Technology

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. Korolev. 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.

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Artificial Satellites





Artificial SatellitesAn artificial satellite is a manufactured object that continuously orbits Earth or some other body in space. Most artificial satellites orbit 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 sun, asteroids, and the planets Venus, Mars, and Jupiter. 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 orbital period -- 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, stars, 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. Image credit: NASA

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 day, 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 constellation, 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. Image credit: NASA

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 year.

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.



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Moon





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 far side of the moon. 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



Euler 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 lunar craters 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.

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Venus





Venus

The surface of Venus was scanned with radar waves beamed from orbiting space probes to produce this image. The colors are based on photos taken by probes that landed on Venus. Image credit: NASA

Venus is known as the Earth's "twin" because the two planets are so similar in size. The diameter of Venus is about 7,520 miles (12,100 kilometers), approximately 400 miles (644 kilometers) smaller than that of the Earth. No other planet comes nearer to the Earth than Venus. At its closest approach, it is about 23.7 million miles (38.2 million kilometers) away.



As seen from the Earth, Venus is brighter than any other planet or even any star. At certain times of the year, Venus is the first planet or star that can be seen in the western sky in the evening. At other times, it is the last planet or star that can be seen in the eastern sky in the morning. When Venus is near its brightest point, it can be seen in daylight.

Ancient astronomers called the object that appeared in the morning Phosphorus, and the object that appeared in the evening Hesperus. Later, they realized these objects were the same planet. They named Venus in honor of the Roman goddess of love and beauty.

Orbit

Venus is closer to the sun than any other planet except Mercury. Its mean (average) distance from the sun is about 67.2 million miles (108.2 million kilometers), compared with about 93 million miles (150 million kilometers) for the Earth and about 36 million miles (57.9 million kilometers) for Mercury.

Venus travels around the sun in a nearly circular orbit. The planet's distance from the sun varies from about 67.7 million miles (108.9 million kilometers) at its farthest point to about 66.8 million miles (107.5 million kilometers) at its closest point. The orbits of all the other planets are more elliptical (oval-shaped). Venus takes about 225 Earth days, or about 71/2 months, to go around the sun once, compared with 365 days, or one year, for the Earth.

Phases

When viewed through a telescope, Venus can be seen going through "changes" in shape and size. These apparent changes are called phases, and they resemble those of the moon. They result from different parts of Venus's sunlit areas being visible from the Earth at different times.

As Venus and the Earth travel around the sun, Venus can be seen near the opposite side of the sun about every 584 days. At this point, almost all its sunlit area is visible. As Venus moves around the sun toward the Earth, its sunlit area appears to decrease and its size seems to increase. After about 221 days, only half the planet is visible. After another 71 days, Venus nears the same side of the sun as the Earth, and only a thin sunlit area can be seen.

When Venus is moving toward the Earth, the planet can be seen in the early evening. When moving away from the Earth, Venus is visible in the early morning.

Rotation

As Venus travels around the sun, it rotates very slowly on its axis, an imaginary line drawn through its center. Venus's axis is not perpendicular (at an angle of 90¡) to the planet's path around the sun. The axis tilts at an angle of approximately 178¡ from the perpendicular position. Unlike the Earth, Venus does not rotate in the same direction in which it travels around the sun. Rather, Venus rotates in the retrograde (opposite) direction and spins around once every 243 Earth days.



Thick clouds of sulfuric acid cover Venus. Because visible light cannot penetrate the clouds, astronomers cannot see the planet's surface with even the most powerful optical telescopes. Image credit: NASA

Surface and Atmosphere



Although Venus is called the Earth's "twin," its surface conditions appear to be very different from those of the Earth. Geologists have had difficulty learning about the surface of Venus because the planet is always surrounded by thick clouds of sulfuric acid. They have used radar, radio astronomy equipment, and space probes to "explore" Venus.

Until recently, much of what geologists knew about the surface of Venus came from ground-based radar observations, the Soviet Union's Venera space probes, and United States Pioneer probes. In 1990, the U.S. space probe Magellan began orbiting Venus, using radar to map the planet's surface.

The surface of Venus is extremely hot and dry. There is no liquid water on the planet's surface because the high temperature would cause any liquid to boil away.



Maat Mons, a mountain on Venus. Image credit: NASA

Venus has a variety of surface features, including level ground, mountains, canyons, and valleys. About 65 percent of the surface is covered by flat, smooth plains. On these plains are thousands of volcanoes, ranging from about 0.5 to 150 miles (0.8 to 240 kilometers) in diameter. Six mountainous regions make up about 35 percent of the surface of Venus. One mountain range, called Maxwell, is about 7 miles (11.3 kilometers) high and about 540 miles (870 kilometers) long. It is the highest feature on the planet. In an area called Beta Regio is a canyon that is 0.6 mile (1.0 kilometer) deep.



There are also impact craters on the surface of Venus. Impact craters form when a planet and asteroid collide. The moon, Mars, and Mercury are covered with impact craters, but Venus has substantially fewer craters. The scarcity of impact craters on Venus has led geologists to conclude that the present surface is less than 1 billion years old.



An impact crater on Venus measures about 23 miles (37 kilometers) across the depression in its center. A computer produced this image in 1991, using information from a radar scan by the U.S. space probe Magellan. Image credit: NASA

A number of surface features on Venus are unlike anything on the Earth. For example, Venus has coronae (crowns), ringlike structures that range from about 95 to 360 miles (155 to 580 kilometers) in diameter. Scientists believe that coronae form when hot material inside the planet rises to the surface. Also on Venus are tesserae (tiles), raised areas in which many ridges and valleys have formed in different directions.



The atmosphere of Venus is heavier than that of any other planet. It consists primarily of carbon dioxide, with small amounts of nitrogen and water vapor. The planet's atmosphere also contains minute traces of argon, carbon monoxide, neon, and sulfur dioxide. The atmospheric pressure (pressure exerted by the weight of the gases) on Venus is estimated at 1,323 pounds per square inch (9,122 kilopascals). This is about 90 times greater than the atmospheric pressure on the Earth, which is about 14.7 pounds per square inch (101 kilopascals).

Temperature

The temperature of the uppermost layer of Venus's clouds averages about 55 degrees F (13 degrees C). However, the temperature of the planet's surface is about 870 degrees F (465 degrees C), higher than that of any other planet and hotter than most ovens.

The plants and animals that live on the Earth could not live on the surface of Venus, because of the high temperature. Astronomers do not know whether any form of life exists on Venus, but they doubt that it does.

Most astronomers believe that Venus's high surface temperature can be explained by what is known as the greenhouse effect. A greenhouse lets in radiant energy from the sun, but it prevents much of the heat from escaping. The thick clouds and dense atmosphere of Venus work in much the same way. The sun's radiant energy readily filters into the planet's atmosphere. But the large droplets of sulfuric acid present in Venus's clouds -- and the great quantity of carbon dioxide in the atmosphere -- seem to trap much of the solar energy at the planet's surface.

Mass and Density

The mass of Venus is about four-fifths that of the Earth. The force of gravity on Venus is slightly less than on the Earth. For this reason, an object weighing 100 pounds on the Earth would weigh about 88 pounds on Venus. Venus is also slightly less dense than the Earth. A portion of Venus would weigh a little less than an equal-sized portion of the Earth.

Flights to Venus

Venus was the first planet to be observed by a passing spacecraft. The unmanned U.S. spacecraft Mariner 2 passed within 21,600 miles (34,760 kilometers) of Venus on Dec. 14, 1962, after traveling through space for more than 31/2 months. It measured various conditions on and near Venus. For example, instruments carried by the spacecraft measured the high temperatures of the planet.

Two unmanned Soviet spacecraft "explored" Venus in 1966. Venera 2 passed within 15,000 miles (24,000 kilometers) of the planet on February 27, and Venera 3 crashed into Venus on March 1.



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

In October 1967, spacecraft from both the United States and the Soviet Union reached Venus. On October 18, the Soviet spacecraft Venera 4 dropped a capsule of instruments into Venus's atmosphere by parachute. On October 19, the U.S. spacecraft Mariner 5 passed within 2,480 miles (3,990 kilometers) of Venus. It did not detect a magnetic field. Both probes reported large amounts of carbon dioxide in the planet's atmosphere. On Dec. 15, 1970, the Soviet spacecraft Venera 7 landed on Venus. The U.S. planetary probe Mariner 10 flew near Venus on Feb. 5, 1974. The probe transmitted the first close-up photographs of the planet.



On Oct. 22, 1975, the unmanned Soviet spacecraft Venera 9 landed on Venus and provided the first close-up photograph on the planet's surface. Three days later, another Soviet space vehicle, Venera 10, reached Venus. It photographed Venus's surface, measured its atmospheric pressure, and determined the composition of rocks on its surface.

Four unmanned spacecraft reached Venus in December 1978. The United States craft Pioneer Venus 1 began orbiting the planet on December 4. This craft transmitted radar images of Venus, produced a map of its surface, and measured temperatures at the top of the planet's clouds. On December 9, the U.S. Pioneer Venus 2 entered the planet's atmosphere and measured its density and chemical composition. On December 21, the Soviet craft Venera 12 landed on Venus. A second Soviet lander, Venera 11, reached the planet's surface four days later. Both probes sent back data on the lower atmosphere of Venus.

Two more Soviet spacecraft landed on Venus in 1982 -- Venera 13 on March 1 and Venera 14 on March 5. Both probes transmitted photographs of Venus and analyzed soil samples. Beginning in October 1983, two additional Soviet spacecraft mapped the region of Venus north of 30¡ north latitude using radar. Venera 15 finished its mapping in July 1984; Venera 16, in April 1984. The two probes provided clear images of features as small as 0.9 mile (1.5 kilometers) across.

The U.S. spacecraft Magellan began orbiting Venus on Aug. 10, 1990. Radar images received from the Magellan show details of features as small as 330 feet (100 meters) across.

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Star





Star

A globular cluster 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

A star is a huge, shining ball in space that produces a tremendous amount of light and other forms of energy. The sun is a star, and it supplies Earth with light and heat energy. The stars look like twinkling points of light -- except for the sun. The sun looks like a ball because it is much closer to Earth than any other star.



The sun and most other stars are made of gas and a hot, gaslike substance known as plasma. But some stars, called white dwarfs and neutron stars, consist of tightly packed atoms or subatomic particles. These stars are therefore much more dense than anything on Earth.

Stars come in many sizes. The sun's radius (distance from its center to its surface) is about 432,000 miles (695,500 kilometers). But astronomers classify the sun as a dwarf because other kinds of stars are much bigger. Some of the stars known as supergiants have a radius about 1,000 times that of the sun. The smallest stars are the neutron stars, some of which have a radius of only about 6 miles (10 kilometers).

About 75 percent of all stars are members of a binary system, a pair of closely spaced stars that orbit each other. The sun is not a member of a binary system. However, its nearest known stellar neighbor, Proxima Centauri, is part of a multiple-star system that also includes Alpha Centauri A and Alpha Centauri B.

The distance from the sun to Proxima Centauri is more than 25 trillion miles (40 trillion kilometers). This distance is so great that light takes 4.2 years to travel between the two stars. Scientists say that Proxima Centauri is 4.2 light-years from the sun. One light-year, the distance that light travels in a vacuum in a year, equals about 5.88 trillion miles (9.46 trillion kilometers).

Stars are grouped in huge structures called galaxies. Telescopes have revealed galaxies throughout the universe at distances of 12 billion to 16 billion light-years. The sun is in a galaxy called the Milky Way that contains more than 100 billion stars. There are more than 100 billion galaxies in the universe, and the average number of stars per galaxy may be 100 billion. Thus, more than 10 billion trillion stars may exist. But if you look at the night sky far from city lights, you can see only about 3,000 of them without using binoculars or a telescope.

Stars, like people, have life cycles -- they are born, pass through several phases, and eventually die. The sun was born about 4.6 billion years ago and will remain much as it is for another 5 billion years. Then it will grow to become a red giant. Late in the sun's lifetime, it will cast off its outer layers. The remaining core, called a white dwarf, will slowly fade to become a black dwarf.

Other stars will end their lives in different ways. Some will not go through a red giant stage. Instead, they will merely cool to become white dwarfs, then black dwarfs. A small percentage of stars will die in spectacular explosions called supernovae.

This article discusses Star (The stars at night) (Names of stars) (Characteristics of stars) (Fusion in stars) (Evolution of stars).

The stars at night

If you look at the stars on a clear night, you will notice that they seem to twinkle and that they differ greatly in brightness. A much slower movement also takes place in the night sky: If you map the location of several stars for a few hours, you will observe that all the stars revolve slowly about a single point in the sky.

Twinkling of stars is caused by movements in Earth's atmosphere. Starlight enters the atmosphere as straight rays. Twinkling occurs because air movements constantly change the path of the light as it comes through the air. You can see a similar effect if you stand in a swimming pool and look down. Unless the water is almost perfectly still, your feet will appear to move and change their shape. This "twinkling" occurs because the moving water constantly changes the path of the light rays that travel from your feet to your eyes.

Brightness of stars. How bright a star looks when viewed from Earth depends on two factors: (1) the actual brightness of the star -- that is, the amount of light energy the star emits (sends out) -- and (2) the distance from Earth to the star. A nearby star that is actually dim can appear brighter than a distant star that is really extremely brilliant. For example, Alpha Centauri A seems to be slightly brighter than a star known as Rigel. But Alpha Centauri A emits only 1/100,000 as much light energy as Rigel. Alpha Centauri A seems brighter because it is only 1/325 as far from Earth as Rigel is -- 4.4 light-years for Alpha Centauri A, 1,400 light-years for Rigel.

Rising and setting of stars

When viewed from Earth's Northern Hemisphere, stars rotate counterclockwise around a point called the celestial north pole. Viewed from the Southern Hemisphere, stars rotate clockwise about the celestial south pole. During the day, the sun moves across the sky in the same direction, and at the same rate, as the stars. These movements do not result from any actual revolution of the sun and stars. Rather, they occur because of the west-to-east rotation of Earth about its own axis. To an observer standing on the ground, Earth seems motionless, while the sun and stars seem to move in circles. But actually, Earth moves.

Names of stars

Ancient people saw that certain stars are arranged in patterns shaped somewhat like human beings, animals, or common objects. Some of these patterns, called constellations, came to represent figures of mythological characters. For example, the constellation Orion (the Hunter) is named after a hero in Greek mythology.

Today, astronomers use constellations, some of which were described by the ancients, in the scientific names of stars. The International Astronomical Union (IAU), the world authority for assigning names to celestial objects, officially recognizes 88 constellations. These constellations cover the entire sky. In most cases, the brightest star in a given constellation has alpha -- the first letter of the Greek alphabet -- as part of its scientific name. For instance, the scientific name for Vega, the brightest star in the constellation Lyra (the Harp), is Alpha Lyrae. Lyrae is Latin for of Lyra.

The second brightest star in a constellation is usually designated beta, the second letter of the Greek alphabet, the third brightest is gamma, and so on. The assignment of Greek letters to stars continues until all the Greek letters are used. Numerical designations follow.

But the number of known stars has become so large that the IAU uses a different system for newly discovered stars. Most new names consist of an abbreviation followed by a group of symbols. The abbreviation stands for either the type of star or a catalog that lists information about the star. For example, PSR J1302-6350 is a type of star known as a pulsar -- hence the PSR in its name. The symbols indicate the star's location in the sky. The 1302 and the 6350 are coordinates that are similar to the longitude and latitude designations used to indicate locations on Earth's surface. The J indicates that a coordinate system known as J2000 is being used.

Characteristics of stars

A star has five main characteristics: (1) brightness, which astronomers describe in terms of magnitude or luminosity; (2) color; (3) surface temperature; (4) size; and (5) mass (amount of matter). These characteristics are related to one another in a complex way. Color depends on surface temperature, and brightness depends on surface temperature and size. Mass affects the rate at which a star of a given size produces energy and so affects surface temperature. To make these relationships easier to understand, astronomers developed a graph called the Hertzsprung-Russell (H-R) diagram. This graph, a version of which appears in this article, also helps astronomers understand and describe the life cycles of stars.

Magnitude and luminosity

Magnitude is based on a numbering system invented by the Greek astronomer Hipparchus in about 125 B.C. Hipparchus numbered groups of stars according to their brightness as viewed from Earth. He called the brightest stars first magnitude stars, the next brightest second magnitude stars, and so on to sixth magnitude stars, the faintest visible stars.

Modern astronomers refer to a star's brightness as viewed from Earth as its apparent magnitude. But they have extended Hipparchus's system to describe the actual brightness of stars, for which they use the term absolute magnitude. For technical reasons, they define a star's absolute magnitude as what its apparent magnitude would be if it were 32.6 light-years from Earth.

Astronomers have also extended the system of magnitude numbers to include stars brighter than first magnitude and dimmer than sixth magnitude. A star that is brighter than first magnitude has a magnitude less than 1. For example, the apparent magnitude of Rigel is 0.12. Extremely bright stars have magnitudes less than zero -- that is, their designations are negative numbers. The brightest star in the night sky is Sirius, with an apparent magnitude of -1.46. Rigel has an absolute magnitude of -8.1. According to astronomers' present understanding of stars, no star can have an absolute magnitude much brighter than -8. At the other end of the scale, the dimmest stars detected with telescopes have apparent magnitudes up to 28. In theory, no star could have an absolute magnitude much fainter than 16.

Luminosity is the rate at which a star emits energy. The scientific term for a rate of energy emission is power, and scientists generally measure power in watts. For example, the luminosity of the sun is 400 trillion trillion watts. But astronomers do not usually measure a star's luminosity in watts. Instead, they express luminosities in terms of the luminosity of the sun. They often say, for instance, that the luminosity of Alpha Centauri A is about 1.3 times that of the sun and that Rigel is roughly 150,000 times as luminous as the sun.

Luminosity is related to absolute magnitude in a simple way. A difference of 5 on the absolute magnitude scale corresponds to a factor of 100 on the luminosity scale. Thus, a star with an absolute magnitude of 2 is 100 times as luminous as a star with an absolute magnitude of 7. A star with an absolute magnitude of -3 is 100 times as luminous as a star whose absolute magnitude is 2 and 10,000 times as luminous as a star that has an absolute magnitude of 7.

Color and temperature

If you look carefully at the stars, even without binoculars or a telescope, you will see a range of color from reddish to yellowish to bluish. For example, Betelgeuse looks reddish, Pollux -- like the sun -- is yellowish, and Rigel looks bluish.

A star's color depends on its surface temperature. Astronomers measure star temperatures in a metric unit known as the kelvin. One kelvin equals exactly 1 Celsius degree (1.8 Fahrenheit degree), but the Kelvin and Celsius scales start at different points. The Kelvin scale starts at -273.15 degrees C. Therefore, a temperature of 0 K equals -273.15 degrees C, or -459.67 degrees F. A temperature of 0 degrees C (32 degrees F) equals 273.15 K.



A spectacular explosion on the star Eta Carinae about 150 years ago produced three huge clouds of gas and dust -- two puffy lobes and a thin disk. Astronomers call Eta Carinae a luminous blue variable star because of its color and because it often becomes very bright -- as it did when the explosion occurred. Image credit: NASA

Dark red stars have surface temperatures of about 2500 K. The surface temperature of a bright red star is approximately 3500 K; that of the sun and other yellow stars, roughly 5500 K. Blue stars range from about 10,000 to 50,000 K in surface temperature.



Although a star appears to the unaided eye to have a single color, it actually emits a broad spectrum (band) of colors. You can see that starlight consists of many colors by using a prism to separate and spread the colors of the light of the sun, a yellow star. The visible spectrum includes all the colors of the rainbow. These colors range from red, produced by the photons (particles of light) with the least energy; to violet, produced by the most energetic photons.

Visible light is one of six bands of electromagnetic radiation. Ranging from the least energetic to the most energetic, they are: radio waves, infrared rays, visible light, ultraviolet rays, X rays, and gamma rays. All six bands are emitted by stars, but most individual stars do not emit all of them. The combined range of all six bands is known as the electromagnetic spectrum.

Astronomers study a star's spectrum by separating it, spreading it out, and displaying it. The display itself is also known as a spectrum. The scientists study thin gaps in the spectrum. When the spectrum is spread out from left to right, the gaps appear as vertical lines. The spectra of stars have dark absorption lines where radiation of specific energies is weak. In a few special cases in the visible spectrum, stars have bright emission lines where radiation of specific energies is especially strong.

An absorption line appears when a chemical element or compound absorbs radiation that has the amount of energy corresponding to the line. For example, the spectrum of the visible light coming from the sun has a group of absorption lines in the green part of the spectrum. Calcium in an outer layer of the sun absorbs light rays that would have produced the corresponding green colors.

Although all stars have absorption lines in the visible band of the electromagnetic spectrum, emission lines are more common in other parts of the spectrum. For instance, nitrogen in the sun's atmosphere emits powerful radiation that produces emission lines in the ultraviolet part of the spectrum.

Size

Astronomers measure the size of stars in terms of the sun's radius. Alpha Centauri A, with a radius of 1.05 solar radii (the plural of radius), is almost exactly the same size as the sun. Rigel is much larger at 78 solar radii, and Antares has a huge size of 776 solar radii.

A star's size and surface temperature determine its luminosity. Suppose two stars had the same temperature, but the first star had twice the radius of the second star. In this case, the first star would be four times as bright as the second star. Scientists say that luminosity is proportional to radius squared -- that is, multiplied by itself. Imagine that you wanted to compare the luminosities of two stars that had the same temperature but different radii. First, you would divide the radius of the larger star by the radius of the smaller star. Then, you would square your answer.

Now, suppose two stars had the same radius but the first star's surface temperature -- measured in kelvins -- was twice that of the second star. In this example, the luminosity of the first star would be 16 times that of the second star. Luminosity is proportional to temperature to the fourth power. Imagine that you wanted to compare the luminosities of stars that had the same radius but different temperatures. First, you would divide the temperature of the warmer star by the temperature of the cooler star. Next, you would square the result. Then, you would square your answer again.

Mass

Astronomers express the mass of a star in terms of the solar mass, the mass of the sun. For example, they give the mass of Alpha Centauri A as 1.08 solar masses; that of Rigel, as 3.50 solar masses. The mass of the sun is 2 Ž 1030 kilograms, which would be written out as 2 followed by 30 zeros.

Stars that have similar masses may not be similar in size -- that is, they may have different densities. Density is the amount of mass per unit of volume. For instance, the average density of the sun is 88 pounds per cubic foot (1,400 kilograms per cubic meter), about 140 percent that of water. Sirius B has almost exactly the same mass as the sun, but it is 90,000 times as dense. As a result, its radius is only about 1/50 of a solar radius.

The Hertzsprung-Russell diagram displays the main characteristics of stars. The diagram is named for astronomers Ejnar Hertzsprung of Denmark and Henry Norris Russell of the United States. Working independently of each other, the two scientists developed the diagram around 1910.

Luminosity classes

Points representing the brightest stars appear toward the top of the H-R diagram; points corresponding to the dimmest stars, toward the bottom. These points appear in groups that correspond to different kinds of stars. In the 1930's, American astronomers William W. Morgan and Philip C. Keenan invented what came to be known as the MK luminosity classification system for these groups. Astronomers revised and extended this system in 1978. In the MK system, the largest and brightest classes have the lowest classification numbers. The MK classes are: Ia, bright supergiant; Ib, supergiant; II, bright giant; III, giant; IV, subgiant; and V, main sequence or dwarf.

Because temperature also affects the luminosity of a star, stars from different luminosity classes can overlap. For example, Spica, a class V star, has an absolute magnitude of -3.2; but Pollux, a class III star, is dimmer, with an absolute magnitude of 0.7.

Spectral classes

Points representing the stars with the highest surface temperatures appear toward the left edge of the H-R diagram; points representing the coolest stars, toward the right edge. In the MK system, there are eight spectral classes, each corresponding to a certain range of surface temperature. From the hottest stars to the coolest, these classes are: O, B, A, F, G, K, M, and L. Each spectral class, in turn, is made up of 10 spectral types, which are designated by the letter for the spectral class and a numeral. The hottest stars in a spectral class are assigned the numeral 0; the coolest stars, the numeral 9.

A complete MK designation thus includes symbols for luminosity class and spectral type. For example, the complete designation for the sun is G2V. Alpha Centauri A is also a G2V star, and Rigel's designation is B8Ia.

Fusion in stars

A star's tremendous energy comes from a process known as nuclear fusion. This process begins when the temperature of the core of the developing star reaches about 1 million K.

A star develops from a giant, slowly rotating cloud that consists almost entirely of the chemical elements hydrogen and helium. The cloud also contains atoms of other elements as well as microscopic particles of dust.

Due to the force of its own gravity, the cloud begins to collapse inward, thereby becoming smaller. As the cloud shrinks, it rotates more and more rapidly, just as spinning ice skaters turn more rapidly when they pull in their arms. The outermost parts of the cloud form a spinning disk. The inner parts become a roughly spherical clump, which continues to collapse.

The collapsing material becomes warmer, and its pressure increases. But the pressure tends to counteract the gravitational force that is responsible for the collapse. Eventually, therefore, the collapse slows to a gradual contraction. The inner parts of the clump form a protostar, a ball-shaped object that is no longer a cloud, but is not yet a star. Surrounding the protostar is an irregular sphere of gas and dust that had been the outer parts of the clump.

Combining nuclei

When the temperature and pressure in the protostar's core become high enough, nuclear fusion begins. Nuclear fusion is a joining of two atomic nuclei to produce a larger nucleus.

Nuclei that fuse are actually the cores of atoms. A complete atom has an outer shell of one or more particles called electrons, which carry a negative electric charge. Deep inside the atom is the nucleus, which contains almost all the atom's mass. The simplest nucleus, that of the most common form of hydrogen, consists of a single particle known as a proton. A proton carries a positive electric charge. All other nuclei have one or more protons and one or more neutrons. A neutron carries no net charge, and so a nucleus is electrically positive. But a complete atom has as many electrons as protons. The net electric charge of a complete atom is therefore zero -- the atom is electrically neutral.

However, under the enormous temperatures and pressures near the core of a protostar, atoms lose electrons. The resulting atoms are known as ions, and the mixture of the free electrons and ions is called a plasma.

Atoms in the core of the protostar lose all their electrons, and the resulting bare nuclei approach one another at tremendous speeds. Under ordinary circumstances, objects that carry like charges repel each other. However, if the core temperature and pressure become high enough, the repulsion between nuclei can be overcome and the nuclei can fuse. Scientists commonly refer to fusion as "nuclear burning." But fusion has nothing to do with ordinary burning or combustion.

Converting mass to energy

When two relatively light nuclei fuse, a small amount of their mass turns into energy. Thus, the new nucleus has slightly less mass than the sum of the masses of the original nuclei. The German-born American physicist Albert Einstein discovered the relationship E = mc-squared (E=mc 2) that indicates how much energy is released when fusion occurs. The symbol E represents the energy; m, the mass that is converted; and c-squared (c2), the speed of light squared.

The speed of light is 186,282 miles (299,792 kilometers) per second. This is such a large number that the conversion of a tiny quantity of mass produces a tremendous amount of energy. For example, complete conversion of 1 gram of mass releases 90 trillion joules of energy. This amount of energy is roughly equal to the quantity released in the explosion of 22,000 tons (20,000 metric tons) of TNT. This is much more energy than was released by the atomic bomb that the United States dropped on Hiroshima, Japan, in 1945 during World War II. The energy of the bomb was equivalent to the explosion of 13,000 tons (12,000 metric tons) of TNT.

Destruction of light nuclei

In the core of a protostar, fusion begins when the temperature reaches about 1 million K. This initial fusion destroys nuclei of certain light elements. These include lithium 7 nuclei, which consist of three protons and four neutrons. In the process involving lithium 7, a hydrogen nucleus combines with a lithium 7 nucleus, which then splits into two parts. Each part consists of a nucleus of helium 4 -- two protons and two neutrons. A helium 4 nucleus is also known as an alpha particle.

Hydrogen fusion

After the light nuclei are destroyed, the protostar continues to contract. Eventually, the core temperature reaches about 10 million K, and hydrogen fusion begins. The protostar is now a star.

In hydrogen fusion, four hydrogen nuclei fuse to form a helium 4 nucleus. There are two general forms of this reaction: (1) the proton-proton (p-p) reaction and (2) the carbon-nitrogen-oxygen (CNO) cycle.

The p-p reaction can occur in several ways, including the following four-step process:

(1) Two protons fuse. In this step, two protons collide, and then one of the protons loses its positive charge by emitting a positron. The proton also emits an electrically neutral particle called a neutrino.

A positron is the antimatter equivalent of an electron. It has the same mass as an electron but differs from the electron in having a positive charge. By emitting the positron, the proton becomes a neutron. The new nucleus therefore consists of a proton and a neutron -- a combination known as a deuteron.

(2) The positron collides with an electron that happens to be nearby. As a result, the two particles annihilate each other, producing two gamma rays.

(3) The deuteron fuses with another proton, producing a helium 3 nucleus, which consists of two protons and one neutron. This step also produces a gamma ray.

(4) The helium 3 nucleus fuses with another helium 3 nucleus. This step produces a helium 4 nucleus, and two protons are released.

The CNO cycle differs from the p-p reaction mainly in that it involves carbon 12 nuclei. These nuclei consist of six protons and six neutrons. During the cycle, they change into nuclei of nitrogen 15 (7 protons and 8 neutrons) and oxygen 15 (8 protons and 7 neutrons). But they change back to carbon 12 nuclei by the end of the cycle.

Fusion of other elements

Helium nuclei can fuse to form carbon 12 nuclei. However, the core temperature must rise to about 100 million K for this process to occur. This high temperature is necessary because the helium nuclei must overcome a much higher repulsive force than the force between two protons. Each helium nucleus has two protons, so the repulsive force is four times as high as the force between two protons.

The fusion of helium is called the triple-alpha process because it combines three alpha particles to create a carbon 12 nucleus. Helium fusion also produces nuclei of oxygen 16 (8 protons and 8 neutrons) and neon 20 (10 protons and 10 neutrons).

At core temperatures of about 600 million K, carbon 12 can fuse to form sodium 23 (11 protons, 12 neutrons), magnesium 24 (12 protons, 12 neutrons), and more neon 20. However, not all stars can reach these temperatures.

As fusion processes produce heavier and heavier elements, the temperature necessary for further processes increases. At about 1 billion K, oxygen 16 nuclei can fuse, producing silicon 28 (14 protons, 14 neutrons), phosphorus 31 (15 protons, 16 neutrons), and sulfur 32 (16 protons, 16 neutrons).

Fusion can produce energy only as long as the new nuclei have less mass than the sum of the masses of the original nuclei. Energy production continues until nuclei of iron 56 (26 protons, 30 neutrons) begin to combine with other nuclei. When this happens, the new nuclei have slightly more mass than the original nuclei. This process therefore uses energy, rather than producing it.

Evolution of stars

The life cycles of stars follow three general patterns, each associated with a range of initial mass. There are (1) high-mass stars, which have more than 8 solar masses; (2) intermediate-mass stars, with 0.5 to 8 solar masses -- the group that includes the sun; and (3) low-mass stars, with 0.1 to 0.5 solar mass. Objects with less than 0.1 solar mass do not have enough gravitational force to produce the core temperature necessary for hydrogen fusion.

The life cycles of single stars are simpler than those of binary systems, so this section discusses the evolution of single stars first. And because astronomers know much more about the sun than any other star, the discussion begins with the development of intermediate-mass stars.

Intermediate-mass stars

A cloud that eventually develops into an intermediate-mass star takes about 100,000 years to collapse into a protostar. As a protostar, it has a surface temperature of about 4000 K. It may be anywhere from a few times to a few thousand times as luminous as the sun, depending on its mass.

T-Tauri phase

When hydrogen fusion begins, the protostar is still surrounded by an irregular mass of gas and dust. But the energy produced by hydrogen fusion pushes away this material as a protostellar wind. In many cases, the disk that is left over from the collapse channels the wind into two narrow cones or jets. One jet emerges from each side of the disk at a right angle to the plane of the disk. The protostar has become a T-Tauri star, a type of object named after the star T in the constellation Taurus (the Bull). A T-Tauri star is a variable star, one that varies in brightness.

Main-sequence phase

The T-Tauri star contracts for about 10 million years. It stops contracting when its tendency to expand due to the energy produced by fusion in its core balances its tendency to contract due to gravity. By this time, hydrogen fusion in the core is supplying all the star's energy. The star has begun the longest part of its life as a producer of energy from hydrogen fusion, the main-sequence phase. The name of this phase comes from a part of the H-R diagram.

Any star -- whatever its mass -- that gets all its energy from hydrogen fusion in its core is said to be "on the main sequence" or "a main-sequence star." The amount of time a star spends there depends on its mass. The greater a star's mass, the more rapidly the hydrogen in its core is used up, and therefore the shorter is its stay on the main sequence. An intermediate-mass star remains on the main sequence for billions of years.

Red giant phase

When all the hydrogen in the core of an intermediate-mass star has fused into helium, the star changes rapidly. Because the core no longer produces fusion energy, gravity immediately crushes matter down upon it. The resulting compression quickly heats the core and the region around it. The temperature becomes so high that hydrogen fusion begins in a thin shell surrounding the core. This fusion produces even more energy than had been produced by hydrogen fusion in the core. The extra energy pushes against the star's outer layers, and so the star expands enormously.

As the star expands, its outer layers become cooler, so the star becomes redder. And because the star's surface area expands greatly, the star also becomes brighter. The star is now a red giant.

Horizontal branch phase

Eventually, the core temperature reaches 100 million K, high enough to support the triple-alpha process. This process begins so rapidly that its onset is known as helium flash.

As the triple-alpha process continues, the core expands, but its temperature drops. This decrease in temperature causes the temperature of the hydrogen-burning shell to drop. Consequently, the energy output of the shell decreases, and the outer layers of the star contract. The star becomes hotter but smaller and fainter than it had been as a red giant. This change occurs over a period of about 100 million years.

At the end of this period, the star is in its horizontal branch phase, named for the position of the point representing the star on the H-R diagram. The star steadily burns helium and hydrogen, and so its temperature, size, and luminosity do not change significantly. This phase lasts for about 10 million years.

Asymptotic giant phase

When all the helium in the core has fused, the core contracts and therefore becomes hotter. The triple-alpha process begins in a shell surrounding the core, and hydrogen fusion continues in a shell surrounding that. Due to the increased energy produced by the burning in the shells, the star's outer layers expand. The star becomes a giant again, but it is bluer and brighter than it was the first time.

On the H-R diagram, the point representing the star has moved upward and to the right along a line known as the asymptotic (as ihm TOT ihk) giant branch (AGB). The star is therefore called an AGB star.

An AGB star's core is so hot and its gravitational grip on its outermost layers is so weak that those layers blow away in a stellar wind. As each layer blows away, a hotter layer is exposed. Thus, the stellar wind becomes even stronger. Out in space, a succession of new, fast winds slam into old, slow winds that are still moving away from the star. The collisions produce dense shells of gas, some of which cool to form dust.

White dwarf phase



A planetary nebula with an unusual textured appearance, the cause of which is unknown. This photo was taken by the Hubble Space Telescope. Image credit: NASA

In just a few thousand years, all but the hot core of an AGB star blows away, and fusion ceases in the core. The core illuminates the surrounding shells. Such shells looked like planets through the crude telescopes of astronomers who studied them in the 1800's. As a result, the astronomers called the shells planetary nebulae -- and today's astronomers still do. The word nebulae is Latin for clouds.



After a planetary nebula fades from view, the remaining core is known as a white dwarf star. This kind of star consists mostly of carbon and oxygen. Its initial temperature is about 100,000 K.

Black dwarf phase

Because a white dwarf star has no fuel remaining for fusion, it becomes cooler and cooler. Over billions of years, it cools more and more slowly. Eventually, it becomes a black dwarf -- an object too faint to detect. A black dwarf represents the end of the life cycle of an intermediate-mass star.

High-mass stars, those with more than 8 solar masses, form quickly and have short lives. A high-mass star forms from a protostar in about 10,000 to 100,000 years.

High-mass stars on the main sequence are hot and blue. They are 1,000 to 1 million times as luminous as the sun, and their radii are about 10 times the solar radius. High-mass stars are much less common than intermediate- and low-mass stars. Because they are so bright, however, high-mass stars are visible from great distances, and so many are known.

A high-mass star has a strong stellar wind. A star of 30 solar masses can lose 24 solar masses by stellar wind before its core runs out of hydrogen and it leaves the main sequence.

As a high-mass star leaves the main sequence, hydrogen begins to fuse in a shell outside its core. As a result, its radius increases to about 100 times that of the sun. However, its luminosity decreases slightly. Because the star is now emitting almost the same amount of energy from a much larger surface, the temperature of the surface decreases. The star therefore becomes redder.

As the star evolves, its core heats up to 100 million K, enough to start the triple-alpha process. After about 1 million years, helium fusion ends in the core but begins in a shell outside the core. And, as in an intermediate-mass star, hydrogen fuses in a shell outside that. The high-mass star becomes a bright red supergiant.

When the contracting core becomes sufficiently hot, carbon fuses, producing neon, sodium, and magnesium. This phase lasts only about 10,000 years. A succession of fusion processes then occur in the core. Each successive process involves a different element and takes less time. Whenever a different element begins to fuse in the core, the element that had been fusing there continues to fuse in a shell outside the core. In addition, all the elements that had been fusing in shells continue to do so. Neon fuses to produce oxygen and magnesium, a process that lasts about 12 years. Oxygen then fuses, producing silicon and sulfur for about 4 years. Finally, silicon fuses to make iron, taking about a week.

Supernovae

At this time, the radius of the iron core is about 1,900 miles (3,000 kilometers). Because further fusion would consume energy, the star is now doomed. It cannot produce any more fusion energy to balance the force of gravity.

When the mass of the iron core reaches 1.4 solar masses, violent events occur. The force of gravity within the core causes the core to collapse. As a result, the core temperature rises to nearly 10 billion K. At this temperature, the iron nuclei break down into lighter nuclei and eventually into individual protons and neutrons. As the collapse continues, protons combine with electrons, producing neutrons and neutrinos. The neutrinos carry away about 99 percent of the energy produced by the crushing of the core.

Now, the core consists of a collapsing ball of neutrons. When the radius of the ball shrinks to about 6 miles (10 kilometers), the ball rebounds like a solid rubber ball that has been squeezed.

All the events from the beginning of the collapse of the core to the rebounding of the neutrons occur in about one second. But more violence is in store. The rebounding of the ball of neutrons sends a spherical shock wave outward through the star. Much of the energy of the wave causes fusion to occur in overlying layers, creating new elements. As the wave reaches the star's surface, it boosts temperatures to 200,000 K. As a result, the star explodes, hurling matter into space at speeds of about 9,000 to 25,000 miles (15,000 to 40,000 kilometers) per second. The brilliant explosion is known as a Type II supernova.

Supernovae enrich the clouds of gas and dust from which new stars eventually form. This enrichment process has been going on since the first supernovae billions of years ago. Supernovae in the first generation of stars enriched the clouds with materials that later went into making newer stars.

Three generations of stars may exist. Astronomers have not found any of what would be the oldest generation, Population III, stars. But they have found members of the other two generations. Population II stars, which would be the second generation, contain relatively small amounts of heavy elements. The more massive ones aged and died quickly, thereby contributing more nuclei of heavy elements to the clouds. For this reason, Population I stars, the third generation, contain the largest amounts of heavy elements. Yet these quantities are tiny compared with the amount of hydrogen and helium in Population I stars. For example, elements other than hydrogen and helium make up from 1 to 2 percent of the mass of the sun, a Population I star.

Neutron stars

After a Type II supernova blast occurs, the stellar core remains behind. If the core has less than about 3 solar masses, it becomes a neutron star. This object consists almost entirely of neutrons. It packs at least 1.4 solar masses into a sphere with a radius of about 6 to 10 miles (10 to 15 kilometers).

Neutron stars have initial temperatures of 10 million K, but they are so small that their visible light is difficult to detect. However, astronomers have detected pulses of radio energy from neutron stars, sometimes at a rate of almost 1,000 pulses per second.

A neutron star actually emits two continuous beams of radio energy. The beams flow away from the star in opposite directions. As the star rotates, the beams sweep around in space like searchlight beams. If one of the beams periodically sweeps over Earth, a radio telescope can detect it as a series of pulses. The telescope detects one pulse for each revolution of the star. A star that is detected in this way is known as a pulsar.

Black holes

If the stellar core remaining after the supernova explosion has about 3 or more solar masses, no known force can support it against its own gravitation. The core collapses to form a black hole, a region of space whose gravitational force is so strong that nothing can escape from it. A black hole is invisible because it traps even light. All its matter is located at a single point in its center. This point, known as a singularity, is much smaller than an atomic nucleus.

Low-mass stars, ranging from 0.1 to 0.5 solar mass, have surface temperatures less than about 4,000 K. Their luminosities are less than 2 percent of the solar luminosity. Low-mass stars use hydrogen fuel so slowly that they may shine as main-sequence stars for 100 billion to 1 trillion years. This life span is longer than the present age of the universe, believed to be 10 billion to 20 billion years. Therefore, no low-mass star has ever died. Nevertheless, astronomers have determined that low-mass stars will never fuse anything but hydrogen. Thus, as these stars die, they will not pass through a red-giant phase. Instead, they will merely cool to become white dwarfs, then black dwarfs.

Binary stars develop from two protostars that form near each other. More than 50 percent of what seem to the unaided eye to be single stars are actually binaries.



Transfer of mass occurs in a binary star system. Matter flows from a sunlike star, in the background in this illustration, to a disk orbiting a white dwarf star, then to the surface of the dwarf. Image credit: Space Telescope Science Institute

One star in a binary system can affect the life cycle of the other if the two stars are sufficiently close together. Between the stars is a location called the Lagrange point, named for the French mathematician Joseph Louis Lagrange, where the star's gravitational forces are exactly equal. If one of the stars expands so much that its outer layers pass the Lagrange point, the other star will begin to strip away those layers and accumulate them on its surface.



This process, called mass transfer, can take many forms. Mass transfer from a red giant onto a main-sequence companion can add absorption lines of carbon or other elements to the spectrum of the main- sequence star. But if the stars are close together, the material will flow in the opposite direction when the giant star becomes a white dwarf. The matter will spiral in toward the dwarf, forming a hot disk around it. The disk will flare brilliantly in visible and ultraviolet radiation.

If the giant star leaves behind a neutron star or a black hole instead of a white dwarf, an X-ray binary may form. In this case, the matter transferred from the main-sequence star will become extremely hot. When this matter strikes the surface of the neutron star or is pulled into the black hole, it will emit X rays.

In a third case, the red giant becomes a white dwarf, and the main-sequence star becomes a red giant. When enough gas from the giant accumulates on the dwarf's surface, gas nuclei will fuse violently in a flash called a nova. In some cases, so much gas will accumulate that its weight will cause the dwarf to collapse. Almost instantly, the dwarf's carbon will fuse, and the entire dwarf will explode in a Type I supernova. This kind of explosion is so bright that it can outshine an entire galaxy for a few months.

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Global Warming





Global WarmingGlobal 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 Celsius. 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.

Posted by DarshaN at 6:34 PM 0 comments Links to this post





Pluto





PlutoPluto, (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.

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AIR CONDITIONER


Air conditioning is a system that introduces cold air into a hot space to make it more comfortable. Air conditioning typically is used when temperatures are above 70 degrees. It is used in cars, houses, offices, retail stores, restaurants, and other indoor facilities. Unlike heat, air conditioning is considered a luxury, and it is possible to go without it. Heat is considered to be a necessity and all homes must have it. When heat warnings exist, however, lack of air conditioning can present a problem for many elderly people who may live in older brick homes. Without air conditioning, these types of homes literally turn into an oven at a certain temperatures and can cause death.Alternatives to air conditioning are fans, either electric or handheld, or open windows. Air conditioning is used primarily in the summer when temperatures are higher. Therefore electric bills are usually higher in the summer months. Many electric companies offer budget billing, which balances out the high and low bills into a constant monthly payment. Budget billing is based on usage and may gradually increase over time as usage goes up.

There are two types of air conditioners that are primarily used in homes. Window style units are placed in a window. Window air conditioning mainly cools the room that they are placed in. More expensive units may cool off more of the surrounding rooms. Window air conditioners run on electricity. They are placed in windows because the heat in the room is passed outside and swapped with the cold air coming in. Alternatives to window air conditioning units are wall air conditioners, which do not need to be placed in a window. Window units are more common in older homes.

New construction typically has central air conditioning built in. Central air conditioning runs the air throughout the entire house with the actual unit located outside. A thermostat controls the temperatures of the house. Older thermostats need to be changed manually to the desired temperature. Newer thermostats are programmable. Temperature settings can be programmed to change at certain times of the day. This can help lower electric bills by allowing the house to be warmer when the house is empty. Running central air conditioning uses a lot of electricity but it is generally more energy efficient than window air conditioning. Many older homes use either window units or have no air conditioning at all.

Infrared (IR) lasers

Infrared or IR lasers have a beam that is in the infrared spectrum which is from 750nm to 1nm in wavelength. This is a longer wavelength than visible light and a shorter wavelength than radio waves. With a beam that is invisible to the naked eye, IR lasers cannot be used for many normal applications such as light shows or alignment. This makes our Stealth IR laser pointer a specialist laser pointer that is not used by the average laser enthusiasts.





Applications

IR lasers are used by military forces and government agencies world wide in a number of uses. IR lasers are ideal for target acquisition on the battle field where a target can be "painted" with out revealing the lasers location. IR lasers are ideal for use with night vision devices that enhance IR. The invisible nature of IR lasers also makes them ideal for covert monitoring and surveillance. IR lasers are also used in both civilian and military capacity for explosive detection. CWE (chemical warfare agents) absorb IR light, giving them a unique fingerprint detectable by IR lasers. Defense Advanced Research Projects Agency (DARPA) in the USA is working on a portable system for using IR lasers to detect CWEs.

Security systems and alarms based on IR laser beams are very effective because intruders are unable to see the beam or know they have triggered an alert. IR lasers have featured in a number of block buster movies in this capacity though normally with technical inaccuracies.



there are also a number of medical applications for IR lasers such as treating soft tissue injuries, promoting healing and as a treatment for acne. There are numerous documented cases where IR lasers such as our Stealth laser pointer have been successfully used to reduce swelling and inflammation of acute and chronic injuries.

Safety





Safety is a very important issue with IR lasers for two major reasons. Firstly because the beam is invisible, there is no way to avoid the beam and the eyes natural protective blink reflex will not work. The second is that normal protective eye wear for lasers will not work on IR lasers. Safety glasses that are opaque to 532nm and provide more than adequate protection against green laser pointers and portable lasers BUT will be completely transparent to IR lasers such as our Stealth laser pointer. IR lasers need safety glasses specific to IR light.v

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• STRATEGIC PERSPECTIVE – Decarbonization over centuries


.

Decarbonization or the changing carbon intensity of primary energy for the world. Carbon intensity is calculated as the ratio of the sum of the carbon content of all fuels to the sum of the energy content of all primary energy sources. Figure prepared by N. M. Victor, Program for the Human Environment, The Rockefeller University, 2003.



• The most important and surprising fact to emerge from energy studies during the past two decades is that, for the last 200 years, the world has progressively pursued a path of decarbonization, a decreasing relative reliance on carbon



• Think of decarbonization as the course over time in the ratio of tons of carbon in the energy supply to the total energy supply, for example, tons of carbon per tons of oil equivalent encompassing all energy supplies.





• Wood is made of much cellulose and some lignin. Heated cellulose leaves charcoal, almost pure carbon. Lignin is a hydrocarbon with a complex benzenic structure. Wood effectively burns about ten carbons for each hydrogen atom.



• Coal approaches parity with one or two C’s per H, depending on the



• Oils are lighter yet, with, for example, with two H’s per C, in kerosene or jet fuel.



• A molecule of methane, the typical natural gas, is a carbon-trim (CH4) that is one carbon for four molecules of hydrogen.



Competition between hydrogen and carbon in primary energy sources. The evolution is seen in the ratio of hydrogen (H) to carbon © in the world fuel mix, graphed on a logarithmic scale, analyzed as a logistic growth process and plotted in the linear transform of the logistic (S) curve. Progression of the ratio above natural gas (methane, CH 4) requires production of large amounts of hydrogen fuel with non-fossil energy. Source: J. H. Ausubel, Can Technology Spare the Earth? American Scientist 84(2):166-178, 1996.



• In 1800 carbon had 90% of the market. In 1935 the elements tied. With business as usual and the rising Pacific Rim economies not considered, hydrogen will garner 90% of the market by 2100.



• Carbon becomes soot or the feared greenhouse gas CO2, and hydrogen becomes only water when combusted, carbon depending on the combustion process combines with nitrogen and oxygen in the air to generate pollutants like CO2, CO, NOX and hydrogen generates water.



• Decarbonization towards a hydrogen economy is only a question of when not if. This transition provides a convergence not conflict between energy and environment.



• The driving force in evolution of the energy system is the increasing spatial density of energy consumption at the level of the end user.



• The British experience demonstrates that, when energy consumption per unit of area rises, the energy sources with higher economies of scale gain an advantage.



• Coal had a long run at the top of the energy heap. Coal-powered automobiles, however, never had much appeal. The weight and volume of the fuel were hard problems, especially for a highly distributed transport system.



• Oil had a higher energy density than coal—and the advantage of flowing through pipelines and into tanks. It is easy to understand why oil gained ascendancy over coal by 1950 as the world’s leading energy source.



• Nevertheless, the share of primary energy used to make electricity has grown steadily in all countries over the past 75 years and now approaches 40%. The Internet economy demands further electrification, with perfect reliability



• The stable dynamics of the energy system permit reliable forecasts. Decarbonization essentially defines the future of energy supply.



• Globally we are destined to use about 50-80 billion tons more coal. This is about one-third what humans have mined in all our earlier history, and about 30 years at present levels of production.



• Coal companies R/D and commercialization is focused on extracting methane from coal seams and sink CO2 there, staying in business without coal extraction. Using CO2 to displace methane (CH4) adsorbed in coal beds provides a two for one bargain



• Globally, drivers and others will consume close to 300 billion tons more oil, before the fleet runs entirely on H2 separated from methane or water. This amount is almost double the petroleum that has so far been extracted, and about 50 years at present production, so oil companies it is business as usual for a while.



• For gas, the next decades will bring enormous growth, matching rising estimates of the gas resource base, which have more than doubled over the past 20 years;



• Between its uses to fuel turbines to make electric power and for fuel cells for transport, Natural gas will dominate the primary energy picture for the next few decades.



• It is expected that methane will provide perhaps 70% of primary energy soon after the year 2030 and to reach a peak absolute use in 2060 of about 30 x 1012 m3, ten times present annual use.





Conclusion



Evolution is a series of replacements. Replacements also mark the evolution of the energy system. Between about 1910 and 1930 cars replaced horses in the United States.



Earlier steam engines had replaced water wheels and later electric drives replaced steam engines. These replacements required about 50 years in the marketplace.



It required about the same amount of time for railways to replace canals as the lead mode of transport and longer for roads to overtake railways and for air to overtake roads.





• GLOBAL OVERVIEW OF ENERGY





• Increased competition between strategic players for energy





• Energy shortfall in USA





• Increasing Energy demands of Pacific Rim Nations





• Major oil producing countries oil production is on a plateau or peaked





• Increasing dependence on Middle Eastern oil.





• The oil markets do not work well without safety net





• Oil price will rise



Greatest Oil Reserves by Country, 2003





2002

rank Country 2003 proved reserves

(billion barrels)

1. Saudi Arabia 261.7

2. Iraq 115.0

3. Iran 100.1

4. Kuwait 98.9

5. United Arab Emirates 63.0

6. Russia 58.8

7. Venezuela 53.1

8. Nigeria 32.0

9. Libya 30.0

10. China 23.7

NOTES: Figures for Russia are “explored reserves,” which are understood to be proved plus some probable. All other figures are proved reserves recoverable with present technology and prices.

Source: World Oil, Vol. 224, No. 8 (Aug. 2003). From: U.S. Energy Information Administration, International Energy Annual 2002 (March–June 2004).





PAKISTAN SITUATION



ENERGY OVERVIEW



Proven Oil Reserves (1/1/02E): 298 million barrels

Oil Production (2001E): 57,000 barrels per day (bbl/d), of which 53,000 bbl/d was crude oil

Oil Consumption (2001E): 359,000 bbl/d

Net Oil Imports (1999E): 302,000 bbl/d

Crude Oil Refining Capacity (1/1/02E): 238,850 bbl/d

Natural Gas Reserves (1/1/02E): 25.1 trillion cubic feet (Tcf)

Natural Gas Production (1999E): 0.8 Tcf

Natural Gas Consumption (1999E): 0.8 Tcf

Coal Production (1999E): 3.8 million short tons (Mmst)

Coal Consumption (1999E): 4.9 Mmst

Net Coal Imports (1999E): 1.1 Mmst

Recoverable Coal Reserves (12/31/96E): 3.2 billion short tons

Electric Generation Capacity (1/1/99E): 17.0 gigawatts (71% thermal, 28% hydro, 1% nuclear)

Electricity Generation (1999E): 62 billion kilowatthours



ENVIRONMENTAL OVERVIEW



Total Energy Consumption (1999E): 1.8 quadrillion Btu* (0.47% of world total energy consumption)

Energy-Related Carbon Emissions (1999E): 27.9 million metric tons of carbon (0.45% of world total carbon emissions)

Per Capita Energy Consumption (1999E): 12.5 million Btu (vs. U.S. value of 355.8 million Btu)

Per Capita Carbon Emissions (1999E): 0.2 metric tons of carbon (vs. U.S. value of 5.5 metric tons of carbon)

Energy Intensity (1999E): 31,193 Btu/$1990 (vs U.S. value of 12,638 Btu/$1990)**

Carbon Intensity (1999E): 0.48 metric tons of carbon/thousand $1990 (vs U.S. value of 0.19 metric tons/thousand $1990)**

Sectoral Share of Energy Consumption (1998E): Residential (48.8%), Industrial (33.4%), Transportation (13.3%), Commercial (4.5%)

Sectoral Share of Carbon Emissions (1998E): Industrial (44.9%), Transportation (27.2%), Residential (22.2%), Commercial (5.7%)

Fuel Share of Energy Consumption (1999E): Oil (41.9%), Natural Gas (40.0%), Coal (5.0%)

Fuel Share of Carbon Emissions (1999E): Oil (54.6%), Natural Gas (37.4%), Coal (8.0%)

Renewable Energy Consumption (1998E): 1,145 trillion Btu* (1% increase from 1997)

Number of People per Motor Vehicle (1998E): 125 (vs. U.S. value of 1.3)

Status in Climate Change Negotiations: Non-Annex I country under the United Nations Framework Convention on Climate Change (ratified June 1st, 1994). Not a signatory to the Kyoto Protocol.

Major Environmental Issues: Water pollution from raw sewage, industrial wastes, and agricultural runoff; limited natural fresh water resources; a majority of the population does not have access to potable water; deforestation; soil erosion and desertification.

Major International Environmental Agreements: A party to Conventions on Biodiversity, Climate Change, Desertification, Endangered Species, Environmental Modification, Hazardous Wastes, Law of the Sea, Nuclear Test Ban, Ozone Layer Protection, Ship Pollution and Wetlands . Has signed, but not ratified, Marine Life Conservation.

* The total energy consumption statistic includes petroleum, dry natural gas, coal, net hydro, nuclear, geothermal, solar, wind, wood and waste electric power. The renewable energy consumption statistic is based on International Energy Agency (IEA) data and includes hydropower, solar, wind, tide, geothermal, solid biomass and animal products, biomass gas and liquids, industrial and municipal wastes. Sectoral shares of energy consumption and carbon emissions are also based on IEA data.

**GDP based on EIA International Energy Annual 1999





ENERGY INDUSTRY



Organization: Oil and Gas Development Corporation (OGDC), a state company, handles oil and gas exploration and development; Water and Power Development Authority (WAPDA) supplies electricity to most of the country; Karachi Electric Supply Corporation Limited (KESC) serves the greater Karachi metropolitan area; Pakistan Atomic Energy Commission (PAEC) operates one nuclear power plant

Major Foreign Energy Company Involvement: AES, Atlantic Richfield, British National Power, Coastal Power, Gaz de France, Total, General Electric, Lasmo Oil (U.K.), Marubeni (Japan), ExxonMobil, Monument Oil & Gas, Premier Oil, Royal Dutch Shell, Xenal (Saudi Arabia)

Major Ports: Gwadar, Karachi, Muhammed bin Qasim, Ormaro

Major Gas Fields: Bhit, Dhodak, Kadanwari, Mari, Prikoh, Qadipur, Sawan, Sui

Major Oil Fields: Dhurnal, Fimkasser, Liari, Mazari, Thora

Major Pipelines: Sui Northern Gas Pipeline; Sui Southern Gas Pipeline; Pak-Arab Refinery Company (PARCO) petroleum product pipeline

Major Refineries (Capacity): Pak-Arab Refinery near Multan (100,000 bbl/d); Attock Refinery in Rawalpindi (35,000 bbl/d), National Refinery in Korangi (62,050 bbl/d), Pakistan Refinery Ltd. in Karachi (46,300 bbl/d)





Pakistan: Environmental Issues



Agricultural runoff exacerbated by ongoing deforestation and industrial runoff have polluted water supplies, factory and vehicle emissions have degraded air quality in the urban centers.



In an attempt to redress the nation's mounting environmental problems, in 1992 the government issued its National Conservation Strategy Report (NCSR).



Building on the Pakistan Environmental Protection Ordinance of 1983, the NCSR stipulated three goals for the country's environmental protection efforts: (1) conservation of natural resources; (2) promotion of sustainable development; and (3) improvement of efficiency in the use and management of resources



In addition, in 1993 Pakistan instituted National Environmental Quality Standards (NEQS) on municipal and liquid industrial effluents and industrial gaseous emissions, motor vehicle exhaust, and noise

The new environmental regulations were implemented in 1996; only 3% of industries were able to pass the test for compliance. National attention towards environmental issues has increased recently because, under provisions of a World Trade Organization (WTO) agreement, Pakistan will have difficulty after 2005 exporting products from industries without adequate environmental safeguards.

Pakistan has not funded environmental protection efforts adequately. A January 2000 report released by the Ministry of Environment showed that Pakistan currently spends about $17 million per year on pollution-related cleanup; however, $84 million is needed to correct the country's environmental problems, and $1.8 billion per year in losses from environmental damage.

Much of the country suffers from a lack of potable water due to industrial waste and agricultural runoff that contaminates drinking water supplies.





Air Pollution



The level of air pollution in Pakistan's two largest cities, Karachi and Lahore, is estimated to be 20 times higher than World Health Organization standards.



As industry has expanded, factories have emitted more and more toxic effluents into the air. Also, as in other developing countries, the number of vehicles in Pakistan has swelled in recent years--from 680,000 in 1980 to 5 million in 2003.

The 1992 National Conservation Strategy Report claims that the average Pakistani vehicle emits 25 times as much carbon dioxide as the average U.S. vehicle, as well as 20 times as many hydrocarbons and more than 3.5 times as many nitrous oxides in grams per kilometer.



Economic damages from urban air pollution are estimated at about $370 million, with 6.4 million people hospitalized annually for air-pollution-related illnesses





Energy Consumption



Pakistan's energy consumption has nearly tripled in the last 20 years, from 0.6 quadrillion Btu in 1980 to 1.9 quads in 2001



In terms of per capita energy consumption, Pakistan's level of 12.9 million Btu in 2001 was higher than Bangladesh's (3.7 million Btu), but virtually on par with India's (12.6 million). In comparison, China's per capita energy consumption in 2001 was 30.9 million Btu, Iran's was 80.3 million Btu, and Russia's was 195.3 million Btu, while U.S. per capita consumption was 341.8 million Btu.





• PAKISTAN – THE THREATS AND OPPORTUNITY



• THREATS





• Imported oil constitutes over 31% of energy consumption 2002/2003 at a cost of $ 3,096 billion.



• The depletion/decline of Pakistan’s natural gas reserves/production will increase dependence on imports.





• Energy prices are expected to increase over the next few years.



• The tipping point in global supply and demand will occur within the next decade if not earlier.





• To reduce poverty and increase prosperity – the energy consumption in Pakistan has to increase.



• The price and availability of natural gas and oil can potentially have grave impacts to Pakistan’s welfare and national security.





• OPPORTUNITIES



• India’s appetite for energy is escalating – natural gas pipelines from either Iran or Central Asia will have to pass through Pakistan. LNG for India is an option but at a much higher price.





• China also has also signed 20 year natural gas agreement with Iran – though for LNG. As China develops its Western regions it may find a pipeline option more attractive not only for economic but also for security reasons.





• Since Pakistan’s situation is more acute than most developed and developing countries it can be a lead player and model in the Changing Global Scenario – Proactive as opposed to traditional economic reactive after the event.





• GLOBAL TECHNOLOGY AND COMMERCIALIZATION REVIEW



• Most alternate energy sources like wind, solar, geothermal, wave, biomass etc are well documented.



• Solar PV panels are still under intense development for increased efficiency and reduced cost. Solar technology is expected to grow to US market of over $ 30 billion in the next 5 to 8 years.



• This presentation will focus more on a technology which produces electric power with no harmful emissions at efficiencies greater than 60% - Fuel Cell.



• Fuel Cell use Hydrogen to produce electric power and water – endless battery.



• Hydrogen can be produced by



o Electrolysis of water – producing hydrogen and oxygen gases.



o Reformation of any Hydrocarbon fuel – producing hydrogen, carbon monoxide and other emissions.



• Hydrogen can be stored by various methods including compressed.



• This stored hydrogen can be used on-demand to generate electric power.



• This storage of hydrogen for subsequent use to generate electricity is called “Hydrocity”.



• Solar and wind power are present in Pakistan but not available 24 hours and 365/366 days per year.



• Hydrogen can be used in Internal Combustion engines, like natural gas, to generate power (22%); though more it more efficient with fuel cell (greater 60%).



• Fuel Cell have two primary Global Markets



o Stationary Power Generation



o Mobile and Automotive applications.



• Present Barriers to fuel cell mass commercialization



o Stationary Power Generation



 Cost per kilowatt/megawatt only competitive in niche markets.



 Currently hundreds of millions in research and development being expended to quickly achieve cost targets for mass commercialization.



 Presently hundreds of commercial applications and pilot projects running in the world.



o Mobile and Automotive applications



 Cost per kilowatt/megawatt is over $500/Kw – target is $25 per Kilowatt.



 Hydrogen Storage:



o 7-10% hydrogen by weight,

o must store 7-10 Kilograms of hydrogen,

o Not more than 30% package penalty than gasoline tank.

o Unit cost less than $30



 Power Electronics:



o Unit cost less than $7 per Kw.



o Packing size be around 20 Kw per litre



o Greater than 98% efficiency.



o Commutation speed greater 20 khz, so as to be above audible range



 Currently GM, Toyota, Mercedes Benz, Ford etc are devoting thousands of engineers and hundreds of millions in research and development being expended to quickly achieve cost target of $30 per kilowatt for mass commercialization.



 The generally expected date for mass consumer commercialization is 2015 but marketing noises are indicating as early as 2010.



 Presently pilot automotive cars are running in the world from GM, Ford, and Toyota, MB etc.





• ROADMAP FOR PAKISTAN





o WHAT



 Decrease dependence on energy imports – displace imported fuel with local energy and hydrogen.

 Reduce Poverty

 Increase prosperity

 Increase energy consumption to sustain growth in GDP

 Mitigate risk to national security

 Increase usage of Hydrogen as a fuel - Eventually achieve congruence between energy and environment.

 Increase efficiency of energy usage.

 Move to decentralized and distributed energy generation and sustaining scenario.

 Globally recognized as an Alternate Energy and Power Generation Technology Nation.

 Global exporter of alternate energy and power generation solutions.





o HOW



 Deletion policies should be expanded to include fuels, energy and power generation.

 Close coordination between energy and environment policies.

 Leverage private sector for implementation.

 Policies to generate establish and foster alternate energy and power industry and research – the markets potential should be clearly visible to MNCs and International investors.

 Establish Alternate Energy and Power Research, Development and Testing facilities – similar to Argonne and NREL in USA.

 Develop global state of manpower and human resources to support cradle to death expertise in the sector.

 Develop Global alliances to secure and dependable supply chain relationships.

 Organization and mechanisms developed to ensure timely implementation at national, provincial and local levels.

 Initiate Pilot programs to benchmark new and emerging technologies – provide forums for private sector awareness of innovative opportunities and solutions.

 Foster niche markets for early commercialization of new technologies.