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Space exploration is our human response to curiosity about Earth, the moon, the planets, the sun and other stars, and the galaxies. Piloted and unpiloted space vehicles venture far beyond the boundaries of Earth to collect valuable information about the universe. Human beings have visited the moon and have lived in space stations for long periods. Space exploration helps us see Earth in its true relation with the rest of the universe. Such exploration could reveal how the sun, the planets, and the stars were formed and whether life exists beyond our own world.

The space age began on Oct. 4, 1957. On that day, the Soviet Union launched Sputnik (later referred to as Sputnik 1), the first artificial satellite to orbit Earth. The first piloted space flight was made on April 12, 1961, when Yuri A. Gagarin, a Soviet cosmonaut, orbited Earth in the spaceship Vostok (later called Vostok 1).

Unpiloted vehicles called space probes have vastly expanded our knowledge of outer space, the planets, and the stars. In 1959, one Soviet probe passed close to the moon and another hit the moon. A United States probe flew past Venus in 1962. In 1974 and 1976, the United States launched two German probes that passed inside the orbit of Mercury, close to the sun. Two other U.S. probes landed on Mars in 1976. In addition to studying planets and their moons, space probes have investigated comets and asteroids.

The first piloted voyage to the moon began on Dec. 21, 1968, when the United States launched the Apollo 8 spacecraft. It orbited the moon 10 times and returned safely to Earth. On July 20, 1969, U.S. astronauts Neil A. Armstrong and Buzz Aldrin landed their Apollo 11 lunar module on the moon. Armstrong became the first person to set foot on the moon. United States astronauts made five more landings on the moon before the Apollo lunar program ended in 1972.

During the 1970's, astronauts and cosmonauts developed skills for living in space aboard the Skylab and Salyut space stations. In 1987 and 1988, two Soviet cosmonauts spent 366 consecutive days in orbit.

On April 12, 1981, the United States space shuttle Columbia blasted off. The shuttle was the first reusable spaceship and the first spacecraft able to land at an ordinary airfield. On Jan. 28, 1986, a tragic accident occurred. The U.S. space shuttle Challenger tore apart in midair, killing all seven astronauts aboard. The shuttle was redesigned, and flights resumed in 1988. A second tragedy struck the shuttle fleet on Feb. 1, 2003. The Columbia broke apart as it reentered Earth's atmosphere, killing all seven of its crew members. The United States did not launch a shuttle again until 2005.

In the early years of the space age, success in space became a measure of a country's leadership in science, engineering, and national defense. The United States and the Soviet Union were engaged in an intense rivalry called the Cold War. As a result, the two nations competed with each other in developing space programs. In the 1960's and 1970's, this "space race" drove both nations to tremendous exploratory efforts. The space race had faded by the end of the 1970's, when the two countries began to pursue independent goals in space.

A major dispute in the development of space programs has been the proper balance of piloted and unpiloted exploration. Some experts favor unpiloted probes because they may be cheaper, safer, and faster than piloted vehicles. They note that probes can make trips that would be too risky for human beings to attempt. On the other hand, probes generally cannot react to unexpected occurrences. Today, most space planners favor a combined, balanced strategy of unpiloted probes and piloted expeditions. Probes can visit uncharted regions of space or patrol familiar regions where the data to be gathered fall within expected limits. But in some cases, people must follow the probes and use human ingenuity, flexibility, and courage to explore the mysteries of the universe.

What is space?

Space is the near-emptiness in which all objects in the universe move. The planets and the stars are tiny dots compared with the vast expanse of space.

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The beginning of space. Earth is surrounded by air, which makes up its atmosphere. As the distance from Earth increases, the air becomes thinner. There is no clear boundary between the atmosphere and outer space. But most experts say that space begins somewhere beyond 60 miles (95 kilometers) above Earth.

Outer space just above the atmosphere is not entirely empty. It contains some particles of air, as well as space dust and occasional chunks of metallic or stony matter called meteoroids. Various kinds of radiation flow freely. Thousands of spacecraft known as artificial satellites have been launched into this region of space.

Earth's magnetic field, the space around the planet in which its magnetism can be observed, extends far out beyond the atmosphere. The magnetic field traps electrically charged particles from outer space, forming zones of radiation called the Van Allen belts.

The region of space in which Earth's magnetic field controls the motion of charged particles is called the magnetosphere. It is shaped like a teardrop, with the point extending away from the sun. Beyond this region, Earth's magnetic field is overpowered by that of the sun. But even such vast distances are not beyond the reach of Earth's gravity. As far as 1 million miles (1.6 million kilometers) from Earth, this gravity can keep a satellite orbiting the planet instead of flying off into space.

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Space between the planets is called interplanetary space. The sun's gravity controls the motion of the planets in this region. That is why the planets orbit the sun.

Huge distances usually separate objects moving through interplanetary space. For example, Earth revolves around the sun at a distance of about 93 million miles (150 million kilometers). Venus moves in an orbit 68 million miles (110 million kilometers) from the sun. Venus is the planet that comes closest to Earth—25 million miles (40 million kilometers) away—whenever it passes directly between Earth and the sun. But this is still 100 times as far away as the moon.

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Space between the stars is called interstellar space. Distances in this region are so great that astronomers do not describe them in miles or kilometers. Instead, scientists measure the distance between stars in units called light-years. For example, the nearest star to the sun is Proxima Centauri, 4.2 light-years away. A light-year equals 5.88 trillion miles (9.46 trillion kilometers). This is the distance light travels in one year at its speed of 186,282 miles (299,792 kilometers) per second.

Various gases, thin clouds of extremely cold dust, and a few escaped comets float between the stars. Interstellar space also contains many objects not yet discovered.

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Getting into space and back

Overcoming gravity is the biggest problem for a space mission. A spacecraft must be launched at a particular velocity (speed and direction).

Gravity gives everything on Earth its weight and accelerates free-falling objects downward. At the surface of Earth, acceleration due to gravity, called g, is about 32 feet (10 meters) per second each second.

A powerful rocket called a launch vehicle or booster helps a spacecraft overcome gravity. All launch vehicles have two or more rocket sections known as stages. The first stage must provide enough thrust (pushing force) to leave Earth's surface. To do so, this stage's thrust must exceed the weight of the entire launch vehicle and the spacecraft. The booster generates thrust by burning fuel and then expelling gases. Rocket engines run on a special mixture called propellant. Propellant consists of solid or liquid fuel and an oxidizer, a substance that supplies the oxygen needed to make the fuel burn in the airlessness of outer space. Lox, or liquid oxygen, is a frequently used oxidizer.

The minimum velocity required to overcome gravity and stay in orbit is called orbital velocity. At a rate of acceleration of 3 g's, or three times the acceleration due to gravity, a vehicle reaches orbital velocity in about nine minutes. At an altitude of 120 miles (190 kilometers), the speed needed for a spacecraft to maintain orbital velocity and thus stay in orbit is about 5 miles (8 kilometers) per second.

In many rocket launches, a truck or tractor moves the rocket and its payload (cargo) to the launch pad. At the launch pad, the rocket is moved into position over a flame pit, and workers load propellants into the rocket through special pipes.

At launch time, the rocket's first-stage engines ignite until their combined thrust exceeds the rocket's weight. The thrust causes the vehicle to lift off the launch pad. If the rocket is a multistage model, the first stage falls away a few minutes later, after its propellant has been used up. The second stage then begins to fire. A few minutes later, it, too, runs out of propellant and falls away. If needed, a small upper stage rocket then fires until orbital velocity is achieved.

The launch of a space shuttle is slightly different. The shuttle has solid-propellant boosters in addition to its main rocket engines, which burn liquid propellant. The boosters combined with the main engines provide the thrust to lift the vehicle off the launch pad. After slightly more than two minutes of flight, the boosters separate from the shuttle and return to Earth by parachute. The main engines continue to fire until the shuttle has almost reached orbital velocity. Small engines on the shuttle push it the remainder of the way to orbital velocity.

To reach a higher altitude, a spacecraft must make another rocket firing to increase its speed. When the spacecraft reaches a speed about 40 percent faster than orbital velocity, it achieves escape velocity, the speed necessary to break free of Earth's gravity.

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Returning to Earth involves the problem of decreasing the spacecraft's great speed. To do this, an orbiting spacecraft uses small rockets to redirect its flight path into the upper atmosphere. This action is called de-orbit. A spacecraft returning to Earth from the moon or from another planet also aims its path to skim the upper atmosphere. Air resistance then provides the rest of the necessary deceleration (speed reduction).

At the high speeds associated with reentering the atmosphere from space, air cannot flow out of the way of the onrushing spacecraft fast enough. Instead, molecules of air pile up in front of it and become tightly compressed. This squeezing heats the air to a temperature of more than 10,000 °F (5,500 °C), hotter than the surface of the sun. The resulting heat that bathes the spacecraft would burn up an unprotected vehicle in seconds. Insulating plates of quartz fiber glued to the skin of some spacecraft create a heat shield that protects against the fierce heat. Refrigeration may also be used. Early spacecraft had ablative shields that absorbed heat by burning off, layer by layer, and vaporizing.

Many people mistakenly believe that the spacecraft skin is heated through friction with the air. Technically, this belief is not accurate. The air is too thin and its speed across the spacecraft's surface is too low to cause much friction.

For unpiloted space probes, deceleration forces can be as great as 60 to 90 g's, or 60 to 90 times the acceleration due to gravity, lasting about 10 to 20 seconds. Space shuttles use their wings to skim the atmosphere and stretch the slowdown period to more than 15 minutes, thereby reducing the deceleration force to about 1¹/2 g's.

When the spacecraft has lost much of its speed, it falls freely through the air. Parachutes slow it further, and a small rocket may be fired in the final seconds of descent to soften the impact of landing. The space shuttle uses its wings to glide to a runway and land like an airplane. The early U.S. space capsules used the cushioning of water and "splashed down" into the ocean.

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Living in space

When people orbit Earth or travel to the moon, they must live temporarily in space. Conditions there differ greatly from those on Earth. Space has no air, and temperatures reach extremes of heat and cold. The sun gives off dangerous radiation. Various types of matter also create hazards in space. For example, particles of dust called micrometeoroids threaten vehicles with destructive high-speed impacts. Debris (trash) from previous space missions can also damage spacecraft.

On Earth, the atmosphere serves as a natural shield against many of these threats. But in space, astronauts and equipment need other forms of protection. They must also endure the physical effects of space travel and protect themselves from high acceleration forces during launch and landing.

The basic needs of astronauts in space must also be met. These needs include breathing, eating and drinking, elimination of body wastes, and sleeping.

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Protection against the dangers of space

Engineers working with specialists in space medicine have eliminated or greatly reduced most of the known hazards of living in space. Space vehicles usually have double hulls for protection against impacts. A particle striking the outer hull disintegrates and thus does not damage the inner hull.

Astronauts are protected from radiation in a number of ways. Missions in earth orbit remain in naturally protected regions, such as Earth's magnetic field. Filters installed on spacecraft windows protect the astronauts from blinding ultraviolet rays.

The crew must also be protected from the intense heat and other physical effects of launch and landing. Space vehicles require a heat shield to resist high temperatures and sturdy construction to endure crushing acceleration forces. In addition, the astronauts must be seated in such a way that the blood supply will not be pulled from their head to their lower body, causing dizziness or unconsciousness.

Aboard a spacecraft, temperatures climb because of the heat given off by electrical devices and by the crew's bodies. A set of equipment called a thermal control system regulates the temperature. The system pumps fluids warmed by the cabin environment into radiator panels, which discharge the excess heat into space. The cooled fluids are pumped back into coils in the cabin.

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Microgravity

Once in orbit, the space vehicle and everything inside it experience a condition called microgravity. The vehicle and its contents fall freely, resulting in an apparently weightless floating aboard the spacecraft. For this reason, microgravity is also referred to as zero gravity. However, both terms are technically incorrect. The gravitation in orbit is only slightly less than the gravitation on Earth. The spacecraft and its contents continuously fall toward Earth. But because of the vehicle's tremendous forward speed, Earth's surface curves away as the vehicle falls toward it. The continuous falling seems to eliminate the weight of everything inside the spacecraft. For this reason, the condition is sometimes referred to as weightlessness.

Microgravity has major effects on both equipment and people. For example, fuel does not drain from tanks in microgravity, so it must be squeezed out by high-pressure gas. Hot air does not rise in microgravity, so air circulation must be driven by fans. Particles of dust and droplets of water float throughout the cabin and only settle in filters on the fans.

The human body reacts to microgravity in a number of ways. In the first several days of a mission, about half of all space travelers suffer from persistent nausea, sometimes accompanied by vomiting. Most experts believe that this "space sickness," called space adaptation syndrome, is the body's natural reaction to microgravity. Drugs to prevent motion sickness can provide some relief for the symptoms of space adaptation syndrome, and the condition generally passes in a few days.

Microgravity also confuses an astronaut's vestibular system—that is, the organs of balance in the inner ear—by preventing it from sensing differences in direction. After a few days in space, the vestibular system disregards all directional signals. Soon after an astronaut returns to Earth, the organs of balance resume normal operation.

Over a period of days or weeks, an astronaut's body experiences deconditioning. In this process, muscles grow weak from lack of use, and the heart and blood vessels "get lazy." Strenuous exercise helps prevent deconditioning. Space travelers ride exercise bikes, use treadmills, and perform other types of physical activity.

After many months in space, a process called demineralization weakens the bones. Most physicians believe that demineralization results from the absence of stress on the bones in a weightless environment. The experiences of Soviet cosmonauts who spent long periods in orbit showed that vigorous exercise and a special diet can minimize demineralization.

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Meeting basic needs in space

Piloted space vehicles have life-support systems designed to meet all the physical needs of the crew members. In addition, astronauts can carry portable life-support systems in backpacks when they work outside the main spacecraft.

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Breathing. A piloted spacecraft must have a source of oxygen for the crew to breathe and a means of removing carbon dioxide, which the crew exhales. Piloted space vehicles use a mixture of oxygen and nitrogen similar to Earth's atmosphere at sea level. Fans circulate air through the cabin and over containers filled with pellets of a chemical called lithium hydroxide. These pellets absorb carbon dioxide from the air. Carbon dioxide can also be combined with other chemicals for disposal. Charcoal filters help control odors.

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Eating and drinking. The food on a spacecraft must be nutritious, easy to prepare, and convenient to store. On early missions, astronauts ate freeze-dried foods—that is, frozen foods with the water removed. To eat, the astronauts simply mixed water into the food. Packaging consisted of plastic tubes. The astronauts used straws to add the water.

Over the years, the food available to space travelers became more appetizing. Today, astronauts enjoy ready-to-eat meals much like convenience foods on Earth. Many space vehicles have facilities for heating frozen and chilled food.

Water for drinking is an important requirement for a space mission. On space shuttles, devices called fuel cells produce pure water as they generate electricity for the spacecraft. On long missions, water must be recycled and reused as much as possible. Dehumidifiers remove moisture from exhaled air. On space stations, this water is usually reused for washing.

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Eliminating body wastes. The collection and disposal of body wastes in microgravity poses a major challenge. Astronauts use a device that resembles a toilet seat. Air flow produces suction that moves the wastes into collection equipment under the seat. On small spacecraft, crew members use funnels for urine and plastic bags for solid wastes. While working outside the spacecraft, astronauts wear special equipment to contain body wastes.

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Bathing. The simplest bathing method aboard a spacecraft is a sponge bath with wet towels. Astronauts on early space stations used a fully enclosed, collapsible plastic shower stall. This allowed the astronauts to spray their bodies with water, then vacuum the stall and towel themselves dry. Newer space stations have permanent shower stalls.

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Sleeping. Space travelers can sleep in special sleeping bags with straps that press them to the soft surface and to a pillow. However, most astronauts prefer to sleep floating in the air, with only a few straps to keep them from bouncing around the cabin. Astronauts may wear blindfolds to block the sunlight that streams in the windows periodically during orbit. Typically, sleep duration in space is about the same as that on Earth.

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Recreation on long space flights is important to the mental health of the astronauts. Sightseeing out the spacecraft window is a favorite pastime. Space stations have small collections of books, tapes, and electronic games. Exercise also provides relaxation.

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Controlling inventory and trash. Keeping track of the thousands of items used during a mission poses a major challenge in space. Drawers and lockers hold some materials. Other equipment is strapped to the walls, ceilings, and floors. Computer-generated lists keep track of what is stored where, and computerized systems check the storage and replacement of materials. The crew aboard the spacecraft may stow trash in unused sections of the vehicle, throw it overboard to burn up harmlessly in the atmosphere, or bring it back to Earth for disposal.

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Communicating with Earth

Communication between astronauts in space and mission control, the facility on Earth that supervises their space flight, occurs in many ways. The astronauts and mission controllers can talk to each other by radio. Television pictures can travel between space vehicles and Earth. Computers, sensors, and other equipment continuously send signals to Earth for monitoring. Facsimile machines on spacecraft also can receive information from Earth.

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Working in space

Once a space vehicle reaches its orbit, the crew members begin to carry out the goals of their mission. They perform a variety of tasks both inside and outside the spacecraft.

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Navigation, guidance, and control. Astronauts use computerized navigation systems and make sightings on stars to determine their position and direction. On Earth, sophisticated tracking systems measure the spacecraft's location in relation to Earth. Astronauts typically use small firings of the spacecraft's rockets to tilt the vehicle or to push it in the desired direction. Computers monitor these changes to ensure they are done accurately.

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Activating equipment. Much of the equipment on a space vehicle is turned off or tied down during launch. Once in space, the astronauts must set up and turn on the equipment. At the end of the mission, they must secure it for landing.

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Conducting scientific observations and research. Astronauts use special instruments to observe Earth, the stars, and the sun. They also experiment with the effects of microgravity on various materials, plants, animals, and themselves.

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Docking. As a spacecraft approaches a target, such as a space station or an artificial satellite, radar helps the crew members control the craft's course and speed. Once the spacecraft reaches the correct position beside the target, it docks (joins) with the target by connecting special equipment. Such a meeting in space is called a rendezvous. A space shuttle can also use its robot arm to make contact with targets.

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Maintaining and repairing equipment. The thousands of pieces of equipment on a modern space vehicle are extremely reliable, but some of them still break down. Accidents damage some equipment. Other units must be replaced when they get old. Astronauts must find out what has gone wrong, locate the failed unit, and repair or replace it.

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Assembling space stations. Astronauts may serve as construction workers in space, assembling a space station from components carried up in the shuttle. On existing space stations, crews often must add new sections or set up new antennas and solar panels. Power and air connectors must be hooked up inside and outside the station.

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Leaving the spacecraft. At times, astronauts must go outside the spacecraft to perform certain tasks. Working outside a vehicle in space is called extravehicular activity (EVA). To prepare for EVA, astronauts put on their space suits and move to a special two-doored chamber called an air lock. They then release the air from the air lock, open the outer hatch, and leave the spacecraft. When they return, they close the outer door and let air into the air lock. Then they open the inner door into the rest of the spacecraft, where they remove their space suits.

A space suit can keep an astronaut alive for six to eight hours. The suit is made from many layers of flexible, airtight materials, such as nylon and Teflon. It provides protection against heat, cold, and space particles. Tight mechanical seals connect the pieces of the space suit. Equipment in a backpack provides oxygen and removes carbon dioxide and moisture. A radio enables the astronaut to communicate with other crew members and with Earth. The helmet must allow good visibility while at the same time blocking harmful solar radiation. Gloves are a crucial part of the space suit. They must be thin and flexible enough for the astronaut to feel small objects and to handle tools.

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The dawn of the space age

As people began to dream of flying above Earth's surface, they realized that objects in the sky could become destinations for human travelers. In the early 1600's, the German astronomer and mathematician Johannes Kepler became the first scientist to describe travel to other worlds. He also developed the laws of planetary motion that explain the orbits of bodies in space. See Kepler, Johannes.

The English scientist Sir Isaac Newton first described the laws of motion in a work published in 1687. These laws enabled scientists to predict the kinds of flight paths needed to orbit Earth and to reach other worlds. Newton also described how an artificial satellite could remain in orbit. His third law, which states that for every action there is an equal and opposite reaction, explains why a rocket works. See Motion (Newton's laws of motion); Newton, Sir Isaac.

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Early dreams of space flight. During the 1700's, scientists realized that air got thinner at higher altitudes. This meant that air probably was entirely absent between Earth and other worlds, so wings would be useless. Many imaginative writers proposed fanciful techniques for travel to these worlds.

In 1903, Konstantin E. Tsiolkovsky, a Russian high-school teacher, completed the first scientific paper on the use of rockets for space travel. Several years later, Robert H. Goddard of the United States and Hermann Oberth of Germany awakened wider scientific interest in space travel. Working independently, these three men addressed many of the technical problems of rocketry and space travel. Together, they are known as the fathers of space flight.

In 1919, Goddard explained how rockets could be used to explore the upper atmosphere in his paper "A Method of Reaching Extreme Altitudes." The paper also described a way of firing a rocket to the moon. In a book called The Rocket into Interplanetary Space (1923), Oberth discussed many technical problems of space flight. He even described what a spaceship would be like. Tsiolkovsky wrote a series of new studies in the 1920's. These works included detailed descriptions of multistage rockets.

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The first space rockets. During the 1930's, rocket research went forward in the United States, Germany, and the Soviet Union. Goddard's team had built the world's first liquid-propellant rocket in 1926, despite a lack of support from the U.S. government. German and Soviet rocket scientists received funding from their governments to develop military missiles.

In 1942, during World War II, German rocket experts under the direction of Wernher von Braun developed the V-2 guided missile. Thousands of V-2's were fired against European cities, especially London, causing widespread destruction and loss of life.

After World War II ended in 1945, many German rocket engineers went to work for the U.S. government to help develop military missiles. The U.S. Navy worked on larger rockets, such as the Aerobee and the Viking. In 1949, the rocket team built and tested the world's first two-stage rocket, with a V-2 missile as a first stage and a small WAC Corporal rocket as a second stage. This rocket reached an altitude of 250 miles (400 kilometers).

By 1947, the Soviet Union had secretly begun a massive program to develop long-range military missiles. In the 1940's, the small but influential British Interplanetary Society published accurate plans for piloted lunar landing vehicles, space suits, and orbital rendezvous. A U.S. group, the American Rocket Society, concentrated on missile engineering. In 1950, a new International Astronautical Federation began to hold annual conferences.

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The first artificial satellites. In 1955, both the United States and the Soviet Union announced plans to launch artificial satellites with scientific instruments on board. The satellites were to be sent into orbit as part of the International Geophysical Year, a period of international cooperation in scientific research beginning in July 1957. The Soviets provided detailed descriptions of the radio equipment to be included on their satellite. But the Soviet rocket program had been kept secret until that time. As a result, many people in other countries did not believe that the Soviets had the advanced technology required for space exploration.

Then, on Oct. 4, 1957, the Soviets stunned the world by succeeding in their promise—and by doing so ahead of the United States. Only six weeks earlier, the Soviet two-stage R-7 missile had made its first 5,000-mile (8,000-kilometer) flight. This time, it carried Sputnik (later referred to as Sputnik 1), the first artificial satellite. Sputnik means traveling companion in Russian. The R-7 booster hurled the 184-pound (83-kilogram) satellite and its main rocket stage into orbit around Earth. Radio listeners worldwide picked up Sputnik's characteristic "beep-beep" signal. See Back in Time: Space exploration (1957).

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The space race begins. The Western world reacted to the launch of Sputnik with surprise, fear, and respect. Soviet Premier Nikita S. Khrushchev ordered massive funding of follow-up projects that would continue to amaze and dazzle the world. In the United States, leaders vowed to do whatever was needed to catch up. Thus the "space race" began.

More Soviet successes followed. A month after Sputnik, another satellite, Sputnik 2, carried a dog named Laika into space. The flight proved that animals could survive the unknown effects of microgravity. In 1959, Luna 2 became the first probe to hit the moon. Later that year, Luna 3 photographed the far side of the moon, which cannot be seen from Earth.

The first United States satellite was Explorer 1, launched on Jan. 31, 1958. This satellite was followed by Vanguard 1, which was launched on March 17, 1958. See Back in Time: Space exploration (1958). These and later U.S. satellites were much smaller than their Soviet counterparts because the rockets the United States used to carry satellites were smaller and less powerful than those used by the Soviet Union. The Soviet Union's rockets gave it an early lead in the space race. Because bigger rockets would be needed for piloted lunar flight, both the United States and the Soviet Union began major programs of rocket design, construction, and testing.

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Organizing and managing space activities. A key to the ultimate success of U.S. space programs was centralized planning. In 1958, a civilian space agency called the National Aeronautics and Space Administration (NASA) was established. NASA absorbed various aviation researchers and military space laboratories. The formation of NASA helped forge agreement among competing interests, including military branches, universities, the aerospace industry, and politicians.

Soviet space activities, on the other hand, were coordinated by special executive commissions. These commissions tried to tie together various space units from military and industrial groups, as well as competing experts and scientists. But the commissions did not coordinate Soviet activities effectively enough to meet the complex challenges of the space race.

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Space probes

A space probe is an unpiloted device sent to explore space. A probe may operate far out in space, or it may orbit or land on a planet or a moon. It may make a one-way journey, or it may bring samples and data back to Earth. Most probes transmit data from space by radio in a process called telemetry.

Lunar and planetary probes that land on their targets may be classified according to their landing method. Impact vehicles make no attempt to slow down as they approach the target. Hard-landers have cushioned instrument packages that can survive the impact of a hard landing. Soft-landers touch down gently. Penetrators ram deeply into the surface of a target.

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How a space probe carries out its mission. Probes explore space in a number of ways. A probe makes observations of temperature, radiation, and objects in space. A probe also observes nearby objects. In addition, a space probe exposes material from Earth to the conditions of space so that scientists can observe the effects. A probe may also perform experiments on its surroundings, such as releasing chemicals or digging into surface dirt. Finally, a probe's motion enables controllers on Earth to determine conditions in space. Changes in course and speed can provide information about atmospheric density and gravity fields.

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Early unpiloted explorations. Beginning in the 1940's, devices called sounding rockets carried scientific instruments into the upper atmosphere and into nearby space. They discovered many new phenomena and took the first photographs of Earth from space.

The 1957 launch of Sputnik 1 marked the beginning of the space age. Sputnik 1 carried only a few instruments and transmitters, but it paved the way for the sophisticated probes that would later explore space.

Many early satellites probed uncharted regions of space. During the late 1950's and the 1960's, the Explorer satellites of the United States and the Kosmos satellites of the Soviet Union analyzed the space environment between Earth and the moon. United States Pegasus satellites recorded the impacts of micrometeorites. During the early 1970's, Soviet Prognoz satellites studied the sun. See Satellite, Artificial.

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Lunar probes. In 1958, both the United States and the Soviet Union began to launch probes toward the moon. The first probe to come close to the moon was Luna 1, launched by the Soviet Union on Jan. 2, 1959. It passed within about 3,700 miles (6,000 kilometers) of the moon and went into orbit around the sun. The United States conducted its own lunar fly-by two months later with the probe Pioneer 4. The Soviet Luna 2 probe, launched on Sept. 12, 1959, was the first probe to hit the moon. One month later, Luna 3 circled behind the moon and photographed its hidden far side. See Back in Time: Space exploration (1959).

The Soviet Union began to test lunar hard-landers in 1963. After many failures, they succeeded with Luna 9, launched in January 1966. The U.S. Surveyor program made a series of successful soft landings beginning in 1966. Between 1970 and 1972, three Soviet probes returned lunar soil samples to Earth in small capsules. Two of them sent remote-controlled jeeps called Lunokhods, which traveled across the lunar surface.

Beginning in 1966, the United States sent five probes called Lunar Orbiters into orbit to photograph the moon's surface. The Lunar Orbiters revealed the existence of irregular "bumps" of gravity in the moon's gravitational field caused by dense material buried beneath the lunar seas. These areas of tightly packed matter were called mascons, which stood for mass concentrations. If the mascons had not been discovered, they might have interfered with the Apollo missions that sent astronauts to the moon.

From 1976 until 1994, no missions went to the moon. In January 1994, the U.S. orbiter Clementine launched. During its short four-month mission, Clementine observed what appeared to be water ice near the moon’s south pole. In the succeeding years, additional missions surveyed the moon. The U.S. probe Lunar Prospector circled the moon from 1998 to 1999 and also found indications of water ice at both poles.

Beginning in 2004, several countries launched their first missions to the moon. The European Space Agency (ESA) launched the SMART-1 spacecraft that orbited the moon from 2004 to 2006. The Japanese spacecraft SELENE remained in lunar orbit from 2007 to 2009. In late 2007, China launched the Chang’e1 spacecraft that remained in orbit around the moon until 2009. India’s Chandrayaan-1 satellite orbited the moon from 2008 to 2009. The satellite released a probe that landed on the surface, making India the fourth country to reach the surface of the moon. All of these missions used instruments to map the surface in detail and gather information about the composition of the lunar surface.

NASA sent two more missions to the moon in 2009. The Lunar Reconnaissance Orbiter (LRO) and the Lunar Crater Observation and Sensing Satellite (LCROSS) were launched on the same rocket but carried out separate missions. LRO entered lunar orbit in June 2009 to map the moon's surface and environment in detail. The LCROSS mission used a section of the launch rocket to crash into the lunar surface near the south pole. The probe then flew through the debris kicked up by the impact. Its sensors confirmed the presence of water ice in the debris cloud in November.

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Solar probes. Beginning in 1965, the United States launched a series of small Pioneer probes into orbit around the sun to study solar radiation. Many of these probes were still operating more than 20 years after launch.

In 1974 and 1976, the United States launched two German-built Helios probes. These probes passed inside the orbit of Mercury to measure solar radiation. The Ulysses probe was launched in 1990 by the United States and the ESA. In 1994, Ulysses became the first probe to observe the sun from an orbit over the sun's poles.

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Probes to Mars. The Soviet Union launched the first probes aimed at another planet, two Mars probes, in 1960. However, neither probe reached orbit. After more Soviet failures, the United States launched two Mariner probes toward Mars in 1964. Mariner 4 flew past the planet on July 14, 1965, and sent back remarkable photographs and measurements. The probe showed that the atmosphere of Mars was much thinner than expected, and the surface resembled that of the moon.

In 1971, the Soviet probe Mars 3 dropped a capsule that made the first soft landing on Mars. However, the capsule failed to return usable data. That same year, the U.S. probe Mariner 9 reached Mars and photographed most of the planet's surface. Mariner 9 also passed near and photographed Mars's two small moons, Phobos and Deimos.

Two U.S. probes, Viking 1 and Viking 2, landed in 1976 and operated for years, measuring surface weather and conducting complex experiments to detect life forms. The probes found no evidence of life.

In 1992, the United States launched the probe Mars Observer. In 1993, NASA lost contact with the probe three days before it would have orbited Mars. Contact was never restored, and the probe was presumed lost.

The United States launched the Pathfinder probe in December 1996. The probe landed on Mars on July 4, 1997. Two days later, a six-wheeled vehicle called Sojourner rolled down a ramp from the probe to the Martian surface. The vehicle was only 24.5 inches long, 18.7 inches wide, and 10.9 inches high (63 by 48 by 28 centimeters). Its mass was 11.5 kilograms, equivalent to a weight of 25.4 pounds on Earth.

The vehicle used a device called an alpha proton X-ray spectrometer to gather data on the chemical makeup of rocks and soil. Sojourner transmitted this information to Pathfinder, and the probe relayed the information to Earth.

Scientists on Earth controlled Sojourner. However, because radio signals take about 10 minutes to travel from Earth to Mars, the scientists could not control Sojourner in real time—that is, as the vehicle moved. To avoid obstacles, Sojourner used a number of automatic devices.

In 1996, the United States launched a probe called the Mars Global Surveyor to map the planet's surface. The probe used a laser device to determine the elevation of the Martian surface. That instrument produced maps of the entire surface that are accurate to within 3 feet (1 meter) of elevation. Another instrument determined the composition of some of the minerals on the surface. A camera revealed layered sediments that may have been deposited in liquid water, and small gullies that appear to have been carved by water.

In 2001, the United States launched the Mars Odyssey probe to Mars. The craft carried instruments to help identify minerals on the surface, to search for evidence of water and ice beneath the surface, and to measure radiation that might harm any future human explorers. In 2002, Mars Odyssey discovered vast quantities of ice within 3 feet (1 meter) of the surface, most of it near the south pole.

In 2003, three probes were launched to Mars, one by the ESA and two by the United States. The ESA's Mars Express probe went into orbit around the planet in December 2003. It transmitted stunning pictures of the planet's surface, confirmed the presence of water ice in the planet's southern region, and detected methane in the Martian atmosphere, a possible indicator of life. Mars Express carried a lander called Beagle 2 that failed to land safely and was lost.

The United States launched rovers nicknamed Spirit and Opportunity. In January 2004, Spirit landed in Gusev Crater, and Opportunity landed in an area called Meridiani Planum. The rovers used cameras and other instruments to analyze soil and rocks. In March 2004, U.S. scientists concluded that Meridiani Planum once held large amounts of liquid water. Opportunity's analysis had shown that the rock there contained minerals and structures normally found in Earth rocks that formed in water.

In August 2005, the United States launched the Mars Reconnaissance Orbiter, which arrived in orbit around Mars in March 2006. The craft was designed to study the planet's structure and atmosphere and to identify potential landing sites for future lander and rover missions.

The U.S. Phoenix Mars lander, launched in August 2007, operated in the north polar region from May to November 2008. The lander revealed the presence of various minerals in the soil. However, chief among its contributions was the confirmation of water ice just below the surface of the Martian soil.

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Probes to Venus and Mercury. The Soviet Union launched the first probes toward Venus in 1961, but these attempts failed. The first successful probe to fly past Venus and return data was the U.S. Mariner 2, on Dec. 14, 1962. Mariner 5 flew past Venus in 1967 and returned important data. Mariner 10 passed Venus and then made three passes near Mercury in 1974 and 1975.

Soviet attempts to obtain data from Venus finally succeeded in 1967. Venera 4 dropped a probe by parachute, and it transmitted data from the planet's extremely dense atmosphere. In 1970, Venera 7 reached the surface of the planet, still functioning. Between 1975 and 1985, several other probes landed and conducted observations for up to 110 minutes before the temperature and pressure destroyed them. In 1978, the United States sent two probes to Venus, Pioneer Venus 1 and 2. Pioneer Venus 1 was an orbiter. Pioneer Venus 2 dropped four probes into the planet's atmosphere.

Probes that orbited Venus generated rough maps of its surface by bouncing radio waves off the ground. Pioneer Venus 1 mapped most of the surface to a resolution of about 50 miles (80 kilometers). This means that objects at least 50 miles apart showed distinctly on the map. In 1983, two Soviet probes carried radar systems that mapped most of the planet's northern hemisphere to a resolution of 0.9 mile (1.5 kilometers). In 1990, the U.S. probe Magellan mapped almost the entire surface to a resolution of about 330 feet (100 meters).

In 2004, the United States launched the Messenger probe to Mercury. Messenger was to enter orbit around Mercury in 2011 after flying by Venus twice and by Mercury three times. The probe was to orbit Mercury for one Earth year while mapping Mercury's surface and studying its composition, interior structure, and magnetic field.

The ESA Venus Express probe, launched in 2005, went into orbit around Venus in 2006. The probe carried instruments used to study Venus’s atmosphere in detail. On June 5, 2007, the Messenger probe flew past Venus on its way to Mercury. During this flyby, the Messenger probe and the Venus Express probe worked together to detail Venus's surface.

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Probes to Jupiter and beyond must meet special challenges. Radiation belts near Jupiter are so intense that camera lenses and computer circuits must be shielded to prevent damage. The dim sunlight at the outer planets requires lengthy camera exposures. And the vast distances mean that radio commands take hours to reach the probes. Probes have visited Jupiter, Saturn, Uranus, and Neptune.

U.S. probes Pioneer 10 and Pioneer 11 were sent to Jupiter in 1972 and 1973. After observing Jupiter, Pioneer 11 was redirected toward Saturn, arriving there in 1979. It was renamed Pioneer-Saturn. From 1979 to 1981, sophisticated Voyager probes provided much more detailed data on Jupiter and Saturn. They still explore space. Voyager 2 flew past Uranus in January 1986 and Neptune in August 1989. The probes sent back spectacular photos of the outer planets and their rings and moons, and recorded a great deal of scientific data. Active volcanoes were found on Io, a moon of Jupiter, and geysers were discovered on Triton, a moon of Neptune. Other moons exhibited bizarre ice and rock formations.

The Galileo space probe, launched on a mission to Jupiter by the United States in 1989, was far more sophisticated than earlier planetary probes. It consisted of two parts—an atmosphere probe and a larger orbiting spacecraft. On the way to Jupiter, Galileo flew past the asteroids Gaspra and Ida. In July 1995, the atmosphere probe separated from the spacecraft. Both parts reached Jupiter five months later. As planned, the probe plunged into Jupiter's atmosphere. The spacecraft orbited Jupiter until 2003, studying the planet, its satellites, and its rings.

In 1997, the United States launched the Cassini probe to investigate Saturn, its rings, and satellites. Cassini began orbiting Saturn in 2004. It then released an ESA probe called Huygens that landed on Saturn's moon Titan in 2005.

In 2006, U.S. scientists launched the New Horizons probe to make the first close observations of Pluto. The probe was expected to fly by Pluto in 2015.

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Probes to comets. Two Soviet probes flew past Venus and dropped instruments into its atmosphere, then intercepted Halley's Comet as it passed by the sun in 1986. In 1985, the ESA launched its first interplanetary probe, called Giotto. It passed closer to the comet's nucleus than any other probe and returned dramatic close-up images. Japan also sent two small probes. After several years of inactivity, Giotto was reactivated to fly past the comet Grigg-Skjellerup in July 1992.

The United States did not send a probe to Halley's Comet due to budget limitations. But NASA scientists used a small probe already in space to explore another comet. The International Sun-Earth Explorer 3 satellite had spent several years between Earth and the sun. In 1983, its course was shifted into interplanetary space, and it was renamed the International Cometary Explorer. On Sept. 11, 1985, it passed a comet named Giacobini-Zinner, becoming the first probe to reach a comet.

In 1999, NASA launched a probe called Stardust to visit Comet Wild 2. In 2004, Stardust passed near the comet and gathered samples from the cloud of dust and gas surrounding the comet's nucleus. Stardust returned the samples to Earth in 2006. Also in 2004, the European Space Agency launched the Rosetta spacecraft, which was to go into orbit around Comet Churyumov-Gerasimenko in 2014. Rosetta carried a small probe designed to land on the comet's nucleus.

In 2005, the United States launched the Deep Impact spacecraft to Comet Tempel 1. The craft consisted of two smaller probes: an impactor and a flyby craft. The impactor intentionally slammed into the comet’s nucleus, while the flyby craft recorded the crash. Analyzing the debris ejected by the collision enabled scientists to study the comet’s composition.

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Probes to asteroids. NASA launched the Near Earth Asteroid Rendezvous (NEAR) probe in 1996. In 1997, the probe flew within 753 miles (1,216 kilometers) of the asteroid Mathilde. NEAR flew past the asteroid Eros at a distance of 2,378 miles (3,829 kilometers) in 1998. It went into orbit around Eros in 2000. The probe, renamed NEAR-Shoemaker in honor of the American astronomer Eugene Shoemaker, landed on Eros in 2001.

In October 1998, NASA launched a probe called Deep Space 1 (DS1). The probe flew within only about 16 miles (26 kilometers) of the asteroid Braille in July 1999.

The flight of DS1 successfully tested several new types of equipment for space probes. This equipment included a navigation system that operates automatically, rather than under the direction of people and computers on Earth. Also included was an ion rocket, which operates by shooting electrically charged particles called ions out its nozzle.

In 2005, Japan's Hayabusa probe visited the asteroid Itokawa. Despite the failure of several of its systems, the craft managed to transmit detailed pictures of the asteroid and to land briefly on its surface.

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Human beings enter space

In 1958, scientists in the United States and the Soviet Union began serious efforts to design a spacecraft that could carry human beings. Both nations chose to develop a wingless capsule atop a launch vehicle that would consist of a modified long-range missile.

The prospect of human beings traveling in space greatly worried scientists. Tests with animals had shown that space travel probably involved no physical danger, but there were serious concerns about possible psychological hazards. Some experts feared that the stresses of launch, flight, and landing might drive a space traveler to terror or unconsciousness.

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Vostok and Mercury: The first human beings in space

The Soviet Union's Vostok (East) program and the Mercury program of the United States represented the first efforts to send a human being into space. The Vostok capsule weighed about 10,000 pounds (4,500 kilograms). It was to be carried into orbit atop a modified R-7 missile. The capsule consisted of a spherical pilot's cabin and a cylindrical service module, the section containing the propulsion system. An ejection seat was designed to provide an escape for the astronaut in case of a mishap during launch. The life-support system used a mixture of oxygen and nitrogen similar to the atmosphere at sea level.

The U.S. Mercury capsule weighed about 3,000 pounds (1,360 kilograms) and was to be carried into space atop a Redstone or Atlas rocket. The cone-shaped capsule would use parachutes to land in the ocean, where the water would provide extra cushioning. The life-support system used pure oxygen at low pressure. In the event of a booster malfunction during launch, the capsule and pilot would be pulled free by a solid-fuel rocket attached to the nose of the capsule.

While U.S. plans proceeded in the glare of publicity, Soviet developments took place in great secrecy. Both nations made unpiloted orbital tests in 1960 and 1961, some of which suffered booster failures. Both nations also sent animals into space during this period. One of these animals was a chimpanzee named Ham, who made an 18-minute flight in a Mercury capsule on Jan. 31, 1961.

The first fatality in a piloted space program occurred on March 23, 1961. A Soviet cosmonaut trainee named Valentin V. Bondarenko burned to death in a pressure chamber fire. Soviet officials covered up the accident.

The first human being in space was a Soviet air force pilot named Yuri A. Gagarin. He was launched aboard Vostok (later referred to as Vostok 1) on April 12, 1961. In 108 minutes, Gagarin orbited Earth once and returned safely. An automatic flight control system managed the spacecraft's operations during the entire flight. See Back in Time: Space exploration (1961). A 25-hour, 17-orbit flight by cosmonaut Gherman Titov aboard Vostok 2 followed in August of that year.

The Mercury program made its first piloted flight on May 5, 1961, when a Redstone rocket launched astronaut Alan B. Shepard, Jr., in a capsule he named Freedom 7. Shepard flew a 15-minute suborbital mission—that is, a mission that did not reach the speed and altitude required to orbit Earth.

A suborbital flight on July 21, 1961, by astronaut Virgil I. Grissom almost ended tragically. The Mercury capsule's side hatch opened too soon after splashdown in the Atlantic Ocean, and the spacecraft rapidly filled with water. Grissom managed to swim to safety.

On Feb. 20, 1962, John H. Glenn, Jr., became the first American to orbit Earth. Glenn completed three orbits in less than five hours. He pointed his capsule in different directions, tested its various systems, and observed Earth.

Three months later, astronaut M. Scott Carpenter repeated Glenn's three-orbit mission. A six-orbit mission by Walter M. Schirra, Jr., in October 1962 further extended the testing of the spacecraft. The final Mercury mission took place in May 1963, with Gordon Cooper aboard. The mission lasted 1¹/2 days.

Meanwhile, the Soviet Union continued to launch Vostok missions. In August 1962, Vostok 3 and Vostok 4 lifted off just a day apart and passed near each other in space. Another two capsules—Vostok 5 and Vostok 6—were launched in June 1963. One of the pilots spent almost five days in orbit, a new record. The other pilot, Valentina Tereshkova, became the first woman in space.

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Voskhod and Gemini: The first multiperson space flights

In 1961, the United States announced the Gemini program, which would send two astronauts into space in an enlarged version of the Mercury capsule. This announcement spurred Soviet planners to modify their Vostok capsule to carry up to three cosmonauts. Political pressure to upstage U.S. efforts was so intense that Soviet engineers sacrificed certain safety features, such as ejection seats, to enlarge the capsule.

The world's first multiperson space capsule, Voskhod (Sunrise)—later referred to as Voskhod 1—was launched on Oct. 12, 1964. Cosmonauts Vladimir M. Komarov, Konstantin P. Feoktistov, and Boris B. Yegorov spent 24 hours in orbit. They became the first space travelers to land inside their capsule on the ground, rather than in the ocean.

In March 1965, cosmonaut Alexei A. Leonov stepped through an inflatable air lock attached to Voskhod 2 to become the first person to walk in space. After the capsule's automatic flight control system failed, Leonov and Pavel I. Belyayev had to land it manually. They missed their planned landing zone and came down in an isolated forest. The cosmonauts had to fend off hungry wolves until rescuers reached them the following day.

The first piloted Gemini mission, Gemini 3, was launched on March 23, 1965. Astronauts Grissom and John W. Young used the capsule's maneuvering rockets to alter its path through space. With Gemini 4, launched on June 3, 1965, copilot Edward H. White II became the first American to walk in space. The astronauts aboard Gemini 5, launched on Aug. 21, 1965, spent almost eight days in space, a record achieved by using fuel cells to generate electricity.

Gemini 6 was originally intended to link up with an Agena rocket sent into space a few hours earlier. After the unpiloted Agena was lost in a booster failure, NASA combined Gemini 6 with an already scheduled 14-day Gemini 7 mission. Gemini 7 was launched as planned, on Dec. 4, 1965, and Gemini 6 took off 11 days later. Within hours, Schirra and Thomas P. Stafford moved their spacecraft to within 1 foot (30 centimeters) of Gemini 7 and its crew, Frank Borman and James A. Lovell, Jr. The two spacecraft orbited Earth together for several hours before separating.

On March 16, 1966, Gemini 8 completed the world's first docking of two space vehicles when it linked up with an Agena rocket in space. However, the spacecraft went into a violent tumble. Astronauts Neil A. Armstrong and David R. Scott managed to regain control of the spacecraft and make an emergency splashdown in the western Pacific Ocean.

Additional tests of docking and extravehicular activity took place on the remaining four Gemini missions. On these missions, astronauts and flight controllers also gained vital experience in preparation for the tremendous challenges of piloted lunar flight.

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Apollo: Mission to the moon

The race to the moon dominated the space race of the 1960's. In a 1961 address to Congress, President John F. Kennedy called for the United States to commit itself to "landing a man on the moon and returning him safely to Earth" before the 1960's ended. This goal was intended to show the superiority of U.S. science, engineering, management, and political leadership.

NASA considered several proposals for a piloted lunar mission. The agency selected a plan known as lunar-orbit rendezvous. A spacecraft would carry three astronauts to an orbit around the moon. Two of the astronauts would then descend to the lunar surface.

The spacecraft would consist of three parts, or modules—a command module (CM), a service module (SM), and a lunar module (LM), which was originally called the lunar excursion module (LEM). The cone-shaped CM would be the spacecraft's main control center. The SM would contain fuel, oxygen, water, and the spacecraft's electric power system and propulsion system. The CM and SM would be joined for almost the entire mission as the command/service module (CSM).

Only the LM would land on the moon. This module would consist of two sections—a descent stage and an ascent stage. The two stages would descend to the lunar surface as a single unit, but only the ascent stage would leave the moon.

A Saturn 5 booster would launch the spacecraft toward the moon. As the craft approached the moon, rockets on the SM would adjust its course so that it would go into a lunar orbit. With the craft in orbit, the LM would separate from the CSM and carry the two astronauts to the surface. After the astronauts completed their activities on the moon, the LM's ascent stage would blast off from the descent stage and rendezvous with the CSM.

After the returning astronauts entered the command module, the CSM would cast off the LM's ascent stage. The CSM would then return to Earth. As the craft approached Earth, the CM would separate from the SM and would splash down in the ocean.

Lunar-orbit rendezvous would be complex but relatively economical. The mission would save a tremendous amount of fuel by landing only the small LM on the moon and then launching only its ascent stage.

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Making ready. Tragedy struck during preparations for the first piloted Apollo flight, a trial run in low earth orbit. During a ground test on Jan. 27, 1967, a flash fire inside the sealed CM killed astronauts Grissom, White, and Roger B. Chaffee. An electrical short circuit probably started the fire, and the pure oxygen atmosphere caused it to burn fiercely.

A few months later, the Soviet space program also suffered a disaster. The Soyuz (Union) 1 capsule was launched with Vladimir Komarov aboard as pilot. It was supposed to link up with a second piloted spaceship, but Soyuz 1 developed problems and the second ship was never launched. Controllers ordered Soyuz 1 to return to Earth. But a parachute failure caused the capsule to crash, killing Komarov.

While the Apollo CSM and the Soyuz capsule were being redesigned, unpiloted tests took place as planned. The United States launched the first Saturn 5 booster on Nov. 9, 1967, with complete success. Early in 1968, an LM was sent into orbit, where it test-fired its engines. Soyuz vehicles linked up automatically in orbit in 1967 and 1968.

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Orbiting the moon. By late 1968, the United States had redesigned the Apollo CSM. However, the lunar module remained far behind schedule.

NASA officials knew about Soviet preparations for a piloted lunar fly-by. To beat the Soviets, NASA decided to fly a piloted mission to orbit the moon, without an LM. The orbital mission would also test navigation and communication around the moon.

Apollo 8, the first piloted expedition to the moon, blasted off from the Kennedy Space Center near Cape Canaveral, Florida, on Dec. 21, 1968. Hundreds of thousands of people crowded nearby beaches to watch the launch. The spacecraft carried astronauts Borman, Lovell, and William A. Anders. After three days, the crew fired the SM engine to change course into a lunar orbit. They made observations and took photographs, then headed back to Earth. Apollo 8 landed safely in the Pacific Ocean near Hawaii on December 27.

Two additional test flights were made to ensure the safety and effectiveness of the lunar module. The LM was tested in low orbit around Earth by the Apollo 9 astronauts and in lunar orbit by the Apollo 10 crew.

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Landing on the moon. Apollo 11 was the first mission to land astronauts on the moon. It blasted off on July 16, 1969, carrying three astronauts—Neil A. Armstrong, Buzz Aldrin, and Michael Collins.

The first two stages of a Saturn 5 rocket carried the spacecraft to an altitude of 115 miles (185 kilometers) and a speed of 15,400 miles (24,800 kilometers) per hour, just short of orbital velocity. The third stage fired briefly to accelerate the vehicle to the required speed. It then shut down while the vehicle coasted in orbit. The astronauts checked the spacecraft and lined up the flight path for the trip to the moon. The third stage was then restarted, increasing the speed to an escape velocity of 24,300 miles (39,100 kilometers) per hour. On the way to the moon, the crew pulled the CSM away from the Saturn rocket. They turned the CSM around and docked it to the LM, which was still attached to the Saturn. The linked vehicles then pulled free of the Saturn.

For three days, Apollo 11 coasted toward the moon. As the spaceship traveled farther from Earth, the pull of Earth's gravity became weaker. But Earth's gravity constantly tugged at the spacecraft, slowing it down. By the time the ship was 215,000 miles (346,000 kilometers) from Earth, its speed had dropped to 2,000 miles (3,200 kilometers) per hour. But then the moon's gravity became stronger than Earth's, and the craft picked up speed again.

Apollo 11 was aimed to pass directly behind the moon. However, it was moving much too fast for the moon's weak gravity to capture it. A braking rocket burn changed its course into a low lunar orbit.

Once in lunar orbit, Armstrong and Aldrin separated the LM from the CSM. They fired the LM's descent stage and began the landing maneuver. They used the LM's rockets to slow its descent. Collins remained in the CSM.

To help NASA mission controllers recognize voice signals from the CSM and the LM, the astronauts used different call signs for the two vehicles. They called the CSM Columbia and the LM Eagle.

The LM's computer controlled all landing maneuvers, but the pilot could override the computer if something unexpected occurred. For the final touchdown, Armstrong looked out the window and selected a level landing site. Probes extended down from the LM's landing legs and signaled when the LM was about 5 feet (1.5 meters) above the surface. The engine shut off, and the LM touched down at a lowland called the Sea of Tranquility on July 20, 1969. Aldrin radioed a brief report on the vehicle's status. Moments later, Armstrong radioed back his famous announcement: "Houston, Tranquility Base here. The Eagle has landed."

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Exploring the moon. Immediately after the LM touched down, the astronauts performed a complete check to make sure that the landing had not damaged any equipment. Then they prepared to go outside.

Armstrong and Aldrin had worn space suits during the landing. They transferred their air hoses from a cabin supply to their backpack units, then released the air from the cabin and opened a small hatch below their front windows. First Armstrong and then Aldrin crawled backward through the hatch. They descended a ladder mounted on one of the LM's legs to a wide pad at the base of the leg.

A television camera mounted on the side of the LM sent blurred images of the astronauts back to Earth. Armstrong stepped off the pad onto the moon and said, "That's one small step for a man, one giant leap for mankind." Most of the huge TV audience did not hear Armstrong say the word a before man because of a gap in the transmission.

The astronauts had no trouble adjusting to the weak lunar gravity. They found rocks and soil samples and photographed their positions before picking them up. The astronauts also set up automatic science equipment on the moon. Meanwhile, from the orbiting CSM, Collins conducted various scientific observations and took photographs.

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Returning to Earth. The LM's descent stage served as a launch pad for the ascent stage liftoff. To lighten the spacecraft, the crew left all extra equipment behind, including backpacks and cameras. The ascent stage rocketed into orbit, where it linked up with the waiting CSM. The astronauts transferred samples and film into the CSM, then cast off the LM ascent stage. The crew fired the on-board rocket again to push the CSM out of lunar orbit and set their course for Earth.

The CM splashed down in the Pacific Ocean on July 24. NASA immediately put the lunar material, the astronauts, and all equipment that had been exposed to the lunar environment into isolation. The purpose of the isolation, which lasted about 17 days for the astronauts, was to determine whether any germs or other harmful material had been brought from the moon. Nothing harmful was found. See Back in Time: Space exploration (1969).

The second flight to the moon was as successful as the first. The Apollo 12 LM made a precision landing on the lunar surface on Nov. 19, 1969. Astronauts Charles (Pete) Conrad, Jr., and Alan L. Bean walked to a landed space probe, Surveyor 3, and retrieved samples for study.

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The flight of Apollo 13, which was supposed to result in the third lunar landing, almost ended in disaster. The flight, from April 11 to 17, 1970, became a mission to save the lives of three astronauts—James A. Lovell, Jr., Fred W. Haise, Jr., and John L. Swigert, Jr.

During the spacecraft's approach to the moon, one of the two oxygen tanks in the SM exploded. The blast also disabled the remaining tank. The tanks provided both breathing oxygen and fuel for the electrical power systems of the CM and the SM. Moments later, Swigert reported "OK, Houston, we've had a problem."

After the explosion, flight controllers at Mission Control in Houston quickly realized that the astronauts probably did not have enough oxygen and battery power to get them back to Earth. The flight controllers ordered the crew to power up the LM, which was still docked with the CSM. The crew then shut down the CSM, saving its power supply until power would be needed for descent to Earth. The LM had its own power and oxygen supplies, but it was not designed to support three astronauts. The astronauts used only minimal electric power during the 3-day return trip to Earth, and all three of them survived.

A NASA investigation later determined the cause of the tank explosion. Months before the launch, wires leading to a fan thermostat inside the tank had been tested at too high a voltage. As a result, the wire's insulation had burned off. When the fan was turned on during the flight, the wires short-circuited. The short caused a fire in the pure oxygen environment of the tank, resulting in the explosion. The blast blew off one side of the SM and broke the feed line to the other tank. See Back in Time: Space exploration (1970). See also Special Reports: Rocket Man.

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Other moon landings. Apollo astronauts landed on the moon six times between 1969 and 1972. Each mission brought various instruments to the moon, which usually included a seismograph—a device that detects and records moonquakes and other small movements of the moon's crust. On later missions, mission controllers sent the empty Saturn third stage and the discarded LM ascent stage hurtling to the moon's surface to create seismic waves. These waves provided information about the moon's internal structure.

An important task of the Apollo astronauts was the recovery of samples from the lunar surface for study. On some flights, they used drills to collect soil samples to a depth of 10 feet (3 meters). Astronauts gathered about 840 pounds (384 kilograms) of samples. Some missions launched small scientific satellites near the moon.

After investigating the Apollo 13 accident, NASA redesigned the CM and SM. The inquiry and modifications set back the Apollo 14 mission from October 1970 to January 1971. The Apollo 14 LM, carrying astronauts Alan B. Shepard, Jr., and Edgar D. Mitchell, landed near Fra Mauro Crater on February 5. Fra Mauro had originally been the target for Apollo 13.

Apollo 15 landed near the Apennine Mountains of the moon on July 30, 1971. Astronauts David R. Scott and James B. Irwin became the first astronauts to drive across the moon's surface. They drove a battery-powered lunar roving vehicle, often called the lunar rover, more than 17 miles (27 kilometers). Apollo 16, carrying John W. Young and Charles M. Duke, Jr., landed in the Descartes region on April 20, 1972. The last lunar mission, Apollo 17, landed in the Taurus Mountains on Dec. 11, 1972. Eugene A. Cernan and Harrison H. Schmitt rode the LM to the surface on this mission.

The Apollo expeditions achieved the goal of demonstrating U.S. technological superiority, and the race to the moon ended with a clear-cut U.S. triumph. Apollo provided unique scientific data, much of which would have been impossible to gather through the use of probes alone. The data enabled scientists to study the origin of the moon and the inner planets of the solar system with much greater certainty than ever before. In addition, the Apollo program forced hundreds of industrial and research teams to develop new tools and technologies that were later applied to more ordinary tasks. For example, microelectronics and new medical monitoring equipment were developed as a result of the Apollo program. These advancements enriched the U.S. economy. Most importantly, the Apollo missions stirred people's imagination and raised their awareness of Earth's place in the universe.

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Soviet attempts to reach the moon

Officials in the Soviet Union publicly denied there had ever been a Soviet equivalent to the Apollo program. This official story became widely accepted around the world. But in the late 1980's, the Soviet Union began to release new information indicating that the Soviet government actually had an ambitious lunar program that failed.

Soviet plans for piloted lunar flight may have been hampered by a lack of central authority. Rivalry among different spacecraft design teams and other space organizations prevented cooperation. The Soviet equivalent of the Apollo CSM was a two-person lunar modification of the Soyuz capsule, called the L-1. The Soviet lunar module, the L-3, resembled the LM developed in the United States. However, it would carry only one cosmonaut. The Soviet booster, the N-1, was bigger than the Saturn 5 but less powerful, because it used less efficient fuels.

Piloted Soviet L-1 capsules were scheduled to fly past the moon as part of a test program. This program was planned for 1966 and 1967, well before the United States could attempt a lunar landing. The Soviet Union conducted unpiloted test flights under the cover name Zond. Three pairs of Soviet cosmonauts trained for a lunar mission.

The Soviet moon ships had serious problems. Many of the boosters for the L-1 lunar fly-by blew up. In addition, the unpiloted L-1 spacecraft developed serious flaws. It was still too dangerous to allow cosmonauts aboard. Soviet efforts to reach the moon were also frustrated by the continued failure of the giant N-1 booster. Four secret test flights were made between 1969 and 1972. However, all of the vehicles exploded.

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The Apollo-Soyuz Test Project

In 1972, the United States and the Soviet Union agreed to participate in the first international piloted space mission. They planned to perform an orbital rendezvous between a Soviet Soyuz capsule and a U.S. Apollo capsule. The Apollo-Soyuz Test Project began on July 15, 1975. The Apollo capsule, commanded by Thomas P. Stafford, successfully linked up with the Soyuz capsule, commanded by Alexei A. Leonov.

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Space stations

A space station is a place where people can live and work in space for long periods. It orbits Earth, usually about 200 to 300 miles (300 to 480 kilometers) high. A space station may serve as an observatory, laboratory, factory, workshop, warehouse, and fuel depot. Space stations are much larger than piloted spacecraft, so they provide more comforts. Piloted spacecraft may transport people between Earth and the space station. Unpiloted spacecraft may supply the station with food, water, equipment, and mail.

Small space stations can be built on Earth and launched into orbit by large rockets. Larger stations are assembled in space. Rockets or space shuttles carry modules (sections) of the station into space, where astronauts assemble them. Old modules can be replaced, and new modules can be added to expand the station.

A space station has at least one docking port to which a visiting spacecraft can attach itself. Most docking ports consist of a rimmed doorway called a hatch that can connect with a hatch on the visiting spacecraft to form an airtight seal. When the two hatches open, they form a pressurized tunnel between the station and the visiting spacecraft.

The main tasks of a space station crew involve scientific research. For example, they might analyze the effects of microgravity on various materials, investigate Earth's surface, or study the stars and planets.

Astronauts at a space station also devote much of their time to the assembly of equipment and the expansion of the station's facilities. This includes erecting beams, connecting electrical and gas lines, and welding permanent joints between sections of the station. The crew must also fix or replace broken equipment.

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Salyut and Skylab

In the 1960's, missions to the moon dominated the U.S. and Soviet space programs. But both countries also developed simple space stations during this period. These early stations had a cylindrical shape, with a docking port at one end and solar power panels sticking out from the sides. The stations were designed to hold enough air, food, and water to last for about 6 to 12 months. The piloted spacecraft originally built for lunar flight—the U.S. Apollo and the Soviet Soyuz—were modified to transport people to the space stations.

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Salyut. The Soviet Union launched the first space station, Salyut (Salute) 1, on April 19, 1971. It consisted of a single module with one docking port. On June 7, 1971, three cosmonauts—Georgi T. Dobrovolsky, Victor I. Patsayev, and Vladislav N. Volkov—linked their Soyuz 11 spacecraft with Salyut 1. They spent 23 days aboard the space station, making medical observations and performing experiments. In a tragic accident, the air leaked out of the Soyuz 11 spacecraft during the return journey, killing all three cosmonauts.

In 1974, Salyut 3 hosted a 15-day mission to photograph Earth. Salyut 4 received two missions in 1975. The second lasted 63 days. In 1976, Salyut 5 repeated the Salyut 3 photography mission.

In 1977, the Soviet Union launched Salyut 6. It had two docking ports, one at either end of the main module. This new design enabled a space station crew to receive a visit from a second crew or a resupply vehicle. A modified, unpiloted Soyuz spacecraft called Progress began delivering new supplies and equipment to Salyut 6 in January 1978. Thus it became the first space station to be resupplied and refueled. These capabilities greatly extended the useful life of space stations and enabled crews to repair and modernize them. Spare parts and more advanced instruments could be sent to the stations as needed. Salyut 6 operated for almost five years. It received visits by 16 crews, who spent up to six months in orbit. Between 1982 and 1986, Salyut 7 housed expeditions lasting up to eight months.

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Skylab. The first U.S. space station was Skylab, launched into orbit by a Saturn 5 booster on May 14, 1973. Skylab was built from the empty third stage of a Saturn 5 rocket, with an attached air lock module, docking port, and solar telescope.

Astronauts Pete Conrad, Joseph P. Kerwin, and Paul J. Weitz arrived at Skylab on May 25. The station had suffered damage during launch, losing most of its thermal insulation and one of its two solar power panels. In addition, debris had jammed the other solar panel so it could not open. The crew worked outside the station several times to free the stuck panel. The success of this 28-day expedition proved the usefulness of people in space for the repair and maintenance of space stations.

Two more crews carried out Skylab missions. These astronauts continued to operate the station while conducting medical experiments, photographing Earth, and observing the sun. The second mission lasted 59 days, and the third ran for 84 days.

United States space officials hoped to keep Skylab in orbit long enough to host a space shuttle mission. However, the station fell from its orbit in July 1979 and broke apart. Fragments of the station landed in western Australia and in the Indian Ocean.

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Mir

The Soviet space station Mir (Peace) was launched on Feb. 20, 1986. Mir featured two docking ports—one at each end—and four other hatches. They were designed for the attachment of laboratory modules, with the original Mir serving as the hub and the modules looking like spokes of a wheel. Mir also had modernized equipment and improved solar power panels.

After the launch of Mir, the Soviet Union sent three laboratory modules into orbit, where they docked with the core module. Many cosmonauts spent several months in space. Beginning in 1987, each crew was relieved by a new crew before leaving Mir, except for a period of a few months in 1989.

Russia took over the operation of Mir after the Soviet Union broke apart in 1991. In 1994, Russia privatized the government-owned business that was in charge of Mir and other rocket and space projects. The business was given the name Energia Rocket and Space Corporation. In 1995, U.S. space shuttles began to dock with Mir. Also in 1995, cosmonaut Valery Polyakov returned to Earth from Mir, setting an international record of 438 days in space. Russia connected an additional science module to Mir in 1995 and another in 1996, completing the station.

A dangerous accident occurred in June 1997. A cosmonaut was practicing maneuvers that would dock a supply craft with the station. The craft collided with a module of Mir called Spektr. The crash opened a small hole in Spektr and damaged one of its four solar panels. Spektr began to leak air. The crew quickly disconnected the solar-power cables that led through the portal connecting Spektr to the rest of Mir. The crew then closed the hatch, sealing off Spektr.

Mir continued to operate on reduced power. In July 1997, Russia sent emergency supplies and equipment to the station.

In March 2001, Russia destroyed Mir by guiding it into the atmosphere. Much of the station burned, and the remainder fell into the Pacific Ocean. See Special Reports: Stepping Stone to the Final Frontier.

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The International Space Station

In 1984, President Ronald Reagan authorized the building of a large, permanent space station "within a decade." Designs for the new station changed often and the estimated cost increased. The promised completion date slipped later and later. In 1993, President Bill Clinton directed NASA to redesign the proposed space station to reduce the cost and amount of time it would take to build. The United States, Canada, Japan, Russia, and the ESA would become the final partners in a program to build the redesigned space station.

Construction began in 1998. Russia launched the first module, called Zarya, in November of that year. A month later, the space shuttle Endeavour carried the module Unity into orbit and docked it with Zarya. A crew of one American astronaut and two Russian cosmonauts moved into the station in 2000. The station expanded to six members in 2009.

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Space shuttles

During the 1950's and the 1960's, aviation researchers worked to develop winged rocket planes. Advocates of winged spaceplanes pointed out that such vehicles could land on ordinary airfields. Adding wings to a spacecraft increases the vehicle's weight, but wings make landing the vehicle much easier and cheaper than splashdowns at sea. Ocean landings require many ships and aircraft, and the salt water usually damages the spacecraft beyond repair.

NASA began to develop a reusable space shuttle while the Apollo program was still underway. In 1972, U.S. President Richard M. Nixon signed an executive order that officially started the space shuttle project. The shuttles were designed to blast off like a rocket and land like an airplane, making up to 100 missions.

The space shuttle system consists of three parts: (1) an orbiter, (2) an external tank, and (3) two solid rocket boosters. The nose of the winged orbiter houses the pressurized crew cabin. From the flight deck at the front of the orbiter, pilots can look through the front and side windows. The middeck, located under the flight deck, contains additional seats, equipment lockers, food systems, sleeping facilities, and a small toilet compartment. An air lock links the middeck with the payload bay, the area that holds the cargo. The tail of the orbiter houses the main engines and a smaller set of engines used for maneuvering in space.

The external tank is attached to the orbiter's belly. It contains the liquid propellants used by the main engines. Two rocket boosters are strapped to the sides of the external tank. They contain solid propellants.

The designers of the space shuttle had to overcome a number of major technological challenges. The shuttle's main engines had to be reusable for many missions. The shuttle needed a flexible but reliable system of computer control. And it required a new type of heat shield that could withstand many reentries into Earth's atmosphere.

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The shuttle era begins

In 1977, NASA conducted flight tests of the first space shuttle, Enterprise, with a modified 747 jumbo jet. The jet carried the orbiter into the air and back on several flights and released it in midair on several more.

The shuttle's first orbital mission began on April 12, 1981. That day, the shuttle Columbia was launched, with astronauts John W. Young and Robert L. Crippen at the controls. The 54-hour mission went perfectly. Seven months later, the vehicle made a second orbital flight, proving that a spacecraft could be reused.

Although the first four shuttle flights each carried only two pilots, the crew size was soon expanded to four, and later to seven or eight. Besides the two pilots, shuttle crews included mission specialists (experts in the operation of the shuttle) and payload specialists (experts in the scientific research to be performed).

The large capacity of the space shuttle's orbiter opened the possibility of including other passengers besides NASA astronauts and scientists. Citizens who participated in shuttle missions included representatives of the companies launching payloads and members of the U.S. Congress.

In 1984, NASA created a special "Space Flight Participant" program to offer the opportunity of space travel to more Americans. President Reagan announced that the first participant would be a schoolteacher. Later flights were expected to carry journalists, artists, and other interested civilians.

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The Soviet space shuttle

The Soviet Union carried out its own shuttle program in great secrecy during the 1980's. The Soviet shuttle, Buran (Snowstorm), resembled the U.S. shuttle, but Soviet engineers made many modifications. For example, Buran had no main engines on board. Instead, an expendable booster provided all its launching power.

On Nov. 15, 1988, a heavy booster called Energia carried Buran into orbit without a crew. An automatic flight control system managed the two-orbit flight. Buran landed on a runway at the Baykonur Cosmodrome in Kazakhstan, then part of the Soviet Union.

Beginning in 1989, shortages of funds caused long delays in further development of the Buran program. In 1993, work on the program ended.

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Types of shuttle missions

Space shuttles carry artificial satellites, space probes, and other heavy loads into orbit around Earth. In addition to launch operations, the shuttles can retrieve artificial satellites that need servicing. Astronauts aboard the shuttle can repair the satellites and then return them to orbit. Shuttle crews can also conduct many kinds of scientific experiments and observations.

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Commercial satellite launches. The first launch of a payload for a customer took place in November 1982. The shuttle Columbia launched two communications satellites. Solid-rocket boosters helped the satellites climb to their designated orbits. Many later satellite launches followed. NASA discovered that using the space shuttle to launch satellites was more flexible than it had expected. However, the length of time required to ready each space shuttle for its next launch was also greater than NASA planners had expected and sometimes caused expensive delays.

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Military missions. About one-fourth of the shuttle missions during the 1980's were conducted for military purposes. Astronauts on these missions sent special observation satellites into orbit and tested various military instruments. To prevent the discovery of information about the capabilities of these satellites, unusual secrecy surrounded the missions. NASA did not reveal launch times of the missions in advance or release any conversations between mission control and the astronauts in space. In the early 1990's, the United States phased out the use of shuttles for such missions and resumed the use of cheaper, single-use rockets.

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Repair missions. The space shuttle enables astronauts to retrieve, repair, and relaunch broken satellites. This important capability was first demonstrated in April 1984, when two astronauts from the shuttle Challenger repaired the Solar Maximum Mission satellite—the only solar observatory in orbit. This success underscored the flexibility and capability of human beings in space. Since then, astronauts have repaired several other satellites in space.

In 1993, a crew from the shuttle Endeavour repaired the orbiting Hubble Space Telescope. After the telescope had been launched in 1990, NASA engineers discovered an error in its primary mirror. The Endeavour astronauts installed optical equipment that cancelled out the effect of the error. The crew also replaced certain scientific instruments, the solar panels, and the gyroscopes, devices used in pointing the telescope. Astronauts traveled by space shuttles to the Hubble Space Telescope to perform four additional servicing missions in 1997, 1999, 2002, and 2009. The final servicing mission, postponed from 2006, left Hubble with a set of instruments that enables the telescope to make the most detailed images in its history. These improvements were meant to leave the telescope operational until at least 2014, when its partial replacement, the James Webb Space Telescope, is scheduled to be launched.

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Spacelab missions. Spacelab was a facility that enabled shuttle crews to perform a wide variety of scientific experiments in space. It was built as a part of the space shuttle program by the European Space Agency. The first Spacelab mission was launched in 1983 in the space shuttle Columbia. In 1998, the same shuttle carried Spacelab on its last mission. Each mission focused on research in a particular area of science or technology, such as astronomy, the life sciences, and microgravity.

Spacelab consisted of a piloted space laboratory and several separate platforms called pallets. The pressurized laboratory was connected to the crew compartment by a tunnel. It had facilities for scientists to conduct experiments in manufacturing, medicine, the production of biological materials, and other areas. The pallets carried large scientific instruments that were used to conduct experiments in astronomy and other fields. Scientists operated the instruments from the laboratory, from the shuttle's orbiter, or from the ground. Spacelab facilities were shared by the ESA and the United States.

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The Challenger disaster

The 10th launch of the space shuttle Challenger was scheduled as the 25th space shuttle mission. Francis R. (Dick) Scobee was the mission commander. The crew included Christa McAuliffe, a high-school teacher from New Hampshire. The five other crew members were Gregory B. Jarvis, Ronald E. McNair, Ellison S. Onizuka, Judith A. Resnik, and Michael J. Smith.

After several launch delays, NASA officials overruled the concerns of engineers and ordered a liftoff on a cold morning, Jan. 28, 1986. The mission ended in tragedy. Challenger disintegrated into a ball of fire. The accident occurred 73 seconds into flight, at an altitude of 46,000 feet (14,020 meters) and at about twice the speed of sound.

Strictly speaking, Challenger did not explode. Instead, various structural failures caused the spacecraft to break apart. Although Challenger disintegrated almost without warning, the crew may have briefly been aware that something was wrong. The crew cabin tore loose from the rest of the shuttle and soared through the air. It took almost three minutes for the cabin to fall to the Atlantic Ocean, where it smashed on impact, killing the seven crew members.

All shuttle missions were halted while a special commission appointed by President Reagan determined the cause of the accident and what could be done to prevent such disasters from happening again. In June 1986, the commission reported that the accident was caused by a failure of O rings in the shuttle's right solid rocket booster. These rubber rings sealed the joint between the two lower segments of the booster. Design flaws in the joint and unusually cold weather during launch caused the O rings to allow hot gases to leak out of the booster through the joint. Flames from within the booster streamed past the failed seal and quickly expanded the small hole. The flaming gases then burned a hole in the shuttle's external fuel tank. The flames also cut away one of the supporting beams that held the booster to the side of the external tank. The booster tore loose and ruptured the tank. The propellants from the tank formed a giant fireball as structural failures tore the vehicle apart.

The commission said NASA's decision to launch the shuttle was flawed. Top-level decision-makers had not been informed of problems with the joints and O rings or of the possible damaging effects of cold weather.

Shuttle designers made several technical modifications, including an improved O-ring design and the addition of a crew bail-out system. Although such a system would not work in all cases, it could save the lives of shuttle crew members in some situations. Procedural changes included stricter safety reviews and more restrictive launching conditions.

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Back into space

The space shuttle resumed flying on Sept. 29, 1988, with the launch of the redesigned shuttle Discovery. The main purpose of the five-man mission was to place a communications satellite into orbit. During the next few years, many long-delayed missions were carried out. Astronauts launched a number of unpiloted space probes, such as Galileo, Magellan, and Ulysses. Large scientific research satellites such as the Hubble Space Telescope, the Compton Gamma Ray Observatory, and the Upper Atmosphere Research Satellite were placed into orbit. In 1993, a shuttle crew flew to the orbiting Hubble Space Telescope and repaired its optical system.

Shuttles also launched military satellites and communications satellites. Spacelab research missions studied astronomy and space medicine. A less ambitious launch schedule was worked out, and major delays became less frequent.

NASA also made improvements in the shuttle fleet. New computers and life-support hardware were installed. A drag parachute and new brakes made landings easier to control. The computerized automatic flight control system and life-support systems were also improved.

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Docking with Mir

Spacecraft from the United States and Russia resumed joint operations in 1995, 20 years after the Apollo-Soyuz mission. On June 29, after three years of negotiations, planning, and practice missions, the space shuttle Atlantis docked with Russia's Mir space station. Atlantis carried a replacement crew of Russian cosmonauts to Mir and brought the station's former crew home to Earth.

Unlike the largely symbolic Apollo-Soyuz mission, the Atlantis-Mir docking was the first in a series of missions. Astronauts began regular visits to Mir, carried up and back by shuttles. The shuttles delivered replacement parts and scientific equipment, as well as water, food, and air. In addition, the astronauts and cosmonauts began to test techniques to be used to build and maintain the International Space Station.

On Sept. 7, 1996, astronaut Shannon Lucid, aboard Mir, broke the record for consecutive days in space by a woman with her 169th day. The previous record of 168 days had been set by cosmonaut Yelena Kondakova in 1995. Lucid had been launched aboard Atlantis on March 22, 1996, and had been on Mir since March 23. On September 26, Lucid returned to Earth aboard Atlantis, having spent 188 consecutive days in space.

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The Columbia disaster

Disaster struck the U.S. space shuttle fleet again on Feb. 1, 2003, when Columbia, the fleet's oldest shuttle, broke apart over the southwestern United States as it reentered Earth's atmosphere. The flight was Columbia's 28th launch and the shuttle fleet's 113th mission. The accident occurred about 16 minutes before the shuttle was due to land. All seven crew members died, including Rick D. Husband, the mission commander, and Ilan Ramon, the first Israeli astronaut. The five other crew members were Michael P. Anderson, David M. Brown, Kalpana Chawla, Laurel Blair Salton Clark, and William C. McCool.

After the disaster, NASA halted shuttle flights and appointed an independent commission to investigate the accident. Investigators collected thousands of pieces of shuttle debris that had fallen to Earth after the accident. They also studied communications, sensor readings, video recordings, and other records from the mission; tested shuttle equipment; and interviewed NASA employees.

In August, the commission reported that the accident had been caused by a chunk of foam insulation that broke away from the shuttle's external fuel tank and struck Columbia's left wing at high speed shortly after liftoff. Investigators concluded that the impact created a hole in the heat-resistant panels that protected the wing from high temperatures during reentry. As the shuttle reentered Earth's atmosphere, the hole allowed superheated air to enter the wing and damage its internal structure. Eventually, the wing was destroyed, and the shuttle went out of control and broke apart.

The commission's report called for NASA to develop systems for inspecting and repairing protective tiles and panels while the shuttle is in orbit. It recommended reinforcing the panels and working to limit or prevent the shedding of foam from the external fuel tank.

The report also criticized NASA's management and safety procedures. Investigators concluded that pressure to meet budgets and deadlines had contributed to a decline in the safety of the shuttle program. The panel also found that NASA management had failed to act on safety concerns raised by engineers. The report recommended that NASA improve safety procedures and create an independent Technical Engineering Authority to oversee and enforce safety standards.

In 2004, U.S. President George W. Bush announced plans to retire the space shuttle fleet by 2010, following the completion of the International Space Station. Bush proposed replacing the shuttle with a new spacecraft designed to carry astronauts to the moon and, eventually, Mars.

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Return to flight

After working more than two years to improve the safety of the space shuttle program, NASA launched the shuttle Discovery on July 26, 2005. The launch was successful, but videotapes showed that the external fuel tank had shed at least one chunk of foam nearly as large as the one that had fatally damaged Columbia. While the foam did not appear to strike Discovery, NASA officials announced that further shuttle launches would be suspended while engineers worked to fix the problem.

The Discovery astronauts continued on their mission to deliver supplies and perform repairs to the International Space Station. The crew also tested methods of inspecting and repairing the shuttle’s heat shield in orbit. The shuttle flipped as it approached the space station so that station astronauts could photograph its underside. After docking, the astronauts used laser imaging equipment mounted on a robotic arm to scan the heat-resistant tiles for damage.

The inspections revealed that two thin strips of ceramic-fiber material used to fill the gaps between the tiles had come loose. Engineers worried that the dangling strips could disrupt the smooth flow of air over the shuttle’s underside during reentry, causing the craft to overheat. Mission managers sent U.S. astronaut Stephen K. Robinson out on a robotic arm to remove the fabric, the first heat shield repair in orbit in shuttle history. The shuttle landed safely on August 9.

On July 4, 2006, Discovery became the next shuttle to travel into space. During the launch, the external fuel tank again shed some small pieces of foam debris. Astronauts and mission managers noticed no immediate damage to the orbiter, but they continued to inspect the craft as it visited the International Space Station. No major safety problems arose during the mission, and NASA managers looked forward to the resumption of more frequent shuttle flights.

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Completing the station

The shuttle Atlantis in September 2006 completed the first of a series of regular missions to finish assembly of the International Space Station. NASA sought to complete the station before the shuttle fleet's planned retirement in 2010. Administrators announced in October that the agency would also launch one shuttle for a final servicing mission to the Hubble Space Telescope.

On June 22, 2007, astronaut Sunita Williams set a record for consecutive days in space by a woman, returning to Earth on Atlantis after 195 days in space. She had been launched with Discovery in December 2006 and served as a flight engineer on the station. Williams broke Shannon Lucid’s record of 188 days, set in 1996.

In August 2007, the shuttle Endeavour carried Barbara Morgan, NASA’s first Educator Astronaut, to space on an assembly mission. Morgan, who had served as backup for the teacher Christa McAuliffe on the disastrous Challenger flight in 1986, led question-and-answer sessions for students on Earth.

The shuttle Discovery carried the Italian-built U.S. Harmony module, designed to link U.S., European, and Japanese modules, to the station in October 2007. In February 2008, Atlantis delivered the European Columbus module. Endeavour brought part of the Japanese Kibo laboratory module to the station in March 2008. The station's first female commander, Peggy Whitson, led the station from October 2007 to April 2008.

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Other nations in space

A number of nations other than the United States and Russia have developed rocket and space programs. These programs are smaller than the U.S. and Russian programs. Most of them concentrate on single applications such as the launching of scientific satellites.

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European nations. Several European nations built boosters to launch small scientific research satellites. In 1965, France became the first nation in western Europe to launch a satellite. The United Kingdom sent another satellite into orbit in 1971.

In 1975, the European Space Agency (ESA) was organized. Its western European member nations combine their financial and scientific resources in the development of spacecraft, instruments, and experiments. The ESA supervised the construction of Spacelab, launched the space probe Giotto toward Halley's Comet, and built the Ulysses solar probe. The ESA also developed a series of Ariane booster rockets to launch communications satellites for paying customers. By the late 1980's, Ariane rockets were launching more commercial satellites than U.S. rockets were. ESA spacecraft lift off from Kourou in French Guiana, on the northern coast of South America. See European Space Agency (ESA).

Besides its activities as a member of the ESA, Germany independently built two solar probes called Helios. One probe was launched in 1974, and another was launched in 1976. These probes flew within 28 million miles (45 million kilometers) of the sun—closer than any other probe had reached.

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Japan became the fourth nation in space when it launched a satellite in February 1970. The nation's space program blossomed in the 1980's. In 1985, Japan fired two probes toward Halley's Comet. Two separate programs developed a family of small, efficient spaceboosters. The H-1 rocket, a medium-sized booster with liquid hydrogen fuel, also became operational. In 1990, Japan launched a lunar probe.

In 1994, Japan launched its first heavy-lifting booster, the H-2. In 1996, an H-2 lofted the Advanced Earth Observing Satellite. The satellite began to gather data on Earth's lands, seas, and atmosphere.

Japan sends small scientific research satellites into orbit from the Uchinoura Space Center on the island of Kyushu. Rockets carrying larger satellites take off from the Tanegashima Space Center on Tanega Island, about 60 miles (95 kilometers) to the south. Japan has developed a laboratory module for the International Space Station.

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China. In April 1970, China sent its first satellite into space aboard a CZ-1 launcher. In the 1980's, China developed impressive space technology that included liquid-hydrogen engines, powerful Long March rockets, and recoverable satellites. China has three satellite launch sites—Jiuquan, Taiyuan, and Xichang.

In the 1990's, China began developing the Shenzhou, a spacecraft designed to carry astronauts. The Shenzhou resembles Russia's Soyuz capsule. In October 2003, China became the third nation to launch a person into space. Chinese astronaut Yang Liwei orbited Earth aboard a Shenzhou craft for 21 hours before landing safely. Another Shenzhou craft carried two astronauts into orbit on a five-day mission in October 2005. On the third piloted Shenzhou flight in September 2008, two astronauts performed the country’s first spacewalk. Astronauts in the Chinese space program are sometimes called taikonauts.

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India first launched a satellite into orbit in July 1980. The Indian Space Research Organisation builds boosters. India launches rockets from the island of Sriharikota, off its eastern coast. In 2008, India launched the Chandrayaan-1 lunar orbiter. The orbiter carried and dropped an impactor, making India’s space program the fifth program to reach the lunar surface.

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Canada has an active space research program and a communications satellite program. That nation took part in the U.S. space shuttle program by designing and building the shuttle's robot arm. Canada also built a larger robot arm that was installed on the International Space Station.

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Other nations. Israel sent its first satellite into orbit in 1988. Australia has launched modified U.S. rockets from Woomera, Australia. Italy has launched U.S. rockets from a platform in the Indian Ocean, off the coast of Kenya. Several countries, including Brazil, Sweden, and South Africa, have sent scientific sounding rockets into space. Iran sent its first satellite into orbit in 2009.

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Plans for the future

In the early 2000's, scientists and engineers were developing new kinds of spacecraft and more efficient rockets. Industrial researchers were working on manufacturing techniques that would use the space environment to advantage. Encouraged by the commercial potential of space activities, private companies had begun to provide launch services.

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Developing new spacecraft. Several organizations were developing technologies for a craft that would replace the space shuttles after 2010. These organizations included NASA, ESA, the Japan Aerospace Exploration Agency, and several private companies. Their chief objective was to cut flight costs.

One way to achieve this goal would be to develop a reusable launch vehicle (RLV). All the main parts of an RLV would be reused, giving the craft an advantage over a shuttle. A shuttle's main fuel tank drops away after use and so must be replaced for each flight. In one RLV design, a special airplane would carry a spacecraft to a high altitude and release it. The spacecraft would then fire its own rockets to go into orbit. After completing its mission, the craft would land as an airplane does. Another type of RLV would be a single-stage-to-orbit (SSTO) craft—a vehicle that would take off by itself and not discard any components. Depending on the design of an SSTO, the craft might take off and land vertically, as a rocket does, or horizontally, as an airplane does.

In the early 2000's, NASA officials decided that an RLV replacement for the shuttle would prove too difficult to develop by 2010. In the meantime, the agency concentrated on developing the Crew Exploration Vehicle (CEV), also known as Orion, a capsule-shaped spacecraft that would carry crew and light cargo to and from the International Space Station. A partially recoverable booster would carry the CEV into space. The agency planned to eventually use Orion to transport crew to the moon.

Also in the 2000's, NASA invested in private companies to help them build spacecraft that could reach low Earth orbit. NASA funded work by Space Exploration Technologies, also known as SpaceX, of Hawthorne, California, and Orbital Sciences Corporation of Dulles, Virginia.

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Developing more efficient rockets. Scientists and engineers were working on alternatives to fuel-burning rockets. Two main alternatives were (1) the ion rocket and (2) the nuclear rocket. For a given amount of fuel, both alternatives can create at least twice as much acceleration as a fuel-burning rocket. In addition, both can operate for a long time before running out of fuel. Neither ion rockets nor nuclear rockets would launch spacecraft; they would create thrust after fuel-burning boosters had performed that task.

An ion rocket is an electrical device. Electric energy heats a fuel, converts its atoms to ions (electrically charged atoms), and expels the ions to create thrust. Designers have already used small ion rockets to keep communications satellites in position above Earth. An ion rocket has also propelled a space probe called Deep Space 1 on a mission to asteroids and comets.

A nuclear rocket uses heat from a nuclear reactor to change a liquid fuel into a gas and expel the gas. This kind of rocket would not be practical as a launcher because some radioactive materials might escape into the atmosphere. However, a small nuclear rocket that created thrust continuously could decrease the time of missions to other planets.

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Expanding space activities. Two major areas of space utilization have been the gathering and communication of information. Satellites monitor weather systems on Earth, and space probes gather information on the other planets and the sun. Since the 1960's, communications satellites have regularly relayed television signals between points on Earth's surface.

The next major area of space utilization may be the manufacture of medicinal and industrial products. Manufacturers may use the low gravity, high-vacuum environment of space to create substances that are purer or stronger than those produced on Earth. These substances might include drugs; semiconductors, the materials of which computer chips are made; and special alloys (mixtures of metals). As profitable manufacturing processes are developed, private companies may even build and operate "orbiting factories."

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Private space flight. Many private companies have begun to develop launch services to compete with the national and international organizations. One firm, Sea Launch Company, boosted a communications satellite from a floating platform in the Pacific Ocean in October 1999. The company used a Ukrainian-built Zenit rocket to launch the satellite. Sea Launch is owned by corporations in the United States, Russia, Norway, and Ukraine.

In 1996, an organization called the X Prize Foundation announced the creation of the X Prize competition to stimulate interest in private space travel. The group offered a $10 million award to the first privately funded team to build and launch a craft capable of carrying a pilot and two passengers into space. To qualify for the prize, the craft had to fly to an altitude of 62 miles (100 kilometers), land safely, and then make a repeat flight within two weeks. More than 20 teams from many countries registered to compete for the prize. The award was later renamed the Ansari X Prize for a family that donated a large portion of the prize money.

On June 21, 2004, one of the X Prize competitors, Scaled Composites of Mojave, California, became the first private company to launch a person into space. The company’s rocket, called SpaceShipOne, carried American test pilot Michael Melvill more than 62 miles above Earth on a brief suborbital test flight. The rocket was launched from a specially designed airplane called the White Knight. SpaceShip0ne went on to win the X Prize with successful launches on Sept. 29 and Oct. 4, 2004.

In September 2007, the X Prize Foundation announced the Google Lunar X Prize for a nongovernmental mission to the moon. The winner would be awarded up to $25 million, paid for by the online-search company Google Inc., for roving a certain distance on the lunar surface and transmitting video and data back to Earth.

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