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Armstrong, Neil

Neil A. Armstrong was an American astronaut. He was the first person to set foot on the . Image credit: NASA Born in 1930, Neil A. Armstrong, a United States astronaut, was the first person to set foot on the moon. On July 20, 1969, Armstrong and Buzz Aldrin landed the 11 lunar module Eagle on the moon. Armstrong left the module and explored the lunar surface. Upon taking his first step onto the moon, he said: "That's one small step for a man, one giant leap for mankind." But the word a was lost in radio transmission. Armstrong was born on Aug. 5, 1930, on his grandparents' farm in Auglaize County, Ohio. He moved with his family to several Ohio communities before they settled in Wapakoneta when Neil was 13 old. Armstrong developed an interest in flying at an early age. His love of airplanes grew when he went for his first plane ride in a Ford Tri-Motor, a "Tin Goose," at the age of 6. From then on, he was fascinated by aviation. In 1947, Armstrong entered Purdue University. He began studies in aeronautical engineering. But in 1949, the United States Navy called him to active duty. Armstrong became a Navy pilot and was sent to Korea in 1950, near the start of the Korean War. In Korea, he flew 78 combat missions in Navy Panther jets. In 1952, Armstrong returned to Purdue. He earned a bachelor's degree in aeronautical engineering there in 1955. Armstrong was a civilian test pilot assigned to test the X-15 rocket airplane before becoming an astronaut in 1962. He made his first space flight in 1966 on Gemini 8 with David R. Scott. The two men performed the first successful docking of two vehicles in space -- the Gemini 8 and an uninhabited Agena rocket. Armstrong resigned from the United States astronaut program in 1970. Also in 1970, he earned a master's degree in aerospace engineering at the University of Southern California. From 1971 to 1979, Armstrong was a professor of aerospace engineering at the University of Cincinnati. In 1986, he was named vice chairman of a presidential commission investigating the breakup of the space shuttle Challenger. From 1982 to 1992, Armstrong served as chairman of the board of Computing Technologies for Aviation, a company that develops software for flight scheduling.

An artificial satellite is a manufactured object that continuously Earth or some other body in space. Most artificial satellites Earth. People use them to study the universe, help forecast the weather, transfer telephone calls over the oceans, assist in the navigation of ships and aircraft, monitor crops and other resources, and support military activities. Artificial satellites also have orbited the moon, the , asteroids, and the planets Venus, Mars, and . Such satellites mainly gather information about the bodies they orbit. Piloted spacecraft in orbit, such as space capsules, space shuttle orbiters, and space stations, are also considered artificial satellites. So, too, are orbiting pieces of "space junk," such as burned-out rocket boosters and empty fuel tanks that have not fallen to Earth. But this article does not deal with these kinds of artificial satellites. Artificial satellites differ from natural satellites, natural objects that orbit a planet. Earth's moon is a natural satellite. The Soviet Union launched the first artificial satellite, Sputnik 1, in 1957. Since then, the United States and about 40 other countries have developed, launched, and operated satellites. Today, about 3,000 useful satellites and 6,000 pieces of space junk are orbiting Earth. Satellite orbits Satellite orbits have a variety of shapes. Some are circular, while others are highly elliptical (egg- shaped). Orbits also vary in altitude. Some circular orbits, for example, are just above the atmosphere at an altitude of about 155 miles (250 kilometers), while others are more than 20,000 miles (32,200 kilometers) above Earth. The greater the altitude, the longer the -- the time it takes a satellite to complete one orbit. A satellite remains in orbit because of a balance between the satellite's velocity (speed at which it would travel in a straight line) and the gravitational force between the satellite and Earth. Were it not for the pull of gravity, a satellite's velocity would send it flying away from Earth in a straight line. But were it not for velocity, gravity would pull a satellite back to Earth. To help understand the balance between gravity and velocity, consider what happens when a small weight is attached to a string and swung in a circle. If the string were to break, the weight would fly off in a straight line. However, the string acts like gravity, keeping the weight in its orbit. The weight and string can also show the relationship between a satellite's altitude and its orbital period. A long string is like a high altitude. The weight takes a relatively long time to complete one circle. A short string is like a low altitude. The weight has a relatively short orbital period. Many types of orbits exist, but most artificial satellites orbiting Earth travel in one of four types: (1) high altitude, geosynchronous; (2) medium altitude, (3) sun-synchronous, polar; and (4) low altitude. Most orbits of these four types are circular. A high altitude, geosynchronous orbit lies above the equator at an altitude of about 22,300 miles (35,900 kilometers). A satellite in this orbit travels around Earth's axis in exactly the same time, and in the same direction, as Earth rotates about its axis. Thus, as seen from Earth, the satellite always appears at the same place in the sky overhead. To boost a satellite into this orbit requires a large, powerful launch vehicle. A medium altitude orbit has an altitude of about 12,400 miles (20,000 kilometers) and an orbital period of 12 hours. The orbit is outside Earth's atmosphere and is thus very stable. Radio signals sent from a satellite at medium altitude can be received over a large area of Earth's surface. The stability and wide coverage of the orbit make it ideal for navigation satellites. A sun-synchronous, polar orbit has a fairly low altitude and passes almost directly over the North and South poles. A slow drift of the orbit's position is coordinated with Earth's movement around the sun in such a way that the satellite always crosses the equator at the same local time on Earth. Because the satellite flies over all latitudes, its instruments can gather information on almost the entire surface of Earth. One example of this type of orbit is that of the TERRA Earth Observing System's NOAA-H satellite. This satellite studies how natural cycles and human activities affect Earth's climate. The altitude of its orbit is 438 miles (705 kilometers), and the orbital period is 99 minutes. When the satellite crosses the equator, the local time is always either 10:30 a.m. or 10:30 p.m. A low altitude orbit is just above Earth's atmosphere, where there is almost no air to cause drag on the spacecraft and reduce its speed. Less energy is required to launch a satellite into this type of orbit than into any other orbit. Satellites that point toward deep space and provide scientific information generally operate in this type of orbit. The Hubble Space Telescope, for example, operates at an altitude of about 380 miles (610 kilometers), with an orbital period of 97 minutes. Types of artificial satellites A weather satellite called the Geostationary Operational Environmental Satellite observes atmospheric conditions over a large area to help scientists study and forecast the weather. Image credit: NASA

Artificial satellites are classified according to their mission. There are six main types of artificial satellites: (1) scientific research, (2) weather, (3) communications, (4) navigation, (5) Earth observing, and (6) military. Scientific research satellites gather data for scientific analysis. These satellites are usually designed to perform one of three kinds of missions. (1) Some gather information about the composition and effects of the space near Earth. They may be placed in any of various orbits, depending on the type of measurements they are to make. (2) Other satellites record changes in Earth and its atmosphere. Many of them travel in sun-synchronous, polar orbits. (3) Still others observe planets, , and other distant objects. Most of these satellites operate in low altitude orbits. Scientific research satellites also orbit other planets, the moon, and the sun. Weather satellites help scientists study weather patterns and forecast the weather. Weather satellites observe the atmospheric conditions over large areas. A communications satellite, such as the Tracking and Data Relay Satellite (TDRS) shown here, relays radio, television, and other signals between different points in space and on Earth.

Some weather satellites travel in a sun-synchronous, polar orbit, from which they make close, detailed observations of weather over the entire Earth. Their instruments measure cloud cover, temperature, air pressure, precipitation, and the chemical composition of the atmosphere. Because these satellites always observe Earth at the same local time of , scientists can easily compare weather data collected under constant sunlight conditions. The network of weather satellites in these orbits also function as a search and rescue system. They are equipped to detect distress signals from all commercial, and many private, planes and ships. Other weather satellites are placed in high altitude, geosynchronous orbits. From these orbits, they can always observe weather activity over nearly half the surface of Earth at the same time. These satellites photograph changing cloud formations. They also produce infrared images, which show the amount of heat coming from Earth and the clouds. Communications satellites serve as relay stations, receiving radio signals from one location and transmitting them to another. A communications satellite can relay several television programs or many thousands of telephone calls at once. Communications satellites are usually put in a high altitude, geosynchronous orbit over a ground station. A ground station has a large dish antenna for transmitting and receiving radio signals. Sometimes, a group of low orbit communications satellites arranged in a network, called a , work together by relaying information to each other and to users on the ground. Countries and commercial organizations, such as television broadcasters and telephone companies, use these satellites continuously. A navigation satellite, like this Global Positioning System (GPS) satellite, sends signals that operators of aircraft, ships, and land vehicles and people on foot can use to determine their location. Image credit: NASA

Navigation satellites enable operators of aircraft, ships, and land vehicles anywhere on Earth to determine their locations with great accuracy. Hikers and other people on foot can also use the satellites for this purpose. The satellites send out radio signals that are picked up by a computerized receiver carried on a vehicle or held in the hand. Navigation satellites operate in networks, and signals from a network can reach receivers anywhere on Earth. The receiver calculates its distance from at least three satellites whose signals it has received. It uses this information to determine its location. Earth observing satellites are used to map and monitor our planet's resources and ever-changing chemical life cycles. They follow sun-synchronous, polar orbits. Under constant, consistent illumination from the sun, they take pictures in different colors of visible light and non-visible radiation. Computers on Earth combine and analyze the pictures. Scientists use Earth observing satellites to locate mineral deposits, to determine the location and size of freshwater supplies, to identify sources of pollution and study its effects, and to detect the spread of disease in crops and forests. An Earth observing satellite surveys our planet's resources. This satellite, Aqua, helps scientists study ocean evaporation and other aspects of the movement and distribution of Earth's water.

Military satellites include weather, communications, navigation, and Earth observing satellites used for military purposes. Some military satellites -- often called "spy satellites" -- can detect the launch of missiles, the course of ships at sea, and the movement of military equipment on the ground. The life and death of a satellite Building a satellite Every satellite carries special instruments that enable it to perform its mission. For example, a satellite that studies the universe has a telescope. A satellite that helps forecast the weather carries cameras to track the movement of clouds. In addition to such mission-specific instruments, all satellites have basic subsystems, groups of devices that help the instruments work together and keep the satellite operating. For example, a power subsystem generates, stores, and distributes a satellite's electric power. This subsystem may include panels of solar cells that gather energy from the sun. Command and data handling subsystems consist of computers that gather and process data from the instruments and execute commands from Earth. A satellite's instruments and subsystems are designed, built, and tested individually. Workers install them on the satellite one at a time until the satellite is complete. Then the satellite is tested under conditions like those that the satellite will encounter during launch and while in space. If the satellite passes all tests, it is ready to be launched. Launching the satellite Space shuttles carry some satellites into space, but most satellites are launched by rockets that fall into the ocean after their fuel is spent. Many satellites require minor adjustments of their orbit before they begin to perform their function. Built-in rockets called thrusters make these adjustments. Once a satellite is placed into a stable orbit, it can remain there for a long time without further adjustment. Performing the mission Most satellites operate are directed from a control center on Earth. Computers and human operators at the control center monitor the satellite's position, send instructions to its computers, and retrieve information that the satellite has gathered. The control center communicates with the satellite by radio. Ground stations within the satellite's range send and receive the radio signals. A satellite does not usually receive constant direction from its control center. It is like an orbiting robot. It controls its solar panels to keep them pointed toward the sun and keeps its antennas ready to receive commands. Its instruments automatically collect information. Satellites in a high altitude, geosynchronous orbit are always in contact with Earth. Ground stations can contact satellites in low orbits as often as 12 times a day. During each contact, the satellite transmits information and receives instructions. Each contact must be completed during the time the satellite passes overhead -- about 10 minutes. If some part of a satellite breaks down, but the satellite remains capable of doing useful work, the satellite owner usually will continue to operate it. In some cases, ground controllers can repair or reprogram the satellite. In rare instances, space shuttle crews have retrieved and repaired satellites in space. If the satellite can no longer perform usefully and cannot be repaired or reprogrammed, operators from the control center will send a signal to shut it off. Falling from orbit A satellite remains in orbit until its velocity decreases and gravitational force pulls it down into a relatively dense part of the atmosphere. A satellite slows down due to occasional impact with air molecules in the upper atmosphere and the gentle pressure of the sun's energy. When the gravitational force pulls the satellite down far enough into the atmosphere, the satellite rapidly compresses the air in front of it. This air becomes so hot that most or all of the satellite burns up. History In 1955, the United States and the Soviet Union announced plans to launch artificial satellites. On Oct. 4, 1957, the Soviet Union launched Sputnik 1, the first artificial satellite. It circled Earth once every 96 minutes and transmitted radio signals that could be received on Earth. On Nov. 3, 1957, the Soviets launched a second satellite, Sputnik 2. It carried a dog named Laika, the first animal to soar in space. The United States launched its first satellite, Explorer 1, on Jan. 31, 1958, and its second, Vanguard 1, on March 17, 1958. In August 1960, the United States launched the first communications satellite, Echo I. This satellite reflected radio signals back to Earth. In April 1960, the first weather satellite, Tiros I, sent pictures of clouds to Earth. The U.S. Navy developed the first navigation satellites. The Transit 1B navigation satellite first orbited in April 1960. By 1965, more than 100 satellites were being placed in orbit each . Since the 1970's, scientists have created new and more effective satellite instruments and have made use of computers and miniature electronic technology in satellite design and construction. In addition, more nations and some private businesses have begun to purchase and operate satellites. By the early 2000's, more than 40 countries owned satellites, and nearly 3,000 satellites were operating in orbit.

Asteroid The asteroid Ida is about 35 miles (55 kilometers) long. It is one of thousands of asteroids in the asteroid belt, a region between the orbits of Mars and Jupiter. Image An asteroid is any of numerous small planetary bodies that revolve around the sun. Asteroids are also called minor planets or planetoids. Most of them are in the asteroid belt between the orbits of Mars and Jupiter. The belt contains more than 200 asteroids larger than 60 miles (100 kilometers) in diameter. Scientists estimate that there are more than 750,000 asteroids in the belt with diameters larger than 3/5 mile (1 kilometer). There are millions of smaller asteroids. The average temperature of the surface of a typical asteroid is -100 degrees F (-73 degrees C). Astronomers are not sure how the asteroids originated. According to the leading theory, however, most known asteroids are the shattered remains of a smaller group of larger objects. These objects were left over from the time the planets formed. Elsewhere in the solar system, other such objects gathered together to form the planets and satellites. Size Asteroids vary greatly in size. The largest and first known asteroid, Ceres, was discovered in 1801. It is 580 miles (933 kilometers) in diameter. Ceres is believed to contain about 1/3 the total of all the asteroids. One of the smallest, discovered in 1991 and named 1991 BA, is only about 20 feet (6 meters) across. Composition Studies of an asteroid's reflected light as well as analyses of meteorites have provided information about the composition of asteroids. Astronomers classify asteroids into two broad groups based on their composition. One group of asteroids dominates the outer part of the belt. These asteroids are rich in carbon. Their composition has not changed much since the solar system formed. Asteroids in the second group, which are located in the inner part of the belt, are rich in minerals. These asteroids formed from melted materials. Measuring asteroids Until the 1990's, astronomers could determine the size of an asteroid in only three ways. In the first method, they use telescopes to determine the asteroid's distance from the sun, the amount of sunlight it reflects, and the amount of heat it gives off. The amount of sunlight or heat reaching the earth depends on the size of the asteroid and its distance from the sun. Therefore, calculations involving distance and either light or heat yield the size of the asteroid. In the second method, astronomers use a telescope to measure an asteroid during an occultation, when the asteroid passes in front of a and is silhouetted against it. The third technique involves the use of radio telescopes to produce images of an asteroid. In 1991, scientists began to use a fourth method -- close-range observation of asteroids by space probes. That year, the United States space probe Galileo took the first detailed photograph of an asteroid. The asteroid, called Gaspra, was an irregularly shaped object measuring about 12 by 7 1/2 by 7 miles (19 by 12 by 11 kilometers). Craters cover the surface of the asteroid Eros. The asteroid is about 21 miles (33 kilometers) long, about 1 1/2 times the length of Manhattan Island. Image credit: NASA

In 1996, the U.S. National Aeronautics and Space Administration (NASA) launched the Near Earth Asteroid Rendezvous (NEAR) probe. The probe flew within 753 miles (1,216 kilometers) of the asteroid Mathilde in 1997. The next year, NEAR flew past the asteroid Eros at a distance of 2,378 miles (3,829 kilometers). NEAR went into orbit around Eros in February 2000. In March 2000, the probe was renamed Near Earth Asteroid Rendezvous-Shoemaker (NEAR Shoemaker) in honor of American astronomer Eugene Shoemaker. In February 2001, NEAR Shoemaker became the first spacecraft to land on an asteroid. In October 1998, NASA launched a probe called Deep Space 1. The probe flew within only about 16 miles (26 kilometers) of the asteroid Braille in July 1999. Orbits Most asteroids follow elliptical (oval-shaped) orbits in the asteroid belt. Groups of asteroids that follow the same orbit are called Hirayama families, named after Kiyotsugu Hirayama, the Japanese astronomer who first discovered them. Many asteroids follow orbits outside the belt. For example, a number of asteroids called Trojans follow the same orbit as does Jupiter. Three groups of asteroids -- Atens, Amors, and Apollos -- orbit in the inner solar system and are known as near-Earth asteroids. Some near-Earth asteroids cross the path of Mars, while others cross Earth's orbit. Asteroid collisions

The Chicxulub Basin along the northern coast of Mexico's Yucatan Peninsula formed when an asteroid hit the earth about 65 million years ago. Debris from the impact may have led to the extinction of the dinosaurs. Image credit: World Book map

Many scientists believe that a near-Earth asteroid collided with Earth about 65 million years ago, triggering widespread environmental changes that led to the extinction of the dinosaurs. The asteroid created a huge circular depression called the Chicxulub (CHEEK shoo loob) Basin centered in Mexico's Yucatan Peninsula. The diameter of the basin is about 190 miles (300 kilometers). In 1908, an object exploded about 6 miles (10 kilometers) above the Tunguska River area of Siberia. The object may have been a comet's nucleus or a large meteorite -- sometimes referred to as a small asteroid. Debris from the explosion flattened forests and burned an area about 50 miles (80 kilometers) across. The gravitational pull of Jupiter and other large planets causes asteroid orbits to change very slowly. Orbital changes lead to collisions that create smaller asteroids and fragments, increasing the chance of more collisions. Some small fragments reach Earth's surface as meteorites. Contributor: Marian E. Rudnyk, B.S., Planetary Photogeologist/Astronomer; Consultant, Jet Propulsion Laboratory, National Aeronautics and Space Administration. How to cite this article: To cite this article, World Book recommends the following format: Rudnyk, Marian E. "Asteroid." World Book Online Reference Center. 2005. World Book, Inc. http://www.worldbookonline.com/wb/Article?id=ar034580.

Astronaut An astronaut is a person who pilots a spacecraft or works in space, particularly in the space program of the United States. In Russia and the other former republics of the Soviet Union, such men and women are called cosmonauts. The cosmonaut program was a project of the Soviet Union until the country broke up in 1991. Russia then took over the program. China sent its first astronaut into space in 2003. Astronauts and cosmonauts operate spacecraft and space stations, launch and recapture satellites, and conduct scientific experiments. The word astronaut comes from Greek words that mean sailor among the stars. Cosmonaut means sailor of the universe. Astronauts in the Chinese space program are sometimes called taikonauts. Taikonaut comes from the Chinese words tai kong (outer space). Most U.S. astronauts work for the National Aeronautics and Space Administration (NASA). They live and train at the Lyndon B. Johnson Space Center in Houston. NASA launches astronauts into space aboard space shuttles. NASA selects two kinds of astronauts for space flights: pilot astronauts and mission specialist astronauts. Pilot astronauts command and pilot shuttles. Most pilot astronauts are test pilots from the United States Air Force, Navy, or Marine Corps. They are paid according to their military rank. Mission specialists work with pilots to maintain spacecraft and the equipment aboard. They also conduct experiments and launch satellites. In addition, they perform spacewalks to work outside the spacecraft. Mission specialists may be engineers, scientists, or physicians who have extensive research experience. Those who are in the armed forces are paid according to their rank. The civilians receive salaries based on an equivalent rank in the civil service system. This system includes almost all the federal government's civilian employees who are appointed rather than elected. A third kind of astronaut is called a payload specialist. This kind of astronaut carries out scientific experiments involving the payload (cargo) on the spacecraft. Most payload specialists are scientists who work for the owner of the payload. They must be approved by NASA. The term astronaut also has a meaning that is not connected with NASA activities. In the 1960's, the United States Department of Defense awarded the rating of astronaut to military and civilian pilots who flew aircraft higher than 50 miles (80 kilometers). Seven test pilots received this rating for flights in the X-15 rocket plane. Flights of the X-15 ended in 1968. Cosmonauts train at the Yuri Gagarin Russian State Scientific-Research Test Center of Cosmonauts Training, in Starry Town, also known as Star City, near Moscow. They travel into space aboard vehicles called Soyuz. Unlike space shuttles, these vehicles are not reusable. Crews lift off from the Baykonur Cosmodrome, near the Aral Sea in south-central Kazakhstan. Landings take place in remote, flat areas of Kazakhstan. A Soyuz carries two or three highly specialized cosmonauts. The commander is almost always a military jet pilot, and the flight engineer is almost always a civilian. The flight engineer is usually a member of the staff of the design bureau responsible for the craft. On about half the Soyuz flights, a third cosmonaut, usually called the cosmonaut researcher, is aboard. This person can be a non-Russian "guest cosmonaut" or a Russian physician. Cosmonauts began making guest flights aboard space shuttles in 1994, and astronauts began visiting Russia's Mir space station in 1995. Both astronauts and cosmonauts helped build, and then worked aboard, the International Space Station. In the 1990's, China began developing a spacecraft designed to carry astronauts. The craft, called the Shenzhou, resembles the Soyuz and lifts off from Jiuquan Space Launch Center in northern China. Landings take place in remote areas of Inner Mongolia. This article discusses Astronaut (Achievements in space) (Accidents in space) (Selecting the astronauts) (A look at the astronauts) (Training the astronauts) (Astronauts on the ground) (The cosmonauts). Achievements in space On April 12, 1961, Yuri A. Gagarin of the Soviet Union became the first person to travel in space. He orbited Earth once in a Vostok capsule.Vostok is Russian for east. Gagarin's flight lasted 1 hour 48 minutes. Twenty-three days later, on May 5, Alan B. Shepard, Jr., became the first American space traveler. He made a 15-minute flight in a Mercury capsule but did not go into orbit. John H. Glenn, Jr., the first American in orbit, circled Earth three times on Feb. 20, 1962. The first woman in space, cosmonaut Valentina Tereshkova, was in space for 3 days in 1963. Twenty years later, astronaut Sally K. Ride became the first American woman in space. In June 1983, Ride orbited Earth with four other crew members on a six-day mission aboard the space shuttle Challenger. In 1964, the Soviet Union placed the first three-person spacecraft in orbit. This design was called Voskhod, which is Russian for sunrise. In 1965 and 1966, the United States conducted a series of 10 two-person flights in Gemini spacecraft. During those flights, the astronauts practiced maneuvering their craft and joining it to other orbiting space vehicles. On March 18, 1965, cosmonaut Alexei A. Leonov became the first human being to step outside a spacecraft and float freely in space. Less than three months later, on June 3, astronaut Edward H. White II made the first spacewalk for the United States. In 1967, cosmonauts began flying the Soyuz series of spacecraft. These are three-seat vehicles, but the first crewed flight carried only one cosmonaut, and other early flights carried two. The Soviet Union also tested spacecraft to send cosmonauts to the moon and land them there. After many failures, however, the Soviets canceled their moon-trip projects. Space flights of the Apollo program, the U.S. project to land astronauts on the moon, began in October 1968. On December 24 and 25 of that year, Frank Borman, James A. Lovell, Jr., and William A. Anders orbited the moon 10 times in 20 hours. In doing so, they became the first people to orbit a celestial body other than Earth. On July 20, 1969, astronauts Neil A. Armstrong and Buzz Aldrin became the first people to set foot on the moon. They landed the Apollo 11 lunar module, called the Eagle, and performed scientific experiments and collected rock samples. Other astronauts made five more moon landings from 1969 to 1972. They left five scientific stations on the moon and brought lunar dust and rock to earth. In June 1971, cosmonauts established the first space station, Salyut 1. In 1973, the United States sent up a team of astronauts to operate its first space station, Skylab. Astronauts Charles Conrad, Jr., Joseph P. Kerwin, and Paul J. Weitz live in Skylab for almost a month. In 1975, the United States and the Soviet Union undertook their first joint space mission, the Apollo- Soyuz Test Project. On July 17, an Apollo spacecraft docked with a Soyuz craft. The Apollo craft carried astronauts Thomas P. Stafford, Vance D. Brand, and Donald K. Slayton. Aboard the Soyuz were cosmonauts Alexei A. Leonov and Valery N. Kubasov. For two days, the five spacefarers conducted experiments in the docked craft. On April 12, 1981, the United States launched the space shuttle Columbia, the first reusable spacecraft to carry a crew. Astronauts John W. Young and Robert L. Crippen orbited Earth more than 36 times during a flight lasting about 2 days 6 hours. On Nov. 28, 1983, Columbia carried the first European- built research laboratory, called Spacelab, into space. Cosmonaut Valery Polyakov completed a record 438 days in space on March 22, 1995. Polyakov spent this time aboard Mir. His mission helped scientists study how extended periods of weightlessness affect the human body. Astronauts first recovered, repaired, and relaunched a disabled satellite in April 1984. Traveling aboard Challenger, they used a Canadian-made robot arm to capture the satellite. In May 1992, astronauts aboard the shuttle Endeavour captured a satellite using only their gloved hands. They then attached a special tool to the satellite so that a robot arm could hold it. In December 1993, astronauts aboard Endeavour repaired the Hubble Space Telescope. They installed a device that made up for a defect in the telescope's main mirror. On Oct. 15, 2003, Yang Liwei became the first astronaut sent into space by China. He orbited Earth aboard a Shenzhou spacecraft for 21 hours before landing safely. On June 21, 2004, the American test pilot Michael Melvill became the first astronaut to be launched into space by a private company. Melvill piloted a rocket called SpaceShipOne, which was built and operated by Scaled Composites of Mojave, California. The craft carried Melvill more than 62 miles (100 kilometers) above Earth on a brief suborbital flight. Accidents in space Space travel is risky, and a number of astronauts and cosmonauts have lost their lives in training or on space flights. The first fatality in a space program occurred on March 23, 1961. Valentin V. Bondarenko, a Soviet cosmonaut trainee, died in a fire in a pressure chamber. During a ground test on Jan. 27, 1967, an Apollo spacecraft caught fire, killing the three astronauts inside. The astronauts -- Virgil I. Grissom, Edward H. White II, and Roger B. -- had been scheduled to fly the first Apollo spacecraft. On April 24, 1967, cosmonaut Vladimir Komarov became the first person to die on a space flight. Komarov's flight was the first in which a Soyuz vehicle carried a cosmonaut into space. When Komarov tried to land the vehicle, its parachutes failed to open properly. Komarov died when the Soyuz crashed to earth. The first mission in which people occupied a space station also ended in disaster. In June 1971, Georgi T. Dobrovolsky, Victor I. Patsayev, and Vladislav N. Volkov boarded the experimental station Salyut 1 from their Soyuz 11 spacecraft. During their 23-day mission, they conducted medical examinations of one another and carried out scientific studies. On the return flight, all three cosmonauts died because of a sudden loss of cabin pressure in the Soyuz. On Jan. 28, 1986, Challenger broke apart shortly after launch. All seven crew members were killed. They included Christa McAuliffe, a teacher, who was aboard as part of a program to make the experience of space flight better known to the public. After the Challenger disaster, NASA canceled this program and suspended all shuttle flights. Astronauts returned to space on Sept. 29, 1988, aboard the shuttle Discovery. Discovery's rocket boosters and many other features of the craft had been redesigned as a result of the Challenger disaster. On Feb. 1, 2003, the shuttle Columbia broke apart as it reentered Earth's atmosphere. All seven astronauts on board were killed. Selecting the astronauts The first seven U.S.astronauts,selected for the Mercury program, were, left to right, Donald K. Slayton, Walter M. Schirra, Jr., Gordon , M. Scott , Virgil I. Grissom, John H. Glenn, Jr., and Alan B. Shepard, Jr. Image credit: NASA

NASA accepts applications for pilot astronauts and mission specialist astronauts on a continuing basis. A selection board normally picks a group of about 15 to 25 candidates every two years. An applicant must be a U.S. citizen and must hold a bachelor's degree or higher in engineering, a biological science, a physical science, or mathematics. There is no age limit, but every candidate must pass the NASA space flight physical examination. Pilot astronaut candidates must have flown for 1,000 hours as a command pilot in high-performance jet aircraft. They must be between 5 feet 4 inches and 6 feet 4 inches (163 and 193 centimeters) tall. Candidates for mission specialist do not need flight experience, but they must have at least three years of related professional experience. They must be between 5 feet and 6 feet 4 inches (152 and 193 centimeters) tall. A look at the astronauts Since 1959, more than 250 astronauts have flown in space. NASA chose seven test pilots as the first group of astronauts and introduced them to the public on April 9, 1959. The group consisted of Air Force officers Gordon Cooper, Virgil I. Grissom, and Donald K. Slayton; Navy pilots M. Scott Carpenter, Walter M. Schirra, Jr., and Alan B. Shepard, Jr.; and Marine Corps pilot John H. Glenn, Jr. In the 1960's, NASA selected an additional 49 experienced jet pilots. From 1965 to 1967, NASA picked 17 scientist astronauts. Shannon Lucid, an American astronaut, set the world record for time in space by a woman. In 1996, she spent 188 days in space, mostly aboard the Russian space station Mir.

In 1978, NASA announced the selection of astronauts for upcoming flights of the space shuttle. In this group were 15 pilot astronauts and the first 20 mission specialists. Among the mission specialists were the first six women selected to become astronauts. All six held doctor's degrees. They were physician Anna L. Fisher, biochemist Shannon Wells Lucid, electrical engineer Judith A. Resnik, physicist Sally K. Ride, physician Margaret R. Seddon, and geologist Kathryn D. Sullivan. In 1990, NASA chose the first woman to become a pilot astronaut, Eileen Marie . In 1983, Canada selected six of its citizens to receive training for NASA missions. The next year, Marc Garneau, a commander in the Canadian Navy, flew aboard Challenger. He thereby became the first Canadian astronaut to travel in space. NASA has also flown payload specialists from Belgium, , , Israel, , Japan, Mexico, the Netherlands, Saudi Arabia, Ukraine, and the former West Germany. In 1985, Senator Edwin J. (Jake) Garn of Utah became the first elected official to fly in space. He was chairman of the Senate committee that had oversight responsibilities for the NASA budget. Garn flew aboard Discovery. The next year, Congressman C. William Nelson of Florida flew aboard Columbia. In 1998, John Glenn, then a U.S. senator, returned to space aboard Discovery. He was 77 years old at the time of the flight, making him the oldest person ever to travel in space. Training the astronauts Candidates for pilot and mission specialist undergo one year of general training at Johnson Space Center. After successfully completing this training, they become astronauts. The training involves two major phases: (1) a general phase, involving classroom work, flight training, and survival training; and then (2) more specific basic mission training and advanced mission training. Classroom work NASA brings in instructors from its research centers and from universities to teach aerodynamics, physics, physiology, computer science, and other subjects. Experienced astronauts lecture on such topics as how to communicate with astronauts in space. Other NASA personnel discuss the people, equipment, and funding that make space flight possible. Mercury and Gemini astronauts took courses in rocket engines, flight mechanics, and navigation. In addition to those subjects, Apollo astronauts studied the geology of the moon. They also traveled to Hawaii, Iceland, Alaska, and other places to study volcanic rocks similar to those on the moon. Skylab crews took classes in astronomy, geology, and life sciences to enable them to perform experiments and make observations. Flight training Flight training takes place in T-38 jet aircraft. Once mission specialist candidates learn to operate the aircraft , they fly about 4 hours per month. Pilot candidates must fly 15 hours. Pilots are also trained on a special airplanes called Shuttle Training Aircraft (STA). These airplanes are designed to perform as a space shuttle does during landing. Survival training Survival training teaches candidates how to survive after an unplanned landing in water or in a forest. Before shuttle flights, returning spacecraft landed in the ocean. The space shuttle lands on a runway, but astronaut candidates prepare for emergency bailout over water from shuttles and T-38's. For example, they are towed through the water in a parachute harness to simulate being dragged by a parachute in a wind. In addition, candidates practice survival training in the wilderness. Basic mission training Basic mission training involves the study of cockpit layout and flight-control systems. During such training, candidates also prepare for the actual conditions of space flight. Candidates for pilot and mission specialist train for weightlessness in two ways. They experience the near absence of gravity as large airplanes fly through a series of arcing climbs and dives. For about 30 seconds during each arc, they float weightlessly in the padded body of the aircraft. Floating in water also simulates (reproduces conditions of) weightlessness. The tanks used for training purposes are known as the Weightless Environmental Training Facility (WETF) and the Neutral Buoyancy Laboratory (NBL). After successful completion of the training program, new astronauts continue to develop their skill while they wait for crew assignments. Some become experts in several support or operational areas. Advanced mission training Once assigned to a crew, astronauts spend most of their time training in simulators. Shuttle astronauts train in the Shuttle Mission Simulator (SMS). This device can reproduce the events of an entire mission. Crew members spend as many as eight hours a day in the simulator. Instructors continually give the crew problems to solve to prepare them for emergency situations. Training in simulators is valuable preparation for what the astronauts may later face on actual flights. For example, in 1970, the Apollo 13 astronauts used the oxygen and power supply of their lunar module to return home safely after an explosion damaged their main spacecraft. This operation was less difficult to carry out because the crew was very knowledgeable about all systems on board. Astronauts also train in mock-ups -- that is, full-sized models of the spacecraft. Mock-ups are used to practice working and living in the close quarters of spacecraft. The astronauts store items, prepare foods, and check equipment in the mock-ups. They also practice entering and leaving the spacecraft. Advanced training prepares astronauts for tasks that are not part of all missions. For example, astronauts involved in the 1975 Apollo-Soyuz project and the visits to Mir in 1995 learned the Russian language. They also studied the operation of Russian space vehicles. Astronauts who worked in the Spacelab practiced operating special equipment and instruments needed to conduct experiments. Astronauts preparing for spacewalks receive extra training in the WETF and the NBL. They also train with virtual reality systems. Astronauts on the ground Astronauts taking part in a space mission work on the ground as well as in space. Those on the ground relay information and instructions from flight controllers, engineers, and scientists to the crew. If problems develop, other astronauts help engineers find solutions. Astronauts have helped change the design of spacecraft and their operating systems. For example, Mercury astronauts insisted on a window in the capsule and a hatch that opened from the inside. Also, skill displayed by the astronauts led designers to give them more control over flying the craft. Shuttle astronauts worked on the location of instruments and the modification of space suits. They also helped develop special equipment, such as satellite repair tools The cosmonauts Since April 1961, about 100 cosmonauts have flown in space. Most of them have been from the Soviet Union and, since 1991, Russia. The first cosmonauts were military pilots. Most were in their middle 20's, and many were sent to college after returning from space. Since 1964, crews of cosmonauts could include civilian engineers and physicians. The first cosmonauts spent less than two years in training. The original training program involved constant athletic activity. It included swimming, running, cycling, and parachute jumping over land and water. The U.S. program did not require such activities, but the astronauts were expected to get into good physical condition on their own. The early Soviet program also included training in heat chambers and an isolation cell. They also sat in a spinning, swinging chair that was designed to test for motion sickness. As the Soviets became more experienced in space travel, they learned that training did not need to be so demanding. They eliminated the heat and isolation chambers, and required less parachute jumping. In addition, motion sickness training became easier. Today, cosmonauts spend most of their time studying complex spacecraft systems and working in simulators. They now spend several years preparing for space flight. The Soviet Union and Russia have sent guest cosmonauts into space since 1978. These cosmonauts' home countries include Afghanistan, Austria, Belgium, Bulgaria, Cuba, Czechoslovakia (now the Czech Republic and Slovakia), the former East Germany, France, Germany, Hungary, India, Italy, Japan, Kazakhstan, Mongolia, Poland, Romania, Slovakia, South Africa, Syria, Ukraine, the United Kingdom, the United States, and Vietnam.

Aurora An aurora is a natural display of light in the sky that can be seen with the unaided eye only at night. An auroral display in the Northern Hemisphere is called the aurora borealis, or the northern lights. A similar phenomenon in the Southern Hemisphere is called the aurora australis. Auroras are the most visible effect of the sun's activity on the earth's atmosphere. Most auroras occur in far northern and southern regions. They appear chiefly as arcs, clouds, and streaks. Some move, brighten, or flicker suddenly. The most common color in an aurora is green. But displays that occur extremely high in the sky may be red or purple. Most auroras occur about 60 to 620 miles (97 to 1,000 kilometers) above the earth. Some extend lengthwise across the sky for thousands of miles or kilometers. A bar magnet has a magnetic field like that of the sun. Field lines, which represent the field, exit the north pole and enter the south pole. Image credit: World Book diagram by Precision Graphics

Auroral displays are associated with the solar wind, a continuous flow of electrically charged particles from the sun. When these particles reach the earth's magnetic field, some get trapped. Many of these particles travel toward the earth's magnetic poles. When the charged particles strike atoms and molecules in the atmosphere, energy is released. Some of this energy appears in the form of auroras. Auroras occur most frequently during the most intense phase of the 11-year sunspot cycle. During this phase, dark patches on the sun's surface, called sunspots, increase in number. Violent eruptions on the sun's surface, known as solar flares, are associated with sunspots. Electrons and protons released by solar flares add to the number of solar particles that interact with the earth's atmosphere. This increased interaction produces extremely bright auroras. It also results in sharp variations in the earth's magnetic field called magnetic storms. During these storms, auroras may shift from the polar regions toward the equator.

Aviation Aviation is a term that includes all the activities involved in building and flying aircraft, especially airplanes. The first successful airplane flights did not take place until 1903. Yet today, airplanes affect the lives of people almost everywhere. Giant airliners carry passengers and cargo between the world's major cities in a matter of hours. Planes and helicopters rush medicine and other supplies to the farthest islands and deepest jungles. Farmers use airplanes to seed fields, count livestock, and spray crops. Aviation has also changed the way nations make war. Modern warfare depends on the instant striking power of jet fighters and bombers and the rapid supply capabilities of jet transports. Helicopters and other special aircraft are also important in military aviation. Hundreds of thousands of airplanes are used throughout the world. They range from small planes with room for only a pilot to enormous jumbo jets, which can carry hundreds of passengers. To produce and operate all these airplanes requires the skills of millions of workers in many countries -- from the engineers who design the planes to the mechanics and pilots who service and fly them. Many government agencies also work to make flying safer and more dependable. All these activities together make up the aviation industry. The industry's two major activities are (1) the manufacture of aircraft and aircraft components, such as engines, and (2) the operation of . The manufacture of aircraft, together with the manufacture of spacecraft, missiles, and related electronic equipment, is often called the aerospace industry. The aviation industry began on Dec. 17, 1903, near Kitty Hawk, North Carolina. That day, Orville and Wilbur Wright -- two brothers who operated a bicycle-manufacturing shop in Dayton, Ohio -- made the world's first successful piloted airplane flights. They had built their airplane after studying the writings of other aviation pioneers and after experimenting with gliders, kites, and wind tunnels. Within a few years, several small factories in Europe and the United States were producing airplanes. Daredevil fliers bought many of these planes and used them to put on thrilling air shows. The governments of various countries also began to buy airplanes to build small air forces. The daring feats of the early fliers and the development of military airplanes greatly encouraged the growth of the aviation industry. By the late 1930's, airplanes had become an important means of transportation. Then, in the 1950's, engineers developed jet airliners -- and air travel grew at an even faster rate. In 1960, the world's airlines carried about 100 million passengers. By the early 2000's, they carried about 1 1/2 billion people annually. Almost from the beginning of the aviation industry, the governments of most nations have been deeply involved in its activities. Airplanes have such great importance as weapons of war that many countries have encouraged and financed improvements in airplane design for military reasons. Most nations have also supported the development of civil aviation (the operation of nonmilitary aircraft). Although aviation includes all types of heavier-than-air craft, this article deals chiefly with airplanes. To learn about the two other main types of heavier-than-air craft. The Airplane article traces the history of human efforts to fly and the development of the airplane. It also describes how a plane flies, how pilots navigate, and how planes are built. This article discusses Aviation (The aviation industry) (Aviation agencies and organizations) (History of the aviation industry) (Careers in aviation). The aviation industry The aviation industry can be divided into five branches: (1) aircraft manufacturing, (2) general aviation activities, (3) operations, (4) airport operations, and (5) aviation support industries. Aircraft manufacturing Aircraft companies produce chiefly airplanes, but many also manufacture gliders, helicopters, and parts for spacecraft. Some parts factories and assembly plants are owned by conglomerates, enormous corporations that control a number of firms in largely unrelated fields. Most of the aircraft used around the world are manufactured in the United States. The Russian aerospace industry produces aircraft and equipment for use throughout the former Eastern bloc -- that is, the former Soviet Union and its Eastern European allies. Russia also exports military aircraft to many other countries. British Aerospace is the United Kingdom's major manufacturer of aircraft. Europe's other leading aircraft manufacturing countries are France, Germany, Italy, and Spain. Other countries with important aerospace industries include Brazil, Canada, China, India, Israel, Japan, and South Africa. Many other nations have facilities for aircraft repair and maintenance. Manufacturers produce three main types of airplanes: (1) general aviation planes, (2) commercial transport planes, and (3) military planes. General aviation activities range from business and personal flying to rescue services. Most general aviation planes are small propeller driven airplanes with one or two engines. Many businesses use jets. Commercial transport planes are large airplanes used to carry both passengers and cargo or cargo only. Airlines operate these planes. The smallest commercial transports carry from 20 to 100 passengers, and the largest, called jumbo jets or airbuses, carry several hundred. Most commercial transports are jet planes with two, three, or four engines. Military planes include bombers, fighters, and military transports owned by the governments of various countries and operated by their armed forces. In some countries, the government wholly or partly owns some or all aircraft companies. All aircraft companies in the United States and some other countries are privately owned. But many depend heavily on government orders for military planes, engines, missiles, or spacecraft. Many U.S. manufacturers -- such as the Boeing Company, General Electric Company, and the Lockheed Martin, Northrop Grumman, and United Technologies corporations -- receive large government contracts. A modern jet airliner costs millions of dollars to build. A small company cannot afford to build such a plane, and even large companies often have trouble acquiring the necessary funds. Many companies have merged (combined) to cut costs. These mergers have produced some of the world's largest aerospace companies, including Boeing, British Aerospace, and the European Aeronautic Defence and Space Company. A number of European nations have cooperated in special aircraft-manufacturing projects. For example, the British and French governments formed a partnership called a consortium to share the cost of building a supersonic transport (SST), the Concorde. SST's were designed to carry passengers at speeds faster than that of sound. General aviation activities include pleasure flying, land surveying, giving flying instructions, inspecting telephone lines, scattering seed, and spraying crops. Another important general aviation activity is using light planes to provide transportation. Most air taxi services, also called commuter airlines, use compact, twin-engine planes to carry passengers -- usually fewer than 20 -- on short flights. They serve small communities and provide connecting flights to large airports. Some air taxi services have planes large enough to carry more than 20 passengers. Some large airlines also provide air taxi service. Many businesses have their own aircraft that are used to fly officials and salespeople to out-of-town assignments. General aviation planes also carry cargo and passengers in areas of the world that do not have highways or railroads. In Australia, a specialized aviation service called the Royal Flying Doctor Service supplies medical treatment to people living in remote areas. People who are ill or require medical advice use radio to contact a doctor at the nearest base. The doctor may advise the patient by radio or may arrange for a light plane to pick up the patient. Air ambulances in other parts of the world provide specially equipped airplanes to fly patients to hospitals. Airline operations Almost every country has at least one airline. In some countries, the government owns one or more airlines. For example, , Italy's national airline, is mostly state-owned. During the 1990's, many governments encouraged privatization of airlines to curb costs and increase efficiency. There are two main types of airline service -- scheduled flights and nonscheduled flights. Scheduled flights are made over certain routes according to a timetable. Nonscheduled flights are mainly charter flights for customers who want to hire a plane to fly to a particular place at a particular time. In the United States, airlines must receive permission from the federal government to use commercial transport planes for scheduled flights. The airlines that the government approves for such flights are called certificated airlines. The term scheduled airlines is often used in the United States for the certificated airlines, though these lines may also make some nonscheduled flights. To receive government certification, an airline's planes and pilots must meet government standards. Most airlines carry both passengers and cargo. Airliners usually carry a certain amount of freight on passenger flights. Many passenger airlines also operate transport planes that carry only cargo. A few certificated airlines in the United States specialize in carrying cargo and do not make any passenger flights. Sometimes, airlines have financial problems due to low passenger traffic, debts from purchasing new aircraft, and increasing costs, such as the rising cost of jet fuel. In the 1970's, many airlines cut their airfares and developed various bargain ticket plans to attract passengers. These steps led to huge increases in passenger traffic. High operating costs led many small airlines to merge with larger airlines. By the late 1990's, many airlines had also formed alliances for ticketing and for scheduling certain routes. In most European countries, the government has combined two or more airlines to form a large national airline. Various European airlines have also formed consortiums to help cut expenses. The members of an airline consortium cooperate in such matters as purchasing aircraft and training pilots. Airport operations Airports provide the fuel and the runways, navigation aids, and other ground facilities needed for air travel. Generally, only a few of a country's airports have the facilities to handle large passenger planes. Additional small airfields serve light planes or specialized aircraft, such as helicopters or seaplanes. Cities or public corporations own most large airports. Most small airports are private airfields owned by organizations or individuals. Aviation support industries provide a wide variety of supplies and services to airlines, airports, pilots, and passengers. Some companies furnish repair services or fuel for airplanes. Freight forwarders make arrangements for shipping air cargo. Various food services prepare meals to be served on passenger flights. Some insurance brokers specialize in flight insurance, and some lawyers specialize in air law. Private weather bureaus supply pilots with specialized information not provided by government weather services. Aviation agencies and organizations Aviation agencies Most countries have government agencies that enforce air safety regulations and handle various economic matters relating to aviation. In the United States, the Federal Aviation Administration (FAA) establishes the rules that all planes must follow when flying in the United States. One of the agency's most important jobs is to operate a network of air route traffic control centers throughout the United States and its territories. Each control center uses radar and radio communications to help planes in its vicinity follow the airways, also called air routes, to which they are assigned. The FAA also issues licenses to pilots. In addition, every newly manufactured airplane must be issued an FAA certificate of airworthiness before it may be flown. This certificate states that the airplane has been inspected and is in good flying condition. Almost every U.S. state has an agency to regulate and improve aviation within its borders. These agencies handle airport construction, registration of airplanes and pilots, and similar matters. Many local governments also have aviation agencies. These agencies deal mainly with the operation and maintenance of local airports. In Canada, the federal government regulates civil aviation. The director of civil aviation, under the supervision of the Department of Transport, deals mainly with such matters as registration of aircraft, licensing of pilots, and establishment of air navigation facilities. The Canadian Transport Commission handles the economic regulation of Canadian airlines. Similar regulatory activities are carried out by national agencies in other countries. Such agencies include the United Kingdom's Civil Aviation Authority (CAA). These agencies are involved in such issues as air-traffic control and registration of airplanes and pilots. The International Civil Aviation Organization is an agency of the United Nations (UN). Almost every country belongs to the ICAO. The organization sets up common air safety standards among member countries and tries to increase cooperation in other matters concerning international aviation. Other aviation organizations include various groups that were formed to further their own special interests. These groups include airline operators, airplane manufacturers, and pilots. For example, U.S. and Canadian airline operators belong to the Air Transport Association of America. Operators of international airlines in countries throughout the world belong to the International Air Transport Association. History of the aviation industry Beginnings The successful piloted flights of a powered airplane by Orville and Wilbur Wright in 1903 marked the beginning of the practical aviation industry. After these flights, the Wright brothers tried to sell the design for their plane to the U.S. and various European governments. But they had never made an official public flight, and government leaders were not convinced that their plane could fly. Meanwhile, a few European inventors had also built airplanes. In the 1890's, the German glider pioneer Otto Lilienthal had manufactured a limited production series of special gliders for experimental use. In 1905, two French fliers, the brothers Charles and Gabriel Voisin, started the world's first airplane- manufacturing company. They began making a few made-to-order planes at a small factory outside Paris. Within a few years, other European fliers also started manufacturing companies. They included Louis Bleriot and the brothers Henri and Maurice Farman in France; and Frederick Handley Page, A. V. Roe, and T. O. M. Sopwith in the United Kingdom. In 1907, Glenn H. Curtiss, an American flier and airplane designer, started the first airplane company in the United States, in Hammondsport, New York. Curtiss sold his first plane to the newly organized Aeronautic Society of New York for $5,000. This was the first sale of a commercial airplane in the United States. The Wright brothers had made their first official public flight in 1908 and amazed the world with their airplane's flying ability. That same year, the U.S. Army Signal Corps ordered a specially built Wright plane, for which the government paid $30,000. This was the world's first military plane. In November 1909, a group of wealthy Americans lent the Wright brothers money to start a manufacturing firm, the Wright Company. The company had its factory in Dayton, Ohio, and its headquarters in New York City. In the autumn of 1909, a young automobile mechanic and salesman named Glenn L. Martin began to manufacture airplanes in an abandoned church in Santa Ana, California. Within a few years, his company became a leading U.S. producer of military planes. The world's first great aviation meeting was held in 1909 near Reims, France. Manufacturers displayed 38 airplanes. Several of the planes on show were offered for sale to the public -- a sign of growing confidence in the airplane. The first flying regulations In 1905, a group of French flying enthusiasts established the Federation Aeronautique Internationale (FAI) in Paris. One of the FAI's main duties was to regulate the sport of flying. It also ruled on world speed, altitude, and other flying records. The FAI still has this function. The Aero Club of America was also founded in 1905. It regulated flying in the United States, sponsored exhibitions and races, and issued licenses to U.S. pilots. In 1908, Kissimmee, Florida, passed the world's first law regulating airplanes. The law required the registration of local aircraft and regulated their speed and altitude when flying over the town. In 1911, Connecticut passed the first state law regulating aviation. The law required anyone who owned or operated an airplane within the state to register the plane and obtain a pilot's license. World War I (1914-1918) When World War I began in Europe, even the largest airplane factories turned out only a few planes a year. But the factories quickly increased their production to meet the demands of the warring nations. Airplane builders used newly designed engines to put fighters and bombers into the skies. Such well- known manufacturers as Farman, Handley Page, and Voisin built many of these planes. Other European manufacturers also became famous for their warplanes. They included Morane-Saulnier and Nieuport in France; Fokker and Junkers in Germany; and Bristol, de Havilland, Hawker, Short, and Vickers in the United Kingdom. By the end of the war, designers had created such aircraft as the British Vickers Vimy bomber and the American Curtiss NC-4, both of which flew across the Atlantic Ocean in 1919. The United States entered the war in 1917 with about 110 military planes. The government immediately set a production goal of 29,000 airplanes a year. But the airplane companies had little or no experience with mass-production methods. The nation's automobile manufacturers, on the other hand, had developed assembly lines before the war and used them to turn out thousands of cars yearly. Various automakers helped set up assembly lines in the airplane factories. The United States had no designs of its own for bombers or fighters. But American engineers designed a powerful airplane engine called the Liberty. Several U.S. companies began to mass-produce the United Kingdom's de Havilland D.H. 4 bombers and equip them with Liberty engines. The principal producer was the Dayton Wright Aeroplane Company, which was organized in 1917. Wilbur Wright had died of typhoid fever in 1912, and Orville sold his interest in the Wright Company to a group of investors in 1915. Although Orville had no financial interest in the Dayton Wright Company, he allowed the firm to use the Wright name in its title. The companies founded by Curtiss and Martin also became major producers of military planes during the war. Although U.S. factories did not meet their production goal of 29,000 planes a year, they had built almost 15,000 military planes by the end of the war. In 1916, two airplane companies were established on the West Coast of the United States. They were the Boeing Company, founded in Seattle by William E. Boeing, and the Lockheed Corporation (now Lockheed Martin Corporation), founded in Santa Barbara, California, by the brothers Allan and Malcolm Loughead. The Boeing and Lockheed companies were too small to make many planes during the war. But in time, they became two of the nation's leading aircraft manufacturers. The first airlines The Wright brothers and other early fliers occasionally took passengers for short plane rides. In 1910, a Wright airplane flew 70 pounds (32 kilograms) of silk from Dayton to Columbus, Ohio -- perhaps the first air freight shipment in history. The world's first regular airplane passenger service began in the United States in 1914, but it lasted only a few months. A pilot named Tony Jannus used a small seaplane to fly passengers across Tampa Bay, between St. Petersburg and Tampa, Florida. On May 15, 1918, the U.S. government started the world's first permanent airmail service. Army pilots flew the mail between New York City, Philadelphia, and Washington, D.C. After World War I, thousands of military planes became available for civilian use. In 1919, bombers were used to start nearly 20 small passenger airlines in France, Germany, the United Kingdom, and several other European countries. One of these airlines, founded by Henri and Maurice Farman, began the world's first regular international airline service. The company used old Farman bombers to make weekly passenger flights between Paris and Brussels, Belgium. By 1924, passenger airlines were operating in 17 European countries as well as in Africa, Australia, and South America. Several of these airlines are still active. They include KLM Royal Dutch Airlines (now part of Air France-KLM) of the Netherlands, Germany's , and Australia's Queensland and Northern Territory Aerial Services (QANTAS). Beginning in the mid-1920's, the governments of many countries started to combine two or more private airlines to form a large national airline. In 1924, the United Kingdom became the first major power to form a national, government-owned airline, Imperial Airways. Aviation progress Many small passenger airlines were formed during the early 1920's. But most lasted only a few months because they could not attract enough customers. Most people considered flying a dangerous sport rather than a safe means of transportation. In the United States, the federal government's main interest in aviation was to improve airmail service. In 1920, airmail routes extended from New York City to San Francisco. Mail planes operated only during the day, however. To help the mail pilots fly their open-cockpit planes at night, the government installed beacon lights at airports along the transcontinental route. Each light could be seen as far as 50 miles (80 kilometers) away. By 1924, night-flying techniques enabled planes to get mail from New York City to San Francisco in 24 hours. In 1925, the U.S. Congress passed the Kelly Air Mail Act, which gave private airlines the job of flying the mail. The government then signed contracts with 11 companies formed to carry the mail. Henry Ford, the famous automobile maker, owned one of these airlines. In 1926, Ford's airline became the first airline to carry U.S. mail. Within a few months, all 11 companies were flying mail between major U.S. cities. Some of the airlines also began carrying passengers. In 1926, airlines in the United States carried only about 6,000 passengers. In 1930, they carried more than 400,000. Several U.S. aircraft companies were also started during the 1920's. In 1920, an engineer named Donald Douglas helped organize an aircraft company in Santa Monica, California. It became the Douglas Company the following year, later part of McDonnell Douglas Corporation, and later still part of the Boeing Company. In 1923, the Consolidated Aircraft Corporation was founded in East Greenwich, Rhode Island. It took over the airplane designs of the Dayton Wright Company. The Pratt and Whitney Company began making aircraft engines in Hartford, Connecticut, in 1925. In 1929, the Curtiss and Wright companies merged to form the Curtiss-Wright Corporation. Grumman Aircraft (now part of Northrop Grumman Corporation) also started business in 1929 on Long Island, New York. The rapid increase in aviation activity led Congress to pass the Air Commerce Act in 1926. This act was the first federal law to regulate aviation in the United States. It provided for a system of airways and navigation aids across the country. The act also called for rules governing the manufacture of airplanes and the licensing of airplanes and pilots. A Bureau of Air Commerce was set up to carry out these measures. The industry comes of age Air transport continued to grow during the early 1930's. By 1935, the United States had four major domestic airlines -- American, Eastern, Transcontinental and Western Air (later called Trans World Airlines), and United. Smaller regional airlines included Braniff, Delta, and Northwest. The country also had a major international airline -- Pan American World Airways (Pan Am) -- which flew to Latin America. Many European governments continued to form large national airlines, such as Air France (now part of Air France-KLM) and Italy's (now Alitalia). To meet the growing demand for faster, larger airliners, manufacturers began to produce twin-engine planes, such as the Boeing 247 and the Douglas DC-3. The DC-3 appeared in 1935 and soon became the world's most popular transport plane. A number of companies, including Martin (now Lockheed Martin Corporation) in the United States and Short in the United Kingdom, started to make large, four- engine seaplanes called flying boats. In the 1930's, flying boats made the first passenger flights across oceans. New firms were also started in the 1930's, such as North American Aviation and United Aircraft (now United Technologies), which took over production of Pratt and Whitney engines. By the late 1930's, flying was an important means of travel in most of the world. In 1938, the world's airlines carried nearly 3 1/2 million passengers. The rapid growth of civil aviation created a need for more effective government regulation. In 1938, the U.S. Congress established the Civil Aeronautics Authority to deal with every aspect of civil aviation. The authority included a five-member board, which, in 1940, became the Civil Aeronautics Board. It also included an administrative office, which became the Civil Aeronautics Administration (CAA) in 1940. World War II (1939-1945) The peace treaty that ended World War I prohibited the manufacture of military aircraft in Germany. Nevertheless, several German aircraft firms were founded during the 1920's. They included the famous Heinkel and Messerschmitt companies. In the mid-1930's, Heinkel, Messerschmitt, and other German firms, such as Dornier and Junkers, secretly made hundreds of bombers and fighters for the German air force. On Sept. 1, 1939, German dive bombers attacked Poland, and World War II began. One European country after another fell to the Germans. Finally, the United Kingdom was left nearly alone to fight off the German air force. British aircraft companies, such as Avro, de Havilland, Handley Page, Hawker, and Supermarine, quickly increased their production of warplanes. The United States produced about 2,100 military planes in 1939. Both Germany and Japan had larger air forces. The huge Mitsubishi corporation produced many of Japan's warplanes, including the famous Zero fighter. After the United States entered the war in December 1941, U.S. airplane production increased greatly. More than 40 companies took part in a gigantic effort to supply the United States and its allies with military planes. Many companies enlarged their factories and hired additional workers. Assembly lines began working round the clock. By 1944, production had reached nearly 100,000 transport planes, bombers, and fighters a year. By the end of the war, U.S. factories had built more than 300,000 aircraft. Germany, Japan, the Soviet Union, and the United Kingdom had also produced many thousands of planes. During the war, aircraft production had become the world's leading manufacturing industry. A new age of flight In 1937, British inventor Frank Whittle built the first successful jet engine. The first jet airplanes were developed for military use. Germany flew the first jet aircraft in 1939. By 1942, both the United Kingdom and the United States had developed experimental jet planes for military use. After World War II, aircraft manufacturers began the development of jet airliners. In 1952, British Overseas Airways Corporation (BOAC), now British Airways, started jet passenger flights with de Havilland Comets. But the flights were stopped after several Comets exploded in the air. Investigators discovered serious flaws in the plane's structure. De Havilland engineers then designed an improved Comet. In 1958, BOAC used the new Comets to begin jet passenger service across the Atlantic Ocean. American companies also built successful jet transports in the late 1950's, and these aircraft quickly dominated international air transportation. In 1959, American Airlines used the first of these -- the Boeing 707 -- to start transcontinental jet service from New York City to Los Angeles. The beginning of jet airline service created new challenges. Large jetliners carried nearly 200 passengers, and the crash of one of these planes could cause heavy loss of life. In addition, new hazards were created along the world's air routes as airplanes flew faster and in greater numbers than ever before. In 1958, the U.S. government combined the CAA and several other agencies to form the Federal Aviation Agency. The agency was given the job of establishing and enforcing air safety regulations and air traffic procedures in the United States. It was renamed the Federal Aviation Administration in 1967. By 1970, jet transports had replaced propeller-driven planes on most major airlines. In 1970, Pan Am became the first airline to offer jumbo jet service, using Boeing 747's. France and the United Kingdom began passenger service with their SST, the Concorde, in 1976. Industry mergers Beginning in the 1950's, several large aerospace companies were formed by mergers. In 1954, the General Dynamics Corporation took control of Consolidated Vultee (Convair). In 1967, McDonnell Aircraft merged with Douglas Aircraft to form the McDonnell Douglas Corporation, and North American Aviation and Rockwell-Standard merged, forming the North American Rockwell Corporation. In 1973, this firm merged with Rockwell Manufacturing Company to become Rockwell International Corporation. Internationalization became an important trend in the aviation industry beginning in the 1960's. The term refers to cooperative manufacturing programs in which firms from different nations share research, development, and production costs. The consortium formed by the British and French to build the Concorde SST was an early program of this type. Another successful program has been Airbus. This consortium, which manufactures commercial transport aircraft, has involved most countries in Western Europe. United States aviation firms moved slowly into internationalization in the 1970's. Manufacturers in Canada, Italy, Japan, and the United Kingdom produced major parts of the McDonnell Douglas DC-10 transport, which began commercial service in 1971. Some U.S. firms have formed partnerships with foreign companies to manufacture European-designed aircraft in the United States. For example, during the 1980's, McDonnell Douglas produced the British-designed Harrier -- a V/STOL (Vertical/Short Take-Off and Landing plane) -- in partnership with British Aerospace. Airline safety concerns Beginning in the 1960's, airliner hijacking, also called air piracy, became a serious problem. In 1970, hijackers throughout the world seized 49 airliners and forced the pilots to fly to destinations off their routes, often to other countries. In the 1980's, terrorist sabotage became a serious risk as several airliners were blown up in flight. In response to the hijackings, aviation authorities tightened airport security regulations. These regulations include the inspection of aircraft, passengers, and baggage for hidden guns, bombs, or other weapons. Most countries have laws against hijacking and terrorism. But laws differ from country to country. The ICAO develops procedures to help member countries establish consistent methods to prevent and investigate hijackings. Deregulation of the U.S. airlines In the late 1970's, the Civil Aeronautics Board began to ease its controls over airline fares and routes in the United States to encourage greater competition and better service. In 1978, Congress passed the Airline Deregulation Act. This law provided for the gradual removal of economic controls of the airline industry. The Civil Aeronautics Board was dissolved in 1984. New airlines soon began to form, and existing ones rapidly expanded their services. Recent developments Deregulation in the United States allowed domestic airlines to compete in many international markets. Many U.S. airlines formed alliances with overseas carriers to simplify ticketing. Many U. S. airlines also developed hub and spoke systems. In such a system, many flights connect at a central airport. In manufacturing, several mergers in the 1990's led to the disappearance of historic U.S. airplane builders, such as McDonnell Douglas, which merged into Boeing. International partnerships became increasingly significant, with Airbus capturing one-third of the world market in jet airliner sales in the 1990's. On Sept. 11, 2001, terrorists hijacked four commercial airplanes, deliberately crashing two into the towers of the World Trade Center in New York City and one into the Pentagon Building outside Washington, D.C. The fourth hijacked plane crashed in Somerset County, Pennsylvania. After the hijackings, U.S. airports and airlines sought new ways to protect against terrorist attacks. Congress passed legislation requiring federal employees to handle all passenger and baggage inspection in U.S. airports by the end of 2002. A newly created agency, the Transportation Security Administration, took over air safety functions from the FAA. Fears of terrorism and a sluggish world economy contributed to a decline in air travel in the early 2000's. In 2003, British Airways and Air France discontinued all Concorde flights because the flights were no longer profitable. Careers in aviation The aviation industry employs many kinds of skilled workers. They include aeronautical engineers, computer specialists, electricians, flight attendants, flight engineers, flying instructors, mechanics, pilots, radar specialists, and radio operators. In the United States, many jobs in the aviation industry require certification from the FAA. For example, air traffic controllers, aviation mechanics, flight engineers, and pilots must have FAA certificates. Some schools offer courses in preparation for such careers as aviation mechanic, computer specialist, and radio operator. Aeronautical engineering and some other highly skilled professions require a college education. Most pilots obtain their training at flying schools or in military service. Some high schools and colleges also offer courses in flying. Jobs in general aviation Many pilots work for air taxi services, business firms, and other organizations that use light planes. In many countries, flying light planes for commercial purposes requires a commercial pilot license. In the United States, the FAA issues these licenses to pilots 18 years old or over who have at least 200 hours of flying experience and who pass the physical, written, and flight examinations. Jobs with airlines and airports In most countries, airline pilots and copilots must obtain a special license. They must pass a thorough physical examination, as well as written and flight examinations. In the United States, airline pilots and copilots must have an FAA airline transport pilot license. To obtain this license, they must be at least 23 years old, and have a commercial pilot license and 1,500 hours of flight time. Some airlines use flight engineers. On long flights, the engineers watch the many instruments in the cockpit that tell how the engines are operating. Most airlines require their flight engineers to have a commercial pilot license. Airlines prefer to hire flight attendants who have some college, business, or nursing training. Skilled mechanics are needed for airliner maintenance. Jobs in the aircraft industry Aircraft manufacturers hire electricians, machine tool operators, mechanics, and other skilled workers to make and assemble the many parts of airplanes. The industry also employs various types of engineers to design aircraft and experienced pilots to test-fly planes. Jobs with government agencies. Government agencies in many countries hire radar and radio operators to work at air route traffic control centers and airport control towers. They also hire mechanics and pilots to serve as safety agents. Many local aviation agencies also require engineers, mechanics, pilots, and other skilled people. Some large cities hire pilots to serve as flying police officers or to perform rescue services.

Black Hole A black hole is a region of space whose gravitational force is so strong that nothing can escape from it. A black hole is invisible because it even traps light. The fundamental descriptions of black holes are based on equations in the theory of general relativity developed by the German-born physicist Albert . The theory was published in 1916. Characteristics of black holes The gravitational force is strong near a black hole because all the black hole's matter is concentrated at a single point in its center. Physicists call this point a singularity. It is believed to be much smaller than an atom's nucleus. The surface of a black hole is known as the event horizon. This is not a normal surface that you could see or touch. At the event horizon, the pull of gravity becomes infinitely strong. Thus, an object can exist there for only an instant as it plunges inward at the speed of light. Astronomers use the radius of the event horizon to specify the size of a black hole. The radius of a black hole measured in kilometers equals three times the number of solar of material in the black hole. One is the mass (amount of matter) of the sun. No one has yet discovered a black hole for certain. To prove that a compact object is a black hole, scientists would have to measure effects that only a black hole could produce. Two such effects would be a severe bending of a light beam and an extreme slowing of time. But astronomers have found compact objects that are almost certainly black holes. The astronomers refer to these objects simply as "black holes" in spite of the small amount of uncertainty. The remainder of this article follows that practice. Formation of black holes According to general relativity, a black hole can form when a massive star runs out of nuclear fuel and is crushed by its own gravitational force. While a star burns fuel, it creates an outward push that counters the inward pull of gravity. When no fuel remains, the star can no longer support its own weight. As a result, the core of the star collapses. If the mass of the core is three or more solar masses, the core collapses into a singularity in a fraction of a second. Galactic black holes Most astronomers believe that the Milky Way -- the galaxy in which our solar system is located -- contains millions of black holes. Scientists have found a number of black holes in the Milky Way. These objects are in binary stars that give off X rays. A binary star is a pair of stars that orbit each other. In a binary system containing a black hole, that object and a normal, visible star orbit one another closely. As a result, the black hole strips gas from the normal star, and the gas falls violently toward the black hole. Friction between the gas atoms heats the gas near the event horizon to several million degrees. Consequently, energy radiates from the gas as X rays. Astronomers have detected this radiation with X-ray telescopes. Astronomers believe that a number of binary star systems contain black holes for two reasons: (1) Each system is a source of intense and variable X rays. The existence of these rays proves that the system contains a compact star -- either a black hole or a less compact object called a neutron star. (2) The visible star orbits the compact object at such a high velocity that the object must be more massive than three solar masses. Supermassive black holes Scientists believe that most have a supermassive black hole at the center. The mass of each of those objects is thought to be between 1 million and 1 billion solar masses. Astronomers suspect that supermassive black holes formed several ago from gas that accumulated in the centers of the galaxies. There is strong evidence that a supermassive black hole lies at the center of the Milky Way. Astronomers believe this black hole is a radio-wave source known as Sagittarius A* (SgrA*). The clearest indication that SgrA* is a supermassive black hole is the rapid movement of stars around it. The fastest of these stars appears to orbit SgrA* every 15.2 years at speeds that reach about 3,100 miles (5,000 kilometers) per second. The star's motion has led astronomers to conclude that an object several million times as massive as the sun must lie inside the star's orbit. The only known object that could be that massive and fit inside the star's orbit is a black hole

Christiaan Huygens Christiaan Huygens, (HY guhnz), (1629-1695), was a Dutch physicist, astronomer, and mathematician. In 1678, Huygens proposed that light consists of series of waves. He used this theory in investigating the refraction (bending) of light. Huygens's wave theory competed for many years with the corpuscular theory of the English scientist Isaac Newton. Newton maintained that light is made up of particles. Today, scientists believe that light behaves as both a particle and a wave. Huygens was born on April 14, 1629, in The Hague, the Netherlands. He studied mathematics and law at the University of Leiden and the College of Orange at Breda. Huygens worked with his brother Constantijn to develop skill in grinding and polishing spherical lenses. With these lenses, they built the most powerful telescopes of their time. Huygens also discovered Saturn's moon Titan and asserted that what astronomers called "Saturn's arms" was a ring. In mathematics, he refined the value of pi . In the 1650's, Huygens invented a clock with a freely suspended pendulum. He died on July 8, 1695. The European Space Agency honored Huygens's discovery of Titan by naming a space probe after him. The Huygens probe, designed to drop through Titan's atmosphere, was launched aboard the spacecraft in 1997

Halley's Comet becomes visible to the unaided eye about every 76 years as it nears the sun. A comet (KOM iht) is an icy body that releases gas or dust. Most of the comets that can be seen from Earth travel around the sun in long, oval orbits. A comet consists of a solid nucleus (core) surrounded by a cloudy atmosphere called the coma and one or two tails. Most comets are too small or too faint to be seen without a telescope. Some comets, however, become visible to the unaided eye for several weeks as they pass close to the sun. We can see comets because the gas and dust in their comas and tails reflect sunlight. Also, the gases release energy absorbed from the sun, causing them to glow. Astronomers classify comets according to how long they take to orbit the sun. Short-period comets need less than 200 years to complete one orbit, while long-period comets take 200 years or longer. Astronomers believe that comets are leftover debris from a collection of gas, ice, rocks, and dust that formed the outer planets about 4.6 billion years ago. Some scientists believe that comets originally brought to Earth some of the water and the carbon-based molecules that make up living things. Parts of a comet The nucleus of a comet is a ball of ice and rocky dust particles that resembles a dirty snowball. The ice consists mainly of frozen water but may include other frozen substances, such as ammonia, carbon dioxide, carbon monoxide, and methane. Scientists believe the nucleus of some comets may be fragile because several comets have split apart for no apparent reason. As a comet nears the inner solar system, heat from the sun vaporizes some of the ice on the surface of the nucleus, spewing gas and dust particles into space. This gas and dust forms the comet's coma. Radiation from the sun pushes dust particles away from the coma. These particles form a tail called the dust tail. At the same time, the solar wind -- that is, the flow of high-speed electrically charged particles from the sun-converts some of the comet's gases into ions (charged particles). These ions also stream away from the coma, forming an ion tail. Because comet tails are pushed by solar radiation and the solar wind, they always point away from the sun. Most comets are thought to have a nucleus that measures about 10 miles (16 kilometers) or less across. Some comas can reach diameters of nearly 1 million miles (1.6 million kilometers). Some tails extend to distances of 100 million miles (160 million kilometers). The life of a comet

Comets that pass near the sun come from two groups of comets near the outer edge of the solar system, according to astronomers. The disk-shaped Kuiper belt contributes comets that orbit the sun in fewer than 200 years. The Kuiper belt lies beyond Pluto's orbit, which extends to about 4.6 billion miles (7.4 billion kilometers) from the sun. The Oort cloud provides comets that take longer to complete their orbits. The outer edge of the Oort cloud may be 1,000 times farther than the orbit of Pluto. Image credit: World Book diagram by Terry Hadler, Bernard Thornto

Scientists think that short-period comets come from a band of objects called the Kuiper belt, which lies beyond the orbit of Pluto. The gravitational pull of the outer planets can nudge objects out of the Kuiper belt and into the inner solar system, where they become active comets. Long-period comets come from the Oort cloud, a nearly spherical collection of icy bodies about 1,000 times farther away from the sun than Pluto's orbit. Gravitational interactions with passing stars can cause icy bodies in the Oort cloud to enter the inner solar system and become active comets. Comets lose ice and dust each time they return to the inner solar system, leaving behind trails of dusty debris. When Earth passes through one of these trails, the debris become meteors that burn up in the atmosphere. Eventually, some comets lose all their ices. They break up and dissipate into clouds of dust or turn into fragile, inactive objects similar to asteroids. The long, oval-shaped orbits of comets can cross the almost circular orbits of the planets. As a result, comets sometimes collide with planets and their satellites. Many of the impact craters in the solar system were caused by collisions with comets. Studying comets Scientists learned much about comets by studying Halley's Comet as it passed near Earth in 1986. Five spacecraft flew past the comet and gathered information about its appearance and chemical composition. Several probes flew close enough to study the nucleus, which is normally concealed by the comet's coma. The spacecraft found a roughly potato-shaped nucleus measuring about 9 miles (15 kilometers) long. The nucleus contains equal amounts of ice and dust. About 80 percent of the ice is water ice, and frozen carbon monoxide makes up another 15 percent. Much of the remainder is frozen carbon dioxide, methane, and ammonia. Scientists believe that other comets are chemically similar to Halley's Comet. The space probe Giotto passed near Halley's Comet on March 14, 1986. Giotto returned dramatic close-up images of the comet, including this one. Image credit: European Space Agency

Scientists unexpectedly found the nucleus of Halley's Comet to be extremely dark black. They now believe that the surface of the comet, and perhaps most other comets, is covered with a black crust of dust and rock that covers most of the ice. These comets release gas only when holes in this crust rotate toward the sun, exposing the interior ice to the warming sunlight. Another comet nucleus that has been seen by spacecraft cameras is that of Comet Borrelly. During a flyby in 2001, the Deep Space 1 spacecraft observed a nucleus about half the size of the nucleus of Halley's Comet. Borrelly's nucleus was also potato-shaped and had a dark black surface. Like Halley's Comet, Comet Borrelly only released gas from small areas where holes in the crust exposed the ice to sunlight. In 1994, astronomers observed a comet named Shoemaker-Levy 9, which had split into more than two dozen pieces, crashing into the planet Jupiter. One of the most active comets seen in more than 400 years was Comet Hale-Bopp, which came within 122 million miles (197 million kilometers) of Earth in 1997. This was not an especially close approach for a comet. However, Hale-Bopp appeared bright to the unaided eye because its unusually large nucleus gave off a great deal of dust and gas. The nucleus was estimated to be about 18 to 25 miles (30 to 40 kilometers) across. In 2004, the U.S. spacecraft Stardust passed near the nucleus of Comet Wild 2 and gathered samples from the comet's coma. Stardust was scheduled to return the samples to Earth in 2006. Also in 2004, the European Space Agency launched the Rosetta spacecraft, which was to go into orbit around Comet Churyumov-Gerasimenko in 2014. Rosetta carried a small probe designed to land on the comet's nucleus. Constellation A constellation (KON stuh LAY shuhn) is a group of stars visible within a particular region of the night sky. The word constellation also refers to the region in which a specific group of stars appears. Astronomers have divided the sky into 88 areas, or . The ancient Greeks, Romans, and people of various other early civilizations observed groups of stars in the northern two-thirds of the sky. They named these groups of stars after animals and mythological characters. For example, the constellation Leo was named for a lion, Pisces for two fish, and Taurus for a bull. The constellations Andromeda, Cassiopeia, Orion, and Perseus are named for heroines and heroes in Greek mythology. Between the early 1400's and the mid-1700's, European navigators explored the Southern Hemisphere and observed many constellations in the southernmost third of the sky. Mapmakers and explorers named these star groups for scientific instruments and other things as well as for animals. For example, the constellation Telescopium was named for the telescope. Musca was named for the fly, and Tucana for the toucan, a large-billed bird of Central and South America. Some well-known groups of stars form only part of a constellation. Such smaller groups are called asterisms. For example, the Big Dipper is an asterism that lies in the constellation Ursa Major (Great Bear). Some constellations can be seen only during certain seasons due to the earth's annual revolution around the sun. The part of the sky visible at night at a particular place gradually changes as the earth moves around the sun. Also, observers at different latitudes see different parts of the sky. An observer at the equator can view all the constellations during the course of a year, but an observer at the North or the South Pole can see only a single hemisphere of constellations.

Earth

Earth, our home planet, has oceans of liquid water, and continents that rise above sea level. NASA scientists combined satellite photographs with surface data to create this detailed image of Earth's land masses and oceans. The swirling mass of clouds west of Mexico is a large hurricane.

Earth is a small planet in the vastness of space. It is one of nine planets that travel through space around the sun. The sun is a star -- one of billions of stars that make up a galaxy called the Milky Way. The Milky Way and as many as 100 billion other galaxies make up the universe. The planet Earth is only a tiny part of the universe, but it is the home of human beings and, in fact, all known life in the universe. Animals, plants, and other organisms live almost everywhere on Earth's surface. They can live on Earth because it is just the right distance from the sun. Most living things need the sun's warmth and light for life. If Earth were too close to the sun, it would be too hot for living things. If Earth were too far from the sun, it would be too cold for anything to live. Living things also must have water to live. Earth has plenty. Water covers most of Earth's surface. The study of Earth is called geology, and scientists who study Earth are geologists. Geologists study different physical features of Earth to understand how they were formed and how they may have changed over time. Much of Earth, such as the deep interior, cannot be studied directly. Geologists must often study samples of rock and use indirect methods to learn about the planet. Today, geologists can also view and study the entire Earth from space. This article discusses Earth (Earth as a planet) (Earth's spheres) (Earth's rocks) (Cycles on and in Earth) (Earth's interior) (Earth's crust) (Earth's changing climate) (History of Earth). Earth as a planet

The sun is much larger than Earth. From the sun's center to its surface, it is about 109 times the radius of Earth. Some of the streams of gas rising from the solar surface are larger than Earth.

Earth ranks fifth in size among the nine planets. It has a diameter of about 8,000 miles (13,000 kilometers). Jupiter, the largest planet, is about 11 times larger in diameter than Earth. Pluto, the smallest planet, has a diameter less than one-fifth that of Earth. Earth, like all the planets in our solar system, travels around the sun in a path called an orbit. Earth is about 93 million miles (150 million kilometers) from the sun. It takes one year for Earth to complete one orbit around the sun. The innermost planet, Mercury, is only about one-third as far from the sun as Earth and circles the sun in only 88 days. Pluto, the outermost planet, is 40 times as far from the sun as Earth and takes 248 Earth years to circle the sun. How Earth moves Earth has three motions. It (1) spins like a top around an imaginary line called an axis that runs from the North Pole to the South Pole, (2) it travels around the sun, and (3) it moves through the Milky Way along with the sun and the rest of the solar system. Earth takes 24 hours to spin completely around on its axis so that the sun is in the same place in the sky. This period is called a solar day. During a solar day, Earth moves a little around its orbit so that it faces the stars a little differently each night. Thus, it only takes 23 hours 56 minutes 4.09 seconds for Earth to spin once so that the stars appear to be in the same place in the sky. This period is called a sidereal day. A sidereal day is shorter than a solar day, so the stars appear to rise about 4 minutes earlier each day. Earth takes 365 days 6 hours 9 minutes 9.54 seconds to circle the sun. This length of time is called a sidereal year. Because Earth does not spin a whole number of times as it goes around the sun, the calendar gets out of step with the seasons by about 6 hours each year. Every four years, a day is added to bring the calendar back into line with the seasons. These years, called leap years, have 366 days. The extra day is added to the end of February and occurs as February 29. The distance around Earth's orbit is 584 million miles (940 million kilometers). Earth travels in its orbit at 66,700 miles (107,000 kilometers) an hour, or 18.5 miles (30 kilometers) a second. Earth's orbit lies on an imaginary flat surface around the sun called the orbital plane. Earth's axis is not straight up and down, but is tilted by about 23 1/2 degrees compared to the orbital plane. This tilt and Earth's motion around the sun causes the change of the seasons. In January, the northern half of Earth tilts away from the sun. Sunlight is spread thinly over the northern half of Earth, and the north experiences winter. At the same time, the sunlight falls intensely on the southern half of Earth, which has summer. By July, Earth has moved to the opposite side of the sun. Now the northern half of Earth tilts toward the sun. Sunlight falls intensely over the northern half of Earth, and the north experiences summer. At the same time, the sunlight falls less intensely on the southern half of Earth, which has winter. Earth's orbit is not a perfect circle. Earth is slightly closer to the sun in early January (winter in the Northern Hemisphere) and farther away in July. In January, Earth is 91.4 million miles (147.1 million kilometers) from the sun, and in July it is 94.5 million miles (152.1 million kilometers) from the sun. This variation has a far smaller effect than the heating and cooling caused by the tilt of Earth's axis. Earth and the solar system are part of a vast disk of stars called the Milky Way Galaxy. Just as the moon orbits Earth and planets orbit the sun, the sun and other stars orbit the tightly packed center of the Milky Way. The solar system is about two-fifths of the way from the center of the Milky Way and revolves around the center at about 155 miles (249 kilometers) per second. The solar system makes one complete revolution around the center of the galaxy in about 220 million years. Earth's size and shape Most people picture Earth as a ball with the North Pole at the top and the South Pole at the bottom. Earth, other planets, large , and stars -- in fact, most objects in space bigger than about 200 miles (320 kilometers) in diameter -- are round because of their gravity. Gravity pulls matter in toward the center of objects. Tiny moons, such as the two moons of Mars, have so little gravity that they do not become round, but remain lumpy instead. To our bodies, "down" is always the direction gravity is pulling. People everywhere on Earth feel "down" is toward the center of Earth and "up" is toward the sky. People in Spain and in New Zealand are on exactly opposite sides of Earth from each other, but both sense their surroundings as "right side up." Gravity works the same way on other planets and moons.

Earth has a diameter of about 7,900 miles (12,700 kilometers). The diameter of Jupiter, the biggest planet in our solar system, is more than 11 times as large as the diameter of Earth. Image credit: NASA/NSSDC Earth, however, is not perfectly round. Earth's spin causes it to bulge slightly at its middle, the equator. The diameter of Earth from North Pole to South Pole is 7,899.83 miles (12,713.54 kilometers), but through the equator it is 7,926.41 miles (12,756.32 kilometers). This difference, 26.58 miles (42.78 kilometers), is only 1/298 the diameter of Earth. The difference is too tiny to be easily seen in pictures of Earth from space, so the planet appears round. Earth's bulge also makes the circumference of Earth larger around the equator than around the poles. The circumference around the equator is 24,901.55 miles (40,075.16 kilometers), but around the poles it is only 24,859.82 miles (40,008.00 kilometers). The circumference is actually greatest just south of the equator, so Earth is slightly pear-shaped. Earth also has mountains and valleys, but these features are tiny compared to the total size of Earth, so the planet appears smooth from space. Earth and its moon Earth has one moon. Pluto also has one moon, while Mercury and Venus have none. All the other planets in our solar system have two or more moons. Earth's moon has a diameter of 2,159 miles (3,474 kilometers) -- about one-fourth of Earth's diameter. View of Earth and the moon from space. Image credit: NASA

The sun's gravity acts on Earth and the moon as if they were a single body with its center about 1,000 miles (1,600 kilometers) below Earth's surface. This spot is the Earth-moon barycenter. It is the point of balance between the heavy Earth and the lighter moon. The path of the barycenter around the sun is a smooth curve. Earth and the moon circle the barycenter as they orbit the sun. The motion of Earth and moon around the barycenter makes them "wobble" in their path around the sun. Earth's spheres Earth is composed of several layers, or spheres, somewhat like the layers of an onion. The solid Earth consists of a thin outer layer, the crust, with a thick rocky layer, the mantle, beneath it. The crust and the upper portion of the mantle are called the lithosphere. At the center of Earth is the core. The outer part of the core is liquid, while the inner part is solid. Much of Earth is covered by a layer of water or ice called the hydrosphere. Earth is surrounded by a thin layer of air, the atmosphere. The portion of the hydrosphere, atmosphere, and solid land where life exists is called the biosphere. The atmosphere Air surrounds Earth and becomes progressively thinner farther from the surface. Most people find it difficult to breathe more than 2 miles (3 kilometers) above sea level. About 100 miles (160 kilometers) above the surface, the air is so thin that satellites can travel without much resistance. Detectable traces of atmosphere, however, can be found as high as 370 miles (600 kilometers) above Earth's surface. The atmosphere has no definite outer edge but fades gradually into space. Nitrogen makes up 78 percent of the atmosphere, while oxygen makes up 21 percent. The remaining 1 percent consists of argon and small amounts of other gases. The atmosphere also contains water vapor, carbon dioxide, water droplets, dust particles, and small amounts of many other chemicals released by volcanoes, fires, living things, and human activities. The lowest layer of the atmosphere is called the troposphere. This layer is in constant motion. The sun heats Earth's surface and the air above it, causing warm air to rise. As the warm air rises, air pressure decreases and the air expands and cools. The cool air is denser than the surrounding air, so it sinks and the cycle starts again. This constant cycle of the air causes the weather. High above the troposphere, about 30 miles (48 kilometers) above Earth's surface, is a layer of still air called the stratosphere. The stratosphere contains a layer where ultraviolet light from the sun strikes oxygen molecules to create a gas called ozone. Ozone blocks most of the harmful ultraviolet rays from reaching Earth's surface. Some ultraviolet rays get through, however. They are responsible for sunburn and can cause skin cancer in people. Tiny amounts of human-made chemicals have caused some of the natural ozone to break down. Many people are concerned that the ozone layer may become too thin, allowing ultraviolet rays to reach the surface and harm people and other living things. Water vapor, carbon dioxide, methane, and other gases in the atmosphere trap heat from the sun, warming Earth. The heat-trapping quality of these gases causes the greenhouse effect. Without the greenhouse effect of the atmosphere, Earth would probably be too cold for life to exist. Ocean waters cover most of Earth's surface. This satellite view shows the Indian Ocean, partly bordered by Africa, Asia, and Australia, and below it the Southern Ocean surrounding Antarctica. The hydrosphere

Ocean waters cover most of Earth's surface. This satellite view shows the Indian Ocean, partly bordered by Africa, Asia, and Australia, and below it the Southern Ocean surrounding Antarctica.

Earth is the only planet in the solar system with abundant liquid water on its surface. Water has chemical and physical properties not matched by any other substance, and it is essential for life on Earth. Water has a great ability to absorb heat. The oceans store much of the heat Earth gets from the sun. The electrical charges on water molecules give water a great ability to attract atoms from other substances. This quality allows water to dissolve many things. Water's ability to dissolve materials makes it a powerful agent in breaking down rocks. Liquid water on Earth affects not just the surface but the interior as well. Water in rocks lowers the melting temperature of rock. Water dramatically weakens rocks and makes them easier to melt beneath Earth's surface. About 71 percent of Earth's surface is covered by water, most of it in the oceans. Ocean water is too salty to drink. Only about 3 percent of Earth's water is fresh water, suitable for drinking. Much of Earth's fresh water is not readily available to people because it is frozen in the polar ice caps or beneath Earth's surface. Polar regions and high mountains stay cold enough for water to remain permanently frozen. The region of permanent ice on Earth is sometimes called the cryosphere. The lithosphere The crust and upper mantle of Earth from the surface to about 60 miles (100 kilometers) down make up the lithosphere. The thin crust is made up of natural chemicals called minerals composed of different combinations of elements. Oxygen is the most abundant chemical element in rocks in Earth's crust, making up about 47 percent of the weight of all rock. The second most abundant element is silicon, 27 percent, followed by aluminum (8 percent), iron (5 percent), calcium (4 percent), and sodium, potassium, and magnesium (about 2 percent each). These eight elements make up 99 percent of the weight of rocks on Earth's surface. Two elements, silicon and oxygen, make up almost three-fourths of the crust. This combination of elements is so important that geologists have a special term for it: silica. Minerals that contain silica are called silicate minerals. The most abundant mineral on Earth's surface is quartz, made up of pure silica. Another plentiful group of silicates are the feldspars, which consist of silica, aluminum, calcium, sodium, and potassium. Other common silicate minerals on Earth's surface are pyroxene (PY rahk seen) and amphibole (AM fuh bohl), which consist of combinations of silica, iron, and magnesium. Another important group of minerals are the carbonates, which contain carbon and oxygen along with small amounts of other elements. The most important carbonate mineral is calcite, made up of calcium, carbon, and oxygen. Limestone, a common rock used for building, is mostly calcite. Another important carbonate is dolomite, composed of carbon, oxygen, calcium, and magnesium. Earth has two kinds of crust. The dry land of the continents is made up mostly of granite and other light silicate minerals, while the ocean floors are composed mostly of a dark, dense volcanic rock called basalt. Continental crust averages about 25 miles (40 kilometers) thick, but it is thicker in some areas and thinner in others. Most oceanic crust is only about 5 miles (8 kilometers) thick. Water fills in the low areas over the thin basalt crust to form the world's oceans. There is more than enough water on Earth to completely fill the oceanic basins, and some of it spreads onto the edges of the continents. This portion of the continents surrounded by a band of shallow ocean is called the continental shelf. The biosphere Earth is the only planet in the universe known to have life. The region containing life extends from the bottom of the deepest ocean to a few miles or kilometers into the atmosphere. There are several million known kinds, called species, of living things, and scientists believe that there are far many more species not yet discovered. Life affects Earth in many ways. Life has actually made the atmosphere around us. Plants take in water and carbon dioxide, both of which contain oxygen. They use the carbon in carbon dioxide and the hydrogen in water to make chemicals of many kinds and give off oxygen as a waste product. Animals eat plants to get energy and return water and carbon dioxide back into the environment. Living things affect the surface of Earth in other ways as well. Plants create chemicals that speed the breakdown of rock. Grasslands and forests slow the erosion of soil. Earth's rocks The solid part of Earth consists of rocks, which are sometimes made up of a single mineral, but more often consist of mixtures of minerals. Geologists classify rocks according to their origin. Igneous rocks form when molten rock cools and solidifies. Sedimentary rocks form when grains of rock or dissolved chemicals are deposited in layers by wind, water, or glaciers. Over time, the layers harden into solid rock. Metamorphic rocks develop deep in Earth's crust when heat or pressure transform other types of rock. Igneous rocks form from molten material called magma. Most of Earth's interior is solid, not molten, but it is extremely hot. At the base of Earth's crust, the temperature is about 1800 degrees F (1000 degrees C). In some portions of the crust, conditions are right for rocks to melt. Rocks can melt more easily near the crust if they contain water, which lowers their melting point. Where conditions are right, small pockets of magma form beneath and within the crust. Some of this magma reaches the surface, where it erupts from volcanoes as lava. Igneous rocks formed this way are called volcanic or extrusive. Vast quantities of magma, however, never reach the surface. They cool slowly within the crust and may only be exposed long afterward by erosion. Such igneous rocks are called plutonic or intrusive. Plutonic rocks cool slowly. During this slow cooling, their minerals form large crystals. Plutonic rocks tend to be much coarser than volcanic rocks. Igneous rocks that are rich in silica tend to be poor in iron and magnesium, and the opposite is also true. Volcanic rocks that are iron-rich and silica-poor are basalt. Plutonic rocks of the same makeup are called gabbro. Silica-rich volcanic rocks are called rhyolite (RY uh lyt), and plutonic rocks of the same composition are granite. Granite lies under most of the continents, while basalt lies under most of the ocean floors. Sedimentary rocks Rocks on Earth's surface are under constant attack by chemicals and mechanical forces. The processes that break down rocks are called weathering. Water is effective at dissolving minerals. When water freezes, it expands, so expanding ice helps pry apart mineral grains in rocks. In addition, living things produce chemicals that help dissolve rocks. Once rocks break apart, the loose material is often carried away by erosion. Running water erodes rocks. Wind and glaciers also contribute to erosion. Erosion is usually a relatively slow process, but over millions of years, erosion can uncover even rocks many miles or kilometers below the surface. Materials derived from weathering and erosion of rocks are eventually deposited to form sedimentary rocks. Rocks that are made up of small pieces of other rocks are called clastic rocks. Rocks containing larger pebbles are called conglomerate. The particles in these rocks are cemented together when minerals dissolved in the water crystallize between the grains. The most abundant sedimentary rocks, called mudrocks, consist of tiny particles. Some of these rocks, called shale, split into thin sheets when broken. Sandstone is a sedimentary rock made up of sand cemented together. Other sedimentary rocks form when dissolved materials undergo chemical reactions and settle out as tiny solid particles. These rocks are called chemical sedimentary rocks. Common chemical sedimentary rocks include some types of limestone and dolomite. Some chemical sedimentary rocks form when water evaporates, leaving dissolved materials behind. Rock salt and a mineral called gypsum form this way. Some sedimentary rocks, called biogenic, are formed by the action of living things. Coal is the remains of woody plants that have been transformed into rock by heat and pressure over time. Most limestone is formed by microscopic marine organisms that secrete protective shells of calcium carbonate. When the animals die, the shells remain and solidify into limestone. Metamorphic rocks When rocks are buried deeply, they become hot. Earth's crust grows hotter by about 70 degrees F per mile (25 degrees C per kilometer) of depth. Pressure also increases with depth. At a depth of 1 mile (1.6 kilometers) beneath the surface, the pressure is about 6,000 pounds per square inch (41,360 kilopascals). As rocks are heated and subjected to pressure, minerals react and the rocks become metamorphic. Shale is transformed to slate, limestone, and eventually into marble under pressure. Many metamorphic rocks contain recognizable features that tell of their origin, but others change so much that only the chemical makeup provides evidence of what they originally were. Cycles on and in Earth Earth can be thought of as a huge system of interacting cycles. In each cycle, matter and energy move from place to place and may change form. Eventually, matter and energy return to their original condition and the cycle begins again. The cycles affect everything on the planet, from the weather to the shape of the landscape. There are many cycles on and within Earth. A few of the most important are (1) atmospheric circulation, (2) ocean currents, (3) the global heat conveyor, (4) the hydrologic cycle, and (5) the rock cycle. Atmospheric circulation Air warmed by the sun near the equator rises and flows toward Earth's poles, returning to the surface and flowing back to the equator. This motion, combined with the rotation of Earth, moves heat and moisture around the planet creating winds and weather patterns. In some areas, the winds change directions with the seasons. These patterns are often called monsoons. In summer, air over Asia is heated by the sun, rises, and draws moist air from the Indian Ocean, causing daily rains over most of southern Asia. In winter, the air over Asia cools, sinks, and flows out, pushing the moist air away and creating dry weather. A similar pattern occurs in the Pacific Ocean near Mexico and brings moist air and afternoon thunderstorms to the southwestern United States in the summer. Ocean currents are driven by the winds and follow the same general pattern. The continents block the flow of water around the globe, so ocean currents flow west near the equator, then turn toward the poles when they strike a continent, turn east, then flow back to the equator on the other side. In all the oceans, the ocean currents form great loops called gyres. The gyres flow clockwise north of the equator and counterclockwise south of it. The global heat conveyor is an enormous cycle of ocean water that distributes the oceans' heat around Earth. Water in the polar regions is very cold, salty, and dense. It sinks and flows along the sea floor toward the equator. Eventually, the water rises along the margins of the continents and merges with the surface water flow. When it reaches the polar regions, it sinks again. This three-dimensional movement of water mixes heat throughout the oceans, warming polar waters. It also brings nutrients up from the deep ocean to the surface, where they are available for marine plants and animals. The hydrologic cycle Water from the oceans evaporates and is carried by the atmosphere, eventually falling as rain or snow. Water that falls on the land helps break rocks down chemically, nourishes plants, and wears down the landscape. Eventually, the water returns to the sea to start the cycle over again. The rock cycle Earth has many more kinds of rocks compared to other planets because there are so many processes acting to form and break down rocks. Geologists sometimes speak of the rock cycle to explain how different rock types are related. The cycle may begin with a flow of lava from a volcano cooling to form new igneous rocks on Earth's surface. As the rock is exposed to water, it breaks down and the resulting materials may be carried away to be deposited as sedimentary rocks. These rocks may eventually be so deeply buried that they change in form to become metamorphic rocks. They may even melt, creating the raw material for the next generation of igneous rocks. Rocks rarely go through the entire rock cycle. Instead, some steps may be skipped or repeated. For example, igneous rocks can be subjected to heat and pressure and transformed directly to metamorphic rocks. Sedimentary rocks can be broken down by weathering and then reassembled into a new generation of sedimentary rocks. Metamorphic rocks can also be weathered to form the raw material for a new generation of sedimentary rocks. Any rock type, igneous, metamorphic, or sedimentary, can be transformed into any other type. Earth's interior

Beneath Earth's solid crust are the mantle, the outer core, and the inner core. Scientists learn about the inside of Earth by studying how waves from earthquakes travel through the planet. Image credit: World Book illustration by Raymond Perlman and Steven Brayfield, Artisan-Chicago

Geologists cannot study the interior of Earth directly. The deepest wells drilled reach less than 8 miles (13 kilometers) below the surface. Geologists know that the whole Earth differs in composition from its thin outer crust. Deep in Earth, pressures are so great that minerals can be compressed into dense forms not found on the surface. One way geologists determine the overall composition of Earth is from chemical analysis of meteorites. Certain types of meteorites, called chondrites, are remains of the early solar system that persisted unchanged in space until they fell to Earth. Geologists can use chondrites to estimate the original chemical composition of the entire Earth. Unlike chondrites, Earth is made up of layers that contain different amounts of various chemical elements. Geologists learn about Earth's interior by studying vibrations generated by earthquakes, using instruments called seismographs. The speed and motion of vibrations traveling through Earth depends on the composition and density of the material they travel through. Geologists can determine many properties of Earth's interior by analyzing such vibrations. The mantle Beneath the crust, extending down about 1,800 miles (2,900 kilometers), is a thick layer called the mantle. The mantle is not perfectly stiff but can flow slowly. Earth's crust floats on the mantle much as a board floats in water. Just as a thick board would rise above the water higher than a thin one, the thick continental crust rises higher than the thin oceanic crust. The slow motion of rock in the mantle moves the continents around and causes earthquakes, volcanoes, and the formation of mountain ranges. The core At the center of Earth is the core. The core is made mostly of iron and nickel and possibly smaller amounts of lighter elements, including sulfur and oxygen. The core is about 4,400 miles (7,100 kilometers) in diameter, slightly larger than half the diameter of Earth and about the size of Mars. The outermost 1,400 miles (2,250 kilometers) of the core are liquid. Currents flowing in the core are thought to generate Earth's magnetic field. Geologists believe the innermost part of the core, about 1,600 miles (2,600 kilometers) in diameter, is made of a similar material as the outer core, but it is solid. The inner core is about four-fifths as big as Earth's moon. Earth gets hotter toward the center. At the bottom of the continental crust, the temperature is about 1800 degrees F (1000 degrees C). The temperature increases about 3 degrees F per mile (1 degrees C per kilometer) below the crust. Geologists believe the temperature of Earth's outer core is about 6700 to 7800 degrees F (3700 to 4300 degrees C). The inner core may be as hot as 12,600 degrees F (7000 degrees C) -- hotter than the surface of the sun. But, because it is under great pressures, the rock in the center of Earth remains solid. Earth's crust The hot rock deep in Earth's mantle flows upward slowly, while cooler rock near the surface sinks because hot materials are lighter than cool materials. The rising and sinking of materials due to differences in temperature is called convection. As Earth's mantle flows, it breaks the crust into a number of large slabs called tectonic plates, much as slabs of ice break apart on a pond. The slow flow of Earth's mantle drags the crust along, causing the continents to move, mountains to form, and volcanoes and earthquakes to occur. This constant motion of Earth's crust is called plate tectonics. In some places, usually under the oceans, Earth's plates are spreading apart. New magma from the mantle rises to fill the cracks between the plates. Places where plates spread apart are called spreading centers. Many volcanoes occur where plates pull apart and magma wells up from within the mantle to fill the gap. The material from the mantle is made of iron and magnesium-rich silicate rocks. It hardens to form rocks and creates oceanic crust made of basalt. Subduction Earth's crust cannot spread apart everywhere. Somewhere, an equal amount of crust must be removed. When two plates push together, one of the plates sinks back into Earth's mantle, a process called subduction. The sinking plate eventually melts into magma in Earth's interior. Much of the magma created in subduction zones does not reach the surface and cools within the crust, forming plutonic rocks. The heat from the magma also helps create metamorphic rocks. Because continental crust is too thick and light to sink into Earth's interior, only plates made of dense oceanic crust are subducted. The boundary where the two plates meet is marked by a deep trench on the ocean floor. The trenches are the deepest places in the oceans, up to 36,000 feet (11,000 meters) deep. The upper plate that remains on the surface may be continental crust or oceanic crust. This plate is also changed by subduction. As the two plates move together, the edge of the upper plate is compressed. The crust becomes thicker and higher, creating a mountain range. When the rocks of the sinking plate reach a depth of about 60 miles (100 kilometers), they begin to melt and form magma. Some of the magma reaches the surface to form volcanoes. Regions with many volcanoes, such as Peru, Japan, and the northwestern United States, lie near areas where subduction is happening. Mountain building Occasionally, as a plate sinks into Earth's mantle, it drags along a continent or a smaller land mass. Continental crust is too thick and light to sink. Instead, it collides with the opposing plate. If the opposing plate is also a continent, neither plate will sink. This type of collision often forms a vast mountain chain in the middle of a continent. The Himalaya were formed in such a way from the collision of two plates of continental crust. The series of events that happen during formation of a mountain range is called orogeny. Orogeny includes the elevation of mountains, folding and crumpling of the rocks, volcanic activity, and formation of plutonic and metamorphic rocks that occur when plates collide. Long after mountains have vanished from erosion, geologists can still see the changes orogeny produces in the rocks. Terrane collisions Smaller pieces of continental crust that collide with another plate are often added to the edge of the larger plate. These small added pieces of crust are called terranes. Most of the land in the United States west of Salt Lake City has been added to North America by terrane collisions in the last 500 million years. Earthquakes Earthquakes occur when rocks on opposite sides of a break in the crust, called a fault, slide past each other. The boundaries between plates are faults, but there are faults within plates as well. Occasionally, forces within the plates cause rocks to fracture and slip even though the rocks are not at a plate boundary. The boundaries between two plates sliding past each other are called transform faults. The San Andreas Fault in California is a transform fault, where a portion of crust called the Pacific Plate is carrying a small piece of California northwest past the rest of North America. The shaping of the continents Several times in Earth's history, collisions between continents have created a huge supercontinent. Although the crust of the continents is thick, it breaks more easily than oceanic crust, and supercontinents broke quickly into smaller pieces. Material from Earth's mantle filled the gaps, creating new oceanic crust. As the continents moved apart, new ocean basins formed between them. About one- third of Earth's surface is covered by continental crust, so the pieces cannot move far before colliding. As two continents collide, an old ocean basin is destroyed. The process of continents breaking apart and rejoining is called the Wilson cycle, after the Canadian geologist John Tuzo Wilson, who first described it. The continents have probably been in motion for at least the past 2 billion years or more. Geologists, however, only have evidence from rocks to understand and reconstruct the motion over the past 800 million years. Most of the oceanic crust older than that has been subducted into the mantle long ago. Geologists have determined that, about 800 million years ago, the continents were assembled into a large supercontinent called Rodinia. What is now North America lay at the center of Rodinia. The flow of material in Earth's mantle caused Rodinia to break apart into many pieces, which collided again between 500 million and 250 million years ago. Collision between what is now North America, Europe, and Africa caused the uplift of the Appalachian Mountains in North America. Collisions between part of present-day Siberia and Europe created the Ural Mountains. By 250 million years ago, the continents reassembled to form another supercontinent called Pangaea. A single, worldwide ocean, called Panthalassa, surrounded Pangaea. About 200 million years ago, Pangaea began to break apart. It split into two large land masses called Gondwanaland and Laurasia. Gondwanaland then broke apart, forming the continents of Africa, Antarctica, Australia, and South America, and the Indian subcontinent. Laurasia eventually split apart into Eurasia and North America. As the continental plates split and drifted apart, new oceanic crust formed between them. The movement of the continents to their present positions took place over millions of years. Earth's changing climate The ice ages

Precambrian time included almost all of Earth's first 4 billion years. The crust, the atmosphere, and the oceans were formed, and the simplest kinds of life appeared.

Throughout the history of Earth, the climate has changed many times. Between 800 million and 600 million years ago, during a time called the Precambrian, Earth experienced several extreme climate changes called ice ages or glacial epochs. The climate grew so cold that some scientists believe Earth nearly or completely froze several times. The theory that the entire Earth froze is sometimes called the snowball Earth. Geologists estimate that Earth experienced up to four such periods of alternate freezing and thawing. Most of the time, Earth has been largely ice free. Brief ice ages occurred about 450 million years ago and again about 250 million years ago. In the last few million years, however, Earth's climate began to cool. Glaciers began forming in Antarctica about 35 million years ago, but the climate there was warm enough for trees to grow until about 5 million years ago. By about 2 million years ago, at the beginning of a time called the Pleistocene , ice had accumulated on other continents as well. Numerous separate ice advances, periods when ice sheets covered vast areas, occurred during the Pleistocene Ice Age. The advances alternated with periods when the climate was warmer and the ice melted. Geologists analyzing sediment deposits from the North Atlantic Ocean determined that there were at least 20 advances and retreats of ice sheets in the past 2 million years. At least four ice advances were big enough to extend over much of Europe, cover most of Canada, and reach deep into the United States. The most recent advance of ice began about 70,000 years ago and reached its farthest extent about 18,000 years ago. The vast glaciers and sheets of ice scoured out the basins of the Great Lakes and blocked rivers, completely changing the courses of the Mississippi, Missouri, and Ohio rivers. So much water was trapped in the form of ice that sea level around Earth dropped as much as 390 feet (120 meters), exposing parts of the present ocean floor. The most recent ice advance ended about 11,500 years ago. Most scientists believe that Earth is currently in an interglacial period, and another ice advance will follow. Why ice ages occur Scientists do not fully understand why Earth has ice ages. Most believe that tiny changes in Earth's orbit and axis due to the gravitational pull of other planets play a part. These changes alter the amount of energy received from the sun. Many scientists also believe that variations in the amount of carbon dioxide in the atmosphere are responsible for long-term changes in the climate. Carbon dioxide, a "greenhouse gas," traps heat from the sun and warms Earth's atmosphere. Most of Earth's carbon dioxide is locked in carbonate rocks, such as limestone and dolomite. Earth's climate today would be much warmer if the carbon dioxide trapped in limestone were released into the atmosphere. When mountains rich in silicate minerals wear down through weathering and erosion, calcium and magnesium erode from the rocks. These elements are carried to the sea by water. There, living organisms absorb the chemicals and use them to make protective carbonate shells. The organisms eventually die and sink to the bottom to form limestone deposits. This process, called the carbonate- silicate cycle, removes carbon dioxide from the atmosphere. With less carbon dioxide in the atmosphere to trap heat from the sun, Earth's climate may cool enough to cause an ice age. Limestone and dolomite deposits exposed to weathering and erosion return carbon dioxide to the atmosphere and contribute to global warming. In addition, some limestone on the ocean floor can be carried down into Earth's mantle by subduction. Beneath the crust, the limestone breaks down into magma under heat and pressure. The carbon dioxide in the limestone can then return to the atmosphere during volcanic eruptions. Scientists theorize that volcanoes continued to emit carbon dioxide into the atmosphere during the Precambrian ice ages. Eventually, the carbon dioxide warmed Earth through the greenhouse effect, causing the ice to melt rapidly. History of Earth The history of Earth is recorded in the rocks of Earth's crust. Rocks have been forming, wearing away, and re-forming ever since Earth took shape. The products of weathering and erosion are called sediment. Sediment accumulates in layers known as strata. Strata contain clues that tell geologists about Earth's past. These clues include the composition of the sediment, the way the strata are deposited, and the kinds of fossils that may occur in the rock. Space exploration has expanded our understanding of Earth's origin. The Hubble Space Telescope has observed what appear to be stars in the process of forming planets. Since the mid-1990's, scientists have found other stars that have planets surrounding them. These discoveries have helped scientists develop theories about the formation of Earth. Age of Earth Scientists think that Earth probably formed at about the same time as the rest of the solar system. They have determined that some chondrite meteorites, the unaltered remains from the formation of the solar system, are up to 4.6 billion years old. Scientists believe that Earth and other planets are probably that old. They can determine the ages of rocks by measuring the amounts of natural radioactive materials, such as uranium, in them. Radioactive elements decay (change into other elements) at a known rate. For example, uranium gives off radiation and decays into lead. Scientists know the time it takes for uranium to change to lead. They can determine the age of a rock by comparing the amount of uranium to the amount of lead. The known history of Earth is divided into four long stretches of time called eons. Starting with the earliest, the eons are Hadean, Archean, Proterozoic, and Phanerozoic. The first three eons, which together lasted nearly 4 billion years, are grouped into a unit called the Precambrian. The Phanerozoic Eon, when life became abundant, is divided into three eras. They are, from the oldest to the youngest, the Paleozoic, Mesozoic, and Cenozoic eras. Eras are divided into periods, and periods are divided into epochs. These divisions and subdivisions are named for places where rocks of each period were studied. Periods are mostly separated by important changes in the types of fossils found in the rocks. As a result, the lengths of eras, periods, and epochs are not equal. A chart showing an outline of Earth's history is called a geological time scale. On such a chart, Earth's earliest history is at the bottom, and its recent history at the top. This arrangement resembles the way rock strata are formed, with the recent over the oldest. Formation of Earth Most scientists believe that the solar system began as a thin cloud of gas and dust in space. The sun itself may have formed from a portion of the cloud that was thicker than the rest. The cloud's own gravity caused it to start contracting, and dust and gas were drawn in toward the center. Much of the cloud collapsed to the center to form a star, the sun, but a great ring of material remained orbiting around the star. Particles in the ring collided to make larger objects, which in turn collided to build up the planets of the solar system in a process called . Scientists believe that many small planets formed and then collided to make larger planets. Earth's early development Scientists theorize that Earth began as a waterless mass of rock surrounded by a cloud of gas. Radioactive materials in the rock and increasing pressure in Earth's interior produced enough heat to melt the interior of Earth. The heavy materials, such as iron, sank. The light silicate rocks rose to Earth's surface and formed the earliest crust. The heat of the interior caused other chemicals inside Earth to rise to the surface. Some of these chemicals formed water, and others became the gases of the atmosphere. In 2001, an international team of scientists announced the discovery of crystals of the mineral zircon that they determined to be 4.4 billion years old. Zircon, made up of the elements zirconium, silicon, and oxygen, is a hard, long lasting mineral that resists erosion and weathering. Through chemical analysis of the zircon, the scientists determined that liquid water probably existed on Earth's surface when the crystal were formed. They concluded that Earth's crust and oceans may have formed within about 200 million years after the planet had taken shape. Astronomers believe that the sun was about 30 percent fainter when Earth first formed than it is today. The oldest rocks on Earth, however, provide evidence that Earth was warm enough for liquid water to exist on the surface. Scientists believe that the atmosphere must have been thicker than it is today, to trap more heat from the sun. Over millions of years, the water slowly collected in low places of the crust and formed oceans. After the main period of planet formation, most of the remaining debris in the solar system was swept up by the newly formed planets. The collisions of the newly formed planets and debris material were explosive. The impacts created the cratered surfaces of the moon, Mars, Venus, and Mercury. Earth was also struck, but the craters produced by the impacts have all been destroyed by erosion and plate tectonics. Geologists believe that large masses of continental crust had formed by 3.5 billion years ago. There is evidence that plate tectonics has been active for at least 2 billion years. Some scientists believe Earth's early atmosphere contained hydrogen, helium, methane, and ammonia, much like the present atmosphere of Jupiter. Others believe it may have contained a large amount of carbon dioxide, as does the atmosphere of Venus. Scientists agree that Earth's earliest atmosphere probably had little oxygen. Geologists have determined that, about 2 billion years ago, a change in Earth's atmosphere occurred. They know this because certain kinds of iron ores created in oxygen-poor environments stopped forming at that time. Instead, large deposits of red sandstone formed. The red color results from iron reacting with oxygen to form iron oxide, or rust. The sandstone deposits are evidence that Earth's atmosphere contained some oxygen. The air was not breathable at that time, but the atmosphere may have had about 1 percent oxygen. The oxygen in the atmosphere today comes mainly from plants and microorganisms such as algae. These organisms use carbon dioxide and give off oxygen through the process of photosynthesis. The amount of oxygen increased in the atmosphere of the early Earth as oxygen-producing organisms developed and became more plentiful. Life on Earth Many rocks contain fossils that reveal the history of life on Earth. A fossil may be an animal's body, a tooth, or a piece of bone. It may simply be an impression of a plant or an animal made in a rock when the rock was soft sediment. Fossils help scientists learn which kinds of plants and animals lived at different times in Earth's history. Scientists who study prehistoric life are called paleontologists. Many scientists believe that life appeared on Earth almost as soon as conditions allowed. There is evidence for chemicals created by living things in rocks from the Archean age, 3.8 billion years old. Fossil remains of microscopic living things about 3.5 billion years old have also been found at sites in Australia and Canada. For most of Earth's history, life consisted mainly of microscopic, single-celled creatures. The earliest fossils of larger creatures with many cells are found in Precambrian rocks that are about 600 million years old. Many of these creatures differed from any living things today. The Paleozoic Era

The Paleozoic Era saw the development of many kinds of animals and plants in the seas and on land. The earliest land plants appeared in the Silurian Period, about 440 million years ago.

Fossils become abundant in Cambrian rocks that are about 544 million to 505 million years old. This apparently sudden expansion in the number of life forms in the fossil record is called the Cambrian Explosion, and it marks the beginning of the Paleozoic Era. The Cambrian Explosion actually occurred over tens of millions of years, but it appears sudden in the fossil record. The earliest abundant fossils consist of only a few kinds of organisms. Over the course of hundreds of millions of years, the number of species increases gradually in the fossil record. Most fossil organisms found in Paleozoic rocks are invertebrates (animals without a backbone), such as corals, mollusks (clams and snails), and trilobites (flat-shelled sea animals). Fish, the earliest vertebrates (animals with a backbone), are first found in Ordovician rocks about 450 million years old. Silurian rocks, about 440 million years old, contain fossils of the first large land plants. Amphibians, animals capable of living on land or in the water, first appear as fossils in Devonian rocks about 380 million years old. Fossil remains preserved in rocks show that by 300 million years ago, large forests and swamps covered the land. The carbon-rich remains of some of these forests are preserved as coal deposits in the United States, Canada, the United Kingdom, and other parts of the world. The Carboniferous Period is named for these enormous deposits of coal. The earliest fossil remains of reptiles are found in rocks of the Carboniferous Period. Unlike amphibians, reptiles have scaly skins that keep them from drying out, and they lay eggs protected by a shell. These features enable reptiles to live their whole lives out of water. Toward the end of the Paleozoic Era, in rocks from the Permian Period, some fossil reptiles begin to show some characteristics of mammals. Several times in Earth's history, there have been great extinctions, periods when many of Earth's living things die out. The greatest of these events, called the Permian extinction, happened about 250 million years ago. Almost 90 percent of the species on Earth during the Permian became extinct in a relatively short time. The cause of this event is a mystery, though many scientists suspect that huge volcanic eruptions in what is now Siberia may have disturbed the climate, causing many organisms to die out. The Mesozoic Era The Mesozoic Era was the Age of Dinosaurs. Plant-eating dinosaurs, such as this Stegosaurus, fed on cycads and conifers, early trees that thrived before modern flowering trees appeared.

Following the Permian extinction, the fossil record shows that reptiles became the dominant animals on land. The most spectacular of these reptiles were the dinosaurs. The Mesozoic is often called the Age of the Dinosaurs, but mammals and birds also appear in the fossil record in rocks from 200 million to 140 million years old. Fossil plants of the Mesozoic Era represent two main groups, gymnosperms and angiosperms. Gymnosperms have naked seeds, and most are cone-bearing. They include conifers, ginkgoes, and cycads. These gymnosperms evolved in the later part of the Paleozoic Era and were dominant into the early Cretaceous Period. Angiosperms have covered seeds and are flowering plants. They became the dominant plant group during the Cretaceous Period and continue to be so today. The dinosaurs died out in another great extinction about 65 million years ago. Most scientists believe that the extinction was caused by the impact of a small asteroid with Earth. The impact would have thrown so much dust into the atmosphere that the surface would have been dark and cold for months, killing off plants and the animals that fed on them. Many scientists believe a large, buried crater in the Yucatan region of Mexico, called Chicxulub (CHEEK shoo loob), is the place the asteroid struck. Debris from the collision has been found all over the world, and deposits created by large sea waves caused by the impact have been found in several places around the Gulf of Mexico. The Cenozoic Era

The Cenozoic Era included the Pleistocene Ice Age, when glaciers swept slowly across large areas before melting. The moving ice created a variety of landscapes in northern lands.

The wide variety of plants and animals that we know today came into existence during the Cenozoic Era. Mammals survived the events that killed off the dinosaurs and expanded to become the dominant land animals of today. The evolutionary history of today's mammals is recorded in the fossil record of the Cenozoic Era. During the Eocene Epoch, ancestors of the horse, rhinoceros, and camel roamed Europe and North America. By the Oligocene Epoch, dogs and cats had appeared, along with three-toed horses about as large as sheep. The mammals grew larger and developed in greater variety as prairies spread over the land during the Miocene Epoch. By the Pliocene Epoch, many kinds of mammals had grown to gigantic size. Elephantlike mammoths and mastodons and giant ground sloths roamed the prairies and forests. These animals died out at the end of the Pleistocene Epoch. Fossils of the first humanlike creatures appeared near the beginning of the Pleistocene Epoch, about 2 million years ago. The first true human beings appeared later, perhaps less than 200,000 years ago. Humanity's years on Earth are only a brief moment among the billions of years during which Earth has developed. Contributor: Steven I. Dutch, Ph.D., Professor of Earth Science, Department of Natural and Applied Sciences, University of Wisconsin, Green Bay. How to cite this article: To cite this article, World Book recommends the following format: Dutch, Steven I. "Earth." World Book Online Reference Center. 2004. World Book, Inc. http://www.worldbookonline.com/wb/Article?id=ar171540.

Earthquake is a shaking of the ground caused by the sudden breaking and shifting of large sections of Earth's rocky outer shell. Earthquakes are among the most powerful events on earth, and their results can be terrifying. A severe earthquake may release energy 10,000 times as great as that of the first atomic bomb. Rock movements during an earthquake can make rivers change their course. Earthquakes can trigger landslides that cause great damage and loss of life. Large earthquakes beneath the ocean can create a series of huge, destructive waves called tsunamis (tsoo NAH meez)that flood coasts for many miles. Earthquakes almost never kill people directly. Instead, many deaths and injuries result from falling objects and the collapse of buildings, bridges, and other structures. Fire resulting from broken gas or power lines is another major danger during a quake. Spills of hazardous chemicals are also a concern during an earthquake. The force of an earthquake depends on how much rock breaks and how far it shifts. Powerful earthquakes can shake firm ground violently for great distances. During minor earthquakes, the vibration may be no greater than the vibration caused by a passing truck. On average, a powerful earthquake occurs less than once every two years. At least 40 moderate earthquakes cause damage somewhere in the world each year. Scientists estimate that more than 8,000 minor earthquakes occur each day without causing any damage. Of those, only about 1,100 are strong enough to be felt. This article discusses Earthquake (How an earthquake begins) (How an earthquake spreads) (Damage by earthquakes) (Where and why earthquakes occur) (Studying earthquakes). How an earthquake begins Most earthquakes occur along a fault -- a fracture in Earth's rocky outer shell where sections of rock repeatedly slide past each other. Faults occur in weak areas of Earth's rock. Most faults lie beneath the surface of Earth, but some, like the San Andreas Fault in California, are visible on the surface. Stresses in Earth cause large blocks of rock along a fault to strain, or bend. When the stress on the rock becomes great enough, the rock breaks and snaps into a new position, causing the shaking of an earthquake. Earthquakes usually begin deep in the ground. The point in Earth where the rocks first break is called the focus, also known as the hypocenter, of the quake. The focus of most earthquakes lies less than 45 miles (72 kilometers) beneath the surface, though the deepest known focuses have been nearly 450 miles (700 kilometers) below the surface. The point on the surface of Earth directly above the focus is known as the epicenter of the quake. The strongest shaking is usually felt near the epicenter. From the focus, the break travels like a spreading crack along the fault. The speed at which the fracture spreads depends on the type of rock. It may average about 2 miles (3.2 kilometers) per second in granite or other strong rock. At that rate, a fracture may spread more than 350 miles (560 kilometers) in one direction in less than three minutes. As the fracture extends along the fault, blocks of rock on one side of the fault may drop down below the rock on the other side, move up and over the other side, or slide forward past the other. How an earthquake spreads When an earthquake occurs, the violent breaking of rock releases energy that travels through Earth in the form of vibrations called seismic waves. Seismic waves move out from the focus of an earthquake in all directions. As the waves travel away from the focus, they grow gradually weaker. For this reason, the ground generally shakes less farther away from the focus. There are two chief kinds of seismic waves: (1) body waves and (2) surface waves. Body waves, the fastest seismic waves, move through Earth. Slower surface waves travel along the surface of Earth. Body waves tend to cause the most earthquake damage. There are two kinds of body waves: (1) compressional waves and (2) shear waves. As the waves pass through Earth, they cause particles of rock to move in different ways. Compressional waves push and pull the rock. They cause buildings and other structures to contract and expand. Shear waves make rocks move from side to side, and buildings shake. Compressional waves can travel through solids, liquids, or gases, but shear waves can pass only through solids. Compressional waves are the fastest seismic waves, and they arrive first at a distant point. For this reason, compressional waves are also called primary (P) waves. Shear waves, which travel slower and arrive later, are called secondary (S) waves. Body waves travel faster deep within Earth than near the surface. For example, at depths of less than 16 miles (25 kilometers), compressional waves travel at about 4.2 miles (6.8 kilometers) per second, and shear waves travel at 2.4 miles (3.8 kilometers) per second. At a depth of 620 miles (1,000 kilometers), the waves travel more than 11/2 times that speed. Surface waves are long, slow waves. They produce what people feel as slow rocking sensations and cause little or no damage to buildings. There are two kinds of surface waves: (1) Love waves and (2) Rayleigh waves. Love waves travel through Earth's surface horizontally and move the ground from side to side. Rayleigh waves make the surface of Earth roll like waves on the ocean. Typical Love waves travel at about 23/4 miles (4.4 kilometers) per second, and Rayleigh waves, the slowest of the seismic waves, move at about 21/4 miles (3.7 kilometers) per second. The two types of waves were named for two British physicists, Augustus E. H. Love and Lord Rayleigh, who mathematically predicted the existence of the waves in 1911 and 1885, respectively. Damage by earthquakes How earthquakes cause damage Earthquakes can damage buildings, bridges, dams, and other structures, as well as many natural features. Near a fault, both the shifting of large blocks of Earth's crust, called fault slippage, and the shaking of the ground due to seismic waves cause destruction. Away from the fault, shaking produces most of the damage. Undersea earthquakes may cause huge tsunamis that swamp coastal areas. Other hazards during earthquakes include rockfalls, ground settling, and falling trees or tree branches. Fault slippage The rock on either side of a fault may shift only slightly during an earthquake or may move several feet or meters. In some cases, only the rock deep in the ground shifts, and no movement occurs at Earth's surface. In an extremely large earthquake, the ground may suddenly heave 20 feet (6 meters) or more. Any structure that spans a fault may be wrenched apart. The shifting blocks of earth may also loosen the soil and rocks along a slope and trigger a landslide. In addition, fault slippage may break down the banks of rivers, lakes, and other bodies of water, causing flooding. Ground shaking causes structures to sway from side to side, bounce up and down, and move in other violent ways. Buildings may slide off their foundations, collapse, or be shaken apart. In areas with soft, wet soils, a process called liquefaction may intensify earthquake damage. Liquefaction occurs when strong ground shaking causes wet soils to behave temporarily like liquids rather than solids. Anything on top of liquefied soil may sink into the soft ground. The liquefied soil may also flow toward lower ground, burying anything in its path. Tsunamis An earthquake on the ocean floor can give a tremendous push to surrounding seawater and create one or more large, destructive waves called tsunamis, also known as seismic sea waves. Some people call tsunamis tidal waves, but scientists think the term is misleading because the waves are not caused by the tide. Tsunamis may build to heights of more than 100 feet (30 meters) when they reach shallow water near shore. In the open ocean, tsunamis typically move at speeds of 500 to 600 miles (800 to 970 kilometers) per hour. They can travel great distances while diminishing little in size and can flood coastal areas thousands of miles or kilometers from their source. Structural hazards Structures collapse during a quake when they are too weak or rigid to resist strong, rocking forces. In addition, tall buildings may vibrate wildly during an earthquake and knock into each other. Picture San Francisco earthquake of 1906 A major cause of death and property damage in earthquakes is fire. Fires may start if a quake ruptures gas or power lines. The 1906 San Francisco earthquake ranks as one of the worst disasters in United States history because of a fire that raged for three days after the quake. Other hazards during an earthquake include spills of toxic chemicals and falling objects, such as tree limbs, bricks, and glass. Sewage lines may break, and sewage may seep into water supplies. Drinking of such impure water may cause cholera, typhoid, dysentery, and other serious diseases. Loss of power, communication, and transportation after an earthquake may hamper rescue teams and ambulances, increasing deaths and injuries. In addition, businesses and government offices may lose records and supplies, slowing recovery from the disaster. Reducing earthquake damage In areas where earthquakes are likely, knowing where to build and how to build can help reduce injury, loss of life, and property damage during a quake. Knowing what to do when a quake strikes can also help prevent injuries and deaths. Where to build Earth scientists try to identify areas that would likely suffer great damage during an earthquake. They develop maps that show fault zones, flood plains (areas that get flooded), areas subject to landslides or to soil liquefaction, and the sites of past earthquakes. From these maps, land-use planners develop zoning restrictions that can help prevent construction of unsafe structures in earthquake-prone areas. How to build

An earthquake-resistant building includes such structures as shear walls, a shear core, and cross- bracing. Base isolators act as shock absorbers. A moat allows the building to sway. Image credit: World Book illustration by Doug DeWitt

Engineers have developed a number of ways to build earthquake-resistant structures. Their techniques range from extremely simple to fairly complex. For small- to medium-sized buildings, the simpler reinforcement techniques include bolting buildings to their foundations and providing support walls called shear walls. Shear walls, made of reinforced concrete (concrete with steel rods or bars embedded in it), help strengthen the structure and help resist rocking forces. Shear walls in the center of a building, often around an elevator shaft or stairwell, form what is called a shear core. Walls may also be reinforced with diagonal steel beams in a technique called cross-bracing. Builders also protect medium-sized buildings with devices that act like shock absorbers between the building and its foundation. These devices, called base isolators, are usually bearings made of alternate layers of steel and an elastic material, such as synthetic rubber. Base isolators absorb some of the sideways motion that would otherwise damage a building. Skyscrapers need special construction to make them earthquake-resistant. They must be anchored deeply and securely into the ground. They need a reinforced framework with stronger joints than an ordinary skyscraper has. Such a framework makes the skyscraper strong enough and yet flexible enough to withstand an earthquake. Earthquake-resistant homes, schools, and workplaces have heavy appliances, furniture, and other structures fastened down to prevent them from toppling when the building shakes. Gas and water lines must be specially reinforced with flexible joints to prevent breaking. Safety precautions are vital during an earthquake. People can protect themselves by standing under a doorframe or crouching under a table or chair until the shaking stops. They should not go outdoors until the shaking has stopped completely. Even then, people should use extreme caution. A large earthquake may be followed by many smaller quakes, called aftershocks. People should stay clear of walls, windows, and damaged structures, which could crash in an aftershock. People who are outdoors when an earthquake hits should quickly move away from tall trees, steep slopes, buildings, and power lines. If they are near a large body of water, they should move to higher ground. Where and why earthquakes occur Scientists have developed a theory, called plate tectonics, that explains why most earthquakes occur. According to this theory, Earth's outer shell consists of about 10 large, rigid plates and about 20 smaller ones. Each plate consists of a section of Earth's crust and a portion of the mantle, the thick layer of hot rock below the crust. Scientists call this layer of crust and upper mantle the lithosphere. The plates move slowly and continuously on the asthenosphere, a layer of hot, soft rock in the mantle. As the plates move, they collide, move apart, or slide past one another. The movement of the plates strains the rock at and near plate boundaries and produces zones of faults around these boundaries. Along segments of some faults, the rock becomes locked in place and cannot slide as the plates move. Stress builds up in the rock on both sides of the fault and causes the rock to break and shift in an earthquake. There are three types of faults: (1) normal faults, (2) reverse faults, and (3) strike-slip faults. In normal and reverse faults, the fracture in the rock slopes downward, and the rock moves up or down along the fracture. In a normal fault, the block of rock on the upper side of the sloping fracture slides down. In a reverse fault, the rock on both sides of the fault is greatly compressed. The compression forces the upper block to slide upward and the lower block to thrust downward. In a strike-slip fault, the fracture extends straight down into the rock, and the blocks of rock along the fault slide past each other horizontally. Most earthquakes occur in the fault zones at plate boundaries. Such earthquakes are known as interplate earthquakes. Some earthquakes take place within the interior of a plate and are called intraplate earthquakes. Interplate earthquakes occur along the three types of plate boundaries: (1) mid-ocean spreading ridges, (2) subduction zones, and (3) transform faults. Mid-ocean spreading ridges are places in the deep ocean basins where the plates move apart. As the plates separate, hot lava from Earth's mantle rises between them. The lava gradually cools, contracts, and cracks, creating faults. Most of these faults are normal faults. Along the faults, blocks of rock break and slide down away from the ridge, producing earthquakes. Near the spreading ridges, the plates are thin and weak. The rock has not cooled completely, so it is still somewhat flexible. For these reasons, large strains cannot build, and most earthquakes near spreading ridges are shallow and mild or moderate in severity. Subduction zones are places where two plates collide, and the edge of one plate pushes beneath the edge of the other in a process called subduction. Because of the compression in these zones, many of the faults there are reverse faults. About 80 per cent of major earthquakes occur in subduction zones encircling the Pacific Ocean. In these areas, the plates under the Pacific Ocean are plunging beneath the plates carrying the continents. The grinding of the colder, brittle ocean plates beneath the continental plates creates huge strains that are released in the world's largest earthquakes. The world's deepest earthquakes occur in subduction zones down to a depth of about 450 miles (700 kilometers). Below that depth, the rock is too warm and soft to break suddenly and cause earthquakes. Transform faults are places where plates slide past each other horizontally. Strike-slip faults occur there. Earthquakes along transform faults may be large, but not as large or deep as those in subduction zones. One of the most famous transform faults is the San Andreas Fault. The slippage there is caused by the Pacific Plate moving past the North American Plate. The San Andreas Fault and its associated faults account for most of California's earthquakes. Intraplate earthquakes are not as frequent or as large as those along plate boundaries. The largest intraplate earthquakes are about 100 times smaller than the largest interplate earthquakes. Intraplate earthquakes tend to occur in soft, weak areas of plate interiors. Scientists believe intraplate quakes may be caused by strains put on plate interiors by changes of temperature or pressure in the rock. Or the source of the strain may be a long distance away, at a plate boundary. These strains may produce quakes along normal, reverse, or strike-slip faults. Studying earthquakes Recording, measuring, and locating earthquakes To determine the strength and location of earthquakes, scientists use a recording instrument known as a seismograph. A seismograph is equipped with sensors called seismometers that can detect ground motions caused by seismic waves from both near and distant earthquakes. Some seismometers are capable of detecting ground motion as small as 0.1 nanometer. One nanometer is 1 billionth of a meter or about 39 billionths of an inch. Scientists called seismologists measure seismic ground movements in three directions: (1) up-down, (2) north-south, and (3) east-west. The scientists use a separate sensor to record each direction of movement. A seismograph produces wavy lines that reflect the size of seismic waves passing beneath it. The record of the wave, called a seismogram, is imprinted on paper, film, or recording tape or is stored and displayed by computers. Probably the best-known gauge of earthquake intensity is the local Richter magnitude scale, developed in 1935 by United States seismologist Charles F. Richter. This scale, commonly known as the Richter scale, measures the ground motion caused by an earthquake. Every increase of one number in magnitude means the energy release of the quake is about 32 times greater. For example, an earthquake of magnitude 7.0 releases about 32 times as much energy as an earthquake measuring 6.0. An earthquake with a magnitude of less than 2.0 is so slight that usually only a seismometer can detect it. A quake greater than 7.0 may destroy many buildings. The number of earthquakes increases sharply with every decrease in Richter magnitude by one unit. For example, there are 8 times as many quakes with magnitude 4.0 as there are with magnitude 5.0. Although large earthquakes are customarily reported on the Richter scale, scientists prefer to describe earthquakes greater than 7.0 on the moment magnitude scale. The moment magnitude scale measures more of the ground movements produced by an earthquake. Thus, it describes large earthquakes more accurately than does the Richter scale. The largest earthquake ever recorded on the moment magnitude scale measured 9.5. It was an interplate earthquake that occurred along the Pacific coast of Chile in South America in 1960. The largest intraplate earthquakes known struck in central Asia and in the Indian Ocean in 1905, 1920, and 1957. These earthquakes had moment magnitudes between about 8.0 and 8.3. The largest intraplate earthquakes in the United States were three quakes that occurred in New Madrid, Missouri, in 1811 and 1812. The earthquakes were so powerful that they changed the course of the Mississippi River. During the largest of them, the ground shook from southern Canada to the Gulf of Mexico and from the Atlantic Coast to the Rocky Mountains. Scientists estimate the earthquakes had moment magnitudes of 7.5. Scientists locate earthquakes by measuring the time it takes body waves to arrive at seismographs in a minimum of three locations. From these wave arrival times, seismologists can calculate the distance of an earthquake from each seismograph. Once they know an earthquake's distance from three locations, they can find the quake's focus at the center of those three locations. Predicting earthquakes Scientists can make fairly accurate long-term predictions of where earthquakes will occur. They know, for example, that about 80 percent of the world's major earthquakes happen along a belt encircling the Pacific Ocean. This belt is sometimes called the Ring of Fire because it has many volcanoes, earthquakes, and other geologic activity. Scientists are working to make accurate forecasts on when earthquakes will strike. Geologists closely monitor certain fault zones where quakes are expected. Along these fault zones, they can sometimes detect small quakes, the tilting of rock, and other events that might signal a large earthquake is about to occur. Exploring Earth's interior Most of what is known about the internal structure of Earth has come from studies of seismic waves. Such studies have shown that rock density increases from the surface of Earth to its center. Knowledge of rock densities within Earth has helped scientists determine the probable composition of Earth's interior. Scientists have found that seismic wave speeds and directions change abruptly at certain depths. From such studies, geologists have concluded that Earth is composed of layers of various densities and substances. These layers consist of the crust, mantle, outer core, and inner core. Shear waves do not travel through the outer core. Because shear waves cannot travel through liquids, scientists believe the outer core is liquid. Scientists believe the inner core is solid because of the movement of compressional waves when they reach the inner core. Edwin Hubble Edwin Powell Hubble, (1889-1953), was an American astronomer whose work revolutionized our understanding of the size and structure of the universe. In the early 1900's, many astronomers believed that all stars and other celestial objects were part of the Milky Way, the galaxy that contains our solar system. In the 1920's, Hubble studied a hazy patch of the sky called the Andromeda . Hubble noticed that it contained stars resembling stars in the Milky Way but much fainter. He concluded that the stars in the nebula must be much farther from Earth than stars in our own galaxy. His work proved that the Andromeda Nebula was actually a distant galaxy separate from our own. Hubble later studied the speed at which galaxies move away from each other. In 1929, he discovered that the farther apart galaxies are from each other, the faster they move away from each other. Based on this observation, Hubble concluded that the universe expands uniformly. Hubble was born on Nov. 20, 1889, in Marshfield, Missouri. He earned a Ph.D. degree from the University of Chicago in 1917. In 1919, he joined the staff of the Mount Wilson Observatory in California, where he remained until his death on Sept. 28, 1953. The Hubble Space Telescope, launched by the National Aeronautics and Space Administration in 1990, was named in his honor.

Europa

The surface of Europa, a moon of Jupiter, consists mostly of huge blocks of ice that have cracked and shifted about, suggesting that there may be an ocean of liquid water underneath.

Europa, (yu ROH puh), is a large moon of Jupiter. Its surface is made of ice, which may have an ocean of water beneath it. Such an ocean could provide a home for living things. The surface layer of ice or ice and water is 50 to 100 miles (80 to 160 kilometers) deep. The satellite has an extremely thin atmosphere. Electrically charged particles from Jupiter's radiation belts continuously bombard Europa. Europa is one of the smoothest bodies in the solar system. Its surface features include shallow cracks, valleys, ridges, pits, blisters, and icy flows. None of them extend more than a few hundred yards or meters upward or downward. In some places, huge sections of the surface have split apart and separated. The surface of Europa has few impact craters (pits caused by collisions with asteroids or comets). The splitting and shifting of the surface and disruptions from below have destroyed most of the old craters. Europa's interior is hotter than its surface. This internal heat comes from the gravitational forces of Jupiter and Jupiter's other large satellites, which pull Europa's interior in different directions. As a result, the interior flexes, producing heat in a process known as tidal heating. The core of Europa may be rich in iron, but most of the satellite is made of rock. Europa's diameter is 1,940 miles (3,122 kilometers), slightly smaller than Earth's moon. Europa takes 3.55 days to orbit Jupiter at a distance of 416,900 miles (670,900 kilometers). The Italian astronomer Galileo discovered Europa in 1610. Much of what is known about it comes from data gathered by a space probe, also named Galileo, that orbited Jupiter from 1995 to 2003. Contributor: William B. McKinnon, Ph.D., Professor of Earth and Planetary SciencesExtraterrestrial

Intelligence Extraterrestrial (EHKS truh tuh REHS tree uhl) intelligence is intelligent life that developed somewhere other than the Earth. No life has been discovered on any planet other than the Earth. However, many scientists have concluded that intelligent life may exist on planets orbiting some of the hundreds of billions of stars in our galaxy, the Milky Way. These scientists base their conclusion on research in such fields as astronomy, biology, planetary science, and paleontology (the study of prehistoric life through fossils). The effort to find evidence that there is extraterrestrial intelligence is often called SETI, which stands for Search for Extraterrestrial Intelligence. SETI researchers believe that the best way to discover other intelligent life in the galaxy is to look for evidence of technology developed by that life. In the belief that intelligent beings on other worlds would eventually develop radio technology, researchers have used large radio telescopes to search the sky. In 1960, the first SETI experiment unsuccessfully examined two stars at a single radio frequency. After several dozen additional searches, the National Aeronautics and Space Administration (NASA) in 1992 began a two-part project known as the High Resolution Microwave Survey. Researchers searched for weak microwave (short radio wave) signals originating near specific stars that are similar to the sun. They also started to scan the entire sky for strong microwave signals. In 1993, the United States Congress, in a budget-cutting measure, instructed NASA to end the project. SETI research continues in the United States under private support. In 1998, astronomers began to search for pulses of laser light. The astronomers reasoned that intelligent beings on a planet orbiting a distant star might have developed powerful lasers. The beings might have transmitted brief pulses of laser light into space as a signal to observers on other planets. They would have used pulses so that the observers could distinguish the laser light from the bright, steady light coming from their star. Astronomers on the earth would be able to distinguish powerful pulses that were a few billionths of a second in duration.

Galaxy A galaxy is a system of stars, dust, and gas held together by gravity. Our solar system is in a galaxy called the Milky Way. Scientists estimate that there are more than 100 billion galaxies scattered throughout the visible universe. Astronomers have photographed millions of them through telescopes. The most distant galaxies ever photographed are as far as 10 billion to 13 billion light-years away. A light-year is the distance that light travels in a vacuum in a year -- about 5.88 trillion miles (9.46 trillion kilometers). Galaxies range in diameter from a few thousand to a half-million light-years. Small galaxies have fewer than a billion stars. Large galaxies have more than a trillion. A spiral galaxy resembles a pinwheel, with spiral arms coiling out from a central bulge. This galaxy, known as M100, looks much like our home galaxy, the Milky Way. However, the Milky Way has a bar of stars, dust, and gas across its center. Image credit: D. Hunter (Lowell Observatory) and Z. Levay (Space Telescope Science

The Milky Way has a diameter of about 100,000 light-years. The solar system lies about 25,000 light- years from the center of the galaxy. There are about 100 billion stars in the Milky Way. Only three galaxies outside the Milky Way are visible with the unaided eye. People in the Northern Hemisphere can see the Andromeda Galaxy, which is about 2 million light-years away. People in the Southern Hemisphere can see the Large Magellanic Cloud, which is about 160,000 light-years from Earth, and the Small Magellanic Cloud, which is about 180,000 light-years away. Groups of galaxies Galaxies are distributed unevenly in space. Some have no close neighbor. Others occur in pairs, with each orbiting the other. But most of them are found in groups called clusters. A cluster may contain from a few dozen to several thousand galaxies. It may have a diameter as large as 10 million light- years. Clusters of galaxies, in turn, are grouped in larger structures called superclusters. On even larger scales, galaxies are arranged in huge networks. The networks consist of interconnected strings or filaments of galaxies surrounding relatively empty regions known as voids. One of the largest structures ever mapped is a network of galaxies known as the Great Wall. This structure is more than 500 million light- years long and 200 million light-years wide. Shapes of galaxies

A is a tightly grouped swarm of stars held together by gravity. This globular cluster is one of the densest of the 147 known clusters in the Milky Way galaxy. Image credit: NASA Astronomers classify most galaxies by shape as either spiral galaxies or elliptical galaxies. A spiral galaxy is shaped like a disk with a bulge in the center. The disk resembles a pinwheel, with bright spiral arms that coil out from the central bulge. The Milky Way is a spiral galaxy. Like pinwheels, all spiral galaxies rotate -- but slowly. The Milky Way, for example, makes a complete revolution once every 250 million years or so. New stars are constantly forming out of gas and dust in spiral galaxies. Smaller groups of stars called globular clusters often surround spiral galaxies. A typical globular cluster has about 1 million stars. Elliptical galaxies range in shape from almost perfect spheres to flattened globes. The light from an elliptical galaxy is brightest in the center and gradually becomes fainter toward its outer regions. As far as astronomers can determine, elliptical galaxies rotate much more slowly than spiral galaxies or not at all. The stars within them appear to move in random orbits. Elliptical galaxies have much less dust and gas than spiral galaxies have, and few new stars appear to be forming in them.

An irregular galaxy, Sextans A does not have a simple shape like a spiral or elliptical galaxy. The bright, yellowish stars in the foreground are part of the Milky Way, Earth's "home" galaxy. Image credit: NASA

Galaxies of a third kind, irregular galaxies, lack a simple shape. Some consist mostly of blue stars and puffy clouds of gas, but little dust. The Magellanic Clouds are irregular galaxies of this type. Others are made up mostly of bright young stars along with gas and dust. Galaxies move relative to one another, and occasionally two galaxies come so close to each other that the gravitational force of each changes the shape of the other. Galaxies can even collide. If two rapidly moving galaxies collide, they may pass right through each other with little or no effect. However, when slow-moving galaxies collide, they can merge into a single galaxy that is bigger than either of the original galaxies. Such mergers can produce spiral filaments of stars that can extend more than 100,000 light-years into space. Emissions from galaxies

An image taken in 2001 with the Hubble Space Telescope reveals the irregular-shaped galaxy ESO 510-13, which astronomers theorize is twisted because of gravitational effects that occurred when it absorbed a smaller galaxy. Image credit: NASA and Hubble Heritage Team

All galaxies emit (give off) energy as waves of visible light and other kinds of electromagnetic radiation. In order of decreasing wavelength (distance between successive wave crests), electromagnetic radiation consists of radio waves, infrared rays, visible light, ultraviolet rays, X rays, and gamma rays. All these forms of radiation together make up the electromagnetic spectrum. The energy emitted by galaxies comes from various sources. Much of it is due to the heat of the stars and of clouds of dust and gas called nebulae. A variety of violent events also provide a great deal of the energy. These events include two kinds of stellar explosions: (1) nova explosions, in which one of the two members of a binary hurls dust and gas into space; (2) much more violent supernova explosions, in which a star collapses, then throws off most of its matter. One supernova may leave behind a compact, invisible object called a black hole, which has such powerful gravitational force that not even light can escape it. Another supernova may leave behind a neutron star, which consists mostly of tightly packed neutrons, particles that ordinarily occur only in the nuclei of atoms. But some supernovae leave nothing behind.

The most distant galaxies yet observed appear as faint patches of light in this photograph taken by the Hubble Space Telescope. The brighter swirls are galaxies somewhat closer to Earth, and the bright orange object is a star in our own galaxy. The telescope photographed this tiny portion of the sky, called the Hubble Ultra Deep Field, in 2004. Image credit: NASA/ESA/S. Beckwith (STScl) The intensity of the radiation emitted by a star at various wavelengths depends on the star's surface temperature. For example, the sun, which has a surface temperature of about 5500 ¡C (10,000 ¡F) emits most of its radiation in the visible part of the electromagnetic spectrum. Radiation of this type, whose intensity depends on temperature as it does for the sun and other normal stars, is called thermal radiation. A small percentage of galaxies called active galaxies emit tremendous amounts of energy. This energy results from violent events occurring in objects at their center. The distribution of the wavelengths of the emissions does not resemble that of normal stars, and so the emissions are known as nonthermal radiation. The most powerful such object is a quasar, which emits a huge amount of radio, infrared, ultraviolet, X-ray, and gamma-ray energy. Some quasars emit 1,000 times as much energy as the entire Milky Way, yet look like stars in photographs. Quasar is short for quasi-stellar radio source. The name comes from the fact that the first quasars identified emit mostly radio energy and look much like stars. A radio galaxy is related to, but appears larger than, a quasar. A Seyfert galaxy is a spiral galaxy that emits large amounts of infrared rays as well as large amounts of radio waves, X rays, or both radio waves and X rays. Seyfert galaxies get their name from American astronomer Carl K. Seyfert, who in 1943 became the first person to discover one. Some active galaxies emit jets and blobs of highly energetic, electrically charged particles. These particles include positively charged protons and positrons and negatively charged electrons. Electrons and protons are forms of ordinary matter, but positrons are antimatter particles. They are the antimatter opposites of electrons -- that is, they have the same mass (amount of matter) as electrons, but they carry the opposite charge. See Antimatter. The cause of the intense activity in active galaxies is thought to arise from a colossal black hole at the galactic center. The black hole can be as much as a billion times as massive as the sun. Because the black hole is so massive and compact, its gravitational force is powerful enough to tear apart nearby stars. The resulting dust and gas fall toward the black hole, adding their mass to a disk of matter called an accretion disk that orbits the black hole. At the same time, matter from the inner edge of the disk falls into the black hole. As the matter falls, it loses energy, thereby producing the radiation and jets that shoot out of the galaxy.

Galaxy M83 The galaxy M83 is shaped much like our home galaxy, the Milky Way. The diameter of the Milky Way is approximately 100,000 light-years -- roughly 700 billion times the sun's diameter. Image credit: noAO/AURA/NSF The Milky Way is not an active galaxy, but it does have a powerful source of radiation called Sagittarius A* at its center. The cause of this radiation may be a black hole a million times as massive as the sun. Origin of galaxies Scientists have proposed two main kinds of theories of the origin of galaxies: (1) bottom-up theories and (2) top-down theories. The starting point for both kinds of theories is the big bang, the explosion with which the universe began 10 billion to 20 billion years ago. Shortly after the big bang, masses of gas began to gather together or collapse. Gravity then slowly compressed these masses into galaxies. The two kinds of theories differ concerning how the galaxies evolved. Bottom-up theories state that much smaller objects such as globular clusters formed first. These objects then merged to form galaxies. According to top-down theories, large objects such as galaxies and clusters of galaxies formed first. The smaller groups of stars then formed within them. But all big bang theories of galaxy formation agree that no new galaxies -- or very few -- have formed since the earliest times. Astronomers have found evidence of what conditions were like before the galaxies formed. In 1965, American physicists Arno Penzias and Robert Wilson detected faint radio waves throughout the sky. According to the big bang theory, the waves are radiation left over from the initial explosion. The strength of the radio waves appeared to be very nearly the same in every direction. But in 1992, a satellite called the Cosmic Background Explorer (COBE) detected tiny differences in the strength of radio waves coming from different directions. The differences in strength arise from tiny increases in the density of matter in the universe shortly after the big bang. The small regions of increased density had a stronger gravitational force than the surrounding matter. Clumps of matter therefore formed in these regions; and the clumps eventually collapsed into galaxies. Most astronomical observations made to date support big bang theories. According to these theories, the universe is still expanding. Two kinds of observations strongly support the idea of an expanding universe. These observations indicate that all galaxies are moving away from one another and that the galaxies farthest from the Milky Way are moving away most rapidly. This relationship between speed and distance is known as the Hubble law of recession (moving backward), or Hubble's law. The law was named after American astronomer Edwin P. Hubble, who reported it in 1929.

Redshift causes a displacement of lines in the spectrum (band of colors) sent out by a galaxy that is moving away from Earth. If the galaxy were motionless relative to Earth, those lines would appear in the positions shown in the upper diagram. The cause of the displacement is known as redshift because the lines are displaced toward the red end of the spectrum. Image credit: World Book diagram by Ernest Norcia Astronomers estimate the speed at which a galaxy is moving away by measuring the galaxy's redshift. The redshift is an apparent lengthening of electromagnetic waves emitted by an object moving away from the observer. A redshift can be measured when light from a galaxy is broken up and spread out into a band of colors called a spectrum. The spectrum of a galaxy contains bright and dark lines that are determined by the galaxy's temperature, density, and chemical composition. These lines are shifted toward the red end of the spectrum if the galaxy is moving away. The greater the amount of redshift, the more rapid the movement. Scientists estimate the distance to galaxies by measuring the galaxies' overall brightness or the brightness of certain kinds of objects within them. These objects include variable stars as well as supernovae. Evolution of spiral galaxies Astronomers do not understand clearly how galactic spirals evolved and why they still exist. The mystery arises when one considers how a spiral galaxy rotates. The galaxy spins much like the cream on the surface of a cup of coffee. The inner part of the galaxy rotates somewhat like a solid wheel, and the arms trail behind. Suppose a spiral arm rotated around the center of its galaxy in about 250 million years -- as in the Milky Way. After a few rotations, taking perhaps 2 billion years, the arms would "wind up," producing a fairly continuous mass of stars. But almost all spiral galaxies are much older than 2 billion years. According to one proposed solution to the mystery, differences in gravitational force throughout the galaxy push and pull at the stars, dust, and gas. This activity produces waves of compression. A familiar example of waves of compression are ordinary sound waves. Because the galaxy is rotating, the waves seem to travel in a spiral path, leading to the appearance of spiral arms of dense dust and gas. Stars then form in the arms.

Giovanni Cassini Giovanni Domenico Cassini, (1625-1712), was an Italian-born French astronomer who discovered four moons of Saturn and a large gap in Saturn's ring system. The gap is now known as the Cassini division. Cassini is also known by the French name Jean Dominique Cassini. Because of his discoveries pertaining to Saturn, a space probe that the United States launched in 1997 to investigate that planet was named after him. Cassini was born on June 8, 1625, in Perinaldo, in what is now northern Italy. In 1650, he became a professor of astronomy at the University of Bologna. He went to Paris in 1669 and soon became the first director of the Paris Observatory. He became a French citizen in 1673. Cassini's tables of the sun, published in 1662, established his reputation as an astronomer. He had precisely measured the sun's apparent motion through the sky. Later, he closely approximated the distance from Earth to the sun. Cassini's observations of Jupiter were so precise that he could distinguish between shadows cast by moons of Jupiter and fixed shadows on Jupiter's surface. Cassini used the moon shadows to create tables of the motions of the moons. He used the fixed shadows to determine the length of Jupiter's day. Cassini died in Paris on Sept. 14, 1712

Global Warming Global warming is an increase in the average temperature of Earth's surface. Since the late 1800's, the global average temperature has increased about 0.7 to 1.4 degrees F (0.4 to 0.8 degrees C). Many experts estimate that the average temperature will rise an additional 2.5 to 10.4 degrees F (1.4 to 5.8 degrees C) by 2100. That rate of increase would be much larger than most past rates of increase. Scientists worry that human societies and natural ecosystems might not adapt to rapid climate changes. An ecosystem consists of the living organisms and physical environment in a particular area. Global warming could cause much harm, so countries throughout the world drafted an agreement called the Kyoto Protocol to help limit it. Causes of global warming Climatologists (scientists who study climate) have analyzed the global warming that has occurred since the late 1800's. A majority of climatologists have concluded that human activities are responsible for most of the warming. Human activities contribute to global warming by enhancing Earth's natural greenhouse effect. The greenhouse effect warms Earth's surface through a complex process involving sunlight, gases, and particles in the atmosphere. Gases that trap heat in the atmosphere are known as greenhouse gases. The main human activities that contribute to global warming are the burning of fossil fuels (coal, oil, and natural gas) and the clearing of land. Most of the burning occurs in automobiles, in factories, and in electric power plants that provide energy for houses and office buildings. The burning of fossil fuels creates carbon dioxide, whose chemical formula is CO2. CO2 is a greenhouse gas that slows the escape of heat into space. Trees and other plants remove CO2 from the air during photosynthesis, the process they use to produce food. The clearing of land contributes to the buildup of CO2 by reducing the rate at which the gas is removed from the atmosphere or by the decomposition of dead vegetation. A small number of scientists argue that the increase in greenhouse gases has not made a measurable difference in the temperature. They say that natural processes could have caused global warming. Those processes include increases in the energy emitted (given off) by the sun. But the vast majority of climatologists believe that increases in the sun's energy have contributed only slightly to recent warming.

The impact of global warming

Thousands of icebergs float off the coast of the Antarctic Peninsula after 1,250 square miles (3,240 square kilometers) of the Larsen B ice shelf disintegrated in 2002. The area of the ice was larger than the state of Rhode Island or the nation of Luxembourg. Antarctic ice shelves have been shrinking since the early 1970's because of climate warming in the region. Image credit: NASA/Earth Observatory Continued global warming could have many damaging effects. It might harm plants and animals that live in the sea. It could also force animals and plants on land to move to new habitats. Weather patterns could change, causing flooding, drought, and an increase in damaging storms. Global warming could melt enough polar ice to raise the sea level. In certain parts of the world, human disease could spread, and crop yields could decline. Harm to ocean life Through global warming, the surface waters of the oceans could become warmer, increasing the stress on ocean ecosystems, such as coral reefs. High water temperatures can cause a damaging process called coral bleaching. When corals bleach, they expel the algae that give them their color and nourishment. The corals turn white and, unless the water temperature cools, they die. Added warmth also helps spread diseases that affect sea creatures. Changes of habitat Widespread shifts might occur in the natural habitats of animals and plants. Many species would have difficulty surviving in the regions they now inhabit. For example, many flowering plants will not bloom without a sufficient period of winter cold. And human occupation has altered the landscape in ways that would make new habitats hard to reach or unavailable altogether. Weather damage Extreme weather conditions might become more frequent and therefore more damaging. Changes in rainfall patterns could increase both flooding and drought in some areas. More hurricanes and other tropical storms might occur, and they could become more powerful. Rising sea level Continued global warming might, over centuries, melt large amounts of ice from a vast sheet that covers most of West Antarctica. As a result, the sea level would rise throughout the world. Many coastal areas would experience flooding, erosion, a loss of wetlands, and an entry of seawater into freshwater areas. High sea levels would submerge some coastal cities, small island nations, and other inhabited regions. Threats to human health Tropical diseases, such as malaria and dengue, might spread to larger regions. Longer-lasting and more intense heat waves could cause more deaths and illnesses. Floods and droughts could increase hunger and malnutrition. Changes in crop yields Canada and parts of Russia might benefit from an increase in crop yields. But any increases in yields could be more than offset by decreases caused by drought and higher temperatures -- particularly if the amount of warming were more than a few degrees . Yields in the tropics might fall disastrously because temperatures there are already almost as high as many crop plants can tolerate. Limited global warming Climatologists are studying ways to limit global warming. Two key methods would be (1) limiting CO2 emissions and (2) carbon sequestration -- either preventing carbon dioxide from entering the atmosphere or removing CO2 already there. Limiting CO2 emissions Two effective techniques for limiting CO2 emissions would be (1) to replace fossil fuels with energy sources that do not emit CO2, and (2) to use fossil fuels more efficiently. Alternative energy sources that do not emit CO2 include the wind, sunlight, nuclear energy, and underground steam. Devices known as wind turbines can convert wind energy to electric energy. Solar cells can convert sunlight to electric energy, and various devices can convert solar energy to useful heat. Geothermal power plants convert energy in underground steam to electric energy. Alternative sources of energy are more expensive to use than fossil fuels. However, increased research into their use would almost certainly reduce their cost. Carbon sequestration could take two forms: (1) underground or underwater storage and (2) storage in living plants. Underground or underwater storage would involve injecting industrial emissions of CO2 into underground geologic formations or the ocean. Suitable underground formations include natural reservoirs of oil and gas from which most of the oil or gas has been removed. Pumping CO2 into a reservoir would have the added benefit of making it easier to remove the remaining oil or gas. The value of that product could offset the cost of sequestration. Deep deposits of salt or coal could also be suitable. The oceans could store much CO2. However, scientists have not yet determined the environmental impacts of using the ocean for carbon sequestration. Storage in living plants Green plants absorb CO2 from the atmosphere as they grow. They combine carbon from CO2 with hydrogen to make simple sugars, which they store in their tissues. After plants die, their bodies decay and release CO2. Ecosystems with abundant plant life, such as forests and even cropland, could tie up much carbon. However, future generations of people would have to keep the ecosystems intact. Otherwise, the sequestered carbon would re-enter the atmosphere as CO2. Agreement on global warming Delegates from more than 160 countries met in Kyoto, Japan, in 1997 to draft the agreement that became known as the Kyoto Protocol. That agreement calls for decreases in the emissions of greenhouse gases. Emissions targets Thirty-eight industrialized nations would have to restrict their emissions of CO2 and five other greenhouse gases. The restrictions would occur from 2008 through 2012. Different countries would have different emissions targets. As a whole, the 38 countries would restrict their emissions to a yearly average of about 95 percent of their 1990 emissions. The agreement does not place restrictions on developing countries. But it encourages the industrialized nations to cooperate in helping developing countries limit emissions voluntarily. Industrialized nations could also buy or sell emission reduction units. Suppose an industrialized nation cut its emissions more than was required by the agreement. That country could sell other industrialized nations emission reduction units allowing those nations to emit the amount equal to the excess it had cut. Several other programs could also help an industrialized nation earn credit toward its target. For example, the nation might help a developing country reduce emissions by replacing fossil fuels in some applications. Approving the agreement The protocol would take effect as a treaty if (1) at least 55 countries ratified (formally approved) it, and (2) the industrialized countries ratifying the protocol had CO2 emissions in 1990 that equaled at least 55 percent of the emissions of all 38 industrialized countries in 1990. In 2001, the United States rejected the Kyoto Protocol. President George W. Bush said that the agreement could harm the U.S. economy. But he declared that the United States would work with other countries to limit global warming. Other countries, most notably the members of the European Union, agreed to continue with the agreement without United States participation. By 2004, more than 100 countries, including nearly all the countries classified as industrialized under the protocol, had ratified the agreement. However, the agreement required ratification by Russia or the United States to go into effect. Russia ratified the protocol in November 2004. The treaty was to come into force in February 2005. Analyzing global warming Scientists use information from several sources to analyze global warming that occurred before people began to use thermometers. Those sources include tree rings, cores (cylindrical samples) of ice drilled from Antarctica and Greenland, and cores drilled out of sediments in oceans. Information from these sources indicates that the temperature increase of the 1900's was probably the largest in the last 1,000 years. Computers help climatologists analyze past climate changes and predict future changes. First, a scientist programs a computer with a set of mathematical equations known as a climate model. The equations describe how various factors, such as the amount of CO2 in the atmosphere, affect the temperature of Earth's surface. Next, the scientist enters data representing the values of those factors at a certain time. He or she then runs the program, and the computer describes how the temperature would vary. A computer's representation of changing climatic conditions is known as a climate simulation. In 2001, the Intergovernmental Panel on Climate Change (IPCC), a group sponsored by the United Nations (UN), published results of climate simulations in a report on global warming. Climatologists used three simulations to determine whether natural variations in climate produced the warming of the past 100 years. The first simulation took into account both natural processes and human activities that affect the climate. The second simulation took into account only the natural processes, and the third only the human activities. The climatologists then compared the temperatures predicted by the three simulations with the actual temperatures recorded by thermometers. Only the first simulation, which took into account both natural processes and human activities, produced results that corresponded closely to the recorded temperatures. The IPCC also published results of simulations that predicted temperatures until 2100. The different simulations took into account the same natural processes but different patterns of human activity. For example, scenarios differed in the amounts of CO2 that would enter the atmosphere due to human activities. The simulations showed that there can be no "quick fix" to the problem of global warming. Even if all emissions of greenhouse gases were to cease immediately, the temperature would continue to increase after 2100 because of the greenhouse gases already in the atmosphere.

Gravitation is the force of attraction that acts between all objects because of their mass. An object's mass is its amount of matter. Because of gravitation, an object that is near Earth falls toward the surface of the planet. An object that is already on the surface experiences a downward force due to gravitation. We experience this force on our bodies as our weight. Gravitation holds together the hot gases that make up the sun, and it keeps the planets in their orbits around the sun. Another term for gravitation is the force of gravity. People misunderstood gravitation for centuries. In the 300's B.C., the Greek philosopher and scientist Aristotle taught the incorrect idea that heavy objects fall faster than light objects. People accepted that idea until the early 1600's, when the Italian scientist Galileo corrected it. Galileo said that all objects fall with the same acceleration unless air resistance or some other force acts on them. An object's acceleration is the rate of change of its velocity (speed in a particular direction). Thus, a heavy object and a light object that are dropped from the same height will reach the ground at the same time. Newton's law of gravitation Ancient astronomers measured the movements of the moon and planets across the sky. However, no one correctly explained those motions until the late 1600's. At that time, the English scientist Isaac Newton described a connection between the movements of the celestial bodies and the gravitation that attracts objects to Earth. In 1665, when Newton was 23 years old, a falling apple caused him to question how far the force of gravity reaches. Newton explained his discovery in 1687 in a work called Philosophiae naturalis principia mathematica (Mathematical Principles of Natural Philosophy). Using laws of planetary motion discovered by the German astronomer Johannes Kepler, Newton showed how the sun's force of gravity must decrease with the distance from the sun. He then assumed that Earth's gravitation decreases in the same way with the distance from Earth. Newton knew that Earth's gravitation holds the moon in its orbit around Earth, and he calculated the strength of Earth's gravitation at the distance of the moon. Using his assumption, he calculated what the strength of that gravitation would be at Earth's surface. The calculated result was the same as the strength of the gravitation that would accelerate an apple. Newton's law of gravitation says that the gravitational force between two objects is directly proportional to their masses. That is, the larger either mass is, the larger is the force between the two objects. The law also says that the gravitational force between two objects is inversely (oppositely) proportional to the distance between the two objects squared (multiplied by itself). For example, if the distance between the two objects doubles, the force between them becomes one-fourth of its original strength. Newton's law is given by the equation F = m1m2 / d 2, where F is the gravitational force between two objects, m1 and m2 are the masses of the objects, and d2 is the distance between them squared. Until the early 1900's, scientists had observed only one movement that could not be described mathematically using Newton's law -- a tiny variation in the orbit of the planet Mercury around the sun. Mercury's orbit -- like the orbits of the other planets -- is an ellipse, a geometric figure with the shape of a flattened hoop. The sun is not at the exact center of the ellipse. So one point in each orbit is closer to the sun than all other points in that orbit. But the location of the closest point changes slightly each time Mercury revolves around the sun. Astronomers refer to that variation as a precession. Scientists used Newton's law to calculate the precession. The calculated amount differed slightly from the observed amount. Einstein's theory of gravitation In 1915, the German-born physicist Albert Einstein announced his theory of space, time, and gravitation, the general theory of relativity. Einstein's theory completely changed scientists' way of thinking about gravitation. However, it expanded upon Newton's law, rather than contradicting it. In many cases, Einstein's theory produced results that differed only slightly from results based on Newton's law. For example, when Einstein used his theory to calculate the precession of Mercury's orbit, the result agreed exactly with the observed motion. That agreement was the first confirmation of Einstein's theory. Einstein based his theory on two assumptions. The first is related to an entity known as space-time, and the second is a rule known as the principle of equivalence. Space-time In the complex mathematics of relativity, time and space are not absolutely separate. Instead, physicists refer to space-time, a combination of time and the three dimensions of space -- length, width, and height. Einstein assumed that matter and energy can distort (change the shape of) space-time, curving it; and that gravitation is an effect of the curvature. The principle of equivalence states that the effects of gravity are equivalent to the effects of acceleration. To understand this principle, suppose you were in a rocket ship so far from any planet, star, or other celestial object that the ship experienced virtually no gravitation. Imagine that the ship was moving forward, but not accelerating -- in other words, that the ship was traveling at a constant speed and in a constant direction. If you held out a ball and released it, the ball would not fall. Instead, it would hover beside you. But suppose the rocket accelerated by increasing its speed. The ball would appear to fall toward the rear of the ship exactly as if gravity had acted upon it. Predictions of general relativity In the years since the calculation of Mercury's precession confirmed Einstein's theory, several observations have verified predictions made with the theory. Some examples include predictions of the bending of light rays and radio waves, the existence of gravity waves and black holes, and the expansion of the universe. Bending of light rays Einstein's theory predicts that gravity will bend the path of a light ray as the ray passes near a massive body. The bending will occur because the body will curve space-time. The sun is massive enough to bend rays by an observable amount, and scientists first confirmed this prediction during a total eclipse of the sun in 1919. Bending and slowing of radio waves The theory also predicts that the sun will bend radio waves and slow them down. Scientists have measured the sun's bending of radio waves emitted (sent out) by quasars, extremely powerful objects at the centers of some galaxies. The measurements agree well with the prediction. Researchers measured a delay of radio waves that pass near the sun by sending signals between Earth and the Viking space probes that reached Mars in 1976. Those measurements still represent one of the most precise confirmations of general relativity. Gravitational waves General relativity also indicates that massive bodies in orbit around each other will emit waves of energy known as gravitational waves. Since 1974, scientists have confirmed the existence of gravitational waves indirectly by observing an object known as a binary pulsar. A binary pulsar is a rapidly rotating neutron star that orbits a similar, but unobserved, companion star. A neutron star consists mostly of tightly packed neutrons, particles that ordinarily occur only in the nuclei of atoms. A pulsar emits two steady beams of radio waves that flow away in opposite directions. As the star rotates on its own axis, the beams sweep around in space like searchlight beams. If one of the radio beams periodically sweeps over Earth, a radio telescope can detect the beam as a series of pulses. By closely observing changes in the pulse rate of a binary pulsar, scientists can determine the pulsar's orbital period -- the time it takes the two stars to completely orbit each other. Observations of the binary pulsar called PSR 1913 + 16 indicate that its orbital period is decreasing, and astronomers have measured the amount of the decrease. Scientists have also used equations of general relativity to calculate the amount by which the orbital period would decrease if the binary pulsar was radiating away energy as gravitational waves. The calculated amount agrees with the measured amount. In addition, the pulsar's orbit precesses as the pulsar revolves around the companion star. General relativity predicts the precession rate, and measurements match the prediction with great precision. Black holes Einstein's theory predicts the existence of objects called black holes. A black hole is a region of space whose gravitational force is so strong that not even light can escape from it. Researchers have found strong evidence that most very massive stars eventually evolve into black holes, and that most large galaxies have a gigantic black hole at their centers. Expansion of the universe In a paper published in 1917, Einstein applied general relativity to cosmology, the study of the universe as a whole. The theory showed that the universe must either expand or contract. In 1917, however, scientists had not yet found any evidence of expansion or contraction. To prevent his theory from disagreeing with the available evidence, Einstein added a term, the cosmological constant, to the theory. That term represented a repulsion (pushing away) of every point in space by the surrounding points, preventing contraction. But in 1929, the American astronomer Edwin Hubble discovered that distant galaxies are moving away from Earth and that, the more distant a galaxy, the more rapidly it is moving away. Hubble's discovery indicated that the universe is expanding. In response to Hubble's discovery and confirming observations by other astronomers, Einstein abandoned the cosmological constant, calling it his greatest blunder. The discovery of the expansion of the universe, together with other observations, led to the development of the big bang theory of the origin of the universe. According to that theory, the universe began with a hot, explosive event -- a "big bang." At the beginning of the event, all the matter in the part of the universe we can see was smaller than a marble. Matter then expanded rapidly, and it is still expanding. Dark energy Although Einstein called the cosmological constant his greatest blunder, it may turn out to be one of his greatest achievements. Measurements reported in 1998 suggest that the universe is expanding more and more rapidly. Furthermore, the rate of expansion has been increasing as predicted by general relativity with a cosmological constant. Until the measurements were reported, astronomers generally thought that the rate of expansion was decreasing due to the gravitational attraction of galaxies for one another. The measurements showed that exploding stars known as supernovae in distant galaxies were dimmer than expected and that the galaxies therefore were farther away then expected. But the galaxies could be so far away only if the rate of expansion had begun to increase in the past. Astronomers have concluded that the increase in the expansion rate is due to an entity that presently opposes gravitation. That entity could be a cosmological constant or something much like it called dark energy. Scientists have not yet developed theories to account for the existence of dark energy, but they know how much of it probably exists. The amount of dark energy in the universe is about twice as much as the amount of matter. The matter in the universe includes both visible matter and a mysterious substance known as dark matter. Scientists do not know the composition of dark matter. But measurements of the motion of stars and gas clouds in galaxies have led scientists to believe that it exists. Those measurements show that the masses of galaxies are many times larger than the masses of the visible objects in them. These and other observations suggest that the universe has at least 30 times as much dark matter as visible matter. Gravitation and the age of the universe Other observations have helped show that the theory of general relativity applies to the whole universe. Cosmologists have calculated the age of the universe using equations of general relativity, the measured rate of expansion of the universe, and estimates of the amounts of dark energy and dark matter. The calculated age, about 14 billion years, agrees well with results determined by two methods that do not involve general relativity: (1) calculations based on the evolution of stars and (2) the radioactive dating of old stars. As a star evolves, its surface temperature and its brightness change in a well-understood way. Astronomers can determine the ages of certain stars by measuring their temperature and brightness, then performing calculations based on their knowledge of stellar evolution. By means of such techniques, astronomers have found stars that may be about 13 billion years old -- but no stars that are clearly older than that. Radioactive dating of stars is based on the fact that certain chemical elements undergo radioactive decay. In radioactive decay, an isotope (form) of an element turns into an isotope of another element. Radioactive isotopes decay at known rates. In 2001, scientists working with the European Southern Observatory's Very Large Telescope in Chile applied the radioactive dating technique to an old star in our galaxy, the Milky Way. The researchers studied the isotope uranium 238, whose nucleus contains 92 protons and 146 neutrons. The scientists knew how much uranium the star must have had when it formed, and they measured how much it has now. They then applied their knowledge of decay rates to calculate the age of the star. The most likely age of the star is 12.5 billion years, so the universe is probably older than that. Measurements of the ages of many old stars using another element, thorium, gave similar results.

Hubble Space Telescope

The Hubble Space Telescope, an orbiting observatory launched in 1990, circles Earth high above the atmosphere. Image credit: NASA

The Hubble Space Telescope is a powerful orbiting telescope that provides sharper images of heavenly bodies than other telescopes do. It is a reflecting telescope with a light-gathering mirror 94 inches (240 centimeters) in diameter. The telescope is named after American astronomer Edwin P. Hubble, who made fundamental contributions to astronomy in the 1920's. Astronomers have used the Hubble Space Telescope to obtain images of celestial objects and phenomena in detail never before observed. These include pictures of stars surrounded by dusty disks that might someday evolve into planetary systems, images of galaxies on the edge of the observable universe, pictures of galaxies colliding and tearing each other apart, and evidence suggesting that most galaxies have massive black holes in their center. How the telescope works In orbit about 380 miles (610 kilometers) above the earth, the Hubble Space Telescope views the heavens without looking through the earth's atmosphere. The atmosphere bends light due to a phenomenon known as diffraction, and the atmosphere is constantly moving. This combination of diffraction and movement causes starlight to jiggle about as it passes through the air, and so stars appear to twinkle. Twinkling blurs images seen through ground-based telescopes. Because an orbiting telescope is above the atmosphere, it can produce pictures in much finer detail than a ground-based telescope can. This false-color image taken by the Hubble Space Telescope using infrared light shows Uranus's rings and clouds. The different colors in the image represent different atmospheric conditions. Image credit: NASA

The Hubble Space Telescope can also observe ultraviolet and infrared light that is blocked by the atmosphere. These forms of light, like visible light, are electromagnetic radiation. The wavelength (distance between successive wave crests) of ultraviolet light is shorter than that of visible light. Infrared light has longer wavelengths than visible light. Ultraviolet light comes from highly energetic processes, such as the formation of disks around black holes and exploding stars. Infrared light provides information about cooler, calmer events, such as the formation of dust clouds around new stars. The United States space agency, the National Aeronautics and Space Administration (NASA), operates the Hubble Space Telescope in cooperation with the European Space Agency (ESA). The telescope is controlled by radio commands relayed from NASA's Goddard Space Flight Center in Greenbelt, Maryland. Astronomers tell the telescope where to point, and computer -- driven instruments aboard the telescope record the resulting observations. The telescope transmits the data by radio to astronomers on the ground. The Hubble Space Telescope has two kinds of instruments: (1) imagers, which take pictures; and (2) spectrographs, which analyze light. Imagers are electronic detectors called charge -- coupled devices (CCD's). The CCD's convert light into electronic signals, which an on -- board computer records and sends to the ground. A spectrograph, like a prism, spreads light into its component colors, much as water droplets spread sunlight into a rainbow. The resulting band of light is called a spectrum (plural spectra). Using spectrographic data from the Hubble Space Telescope, astronomers can determine the composition of stars and galaxies--measuring, for example, the amounts of hydrogen, carbon, and other chemical elements in them. History

The most distant galaxies yet observed appear as faint patches of light in this photograph taken by the Hubble Space Telescope. The brighter swirls are galaxies somewhat closer to Earth, and the bright orange object is a star in our own galaxy. The telescope photographed this tiny portion of the sky, called the Hubble Ultra Deep Field, in 2004. Image credit: ASA/ESA/S. Beckwith (STScl) and the HUDF Team The space shuttle Discovery launched the telescope into orbit in 1990. Soon after launch, engineers discovered a flaw in the telescope's light -- gathering mirror. The flaw made the images less clear than they otherwise would have been. Engineers designed an optical device to bend light reflected by the mirror in a way that would make up for the error. Astronauts from the space shuttle Endeavour installed the device on the telescope in 1993, and it worked as planned. During the 1993 mission, astronauts also mounted new instruments on the telescope. As part of a continuing program to upgrade the telescope, astronauts installed additional components in 1997, 1999, and 2002. In 2004, NASA administrators canceled a final servicing mission to the telescope that had been scheduled for 2006. The officials based the decision on concern for the safety of astronauts after the loss of the space shuttle Columbia in 2003. Scientists, politicians, and astronomy enthusiasts protested that the decision would bring an early end to the telescope's observations. NASA officials then agreed to study the possibility of sending a robotic craft to perform needed repairs.

Hurricane A hurricane is a powerful, swirling storm that begins over a warm sea. Hurricanes form in waters near the equator, and then they move toward the poles.

Hurricane winds swirl about the eye, a calm area in the center of the storm. The main mass of clouds shown in this photograph measures almost 250 miles (400 kilometers) across. The hurricane, named Andrew, struck the Bahamas, Florida, and Louisiana in 1992, killing 65 people and causing billions of dollars in damage. Image credit: NASA The winds of a hurricane swirl around a calm central zone called the eye surrounded by a band of tall, dark clouds called the eyewall. The eye is usually 10 to 40 miles (16 to 64 kilometers) in diameter and is free of rain and large clouds. In the eyewall, large changes in pressure create the hurricane's strongest winds. These winds can reach nearly 200 miles (320 kilometers) per hour. Damaging winds may extend 250 miles (400 kilometers) from the eye. Hurricanes are referred to by different labels, depending on where they occur. They are called hurricanes when they happen over the North Atlantic Ocean, the Caribbean Sea, the Gulf of Mexico, or the Northeast Pacific Ocean. Such storms are known as typhoons if they occur in the Northwest Pacific Ocean, west of an imaginary line called the International Date Line. Near Australia and in the Indian Ocean, they are referred to as tropical cyclones. Hurricanes are most common during the summer and early fall. In the Atlantic and the Northeast Pacific, for example, August and September are the peak hurricane months. Typhoons occur throughout the year in the Northwest Pacific but are most frequent in summer. In the North Indian Ocean, tropical cyclones strike in May and November. In the South Indian Ocean, the South Pacific Ocean, and off the coast of Australia, the hurricane season runs from December to March. Approximately 85 hurricanes, typhoons, and tropical cyclones occur in a year throughout the world. In the rest of this article, the term hurricane refers to all such storms. Hurricane conditions Hurricanes require a special set of conditions, including ample heat and moisture, that exist primarily over warm tropical oceans. For a hurricane to form, there must be a warm layer of water at the top of the sea with a surface temperature greater than 80 degrees F (26.5 degrees C). Warm seawater evaporates and is absorbed by the surrounding air. The warmer the ocean, the more water evaporates. The warm, moist air rises, lowering the atmospheric pressure of the air beneath. In any area of low atmospheric pressure, the column of air that extends from the surface of the water -- or land -- to the top of the atmosphere is relatively less dense and therefore weighs relatively less. Air tends to move from areas of high pressure to areas of low pressure, creating wind. In the Northern Hemisphere, the earth's rotation causes the wind to swirl into a low-pressure area in a counterclockwise direction. In the Southern Hemisphere, the winds rotate clockwise around a low. This effect of the rotating earth on wind flow is called the effect. The Coriolis effect increases in intensity farther from the equator. To produce a hurricane, a low-pressure area must be more than 5 degrees of latitude north or south of the equator. Hurricanes seldom occur closer to the equator. For a hurricane to develop, there must be little wind shear -- that is, little difference in speed and direction between winds at upper and lower elevations. Uniform winds enable the warm inner core of the storm to stay intact. The storm would break up if the winds at higher elevations increased markedly in speed, changed direction, or both. The wind shear would disrupt the budding hurricane by tipping it over or by blowing the top of the storm in one direction while the bottom moved in another direction. The life of a hurricane Meteorologists (scientists who study weather) divide the life of a hurricane into four stages: (1) tropical disturbance, (2) tropical depression, (3) tropical storm, and (4) hurricane. Tropical disturbance is an area where rain clouds are building. The clouds form when moist air rises and becomes cooler. Cool air cannot hold as much water vapor as warm air can, and the excess water changes into tiny droplets of water that form clouds. The clouds in a tropical disturbance may rise to great heights, forming the towering thunderclouds that meteorologists call cumulonimbus clouds. Cumulonimbus clouds usually produce heavy rains that end after an hour or two, and the weather clears rapidly. If conditions are right for a hurricane, however, there is so much heat energy and moisture in the atmosphere that new cumulonimbus clouds continually form from rising moist air. Tropical depression is a low-pressure area surrounded by winds that have begun to blow in a circular pattern. A meteorologist considers a depression to exist when there is low pressure over a large enough area to be plotted on a weather map. On a map of surface pressure, such a depression appears as one or two circular isobars (lines of equal pressure) over a tropical ocean. The low pressure near the ocean surface draws in warm, moist air, which feeds more thunderstorms. The winds swirl slowly around the low-pressure area at first. As the pressure becomes even lower, more warm, moist air is drawn in, and the winds blow faster. Tropical storm When the winds exceed 38 miles (61 kilometers) per hour, a tropical storm has developed. Viewed from above, the storm clouds now have a well-defined circular shape. The seas have become so rough that ships must steer clear of the area. The strong winds near the surface of the ocean draw more and more heat and water vapor from the sea. The increased warmth and moisture in the air feed the storm. A tropical storm has a column of warm air near its center. The warmer this column becomes, the more the pressure at the surface falls. The falling pressure, in turn, draws more air into the storm. As more air is pulled into the storm, the winds blow harder. Each tropical storm receives a name. The names help meteorologists and disaster planners avoid confusion and quickly convey information about the behavior of a storm. The World Meteorological Organization (WMO), an agency of the United Nations, issues four alphabetical lists of names, one for the North Atlantic Ocean and the Caribbean Sea, and one each for the Eastern, Central, and Northwestern Pacific. The lists include both men's and women's names that are popular in countries affected by the storms. Except in the Northwestern and Central Pacific, the first storm of the year gets a name beginning with A -- such as Tropical Storm Alberto. If the storm intensifies into a hurricane, it becomes Hurricane Alberto. The second storm gets a name beginning with B, and so on through the alphabet. The lists do not use all the letters of the alphabet, however, since there are few names beginning with such letters as Q or U. For example, no Atlantic or Caribbean storms receive names beginning with Q, U, X, Y, or Z. Because storms in the Northwestern Pacific occur throughout the year, the names run through the entire alphabet instead of starting over each year. The first typhoon of the year might be Typhoon Nona, for example. The Central Pacific usually has fewer than five named storms each year. The system of naming storms has changed since 1950. Before that year, there was no formal system. Storms commonly received women's names and names of saints of both genders. From 1950 to 1952, storms were given names from the United States military alphabet -- Able, Baker, Charlie, and so on. The WMO began to use only the names of women in 1953. In 1979, the WMO began to use men's names as well. Hurricane

Hurricane winds on the ocean surface swirl counterclockwise around a calm eye in the Northern Hemisphere. Image credit: World Book illustrations by Bruce Kerr A storm achieves hurricane status when its winds exceed 74 miles (119 kilometers) per hour. By the time a storm reaches hurricane intensity, it usually has a well-developed eye at its center. Surface pressure drops to its lowest in the eye. In the eyewall, warm air spirals upward, creating the hurricane's strongest winds. The speed of the winds in the eyewall is related to the diameter of the eye. Just as ice skaters spin faster when they pull their arms in, a hurricane's winds blow faster if its eye is small. If the eye widens, the winds decrease. Heavy rains fall from the eyewall and bands of dense clouds that swirl around the eyewall. These bands, called rainbands, can produce more than 2 inches (5 centimeters) of rain per hour. The hurricane draws large amounts of heat and moisture from the sea. The path of a hurricane Hurricanes last an average of 3 to 14 days. A long-lived storm may wander 3,000 to 4,000 miles (4,800 to 6,400 kilometers), typically moving over the sea at speeds of 10 to 20 miles (16 to 32 kilometers) per hour. Hurricanes in the Northern Hemisphere usually begin by traveling from east to west. As the storms approach the coast of North America or Asia, however, they shift to a more northerly direction. Most hurricanes turn gradually northwest, north, and finally northeast. In the Southern Hemisphere, the storms may travel westward at first and then turn southwest, south, and finally southeast. The path of an individual hurricane is irregular and often difficult to predict. All hurricanes eventually move toward higher latitudes where there is colder air, less moisture, and greater wind shears. These conditions cause the storm to weaken and die out. The end comes quickly if a hurricane moves over land, because it no longer receives heat energy and moisture from warm tropical water. Heavy rains may continue, however, even after the winds have diminished. Hurricane damage Hurricane damage results from wind and water. Hurricane winds can uproot trees and tear the roofs off houses. The fierce winds also create danger from flying debris. Heavy rains may cause flooding and mudslides. The most dangerous effect of a hurricane, however, is a rapid rise in sea level called a storm surge. A storm surge is produced when winds drive ocean waters ashore. Storm surges are dangerous because many coastal areas are densely populated and lie only a few feet or meters above sea level. A 1970 cyclone in East Pakistan (now Bangladesh) produced a surge that killed about 266,000 people. A hurricane in Galveston, Texas, in 1900 produced a surge that killed about 6,000 people, the worst natural disaster in United States history. Hurricane watchers rate the intensity of storms on a scale called the Saffir-Simpson scale, developed by American engineer Herbert S. Saffir and meteorologist Robert H. Simpson. The scale designates five levels of hurricanes, ranging from Category 1, described as weak, to Category 5, which can be devastating. Category 5 hurricanes have included Hurricane Camille, which hit the United States in 1969; Hurricane Gilbert, which raked the West Indies and Mexico in 1988; and Hurricane Andrew, which struck the Bahamas, Florida, and Louisiana in 1992. Forecasting hurricanes Meteorologists use weather balloons, satellites, and radar to watch for areas of rapidly falling pressure that may become hurricanes. Specially equipped airplanes called hurricane hunters investigate budding storms. If conditions are right for a hurricane, the National Weather Service issues a hurricane watch. A hurricane watch advises an area that there is a good possibility of a hurricane within 36 hours. If a hurricane watch is issued for your location, check the radio or television often for official bulletins. A hurricane warning means that an area is in danger of being struck by a hurricane in 24 hours or less. Keep your radio tuned to a news station after a hurricane warning. If local authorities recommend evacuation, move quickly to a safe area or a designated hurricane shelter.

Iceberg

Icebergs form where chunks of ice break away from a glacier as it flows into the sea. The sun and wind melt the top of an iceberg. The bottom, which is under water, melts much more slowly. As the top melts away, leaving the bottom hidden beneath the surface, the iceberg becomes extremely dangerous to ships. Image credit: World Book diagrams by Marion Pahl Icebergs are huge masses of ice that break off the lower end of a glacier and fall into the sea. These masses are are made of frozen fresh water. Fog and icebergs are two of the greatest natural dangers to ships. (See the Life on Earth section for more about icebergs.) Explorers have written vivid descriptions of the color and beauty of icebergs, and compared them to towers, spires, pyramids, cathedrals, and palaces. Large icebergs weigh more than 1 million tons (910,000 metric tons), and some are many miles or kilometers long. The biggest ones tower as much as 400 feet (120 meters) above the surface of the ocean. But this is only a small part of the whole iceberg. Only one-seventh to one-tenth of the iceberg's total mass is above water. The white color of icebergs is caused by tiny, closely spaced gas cavities throughout the ice. When the sun is shining, streams of water form on the slopes of icebergs and drop over their edges in waterfalls. Icebergs often carry away large boulders and quantities of gravel from their glaciers. These are carried for long distances and finally dumped in the sea when the iceberg melts. North Atlantic icebergs come from the island of Greenland. A huge ice sheet covers nearly all of Greenland. It has an area of about 672,000 square miles (1,740,000 square kilometers) and an average thickness of 5,000 feet (1,500 meters). Long tongues of ice extend from the edge of this ice sheet into the sea. Cracks in the ice, and the action of rough sea waves, cause the icebergs to break off from the ice tongues. Noises like great explosions and rolling thunder accompany the beginning of an iceberg as it cracks loose. If the iceberg drops into an enclosed bay, it may cause huge waves. Most of the icebergs in the North Atlantic drift across Baffin Bay and Davis Strait to the coast of Labrador. Some of them are carried by the wind and the Labrador Current through the Newfoundland Banks into the Atlantic Ocean. In this region of the ocean, the icebergs melt rapidly because of the sunshine and warm ocean water. Parts of the icebergs may break off to form bergy bits, which are the size of an average house, or smaller pieces called growlers. Growlers are named for the noise they make as they float in the waves. Icebergs disappear about 400 miles (640 kilometers) south of Newfoundland. The greatest numbers of icebergs reach the routes of transatlantic liners in April, May, and June. That is why ships crossing the Atlantic follow a more southerly course during these months. Antarctic icebergs Many icebergs drift out to sea from the great Antarctic ice sheets. Some of these icebergs are many times larger than those found in the North Atlantic. The largest one ever seen in the Antarctic region had a length of about 200 miles (320 kilometers) at its longest point and a width of about 60 miles (97 kilometers) at its widest point. It covered about 5,000 square miles (13,000 square kilometers). Icebergs that are 10 miles (16 kilometers) long are common in the Antarctic. In contrast, the largest iceberg measured in the North Atlantic was 4 miles (6.4 kilometers) long. Ice patrols Icebergs can be extremely dangerous to ships. One of the greatest sea disasters in history was the sinking of the Titanic during the night of April 14 and 15, 1912. The Titanic was the largest ship afloat then, and was on its first trip from England to New York. The ship struck an iceberg, and about 1,500 people died. The wreck of the Titanic led to the establishment of an International Ice Patrol along the North Atlantic ship lanes. Since 1914, the patrol has been maintained through a sharing of its expenses by the principal shipping nations of the world. The United States Coast Guard does the actual patrolling. The patrol reports the position of icebergs and estimates their probable courses. Planes, ships, and satellites are used in patrolling. There is little human beings can do to control icebergs. It is difficult to destroy an iceberg by blasting, or to steer it into a different course which would take it out of an ocean shipping lane. It is even difficult to approach an iceberg, because the submerged parts may tear open a ship's bottom.

Two modules of the International Space Station were launched and assembled in 1998 by the United States and Russia. Image credit: NASA

The International Space Station is a large, inhabited Earth satellite that more than 15 nations are building in space. The first part of the station was launched in 1998, and the first full-time crew -- one American astronaut and two Russian cosmonauts -- occupied the station in 2000. The International Space Station orbits Earth at an altitude of about 250 miles (400 kilometers). The orbit extends from 52 degrees north latitude to 52 degrees south latitude. The station will include about eight large cylindrical sections called modules. Each module is being launched from Earth separately, and astronauts and cosmonauts are connecting the sections in space. Eight solar panels will supply more than 100 kilowatts of electric power to the station. The panels are being mounted on a metal framework 360 feet (109 meters) long.

The International Space Station will function as an observatory, laboratory, and workshop. Astronauts and cosmonauts will live and work in cylindrical modules, and solar panels will furnish electric power. Fifteen countries are building the station, shown here as it will look when finished. Image credit: NASA The United States and Russia are providing most of the modules and other equipment. Canada built a mobile robot arm, which was installed in 2001. Other participants include Japan and the member nations of the European Space Agency (ESA). Brazil signed a separate agreement with the United States to provide equipment. In exchange, Brazil will have access to U.S. equipment and permission to send a Brazilian astronaut to the station. More than 80 flights of U.S. space shuttles and Russian rockets will be necessary to complete the International Space Station. The ESA and Japan plan to develop supply vehicles to be launched on the ESA's Ariane 5 and Japan's H-2A booster rockets. The space station was originally scheduled for completion in 2006, but unpredicted expenses have created major delays. Missions The crew and scientists on Earth -- using radio signals -- operate laboratory equipment on the station. Some of the equipment measures the effects of space conditions, such as apparent weightlessness, on biological specimens -- including the crew. Other equipment produces various materials, including protein crystals for medical research. Crystals grown in space have fewer imperfections than those grown on Earth and are therefore easier to analyze. Medical researchers will use results of protein analyses to determine which crystals to mass-produce on Earth. The major value of having a space station is that all the equipment needs to be carried into space only once. Also, the station can be used again and again by visiting astronauts and cosmonauts. Scientists on Earth can analyze experimental results and modify follow-up investigations much more quickly than before. The station has been designed to operate for at least 15 years. But it could last for decades if parts are repaired and replaced as they wear out or are damaged. History The International Space Station is the ninth inhabited space station to orbit Earth. The first such stations, consisting of six models of the Soviet Salyut station and the U.S. Skylab, were launched in the 1970's. In 1986, the Soviet Union began operating Mir, the first space station to use a modular design. The Soviets developed a reliable, economical transportation system, based on Soyuz spacecraft, for the station. The system enabled them to deliver supplies, equipment, and crew members to Mir. Following the collapse of the Soviet Union in 1991, Russia took over the operation of Mir. Russian cosmonauts handled major breakdowns on the station, which was wearing out. In March 2001, Russia took Mir out of orbit and sent it plunging to Earth. Russia had intended to construct a station known as Mir 2 in the 1990's. The United States had planned to build a station called Freedom in partnership with space agencies in Europe, Canada, and Japan. But due to funding difficulties, the United States and Russia agreed in 1993 to build a combined station -- the International Space Station. To prepare for the project, shuttles flew to Mir from 1995 to 1998. United States astronauts served on board the Russian station as researchers for as long as six months. Major delays occurred in the construction of the International Space Station due to cutbacks in funding by the Russian government. A Russian Proton rocket finally launched the first module in November 1998. The module was a Russian-built and United States-funded unit called Zarya or the FGB. Zarya means sunrise in Russian, and FGB stands for functional cargo block. The second module, Unity, was built by the United States. The space shuttle Endeavour carried Unity into orbit in December 1998, and it was then joined to Zarya. Unity has six hatches. One is connected to Zarya, and others serve as connectors for other modules. In July 2000, a Proton rocket launched the Russian-built Zvezda (Star), or Service Module. Zvezda has living and working quarters for astronauts and cosmonauts. In October 2000, the shuttle Discovery carried up several more pieces. Those included a support truss for solar panels and a connecting unit called a Pressurized Mating Adapter (PMA). The PMA provided a docking port for shuttles. The first full-time crew, known as Expedition One, arrived in a Soyuz in November 2000. The crew commander was astronaut William Shepherd, and the other members were cosmonauts Yuri Gidzenko and Sergei Krikalev. Later that month, Endeavour carried the first four U.S.-built solar panels into space to supplement the small panels on the Russian modules.

The first two modules of the International Space Station were assembled in December 1998. In the foreground is the Unity module, which was built by the United States. Behind Unity, with solar panels attached to it, is a Russian-built module named Zarya -- the Russian word for sunrise. Image credit: NASA The shuttle Atlantis carried the U.S.-built Destiny Laboratory Module to the station in February 2001. Over the next few months, Destiny was activated, and scientific research began. Also in 2001, two additional modules -- a U.S. airlock and a Russian airlock and docking port -- were added. In April 2001, the first space tourist traveled as a passenger in a Soyuz-replacement mission. Dennis Tito, an investment consultant from California, purchased the trip for an undisclosed price. He trained in Moscow for six months before the flight and spent six days aboard the station. South African businessman Mark Shuttleworth became the second space tourist when he purchased a ride on a Soyuz- replacement mission launched in April 2002. Other Soyuz missions carried astronauts selected and sponsored by the ESA. The next stages of construction were to expand the station's power and life-support systems to support a full-time crew of six or seven. To provide emergency return capability for a crew that large, NASA planned to build a seven-person escape craft. But during 2001, it became clear that NASA had greatly underestimated the cost of developing and operating the station. NASA's cost for the station was about $5 billion over budget. As a result, NASA suspended the plan to enlarge the crew and build the escape craft. NASA's partners in Europe and Japan strongly objected to that decision. In 2002, the station continued to operate with a crew of three. Space shuttles replaced the crew every four or five months. Russian cosmonauts also flew a new Soyuz spacecraft to the station every six months. The Soyuz would serve as a "bail-out capsule" in case of a life-threatening emergency. The expansion of the power system continued, but more slowly than originally planned. On Feb. 1, 2003, the space shuttle Columbia broke apart on reentry into Earth's atmosphere, killing all seven crew members. NASA halted shuttle flights until it could ensure the safety of future flights. Soyuz missions then began carrying crews to and from the station. The station's crew was reduced to two people to conserve supplies normally carried to the station by shuttles.

Jupiter

The layers of dense clouds around Jupiter appear in a photograph of the planet taken by the Voyager 1 space probe. The large, oval-shaped mark on the clouds is the Great Red Spot. The spot is believed to be an intense atmospheric disturbance. Image credit: Jet Propulsion Laboratory

Jupiter is the largest planet in the solar system. Its diameter is 88,846 miles (142,984 kilometers), more than 11 times that of Earth, and about one-tenth that of the sun. It would take more than 1,000 Earths to fill up the volume of the giant planet. When viewed from Earth, Jupiter appears brighter than most stars. It is usually the second brightest planet -- after Venus. Jupiter is the fifth planet from the sun. Its mean (average) distance from the sun is about 483,780,000 miles (778,570,000 kilometers), more than five times Earth's distance. Ancient astronomers named Jupiter after the king of the Roman gods. Astronomers have studied Jupiter with telescopes based on Earth and aboard artificial satellites in orbit around Earth. In addition, the United States has sent six space probes (crewless exploratory craft) to Jupiter. Astronomers witnessed a spectacular event in July 1994, when 21 fragments of a comet named Shoemaker-Levy 9 crashed into Jupiter's atmosphere. The impacts caused tremendous explosions, some scattering debris over areas larger than the diameter of Earth. Physical features of Jupiter Jupiter is a giant ball of gas and liquid with little, if any, solid surface. Instead, the planet's surface is composed of dense red, brown, yellow, and white clouds. The clouds are arranged in light-colored areas called zones and darker regions called belts that circle the planet parallel to the equator. Orbit and rotation Jupiter travels around the sun in a slightly elliptical (oval-shaped) orbit. The planet completes one orbit in 4,333 Earth days, or almost 12 Earth years. As Jupiter orbits the sun, the planet rotates on its axis, an imaginary line through its center. The axis is tilted about 3¡. Scientists measure tilt relative to a line at a right angle to the orbital plane, an imaginary surface touching all points of the orbit. Jupiter rotates faster than any other planet. It takes 9 hours 56 minutes to spin around once on its axis, compared with 24 hours for Earth. Scientists cannot measure the rotation of the interior of the giant planet directly, so they have calculated the speed from indirect measurements. They first calculated the speed using an average of the speeds of the visible clouds that move with interior currents, except for a more rapid zone near the equator. Jupiter sends out radio waves strong enough to be picked up by radio telescopes on Earth. Scientists now measure these waves to calculate Jupiter's rotational speed. The strength of the waves varies under the influence of Jupiter's magnetic field in a pattern that repeats every 9 hours 56 minutes. Because the magnetic field originates in Jupiter's core, this variation shows how fast the plant's interior spins. Jupiter's rapid rotation makes it bulge at the equator and flatten at the poles. The planet's diameter is about 7 percent larger at the equator than at the poles. Mass and density Jupiter is heavier than any other planet. Its mass (quantity of matter) is 318 times larger than that of Earth. Although Jupiter has a large mass, it has a relatively low density. Its density averages 1.33 grams per cubic centimeter, slightly more than the density of water. The density of Jupiter is about 1/4 that of Earth. Because of Jupiter's low density, astronomers believe that the planet consists primarily of hydrogen and helium, the lightest elements. Earth, on the other hand, is made up chiefly of metals and rock. Jupiter's mix of chemical elements resembles that of the sun, rather than that of Earth. Jupiter may have a core made up of heavy elements. The core may be of about the same chemical composition as Earth, but 20 or 30 times more massive. The force of gravity at the surface of Jupiter is up to 2.4 times stronger than on Earth. Thus, an object that weighs 100 pounds on Earth would weigh as much as 240 pounds on Jupiter. The atmosphere of Jupiter is composed of about 86 percent hydrogen, 14 percent helium, and tiny amounts of methane, ammonia, phosphine, water, acetylene, ethane, germanium, and carbon monoxide. The percentage of hydrogen is based on the number of hydrogen molecules in the atmosphere, rather than on their total mass. Scientists have calculated these amounts from measurements taken with telescopes and other instruments on Earth and aboard spacecraft. These chemicals have formed colorful layers of clouds at different heights. The highest white clouds in the zones are made of crystals of frozen ammonia. Darker, lower clouds of other chemicals occur in the belts. At the lowest levels that can be seen, there are blue clouds. Astronomers had expected to detect water clouds about 44 miles (70 kilometers) below the ammonia clouds. However, none have been discovered at any level.

The planet Jupiter's Great Red Spot is a huge mass of swirling gas. At its widest, it is about three times the diameter of the Earth. Image credit: NASA

Jupiter's most outstanding surface feature is the Great Red Spot, a swirling mass of gas resembling a hurricane. The widest diameter of the spot is about three times that of Earth. The color of the spot usually varies from brick-red to slightly brown. Rarely, the spot fades entirely. Its color may be due to small amounts of sulfur and phosphorus in the ammonia crystals. The edge of the Great Red Spot circulates at a speed of about 225 miles (360 kilometers) per hour. The spot remains at the same distance from the equator but drifts slowly east and west. The zones, belts, and the Great Red Spot are much more stable than similar circulation systems on Earth. Since astronomers began to use telescopes to observe these features in the late 1600's, the features have changed size and brightness but have kept the same patterns. Temperature The temperature at the top of Jupiter's clouds is about -230 degrees F (-145 degrees C). Measurements made by ground instruments and spacecraft show that Jupiter's temperature increases with depth below the clouds. The temperature reaches 70 degrees F (21 degrees C) -- "room temperature" -- at a level where the atmospheric pressure is about 10 times as great as it is on Earth. Scientists speculate that if Jupiter has any form of life, the life form would reside at this level. Such life would need to be airborne, because there is no solid surface at this location on Jupiter. Scientists have discovered no evidence for life on Jupiter. Near the planet's center, the temperature is much higher. The core temperature may be about 43,000 degrees F (24,000 degrees C) -- hotter than the surface of the sun. Jupiter is still losing the heat produced when it became a planet. Most astronomers believe that the sun, the planets, and all the other bodies in the solar system formed from a spinning cloud of gas and dust. The gravitation of the gas and dust particles packed them together into dense clouds and solid chunks of material. By about 4.6 billion years ago, the material had squeezed together to form the various bodies in the solar system. The compression of material produced heat. So much heat was produced when Jupiter formed that the planet still radiates about twice as much heat into space as it receives from sunlight. Magnetic field Like Earth and many other planets, Jupiter acts like a giant magnet. The force of its magnetism extends far into space in a region surrounding the planet called its magnetic field. Jupiter's magnetic field is about 14 times as strong as Earth's, according to measurements made by spacecraft. Jupiter's magnetic field is the strongest in the solar system, except for fields associated with sunspots and other small regions on the sun's surface. Scientists do not fully understand how planets produce magnetic fields. They suspect, however, that the movement of electrically charged particles in the interior of planets generates the fields. Jupiter's field would be so much stronger than Earth's because of Jupiter's greater size and faster rotation. Jupiter's magnetic field traps electrons, protons, and other electrically charged particles in radiation belts around the planet. The particles are so powerful that they can damage instruments aboard spacecraft operating near the planet. Within a region of space called the magnetosphere, Jupiter's magnetic field acts as a shield. The field protects the planet from the solar wind, a continuous flow of charged particles from the sun. Most of these particles are electrons and protons traveling at a speed of about 310 miles (500 kilometers) per second. The field traps the charged particles in the radiation belts. The trapped particles enter the magnetosphere near the poles of the magnetic field. On the side of the planet away from the sun, the magnetosphere stretches out into an enormous magnetic tail, often called a magnetotail, that is at least 435 million miles (700 million kilometers) long. Radio waves given off by Jupiter reach radio telescopes on Earth in two forms -- bursts of radio energy and continuous radiation. Strong bursts occur when Io, the closest of Jupiter's four large moons, passes through certain regions in the planet's magnetic field. Continuous radiation comes from Jupiter's surface as well as from high-energy particles in the radiation belts.

Callisto, a moon of Jupiter, is covered with craters produced when asteroids and comets struck its icy surface. Beneath the surface may be an ocean of salty liquid water. Image credit: NASA

Satellites Jupiter has 16 satellites that measure at least 6 miles (10 kilometers) in diameter. It also has many smaller satellites. Jupiter's four largest satellites, in order of their distance from Jupiter, are Io, Europa, Ganymede, and Callisto. These four moons are called the Galilean satellites. The Italian astronomer Galileo discovered them in 1610 with one of the earliest telescopes. Io has many active volcanoes, which produce gases containing sulfur. The yellow-orange surface of Io probably consists largely of solid sulfur that was deposited by the eruptions. Europa ranks as the smallest of the Galilean satellites, with a diameter of 1,945 miles (3,130 kilometers). Europa has a smooth, cracked, icy surface. The largest Galilean satellite is Ganymede, with a diameter of 3,273 miles (5,268 kilometers). Ganymede is larger than the planet Mercury. Callisto, with a diameter of 2,986 miles (4,806 kilometers), is slightly smaller than Mercury. Ganymede and Callisto appear to consist of ice and some rocky material. The two satellites have many craters.

Ganymede, a moon of Jupiter, has craters and cracks on its surface. Asteroids and comets that hit Ganymede made the craters. The cracks are due to expansion and contraction of the surface. Image credit: NASA Jupiter's remaining satellites are much smaller than the Galilean moons. Amalthea and Himalia are the next largest. Potato-shaped Amalthea is about 163 miles (262 kilometers) in its long dimension. Himalia is 106 miles (170 kilometers) in diameter. Most of the remaining satellites were discovered by astronomers using large telescopes on Earth. Scientists discovered Metis and Adrastea in 1979 by studying pictures that had been taken by the Voyager spacecraft. Rings Jupiter has three thin rings around its equator. They are much fainter than the rings of Saturn. Jupiter's rings appear to consist mostly of fine dust particles. The main ring is about 20 miles (30 kilometers) thick and more than 4,000 miles (6,400 kilometers) wide. It circles the planet inside the orbit of Amalthea. The impact of Comet Shoemaker-Levy 9 In March 1993, astronomers Eugene Shoemaker, Carolyn Shoemaker, and David H. Levy discovered a comet near Jupiter. The comet, later named Shoemaker-Levy 9, probably once orbited the sun independently, but had been pulled by Jupiter's gravity into an orbit around the planet. When the comet was discovered, it had broken into 21 pieces. The comet probably had broken apart when it passed close to Jupiter. Calculations based on the comet's location and velocity showed that the fragments would crash into Jupiter's atmosphere in July 1994. Scientists hoped to learn much about the effects of a collision between a planet and a comet. Scars from the crash of Comet Shoemaker-Levy 9 appear on Jupiter's surface as a series of maroon blotches in this photo. The comet broke into 21 pieces before it hit Jupiter in 1994. Image credit: Hubble Space Telescope Comet Team and NASA Astronomers at all the major telescopes on Earth turned their instruments toward Jupiter at the predicted collision times. Scientists also observed Jupiter with the powerful Hubble Space Telescope, which is in orbit around Earth; and the remotely controlled space probe Galileo, which was on its way to Jupiter. The fragments fell on the back side of Jupiter as viewed from Earth and the Hubble Space Telescope. But the rotation of Jupiter carried the impact sites around to the visible side after less than half an hour. Scientists estimate that the largest fragments were about 0.3 to 2.5 miles (0.5 to 4 kilometers) in diameter. The impacts were directly observable from Galileo, which was within about 150 million miles (240 million kilometers) from Jupiter. However, damage to certain of the probe's instruments limited its ability to record and send data. The impacts caused large explosions, probably due to the compression, heating, and rapid expansion of atmospheric gases. The explosions scattered comet debris over large areas, some with diameters larger than that of Earth. The debris gradually spread into a dark haze of fine material that remained suspended for several months in Jupiter's upper atmosphere. If a similar comet ever collided with Earth, it might produce a haze that would cool the atmosphere and darken the planet by absorbing sunlight. If the haze lasted long enough, much of Earth's plant life could die, along with the people and animals that depend on plants. Flights to Jupiter The United States has sent six space probes to Jupiter: (1) Pioneer 10, (2) Pioneer-Saturn, (3) Voyager 1, (4) Voyager 2, (5) Ulysses, and (6) Galileo. Pioneer 10 was launched in 1972 and flew within 81,000 miles (130,000 kilometers) of Jupiter on Dec. 3, 1973. The probe revealed the severe effects of Jupiter's radiation belt on spacecraft. Pioneer 10 also reported the amount of hydrogen and helium in the planet's atmosphere. In addition, the probe discovered that Jupiter has an enormous magnetosphere. Pioneer-Saturn flew within 27,000 miles (43,000 kilometers) of Jupiter in December 1974. The craft provided close-up photographs of Jupiter's polar regions and data on the Great Red Spot, the magnetic field, and atmospheric temperatures. Voyager 1 and Voyager 2 flew past Jupiter in March and July 1979, respectively. These craft carried more sensitive instruments than did the Pioneers, and transmitted much more information. Astronomers used photographs taken by the Voyagers to make the first detailed maps of the Galilean satellites. The Voyagers also revealed sulfur volcanoes on Io, discovered lightning in Jupiter's clouds, and mapped flow patterns in the cloud bands. Ulysses was launched in October 1990 and passed by Jupiter in February 1992. The European Space Agency, an organization of Western European nations, had built the probe mainly to study the sun's polar regions. Scientists used the tremendous gravitational force of Jupiter to put Ulysses into an orbit that would take it over the sun's polar regions. As Ulysses passed by Jupiter, it gathered data indicating that the solar wind has a much greater effect on Jupiter's magnetosphere than earlier measurements had suggested. Galileo began its journey to Jupiter in October 1989. The craft released an atmospheric probe in July 1995. In December 1995, the probe plunged into Jupiter's atmosphere. The probe penetrated deep into the cloud layers and measured the amount of water and other chemicals in the atmosphere. Also in December 1995, Galileo went into orbit around Jupiter. Over the next several years, the craft monitored Jupiter's atmosphere and observed the planet's major satellites. Galileo's mission was extended in 1997 and again in 1999. Eventually, however, the craft ran low on fuel. In September 2003, mission managers intentionally crashed Galileo into Jupiter's atmosphere to avoid any risk of the craft crashing into and contaminating Jupiter's moon Europa. Galileo's observations of Europa had shown that it might have an ocean below its surface capable of supporting life.

Mars

The planet Mars, like Earth, has clouds in its atmosphere and a deposit of ice at its north pole. But unlike Earth, Mars has no liquid water on its surface. The rustlike color of Mars comes from the large amount of iron in the planet's soil. Image credit: NASA/JPL/Malin Space Science Systems Mars is the fourth planet from the sun. The planet is one of Earth's "next-door neighbors" in space. Earth is the third planet from the sun, and Jupiter is the fifth. Like Earth, Jupiter, the sun, and the remainder of the solar system, Mars is about 4.6 billion years old. Mars is named for the ancient Roman god of war. The Romans copied the Greeks in naming the planet for a war god; the Greeks called the planet Ares (AIR eez). The Romans and Greeks associated the planet with war because its color resembles the color of blood. Viewed from Earth, Mars is a bright reddish-orange. It owes its color to iron-rich minerals in its soil. This color is also similar to the color of rust, which is composed of iron and oxygen. Scientists have observed Mars through telescopes based on Earth and in space. Space probes have carried telescopes and other instruments to Mars. Early probes were designed to observe the planet as they flew past it. Later, spacecraft orbited Mars and even landed there. But no human being has ever set foot on Mars. Scientists have found strong evidence that water once flowed on the surface of Mars. The evidence includes channels, valleys, and gullies on the planet's surface. If this interpretation of the evidence is correct, water may still lie in cracks and pores in subsurface rock. A space probe has also discovered vast amounts of ice beneath the surface, most of it near the south pole. In addition, a group of researchers has claimed to have found evidence that living things once dwelled on Mars. That evidence consists of certain materials in meteorites found on Earth. But the group's interpretation of the evidence has not convinced most scientists. The Martian surface has many spectacular features, including a canyon system that is much deeper and much longer than the Grand Canyon in the United States. Mars also has mountains that are much higher than Mount Everest, Earth's highest peak. Above the surface of Mars lies an atmosphere that is about 100 times less dense than the atmosphere of Earth. But the Martian atmosphere is dense enough to support a weather system that includes clouds and winds. Tremendous dust storms sometimes rage over the entire planet. Mars is much colder than Earth. Temperatures at the Martian surface vary from as low as about -195 degrees F (-125 degrees C) near the poles during the winter to as much as 70 degrees F (20 degrees C) at midday near the equator. The average temperature on Mars is about -80 degrees F (-60 degrees C).

A sunset on Mars creates a glow due to the presence of tiny dust particles in the atmosphere. This photo is a combination of four images taken by Mars Pathfinder, which landed on Mars in 1997. Image credit: NASA/JPL Mars is so different from Earth mostly because Mars is much farther from the sun and much smaller than Earth. The average distance from Mars to the sun is about 141,620,000 miles (227,920,000 kilometers). This distance is roughly 1 1/2 times the distance from Earth to the sun. The average radius (distance from its center to its surface) of Mars is 2,107 miles (3,390 kilometers), about half the radius of Earth. Characteristics of Mars Orbit and rotation Like the other planets in the solar system, Mars travels around the sun in an elliptical (oval) orbit. But the orbit of Mars is slightly more "stretched out" than the orbits of Earth and most of the other planets. The distance from Mars to the sun can be as little as about 128,390,000 miles (206,620,000 kilometers) or as much as about 154,860,000 miles (249,230,000 kilometers). Mars travels around the sun once every 687 Earth days; this is the length of the Martian year. The distance between Earth and Mars depends on the positions of the two planets in their orbits. It can be as small as about 33,900,000 miles (54,500,000 kilometers) or as large as about 249,000,000 miles (401,300,000 kilometers). Like Earth, Mars rotates on its axis from west to east. The Martian solar day is 24 hours 39 minutes 35 seconds long. This is the length of time that Mars takes to turn around once with respect to the sun. The Earth day of 24 hours is also a solar day. The axis of Mars is not perpendicular to the planet's orbital plane, an imaginary plane that includes all points in the orbit. Rather, the axis is tilted from the perpendicular position. The angle of the tilt, called the planet's obliquity, is 25.19¡ for Mars, compared with 23.45¡ for Earth. The obliquity of Mars, like that of Earth, causes the amount of sunlight falling on certain parts of the planet to vary widely during the year. As a result, Mars, like Earth, has seasons. Mass and density Mars has a mass (amount of matter) of 7.08 X 1020 tons (6.42 X 1020 metric tons). The latter number would be written out as 642 followed by 18 zeroes. Earth is about 10 times as massive as Mars. Mars's density (mass divided by volume) is about 3.933 grams per cubic centimeter. This is roughly 70 percent of the density of Earth. Gravitational force Because Mars is so much smaller and less dense than Earth, the force due to gravity at the Martian surface is only about 38 percent of that on Earth. Thus, a person standing on Mars would feel as if his or her weight had decreased by 62 percent. And if that person dropped a rock, the rock would fall to the surface more slowly than the same rock would fall to Earth. Physical features of Mars Scientists do not yet know much about the interior of Mars. A good method of study would be to place a network of motion sensors called seismometers on the surface. Those instruments would measure tiny movements of the surface, and scientists would use the measurements to learn what lies beneath. Researchers commonly use this technique to study Earth's interior. Scientists have four main sources of information on the interior of Mars: (1) calculations involving the planet's mass, density, gravity, and rotational properties; (2) knowledge of other planets; (3) analysis of Martian meteorites that fall to Earth; and (4) data gathered by orbiting space probes. They think that Mars probably has three main layers, as Earth has: (1) a crust of rock, (2) a mantle of denser rock beneath the crust, and (3) a core made mostly of iron. Crust Scientists suspect that the average thickness of the Martian crust is about 30 miles (50 kilometers). Most of the northern hemisphere lies at a lower elevation than the southern hemisphere. Thus, the crust may be thinner in the north than in the south.

The surface of Mars was sampled for signs of life by the Viking 2 lander in 1976. A mechanical sampling arm dug the grooves near the round rock at the lower left. The cylinder at the right covered the sampling device and was ejected after landing. The cylinder is about 12 inches (30 centimeters) long. Image credit: NASA/National Space Science Data Center Much of the crust is probably composed of a volcanic rock called basalt (buh SAWLT). Basalt is also common in the crusts of Earth and the moon. Some Martian crustal rocks, particularly in the northern hemisphere, may be a form of andesite. Andesite is also a volcanic rock found on Earth, but it contains more silica than basalt does. Silica is a compound of silicon and oxygen. Mantle The mantle of Mars is probably similar in composition to Earth's mantle. Most of Earth's mantle rock is peridotite (PEHR uh DOH tyt), which is made up chiefly of silicon, oxygen, iron, and magnesium. The most abundant mineral in peridotite is olivine (OL uh veen). The main source of heat inside Mars must be the same as that inside Earth: radioactive decay, the breakup of the nuclei of atoms of elements such as uranium, potassium, and thorium. Due to radioactive heating, the average temperature of the Martian mantle may be roughly 2700 degrees F (1500 degrees C). Core Mars probably has a core composed of iron, nickel, and sulfur. The density of Mars gives some indication of the size of the core. Mars is much less dense than Earth. Therefore, the radius of Mars's core relative to the overall radius of Mars must be smaller than the radius of Earth's core relative to the overall radius of Earth. The radius of the Martian core is probably between 900 and 1,200 miles (1,500 and 2,000 kilometers). Unlike Earth's core, which is partially molten (melted), the core of Mars probably is solid. Scientists suspect that the core is solid because Mars does not have a significant magnetic field. A magnetic field is an influence that a magnetic object creates in the region around it. Motion within a planet's molten core makes the core a magnetic object. The motion occurs due to the rotation of the planet. Data from Mars Global Surveyor show that some of the planet's oldest rocks formed in the presence of a strong magnetic field. Thus, in the distant past, Mars may have had a hotter interior and a molten core. Surface features Mars has many of the kinds of surface features that are common on Earth. These include plains, canyons, volcanoes, valleys, gullies, and polar ice. But craters occur throughout the surface of Mars, while they are rare on Earth. In addition, fine-grained reddish dust covers almost all the Martian surface. Plains Many regions of Mars consist of flat, low-lying plains. Most of these areas are in the northern hemisphere. The lowest of the northern regions are among the flattest, smoothest places in the solar system. They may be so smooth because they were built up from deposits of sediment (tiny particles that settle to the bottom of a liquid). There is ample evidence that water once flowed across the Martian surface. The water would have tended to collect in the lowest spots on the planet and thus would have deposited sediments there.

Canyons The Valles Marineris system of valleys is about 2,500 miles (4,000 kilometers) long -- roughly one-fifth the distance around the planet Mars. Parts of the system are 6 miles (10 kilometers) deep. Image credit: NASA/National Space Science Data Center

Along the equator lies one of the most striking features on the planet, a system of canyons known as the Valles Marineris. The name is Latin for Valleys of Mariner; a space probe called Mariner 9 discovered the canyons in 1971. The canyons run roughly east-west for about 2,500 miles (4,000 kilometers), which is close to the width of Australia or the distance from Philadelphia to San Diego. Scientists believe that the Valles Marineris formed mostly by rifting, a splitting of the crust due to being stretched. Individual canyons of the Valles Marineris are as much as 60 miles (100 kilometers) wide. The canyons merge in the central part of the system, in a region that is as much as 370 miles (600 kilometers) wide. The depth of the canyons is enormous, reaching 5 to 6 miles (8 to 10 kilometers) in some places. Large channels emerge from the eastern end of the canyons, and some parts of the canyons have layered sediments. The channels and sediments indicate that the canyons may once have been partly filled with water. Volcanoes Mars has the largest volcanoes in the solar system. The tallest one, Olympus Mons (Latin for Mount Olympus), rises 17 miles (27 kilometers) above the surrounding plains. It is about 370 miles (600 kilometers) in diameter. Three other large volcanoes, called Arsia Mons, Ascraeus Mons, and Pavonis Mons, sit atop a broad uplifted region called Tharsis. All these volcanoes have slopes that rise gradually, much like the slopes of Hawaiian volcanoes. Both the Martian and Hawaiian volcanoes are shield volcanoes. They formed from eruptions of lavas that can flow for long distances before solidifying. Mars also has many other types of volcanic landforms. These range from small, steep-sided cones to enormous plains covered in solidified lava. Scientists do not know how recently the last volcano erupted on Mars -- some minor eruptions may still occur. Craters and impact basins Many meteoroids have struck Mars over its history, producing impact craters. Impact craters are rare on Earth for two reasons: (1) Those that formed early in the planet's history have eroded away, and (2) Earth developed a dense atmosphere, preventing meteorites that could have formed craters from reaching the planet's surface. Martian craters are similar to craters on Earth's moon, the planet Mercury, and other objects in the solar systems. The craters have deep, bowl-shaped floors and raised rims. Large craters can also have central peaks that form when the crater floor rebounds upward after an impact. On Mars, the number of craters varies dramatically from place to place. Much of the surface of the southern hemisphere is extremely old, and so has many craters. Other parts of the surface, especially in the northern hemisphere, are younger and thus have fewer craters. Some volcanoes have few craters, indicating that they erupted recently. The lava from the volcanoes would have covered any craters that existed at the time of the eruptions. And not enough time has passed since the eruptions for many new craters to form. Some of the impact craters have unusual-looking deposits of ejecta, material thrown out of the craters at impact. These deposits resemble mudflows that have solidified. This appearance suggests that the impacting bodies may have encountered water or ice beneath the ground. Mars has a few large impact craters. The largest is Hellas Planitia in the southern hemisphere. Planitia is a Latin word that can mean low plain or basin; Hellas Planitia is also known as the Hellas impact basin. The crater has a diameter of about 1,400 miles (2,300 kilometers). The crater floor is about 5.5 miles (9 kilometers) lower than the surrounding plain.

Channels in a Martian crater, in an image taken in 2000 by the Mars Global Surveyor, suggest to scientists that liquid water may have flowed across the surface of Mars in recent times. Image credit: NASA Channels, valleys, and gullies occur in many regions of Mars, apparently as a result of water erosion. The most striking of these features are known as outflow channels. These channels can be as wide as 60 miles (100 kilometers) and as long as 1,200 miles (2,000 kilometers). They appear to have been carved by enormous floods that rushed across the surface. In many cases, the water seems to have escaped suddenly from underground. Many of the channels do not look like river systems on Earth, with the main river formed from smaller rivers and streams. Rather, those Martian channels arise fully formed from low-lying areas. Other regions of Mars have much smaller features called valley networks. These networks look more like river systems on Earth. Martian valley networks are up to a few miles or kilometers wide and up to a few hundred miles or kilometers long. The networks are mostly ancient features. They suggest that the Martian climate may once have been warm enough to enable water to exist as a liquid. The gullies are smaller still. Most of them lie at high latitudes. They may be a result of a leakage of a small amount of ground water to the surface within the past few million years. Polar deposits The most interesting features in the polar regions of Mars are thick stacks of finely layered deposits of material. Scientists believe that the layers consist of mixtures of water ice and dust. The deposits extend from the poles to latitudes of about 80 degrees in both hemispheres. The atmosphere probably deposited the layers over long periods. The layers may provide evidence of seasonal weather activity and long-term changes in the Martian climate. One possible cause of climate changes is variation in the planet's obliquity. This variation alters the amount of sunlight falling on different parts of Mars. The variation in sunlight, in turn, may change the climate. Past climate changes could have affected the rate at which the atmosphere deposited dust and ice into layers. Lying atop much of the layered deposits in both hemispheres are caps of water ice that remain frozen all year. The layers and overlying caps are several miles or kilometers thick. In the wintertime, additional seasonal caps form from layers of . The seasonal caps are clearly visible through Earth-based telescopes. The frost consists of solid carbon dioxide (CO2) -- also known as "dry ice" -- that has condensed from CO2 gas in the atmosphere. In the deepest part of the winter, the frost extends from the poles to latitudes as low as 45 degrees -- halfway to the equator. Atmosphere The atmosphere of Mars contains much less oxygen (O2) than that of Earth. The O2 content of the Martian atmosphere is only 0.13 percent, compared with 21 percent in Earth's atmosphere. Carbon dioxide makes up 95.3 percent of the gas in the atmosphere of Mars. Other gases include nitrogen (N2), 2.7 percent; argon (Ar), 1.6 percent; carbon monoxide (CO), 0.07 percent; and water vapor (H2O), 0.03 percent. Pressure At the surface of Mars, the atmospheric pressure is typically only about 0.10 pound per square inch (0.7 kilopascal). This is roughly 0.7 percent of the atmospheric pressure at Earth's surface. When the seasons change on Mars, the atmospheric pressure at the surface there varies by 20 to 30 percent. Each winter, the condensation of CO2 at the poles removes much gas from the atmosphere. When this happens, the atmospheric pressure due to CO2 gas decreases sharply. The opposite process occurs each summer. In addition, the atmospheric pressure varies as the weather changes during the day, much as on Earth. Temperature The atmosphere of Mars is coldest at high altitudes, from about 40 to 78 miles (65 to 125 kilometers) above the surface. At those altitudes, typical temperatures are below -200 degrees F (-130 degrees C). The temperature increases toward the surface, where daytime temperatures of -20 to -40 degrees F (-30 to -40 degrees C) are typical. In the lowest few miles or kilometers of the atmosphere, the temperature varies widely during the day. It can reach -150 degrees F (-100 degrees C) late at night, even near the equator. Atmospheric temperatures can be warmer than normal when the atmosphere contains much dust. The dust absorbs sunlight and then transfers much of the resulting heat to the atmospheric gases. Clouds In the Martian atmosphere, thin clouds made up of particles of frozen CO2 can form at high altitudes. In addition, clouds, haze, and fog composed of particles of water ice are common. Haze and fog are especially frequent in the early morning. At that time, temperatures are the lowest, and water vapor is therefore most likely to condense. Wind The Martian atmosphere, like that of Earth, has a general circulation, a wind pattern that occurs over the entire planet. Scientists have studied the global wind patterns of Mars by observing the motions of clouds and changes in the appearance of wind-blown dust and sand on the surface. Global-scale winds occur on Mars as a result of the same process that produces such winds on Earth. The sun heats the atmosphere more at low latitudes than at high latitudes. At low latitudes, the warm air rises, and cooler air flows in along the surface to take its place. The warm air then travels toward the cooler regions at higher latitudes. At the higher latitudes, the cooler air sinks, then travels toward the equator. On Mars, the condensation and evaporation of CO2 at the poles influence the general circulation. When winter begins, atmospheric CO2 condenses at the poles, and more CO2 flows toward the poles to take its place. When spring arrives, CO2 frost evaporates, and the resulting gas flows away from the poles. Surface winds on Mars are mostly gentle, with typical speeds of about 6 miles (10 kilometers) per hour. Scientists have observed wind gusts as high as 55 miles (90 kilometers) per hour. However, the gusts exert much less force than do equally fast winds on Earth. The winds of Mars have less force because of the lower density of the Martian atmosphere. Dust storms Some of the most spectacular weather occurs on Mars when dust blows in the wind. Small, swirling winds can lift dust off the surface for brief intervals. These winds create dust devils, tiny storms that look like tornadoes. Large dust storms begin when wind lifts dust into the atmosphere. The dust then absorbs sunlight, warming the air around it. As the warmed air rises, more winds occur, lifting still more dust. As a result, the storm becomes stronger. At larger scales, dust storms can blanket areas from more than 200 miles (320 kilometers) to a few thousand miles or kilometers across. The largest storms can cover the entire surface of Mars. Storms of that size are unusual, but they can last for months. The strongest storms can block almost the entire surface from view. Such storms occurred in 1971 and 2001. Dust storms are most common when Mars is closest to the Sun. More storms occur then because that is when the sun heats the atmosphere the most. Satellites Mars has two tiny moons, Phobos and Deimos. The American astronomer Asaph Hall discovered them in 1877 and named them for the sons of Ares. Both satellites are irregularly shaped. The largest diameter of Phobos is about 17 miles (27 kilometers); that of Deimos, about 9 miles (15 kilometers). The two satellites have many craters that formed when meteoroids struck them. The surface of Phobos also has a complicated pattern of grooves. These may be cracks that developed when an impact created the satellite's largest crater. Scientists do not know where Phobos and Deimos formed. They may have come into existence in orbit around Mars at the same time the planet formed. Another possibility is that the satellites formed as asteroids near Mars. The gravitational force of Mars then pulled them into orbit around the planet. The color of both satellites is a dark gray that is similar to the color of some kinds of asteroids. Evolution of Mars Scientists know generally how Mars evolved after it formed about 4.6 billion years ago. Their knowledge comes from studies of craters and other surface features. Features that formed at various stages of the planet's evolution still exist on different parts of the surface. Researchers have developed an evolutionary scenario that accounts for the sizes, shapes, and locations of those features. Researchers have ranked the relative ages of surface regions according to the number of impact craters observed. The greater the number of craters in a region, the older the surface there. However, scientists have not yet determined exactly when the various evolutionary stages occurred. To do that, they would need to know the ages of rocks of surface features representing those stages. They could determine how old such rocks are if they could analyze samples of them in a laboratory. But no space probe has ever brought Martian rocks to Earth. Scientists have divided the "lifetime" of Mars into three periods. From the earliest to the most recent, the periods are: (1) The Noachian (noh AY kee uhn), (2) the Hesperian, and (3) the Amazonian. Each period is named for a surface region that was created during that period. The Noachian Period is named for Noachis Terra, a vast highland in the southern hemisphere. During the Noachian Period, a tremendous number of rocky objects of all sizes, ranging from small meteoroids to large asteroids, struck Mars. The impact of those objects created craters of all sizes. The Noachian was also a time of great volcanic activity. In addition, water erosion probably carved the many small valley networks that mark Mars's surface during the Noachian Period. The presence of those valleys suggests that the climate may have been warmer during the Noachian Period than it is today. The Hesperian Period The intense meteoroid and asteroid bombardment of the Noachian Period gradually tapered off, marking the beginning of the Hesperian Period. This period is named for Hesperia Planum, a high plain in the lower latitudes of the southern hemisphere. During the Hesperian Period, volcanic activity continued. Volcanic eruptions covered over Noachian craters in many parts of Mars. Most of the largest outflow channels on the planet are of Hesperian age. The Amazonian Period, which is characterized by a low rate of cratering, continues to this day. The period is named for Amazonis Planitia, a low plain that is in the lower latitudes of the northern hemisphere. Volcanic activity has occurred throughout the Amazonian Period, and some of the largest volcanoes on Mars are of Amazonian age. The youngest geologic materials on Mars, including the ice deposits at the poles, are also Amazonian. Possibility of life Mars might once have harbored life, and living things might exist there even today. Mars almost certainly has three ingredients that scientists believe are necessary for life: (1) chemical elements such as carbon, hydrogen, oxygen, and nitrogen that form the building blocks of living things, (2) a source of energy that living organisms can use, and (3) liquid water. The essential chemical elements likely were present throughout the planet's history. Sunlight could be the energy source, but a second source of energy could be the heat inside Mars. On Earth, internal heat supports life in the deep ocean and in cracks in the crust. Liquid water apparently carved Mars's large channels, its smaller valleys, and its young gullies. In addition, there are vast quantities of ice within about 3 feet (1 meter) of the surface near the south pole and perhaps near the north pole. Thus, water apparently has existed near the surface over much of the planet's history. And water is probably present beneath the surface today, kept liquid by Mars's internal heat.

A curved, rodlike structure shown in the center of this photo has been referred to as a fossilized Martian creature by some scientists. The structure is about 200 billionths of a meter long and is part of a Martian rock that was found on Earth. Image credit: NASA/Johnson Space Center In 1996, scientists led by David S. McKay, a geologist at the National Aeronautics and Space Administration's Johnson Space Center in Houston, reported that scientists there had found evidence of microscopic Martian life. They discovered this evidence inside a meteorite that had made its way to Earth. The meteorite had been blasted from the surface of Mars, almost certainly by the impact of a much larger meteorite. The small meteorite had then journeyed to Earth, attracted by Earth's gravity. The trip may have taken millions of years. The evidence included complex organic molecules, grains of a mineral called magnetite that can form within some kinds of bacteria, and tiny structures that resemble fossilized microbes. The scientists' conclusions are controversial, however. There is no general scientific agreement that Mars has ever harbored life. History of Mars study Observation from Earth Observing Mars through Earth-based telescopes, early astronomers discovered polar caps that grow and shrink with the seasons. They also found light and dark markings that change their shape and location. In the late 1800's, the Italian astronomer Giovanni V. Schiaparelli reported that he saw a network of fine dark lines. He called these lines canali, which is Italian for channels. But canali was generally mistranslated as canals. Many other astronomers also reported seeing such features. Among those observers was the American astronomer Percival Lowell, who referred to the features as canals. Lowell speculated that the canals had been built by a Martian civilization. The canals turned out not to exist. In some cases, the observers had misinterpreted dark, blurry regions that they had actually seen. In other cases, there was no relationship between "canals" and real features. However, the changing dark and light markings were real. Some scientists thought that the changing patterns might result from the growth and death of vegetation. Much later, other scientists suspected correctly that the cause was the Martian winds. Light and dark materials blow to and fro across the surface. Observation by spacecraft Robotic spacecraft began detailed observation of Mars in the 1960's. The United States launched Mariner 4 to Mars in 1964 and Mariners 6 and 7 in 1969. Each flew by Mars about half a year after its launch. The craft took pictures showing that Mars is a barren world, with craters like those on the moon. There was no sign of liquid water or life. The spacecraft observed few of the planet's most interesting features because they happened to fly by only heavily cratered regions. In 1971, Mariner 9 went into orbit around Mars. This craft mapped about 80 percent of Mars. It made the first discoveries of the planet's canyons and volcanoes. It also found what appear to be dry riverbeds.

The Sojourner Rover examines a rock on Mars. The rover traveled from Earth aboard the Mars Pathfinder space probe, then rolled down a ramp to the surface. Sojourner is only 24 3/4 inches (63 centimeters) long. Image credit: NASA The next major mission to Mars was Viking, launched by the United States in 1975. Viking consisted of two orbiters and two landers. Its main goal was to search for life. The orbiters scouted out landing sites for the landers, which touched down in July and September 1976. The landers took the first close-up pictures of the Martian surface, and they sampled the soil. They found no strong evidence for life. The next two successful probes were Mars Pathfinder, which was a lander, and Mars Global Surveyor, an orbiter. The United States launched both craft in 1996. The main objective of Pathfinder was to demonstrate a new landing system. Inflated air bags cushioned the probe's landing in July 1997. Pathfinder also carried a small roving vehicle called Sojourner. The rover rolled down a ramp to the surface, and then moved from rock to rock. Pathfinder sent spectacular photos back to Earth, and Sojourner analyzed rocks and soil. People throughout the world watched television pictures of Sojourner doing its work.

Mars Global Surveyor studied the composition of the Martian surface, photographed the surface in detail, and measured its elevation. The space probe went into orbit around Mars in 1997. Image credit: NASA/JPL

Mars Global Surveyor carried a group of sophisticated scientific instruments. A laser altimeter used laser beams to determine the elevation of the Martian surface. This instrument produced maps of the entire surface that are accurate to within 1 yard or meter of elevation. An infrared spectrometer determined the composition of some of the minerals on the surface. A high-resolution camera revealed a host of new geologic features. These include layered sediments that may have been deposited in liquid water, and small gullies that appear to have been carved by water. In April 2001, the United States launched the Mars Odyssey probe. The probe carried instruments to analyze the chemical composition of the Martian surface and the rocks just below the surface, to determine whether there is water ice on or beneath the surface, and to study the radiation near Mars. Mars Odyssey went into orbit around the planet in October 2001. In 2002, the probe discovered vast amounts of water ice beneath the surface. Most of the ice found is in the far southern part of the planet, south of 60 degrees south latitude. Scientists also suspect that there are large amounts of water ice north of 60 degrees north latitude. However, when the discovery was made, CO2 frost covered most of that area, preventing the probe from detecting underlying ice. The water ice found in the south is in the upper 3 feet (1 meter) of soil. That soil is more than 50 percent water ice by volume. The total volume of the water ice discovered is roughly 2,500 cubic miles (10,400 cubic kilometers), more than enough to fill Lake Michigan twice. The probe cannot detect evidence of water at depths greater than 3 feet. Thus, scientists cannot yet determine the total depth or the total volume of all the water ice on Mars.

Mars was photographed by the Hubble Space Telescope in August 2003 as the planet passed closer to Earth than it had in nearly 60,000 years. The photographs captured many features of the Martian surface, including dark, circular impact craters and the bright ice of the southern polar cap. Image credit: NASA, J. Bell (Cornell U.) and M. Wolff (SSI) Mars passed closer to Earth in August 2003 than it had in nearly 60,000 years. In that year, scientists launched three new probes. The European Space Agency's Mars Express mission included an orbiter that carried scientific instruments and a lander designed to analyze the planet's soil for evidence of life. The United States launched two rovers, nicknamed Spirit and Opportunity, to explore different regions of the planet's surface. In December 2003, Mars Express went into orbit around the planet and released its lander, Beagle 2. Mars Express immediately began transmitting pictures and other information about the planet, but mission managers could not contact Beagle 2 and feared it was lost. In early January 2004, the U.S. rover Spirit landed safely in an area called Gusev Crater. The rover Opportunity landed later that month in an area called Meridiani Planum. The rovers transmitted detailed photographs of Martian ground features and began analyzing rocks and soil for evidence that large amounts of liquid water once existed on the planet's surface. In March 2004, U.S. scientists announced that they had concluded that Meridiani Planum once held large amounts of liquid water. Their evidence came from an outcropping of Martian bedrock found in the small crater in which Opportunity landed. The rover's analysis showed that the rock contained large amounts of sulfate salts, which contain sulfur and oxygen. On Earth, such high concentrations of sulfate salts occur only in rocks that formed in water or were exposed to water for long periods. The outcropping's surface also bore tiny pits similar to those found on Earth where salt crystals formed in wet rock and later dissolved or eroded away. The rover Spirit rests on Mars in a composite image made up of photographs taken by a camera mounted above the rover's body. Spirit landed on Mars in early January 2004. The pole at the lower left is one of the antennas Spirit uses to communicate with NASA controllers. Image credit: NASA The rover mission was scheduled to last only 90 days, but it was extended because Spirit and Opportunity continued to function well. In June 2004, Opportunity descended into a large crater that mission managers called Endurance and analyzed the layers of bedrock there. Also in June, Spirit arrived at a group of hills, called Columbia Hills, after a drive of over 2 miles (3 kilometers). The rovers continued to explore these sites for several months.

Mercury

The planet Mercury was first photographed in detail on March 29, 1974, by the U.S. probe Mariner 10. The probe was about 130,000 miles (210,000 kilometers) from Mercury. Image credit: NASA

Mercury is the planet nearest the sun. It has a diameter of 3,032 miles (4,879 kilometers), about two- fifths of Earth's diameter. Mercury orbits the sun at an average distance of about 36 million miles (58 million kilometers), compared with about 93 million miles (150 million kilometers) for Earth. Because of Mercury's size and nearness to the brightly shining sun, the planet is often hard to see from the Earth without a telescope. At certain times of the year, Mercury can be seen low in the western sky just after sunset. At other times, it can be seen low in the eastern sky just before sunrise. Orbit Mercury travels around the sun in an elliptical (oval-shaped) orbit. The planet is about 28,580,000 miles (46,000,000 kilometers) from the sun at its closest point, and about 43,380,000 miles (69,820,000 kilometers) from the sun at its farthest point. Mercury is about 48,000,000 miles (77,300,000 kilometers) from Earth at its closest approach. Mercury moves around the sun faster than any other planet. The ancient Romans named it Mercury in honor of the swift messenger of their gods. Mercury travels about 30 miles (48 kilometers) per second, and goes around the sun once every 88 Earth days. The Earth goes around the sun once every 365 days, or one year. Rotation As Mercury moves around the sun, it rotates on its axis, an imaginary line that runs through its center. The planet rotates once about every 59 Earth days -- a rotation slower than that of any other planet except Venus. As a result of the planet's slow rotation on its axis and rapid movement around the sun, a day on Mercury -- that is, the interval between one sunrise and the next -- lasts 176 Earth days. Until the mid-1960's, astronomers believed that Mercury rotated once every 88 Earth days, the same time the planet takes to go around the sun. If Mercury did this, one side of the planet would always face the sun, and the other side would always be dark. However, radar studies conducted in 1965 showed that the planet rotates once in about 59 days. Phases When viewed through a telescope, Mercury can be seen going through "changes" in shape and size. These apparent changes are called phases, and resemble those of the moon. They result from different parts of Mercury's sunlit side being visible from the Earth at different times. As Mercury and the Earth travel around the sun, Mercury can be seen near the other side of the sun about every 116 days. At this point, almost all its sunlit area is visible from the Earth. It looks like a bright, round spot with almost no visible marks. As Mercury moves around the sun toward the Earth, less and less of its sunlit area can be seen. After about 36 days, only half its surface is visible. After another 22 days, it nears the same side of the sun as the Earth, and only a thin sunlit area is visible. The amount of sunlit area that can be seen increases gradually after Mercury passes in front of the sun and begins moving away from the Earth. When Mercury is on the same side of the sun as the Earth is, its dark side faces the Earth. The planet is usually not visible at this point because Mercury and the Earth orbit the sun at different angles. As a result, Mercury does not always pass directly between the Earth and the sun. Sometimes Mercury is directly between the Earth and the sun. When this occurs, every 3 to 13 years, the planet is in transit and can be seen as a black spot against the sun. Surface and atmosphere

The surface of Mercury consists of cratered terrain and smooth plains. Image credit: NASA

Mercury's surface appears to be much like that of the moon. It reflects approximately 6 percent of the sunlight it receives, about the same as the moon's surface reflects. Like the moon, Mercury is covered by a thin layer of minerals called silicates in the form of tiny particles. It also has broad, flat plains; steep cliffs; and many deep craters similar to those on the moon. The craters formed when meteors or small comets crashed into the planet. Mercury does not have enough atmosphere to slow down meteoroids and burn them up by friction. The Caloris Basin, Mercury's largest crater, measures about 800 miles (1,300 kilometers) across. Mercury's interior appears to resemble that of the Earth. Both planets have a rocky layer called a mantle beneath their crust, and both planets have an iron core. Based on Mercury's size and mass, scientists believe the planet's core makes up about three-fourths of its radius. Earth's core makes up about half of its radius. The discovery of a magnetic field around Mercury led some scientists to believe that the planet's outer core, like Earth's, consists of liquid iron. Mercury is dry, extremely hot, and almost airless. The sun's rays are approximately seven times as strong on Mercury as they are on the Earth. The sun also appears about 2 1/2 times as large in Mercury's sky as in the Earth's. Mercury does not have enough gases in its atmosphere to reduce the amount of heat and light it receives from the sun. The temperature on the planet may reach 840 degrees F (450 degrees C) during the day. But at night, the temperature may drop as low as -275 degrees F (-170 degrees C). Because of the lack of atmosphere, Mercury's sky is black. Stars probably would be visible from the surface during the day. Scans of Mercury made by Earth-based radar indicate that craters at Mercury's poles contain water ice. The floors of the craters are permanently shielded from sunlight, so the temperature never gets high enough to melt the ice. Mercury is surrounded by an extremely small amount of helium, hydrogen, oxygen, and sodium. This envelope of gases is so thin that the greatest possible atmospheric pressure (force exerted by the weight of gases) on Mercury would be about 0.00000000003 pound per square inch (0.000000000002 kilogram per square centimeter). The atmospheric pressure on the Earth is about 14.7 pounds per square inch (1.03 kilograms per square centimeter). The plant and animal life of the Earth could not live on Mercury because of the lack of oxygen and the intense heat. Scientists doubt that the planet has any form of life. Density and mass Mercury's density is slightly less than the Earth's (see Density). That is, a portion of Mercury would weigh slightly less than an equal portion of the Earth. Mercury is smaller than the Earth and therefore has much less mass (see Mass). Mercury's smaller mass makes its force of gravity only about a third as strong as that of the Earth. An object that weighs 100 pounds on the Earth would weigh only about 38 pounds on Mercury. Flights to Mercury

Mariner 10 is the only space probe that has visited the planet Mercury. It flew past Venus in 1974, then made three passes near Mercury in 1974 and 1975. A probe called Messenger, launched in 2004, was scheduled to make its first visit to Mercury in 2008. Image credit: NASA The United States Mariner 10 became the first and only spacecraft to reach Mercury. The remotely controlled spacecraft flew to within 460 miles (740 kilometers) of Mercury on March 29, 1974. It swept past the planet again on Sept. 24, 1974, and on March 16, 1975. During those flights, the spacecraft photographed portions of the surface of Mercury. It also detected Mercury's magnetic field. Mariner 10 became the first spacecraft to study two planets. The probe photographed and made scientific measurements of Venus while traveling to Mercury. As the probe flew near Venus, the planet's gravity pulled on the spacecraft, causing it to move faster. Thus, Mariner 10 reached Mercury in less time and by using less fuel than if it had flown directly from the Earth. In 2004, the United States launched the Messenger probe to Mercury. Messenger was scheduled to fly by Mercury twice in 2008 and once in 2009 before going into orbit around the planet in 2011. The probe was then to orbit Mercury for one Earth year while mapping Mercury's surface and studying its composition, interior structure, and magnetic field. Meteor

A meteor is a bright streak of light that appears briefly in the sky. Observers often call meteors shooting stars or falling stars because they look like stars falling from the sky. People sometimes call the brightest meteors fireballs. A meteor appears when a particle or chunk of metallic or stony matter called a meteoroid enters the earth's atmosphere from outer space. Air friction heats the meteoroid so that it glows and creates a shining trail of gases and melted meteoroid particles. The gases include vaporized meteoroid material and atmospheric gases that heat up when the meteoroid passes through the atmosphere. Most meteors glow for about a second. Most meteoroids disintegrate before reaching the earth. But some leave a trail that lasts several minutes. Meteoroids that reach the earth are called meteorites. Millions of meteors occur in the earth's atmosphere every day. Most meteoroids that cause meteors are about the size of a pebble. They become visible between about 40 and 75 miles (65 and 120 kilometers) above the earth. They disintegrate at altitudes of 30 to 60 miles (50 to 95 kilometers). Meteoroids travel around the sun in a variety of orbits and at various velocities. The fastest ones move at about 26 miles per second (42 kilometers per second). The earth travels at about 18 miles per second (29 kilometers per second). Thus, when meteoroids meet the earth's atmosphere head-on, the combined speed may reach about 44 miles per second (71 kilometers per second). Meteor showers The earth meets a number of streams (trails) or swarms (clusters) of tiny meteoroids at certain times every year. At such times, the sky seems filled with a shower of sparks. Streams and swarms have orbits like those of comets and are believed to be fragments of comets. The most brilliant meteor shower known took place on Nov. 12-13, 1833. It was one of the Leonid showers, which occur every November and seem to come from the direction of the constellation Leo. Meteorites There are three kinds of meteorites, stony, iron, and stony-iron. Stony meteorites consist of minerals rich in silicon and oxygen, with smaller amounts of iron, magnesium, and other elements. One group of stony meteorites, called chondrites, are pieces of the same material from which the planets formed. Another group of stony meteorites, the achondrites, were once part of a parent body, such as an asteroid, that was large enough to have melted and separated into an iron-rich core and a stony crust. Achondrites come from the outer crust; stony-iron meteorites, from the inner crust; and iron meteorites, from the metallic core. Iron meteorites consist mostly of iron and nickel. Stony-iron meteorites have nearly equal amounts of silicon-based stone and iron-nickel metal. The size of meteorites varies greatly. Most of them are relatively small. The largest meteorite ever found weighs about 66 short tons (60 metric tons). It fell at Hoba West, a farm near Grootfontein, Namibia. However, much larger bodies, such as asteroids and comets, can also strike the earth and become meteorites. Meteorites reach the earth's surface because they are the right size to travel through the atmosphere. If they are too small, they will disintegrate in the atmosphere. If they are too large, they may explode before reaching the earth's surface. One such object exploded about 6 miles (10 kilometers) above the Tunguska River in Siberia in 1908, leaving a 20-mile (32-kilometer) area of felled and scorched trees. Thousands of small meteorites have been found in Antarctica, providing a rich supply of specimens for scientists to study. Scientists study meteorites for clues to the types of material that formed the planets. Impact craters and basins When large bodies such as asteroids and comets strike a planet, they produce an or impact basin. Impact craters are bowl-shaped depressions that measure up to about 10 miles (25 kilometers) in diameter. They have shallow, flat floors and uplifted centers. Impact basins are larger, and inside their rims there are one or more rings on the planet's surface. Scientists have found more than 120 impact craters and basins on the earth. One of the most famous, the Meteor Crater in Arizona, is about 4,180 feet (1,275 meters) across and 570 feet (175 meters) deep. It formed nearly 50,000 years ago when an iron meteorite weighing 330,000 short tons (300,000 metric tons) struck the earth. Most impact craters and basins larger than the Meteor Crater are heavily worn away or have been buried by rocks and dirt as the earth's surface changed. The largest known of these is the Chicxulub (CHEEK shoo loob) Basin centered in Mexico's Yucatan Peninsula. The diameter of the basin is about 190 miles (300 kilometers). Rock samples obtained by drilling into the basin indicate that an asteroid struck the earth there about 65 million years ago. This was about the time the last dinosaurs became extinct. The impact hurled much debris into the sky. Many scientists believe this debris caused climate changes that the dinosaurs could not survive. Moon The moon's surface shows striking contrasts of light and dark. The light areas are rugged highlands. The dark zones were partly flooded by lava when volcanoes erupted billions of years ago. The lava froze to form smooth rock. Image credit: Lunar and Planetary Institute Moon is Earth's only natural satellite and the only astronomical body other than Earth ever visited by human beings. The moon is the brightest object in the night sky but gives off no light of its own. Instead, it reflects light from the sun. Like Earth and the rest of the solar system, the moon is about 4.6 billion years old. The moon is much smaller than Earth. The moon's average radius (distance from its center to its surface) is 1,079.6 miles (1,737.4 kilometers), about 27 percent of the radius of Earth. The moon is also much less massive than Earth. The moon has a mass (amount of matter) of 8.10 x 1019 tons (7.35 x 1019 metric tons). Its mass in metric tons would be written out as 735 followed by 17 zeroes. Earth is about 81 times that massive. The moon's density (mass divided by volume) is about 3.34 grams per cubic centimeter, roughly 60 percent of Earth's density. Because the moon has less mass than Earth, the force due to gravity at the lunar surface is only about 1/6 of that on Earth. Thus, a person standing on the moon would feel as if his or her weight had decreased by 5/6. And if that person dropped a rock, the rock would fall to the surface much more slowly than the same rock would fall to Earth. Despite the moon's relatively weak gravitational force, the moon is close enough to Earth to produce tides in Earth's waters. The average distance from the center of Earth to the center of the moon is 238,897 miles (384,467 kilometers). That distance is growing -- but extremely slowly. The moon is moving away from Earth at a speed of about 1 1/2 inches (3.8 centimeters) per year.

The distance to the moon is measured to an accuracy of 5 centimeters by a laser beam sent from Earth. The beam bounces off a laser reflector placed on the moon by astronauts, and returns to Earth. Image credit: World Book diagram by Bensen Studios The temperature at the lunar equator ranges from extremely low to extremely high -- from about -280 degrees F (-173 degrees C) at night to +260 degrees F (+127 degrees C) in the daytime. In some deep craters near the moon's poles, the temperature is always near -400 degrees F (-240 degrees C). The moon has no life of any kind. Compared with Earth, it has changed little over billions of years. On the moon, the sky is black -- even during the day -- and the stars are always visible. A person on Earth looking at the moon with the unaided eye can see light and dark areas on the lunar surface. The light areas are rugged, cratered highlands known as terrae (TEHR ee). The word terrae is Latin for lands. The highlands are the original crust of the moon, shattered and fragmented by the impact of meteoroids, asteroids, and comets. Many craters in the terrae exceed 25 miles (40 kilometers) in diameter. The largest is the South Pole-Aitken Basin, which is 1,550 miles (2,500 kilometers) in diameter. The dark areas on the moon are known as maria (MAHR ee uh). The word maria is Latin for seas; its singular is mare (MAHR ee). The term comes from the smoothness of the dark areas and their resemblance to bodies of water. The maria are cratered landscapes that were partly flooded by lava when volcanoes erupted. The lava then froze, forming rock. Since that time, meteoroid impacts have created craters in the maria. The moon has no substantial atmosphere, but small amounts of certain gases are present above the lunar surface. People sometimes refer to those gases as the lunar atmosphere. This "atmosphere" can also be called an exosphere, defined as a tenuous (low-density) zone of particles surrounding an airless body. Mercury and some asteroids also have an exosphere. In 1959, scientists began to explore the moon with robot spacecraft. In that year, the Soviet Union sent a spacecraft called Luna 3 around the side of the moon that faces away from Earth. Luna 3 took the first photographs of that side of the moon. The word luna is Latin for moon.

The first people on the moon were U.S. astronauts Neil A. Armstrong, who took this picture, and Buzz Aldrin, who is pictured next to a seismograph. A television camera and a United States flag are in the background. Their lunar module, Eagle, stands at the right. Image credit: NASA

On July 20, 1969, the U.S. Apollo 11 lunar module landed on the moon in the first of six Apollo landings. Astronaut Neil A. Armstrong became the first human being to set foot on the moon. In the 1990's, two U.S. robot space probes, Clementine and Lunar Prospector, detected evidence of frozen water at both of the moon's poles. The ice came from comets that hit the moon over the last 2 billion to 3 billion years. The ice apparently has lasted in areas that are always in the shadows of crater rims. Because the ice is in the shade, where the temperature is about -400 degrees F (-240 degrees C), it has not melted and evaporated. This article discusses Moon (The movements of the moon) (Origin and evolution of the moon) (The exosphere of the moon) (Surface features of the moon) (The interior of the moon) (History of moon study). The movements of the moon The moon moves in a variety of ways. For example, it rotates on its axis, an imaginary line that connects its poles. The moon also orbits Earth. Different amounts of the moon's lighted side become visible in phases because of the moon's orbit around Earth. During events called eclipses, the moon is positioned in line with Earth and the sun. A slight motion called libration enables us to see about 59 percent of the moon's surface at different times. Rotation and orbit The moon rotates on its axis once every 29 1/2 days. That is the period from one sunrise to the next, as seen from the lunar surface, and so it is known as a lunar day. By contrast, Earth takes only 24 hours for one rotation. The moon's axis of rotation, like that of Earth, is tilted. Astronomers measure axial tilt relative to a line perpendicular to the ecliptic plane, an imaginary surface through Earth's orbit around the sun. The tilt of Earth's axis is about 23.5 degrees from the perpendicular and accounts for the seasons on Earth. But the tilt of the moon's axis is only about 1.5 degrees, so the moon has no seasons. Another result of the smallness of the moon's tilt is that certain large peaks near the poles are always in sunlight. In addition, the floors of some craters -- particularly near the south pole -- are always in shadow. The moon completes one orbit of Earth with respect to the stars about every 27 1/3 days, a period known as a sidereal month. But the moon revolves around Earth once with respect to the sun in about 29 1/2 days, a period known as a synodic month. A sidereal month is slightly shorter than a synodic month because, as the moon revolves around Earth, Earth is revolving around the sun. The moon needs some extra time to "catch up" with Earth. If the moon started on its orbit from a spot between Earth and the sun, it would return to almost the same place in about 29 1/2 days. A synodic month equals a lunar day. As a result, the moon shows the same hemisphere -- the near side -- to Earth at all times. The other hemisphere -- the far side -- is always turned away from Earth. People sometimes mistakenly use the term dark side to refer to the far side. The moon does have a dark side -- it is the hemisphere that is turned away from the sun. The location of the dark side changes constantly, moving with the terminator, the dividing line between sunlight and dark. The lunar orbit, like the orbit of Earth, is shaped like a slightly flattened circle. The distance between the center of Earth and the moon's center varies throughout each orbit. At perigee (PEHR uh jee), when the moon is closest to Earth, that distance is 225,740 miles (363,300 kilometers). At apogee (AP uh jee), the farthest position, the distance is 251,970 miles (405,500 kilometers). The moon's orbit is elliptical (oval-shaped). Phases As the moon orbits Earth, an observer on Earth can see the moon appear to change shape. It seems to change from a crescent to a circle and back again. The shape looks different from one day to the next because the observer sees different parts of the moon's sunlit surface as the moon orbits Earth. The different appearances are known as the phases of the moon. The moon goes through a complete cycle of phases in a synodic month. The moon has four phases: (1) new moon, (2) first quarter, (3) full moon, and (4) last quarter. When the moon is between the sun and Earth, its sunlit side is turned away from Earth. Astronomers call this darkened phase a new moon. The next night after a new moon, a thin crescent of light appears along the moon's eastern edge. The remaining portion of the moon that faces Earth is faintly visible because of earthshine, sunlight reflected from Earth to the moon. Each night, an observer on Earth can see more of the sunlit side as the terminator, the line between sunlight and dark, moves westward. After about seven days, the observer can see half a full moon, commonly called a half moon. This phase is known as the first quarter because it occurs one quarter of the way through the synodic month. About seven days later, the moon is on the side of Earth opposite the sun. The entire sunlit side of the moon is now visible. This phase is called a full moon. About seven days after a full moon, the observer again sees a half moon. This phase is the last quarter, or third quarter. After another seven days, the moon is between Earth and the sun, and another new moon occurs. As the moon changes from new moon to full moon, and more and more of it becomes visible, it is said to be waxing. As it changes from full moon to new moon, and less and less of it can be seen, it is waning. When the moon appears smaller than a half moon, it is called crescent. When it looks larger than a half moon, but is not yet a full moon, it is called gibbous (GIHB uhs). Like the sun, the moon rises in the east and sets in the west. As the moon progresses through its phases, it rises and sets at different times. In the new moon phase, it rises with the sun and travels close to the sun across the sky. Each successive day, the moon rises an average of about 50 minutes later. Eclipses occur when Earth, the sun, and the moon are in a straight line, or nearly so. A lunar eclipse occurs when Earth gets directly -- or almost directly -- between the sun and the moon, and Earth's shadow falls on the moon. A lunar eclipse can occur only during a full moon. A solar eclipse occurs when the moon gets directly -- or almost directly -- between the sun and Earth, and the moon's shadow falls on Earth. A solar eclipse can occur only during a new moon. During one part of each lunar orbit, Earth is between the sun and the moon; and, during another part of the orbit, the moon is between the sun and Earth. But in most cases, the astronomical bodies are not aligned directly enough to cause an eclipse. Instead, Earth casts its shadow into space above or below the moon, or the moon casts its shadow into space above or below Earth. The shadows extend into space in that way because the moon's orbit is tilted about 5 degrees relative to Earth's orbit around the sun. Libration People on Earth can sometimes see a small part of the . That part is visible because of lunar libration, a slight rotation of the moon as viewed from Earth. There are three kinds of libration: (1) libration in longitude, (2) diurnal (daily) libration, and (3) libration in latitude. Over time, viewers can see more than 50 percent of the moon's surface. Because of libration, about 59 percent of the lunar surface is visible from Earth. Libration in longitude occurs because the moon's orbit is elliptical. As the moon orbits Earth, its speed varies according to a law discovered in the 1600's by the German astronomer Johannes Kepler. When the moon is relatively close to Earth, the moon travels more rapidly than its average speed. When the moon is relatively far from Earth, the moon travels more slowly than average. But the moon always rotates about its own axis at the same rate. So when the moon is traveling more rapidly than average, its rotation is too slow to keep all of the near side facing Earth. And when the moon is traveling more slowly than average, its rotation is too rapid to keep all of the near side facing Earth.

Diurnal libration enables an observer on Earth to see around one edge of the moon, then the other, during a single night. The libration occurs because Earth's rotation changes the observer's viewpoint by a distance equal to the diameter of Earth. Image credit: World Book illustration

Diurnal libration is caused by a daily change in the position of an observer on Earth relative to the moon. Consider an observer who is at Earth's equator when the moon is full. As Earth rotates from west to east, the observer first sees the moon when it rises at the eastern horizon and last sees it when it sets at the western horizon. During this time, the observer's viewpoint moves about 7,900 miles (12,700 kilometers) -- the diameter of Earth -- relative to the moon. As a result, the moon appears to rotate slightly to the west. While the moon is rising in the east and climbing to its highest point in the sky, the observer can see around the western edge of the near side. As the moon descends to the western horizon, the observer can see around the eastern edge of the near side. Libration in latitude occurs because the moon's axis of rotation is tilted about 6 1/2 degrees relative to a line perpendicular to the moon's orbit around Earth. Thus, during each lunar orbit, the moon's north pole tilts first toward Earth, then away from Earth. When the lunar north pole is tilted toward Earth, people on Earth can see farther than normal along the top of the moon. When that pole is tilted away from Earth, people on Earth can see farther than normal along the bottom of the moon. Origin and evolution of the moon Scientists believe that the moon formed as a result of a collision known as the Giant Impact or the "Big Whack." According to this idea, Earth collided with a planet-sized object 4.6 billion years ago. As a result of the impact, a cloud of vaporized rock shot off Earth's surface and went into orbit around Earth. The cloud cooled and condensed into a ring of small, solid bodies, which then gathered together, forming the moon. The rapid joining together of the small bodies released much energy as heat. Consequently, the moon melted, creating an "ocean" of magma (melted rock). The magma ocean slowly cooled and solidified. As it cooled, dense, iron-rich materials sank deep into the moon. Those materials also cooled and solidified, forming the mantle, the layer of rock beneath the crust. As the crust formed, asteroids bombarded it heavily, shattering and churning it. The largest impacts may have stripped off the entire crust. Some collisions were so powerful that they almost split the moon into pieces. One such collision created the South Pole-Aitken Basin, one of the largest known impact craters in the solar system. A basalt rock that astronauts brought to Earth from the moon formed from lava that erupted from a lunar volcano. Escaping gases created the holes before the lava solidified into rock. Image credit: Lunar and Planetary Institute About 4 billion to 3 billion years ago, melting occurred in the mantle, probably caused by radioactive elements deep in the moon's interior. The resulting magma erupted as dark, iron-rich lava, partly flooding the heavily cratered surface. The lava cooled and solidified into rocks known as basalts (buh SAWLTS). Small eruptions may have continued until as recently as 1 billion years ago. Since that time, only an occasional impact by an asteroid or comet has modified the surface. Because the moon has no atmosphere to burn up meteoroids, the bombardment continues to this day. However, it has become much less intense. Impacts of large objects can create craters. Impacts of micrometeoroids (tiny meteoroids) grind the surface rocks into a fine, dusty powder known as the regolith (REHG uh lihth). Regolith overlies all the bedrock on the moon. Because regolith forms as a result of exposure to space, the longer a rock is exposed, the thicker the regolith that forms on it. The exosphere of the moon The lunar exosphere -- that is, the materials surrounding the moon that make up the lunar "atmosphere" -- consists mainly of gases that arrive as the solar wind. The solar wind is a continuous flow of gases from the sun -- mostly hydrogen and helium, along with some neon and argon. The remainder of the gases in the exosphere form on the moon. A continual "rain" of micrometeoroids heats lunar rocks, melting and vaporizing their surface. The most common atoms in the vapor are atoms of sodium and potassium. Those elements are present in tiny amounts -- only a few hundred atoms of each per cubic centimeter of exosphere. In addition to vapors produced by impacts, the moon also releases some gases from its interior. Most gases of the exosphere concentrate about halfway between the equator and the poles, and they are most plentiful just before sunrise. The solar wind continuously sweeps vapor into space, but the vapor is continuously replaced. During the night, the pressure of gases at the lunar surface is about 3.9 x 10-14 pound per square inch (2.7 x 10-10 pascal). That is a stronger vacuum than laboratories on Earth can usually achieve. The exosphere is so tenuous -- that is, so low in density -- that the rocket exhaust released during each Apollo landing temporarily doubled the total mass of the entire exosphere. The surface of the moon is covered with bowl-shaped holes called craters, shallow depressions called basins, and broad, flat plains known as maria. A powdery dust called the regolith overlies much of the surface of the moon. Craters Crater has central peaks and slumped walls. The peaks almost certainly formed quickly after the impact that produced the crater compressed the ground. The ground rebounded upward, forming the peaks. The crater walls are slumped because the original walls were too steep to withstand the force of gravity. Material fell inward, away from the walls. This crater, in Mare Imbrium (Sea of Rains), is about 17 1/2 miles (28 kilometers) across. Image credit: Lunar and Planetary Institute The vast majority of the moon's craters are formed by the impact of meteoroids, asteroids, and comets. Craters on the moon are named for famous scientists. For example, Copernicus Crater is named for Nicolaus Copernicus, a Polish astronomer who realized in the 1500's that the planets move about the sun. Archimedes Crater is named for the Greek mathematician Archimedes, who made many mathematical discoveries in the 200's B.C. The shape of craters varies with their size. Small craters with diameters of less than 6 miles (10 kilometers) have relatively simple bowl shapes. Slightly larger craters cannot maintain a bowl shape because the crater wall is too steep. Material falls inward from the wall to the floor. As a result, the walls become scalloped and the floor becomes flat. Still larger craters have terraced walls and central peaks. Terraces inside the rim descend like stairsteps to the floor. The same process that creates wall scalloping is responsible for terraces. The central peaks almost certainly form as did the central peaks of impact craters on Earth. Studies of the peaks on Earth show that they result from a deformation of the ground. The impact compresses the ground, which then rebounds, creating the peaks. Material in the central peaks of may come from depths as great as 12 miles (19 kilometers). Surrounding the craters is rough, mountainous material -- crushed and broken rocks that were ripped out of the crater cavity by shock pressure. This material, called the crater ejecta blanket, can extend about 60 miles (100 kilometers) from the crater. Farther out are patches of debris and, in many cases, irregular secondary craters, also known as secondaries. Those craters come in a range of shapes and sizes, and they are often clustered in groups or aligned in rows. Secondaries form when material thrown out of the primary (original) crater strikes the surface. This material consists of large blocks, clumps of loosely joined rocks, and fine sprays of ground-up rock. The material may travel thousands of miles or kilometers. Crater rays are light, wispy deposits of powder that can extend thousands of miles or kilometers from the crater. Rays slowly vanish as micrometeoroid bombardment mixes the powder into the upper surface layer. Thus, craters that still have visible rays must be among the youngest craters on the moon. Craters larger than about 120 miles (200 kilometers) across tend to have central mountains. Some of them also have inner rings of peaks, in addition to the central peak. The appearance of a ring signals the next major transition in crater shape -- from crater to basin. Basins are craters that are 190 miles (300 kilometers) or more across. The smaller basins have only a single inner ring of peaks, but the larger ones typically have multiple rings. The rings are concentric -- that is, they all have the same center, like the rings of a dartboard. The spectacular, multiple-ringed basin called the Eastern Sea (Mare Orientale) is almost 600 miles (1,000 kilometers) across. Other basins can be more than 1,200 miles (2,000 kilometers) in diameter -- as large as the entire western United States. Basins occur equally on the near side and far side. Most basins have little or no fill of basalt, particularly those on the far side. The difference in filling may be related to variations in the thickness of the crust. The far side has a thicker crust, so it is more difficult for molten rock to reach the surface there. In the highlands, the overlying ejecta blankets of the basins make up most of the upper few miles or kilometers of material. Much of this material is a large, thick layer of shattered and crushed rock known as breccia (BREHCH ee uh). Scientists can learn about the original crust by studying tiny fragments of breccia. Maria, the dark areas on the surface of the moon, make up about 16 percent of the surface area. Some maria are named in Latin for weather terms -- for example, Mare Imbrium (Sea of Rains) and Mare Nubium (Sea of Clouds). Others are named for states of mind, as in Mare Serenitatus (Sea of Serenity) and Mare Tranquillitatis (Sea of Tranquility). Landforms on the maria tend to be smaller than those of the highlands. The small size of mare features relates to the scale of the processes that formed them -- volcanic eruptions and crustal deformation, rather than large impacts. The chief landforms on the maria include wrinkle ridges and rilles and other volcanic features. Wrinkle ridges are blisterlike humps that wind across the surface of almost all maria. The ridges are actually broad folds in the rocks, created by compression. Many wrinkle ridges are roughly circular, aligned with small peaks that stick up through the maria and outlining interior rings. Circular ridge systems also outline buried features, such as rims of craters that existed before the maria formed.

A lunar rover is parked near the edge of Hadley Rille, a long channel probably formed by lava 4 billion to 3 billion years ago. The slopes in the background are part of a formation called the Swann Hills. This photo was taken during the Apollo 15 mission in 1971. Astronaut David R. Scott is reaching under a seat to get a camera. Image credit: NASA

Rilles are snakelike depressions that wind across many areas of the maria. Scientists formerly thought the rilles might be ancient riverbeds. However, they now suspect that the rilles are channels formed by running lava. One piece of evidence favoring this view is the dryness of rock samples brought to Earth by Apollo astronauts; the samples have almost no water in their molecular structure. In addition, detailed photographs show that the rilles are shaped somewhat like channels created by flowing lava on Earth. Volcanic features Scattered throughout the maria are a variety of other features formed by volcanic eruptions. Within Mare Imbrium, scarps (lines of cliffs) wind their way across the surface. The scarps are lava flow fronts, places where lava solidified, enabling lava that was still molten to pile up behind them. The presence of the scarps is one piece of evidence indicating that the maria consist of solidified basaltic lava. Small hills and domes with pits on top are probably little volcanoes. Both dome-shaped and cone- shaped volcanoes cluster together in many places, as on Earth. One of the largest concentrations of cones on the moon is the Marius Hills complex in Oceanus Procellarum (Ocean of Storms). Within this complex are numerous wrinkle ridges and rilles, and more than 50 volcanoes. Large areas of maria and terrae are covered by dark material known as dark mantle deposits. Evidence collected by the Apollo missions confirmed that dark mantling is volcanic ash. Much smaller dark mantles are associated with small craters that lie on the fractured floors of large craters. Those mantles may be cinder cones -- low, broad, cone-shaped hills formed by explosive volcanic eruptions. The interior of the moon The moon, like Earth, has three interior zones -- crust, mantle, and core. However, the composition, structure, and origin of the zones on the moon are much different from those on Earth. Most of what scientists know about the interior of Earth and the moon has been learned by studying seismic events -- earthquakes and moonquakes, respectively. The data on moonquakes come from scientific equipment set up by Apollo astronauts from 1969 to 1972. Crust The average thickness of the lunar crust is about 43 miles (70 kilometers), compared with about 6 miles (10 kilometers) for Earth's crust. The outermost part of the moon's crust is broken, fractured, and jumbled as a result of the large impacts it has endured. This shattered zone gives way to intact material below a depth of about 6 miles. The bottom of the crust is defined by an abrupt increase in rock density at a depth of about 37 miles (60 kilometers) on the near side and about 50 miles (80 kilometers) on the far side. Mantle The mantle of the moon consists of dense rocks that are rich in iron and magnesium. The mantle formed during the period of global melting. Low-density minerals floated to the outer layers of the moon, while dense minerals sank deeper into it. Later, the mantle partly melted due to a build-up of heat in the deep interior. The source of the heat was probably the decay (breakup) of uranium and other radioactive elements. This melting produced basaltic magmas -- bodies of molten rock. The magmas later made their way to the surface and erupted as the mare lavas and ashes. Although mare volcanism occurred for more than 1 billion years -- from at least 4 billion years to fewer than 3 billion years ago -- much less than 1 percent of the volume of the mantle ever remelted. Core Data gathered by Lunar Prospector confirmed that the moon has a core and enabled scientists to estimate its size. The core has a radius of only about 250 miles (400 kilometers). By contrast, the radius of Earth's core is about 2,200 miles (3,500 kilometers). The lunar core has less than 1 percent of the mass of the moon. Scientists suspect that the core consists mostly of iron, and it may also contain large amounts of sulfur and other elements. Earth's core is made mostly of molten iron and nickel. This rapidly rotating molten core is responsible for Earth's magnetic field. A magnetic field is an influence that a magnetic object creates in the region around it. If the core of a planet or a satellite is molten, motion within the core caused by the rotation of the planet or satellite makes the core magnetic. But the small, partly molten core of the moon cannot generate a global magnetic field. However, small regions on the lunar surface are magnetic. Scientists are not sure how these regions acquired magnetism. Perhaps the moon once had a larger, more molten core. There is evidence that the lunar interior formerly contained gas, and that some gas may still be there. Basalt from the moon contains holes called vesicles that are created during a volcanic eruption. On Earth, gas that is dissolved in magma comes out of solution during an eruption, much as carbon dioxide comes out of a carbonated beverage when you shake the drink container. The presence of vesicles in lunar basalt indicates that the deep interior contained gases, probably carbon monoxide or gaseous sulfur. The existence of volcanic ash is further evidence of interior gas; on Earth, volcanic eruptions are largely driven by gas. History of moon study Ancient ideas Some ancient peoples believed that the moon was a rotating bowl of fire. Others thought it was a mirror that reflected Earth's lands and seas. But philosophers in ancient Greece understood that the moon is a sphere in orbit around Earth. They also knew that moonlight is reflected sunlight. Some Greek philosophers believed that the moon was a world much like Earth. In about A.D. 100, Plutarch even suggested that people lived on the moon. The Greeks also apparently believed that the dark areas of the moon were seas, while the bright regions were land. In about A.D. 150, Ptolemy, a Greek astronomer who lived in Alexandria, Egypt, said that the moon was Earth's nearest neighbor in space. He thought that both the moon and the sun orbited Earth. Ptolemy's views survived for more than 1,300 years. But by the early 1500's, the Polish astronomer Nicolaus Copernicus had developed the correct view -- Earth and the other planets revolve about the sun, and the moon orbits Earth. Early observations with telescopes The Italian astronomer and physicist Galileo wrote the first scientific description of the moon based on observations with a telescope. In 1609, Galileo described a rough, mountainous surface. This description was quite different from what was commonly believed -- that the moon was smooth. Galileo noted that the light regions were rough and hilly and the dark regions were smoother plains. The presence of high mountains on the moon fascinated Galileo. His detailed description of a large crater in the central highlands -- probably Albategnius -- began 350 years of controversy and debate about the origin of the "holes" on the moon. Other astronomers of the 1600's mapped and cataloged every surface feature they could see. Increasingly powerful telescopes led to more detailed records. In 1645, the Dutch engineer and astronomer Michael Florent van Langren, also known as Langrenus, published a map that gave names to the surface features of the moon, mostly its craters. A map drawn by the Bohemian-born Italian astronomer Anton M. S. de Rheita in 1645 correctly depicted the bright ray systems of the craters Tycho and Copernicus. Another effort, by the Polish astronomer Johannes Hevelius in 1647, included the moon's libration zones. By 1651, two Jesuit scholars from Italy, the astronomer Giovanni Battista Riccioli and the mathematician and physicist Francesco M. Grimaldi, had completed a map of the moon. That map established the naming system for lunar features that is still in use. Determining the origin of craters Until the late 1800's, most astronomers thought that volcanism formed the craters of the moon. However, in the 1870's, the English astronomer Richard A. Proctor proposed correctly that the craters result from the collision of solid objects with the moon. But at first, few scientists accepted Proctor's proposal. Most astronomers thought that the moon's craters must be volcanic in origin because no one had yet described a crater on Earth as an impact crater, but scientists had found dozens of obviously volcanic craters. In 1892, the American geologist Grove Karl Gilbert argued that most lunar craters were impact craters. He based his arguments on the large size of some of the craters. Those included the basins, which he was the first to recognize as huge craters. Gilbert also noted that lunar craters have only the most general resemblance to calderas (large volcanic craters) on Earth. Both lunar craters and calderas are large circular pits, but their structural details do not resemble each other in any way. In addition, Gilbert created small craters experimentally. He studied what happened when he dropped clay balls and shot bullets into clay and sand targets. Gilbert was the first to recognize that the circular Mare Imbrium was the site of a gigantic impact. By examining photographs, Gilbert also determined which nearby craters formed before and after that event. For example, a crater that is partially covered by ejecta from the Imbrium impact formed before the impact. A crater within the mare formed after the impact. Describing lunar evolution Gilbert suggested that scientists could determine the relative age of surface features by studying the ejecta of the Imbrium impact. That suggestion was the key to unraveling the history of the moon. Gilbert recognized that the moon is a complex body that was built up by innumerable impacts over a long period. In his book The Face of the Moon (1949), the American astronomer and physicist Ralph B. Baldwin further described lunar evolution. He noted the similarity in form between craters on the moon and bomb craters created during World War II (1939-1945) and concluded that lunar craters form by impact. Baldwin did not say that every lunar feature originated with an impact. He stated correctly that the maria are solidified flows of basalt lava, similar to flood lava plateaus on Earth. Finally, independently of Gilbert, he concluded that all circular maria are actually huge impact craters that later filled with lava. In the 1950's, the American chemist Harold C. Urey offered a contrasting view of lunar history. Urey said that, because the moon appears to be cold and rigid, it has always been so. He then stated -- correctly -- that craters are of impact origin. However, he concluded falsely that the maria are blankets of debris scattered by the impacts that created the basins. And he was mistaken in concluding that the moon never melted to any significant extent. Urey had won the 1934 Nobel Prize in chemistry and had an outstanding scientific reputation, so many people took his views seriously. Urey strongly favored making the moon a focus of scientific study. Although some of his ideas were mistaken, his support of moon study was a major factor in making the moon an early goal of the U.S. space program. In 1961, the U.S. geologist Eugene M. Shoemaker founded the Branch of Astrogeology of the U.S. Geological Survey (USGS). Astrogeology is the study of celestial objects other than Earth. Shoemaker showed that the moon's surface could be studied from a geological perspective by recognizing a sequence of relative ages of rock units near the crater Copernicus on the near side. Shoemaker also studied the Meteor Crater in Arizona and documented the impact origin of this feature. In preparation for the Apollo missions to the moon, the USGS began to map the geology of the moon using telescopes and pictures. This work gave scientists their basic understanding of lunar evolution. Apollo missions Beginning in 1959, the Soviet Union and the United States sent a series of robot spacecraft to examine the moon in detail. Their ultimate goal was to land people safely on the moon. The United States finally reached that goal in 1969 with the landing of the Apollo 11 lunar module. The United States conducted six more Apollo missions, including five landings. The last of those was Apollo 17, in December 1972. The Apollo missions revolutionized the understanding of the moon. Much of the knowledge gained about the moon also applies to Earth and the other inner planets -- Mercury, Venus, and Mars. Scientists learned, for example, that impact is a fundamental geological process operating on the planets and their satellites. After the Apollo missions, the Soviets sent four Luna robot craft to the moon. The last, Luna 24, returned samples of lunar soil to Earth in August 1976. Recent exploration The Clementine orbiter used radar signals to find evidence of a large deposit of frozen water on the moon. The orbiter sent radar signals to various target points on the lunar surface. The targets reflected some of the signals to Earth, where they were received by large antennas and analyzed. Image credit: Lunar and Planetary Institute No more spacecraft went to the moon until January 1994, when the United States sent the orbiter Clementine. From February to May of that year, Clementine's four cameras took more than 2 million pictures of the moon. A laser device measured the height and depth of mountains, craters, and other features. Radar signals that Clementine bounced off the moon provided evidence of a large deposit of frozen water. The ice appeared to be inside craters at the south pole. The U.S. probe Lunar Prospector orbited the moon from January 1998 to July 1999. The craft mapped the concentrations of chemical elements in the moon, surveyed the moon's magnetic fields, and found strong evidence of ice at both poles. Small particles of ice are apparently part of the regolith at the poles. The SMART-1 spacecraft, launched by the European Space Agency in 2003, went into orbit around the moon in 2004. The craft's instruments were designed to investigate the moon's origin and conduct a detailed survey of the chemical elements on the lunar surface.

Nebula

The Great Nebula in the constellation Orion is a huge cloud of dust and gas that appears as a misty spot in Orion's sword. The nebula's bright central area is shown in this photograph. Image credit: National Optical Astronomy Observatories A nebula (NEHB yuh luh) is a cloud of dust particles and gases in space. The term nebula comes from the Latin word for cloud. Early astronomers also used the term for distant galaxies outside the earth's galaxy, the Milky Way. Such galaxies, called extragalactic nebulae, looked like hazy patches of light among the stars. But modern telescopes showed that extragalactic nebulae are actually systems of stars similar to the Milky Way. Today, most astronomers use the term nebulae only for the clouds of dust and gases in the Milky Way and other galaxies. They classify these masses into two general types: diffuse nebulae and planetary nebulae. Both types are also called gaseous nebulae. Diffuse nebulae are the larger of the two types. Some diffuse nebulae contain enough dust and gases to form as many as 100,000 stars the size of the sun. A diffuse nebula may occur near an extremely hot, bright star. The intense ultraviolet light from the star energizes the gas atoms of the nebula and enables the mass to emit light. A diffuse nebula of this kind is called an emission nebula. Astronomers believe some emission nebulae are places where new stars are forming. Gravity causes a portion of a nebula's dust and gases to contract into a much smaller, denser mass. Pressure and temperature build up within the mass of dust and gases as contraction continues for millions of years. In time, the mass becomes hot enough to shine -- and forms a new star. A with an unusual textured appearance, the cause of which is unknown. This photo was taken by the Hubble Space Telescope. Image credit: NASA A diffuse nebula also may occur near a cool star. In this case, the ultraviolet light from the star is too weak to make the nebula's gas atoms give off light. But the dust particles in the diffuse nebula reflect the starlight. Astronomers refer to this kind of diffuse nebula as a reflection nebula. If a diffuse nebula occurs in an area that has no nearby stars, it neither emits nor reflects enough light to be visible. In fact, its dust particles blot out the light from the stars behind them. Astronomers call such a diffuse nebula a . Planetary nebulae are ball-like clouds of dust and gases that surround certain stars. They form when a star begins to collapse and throw off the outer layers of its atmosphere. When viewed through a small telescope, this type of nebula appears to have a flat, rounded surface like that of a planet. Neptune The blue clouds of Neptune are mostly frozen methane, the main chemical in natural gas -- a fuel for heating and cooking on Earth. The other object shown is Neptune's moon Triton. Image credit: NASA/JPL

Neptune is one of the two planets that cannot be seen without a telescope. The other is Pluto. Neptune is about 30 times as far from the sun as is Earth. Pluto is the only planet farther from the sun than Neptune. But every 248 years Pluto moves inside Neptune's orbit for about a 20-year period, during which it is closer to the sun than Neptune. Pluto last crossed Neptune's orbit on Jan. 23, 1979, and remained within it until Feb. 11, 1999. Neptune's diameter at the equator is 30,775 miles (49,528 kilometers), or almost 4 times that of Earth. It is about 17 times as massive (heavy) as Earth, but is not so dense as Earth. Neptune has 11 satellites (moons) and several rings around it. Neptune travels around the sun in an elliptical (oval-shaped) orbit. Its average distance from the sun is about 2,793,100 miles (4,495,060,000 kilometers). Neptune goes around the sun once about every 165 Earth years, compared with once a year for Earth. As Neptune orbits the sun, it spins on its axis, an imaginary line through its center. Neptune's axis is not perpendicular (at an angle of 90 degrees) to the planet's path around the sun. The axis tilts about 28 degrees from the perpendicular position. Neptune spins around once in about 16 hours and 7 minutes.

Bright blue clouds that surround the planet Neptune consist mainly of frozen methane. Winds that carry these clouds may reach speeds up to 700 miles (1,100 kilometers) per hour. Image credit: NASA/JPL Surface and atmosphere Scientists believe that Neptune is made up chiefly of hydrogen, helium, water, and silicates. Silicates are the minerals that make up most of Earth's rocky crust, though Neptune does not have a solid surface like Earth. Thick clouds cover Neptune's surface. Its interior begins with a region of heavily compressed gases. Deep in the interior, these gases blend into a liquid layer that surrounds the planet's central core of rock and ice. The tilt of its axis causes the sun to heat the Neptune's northern and southern halves alternately, resulting in seasons and temperature changes. Neptune is surrounded by thick layers of clouds in rapid motion. Winds blow these clouds at speeds up to 700 miles (1,100 kilometers) per hour. The clouds farthest from Neptune's surface consist mainly of frozen methane. Scientists believe that Neptune's darker clouds, which lie below the clouds of methane, are composed of hydrogen sulfide. In 1989, the Voyager 2 spacecraft found that Neptune had a dark area made up of violently swirling masses of gas resembling a hurricane. This area, called the Great Dark Spot, was similar to the Great Red Spot on Jupiter. But in 1994, the Hubble Space Telescope found that the Great Dark Spot had vanished.

The icy crust of Triton, Neptune's largest satellite, has ridges and valleys that were revealed in photographs taken by the U.S. space probe Voyager 2. Image credit: NASA/JPL Satellites and rings Neptune has 11 known satellites. Triton, Neptune's largest satellite, is about 1,681 miles (2,705 kilometers) in diameter and about 220,440 miles (354,760 kilometers) from Neptune. It is the only major satellite in the solar system that orbits in a direction opposite to that of its planet. Triton has a circular orbit and travels once around Neptune every six days. Triton may once have been a large comet that traveled around the sun. At some point, Neptune's gravity probably captured the comet, and it became a satellite of Neptune. Scientists have discovered evidence that volcanoes on Triton once spewed a slushy mixture of water and ammonia. This mixture is now frozen on Triton's surface. Triton has a surface temperature of -390 degrees F (-235 degrees C), the coldest known temperature in the solar system. Some volcanoes on Triton remain active, shooting crystals of nitrogen ice as high as 6 miles (10 kilometers) above the moon's surface.

In Neptune's outermost ring, 39,000 miles (63,000 kilometers) from the planet, material mysteriously clumps into three bright, dense arcs. Image credit: NASA Neptune has three conspicuous rings and one faint ring. All of these rings are much fainter and darker than the rings of Saturn. They appear to consist of particles of dust. Neptune's outer ring is unlike any other planetary ring in the solar system. It has three curved segments that are brighter and denser than the rest of the ring. Scientists do not know why the dust is spread unevenly in the ring. Discovery Neptune was discovered by means of mathematics before being seen through a telescope. Astronomers had noticed that Uranus, which they thought was the most distant planet, was not always in the position they predicted for it. The force of gravity of some unknown planet seemed to be influencing Uranus. In 1843, John C. Adams, a young English astronomer and mathematician, began working to find the location of the unknown planet. Adams predicted the planet would be about 1 billion miles (1.6 billion kilometers) farther from the sun than Uranus. He completed his remarkably accurate work in September 1845. Adams sent it to Sir George B. Airy, the Astronomer Royal of England. However, Airy did not look for the planet with a telescope. Apparently, he lacked confidence in Adams. Meanwhile, Urbain J. J. Leverrier, a young French mathematician unknown to Adams, began working on the project. By mid-1846, Leverrier also had predicted Neptune's position. He sent his predictions, which were similar to those of Adams, to the Urania Observatory in , Germany. Johann G. Galle, the director of the observatory, had just charted the fixed stars in the area where the planet was believed to be. On Sept. 23, 1846, Galle and his assistant, Heinrich L. d'Arrest, found Neptune near the position predicted by Leverrier. Today, both Adams and Leverrier are credited with the discovery. The planet was named for Neptune, the Roman sea god. In August 1989, the Voyager 2 spacecraft provided the first close-up views of Neptune and most of its moons. The spacecraft also discovered the planet's rings and six of its moons -- Despina, Galatea, Larissa, Naiad, Proteus, and Thalassa.

Ozone Ozone (OH zohn) is a gas that is present in small amounts in Earth's atmosphere. In the troposphere (the lowest level of the atmosphere), ozone is a pollutant. It can harm plant and animal tissues, and it can damage rubber and plastic. Ozone in the stratosphere (the layer above the troposphere) blocks harmful ultraviolet rays from the sun, protecting life on Earth. Every spring in the Southern Hemisphere since the late 1970's, scientists have observed a depletion of the ozone layer over Antarctica. The region of decreased ozone is known as the ozone hole. Ozone is related to the oxygen molecules that sustain life. An oxygen molecule has two oxygen atoms and the chemical formula O2. An ozone molecule has three oxygen atoms and the formula O3. Pure ozone is a pale blue gas. The word ozone comes from a Greek word meaning to smell, reflecting ozone's sharp, irritating odor. Ozone occurs naturally through photochemical reactions and by electrical discharges. In photochemical reactions in the stratosphere, ultraviolet rays from the sun strike oxygen molecules, breaking each molecule into two oxygen atoms. The oxygen atoms combine with other oxygen molecules, forming ozone. Electrical discharges include lightning and sparks from motors. Such discharges can break up oxygen molecules, leading to ozone formation. Near Earth's surface, reactions between such gases as nitrogen oxides and hydrocarbons create ozone and other ingredients of a dangerous pollutant called photochemical smog. Commercial applications of ozone include the bleaching of pulp in paper mills and the purification of water. Manufacturers produce ozone by creating electrical discharges in a machine. Ozone's relative molecular mass (formerly called molecular weight) -- that is, its amount of matter compared with that of the most common form of carbon -- is 47.998. The German chemist Christian Friedrich Schonbein discovered ozone in 1840.

Planets A planet is a large, round heavenly body that orbits a star and shines with light reflected from the star. Eight planets orbit the sun in our solar system. In order of increasing distance from the sun, they are: (1) Mercury, (2) Venus, (3) Earth, (4) Mars, (5) Jupiter, (6) Saturn, (7) Uranus, and (8) Neptune. Many nearly planet-sized objects, called dwarf planets, also orbit the sun. Dwarf planets include Pluto and Ceres. Since 1992, astronomers have discovered many planets orbiting other stars. The sun blazes with energy. On its surface, magnetic forces create loops and streams of gas that extend tens of thousands of miles or kilometers into space. This image was made by photographing ultraviolet radiation given off by atoms of iron gas that are hotter than 9 million degrees F (5 million degrees C). Image credit: NASA/Transition Region & Coronal Explorer Traditionally, the term planet has had no formal definition in astronomy. Millions of objects orbit the sun—the most basic characteristic of a planet. But scholars have struggled to devise a simple classification system that distinguishes the smallest worlds from the largest comets, asteroids, and other bodies. The International Astronomical Union (IAU), the recognized authority in naming heavenly bodies, divides objects that orbit the sun into three major classes: (1) planets, (2) dwarf planets, and (3) small solar system bodies. A planet orbits the sun and no other body. It has so much mass (amount of matter) that its own gravitational pull compacts it into a round shape. In addition, a planet has a strong enough gravitational pull to sweep the region of its orbit relatively free of other objects. A dwarf planet also orbits the sun and is large enough to be round. However, it does not have a strong enough gravitational pull to clear the region of its orbit. Small solar system bodies, including most asteroids and comets, have too little mass for gravity to round their irregular shapes. Many planets, dwarf planets, and other bodies have smaller objects orbiting them called satellites or moons. The planets in our solar system can be divided into two groups. The innermost four planets—Mercury, Venus, Earth, and Mars—are small, rocky worlds. They are called the terrestrial (earthlike) planets, from the Latin word for Earth, terra. Earth is the largest terrestrial planet. The other Earthlike planets have from 38 to 95 percent of Earth's diameter and from 5.5 to 82 percent of Earth's mass. The outer four planets—Jupiter, Saturn, Uranus, and Neptune—are called gas giants or Jovian (Jupiterlike) planets. They have gaseous atmospheres and no solid surfaces. All four Jovian planets consist mainly of hydrogen and helium. Smaller amounts of other materials also occur, including traces of ammonia and methane in their atmospheres. They range from 3.9 times to 11.2 times Earth's diameter and from 15 times to 318 times Earth's mass. Jupiter, Saturn, and Neptune give off more energy than they receive from the sun. Most of this extra energy takes the form of infrared radiation, which is felt as heat, instead of visible light. Scientists think the source of some of the energy is probably the slow compression of the planets by their own gravity. From its discovery in 1930, Pluto was generally considered a planet. However, its small size and irregular orbit led many astronomers to question whether Pluto should be grouped with worlds such as Earth and Jupiter. Pluto more closely resembles other icy objects found in a region of the outer solar system called the Kuiper belt. In the early 2000’s, astronomers found several such Kuiper belt objects (KBO’s) comparable in size to Pluto. The IAU created the “dwarf planetâ€ classification to describe Pluto and other nearly planet-sized objects. Observing the planets People have known the inner six planets of our solar system for thousands of years because they are visible from Earth without a telescope. The outermost three planets—Uranus and Neptune—were discovered by astronomers, beginning in the 1780's. These planets can be seen from Earth with a telescope. To the unaided eye, the planets look much like the background stars in the night sky. However, the planets move slightly from night to night in relation to the stars. The name planet comes from a Greek word meaning to wander. The planets and the moon follow the same apparent path through the sky. This path, known as the zodiac, is about 16° wide. At its center is the ecliptic, the apparent path of the sun. If you see a bright object near the ecliptic at night or near sunrise or sunset, it is most likely a planet. You can even see the brightest planets in the daytime, if you know where to look. Planets and stars also differ in the steadiness of their light when viewed from Earth's surface. Planets shine with a steady light, but stars seem to twinkle. The twinkling is due to the moving layers of air that surround Earth. Stars are so far away that they are mere points of light in the sky, even when viewed through a telescope. The atmosphere bends the starlight passing through it. As small regions of the atmosphere move about, the points of light seem to dance and change in brightness. Planets, which are much closer, look like tiny disks through a telescope. The atmosphere scatters light from different points on a planet's disk. However, enough light always arrives from a sufficient number of points to provide a steady appearance. Orbits Viewed from Earth's surface, the planets of the solar system and the stars appear to move around Earth. They rise in the east and set in the west each night. Most of the time, the planets move westward across the sky slightly more slowly than the stars do. As a result, the planets seem to drift eastward relative to the background stars. This motion is called prograde. For a while each year, however, the planets seem to reverse their direction. This backward motion is called retrograde. In ancient times, most scientists thought that the moon, sun, planets, and stars actually moved around Earth. One puzzle that ancient scientists struggled to explain was the annual retrograde motion of the planets. In about A.D. 150, the Greek astronomer Ptolemy developed a theory that the planets orbited in small circles, which in turn orbited Earth in larger circles. Ptolemy thought that retrograde motion was caused by a planet moving on its small circle in an opposite direction from the motion of the small circle around the big circle. In 1543, the Polish astronomer Nicolaus Copernicus showed that the sun is the center of the orbits of the planets. Our term solar system is based on Copernicus's discovery. Copernicus realized that retrograde motion occurs because Earth moves faster in its orbit than the planets that are farther from the sun. The planets that are closer to the sun move faster in their orbits than Earth travels in its orbit. Retrograde motion occurs whenever Earth passes an outer planet traveling around the sun or an inner planet passes Earth. In the 1600's, the German astronomer Johannes Kepler used observations of Mars by the Danish astronomer Tycho Brahe to figure out three laws of planetary motion. Although Kepler developed his laws for the planets of our solar system, astronomers have since realized that Kepler's laws are valid for all heavenly bodies that orbit other bodies. Kepler's first law says that planets move in elliptical (oval-shaped) orbits around their parent star—in our solar system, the sun. An ellipse is a closed curve formed around two fixed points called foci. The ellipse is formed by the path of a point moving so that the sum of its distances from the two foci remains the same. The orbital paths of the planets form such curves, with the parent star at one focus of the ellipse. Before Kepler, scientists had assumed that the planets moved in circular orbits. Kepler's second law says that an imaginary line joining the parent star to its planet sweeps across equal areas of space in equal amounts of time. When a planet is close to its star, it moves relatively rapidly in its orbit. The line therefore sweeps out a short, fat, trianglelike figure. When the planet is farther from its star, it moves relatively slowly. In this case, the line sweeps out a long, thin figure that resembles a triangle. But the two figures have equal areas. Kepler's third law says that a planet's period (the time it takes to complete an orbit around its star) depends on its average distance from the star. The law says that the square of the planet's period—that is, the period multiplied by itself—is proportional to the cube of the planet's average distance from its star—the distance multiplied by itself twice—for all planets in a solar system. The English scientist, astronomer, and mathematician Isaac Newton presented his theory of gravity and explained why Kepler's laws work in a treatise published in 1687. Newton showed how his expanded version of Kepler's third law could be used to find the mass of the sun or of any other object around which things orbit. Using Newton's explanation, astronomers can determine the mass of a planet by studying the period of its moon or moons and their distance from the planet. Rotation Planets rotate at different rates. One day is defined as how long it takes Earth to rotate once. Jupiter and Saturn spin much faster, in only about 10 hours. Venus rotates much slower, in about 243 Earth days. Most planets rotate in the same direction in which they revolve around the sun, with their axis of rotation standing upright from their orbital path. A law of physics holds that such rotation does not change by itself. So astronomers think that the solar system formed out of a cloud of gas and dust that was already spinning. Uranus is tipped on its side, however, so that its axes lies nearly level with its paths around the sun. Venus is tipped all the way over. Its axis is almost completely upright, but the planet rotates in the direction opposite from the direction of its revolution around the sun. Most astronomers think that some other objects in the solar system must have collided with Uranus, Pluto, and Venus and tipped them.

The planet Mercury was first photographed in detail on March 29, 1974, by the U.S. probe Mariner 10. Image credit: NASA

The planets of our solar system Astronomers measure distances within the solar system in astronomical units (AU). One is the average distance between Earth and the sun, which is about 93 million miles (150 million kilometers). The inner planets have orbits whose diameters are 0.4, 0.7, 1.0, and 1.5 AU, respectively. The orbits of the gas giants are much larger: 5, 10, 20, and 30 AU, respectively. Because of their different distances from the sun, the temperature, surface features, and other conditions on the planets vary widely. Mercury, the innermost planet, has no moon and almost no atmosphere. It orbits so close to the sun that temperatures on its surface can climb as high as 800 degrees F (430 degrees C). But some regions near the planet's poles may be always in shadow, and astronomers speculate that water or ice may remain there. No spacecraft has visited Mercury since the 1970's, when Mariner 10 photographed about half the planet's surface at close range. The Messenger spacecraft, launched in 2004, was scheduled to fly by Mercury three times before going into orbit around the planet in 2011.

Thick clouds of sulfuric acid cover Venus. Image credit: NASA

Venus is known as Earth's twin because it resembles Earth in size and mass, though it has no moon. Venus has a dense atmosphere that consists primarily of carbon dioxide. The pressure of the atmosphere on Venus's surface is 90 times that of Earth's atmosphere. Venus's thick atmosphere traps energy from the sun, raising the surface temperature on Venus to about 870 degrees F (465 degrees C), hot enough to melt lead. This trapping of heat is known as the greenhouse effect. Scientists have warned that a similar process on Earth is causing permanent global warming. Several spacecraft have orbited or landed on Venus. In the 1990's, the Magellan spacecraft used radar -- radio waves bounced off the planet -- to map Venus in detail. Earth, our home planet, has oceans of liquid water, and continents that rise above sea level. Image credit: NASA/Goddard Space Flight Center Earth, our home planet, has an atmosphere that is mostly nitrogen with some oxygen. Earth has oceans of liquid water and continents that rise above sea level. Many measuring devices on the surface and in space monitor conditions on our planet. In 1998, the National Aeronautics and Space Administration (NASA) launched the first of a series of satellites called the Earth Observing System (EOS). The EOS satellites will carry remote-sensing instruments to measure climate changes and other conditions on Earth's surface. The planet Mars has clouds in its atmosphere and a deposit of ice at its north pole. Image credit: NASA/JPL/Malin Space Science Systems Mars is known as the red planet because of its reddish-brown appearance, caused by rusty dust on the Martian surface. Mars is a cold, dry world with a thin atmosphere. The atmospheric pressure (pressure exerted by the weight of the gases in the atmosphere) on the Martian surface is less than 1 percent the atmospheric pressure on Earth. This low surface pressure has enabled most of the water that Mars may once have had to escape into space. The surface of Mars has giant volcanoes, a huge system of canyons, and stream beds that look as if water flowed through them in the past. Mars has two tiny moons, Phobos and Deimos. Many spacecraft have landed on or orbited Mars.

The layers of dense clouds around Jupiter appear in a photograph of the planet taken by the Voyager 1 space probe. Image credit: JPL Jupiter, the largest planet in our solar system, has more mass than the other planets combined. Like the other Jovian planets, it has gaseous outer layers and may have a rocky core. A huge storm system called the Great Red Spot in Jupiter's atmosphere is larger than Earth and has raged for hundreds of years. Jupiter's four largest moons -- Io, Europa, Ganymede, and Callisto -- are larger than Pluto, and Ganymede is also bigger than Mercury. Circling Jupiter's equator are three thin rings, consisting mostly of dust particles. A pair of Voyager spacecraft flew by Jupiter in 1979 and sent back close-up pictures. In 1995, the Galileo spacecraft dropped a probe into Jupiter's atmosphere. Galileo orbited Jupiter from 1995 to 2003. Saturn is encircled by seven major rings. Image credit: NASA/JPL/Space Science Institute Saturn, another giant planet, has a magnificent set of gleaming rings. Its gaseous atmosphere is not as colorful as Jupiter's, however. One reason Saturn is relatively drab is that its hazy upper atmosphere makes the cloud patterns below difficult to see. Another reason is that Saturn is farther than Jupiter from the sun. Because of the difference in distance, Saturn is colder than Jupiter. Due to the temperature difference, the kinds of chemical reactions that color Jupiter's atmosphere occur too slowly to do the same on Saturn. Saturn's moon Titan is larger than Pluto and Mercury. Titan has a thick atmosphere of nitrogen and methane. In 1980 and 1981, the Voyager 2 spacecraft sent back close-up views of Saturn and its rings and moons. The Cassini spacecraft began orbiting Saturn in 2004. It carried a small probe that was designed to be dropped into Titan's atmosphere.

Uranus appears in true colors, left, and false colors, right, in images produced by combining numerous pictures taken by the Voyager 2 spacecraft. Image credit: JPL Uranus was the first planet discovered with a telescope. German-born English astronomer William Herschel found it in 1781. He at first thought he had discovered a comet. Almost 200 years later, scientists detected 10 narrow rings around Uranus when the planet moved in front of a star and the rings became visible. Voyager 2 studied Uranus and its rings and moons close-up in 1986.

The blue clouds of Neptune are mostly frozen methane. The other object shown is Neptune's moon Triton. Image credit: NASA/JPL Neptune was first observed in 1846 by German astronomer Johann G. Galle after other astronomers predicted its position by studying how it affected Uranus's orbit. In 1989, Voyager 2 found that Neptune had a storm system called the Great Dark Spot, similar to Jupiter's Great Red Spot. But five years later, in 1994, the Hubble Space Telescope found that the Great Dark Spot had vanished. Neptune has four narrow rings, one of which has clumps of matter. Neptune's moon Triton is one of the largest in the solar system and has volcanoes that emit plumes of frozen nitrogen. The Dwarf Planets

Pluto is so far from Earth that even powerful telescopes reveal little detail of its surface. The Hubble Space Telescope gathered the light for the pictures of Pluto shown here. Image credit: NASA The solar system’s dwarf planets consist primarily of rock and ice and feature little or no atmosphere. They lack the mass to sweep their orbits clear, so they tend to be found among populations of similar, smaller bodies. Ceres ranks as the largest of millions of asteroids found between the orbits of Mars and Jupiter. Ceres has a rocky composition and resembles a slightly squashed sphere. Its longest diameter measures 596 miles (960 kilometers). The Italian astronomer Giuseppe Piazzi discovered Ceres in 1801. As with Pluto, people once widely considered Ceres a planet. The outer dwarf planets generally lie beyond the orbit of Neptune. Astronomers have had difficulty studying bodies in this region because they are extremely far from Earth. Dozens of them probably fit the IAU’s definition of a dwarf planet. Most of these bodies belong to the Kuiper belt. Some astronomers have suggested calling the outer dwarf planets plutonians in honor of Pluto, the first one discovered. A body designated 2003 UB313 ranks as the largest dwarf planet, with a diameter of around 1,500 miles (2,400 kilometers). Quaoar «KWAH oh wahr» , a KBO discovered in 2002, measures roughly half the size of Pluto. Sedna, discovered in 2004, measures about three-fourths the size of Pluto and lies nearly three times as far from the sun. Some scientists think Sedna belongs to population of cometlike objects called the Oort cloud, which lies beyond the Kuiper belt. 2003 UB 313 has a surface that contains methane ice and remains around –406 °F (–243 °C). 2003 UB313 has a small moon about 1â„8 its diameter. The American astronomers Michael E. Brown, A. Trujillo, and David L. Rabinowitz announced the discovery of 2003 UB313 in 2005. Pluto was long considered the ninth planet. The American astronomer Clyde W. Tombaugh discovered Pluto in 1930. Pluto is slightly smaller than 2003 UB313. Its surface also features methane ice. Pluto’s largest satellite, Charon, measures about half the dwarf planet’s diameter. The New Horizons probe, launched in 2006, was designed to observe Pluto during a flyby in 2015. Planets in other solar systems Even with the most advanced telescopes, astronomers cannot see planets orbiting other stars directly. The planets shine only by reflected light and are hidden by the brilliance of their parent stars. The planets and their stars are also much farther away than our sun. The nearest star is 4.2 light-years away, compared to 8 light-minutes for the sun. One light-year is the distance that light travels in one year -- about 5.88 trillion miles (9.46 trillion kilometers). Thus, it takes light 4.2 years to reach Earth from the nearest star beyond the sun and only 8 minutes to reach Earth from the sun. Scientists know of more than 100 stars other than the sun that have planets. Astronomers cannot see planets around distant stars. However, they can detect the planets from tiny changes in the stars' movement and tiny decreases in the amount of light coming from the stars. The changes in a star's movement are caused by the slight pull of the planet's gravity on its parent star. To find new planets, astronomers use a technique called spectroscopy, which breaks down the light from stars into its component rainbow of colors. The scientists look for places in the rainbow where colors are missing. At these places, dark lines known as spectral lines cross the rainbow. The spectral lines change their location in the rainbow slightly as a star is pulled by the gravity of an orbiting planet toward and away from Earth. These apparent changes in a star's light as the star moves are due to a phenomenon known as the effect. The changes not only show that a planet is present but also indicate how much mass it has. The amount of light coming from the star decreases when the planet passes in front of the star. The planet blocks some of the starlight, dimming the star. The first discoveries Astronomers announced the discovery of the first planets around a star other than our sun in 1992. The star is a pulsar named PSR B1257+12 in the constellation Virgo. Pulsars are dead stars that have collapsed until they are only about 12 miles (20 kilometers) across. They spin rapidly on their axes, sending out radio waves that arrive on Earth as pulses of radio energy. Some pulsars spin hundreds of times each second. If a pulsar has a planet, the planet pulls the star to and fro slightly as it orbits. These pulls cause slight variations in the radio pulses. From measurements of these variations, the Polish-born American astronomer Alexander Wolszczan and American A. Frail discovered three planets in orbit around PSR B1257+12. The star emits such strong X rays, however, that no life could survive on its planets. Astronomers soon began to find planets around stars more like the sun. In 1995, Swiss astronomers Michel Mayor and Didier Queloz found the first planet orbiting a sunlike star, 51 Pegasi, in the constellation Pegasus. American astronomers Geoffrey W. Marcy and R. Paul Butler confirmed the discovery and found planets of their own around other stars. In 1999, astronomers announced the first discovery of a multiple-planet system belonging to a sunlike star. They determined that three planets orbit the star Upsilon Andromedae, which is 44 light-years from Earth in the constellation Andromeda. Also in 1999, American astronomer Gregory W. Henry first detected a dimming of starlight due to the presence of a planet. The star that Henry observed is known as HD 209458, and it is located in Pegasus. Henry measured the star's brightness at the request of Marcy, Butler, and American astronomer Steven S. Vogt, who had previously used the spectroscopic technique to identify this star as a parent of a planet. Some stars have a planet orbiting them at a distance at which living things could exist. Most scientists consider liquid water essential for life, so a region that is neither too hot nor too cold for liquid water is known as a habitable zone. Although astronomers have found stars with planets in their habitable zones, all the planets found so far are probably gaseous with no solid surface. But they may have solid moons. In 2001, Marcy announced the discovery of a solar system containing an extremely unusual object. That object and an ordinary planet orbit the star HD 168443, which is 123 light-years away in the constellation . The object is so unusual because of its mass. It is at least 17 times as massive as Jupiter. Astronomers are not yet sure how to classify the object. They had not thought that a planet could be as massive as the object is. Before this discovery, the only known heavenly bodies of such mass were dim objects called brown dwarfs. But brown dwarfs form by means of the same process that forms stars, not planets. Astronomers also have been surprised to find that other solar systems have huge, gaseous planets in close orbits. In our own solar system, the inner planets are rocky and small, and only the outer planets, except for Pluto, are huge and gassy. But several newly discovered planets have at least as much mass as Jupiter, the largest planet in our solar system. Unlike Jupiter, however, these massive planets race around their stars in only a few weeks. Kepler's third law says that for a planet to complete its orbit so quickly, it must be close to its parent star. Several of these giant planets, therefore, must travel around their stars even closer than our innermost planet, Mercury, orbits our sun. Such close orbits would make their surfaces too hot to support life as we know it. Some newly discovered planets follow unusual orbits. Most planets travel around their stars on nearly circular paths, like those of the planets in our solar system. But a planet around the star 16 Cygni B follows an extremely elliptical orbit. It travels farther from its star than the planet Mars does from our sun, and then draws closer to the star than Venus does to our sun. If a planet in our solar system traveled in such an extreme oval, its gravity would disrupt the orbits of the other planets and toss them out of their paths. hroughout the early 2000's, astronomers continued to improve techniques for detecting planets, enabling them to discover an increasing variety of planets around other stars. In 2004, astronomers announced the first discoveries of planets much smaller than Jupiter. The newly discovered planets were about the size of Uranus or Neptune. Despite the planets' huge size, astronomers theorized that some of them might be rocky planets rather than gas giants. How the planets formed Astronomers have developed a theory about how our solar system formed that explains why it has small, rocky planets close to the sun and big, gaseous ones farther away. Astronomers believe our solar system formed about 4.6 billion years ago from a giant, rotating cloud of gas and dust called the solar nebula. Gravity pulled together a portion of gas and dust at the center of the nebula that was denser than the rest. The material accumulated into a dense, spinning clump that formed our sun. The remaining gas and dust flattened into a disk called a swirling around the sun. Protoplanetary disks around distant stars were first observed through telescopes in 1983. Rocky particles within the disk collided and stuck together, forming bodies called planetesimals. Planetesimals later combined to form the planets. At the distances of the outer planets, gases froze into ice, creating huge balls of frozen gas that formed the Jovian planets. Hot gases and electrically charged particles flow from our sun constantly, forming a stream called the solar wind. The solar wind was stronger at first than it is today. The early solar wind drove the light elements -- hydrogen and helium -- away from the inner planets like Earth. But the stronger gravity of the giant outer planets held on to more of the planets' hydrogen and helium, and the solar wind was weaker there. So these outer planets kept most of their light elements and wound up with much more mass than Earth. Astronomers developed these theories when they thought that rocky planets always orbited close to the parent star and giant planets farther out. But the "rule" was based only on our own solar system. Now that astronomers have learned something about other solar systems, they have devised new theories. Some scientists have suggested that the giant planets in other solar systems may have formed far from their parent stars and later moved in closer.

Pluto Pluto, (PLOO toh), is a dwarf planet that orbits far from the sun. It shares the region of its orbit, known as the Kuiper belt, with a collection of similar icy bodies called Kuiper belt objects (KBO’s). From its discovery in 1930, people widely considered Pluto to be the ninth planet of our solar system. However, because of its small size and irregular orbit, many astronomers questioned whether Pluto should be grouped with worlds like Earth and Jupiter. Pluto seemed to share more similarities with KBO’s. In 2006, this debate led the International Astronomical Union, the recognized authority in naming heavenly objects, to formally classify Pluto as a dwarf planet. Pluto cannot be seen without a telescope. Pluto is so far from Earth that even powerful telescopes reveal little detail of its surface. The Hubble Space Telescope gathered the light for the pictures of Pluto shown here. Image credit: NASA Pluto is about 39 times as far from the sun as Earth is. Its average distance from the sun is about 3,647,240,000 miles (5,869,660,000 kilometers). Pluto travels around the sun in an elliptical (oval- shaped) orbit. At some point in its orbit, it comes closer to the sun than Neptune, the outermost planet. It stays inside Neptune's orbit for about 20 Earth years. This event occurs every 248 Earth years, which is about the same number of Earth years it takes Pluto to travel once around the sun. Pluto entered Neptune's orbit on Jan. 23, 1979, and remained there until Feb. 11, 1999. As it orbits the sun, Pluto spins on its axis, an imaginary line through its center. It spins around once in about six Earth days. Astronomers know little about Pluto's size or surface conditions because it is so far from Earth. Pluto has an estimated diameter of about 1,400 miles (2,300 kilometers), less than a fifth that of Earth. Pluto's surface is one of the coldest places in our solar system. Astronomers believe the temperature on Pluto may be about –375 °F (–225 °C). Pluto is mostly brown. The planet appears to be partly covered with frozen methane gas and to have a thin atmosphere composed mostly of methane. Because Pluto's density is low, astronomers think Pluto is mainly icy. Scientists doubt Pluto has any form of life. In 1905, Percival Lowell, an American astronomer, found that the force of gravity of some unknown object seemed to be affecting the orbits of Neptune and Uranus. In 1915, he predicted the location of a new planet and began searching for it from his observatory in Flagstaff, Arizona. He used a telescope to photograph the area of the sky where he thought the planet would be found. He died in 1916 without finding it. In 1929, Clyde W. Tombaugh, an assistant at the Lowell Observatory, used predictions made by Lowell and other astronomers and photographed the sky with a more powerful, wide-angle telescope. In 1930, Tombaugh found Pluto's image on three photographs. The planet was named after the Roman god of the dead. The name also honors Percival Lowell, whose initials are the first two letters of Pluto. In 1978, astronomers at the U.S. Naval Observatory substation in Flagstaff detected a satellite of Pluto. They named it Charon. This satellite has a diameter of about 750 miles (1,210 kilometers). In 1996, astronomers published the first detailed images of Pluto's surface. The images, taken by the Hubble Space Telescope, show about 12 large bright or dark areas. The bright regions, which include polar caps, are probably frozen nitrogen. The dark areas may be methane frost that has been broken down chemically by ultraviolet radiation from the sun. In 2005, a team of astronomers studying images from the Hubble Space Telescope discovered two previously unknown moons of Pluto. The satellites, later named Hydra and Nix, had diameters of up to 100 miles (160 kilometers) and lay well outside the orbit of Charon. In 2006, the U.S. National Aeronautics and Space Administration (NASA) launched the New Horizons probe. The probe was expected to fly by Pluto in 2015.

Relativity Relativity is either of two theories of physics developed by the German-born American physicist Albert Einstein. Those theories are (1) the special theory of relativity, which was published in 1905; and (2) the general theory of relativity, announced in 1915. Einstein's theories explain the behavior of matter, energy, and even time and space. They are two of the "foundation blocks" upon which modern physics is built. The theories of relativity describe events so strange that people find it difficult to understand how they could possibly occur. For example, one person can observe that two events happen at the same time, while another person observes that they occur at different times. A clock can appear to one observer to be running at a given rate, yet seem to another observer to run at a different rate. Two observers can measure the length of the same rod correctly but obtain different results. Matter can turn into energy, and energy can turn into matter. Galilean relativity In developing his theories, Einstein used ideas from a principle of relativity developed by the Italian astronomer and physicist Galileo. That principle is now known as Galilean relativity. Undetectable motion Galileo presented the main idea behind Galilean relativity in the Dialogue Concerning the Two Chief World Systems (1632). In this work, a character named Salvatius describes two scenarios involving a ship's cabin. In both scenarios, two friends are in the cabin, along with butterflies and other small flying animals, fish swimming in a bowl, a bottle from which drops of water fall into another container, and a ball. The cabin is below deck, so neither person can see outside. In the first scenario, the ship is at rest. The animals move about naturally, and the two friends throw the ball to each other and jump about. The friends observe that the flying animals fly with equal speed to all sides of the cabin, the fish swim in all directions, and the drops of water fall straight downward. When one friend throws the ball to the other, the effort required for the throw does not depend on the direction of the throw. When either person jumps forward, the effort required for the jump does not depend on the direction of the jump. In the second scenario, the ship is traveling at a constant velocity. That is, both the speed and direction of the ship are unchanging. All the events that occurred in the first scenario happen again: The small creatures fly and swim, the water drips, and the two friends throw the ball and jump. The motion of the ship has no effect on any of these events. Salvatius explains why this is so: All the objects in the cabin, including the living things, share in the motion of the ship. Because the ship's motion has no effect on the events in the cabin, neither friend can tell by observing those events whether the ship is at rest or moving. This is the main idea behind Galilean relativity. Strictly speaking, an actual ship would not travel at a constant velocity. For example, the ship would travel in a curve because Earth's surface -- including the surface of the water -- is curved. The ship would also curve due to Earth's rotation on its axis and its revolution around the sun. During periods of a few seconds, however, the ship's velocity could be almost perfectly constant. Inertial frames Physicists would refer to the cabin as an inertial (ihn UR shuhl) frame. This term comes from the fact that, in the cabin, the principle of inertia would apply relative to the cabin. Inertia is a body's resistance to a change in its motion. A body that is at rest tends to remain at rest due to inertia. A moving body tends to maintain its velocity. For example, the fishbowl would be at rest relative to the cabin. Due to inertia, the bowl would tend to remain at rest relative to the cabin. But suppose the ship suddenly gained speed, causing the bowl to slide. The friends in the cabin would observe that the principle of inertia no longer applied relative to the cabin. The cabin would no longer be moving at a constant velocity, so it would no longer be an inertial frame. Because the cabin was accelerating (gaining speed) it would be an accelerating frame of reference. The principle of inertia is also known as Newton's first law of motion. It is one of three laws of motion discovered by the English scientist Isaac Newton. Those laws were published in 1687 in Philosophiae naturalis principia mathematica (Mathematical Principles of Natural Philosophy), a work usually called simply Principia or Principia mathematica. Until the late 1800's, most scientists thought that all natural events could be explained by Newton's laws. So the principle of Galilean relativity could be stated as: "The laws of nature are the same in all inertial frames," where the laws of nature were understood to be Newton's laws of motion and any laws based on them. Galilean transformations Certain kinds of calculations involving Galilean relativity are an important part of the background of Einstein's theories. Such calculations are known as Galilean transformations. They show how an event occurring in one inertial frame would appear to an observer in another inertial frame. Galilean transformations apply a principle that is based on Newton's first law: Any frame of reference that is moving at a constant velocity relative to an inertial frame is also an inertial frame. Suppose, for example, two jet aircraft, Jet A and Jet B, are flying in the same direction. Jet A is traveling 30 kilometers per hour (kph) faster than Jet B. A flight attendant in Jet A is walking at a speed of 5 kph in Jet A's direction of flight. A Galilean transformation will give the speed of the flight attendant relative to Jet B. The transformation will be an addition: 30 kph + 5 kph = 35 kph. Now, suppose the attendant turns around and walks at a speed of 5 kph in the opposite direction. The Galilean transformation will be a subtraction: 30 kph - 5 kph = 25 kph. The Michelson-Morley experiment In 1887, an experiment conducted by two American physicists showed that there was something incorrect about Galileo's principle of relativity. The physicists, Albert A. Michelson and Edward W. Morley, performed their experiment on light rays. The Michelson-Morley experiment can be traced back to a theory produced in 1864 by the Scottish scientist James Clerk Maxwell. Part of this theory describes the relationship between electric and magnetic fields. An electric field is an influence that an electrically charged object creates in the region around it. Electrically charged objects can act through their electric fields to attract or repel one another. Similarly, a magnetic field is an influence that a magnet or an electric current creates in the region around it. And similarly, magnets and objects that carry current can act through their magnetic fields to attract or repel one another. Maxwell developed equations showing that electric and magnetic fields can combine in ways that create waves. The equations also indicate that these electromagnetic waves travel at the speed of light. Maxwell said that light itself consists of electromagnetic waves -- a statement later proved to be true. He also said that other kinds of electromagnetic waves exist. The German physicist Heinrich Hertz discovered such waves -- now known as radio waves -- between 1886 and 1888. Physicists reasoned that, if light consisted of waves, the waves had to travel through some substance, just as water waves travel through water. They called the substance ether, and they imagined that it filled all space. Although the ether could transmit waves, they said, it could not move from place to place. The ether's immovability made it a special inertial frame. Maxwell's equations indicate that light moves at a particular speed, represented by the letter c. The value of c is now known to be 186,282 miles (299,792 kilometers) per second. Maxwell assumed that c was the speed of light relative to the ether. According to this assumption, light would travel faster or slower than c in an inertial frame moving relative to the ether. Physicists also reasoned that Earth moved through the ether as the planet spun on its axis and circled the sun. Thus, any object on Earth's surface -- including Michelson and Morley's laboratory -- moved relative to the ether. The speed of light relative to the lab would therefore be different for light rays moving in different directions relative to the lab. And one could use Galilean transformations to calculate the speed of various rays relative to the lab. For example, suppose the lab moved through the ether at a speed of 150 kilometers per second (kps). Imagine that a ray of light were emitted in the direction of the lab's movement. A Galilean transformation would show that the expected speed of the light relative to the lab would be c - 150 kps. Now, imagine that a light ray were emitted in the opposite direction. The expected speed of the light relative to the lab would be c + 150 kps. Michelson and Morley conducted their experiment to measure expected differences in the speed of light relative to their laboratory. Although light travels extremely rapidly, their experiment could measure tiny differences in speed. Surprisingly, Michelson and Morley found no difference at all. This result was a great puzzle. Physicists tried without success to determine how light could act in a manner consistent with both Galilean relativity and the Michelson-Morley experiment. Special relativity Einstein noted that there was no evidence for the existence of the ether. He therefore eliminated the ether from consideration. He argued that Maxwell's equations mean that the speed of light must be the same in all inertial frames. Therefore, Galileo's principle cannot be absolutely correct. Accordingly, Einstein introduced a new principle, the special principle of relativity. This principle has two parts: (1) There is no ether, and the speed of light is the same for all observers, whatever their relative motion. (2) The laws of nature are the same in all inertial frames, where the laws are understood to include those described by Maxwell. Einstein based his special theory of relativity on this principle. The theory solved the puzzle of the Michelson-Morley experiment. It also made dramatic new predictions that were verified by later experiments. Lorentz transformations Special relativity uses equations known as Lorentz transformations to describe how an event occurring in one inertial frame would appear to an observer in another inertial frame. The equations are named for the Dutch physicist Hendrik A. Lorentz, who first wrote them down in 1895. Lorentz developed the equations in an attempt to understand the Michelson-Morley experiment. In the complex mathematics of special relativity, time and space are not absolutely separate. Instead, physicists refer to a single entity, space-time. This entity is a combination of the dimension of time and the three dimensions of space -- length, width, and height. Thus, space-time is four-dimensional. Time dilation The Lorentz transformations show that a number of strange effects can occur. One of these is known as time dilation (dilation means widening). For an example of this effect, consider two spaceships, A and B. The ships are moving relative to each other at a speed close to c. There is a clock in each ship. Both clocks keep time accurately, and people in both ships can see both clocks. Strangely, the people in the two ships will read the clocks differently. The people in Spaceship A will observe that the clock in Spaceship B is running more slowly than the clock in Spaceship A. But the people in Spaceship B will observe that the clock in Spaceship A is running more slowly than the clock in Spaceship B. Time dilation actually occurs at all relative velocities. But at everyday velocities, even the most sensitive instruments cannot detect it. Thus, people are not aware of time dilation as they go about their normal activities. However, time dilation is important in the study of cosmic rays, high-energy particles that travel through space. Some cosmic rays that originate in outer space collide with atoms at the top of Earth's atmosphere. The collisions create a variety of particles, including muons. The muons travel at almost the speed of light. In addition, they are radioactive -- that is, they break apart as they travel. Each muon can be considered to be its own reference frame. Physicists have measured how quickly muons break apart in terms of the passage of time in their reference frames. They break apart so rapidly that one might conclude that hardly any of them could ever reach Earth's surface. However, due to time dilation, the muons break apart much more slowly relative to the reference frame of Earth. As a result, many of them reach the surface. Lorentz- contraction Another strange effect of special relativity is the Lorentz-Fitzgerald contraction, or simply the Fitzgerald contraction. Lorentz proposed that contraction occurred as an effect of the Lorentz transformations. In 1889, the Irish physicist George F. Fitzgerald had made a similar proposal. For an example of the Lorentz-Fitzgerald contraction, again consider the two spaceships. The people in Spaceship A will observe that Spaceship B and all the objects in it have become shorter in the direction of Spaceship B's motion relative to Spaceship A. But they will observe no change in the size of Spaceship B or any of the objects as measured from top to bottom or from side to side. This effect, like time dilation, also occurs in reverse: The people in Spaceship B will observe that Spaceship A and all the objects in it have shrunk in the direction of Spaceship A's motion relative to Spaceship B. The Lorentz-Fitzgerald contraction also occurs at all relative velocities. Mass-energy relationship One of the most famous effects of special relativity is the relation between mass and energy: E = mc- squared (E = mc2). Mass can be thought of as the amount of matter in an object. The equation says that an object at rest has an energy E equal to its mass m times the speed of light c multiplied by itself, or squared. The speed of light is so high that the conversion of a tiny quantity of mass releases a tremendous amount of energy. For example, the complete conversion of an object with a mass of 1 gram would release 90 trillion joules of energy. This quantity is roughly equal to the energy released in the explosion of 22,000 tons (20,000 metric tons) of TNT. The conversion of mass creates energy in the sun and other stars. It also produces the heat energy that is converted to electric energy in nuclear power plants. In addition, mass-to-energy conversion is responsible for the tremendous destructive force of nuclear weapons. General relativity Einstein developed the general theory of relativity to modify Newton's law of gravitation so that it would agree with special relativity. The key disagreement lay in descriptions of how objects exert forces on one another. In special relativity, nothing can travel between two points faster than the speed of light. This principle applies to forces as well as rays of light. Consider, for example, an atom of the simplest form of hydrogen. This atom consists of a single electron in orbit around a single proton. The electron carries a negative electric charge, while the proton is positively charged. The position of the proton determines the motion of the electron. It does so by exerting a force of attraction on the electron -- an application of the familiar principle "opposite charges attract." The proton exerts the force by means of electromagnetic waves that can be thought of as light rays. The proton emits (sends out) a ray, which the electron then absorbs. Thus, the electron's motion depends on what the position of the proton was when the proton emitted the ray.

2 In the Principia, Newton had given the law of gravity as F = m1m2 ÷ d , where F is the gravitational 2 force between two objects, m1 and m2 are the masses of the objects, and d is the distance between them squared. This law explained the motion of the planets. According to the law, a planet's motion depends on the position of the sun and the other planets. All these objects influence one another by means of gravitational force. But Newton's law says that the force between two objects is transmitted instantaneously, no matter how far apart the objects are. That is, the law describes a gravitational action at a distance. This description disagrees with special relativity, which says that there is no action at a distance. Principle of equivalence To eliminate action at a distance from Newton's laws, Einstein began with an observation that he called the principle of equivalence. According to this principle, an object's gravitational mass equals its inertial mass.

Gravitational mass helps determine the force of gravity on an object. The masses m1 and m2 in Newton's law of gravity are gravitational masses. Inertial mass is a measure of an object's inertia. Inertial mass is given in the equation for Newton's second law of motion: F = ma, where F is the force exerted on an object, m is the inertial mass of the object, and a is the acceleration of the object. This equation applies, for example, when you push an object across the floor. If your force is greater than the force of friction between the object and the floor and any other force that is working against you, the object will go faster and faster. The amount of acceleration will depend on the mass of the object and on your force minus the opposing forces. The Hungarian physicist Lorand Eotvos had verified the principle of equivalence experimentally in 1889. Einstein saw that the principle reveals a close connection between the way an object moves through space-time and the gravitational force that acts on the object. He recognized that gravity is therefore related to the structure of space-time. A "thought experiment." To describe how he would work to eliminate action at a distance, Einstein offered an example called a "thought experiment": First, consider an elevator that is falling freely toward Earth's surface. Suppose a person in the elevator drops a rock. The rock will fall with the person, and so it will merely hover in the air beside the person. Now, imagine that the elevator is in outer space -- so far from any planet or star that almost no gravitational force is present. The person drops the rock and, again, the rock hovers beside the person. Einstein said that the "thought experiment" reveals a general truth: A person in free fall cannot determine by observation within his or her reference frame that gravitation is present. Thus, gravitation must be a characteristic of the space-time in which the observer is falling. Nowadays, the principle that underlies Einstein's example is familiar in the phenomenon of weightlessness. Astronauts in the space shuttle are so close to Earth that the planet's gravity acts on them. But, like the rock in the elevator, the shuttle and its passengers are in free fall. Therefore, their experience is the same as it would be if there were no gravity at all. Distortions in space-time Einstein translated this principle into mathematical terms in his general theory of relativity. In this theory, matter and energy distort (change the shape of) space-time, and the distortion is experienced as gravity. A more common -- but less precise -- way of explaining the distortion is "Mass curves space." Einstein suggested that astronomers could make certain observations to test the general theory of relativity. The most dramatic of these would be a bending of light rays by the sun's gravitation. In relativity, mass and energy are equivalent; and, because light carries energy, it also is affected by gravity. The light-bending effect is small, but Einstein calculated that it could be observed during a solar eclipse. In 1919, the British astronomer Arthur S. observed it, thereby making Einstein world-famous. Gravitational waves General relativity indicates that gravitational waves transmit gravitational force, just as electromagnetic waves transmit electric and magnetic forces. Scientists have observed gravitational waves indirectly in a pair of neutron stars that orbit each other. Neutron stars are the smallest and densest stars known. A neutron star measures only about 12 miles (20 kilometers) across, but has more mass then the sun. By observing the pair of stars for several years, the scientists determined that the stars' orbit is becoming smaller. Calculations involving equations of general relativity show that the orbit is shrinking because the stars are emitting gravitational waves. Most gravitational waves produce such small distortions of space-time that they are impossible to detect directly. However, collisions between neutron stars and even more compact objects called black holes create tremendous distortions. Physicists are building observatories to detect the resulting waves directly. An observatory known as the Laser Interferometer Gravitational-Wave Observatory (LIGO) has three facilities -- two in Hanford, Washington, and one in Livingston, Louisiana. Each facility is designed to detect gravitational waves by sensing their effect on two metal tubes that are 2 1/2 miles (4 kilometers) long. The tubes are built along the ground, and they are connected to each other in the shape of an L. When a gravitational wave passes through them, it changes their lengths by an amount much smaller than an atomic nucleus. A laser system detects changes in the lengths. Robot

Robots weld car bodies at a manufacturing plant in Wixom. Michigan ranks as the leading U.S. automobile manufacturer. Image credit: Ford Motor Company

A robot is a mechanical device that operates automatically. Robots can perform a variety of tasks. They are especially suitable for doing jobs too boring, difficult, or dangerous for people. The term robot comes from the Czech word robota, meaning drudgery. Robots efficiently carry out such routine tasks as welding, drilling, and painting automobile body parts. They also do such jobs as making plastic containers, wrapping ice cream bars, and assembling electronic circuits. The science and technology that deals with robots is called robotics. A typical robot performs a task by following a set of instructions that specifies exactly what it must do to complete the job. These instructions are stored in the robot's control center, a computer or part of a computer. The computer, in turn, sends commands to the robot's motorized joints, which function much like human joints to move various parts of the robot. Robots vary in design and size, but few resemble the humanlike machines that appear in works of science fiction. Most are stationary structures with a single arm capable of lifting objects and using tools. Engineers have also developed mobile robots equipped with television cameras for sight and electronic sensors for touch. These robots are controlled by stored instructions, feedback from sensors, and remote control. Scientists have used such robots to explore the sea floor on Earth and the surface of Mars.

Rocket A rocket is a type of engine that pushes itself forward or upward by producing thrust. Unlike a jet engine, which draws in outside air, a rocket engine uses only the substances carried within it. As a result, a rocket can operate in outer space, where there is almost no air. A rocket can produce more power for its size than any other kind of engine. For example, the main rocket engine of the space shuttle weighs only a fraction as much as a train engine, but it would take 39 train engines to produce the same amount of power. The word rocket can also mean a vehicle or object driven by a rocket engine. Rockets come in a variety of sizes. Some rockets that shoot fireworks into the sky measure less than 2 feet (60 centimeters) long. Rockets 50 to 100 feet (15 to 30 meters) long serve as long-range missiles that can be used to bomb distant targets during wartime. Larger and more powerful rockets lift spacecraft, artificial satellites, and scientific probes into space. For example, the Saturn 5 rocket that carried astronauts to the moon stood about 363 feet (111 meters) tall. Rocket engines generate thrust by expelling gas. Most rockets produce thrust by burning a mixture of fuel and an oxidizer, a substance that enables the fuel to burn without drawing in outside air. This kind of rocket is called a chemical rocket because burning fuel is a chemical reaction. The fuel and oxidizer are called the propellants. A chemical rocket can produce great power, but it burns propellants rapidly. As a result, it needs a large amount of propellants to work for even a short time. The Saturn 5 rocket burned more than 560,000 gallons (2,120,000 liters) of propellants during the first 2 3/4 minutes of flight. Chemical rocket engines become extremely hot as the propellants burn. The temperature in some engines reaches o 6000 degrees F (3300 degrees C), much higher than the temperature at which steel melts. Jet engines also burn fuel to generate thrust. Unlike rocket engines, however, jet engines work by drawing in oxygen from the surrounding air. For more information on jet engines, see Jet propulsion. Researchers have also developed rockets that do not burn propellants. Nuclear rockets use heat generated by a nuclear fuel to produce thrust. In an electric rocket, electric energy produces thrust. Military forces have used rockets in war for hundreds of years. In the 1200's, Chinese soldiers fired rockets against attacking armies. British troops used rockets to attack Fort McHenry in Maryland during the War of 1812 (1812-1815). After watching the battle, the American lawyer Francis Scott Key described "the rocket's red glare" in the song "The Star-Spangled Banner." During World War I (1914-1918), the French used rockets to shoot down enemy observation balloons. Germany attacked London with V-2 rockets during World War II (1939-1945). In the Persian Gulf War of 1991 and the Iraq War, which began in 2003, United States troops launched rocket-powered Patriot missiles to intercept and destroy Iraqi missiles. Rockets are the only vehicles powerful enough to carry people and equipment into space. Since 1957, rockets have lifted hundreds of artificial satellites into orbit around Earth. These satellites take pictures of Earth's weather, gather information for scientific study, and transmit communications around the world. Rockets also carry scientific instruments far into space to explore and study other planets. Since 1961, rockets have launched spacecraft carrying astronauts and cosmonauts into orbit around Earth. In 1969, rockets carried astronauts to the first landing on the moon. In 1981, rockets lifted the first space shuttle into Earth orbit. This article discusses Rocket (How rockets work) (How rockets are used) (Kinds of rocket engines) (History). How rockets work Rocket engines generate thrust by putting a gas under pressure. The pressure forces the gas out the end of the rocket. The gas escaping the rocket is called exhaust. As it escapes, the exhaust produces thrust according to the laws of motion developed by the English scientist Isaac Newton. Newton's third law of motion states that for every action, there is an equal and opposite reaction. Thus, as the rocket pushes the exhaust backward, the exhaust pushes the rocket forward. The amount of thrust produced by a rocket depends on the momentum of the exhaust -- that is, its total amount of motion. The exhaust's momentum equals its mass (amount of matter) multiplied by the speed at which it exits the rocket. The more momentum the exhaust has, the more thrust the rocket produces. Engineers can therefore increase a rocket's thrust by increasing the mass of exhaust it produces. Alternately, they can increase the thrust by increasing the speed at which the exhaust leaves the rocket. Parts of a rocket include the rocket engine and the equipment and cargo the rocket carries. The four major parts of a rocket are (1) the payload, (2) propellants, (3) the chamber, and (4) the nozzle. The payload of a rocket includes the cargo, passengers, and equipment the rocket carries. The payload may consist of a spacecraft, scientific instruments, or even explosives. The space shuttle's payload, for example, is the shuttle orbiter and the mission astronauts and any satellites, scientific experiments, or supplies the orbiter carries. The payload of a missile may include explosives or other weapons. This kind of payload is called a warhead. Propellants generally make up most of the weight of a rocket. For example, the fuel and oxidizer used by the space shuttle account for nearly 90 percent of its weight at liftoff. The shuttle needs such a large amount of propellant to overcome Earth's gravity and the resistance of the atmosphere. The space shuttle and many other chemical rockets use liquid hydrogen as fuel. Hydrogen becomes a liquid only at extremely low temperatures, requiring powerful cooling systems. Kerosene, another liquid fuel, is easier to store because it remains liquid at room temperature. Many rockets, including the space shuttle, use liquid oxygen, or lox, as their oxidizer. Like hydrogen, oxygen must be cooled to low temperatures to become a liquid. Other commonly used oxidizers include nitrogen tetroxide and hydrogen peroxide. These oxidizers remain liquid at room temperature and do not require cooling. An electric or nuclear rocket uses a single propellant. These rockets store the propellant as a gas or liquid. The chamber is the area of the rocket where propellants are put under pressure. Pressurizing the propellants enables the rocket to expel them at high speeds. In a chemical rocket, the fuel and oxidizer combine and burn in an area called the combustion chamber. As they burn, the propellants expand rapidly, creating intense pressure. Burning propellants create extreme heat and pressure in the combustion chamber. Temperatures in the chamber become hot enough to melt the steel, nickel, copper, and other materials used in its construction. Combustion chambers need insulation or cooling to survive the heat. The walls of the chamber must also be strong enough to withstand intense pressure. The pressure inside a rocket engine can exceed 3,000 pounds per square inch (200 kilograms per square centimeter), nearly 100 times the pressure in the tires of a car or truck. In a nuclear rocket, the chamber is the area where nuclear fuel heats the propellant, producing pressure. In an electric rocket, the chamber contains the electric devices used to force the propellant out of the nozzle. The nozzle is the opening at the end of the chamber that allows the pressurized gases to escape. It converts the high pressure of the gases into thrust by forcing the exhaust through a narrow opening, which accelerates the exhaust to high speeds. The exhaust from the nozzle can travel more than 1 mile (1.6 kilometers) per second. Like the chamber, the nozzle requires cooling or insulation to withstand the heat of the exhaust. Multistage rockets A two-stage rocket carries a propellant and one or more rocket engines in each stage. The first stage launches the rocket. After burning its supply of propellant, the first stage falls away from the rest of the rocket. The second stage then ignites and carries the payload into earth orbit or even farther into space. A balloon and a rocket work in much the same way. Gas flowing from the nozzle creates unequal pressure that lifts the balloon or the rocket off the ground. Image credit: World Book diagram

Many chemical rockets work by burning propellants in a single combustion chamber. Engineers refer to these rockets as single-stage rockets. Missions that require long-distance travel, such as reaching Earth orbit, generally require multiple-stage or multistage rockets. A multistage rocket uses two or more sets of combustion chambers and propellant tanks. These sets, called stages, may be stacked end to end or attached side by side. When a stage runs out of propellant, the rocket discards it. Discarding the empty stage makes the rocket lighter, allowing the remaining stages to accelerate it more strongly. Engineers have designed and launched rockets with as many as five separate stages. The space shuttle uses two stages. How rockets are used People use rockets for high-speed, high-power transportation both within Earth's atmosphere and in space. Rockets are especially valuable for (1) military use, (2) atmospheric research, (3) launching probes and satellites, and (4) space travel. Military use Rockets used by the military vary in size from small rockets used on the battlefield to giant guided missiles that can fly across oceans. The bazooka is a small rocket launcher carried by soldiers for use against armored vehicles. A person using a bazooka has as much striking power as a small tank. Armies use larger rockets to fire explosives far behind enemy lines and to shoot down enemy aircraft. Fighter airplanes carry rocket-powered guided missiles to attack other planes and ground targets. Navy ships use guided missiles to attack other ships, land targets, and planes. Powerful rockets propel a type of long-range guided missile called an intercontinental ballistic missile (ICBM). Such a missile can travel 3,400 miles (5,500 kilometers) or more to bomb an enemy target with nuclear explosives. An ICBM generally employs two or three separate stages to propel it during the early part of its flight. The ICBM coasts the rest of the way to its target. Atmospheric research Scientists use rockets to explore Earth's atmosphere. Sounding rockets, also called meteorological rockets, carry such equipment as barometers, cameras, and thermometers high into the atmosphere. These instruments collect information about the atmosphere and send it by radio to receiving equipment on the ground. Rockets also provide the power for experimental research airplanes. Engineers use these planes in the development of spacecraft. By studying the flights of such planes as the rocket-powered X-1 and X-15, engineers learned how to control vehicles flying many times as fast as the speed of sound. Launching probes and satellites Rockets carry crewless spacecraft called space probes on long voyages to explore the solar system. Probes have explored the sun, the moon, and all the planets in our solar system except Pluto. They carry scientific instruments that gather information about the planets and transmit data back to Earth. Probes have landed on the surface of the moon, Venus, and Mars. Rockets lift artificial satellites into orbit around Earth. Some orbiting satellites gather information for scientific research. Others relay telephone conversations and radio and television broadcasts across the oceans. Weather satellites track climate patterns and help scientists predict the weather. Navigation satellites, such as those that make up the Global Positioning System (GPS), enable receivers anywhere on Earth to determine their locations with great accuracy. The armed forces use satellites to observe enemy facilities and movements. They also use satellites to communicate, monitor weather, and watch for missile attacks. Not only are satellites launched by rockets, but many satellites use small rocket engines to maintain their proper orbits. Rockets that launch satellites and probes are called launch vehicles. Most of these rockets have from two to four stages. The stages lift the satellite to its proper altitude and give it enough speed -- about 17,000 miles (27,000 kilometers) per hour -- to stay in orbit. A space probe's speed must reach about 25,000 miles (40,000 kilometers) per hour to escape Earth's gravity and continue on its voyage. Engineers created the first launch vehicles by altering military rockets or sounding rockets to carry spacecraft. For example, they added stages to some of these rockets to increase their speed. Today, engineers sometimes attach smaller rockets to a launch vehicle. These rockets, called boosters, provide additional thrust to launch heavier spacecraft. Space travel Rockets launch spacecraft carrying astronauts that orbit Earth and travel into space. These rockets, like the ones used to launch probes and satellites, are called launch vehicles. The Saturn 5 rocket, which carried astronauts to the moon, was the most powerful launch vehicle ever built by the United States. Before launch, it weighed more than 6 million pounds (2.7 million kilograms). It could send a spacecraft weighing more than 100,000 pounds (45,000 kilograms) to the moon. The Saturn 5 used 11 rocket engines to propel three stages. Space shuttles are reusable rockets that can fly into space and return to Earth repeatedly. Engineers have also worked to develop space tugs, smaller rocket-powered vehicles that could tow satellites, boost space probes, and carry astronauts over short distances in orbit. For more information on rockets used in space travel, see Space exploration. Other uses People have fired rockets as distress signals from ships and airplanes and from the ground. Rockets also shoot rescue lines to ships in distress. Small rockets called JATO (jet-assisted take-off) units help heavily loaded airplanes take off. Rockets have long been used in fireworks displays. Kinds of rocket engines The vast majority of rockets are chemical rockets. The two most common types of chemical rockets are solid-propellant rockets and liquid-propellant rockets. Engineers have tested a third type of chemical rocket, called a hybrid rocket, that combines liquid and solid propellants. Electric rockets have propelled space probes and maneuvered orbiting satellites. Researchers have designed experimental nuclear rockets.

A solid-propellant rocket burns a solid material called the grain. Engineers design most grains with a hollow core. The propellant burns from the core outward. Unburned propellant shields the engine casing from the heat of combustion. Image credit: World Book diagram by Precision Graphics

Solid-propellant rockets burn a rubbery or plastic-like material called the grain. The grain consists of a fuel and an oxidizer in solid form. It is shaped like a cylinder with one or more channels or ports that run through it. The ports increase the surface area of the grain that the rocket burns. Unlike some liquid propellants, the fuel and oxidizer of a solid-propellant rocket do not burn upon contact with each other. Instead, an electric charge ignites a smaller grain. Hot exhaust gases from this grain ignite the main propellant surface. The temperature in the combustion chamber of a solid-propellant rocket ranges from 3000 to 6000 degrees F (1600 to 3300 degrees C). In most of these rockets, engineers build the chamber walls from high-strength steel or titanium to withstand the pressure and heat of combustion. They also may use composite materials consisting of high-strength fibers embedded in rubber or plastic. Composite chambers made from high-strength graphite fibers in a strong adhesive called epoxy weigh less than steel or titanium chambers, enabling the rocket to accelerate its payload more efficiently. Solid propellants burn at a rate of about 0.6 inch (1.5 centimeters) per second. Solid propellants can remain effective after long storage and present little danger of combusting or exploding until ignited. Furthermore, they do not need the pumping and injecting equipment required by liquid propellants. On the other hand, rocket controllers cannot easily stop or restart the burning of solid propellant. This can make a solid-propellant rocket difficult to control. One method used to stop the burning of solid propellant involves blasting the entire nozzle section from the rocket. This method, however, prevents restarting. Rocket designers often choose solid propellants for rockets that must be easy to store, transport, and launch. Military planners prefer solid-propellant rockets for many uses because they can be stored for a long time and fired with little preparation. Solid-propellant rockets power ICBM's, including the American Minuteman 2 and MX and the Russian RT-2. They also propel such smaller missiles as the American Hellfire, Patriot, Sparrow, and Sidewinder, and the French SSBS. Solid-propellant rockets often serve as sounding rockets and as boosters for launch vehicles and cruise missiles. They are also used in fireworks.

A liquid-propellant rocket carries fuel and an oxidizer in separate tanks. The fuel circulates through the engine's cooling jacket before entering the combustion chamber. This circulation preheats the fuel for combustion and helps cool the rocket. Image credit: World Book diagram by Precision Graphics

Liquid-propellant rockets burn a mixture of fuel and oxidizer in liquid form. These rockets carry the fuel and the oxidizer in separate tanks. A system of pipes and valves feeds the propellants into the combustion chamber. In larger engines, either the fuel or the oxidizer flows around the outside of the chamber before entering it. This flow cools the chamber and preheats the propellant for combustion. A liquid-propellant rocket feeds the fuel and oxidizer into the combustion chamber using either pumps or high-pressure gas. The most common method uses pumps to force the fuel and oxidizer into the combustion chamber. Burning a small portion of the propellants provides the energy to drive the pumps. In the other method, high-pressure gas forces the fuel and oxidizer into the chamber. The gas may be nitrogen or some other gas stored under high pressure or may come from the burning of a small amount of propellants. Some liquid propellants, called hypergols, ignite when the fuel and the oxidizer mix. But most liquid propellants require an ignition system. An electric spark may ignite the propellant, or the burning of a small amount of solid propellant in the combustion chamber may do so. Liquid propellants continue to burn as long as fuel and oxidizer flow into the combustion chamber. Engineers use thin, high-strength steel or aluminum to construct most tanks that hold liquid propellants. They may also reinforce tanks with composite materials like those used in solid-propellant rocket chambers. Most combustion chambers in liquid-propellant rockets are made of steel or nickel. Launch vehicles used by European nations include the European Space Agency's Ariane 5 rocket and Russia's A class and Proton rockets. These vehicles carry space probes and artificial satellites into outer space. The A Class rocket has also carried people into space, and the Proton rocket has carried International Space Station modules. Image credit: World Book illustrations by Oxford Illustrators Limited

Liquid propellants usually produce greater thrust than do equal amounts of solid propellants burned in the same amount of time. Controllers can easily adjust or stop burning in a liquid-propellant rocket by increasing or decreasing the flow of propellants into the chamber. Liquid propellants, however, are difficult to handle. If the fuel and oxidizer blend without igniting, the resulting mixture often will explode easily. Liquid propellants also require complicated pumping machinery. Scientists use liquid-propellant rockets for most space launch vehicles. Liquid-propellant rockets serve as the main engines of the space shuttle as well as Europe's Ariane rocket, Russia's Soyuz rocket, and China's Long March rocket. Hybrid rockets combine some of the advantages of both solid-propellant and liquid-propellant rockets. A hybrid rocket uses a liquid oxidizer, such as liquid oxygen, and a solid-fuel grain made of plastic or rubber. The solid-fuel grain lines the inside of the combustion chamber. A pumping system sprays the oxidizer onto the surface of the grain, which is ignited by a smaller grain or torch. Hybrid rockets are safer than solid-propellant rockets because the propellants are not premixed and so will not ignite accidentally. Also, unlike solid-propellant rockets, hybrid rockets can vary thrust or even stop combustion by adjusting the flow of oxidizer. Hybrid engines require only half the pumping gear of liquid-propellant rockets, making them simpler to build. A key disadvantage of hybrid rockets is that their fuel burns slowly, limiting the amount of thrust they can produce. A hybrid rocket burns grain at a rate of about 0.04 inch (1 millimeter) per second. For a given amount of propellant, hybrid rockets typically produce more thrust than solid rockets and less than liquid engines. To generate more thrust, engineers must manufacture complex fuel grains with many separate ports through which oxidizer can flow. This exposes more of the grain to the oxidizer. Researchers have used hybrid rockets to propel targets used in missile testing and to accelerate experimental motorcycles and cars attempting land speed records. Their safety has led designers to attempt to develop hybrid rockets for use in human flight. One such rocket would launch from an airplane to carry people to an altitude of about 60 miles (100 kilometers). Researchers have not yet developed hybrid rockets powerful enough to launch human beings into space. Hybrid rockets can produce enough thrust, however, to boost planetary probes or maneuver satellites in orbit. Hybrid rockets could also power escape mechanisms being developed for new launch vehicles that would carry crews. The safety of hybrid rockets has led engineers to develop them for use in human flight. The Scaled Composites company of Mojave, California, developed a hybrid rocket called SpaceShipOne that launched from an airplane. On June 21, 2004, SpaceShipOne became the first privately funded craft to carry a person into space. It carried the American test pilot Michael Melvill more than 62 miles (100 kilometers) above Earth's surface during a brief test flight. Researchers have also used hybrid rockets to propel targets used in missile testing and to accelerate experimental motorcycles and cars attempting land speed records. In addition, they have worked to develop hybrid rockets to boost planetary probes, maneuver satellites in orbit, and power crew escape mechanisms for launch vehicles.

An ion rocket is a kind of electric rocket. Heating coils in the rocket change a fuel, such as xenon, into a vapor. A hot platinum or tungsten ionization grid changes the flowing vapor into a stream of electrically charged particles called ions. Image credit: World Book diagram by Precision Graphics

Electric rockets use electric energy to expel ions (electrically charged particles) from the nozzle. Solar panels or a nuclear reactor can provide the energy. In one design, xenon gas passes through an electrified metal grid. The grid strips electrons from the xenon atoms, turning them into positively charged ions. A positively charged screen repels the ions, focusing them into a beam. The beam then enters a negatively charged device called an accelerator. The accelerator speeds up the ions and shoots them out through a nozzle. The exhaust from such rockets travels extremely fast. However, the stream of xenon ions has a relatively low mass. As a result, an electric rocket cannot produce enough thrust to overcome Earth's gravity. Electric rockets used in space must therefore be launched by chemical rockets. Once in space, however, the low rate of mass flow becomes an advantage. It enables an electric rocket to operate for a long time without running out of propellant. The xenon rocket that powered the U.S. space probe Deep Space 1, launched in 1998, fired for a total of over 670 days using only 160 pounds (72 kilograms) of propellant. In addition, small electric rockets using xenon propellant have provided the thrust to keep communications satellites in position above Earth's surface. Another type of electric rocket uses electromagnets rather than charged screens to accelerate xenon ions. This type of rocket powers the SMART-1 lunar probe, launched by the European Space Agency in 2003.

A nuclear rocket uses the heat from a nuclear reactor to change a liquid fuel into a gas. Most of the fuel flows through the reactor. Some of the fuel, heated by the nozzle of the rocket, flows through the turbine. The turbine drives the fuel pump. Image credit: World Book diagram by Precision Graphics

Nuclear rockets use the heat energy of a nuclear reactor, a device that releases energy by splitting atoms. Some proposed designs would use hydrogen as propellant. The rocket would store the hydrogen as a liquid. Heat from the reactor would boil the liquid, creating hydrogen gas. The gas would expand rapidly and push out from the nozzle. The exhaust speed of a nuclear rocket might reach four times that of a chemical rocket. By expelling a large quantity of hydrogen, a nuclear rocket could therefore achieve high thrust. However, a nuclear rocket would require heavy shielding because a nuclear reactor uses radioactive materials. The shielding would weigh so much that the rocket could not be practically used to boost a launch vehicle. More practical applications would use small nuclear engines with low, continuous thrust to decrease flight times to Mars or other planets. Nuclear rocket developers must also overcome public fears that accidents involving such devices could release harmful radioactive materials. Before nuclear rockets can be launched, engineers must convince the public that such devices are safe. History Historians believe the Chinese invented rockets, but they do not know exactly when. Historical accounts describe "arrows of flying fire" -- believed to have been rockets -- used by Chinese armies in A.D. 1232. By 1300, the use of rockets had spread throughout much of Asia and Europe. These first rockets burned a substance called black powder, which consisted of charcoal, saltpeter, and sulfur. For several hundred years, the use of rockets in fireworks displays outranked their military use in importance During the early 1800's, Colonel William Congreve of the British Army developed rockets that could carry explosives. Many of these rockets weighed about 32 pounds (15 kilograms) and could travel 1 3/4 miles (2.7 kilometers). British troops used Congreve rockets against the United States Army during the War of 1812. Austria, Russia, and several other countries also developed military rockets during the early 1800's. The English inventor William Hale improved the accuracy of military rockets. He substituted three fins for the long wooden tail that had been used to guide the rocket. United States troops used Hale rockets in the Mexican War (1846-1848). During the American Civil War (1861-1865), both sides used rockets. Rockets of the early 1900's The Russian school teacher Konstantin E. Tsiolkovsky first stated the correct theory of rocket power. He described his theory in a scientific paper published in 1903. Tsiolkovsky also first presented the ideas of the multistage rocket and rockets using liquid oxygen and hydrogen propellants. In 1926, the American rocket pioneer Robert H. Goddard conducted the first successful launch of a liquid- propellant rocket. The rocket climbed 41 feet (13 meters) into the air at a speed of about 60 miles (97 kilometers) per hour and landed 184 feet (56 meters) away. During the 1930's, rocket research advanced in Germany, the Soviet Union, and the United States. Hermann Oberth led a small group of German engineers and scientists that experimented with rockets. Leading Soviet rocket scientists included Fridrikh A. Tsander and Sergei P. . Goddard remained the most prominent rocket researcher in the United States.

The vehicles shown here helped the United States and the Soviet Union achieve milestones in the exploration of space. The United States no longer builds these rockets, but Russia continues to use the Soviet A Class design in the Soyuz rocket. • Jupiter C, U.S. Lifted Explorer I, the first U.S. satellite, in 1958. 68 feet (21 meters) • Mercury-Redstone, U.S. Launched Alan Shepard in 1961. 83 feet (25 meters) • A Class (Sputnik), Soviet. Boosted Sputnik 1, the first artificial satellite, in 1957. 98 feet (29 meters) Image credit: WORLD BOOK illustrations by Oxford Illustrators Limited During World War II, German engineers under the direction of Wernher von Braun developed the powerful V-2 guided missile. Germany bombarded London and Antwerp, Belgium, with hundreds of V-2's during the last months of the war. American forces captured many V-2 missiles and sent them to the United States for use in research. After the war, von Braun and about 150 other German scientists moved to the United States to continue their work with rockets. Some other German rocket experts went to the Soviet Union. High-altitude rockets For several years after World War II, U.S. scientists benefited greatly by conducting experiments with captured German V-2's. These V-2's became the first rockets used for high-altitude research. The first high-altitude rockets designed and built in the United States included the WAC Corporal, the Aerobee, and the Viking. The 16-foot (4.9-meter) WAC Corporal reached altitudes of about 45 miles (72 kilometers) during test flights in 1945. Early models of the Aerobee climbed about 70 miles (110 kilometers). In 1949, the U.S. Navy launched the Viking, an improved liquid-propellant rocket based chiefly on the V-2. The Viking measured more than 45 feet (14 meters) long, much longer than the Aerobee. But the first models of the Viking rose only about 50 miles (80 kilometers). Rockets developed by the U.S. armed forces during the 1950's included the Jupiter and the Pershing. The Jupiter had a range of about 1,600 miles (2,600 kilometers), and the Pershing could travel about 450 miles (720 kilometers).

The vehicles shown here helped the United States and the Soviet Union achieve milestones in the exploration of space. The United States no longer builds these rockets, but Russia continues to use the Soviet A Class design in the Soyuz rocket. • A Class (Vostok), Soviet. Carried Yuri Gagarin, the first person to orbit the earth, in 1961. 126 feet (38 meters) • Saturn 5, U.S. Launched Neil Armstrong, the first person to set foot on the moon, in 1969. 363 feet (111 meters) Image credit: WORLD BOOK illustrations by Oxford Illustrators Limited The U.S. Navy conducted the first successful launch of a Polaris underwater missile in 1960. United States space scientists later used many military rockets developed in the 1950's as the basis for launch vehicles. Rocket-powered airplanes On Oct. 14, 1947, Captain Charles E. Yeager of the U.S. Air Force made the first supersonic (faster than sound) flight. He flew a rocket-powered airplane called the X-1. A rocket engine also powered the X-15, which set an unofficial airplane altitude record of 354,200 feet (107,960 meters) in 1963. In one flight, the X-15 reached a peak speed of 4,520 miles (7,274 kilometers) per hour -- more than six times the speed of sound. A privately owned and developed rocket-powered plane called the EZ-Rocket began piloted test flights in 2001. The space age began on Oct. 4, 1957, when the Soviet Union launched the first artificial satellite, Sputnik 1, aboard a two-stage rocket. On Jan. 31, 1958, the U.S. Army launched the first American satellite, Explorer 1, into orbit with a Juno I rocket. On April 12, 1961, a Soviet rocket put a cosmonaut, Major Yuri A. Gagarin, into orbit around Earth for the first time. On May 5, 1961, a Redstone rocket launched Commander Alan B. Shepard, Jr., the first American to travel in space. On April 12, 1981, the United States launched the rocket-powered Columbia, the first space shuttle to orbit Earth. For more information on the history of rockets in space travel, see Space exploration. Rocket research In the early 2000's, engineers and scientists worked to develop lightweight rocket engines that used safer propellants. They also searched for more efficient propellants that did not require refrigeration. Engineers began designing and testing smaller rocket engines for use in smaller vehicles, such as tiny satellites that may weigh only a few pounds or kilograms when fully loaded. Contributor: Stephen Heister, Ph.D., Professor of Aeronautics and Astronautics, Purdue University. How to cite this article: To cite this article, World Book recommends the following format: Heister, Stephen. "Rocket." World Book Online Reference Center. 2005. World Book, Inc. http://www.worldbookonline.com/wb/Article?id=ar472580.