Week 2 - Our Solar System

OUR SOLAR SYSTEM

Are you ready for a trip through our solar system? Get ready to blast off into space!

We leave our planet Earth at the speed of light. Light travels 186,282 miles (299,792 kilometers) per second. As we leave the earth's atmosphere, we notice that the sky is black and that the stars can be seen all day.

In less than two seconds we reach our nearest neighbor, the moon. Although we do not plan to stop here, we get a quick glimpse as we whiz by. We take a quick snapshot of the moon's surface from our window. We notice the moon's deep craters, broad, dark plains, and lofty mountains.

At the moon we are about 240,000 miles (386,160 kilometers) from Earth. Our trip will take us toward the planets, but away from the sun. The sun is a glowing ball of hot gases with a surface temperature of 11,000° Fahrenheit (6,092° Celsius). It is best to pass it at a safe distance.

As we travel away from the sun, we leave behind two planets—Mercury and Venus. Mercury is the smallest of the planets. It is closest to the sun and is very rocky. Venus is the second planet from the sun. It has a thick atmosphere of poisonous gases and is the hottest planet in the solar system. About four minutes after leaving Earth, we pass Mars. We notice what looks like a dust storm blowing over a desert. Mars is often called the "Red Planet" because of its reddish color. Although it is dry now, scientists believe Mars once had liquid water on its surface.

Past Mars is Jupiter, the largest planet in the solar system. Jupiter is so big that 318 Earths could fit inside it! Jupiter is made mostly of gas. Its most recognizable feature is the Great Red Spot. The spot is a giant storm that has been raging for hundreds and hundreds of years.

As we speed past Jupiter, we come to Saturn, the second largest planet. Saturn is famous for its beautiful rings. The rings are made of tiny particles of ice and dust. The rings were first seen by Galileo Galilei in 1610. Galileo's telescope was only strong enough to make out fuzzy outlines of the rings. Today, pictures taken by the Hubble Telescope allow us to see the rings very clearly.

In a few moments we fly past Uranus and Neptune. Uranus is the only planet in the solar system which rotates on its side. It is a very cold planet. Neptune is the last planet in the solar

system. Neptune has a gas called methane in its atmosphere. Methane gives the planet a bluish glow. Both Uranus and Neptune have rings, but they are faint and hard to see.

Our spacecraft flies past Neptune and on to Pluto. For many years, Pluto was the ninth planet in the solar system. In 2006, the International Astronomical Union (IAU) created new standards for planets. According to the IAU, Pluto does not meet the standards because its orbit overlaps Neptune's. Every few years, Pluto trades places with Neptune and orbits closer to the sun. For this reason, Pluto is now called a dwarf planet. This new classification shows us that our understanding of science is always changing.

We have now traveled in imagination more than three-and-a-half billion (3,500,000,000) miles (5,600,000,000 kilometers) in about five hours. Looking back, we notice that our sun is only a bright star shining among millions of other stars.

It is time to start back to planet Earth. If we were to visit one of the nearest stars in our galaxy, Alpha Centauri, we would have to travel for more than 4 years at the speed of light. If we wish to go to the farthest edge of our galaxy, the Milky Way, it would take us about 120,000 years. Then, if we could continue on to other galaxies beyond, we would need to travel for another 20 million years! Even at the speed of light, that would be a long, long trip.

THE SUN

The sun is one of many billions of stars in the universe. It is similar to the other stars we see at night. The sun is the center of our solar system. Everything in the solar system revolves around the sun. It provides most of the light, heat, and power for the earth. Without the sun, there could be no life.

Ancient myths. The sun's importance was known long before the first telescope was invented. Ancient civilizations made up stories called myths about the sun. In many myths, the sun was thought to be a powerful god. Many ancient civilizations worshipped the sun and made sacrifices to it.

Ancient Egyptians believed in three sun gods. Har was the rising sun, Ra represented the noon sun, and Tum was the setting sun. Ancient Greeks believed that the sun god Helios drove his golden chariot from his palace in the east to his palace in the west. This daily trip explained the movement of the rising and setting sun.

Characteristics. The sun is the closest star to Earth. It

is about 93 million miles (150,000,000 kilometers) away. It is the perfect distance to support life on our earth. If the sun were farther away, the earth would be too cold. The planet would freeze. On

the other hand, if the sun were any closer, the earth would be too hot to support life. Just the right amount of sunlight reaches Earth. The energy from the sun is used by plants to grow and make food.

The sun is very large. It contains 99 percent of the mass of the entire solar system! The sun's mass gives it an extremely strong gravitational pull. It is this pull which keeps the planets in orbit. The sun is also very hot. The center is about 15 million degrees Celsius (27 million degrees Fahrenheit). The sun is made mostly of hydrogen and helium.

Weather. Without the sun there could be no weather. The tilt of the earth causes the seasons.

As the earth moves closer to, or farther away from the sun, the earth's surface is heated differently. This causes the air to be heated unevenly, creating wind. The earth's tilt also causes the light to be less direct in the winter, causing the earth to be heated less. The sunlight is more direct in the summer, heating the earth more.

GRAVITY AND MOTION

We say that the sun rises in the east and sets in the west. When we say these words, we know that the sun is not really moving across the sky. In this lesson, you will learn about the motion of objects in space.

Motion. The sun moves in two ways. First, it moves along the Milky Way with billions of other

stars. The sun travels through space at a speed of about 43,000 miles (69,187 kilometers) an hour. This speed is more than 700 miles (900 kilometers) per second. The long trip to circle the Milky Way takes more than 250 million years. This movement is called a revolution or an orbit.

The sun also spins on its axis as it moves through part of the universe. This movement is called a rotation. It takes about twenty-five days for the sun to make one rotation. Because it is made up of gas, parts of the sun spin around in less time than others. The sun's equator takes the least amount of time to spin on its axis.

The sun is not the only object in our solar system with a revolution and rotation. All the planets revolve around the sun and rotate on axes. Similarly, moons rotate and revolve around planets. On Earth, rotation takes 24 hours. It is this rotation which makes the sun seem to travel across the sky. This rotation also causes day and night. The Earth takes much longer to complete a revolution. It takes 365 days, or one year, to orbit the sun.

Some of the planets in our solar system have interesting rotations. Venus, for example, rotates backwards. Instead of spinning west to east, Venus spins east to west. Seen from Venus, the sun would rise in the west and set in the east. Uranus has another interesting rotation. Its axis is almost completely horizontal. This means that Uranus spins on its side.

The planets also have interesting orbits. Mercury revolves around the sun faster than any other planet. One year on Mercury is equal to only 58 days on Earth. Venus' revolution is interesting because it is shorter than its rotation. It takes about 240 days to rotate, but only 220 days to orbit the sun. This means that on Venus, the days are longer than the years.

Gravity. Gravity is the pull or force that holds us to the earth. The sun has gravity the same as the earth. The pull of the sun is greater than the pull of the earth because the sun is so much bigger. The diameter of the sun is about 865,000 miles (1,391,785 kilometers). This distance is 109 times the diameter of the earth. The diameter of the earth is about 8,000 miles (12,872 kilometers).

The gravity of the sun keeps the planets going around the sun. If there were no gravity pulling them toward the sun, the planets, including the earth, would move out into space in a straight

line.

Not much was known about gravity until the year 1687. In this year Sir Isaac Newton, an English astronomer and scientist, discovered some of the secrets of light and color. He also discovered how the universe is held together by gravity. The idea of gravity came to him while he was in the garden drinking tea. An apple fell to the ground at his feet. He realized that the same force that drew the apple to the ground keeps the moon in its orbit around the earth. He later wrote a book explaining the laws of motion and gravity. Newton's book became one of the most important books in the history of science.

PLANETS AND OTHER CELESTIAL BODIES

You have learned about the sun's size, temperature, and motion. In this lesson, you will learn about the planets and other celestial bodies in our solar system.

Planets. All the planets revolve around the sun in an orbit or path. This orbit is oval-shaped, or elliptical. The elliptical shape causes some planets to come closer to the sun at some points in their orbit than at other points. Mercury, for example, is sometimes more than 43 million miles away from the sun. Other times, it is closer and is only about 28 million miles away.

Each planet orbits at its own speed. A planet nearer the sun moves faster than one farther away from it. The nearest planet to the sun, Mercury, travels 30 miles (48.27 kilometers) per second. It goes around the sun in

less than three months. The dwarf planet Pluto, farther from the sun than any of the eight planets, takes 248 years to complete its round trip.

As each planet revolves around the sun, it is also rotating on its axis. The time it takes to spin around once varies with the different planets. Mars rotates once about every 25 hours. Venus takes 243 days, while Jupiter takes only 10 hours. The time it takes for a planet to rotate once is called its rotation period.

The temperature of the planets varies a great deal from one to another. Mercury, which is closest to the sun, reaches a temperature of 625 degrees Fahrenheit (329° Celsius) during the daytime. The average temperature on Mars is 80 degrees below zero Fahrenheit (-62° Celsius). Although it is not closest to the sun, the hottest planet in the solar system is Venus. Its thick, cloudy atmosphere traps almost all of the sun's heat. Temperatures on the planet's surface can reach 880 degrees Fahrenheit (470° Celsius).

Satellites. Most of the planets in our solar system have satellites, or moons. Satellites revolve around planets. The moon is Earth's only satellite. It rotates on its axis in about the same amount of time that it orbits Earth. The moon has a rocky

surface covered with craters. Other planets have more than one satellite. Jupiter, for example, has sixty-three. Some planets, such as Venus and Mercury, have no satellites.

Other celestial bodies. In addition to the planets and their satellites, there are other types of objects that revolve around the sun. These other celestial bodies include dwarf planets, meteoroids, comets, and asteroids.

In 1930 Pluto was discovered and photographed. Scientists originally thought it was a planet, but in 2006 decided it was not. Pluto is now called a dwarf planet.

Meteoroids are pieces of metal or stone that come from outer space into our atmosphere. Most of them are as small as a grain of sand. As they travel from outer space, they cannot be seen until they enter our atmosphere. They then begin to heat up and glow. As they reach the

Venus*

Mercury*

heavier atmosphere, they burn with a white light. We call them meteors as they streak across the sky. They are sometimes mistakenly called "falling stars."

Meteors usually break and burn up completely before reaching the ground. A few very large ones have been know to hit the ground. Meteors that hit the ground are called meteorites. Between Flagstaff and Winslow, Arizona, is a huge crater measuring 4,150 feet (1,264.92 meters) wide and 570 feet (158.536 meters) deep.

Comets look like stars with tails. The head of a comet may be as much as 100,000 miles (160,900 kilometers) in diameter. The comet's tail may be 100 million miles long (160.9 million kilometers). Comets travel around the sun in elliptical paths. The length of their orbits varies greatly. One comet completes its orbit in three and one-third years. Another one, called Halley's comet, circles the sun once in about seventy-seven years.

From July 16, through July 22, 1994, pieces of the Shoemaker-Levy Comet collided with Jupiter. The animation below shows what that looked like.

Asteroids are like planets except they are much smaller. The first one was discovered by accident in 1801. Since then, more than 1,600 have been recorded. Most of the asteroids in our solar system are found in orbits between Mars and Jupiter.

*All indicated photographs and animations courtesy of NASA.

STARS AND SPACE

The sun is only one of billions of stars in the Milky Way galaxy. Although it appears very large from Earth, the sun is a medium-sized star. Many stars are much larger. In this lesson, you will learn about other stars.

At night, we can see between 3,000 and 5,000 stars. With a high-powered telescope, astronomers can see many millions of stars. Astronomers think there may be more than 30 billion stars (30,000,000,000) in our galaxy. This number does not include the stars in other galaxies. Scientists are asking the question, "What lies beyond our universe?" Some have suggested that there could be an infinite number of universes.

The brightest star we can see is Sirius. Sirius is often called the Dog Star. If you wanted to travel to Sirius, you would have to fly at the speed of light for eight-and-a-half years. This distance and speed is impossible for us to imagine. Sirius is so far from us that it takes almost nine years for its light to reach the earth. If you are nine years old, the light which you see tonight started to travel from Sirius toward the earth at about the time you were born. This star radiates about thirty times as much light as the sun, but because it is so far away, it appears much less bright.

The closest known star, other than the sun, is Alpha Centauri C. This star is part of a star system made of two other stars—Alpha Centauri A and Alpha Centauri B. Although it is smaller and less bright than Sirius, Alpha Centauri C appears bright because it is very close.

An important star is the North Star. It is always in the same place nearly over the earth's North Pole. For centuries this North Star, or pole star, has been a guide to everyone in the Northern Hemisphere who has needed directions. It is easy to find the North Star. First, find the Big Dipper. Then follow the pointer stars of the bowl of the dipper to the North Star.

Another important star is Betelgeuse (pronounced "beetle juice" by most astronomers). This star is so large it is called a supergiant. Its diameter is 650 times as big as the sun's. It is 430 light years away from Earth, which makes it less bright than other smaller stars. If Betelgeuse were

closer, it would certainly stand out in the sky. It is as bright as 55,000 suns! Betelgeuse was the first star to have its surface closely examined by the Hubble Space Telescope.

The stars seem to rise in the east and set in the west because the earth is rotating on its axis. They also seem to change position in the sky with the seasons. The earth's revolution makes the stars look like they move with the seasons. In the Northern Hemisphere, we can see groups of stars called constellations. Many, many years ago, people imagined they saw pictures in the sky. They made up stories about these "star pictures." We call these pictures constellations. Each constellation appears at a different season of the year. For instance, in April, May, and June, one can see the constellation Leo. This group of stars is imagined to take the shape of a roaring lion.

BEYOND THE MILKY WAY

You have learned about the planets, stars, and other celestial bodies in our solar system. You have also learned about stars in the Milky Way galaxy. In this lesson, you will learn about space beyond our galaxy.

A galaxy is a system or group of stars. It is sometimes called a stellar universe. Each group is made up of many, many stars. Some galaxies contain as many as a billion stars. Most galaxies are too far away for us to see. The earth is in a galaxy called the Milky Way. All the stars we see on a clear night are part of our galaxy. The Milky Way is thought to be spiral-shaped. It would take about two hundred fifty million years for the Milky Way to make one rotation.

The ancient Greeks looked at the sky and thought the cluster of stars looked like freshly spilled milk. The Greek word for milk is galaktos. We get our word galaxy from the Greek word. Our sun is on the edge of the Milky Way about two-thirds the distance from the center in one of the spiral arms.

Astronomers know that billions of galaxies exist. They have taken pictures of many of them through huge telescopes. Some are larger than the Milky Way. Other galaxies are smaller. Galaxies have different shapes. Some are round. Others are spiral like a large pinwheel with

giant arms. We do not know if these galaxies have planets in them because they are trillions of miles away. Our modern instruments are not able to tell us much about celestial bodies that far away.

Magellanic Clouds. The closest galaxy to the Milky Way is the Magellanic Clouds. The clouds are actually two small galaxies called the Large Magellanic Cloud and the Small Magellanic Cloud. Both galaxies are called satellite galaxies because they orbit the Milky Way.

The Magellanic Clouds can only be seen from the Southern Hemisphere. They were named after the explorer Ferdinand Magellan, who noticed them in 1519 during his voyage around the world. The Large Magellanic Cloud is the closer of the two galaxies. It is smaller than the Milky Way and has about 15 billion stars.

Andromeda galaxy. The nearest spiral galaxy is Andromeda. Andromeda is similar to the Milky Way, but it is almost twice as large. Andromeda's size gives it a very strong gravitational pull. Like the Milky Way, it has several satellite galaxies. Two of these galaxies can be seen with a small telescope.

Andromeda is about two million light-years away and has an estimated 300 billion stars. It is slowly moving toward the Milky Way. Some astronomers believe the two galaxies could eventually collide and merge. However it would take more than two billion years for that to happen.

As we learn more about the universe, we can better understand how things work. We can better understand and appreciate the earth, the solar system, and the galaxies in the universe.

THE SOLAR SYSTEM

All of the celestial bodies that revolve around the sun make up our solar system. This system, made up of one star and eight planets, is held together by the force of gravity. The sun's gravity pulls on each of the planets to keep them in orbit. Gravity is also the force that keeps each planet's moons orbiting around them. In this lesson you will learn about our solar system.

Solar system. Our solar system is located in the Milky Way Galaxy. It is only a tiny speck of dust compared to the entire Milky Way. In addition to our solar system, the Milky Way includes billions of stars. Similarly, the Milky Way is only one of millions, perhaps billions, of other galaxies in space. Each of these galaxies contains billions of stars. The universe is infinite.

The sun is one of only a very few stars that are known to have planets surrounding it. Our solar system is the only system known to us that can support life. Our solar system consists of the sun, eight planets, and their moons. Asteroids, comets, and meteors also are a part of the solar system. The sun is composed of an enormous quantity of gas. It produces its heat and energy by the fusion of hydrogen gas into helium. This process is similar to the reaction of a hydrogen bomb on Earth.

Planets. Planets are major bodies that revolve around the sun. They do not produce their own light but some of the planets are warm enough to give off a small amount of heat. Mercury is closest planet to the sun. It is small and rocky. Venus is the second planet from the sun. It has a thick atmosphere of poisonous gas. The atmosphere traps the sun's heat like a greenhouse. Venus' atmosphere makes it the hottest planet in the solar system. The atmosphere also reflects a lot of the sun's rays, making Venus a very bright planet. In the early evening or early morning it looks like a star.

Earth is the third planet from the sun and the only known planet to support life. The amount of water and oxygen present on Earth makes it different from the other planets. After Earth is

Mars. Mars has a red glow and can usually be identified by this unusual color. Its rugged, eroded surface has been revealed by orbiting satellites and surface landers. Numerous detailed images show that the entire surface of Mars has been affected by massive flood waters at some time in its history. No one knows, at this time, where all the water went.

Jupiter is the fifth planet from the sun. It is also the largest planet in the solar system. Jupiter is over 300 times more massive than Earth. Jupiter is most recognized by its Great Red Spot. The spot is actually the eye of a hurricane that has been raging for hundreds of years. Although they are difficult to see, Jupiter has three rings. Jupiter's closest neighbor is Saturn. Saturn is usually the most easily recognized planet because of its many beautiful rings. Saturn has been observed to have hundreds of fine individual rings. The rings were first observed by Galileo in 1610.

Past Saturn is Uranus, the seventh planet from the sun. Uranus is unique in that is the only planet in the solar system with an almost horizontal axis. The extreme tilt of Uranus' axis causes the planet to rotate nearly on its side. The last planet in the solar system is Neptune. Neptune was the first planet located through mathematical predictions. Scientists observed strange patterns in the orbit of Uranus and determined that another gravitational force must be affecting the orbit. Neptune's location was calculated by French a mathematician named Le Verrier. Neptune is a cold, blue planet. Its color is the result of high concentrations of methane gas in the atmosphere.

Beyond Neptune is a dwarf planet, Pluto. It is usually farther from the sun than Neptune, however, Pluto's orbit is highly elliptical. At certain times, Pluto's orbit brings the dwarf planet inside Neptune's orbit. In January 1979, Pluto's unusual orbit brought it closer to the sun than Neptune. They remained in this order until 1999, when its orbit took it farther away from the sun once again. Information is being gathered on Uranus, Neptune, and Pluto by satellites and powerful telescopes. The planets in order from the sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

The solar system consists of 9 planets, more than 31 moons, thousands of asteroids, millions of comets, and countless meteoroids. The feature that all members of the sun's family have in common is their gravitational attraction to the sun while they travel with the sun through space.

The order of the planets, beginning closest to the sun, is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Mercury, Venus, Earth, and Mars are called inner, or terrestrial planets. Jupiter, Saturn, Uranus, and Neptune, are called the outer, or Jovian planets.

Beyond the Jovian planets is Pluto.* Pluto is probably like the terrestrial planets, but it is too far away to be seen in detail. Inner and outer planets are separated by the asteroid belt. Moons orbit seven of the eight planets. Comets travel in elliptical or parabolic paths around the sun. Meteoroids are visitors seen only when they enter the earth's atmosphere as meteors.

As you read the text, notice that the inclination of the axes of the planets have a wide variation. Venus and Neptune, as well as Pluto, rotate in an opposite direction to the remaining planets. Jupiter, Saturn and Neptune have satellites which rotate in the opposite direction. This is called retrograde motion.

The sun and the stars are giant spheres of hot gases. The planets are dark, relatively cold, bodies, much smaller than the sun. The stars produce their own heat and light, but the planets do not. All light and nearly all heat on the planets comes from the sun. The planets can be seen only because they reflect sunlight.

Planets and stars look much alike in the night sky, but planets have two distinguishing characteristics: First, the planets shine with a steady light, but the stars seem to twinkle. Second, the planets change positions and directions in relation to the stars. This movement was first noted by the ancient Greeks, who called the moving objects planetae, which means wanderers.

*Pluto's elliptical orbit brought it closer to the sun than Neptune in January 1979. They remained in this order until 1999.

INNER PLANETS

Earth and the three other planets nearest the sun are somewhat similar. They are called the terrestrial (earth-like) planets. Astronomers think Pluto might be terrestrial in characteristics, but it is too far away to be examined accurately. Suppose the solar system could be shrunk so that the sun was the size of a half dollar. If the sun were at home plate on a baseball diamond, the inner terrestrial planets would be within five meters of home plate.

Mercury. Mercury is the second smallest planet and is closest to the sun. Its average distance from the sun is 57,900,000 kilometers. Early astronomers thought that Mercury both revolved around the sun and rotated on its axis, once in 88 earth days. If its day and its year were the same length, one side of Mercury would be in constant sunlight; the other side, in constant darkness. In 1965 new data from the giant 1,000-foot radio telescope at Arecibo, Puerto Rico, showed that Mercury's period of rotation might be 59 earth days. This theory was proved when radar beams were bounced back from Mercury. Mercury does not keep one side facing the sun as originally thought. It has phases like our moon.

Because of the long length of the Mercurial day, the temperatures on Mercury are extreme. The day is extremely hot with temperatures about 750°F, and the night temperatures are about -275°F. The difference between the two temperatures is, obviously, very great.

Mercury has no atmosphere, and its surface is like that of the moon. No life could exist on Mercury. Mercury has no satellites and can be seen only at dusk or dawn--and then only for a few days each month. The Romans named Mercury in honor of the swift messenger of their gods.

The diameter of Mercury is 4,878 kilometers. Because it is so small, its gravity is less than that of the earth. Gravity on Mercury is 0.378 times that on the earth--slightly over one-third. The pull of gravity holds the clouds and air around the earth. Mercury has no atmosphere, possibly because its gravity is so low. Without clouds, the planet lacks rain and water. Without an atmosphere, Mercury's sky is black and stars can probably be seen in the day.

The mass of Mercury is 0.05--about one-twentieth--the mass of the earth.

On March 29, 1974, Mariner X became the first United States spacecraft to reach Mercury. The unmanned spacecraft flew to within 740 kilometers of the surface. It swept past the planet again on September 24, 1974, and on March 16, 1975. During those flights, Mariner X photographed the surface of Mercury and made scientific measurements of the planet. It detected a magnetic field around Mercury.

Venus. Venus is located between Mercury and Earth. It was named by the Romans in honor of their goddess of love and beauty. It is the most brilliant of all the planets and is called the Morning Star when seen before sunrise and the Evening Star when seen after sunset.

Venus is 108,230,000 kilometers from the sun--over twice as far as Mercury. During its orbit it comes within 40,200,000 kilometers of the earth. Photographs taken of Venus show that it has phases like the earth's moon. Venus has a diameter of 12,100 kilometers, approximately that of the earth. Like Mercury, it has no satellites. The surface gravity on Venus is 0.907 compared to Earth's 1.00. The mass is 0.82 compared to the earth.

The Venusian surface is hidden by a dense atmosphere and heavy clouds of sulfuric acid, making observations almost impossible. In December of 1962, the unmanned Mariner II relayed information back to the earth about this mysterious planet. Instruments measured the high surface temperatures of the planet.

Between February, 1966, and October, 1975, several space probes from both Russia and the United States either landed on or circled Venus, sending back information about the planet. These probes have photographed Venus' surface and have determined that its atmospheric pressure is 1300 pounds per square inch. Compare this enormous pressure with

Earth, which is only 15 pounds per square inch. No magnetic field was found.

Venus is about the same size as the earth and is often called our sister planet.

The atmosphere of Venus is composed of dust, ice crystals, carbon dioxide, and a small amount of hydrogen. The carbon dioxide creates a "greenhouse effect," which causes the 800°F temperatures. Some scientists are worried that the earth's temperature will become greater if too much carbon dioxide from burning fuels gets into our atmosphere. Venusian clouds do not change shape, which has led scientists to conclude that Venus has no great oceans or continents. Large oceans and big land areas would produce air currents similar to those found on the earth.

Venus spins very slowly on its axis, once every 243 days. This is even longer than the time it takes to orbit the sun, 225 days. Therefore, a day on Venus is longer than a year. It is the only planet that takes longer to spin on its axis than to orbit the sun. Another odd fact is that it spins from east to west, the opposite direction to the spin of all the other planets. No one knows why the spin of Venus should be so strange. Clouds on Venus also move from east to west, but they do so every four days, blown by howling winds of 220 miles per hour.

Earth. The earth is an average of 150,000,000 kilometers from the sun and has a diameter of 13,000 kilometers. The moon is its only natural satellite. The earth rotates once in 23 hours, 56 minutes and takes 365.25 days to make one complete trip around the sun. The mass and surface gravity of the earth are both defined as 1.00 to provide a standard of comparison with other planets.

The gaseous atmosphere is composed primarily of nitrogen with some oxygen and with small amounts of carbon dioxide and argon. Temperature extremes are -129°F at the poles to 136°F in the equatorial deserts. The axial tilt causes seasonal changes in the temperature zones.

The planet earth is completely unique among all the other objects in the solar system. It's exact position in space relative to the sun permits the atmosphere, hydrosphere, and biosphere to sustain each other through complex interrelationships. These chemical and biologic interactions are the result of receiving just the right amount of solar energy which permits life on the planet earth to exist.

Mars. Mars is the planet that has characteristics most similar to the earth. Mars was once thought to have life. It is only half the size of the earth with a diameter of 6,791 kilometers. In some ways it seems much like our home planet. The day on Mars is 24 hours and 37 minutes, just over half an hour longer than our day. It has a thin atmosphere with clouds, but its atmosphere is chiefly carbon dioxide with traces of nitrogen, argon, and oxygen. Mars has polar caps that melt in summer and spread in winter, but they may contain more frozen carbon dioxide than frozen water. It also has deserts, and it has seasons similar to ours because the axis is tilted at about 25°. Earth's axis is inclined 23½°.

To the naked eye, Mars appears to be a red star. It was named by the Romans for their bloody red god of war. Early astronomers described large bright areas, reddish in color, which they believed to be deserts. They described large dark areas which they believed to be oceans. Today, scientists think that both areas are different rock types or different textures in the surface dust.

One of the leading reasons some people believed that intelligent life existed on Mars is the presence of what seemed to be canals. Many people believed that some reasoning being like man lived on Mars and built the canals to irrigate the lands. The canals are the same color as the areas once thought to be oceans. Now, with high powered telescopes and satellite probes, we

now know that the imagined canals do not exist, and all the surface features on Mars appear to have formed from natural physical processes. Space exploration continues to confirm that plant and animal life exist only on the earth.

Mars does not provide an environment that would provide a comfortable life for people like us. It is too cold. Even on a summer day the temperature never rises above the freezing point. What little air there is on Mars is too thin to breathe. It is as thin as the earth's air 35 kilometers up. That is higher than airplanes fly. At this altitude we need pressurized cabins on commercial

flights. There is too much ultraviolet radiation from the sun. On the earth we only get a sunburn from the ultraviolet light, but on Mars it would kill any living thing.

Mars is an average distance of 228,000,000 kilometers from the sun. Mars receives less than half the light and heat from the sun that the Earth receives. Because of its smaller mass and lower density, Mars has a gravitational pull less than the earth. In fact, the gravitational pull on Mars is only slightly greater than that on Mercury. The surface gravity is 0.38 times that on the earth. The mass of Mars is 0.11 times that of the earth.

Mars has two satellites. They are both small and oddly shaped. Phobos is only 28 kilometers at its widest. Deimos is 16 kilometers. Both moons have craters created by meteorites. Both Phobos and Deimos are thought to be ex-asteroids that were captured by the gravitational pull of Mars and pulled into orbit around Mars.

There have been a number of unmanned space flights to Mars. The following is a summary of these flights and the information learned:

1965 -- Mariner IV sent back photographs of meteor craters. 1969 -- Mariner VI and Mariner VII photographed the surface. 1971-1972 -- Mariner IX orbited Mars photographing both moons, a dust storm, and

many surface details. 1971 -- A Russian probe, Mars 3, made the first soft landing on Mars, but after 20

seconds it stopped transmitting information. 1976 -- Viking I and Viking II landed on Mars. They sent back images of the surface. The

surface is red because it is rusty. There is a lot of iron in the rocks. It has combined with oxygen from the air to make rust. Both Viking probes had sophisticated equipment to test for evidence of life within the Martian soil. No evidence of life was found.

1997 -- After a seven-month journey from the earth, Pathfinder landed on Mars on July 4, 1997. The Pathfinder spacecraft contained a small rover called Sojourner which roamed the Martian terrain, sending back video images as well as spectrographic analysis of rocks and soil. The landing site was in a dry flood channel called Ares Valles. Geologic evidence seems to indicate that catastrophic floods once ran through the landing site. The details of many surface features on Mars indicate that the planet's surface has undergone massive erosion by flood waters at some time in its history. Geologic features such as shorelines, river channels, and tear drop shaped erosion remnants are evident. Although the remnant of some of the water can be seen in the frozen ice caps at the Martian poles, scientists are uncertain where all the water has gone. Perhaps it is located underground or now consists of ice crystals within the Martian soil.

At one time the huge Valles Mariners Canyon, which averages 250 miles wide and 3100 miles long, was formed on Mars. If on the earth it would stretch from San Francisco to New York. The Grand Canyon in Arizona is very small in comparison, being "only" 18 miles wide and 277 miles long! The odd hue of the Martian sky results from dust particles suspended in the atmosphere.

OUTER PLANETS

The outer planets--Jupiter, Saturn, Uranus, and Neptune,--are much farther from the sun and are much farther apart than the inner planets. In addition to the planets is Pluto, which is usually farther from the sun than Neptune. If a half-dollar-sized sun were placed at home plate, Jupiter, the nearest, would be at the pitcher's mound, 18.44 meters away (60 feet, 6 inches). Pluto, the farthest, would be 128 meters (almost 400 feet) out in the outfield.

The four largest planets--Jupiter, Saturn, Uranus, and Neptune--are called the Jovian planets. Jovian comes from Jove, a form of the name Jupiter. The Jovian planets are not like the inner planets. They are much larger and are probably made of gas instead of rock. They may be very much like our sun. They even give off some heat, but no light.

Jupiter. Jupiter is the closest of the outer planets. Jupiter was named for the king of the Roman gods. It is the fifth planet, 778,400,000 kilometers from the sun, and is the largest planet in the solar system. Its diameter is 142,700 kilometers.

About 1,300 Earths could fit inside Jupiter! In spite of its size, its mass is only 318.3 times the mass of the earth because the planet is composed chiefly of dense gases. The surface gravity of Jupiter is 2.364 times that of the earth. Jupiter rotates very fast--as do all the larger planets--once every 9 hours, 55 minutes. It

revolves around the sun once every 12 years. Jupiter's atmosphere is banded with colored light and dark belts that sometimes appear to merge. The atmosphere is chiefly composed of hydrogen, helium, and lesser amounts of methane and ammonia. A large, red, oval region seems to be suspended in it.

The red spot is three times the diameter of the earth and was first seen in 1875. The red spot appears to be an enormous hurricane within the turbulent Jovian atmosphere

Jupiter receives very little light or heat from the sun. Its surface is believed to be thick ice that never melts. The surface temperature averages -185°F. Space probes that have passed close to Jupiter have detected a significantly higher radiation of heat from the planet than expected. It would appear from these measurements that the planet is relatively young.

Galileo discovered 4 of Jupiter's 16 satellites in 1610. Careful observations have shown that these moons are eclipsed by the shadow of Jupiter as well as by each other's shadows. Four of these satellites revolve in orbits in the direction opposite to the remaining moons.

For many years, Jupiter was called a gas giant. A question existed whether it was solid below the ice. In recent years, using radio telescopes, astronomers have found that the inner core of Jupiter is composed of hot liquid. Reason says that the heat would melt the ice. Astronomers are not sure of what is between. Some believe that insulating material separates the hot material inside from the ice outside. Others believe that a layer of water lies between the two.

In 1972 the United States launched Pioneer X, which became the first space probe to fly past Jupiter. This unmanned probe flew within 130,000 kilometers of the planet on December 3, 1973. The probe measured Jupiter's radiation belts and reported amounts of hydrogen and helium in the atmosphere. On December 3, 1974, the unmanned Pioneer-Saturn flew within 42,000 kilometers. It provided scientists with close-up photographs of the planet's polar regions, which had never been studied before. It enabled scientists to obtain new data on Jupiter's red spot, magnetic field, and temperature.

Voyager 1 and 2 were launched in 1977 and both flew by Jupiter in 1979 before heading for Saturn. Each probe carried instruments to study the planet's magnetic fields, examine its atmosphere and to detect Jupiter's loud and powerful radio radiation. Many photographs or images of the planet were collected and are still being examined to learn more about Jupiter and its moons. One of the most interesting satellites of Jupiter is Io. Beyond planet earth, Io is the only evidence of active volcanic activity. Space probes have captured images of several

eruptions spewing material as high as 60 miles above the surface. This may be caused by distortion of the surface due to the intense pull of gravity because Io's orbit is very close to the giant planet.

The first evidence of a very fine ring around Jupiter was transmitted by Voyager I.

Saturn. Saturn is the sixth planet from the sun. It has an orbital radius of approximately 1,424,600,000 kilometers. It is the second largest planet and was named for the Roman god of agriculture. Its diameter is about 120,000 kilometers. Its period

of rotation is slightly more than 10 hours. Saturn makes one complete revolution around the sun every 29.5 years. Saturn's surface gravity is 0.916 times that of the earth. The mass of Saturn is 95.3 times that of the earth.

Around Saturn's equator are seven rings which consist of thousands of narrow ringlets. The ringlets consist of billions of pieces of ice which orbit the planet like satellites. Again, the uniformity and preserved fine details of Saturn's rings may indicate that the planet is relatively young. The rings have not been disrupted by meteoroids. The rings are a very fine physical feature which are only about 15 km thick. It seems that the individual particles of the rings are only a centimeter to several meters in size. In addition to the rings of

particles, Saturn has 23 moons.

The largest of Saturn's moons, Titan, is about the size of Mercury and has an atmosphere. One of the moons of Saturn revolves clockwise around the planet, while the other moons revolve in normal solar system direction, counterclockwise. Why this moon behaves differently is not known.

Manned space probes to Saturn will probably not be attempted for many years. Saturn is so far away that a spacecraft would take at least five years to reach the planet. It will be difficult to equip a spacecraft with enough power for such a long time. The spacecraft would be too far from the sun to use solar energy. However, several unmanned probes (Voyager 1 and Voyager 2) have revealed much new information about outer space. These probes were the fastest man-

made objects to ever leave the earth. They were propelled into the solar system at 32,000 Mph from their launch rockets.

An advanced space probe sent to explore Saturn and its moon Titan is called Cassini. It was launched on October 15, 1997. Its trajectory took it around Earth, Venus, and past Jupiter to get additional gravity assists from other planets to accelerate it faster toward its journey to Saturn. Cassini entered into orbit around Saturn on July, 1, 2004.

Uranus. In 1781, an outstanding astronomer named Sir William Herschel discovered the planet Uranus through his large reflecting telescope. Herschel had constructed the largest reflecting telescope of his day.

Uranus is 2,866,900,000 kilometers from the sun and has a diameter of 50,800 kilometers. The length of a day on Uranus is about 17 hours. Its period of revolution is about 84 years. Uranus has 15 satellites which revolve around it in a clockwise direction in planes almost perpendicular to its orbit.

When first seen by astronomers in 1690, Uranus was thought to be a star. Uranus's rotational axis points almost directly at the sun. Its orientation is much the same as the earth's would be were it tilted with the North Pole pointing almost at the sun. The north pole of Uranus faces the sun for almost 20 years. Then the planet shifts and the rays of the sun move over the equator and shine over the south pole for about 20 years. Because of its great distance from the sun, little heat reaches that planet.

Although Uranus has four times the diameter of the earth, its mass is only 14.7 times that of the earth. Its surface gravity is 0.889 times that of the earth. In addition, its atmosphere is probably made of ammonia and methane.

Astronomers were puzzled by "detours" in the orbit of Uranus. Both the French astronomer Leverrier and the Englishman Adams decided that another planet must be orbiting beyond Uranus, causing a disturbance in its orbit. Only in that way could the orbit of Uranus be explained.

When Voyager 2 flew past the planet in 1986, it found that the greenish blue clouds show little sign of activity. There were few cloud features of note. Voyager discovered a number of new moons, all of which are too small to be seen from the earth. Voyager 2 also discovered two new rings around Uranus, making a total of 11 rings. The rings are black as coal, which is why they cannot be seen from the earth. The first rings were discovered in 1977 when Uranus moved in front of a star. The rings made the star's light appear to flash off and on as they passed across it.

Neptune. Neptune is 4,486,100,000 kilometers from the sun. Neptune cannot be seen with the naked eye, but appears as an eighth-magnitude star through the telescope. It has a diameter of 48,600 kilometers. It makes one rotation about every 16 hours. It revolves around the sun once every 165 years. Neptune has eight moons and was named for the Roman sea god.

In 1846, the German astronomer Johann Galle located the mystery planet, Neptune, exactly where Leverrier had predicted it would be. Neptune is more than three times larger than the earth and has a mass of 17.3 times that of the earth. Its surface gravity is 1.125 times that of the earth's surface gravity.

In some ways Neptune and Uranus are more similar than Earth and Venus. Two main differences distinguish those two planets: (1) Neptune is somewhat colder and slightly larger, and (2) Neptune appears bluish, while Uranus seems to have a greenish hue.

After Neptune was discovered, astronomers found that they still could not fully explain the orbit of Uranus. Another heavenly body had to be exerting a gravitational pull on Uranus so that it followed its strange path around the sun. A search was begun to find the missing planet. The letter X, used by mathematicians to signify an unknown quantity, was used as the name of the missing planet.

Little was known about Neptune until the Voyager 2 space probe passed it in 1989. Neptune is so far away that the radio messages, traveling at the speed of light, took over four hours to reach the earth. Neptune has blue-green clouds with a dark spot that is thought to be a storm cloud.

Neptune, like Uranus, has faint rings. Voyager discovered six new moons to add to the two already discovered. Triton, the largest moon, consists of frozen nitrogen and methane and revolves in a retrograde orbit opposite to the rotation of Neptune. Triton also has volcanoes of liquid nitrogen that erupt spraying material nearly as high as Mount Everest.

Pluto. Originally believed to be a planet, Pluto has the most elliptical orbit. Its aphelion is 7,356,000,000 kilometers and its perihelion is 4,423,200,000 kilometers, which occasionally brings it inside Neptune's orbit. This phenomenon occurred from 1979 to 1999. This event occurs every 248 years.

Pluto's average distance from the sun is 5,890,000,000 kilometers. Little is known about its surface conditions because it is so far away. It is believed to have a diameter of approximately 3,000 kilometers, which makes it the smallest planet. It has one satellite. Its period of rotation is six days, nine hours, and its period of revolution is 248 years. Pluto's orbit has a strange tilt when compared to the orbits of the eight planets. Because of its effect on the orbits of Uranus and Neptune, its mass is thought to be about the same as that of the earth. Pluto's surface gravity is 0.0675 times that of the earth.

In 1905, an American, Percival Lowell, predicted where Pluto would be found because of the irregularity of Uranus and Neptune. He died before Pluto was discovered in 1930. The initials PL were used at first to name it, and then it was called Pluto for the Roman god of the underworld.

Pluto, like Earth, has only one moon, Charon. Discovered in 1978, Pluto's moon in unique because of its size. Most moons in the solar system are a great deal smaller than the planets they orbit. Our moon, for example, is only about one-fourth the width of Earth. Charon, on the other hand, is roughly half the size of Pluto. Charon orbits Pluto every 6.4 days, the same time the planet takes to spin on its own axis. Therefore, the moon remains over one side of Pluto all the time and can never be seen from the other side of Pluto. Like the satellites of Uranus, Charon's orbit is perpendicular to Pluto's orbital plane.

No space probe has yet penetrated far enough into the solar system to inspect Pluto. The most detailed images have been made from the Hubble Space Telescope.

Light-year. Distances to the planets are given in miles. Stars and galaxies are too far away to use miles. They are measured in light-years. A light-year is the distance light travels in one year. Light travels almost 6 trillion miles per year (the actual distance is rounded for convenience). Therefore, one light-year is equal to 6 trillion miles.

The sun is 93 million miles from the earth. All other star distances are given in light-years. Alpha Centauri is the closest star to our solar system. It is 4.3 light-years away from earth. In miles, Alpha Centauri is over 24 trillion miles away. The galaxy Andromeda is 2.2 million light-years away, or approximately 12 trillion miles away.

My Very Efficient Mother Just Served Us Nachos. It may sound like a crazy sentence, but it is not designed to make perfect sense. It is a mnemonic designed to remember the planets in our solar system in order from closest to the sun to farthest away - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Our solar system is made of the sun and all the other celestial bodies that orbit it, including planets, moons, dwarf planets, asteroids, comets, meteoroids, and dust. In this lesson, you will learn about our solar system.

Structure

What do you think the solar system is mostly made of? The answer is not planets, moons, or stars, but empty space. Compared to the amount of space between them, the planets are very small. Most models do not show the solar system to scale, which makes the distances between planets is inaccurate. If the distances between the planets were measured in feet, Mercury

would be located 15 feet from the sun. Neptune would be located over 1160 feet from the sun. Neptune's distance from the sun is over 77 times that of Mercury.

The solar system

Image courtesy of NASA/JPL-Caltech

A system is an organized group of related objects that form a whole. Our solar system is an organized group of celestial bodies that form a distinct unit in space. It consists of one star, eight planets, three known dwarf planets, over 250 satellites, and other small celestial bodies, such as comets and asteroids. The planets, dwarf planets, and small celestial bodies revolve around the sun in elliptical orbits. Satellites orbit planets and other large objects. All the planets orbit the sun in a counter-clockwise direction. Although scientists have discovered other solar systems in distant galaxies, we are just beginning to explore these systems of planets.

The Sun

Which object in our solar system is the most important? If you said Earth, you're close, but the title belongs to the sun. Without the sun's energy, life on Earth could not exist. Compared to other stars in the universe, the sun is only medium in size, but it accounts for 99 percent of the mass of our solar system. The sun's incredible mass creates a powerful gravitational pull that keeps planets and other celestial objects in orbit. Without the sun's gravity, these objects would drift off into space in straight lines.

The sun is made of mostly hydrogen and helium. Nuclear reactions occurring on and inside the sun cause the radiation of solar energy. Like other stars, the sun is its own source of light and heat. Energy from the sun warms and illuminates other objects in the solar system. Since planets cannot radiate their own light, they depend on reflected light from the sun. Sometimes,

Venus and Jupiter appear brighter than stars in the night sky. This brightness is due to the reflection of sunlight.

Planets

Eight planets revolve around the sun. Their orbital distances range from 36 million miles (Mercury) to over two billion miles (Neptune) away from the sun. Scientists often use astronomical units (AU) to describe a planet's distance from the sun. One AU is equal to the average distance between the sun and the Earth, or roughly 93 million miles. Planets closer to the sun than Earth have an AU less than one, while those farther away have AUs larger greater than one.

The first four planets in the solar system, Mercury, Venus, Earth, and Mars, are called the inner planets because they are found between the sun and the asteroid belt. The inner planets are terrestrial, meaning they have hard surfaces made mostly of iron and rock. The terrestrial planets lack rings and have few or no satellites. Jupiter, Saturn, Uranus, and Neptune, are called the outer planets because they are found outside the asteroid belt. Unlike the inner planets, the outer planets do not have rocky surfaces. Instead, they have thick layers of gas and no solid surfaces. Most of the gas is hydrogen and helium, making them similar in composition to the sun. Although theses planets, sometimes called Jovian planets, have no surface, they may have icy or rock cores. Jovian planets usually have rings and many satellites.

Of the planets, Earth is special and unique. It is the only planet in our solar system able to support life. It is the perfect distance from the sun to receive just the right amount of solar radiation. If the Earth were any closer, its surface water would boil and evaporate. If the Earth were any farther away, its water would freeze into unusable blocks of ice. Water is essential for all known forms of life.

Did You Know Scientists have recently discovered an Earth-like planet outside our solar system. The new planet, called Gliese 581 C, is just the right distance from the star it orbits to keep water in liquid form. Scientists will continue to study the planet in hopes of finding a hospitable habitat.

Dwarf Planets

A few years ago, the mnemonic used to remember the planets would have substituted "Nine Pickles" for "Nachos" because Pluto was still considered a planet. In 2006, the International Astronomical Union (IAU) revised the description of a planet, which demoted Pluto to dwarf

planet status. How are planets and dwarf planets different? To be classified as a planet, a celestial object must meet three criteria. First, it must orbit the sun. Second, it must have enough mass for its gravity to pull its matter into a rounded shape. Third, it must orbit the sun in a clear path unobstructed by other celestial bodies. Pluto orbits the sun inside a disk-shaped region of ice and rock called the Kuiper (pronounced Ki-Per) Belt. Since it does not have a clear orbital area, Pluto is considered a dwarf planet. In addition to Pluto, two other dwarf planets have been recognized - Ceres and Eris.

Outer planets and Kuiper Belt

Image courtesy of NASA

Satellites

What do you think about when you hear the word "satellite"? Most people imagine a man-made object orbiting the Earth or nearby planet. Artificial satellites are objects made by humans and sent off into space. Natural satellites are object that orbits another planet or large celestial body, such as a star. Like planets revolving around the sun, satellites are kept in orbit by gravitational force. Without the gravitational pull of the earth, the moon would drift off into space in a straight line.

The most familiar satellite is Earth's moon, which is why most other satellites are called moons. Scientists have discovered almost 250 moons in our solar system, most of which orbit planets or dwarf planets. Of the planets, only Mercury and Venus have no satellites. Jupiter has 63 known satellites, more than any other planet. Jupiter also has the largest moon in the solar system, called Ganymede, which is larger than the planet Mercury. The smallest moon is Mars' Deimos, which is only 6 km in diameter. The Earth has the largest satellite in relation to its planet. The moon is about one-fourth the size of Earth, whereas Ganymede is only 1/27th the size of Jupiter.

The moon with Earth

Image courtesy of NASA/JPL-Caltech

Small Solar Bodies

Imagine you are categorizing your CD collection. You have a category for country, rock, and pop. What do you do with music that is neither country, rock, nor pop? One solution is to make a miscellaneous category. In 2006, the IAU created a miscellaneous-like category for objects in the solar system that are neither stars, planets, dwarf planets, nor satellites. The category, called small solar system bodies, includes asteroids, comets, meteoroids, and other small objects.

Asteroids are small, irregularly shaped chunks of rock. Like the planets, asteroids revolve around the sun. Most asteroids are found in the asteroid belt between Mars and Jupiter. Comets are bright balls of dust, rock, and frozen gas. Like planets and asteroids, comets revolve around the sun, but their orbits are highly elliptical. The orbit of a comet may take it to the outer edge of the solar system, which is why most are only seen once in a lifetime. Meteoroids are chunks of rock moving through space. Unlike asteroids and comets, meteoroids fall to Earth everyday. Most are burned up in the atmosphere, often looking like "shooting stars" as they vaporize.

Let's Review

Our solar system includes the sun, planets, dwarf planets, satellites, and small solar bodies.

Life on Earth is made possible by solar energy that radiates from the sun. Planets must revolve around the sun, have a spherical shape, and maintain a clear orbit.

Dwarf planets do not have clear orbits. Satellites orbit around planets or other large bodies. They are usually called moons. Small solar bodies include asteroids, comets, and meteoroids.