from the Hubble Space Telescope. This telescope orbits high above Earth. They use other big telescopes on Earth to study Uranus.

*Neptune:-

In ancient times, people didn’t even know the planet Neptune existed. Astronomers, scientists who study space, didn’t discover the big blue planet until the 1840s.

Neptune is usually the eighth planet from the Sun. So far as astronomers know, only the planet Pluto is farther from the Sun. However, Pluto’s orbit sometimes brings it closer to the Sun than Neptune is, making Neptune the planet farthest from the Sun. This happens every 248 years.

HOW DID WE FIND NEPTUNE?

Astronomers found Neptune by watching the planet Uranus. Uranus wobbles as it orbits the Sun. A British astronomer and a French astronomer used math to figure out that the gravity of another planet was making Uranus wobble. A German astronomer finally saw Neptune through a telescope in 1846. Astronomers named the newly found planet Neptune after the ancient Roman god of the sea.

STUDYING NEPTUNE

It took astronomers a long time to learn anything about Neptune because the planet is so far away. Neptune is about 2.8 billion miles (about 4.5 billion kilometers) from the Sun. That’s 30 times as far as Earth is from the Sun! It takes Neptune almost 165 years to go around the Sun once.

Looking through telescopes, astronomers can tell that Neptune is a big planet. It is about four times larger than Earth. They call Neptune a giant planet. It is the smallest of four huge planets made mostly of gas. Jupiter, Saturn, and Uranus are the other gas giants.

Astronomers learned most of what they know about Neptune from a spacecraft named Voyager 2. Voyager 2 is the only spacecraft that has visited Neptune. It flew past in 1989. It took pictures and made measurements of Neptune.

Astronomers also study pictures of Neptune made by the Hubble Space Telescope. Hubble orbits high above Earth.

WHAT IS IT LIKE ON NEPTUNE?

You could not breathe on Neptune, and the fierce winds would blow you away. Even without the winds, you could not walk on Neptune because the planet has no solid surface.

A thick layer of clouds surrounds Neptune. The clouds are part of the planet’s atmosphere. An atmosphere is made of gases. Neptune’s atmosphere is made mostly of hydrogen and helium gases. Some methane gas high in the atmosphere gives Neptune its bluish color. You could not breathe the gases in Neptune’s atmosphere. You need to breathe oxygen in order to live. There is no oxygen in Neptune’s atmosphere.

Blowing clouds make striped patterns around Neptune. Sometimes there are spots in the clouds. The spots are storms. Neptune has the fastest winds in the solar system. The winds on Neptune can blow at 1,200 miles per hour (2,000 kilometers per hour).

Scientists have made guesses about what the inside of Neptune is like. Scientists think the planet’s thick atmosphere blends into an ocean of water. The core (center) of Neptune may be made of ice and rock.

DOES NEPTUNE HAVE MOONS?

Astronomers have found 13 moons around Neptune. The biggest moon is Triton. Triton is mostly made of ice. There may be some rock in its core. Triton is one of the coldest spots in the solar system. The temperature there can drop to -390° Fahrenheit (–235° Celsius).

Triton once had volcanoes. Its volcanoes did not shoot out melted rock as volcanoes on Earth do. Triton’s volcanoes shot out slushy, half-melted ice.

Astronomers also have found four rings around Neptune. The rings are dark and hard to see, even in pictures from Voyager 2. They may be made of dust and rocks.

HOW DID NEPTUNE FORM?

Astronomers think the Sun and all the planets formed from a disk of gas and tiny particles of rock, metal, and ice. The rocky planets closest to the Sun formed mainly from rock and metal. The planets farthest from the Sun formed mostly from gas and chunks of ice.

The Haviest Elements

             *The Haviest Elements:-

1. OSMIUM-  It is the heaviest element of all (22.61g/cub cm). It was discovered by the British chemist Smithson tannant in 1803. He names it after the Greek word for smell because it smell bad. Osmium is very hard , and is used to  make hard- wearing points such as the nibs of fountain pens.
2. Platinum- Platinum was  used before anyone realized that it was an element. It weighs almost as much as osmium(21.45g/cub cm) and is used to make jewellery that s even more expansive than gold. It is also used in catalytic converters in cars to reduce the pollution from exhaust gases.
3. Plutonium- It was discovered  in 1941and is a heavy and highly radioactive metal. It is used as a nuclear fuel and in nuclear weapons.
4. Gold- It is the best know of all heavy metals(19.299g/cub cm).It is used to make jewellary and coins etc.

         *Elements in sea water :-

                Mg/L
1. Oxygen - 857,000
2. Hydrogen - 107,800
3. Chlorine - 19,870
4. Sodium - 11,0050
5. Magnesium - 1,326
     
  *Highest melting point:-
1. Carbon - 3,527 C
2. Tungsten - 3,433 C
3. Rhenium - 3,186 C
4. Osmium - 3,033 C
5. Tantalum - 3,017 C

The seven continents

*Oceans & Seas:-
                           Area(sq Km)
1. Pacific ocean           166,240,000
2. Atlantic ocean          86,560,000
3. Indian ocean            73,430,000
4. Arctic ocean             13,230,000
5. south china sea        2,974,600
6. Caribbean sea          2,753,000
7. Mediterranean sea    2,510,000
8. Bering sea                 2,261,000
9. Gulf of Mexico           1,542,985   

*The seven Continents:-
                    Land Area(Sq Km)       % of worlds total
1. Asia:-                       43,608,000                  27.3%
2. Africa:-                    30,335,000                  20.4%
3. North America:-     24,349,000                 16.4%
4. South America:-     7,611,000                   11.8%
5. Antarctica:-             13,340,000                  9%
6. Europe:-                   10,498,000                 7.1%
7. Australia:-                8,923,000                    6%      

*Oldest Trees:-
1. Japanese cedar        7,200 years
2. Baobab                      5,150 years
3. Yew                            5,000years
4. Sequoia                3,000-4000yrs
5. Olive                     200-4000yrs
6. Oak                       500-600Years