the investigation, by means of manned and unmanned spacecraft, of the reaches of the universe beyond Earth's atmosphere and the use of the information so gained to increase knowledge of the cosmos and benefit humanity.

Humans have always looked at the heavens and wondered about the nature of the objects seen in the night sky. With the development of rockets and the advances in electronics and other technologies in the 20th century, it became possible to send machines and animals and then people above Earth's atmosphere into outer space. Well before technology made these achievements possible, however, space exploration had already captured the minds of many people, not only aircraft pilots and scientists but also writers and artists. The strong hold that space travel has always had on the imagination may well explain why professional astronauts and laypeople alike consent at their great peril, in the words of Tom Wolfe (Wolfe, Tom) in the The Right Stuff (1979), to sit “on top of an enormous Roman candle, such as a Redstone, Atlas, Titan or Saturn rocket, and wait for someone to light the fuse.” It perhaps also explains why space exploration has been a common and enduring theme in literature and art. As centuries of speculative fiction in books and more recently in films make clear, “one small step for man, one giant leap for mankind” was taken by the human spirit many times and in many ways before Neil Armstrong (Armstrong, Neil) stamped humankind's first footprint on the Moon.

Even before the first satellite was launched, U.S. leaders recognized that the ability to observe military activities around the world from space would be an asset to national security. Following on the success of its photoreconnaissance satellites, which began operation in 1960, the United States built increasingly complex observation and electronic-intercept intelligence satellites. The Soviet Union also quickly developed an array of intelligence satellites, and later a few other countries instituted their own satellite observation programs. Intelligence-gathering satellites have been used to verify arms-control agreements, provide warnings of military threats, and identify targets during military operations, among other uses.

Governments realized early on that the ability to observe Earth from space could provide significant benefits to the general public apart from security and military uses. The first application to be pursued was the development of satellites for assisting in weather forecasting. A second application involved remote observation of land and sea surfaces to gather imagery and other data of value in crop forecasting, resource management, environmental monitoring, and other applications. The U.S. and Soviet governments also developed their own satellite-based global positioning system (GPS)s, originally for military purposes, that could pinpoint a user's exact location, help in navigating from one point to another, and provide very precise time signals. These satellites quickly found numerous civilian uses in such areas as personal navigation, surveying and cartography, geology, air traffic control, and the operation of information-transfer networks. They illustrate a reality that has remained constant for the past half century—as space capabilities are developed, they often can be used for both military and civilian purposes.

The first artificial Earth satellite, Sputnik 1, was launched by the Soviet Union on October 4, 1957. The first human to go into space, Yury Gagarin (Gagarin, Yury Alekseyevich), was launched, again by the Soviet Union, for a one-orbit journey around Earth on April 12, 1961. Within 10 years of that first human flight, American astronauts walked on the surface of the Moon. Apollo (Apollo program) 11 crew members Neil Armstrong (Armstrong, Neil) and Edwin (“Buzz”) Aldrin (Aldrin, Edwin Eugene, Jr.) made the first lunar landing on July 20, 1969. A total of 12 Americans on six separate Apollo missions set foot on the Moon between July 1969 and December 1972. Since then, no humans have left Earth orbit, but several hundred men and women have spent as many as 438 consecutive days in space. Starting in the early 1970s, a series of Soviet (Russian from December 1991) space stations (space station), the U.S. Skylab station, and numerous space shuttle flights have provided Earth-orbiting bases for varying periods of human occupancy and activity. With the development of the International Space Station (ISS) in the 1990s, the intent has been to have humans living and working in space on a permanent basis. The first ISS crew took up long-duration residence on November 2, 2000.

Since 1957 Earth-orbiting satellites and robotic spacecraft journeying away from Earth have gathered valuable data about the Sun, Earth, other bodies in the solar system, and the universe beyond. Robotic spacecraft have landed on the Moon, Venus, Mars, and the asteroid Eros, have visited the vicinity of all the major planets, and have flown by the nuclei of comets, including Halley's Comet, traveling in the inner solar system. Scientists have used space-derived data to deepen human understanding of the origin and evolution of galaxies, stars, planets, and other cosmological phenomena.

Significant milestones in space explorationOrbiting satellites also have provided, and continue to provide, important services to the everyday life of many people on Earth. Meteorologic satellites deliver information on short- and long-term weather patterns and their underlying causes. Other Earth-observation satellites remotely sense land and ocean areas, gathering data that improve management of Earth's resources. Telecommunications satellites allow essentially instantaneous transfer of voice, images, and data on a global basis. Satellites operated by the United States and Russia give precision navigation, positioning, and timing information that has become essential to many terrestrial users. Earth-observation satellites have also become extremely useful to the military authorities of several countries as complements to their land, sea, and air forces. (For a list of significant milestones in space exploration, see the table (Significant milestones in space exploration).)

It was space exploration that motivated the members of the German VfR to build their rockets, but in the early 1930s their work came to the attention of the German military. In 1932 Wernher von Braun (Braun, Wernher von), at age 20, became chief engineer of a rocket-development team for the German army. After Adolf Hitler came to power in 1933, Braun was named the civilian head of that team, under the military command of Walter Robert Dornberger (Dornberger, Walter Robert). To give Braun's engineers the needed space and secrecy for their work, the German government erected a development and test centre at Peenemünde on the coast of the Baltic Sea. There they developed, among other devices, the V-2 (V-2 missile) (originally designated the A-4) ballistic missile. First launched successfully in 1942, the V-2 was used on targets in Europe beginning in September 1944. Although built as a weapon of war, the V-2 later served as the predecessor of many of the rockets used in the early space programs of the United States and the Soviet Union. As World War II neared its end in early 1945, Braun and many of his associates chose to surrender to the United States, where they believed they would likely receive support for their rocket research and space exploration plans. Later in the year, they were taken to the United States, as were their engineering plans and the parts needed to construct a number of V-2s. The German rocket team played a central role in the early development of space launchers for the United States.

Between 1946 and 1951, the U.S. Army (United States Army, The) conducted test firings of captured German V-2 rockets at White Sands, New Mexico. These sounding-rocket flights reached high altitudes (120–200 km 【75–125 miles】) before falling back to Earth. Although the primary purpose of the tests was to advance rocket technology, the army invited American scientists interested in high-altitude research to put experiments aboard the V-2s. An Upper Atmosphere Research Panel, chaired by the physicist James Van Allen (Van Allen, James A.), was formed to coordinate the scientific use of these rocket launchings. The panel had a central role in the early years of American space science, which focused on experiments on solar and stellar ultraviolet radiation, the aurora, and the nature of the upper atmosphere. As the supply of V-2s dwindled, other U.S.-built sounding rockets such as the WAC Corporal, Aerobee, and Viking were put into use. In other countries, particularly the Soviet Union, rocket-based upper-atmosphere research also took place after World War II.

As scientific and military planners contemplated initial space projects and engineers worked on developing the needed launch vehicles, the idea that humans would soon begin the exploration of space entered popular imagination. In Europe, since the 1930s, the British Interplanetary Society had been actively promoting the idea that human space travel was soon to happen. American movies such as The Day the Earth Stood Still (1950), Destination Moon (1950), and When Worlds Collide (1951) contained vivid images of such journeys. Reports were widespread of sightings of unidentified flying objects (unidentified flying object) (UFOs), which were thought by some to be spacecraft from alien worlds.

Although Soviet plans to orbit a satellite during the IGY had been discussed extensively in technical circles, the October 4, 1957, launch of Sputnik 1 came as a surprise, and even a shock, to most people. Prior to the launch, skepticism had been widespread about the U.S.S.R.'s technical capabilities to develop both a sophisticated scientific satellite and a rocket powerful enough to put it into orbit. Under Korolyov's direction, however, the Soviet Union had been building an intercontinental ballistic missile (ICBM), with engines designed by Glushko, that was capable of delivering a heavy nuclear warhead to American targets. That ICBM, called the R-7 or Semyorka (“Number 7”), was first successfully tested on August 21, 1957, which cleared the way for its use to launch a satellite. Fearing that development of the elaborate scientific satellite intended as the Soviet IGY contribution would keep the U.S.S.R. from being the first into space, Korolyov and his associates, particularly Tikhonravov, designed a much simpler 83.6-kg (184-pound) sphere carrying only two radio transmitters and four antennas. After the success of the R-7 in August, that satellite was rushed into production and became Sputnik 1. A second, larger satellite carrying scientific instruments and the dog Laika, the first living creature in orbit, was launched November 3. The even larger, instrumented spacecraft originally intended to be the first Soviet satellite went into orbit in May 1958 as Sputnik 3. (For additional information on Korolyov's contribution to the Soviet space program, see Energia.)

Braun and his army superiors had not agreed with the decision to assign the satellite mission to the navy. After the launches of the first two Sputniks, they secured permission to attempt their own satellite launch. In anticipation of such a situation, they had kept in touch with JPL and Van Allen and so were able to prepare a satellite quickly. On January 31, 1958, Braun's Jupiter-C launch vehicle, a modified Redstone ballistic missile, carried into orbit Explorer 1, the first U.S. satellite. Designed at JPL, Explorer 1 carried Van Allen's experiment to measure cosmic rays (cosmic ray). The results from this experiment and similar ones aboard other U.S. and Soviet satellites launched that same year revealed that Earth was surrounded by two zones of radiation, now known as the Van Allen radiation belts (Van Allen radiation belt), comprising energetic particles trapped by Earth's magnetic field.

In the United States, the air force developed a rocket-powered experimental aircraft, the X-15, which, after being dropped from an in-flight B-52 bomber, could reach altitudes as high as 108 km (67 miles), the edge of outer space. Nevertheless, the X-15 could not achieve the velocity and altitude needed for orbital flight. That was the mission of Dyna-Soar, another air force project. Dyna-Soar was to be a piloted reusable delta-winged vehicle that would be launched into orbit by a modified Titan (Titan rocket) ICBM and could carry out either bombing or reconnaissance missions over the Soviet Union or intercept a Soviet satellite in orbit. Although a full-scale vehicle was built and six people were chosen to train as Dyna-Soar crew, the project was canceled in 1963.

Soon after the success of the first Sputniks, Korolyov (Korolyov, Sergey Pavlovich) and his associate Tikhonravov began work on the design of an orbital spacecraft that could be used for two purposes. One was to conduct photoreconnaissance missions and then return the exposed film to Earth. The other was to serve as a vehicle for the first human spaceflight missions, in which a human being would replace the reconnaissance camera. The spacecraft was known as Object K, but it was called Vostok when it was used to carry a human into space. Vostok had two sections—a spherical capsule in which the person would ride and a conical module that contained the instruments needed for its flight. The spacecraft was large for the time, weighing 4.73 metric tons. The crew capsule was completely covered by a thermal coating to protect it during reentry. Vostok was designed so that the human aboard need not touch any control from launch to touchdown; he would be essentially just a passenger. Nor would he land with the spacecraft. Rather, he would be ejected from it at an altitude of 7 km (4.3 miles) and parachute to dry land, while the spacecraft landed nearby with its own parachutes.

The United States used chimpanzees (chimpanzee), rather than dogs, as test subjects prior to human flights. In what was intended to be the final test flight before a human launch, the chimpanzee Ham rode a suborbital trajectory on January 31, 1961, using a Redstone rocket developed by Braun's team. Because the flight had experienced minor problems, Braun insisted on one more test flight with an unoccupied dummy spacecraft. If instead, as originally scheduled, that March 1961 flight had carried an astronaut, the United States would have been first with a human in space, although not in orbit. Alan B. Shepard, Jr. (Shepard, Alan B., Jr.), made the first manned Mercury flight atop a Redstone rocket on May 5, 1961. A second suborbital Mercury mission, carrying Virgil I. Grissom (Grissom, Virgil I.), followed in July.

John H. Glenn, Jr. (Glenn, John H., Jr.), became the first American astronaut to orbit Earth in his three-orbit mission on February 20, 1962. His Mercury spacecraft was launched by a modified air force Atlas (Atlas rocket) ICBM. Three more one-man Mercury orbital flights, carrying astronauts M. Scott Carpenter (Carpenter, M. Scott), Walter M. Schirra, Jr. (Schirra, Walter M., Jr.), and L. Gordon Cooper, Jr. (Cooper, L. Gordon, Jr.), were conducted, the last being a 22-orbit mission in May 1963.

The first manned Gemini mission lifted into space in March 1965; nine more missions followed, the last in November 1966. On the second mission in June 1965, Edward H. White II (White, Edward H., II) became the first American astronaut to operate outside a spacecraft. His 20-minute space walk—also known as extravehicular activity (EVA)—was without incident. Although problems developed on many of the Gemini flights, the program demonstrated that people could live and work in space for as long as 14 days, more than the time needed for a round trip to the Moon. It also showed that astronauts could carry out rendezvous in space and could make useful observations of Earth, both visually and photographically.

Korolyov and his associates began work in 1962 on a second-generation spacecraft, to be called Soyuz. It was to be a much more complex vehicle than Vostok, holding as many as three people in an orbital crew compartment, with a separate module for crew reentry and a third section containing spacecraft equipment and rocket engines for in-orbit and reentry maneuvers. Soyuz was to be capable not only of flights in Earth orbit but also, in modified versions, of flights around the Moon and even a lunar landing.

By the end of 1962, the basic elements of what was called Project Apollo (Apollo program) were in place. The launch vehicle would be a powerful Saturn V rocket, 110.6 metres (363 feet) tall and power-driven by five huge engines generating a total of 33,000 kilonewtons (7.5 million pounds) of lifting power at takeoff—100 times the takeoff thrust of the Redstone rocket that had launched Shepard. After an intense debate, NASA chose a spacecraft configuration for Apollo that could be sent up in one launch, rather than a larger spacecraft that would need to be assembled in a series of rendezvous in Earth orbit. The Apollo spacecraft would have three sections. A Command Module would house the three-person crew on liftoff and landing and during the trip to and from the Moon. A Service Module would carry various equipment and the rocket engine needed to guide the spacecraft into lunar orbit and then send it back to Earth. A Lunar Module, comprising a descent stage and an ascent stage, would carry two people from lunar orbit to the Moon's surface and back to the Command Module. The ability of the Lunar Module's ascent stage to rendezvous and dock in lunar orbit with the Command Module after takeoff from the Moon was critical to the success of the mission. NASA also created a large new launch facility on Merritt Island, near Cape Canaveral (Canaveral, Cape), Florida, as the Apollo spaceport.

In the United States, Apollo moved forward as a high-priority program. A major setback occurred on January 27, 1967, when astronauts Grissom, White, and Roger Chaffee (Chaffee, Roger B.) were killed after their Apollo 1 Command Module caught fire during a ground test. The first manned Apollo mission, designated Apollo 7 and intended to test the redesigned Command Module, was launched into Earth orbit on October 11, 1968. The launcher used was a Saturn IB, a less-powerful rocket than the Saturn V needed to reach the Moon. The mission's success cleared the way for a bold step—the first launch of a crew atop a Saturn V to the lunar vicinity. On December 21, 1968, the Apollo 8 Command and Service modules were put on a trajectory that sent them into orbit around the Moon on Christmas Eve, December 24. The three astronauts—Frank Borman (Borman, Frank), James A. Lovell, Jr. (Lovell, James A(rthur), Jr.), and William A. Anders (Anders, William A.)—sent back close-up images of the lunar surface, read from the biblical book of Genesis, and brought back vivid colour photographs of a blue planet Earth rising over the desolate lunar landscape. By the end of the mission, it was clear that the first lunar landing was only months away.

By contrast with the Soviet lunar landing efforts, during 1969 all went well for the Apollo program. In March the Apollo 9 crew sucessfully tested the Lunar Module in Earth orbit, and in May the Apollo 10 crew carried out a full dress rehearsal for the landing, coming within 15,200 metres (50,000 feet) of the lunar surface. On July 16, 1969, astronauts Armstrong (Armstrong, Neil), Aldrin (Aldrin, Edwin Eugene, Jr.), and Michael Collins (Collins, Michael) set off on the Apollo 11 mission, the first lunar landing attempt. While Collins remained in lunar orbit in the Command Module, Armstrong piloted the Lunar Module, nicknamed Eagle, away from boulders on the lunar surface and to a successful landing on a flat lava plain called the Sea of Tranquillity at 4:18 PM U.S. Eastern Daylight Time on July 20. He reported to mission control, “Houston. Tranquillity Base here. The Eagle has landed.” Six and a half hours later, Armstrong, soon followed by Aldrin, left the Lunar Module and took the first human step on the surface of another celestial body. As he did so, he noted, “That's one small step for 【a】 man, one giant leap for mankind.” (In the excitement of the moment, Armstrong skipped the “a” in the statement he had prepared.) Concluding 2.5 hours of activity on the lunar surface, the two men returned to the Lunar Module with 21.7 kg (47.8 pounds) of lunar samples. Twelve hours later, they blasted off the Moon in the Lunar Module's ascent stage and rejoined Collins in the Command Module. The crew returned to Earth on July 24, splashing down in the Pacific Ocean.

The successful Apollo 12 mission followed in November 1969. The Apollo 13 mission, launched in April 1970, experienced an explosion of the oxygen tank in its Service Module on the outbound trip to the Moon. The crew survived this accident only through the improvised use of the Lunar Module as living quarters in order to preserve the remaining capabilities of the Command Module for reentering Earth's atmosphere after they had returned from their circumlunar journey. Four more Apollo missions followed. On each of the final three, the crew had a small cartlike rover that allowed them to travel several kilometres from their landing site. The final mission, Apollo 17, which was conducted in December 1972, included geologist Harrison Schmitt, the only trained scientist to set foot on the Moon.

The United States had won the race to the Moon, but that race had been motivated primarily by political considerations. After the early 1970s there was no interest within the U.S. government for the next three decades in additional lunar exploration or in sending people to Mars or any other distant destination. No human has traveled beyond near-Earth orbit since Apollo 17.

By 1969, even though the U.S.S.R. was still moving forward with its lunar landing program, it had begun to shift its emphasis in human spaceflight to the development of Earth-orbiting stations in which cosmonaut crews could carry out extended observations and experiments on missions that lasted weeks or months rather than a few days. The first Soviet space station, called Salyut 1, was launched April 19, 1971. The first crew to occupy the station—Georgy Dobrovolsky (Dobrovolsky, Georgy Timofeyevich), Viktor Patsayev (Patsayev, Viktor Ivanovich), and Vladislav Volkov (Volkov, Vladislav Nikolayevich)—spent 23 days aboard carrying out scientific studies but perished when their Soyuz spacecraft depressurized during reentry.

With similar objectives for a long-term manned platform in space, the United States converted the third stage of a Saturn V rocket into an orbital workshop for solar and biomedical studies. This first U.S. space station, called Skylab, was launched May 14, 1973. Over a period of eight and a half months, three three-person crews using Apollo spacecraft for transport spent time aboard Skylab, with the final crew staying for 84 days. Skylab was abandoned in February 1974 and reentered Earth's atmosphere in July 1979, with some portions of the station surviving reentry and landing in Australia.

Whereas, because of budgetary cuts, the United States did not launch a planned second Skylab, the Soviet Union orbited and successfully occupied five more Salyut stations in a program that continued through the mid-1980s. Two of these stations had a military reconnaissance mission, but the others were devoted to scientific studies, particularly biomedical research. The Soviet Union also launched guest cosmonauts from allied countries for short stays aboard Salyut 6 and 7.

International space endurance recordsThe Soviet Union followed its Salyut station series with the February 1986 launch of the core element of the modular Mir space station. Additional modules carrying scientific equipment and expanding the living space were attached to Mir in subsequent years. In 1994–95 Valery Polyakov (Polyakov, Valery Vladimirovich), a medical doctor, spent 438 continuous days aboard the station. (For a list of human endurance records in space, see the table (International space endurance records).) More than 100 different people from 12 countries visited Mir, including seven American astronauts in the 1995–98 period. The station, which was initially scheduled to operate for only five years, supported human habitation until mid-2000 (continuously between 1989 and 1999), although it experienced a number of accidents and other serious problems. In March 2001 it made a controlled atmospheric reentry, with the surviving pieces falling into the Pacific Ocean.

Space stations, from 1971The United States did not follow up on Skylab until 1984, when President Ronald Reagan (Reagan, Ronald W.) approved a space station program and invited U.S. allies to participate. By 1988, 11 countries—Canada, Japan, and 9 countries from Europe—had decided to join what was known as Space Station Freedom. Progress in developing the station was slow, however, and in 1993 newly elected President Bill Clinton (Clinton, Bill) ordered a sweeping redesign of the program. The United States and its existing partners invited Russia, which had inherited most of the Soviet Union's space efforts after the U.S.S.R.'s collapse in 1991, to participate in the multinational program, renamed the International Space Station (ISS). Three additional countries joined during the 1990s and thereby made the 16-country project the largest-ever cooperative technological undertaking. The first two elements of the ISS were launched and connected in space in late 1998, and several modules and other equipment were subsequently added. Assembly and initial operation of the facility were scheduled to take place over much of the first decade of the 21st century. (For a summary of space stations launched since 1971, see table (Space stations, from 1971).)

The space shuttle design had three major components. A reusable winged orbiter would carry crew and cargo and be able to glide to a landing on a conventional runway at the end of its mission. A large external tank would carry the liquid-oxygen and liquid-hydrogen propellants for the orbiter's three powerful engines. The tank would be used only during the first eight minutes of flight; once the fuel was exhausted, it would be discarded and burn up on reentry. Two solid-fuel rockets would assist in accelerating the vehicle during the first two minutes of flight; they would then be detached and parachute into the ocean, where they would be recovered for future use. A fleet of four operational orbiters, named Columbia, Challenger, Atlantis, and Discovery, was built in order to allow multiple shuttle flights each year. Facilities in Florida originally constructed for the Apollo program were remodeled for shuttle use.

The optimism surrounding the space shuttle program was publicly shattered on January 28, 1986, when the Challenger orbiter was destroyed in a catastrophic explosion 73 seconds after liftoff. Its seven-person crew perished; among them was schoolteacher Christa McAuliffe (McAuliffe, Christa Corrigan), on board as the first private citizen in space. The launch had taken place in unusually cold weather, and a sealing ring within a segment joint of one of the solid rocket boosters failed. The solid rocket broke loose and hit the external tank, rupturing it. The flame from the leaking booster ignited the shuttle's fuel, causing the explosion.

The shuttle program suffered its second fatal disaster on February 1, 2003, when the orbiter Columbia broke up over Texas at an altitude of about 60 km (40 miles) as it was returning from an orbital mission. All seven crew members died, including Ilan Ramon, the first Israeli astronaut to go into space. The shuttle fleet was once again grounded during the ensuing investigation into the cause of the accident, and critical flights to the ISS were conducted by Russian expendable launch vehicles.

From the start of human spaceflight efforts, some have argued that the benefits of sending humans into space do not justify either the risks or the costs. They contend that robotic missions can produce equal or even greater scientific results with lower expenditures and that human presence in space has no other valid justification. Those who support human spaceflight cite the still unmatched ability of human intelligence, flexibility, and reliability in carrying out certain experiments in orbit, in repairing and maintaining robotic spacecraft and automated instruments in space, and in acting as explorers in initial journeys to other places in the solar system. They also argue that astronauts serve as excellent role models for younger people and act as vicarious representatives of the many who would like to fly in space themselves. In addition is the long-held view that eventually some humans will leave Earth to establish permanent outposts and larger settlements on the Moon, Mars, or other locations.

Most of the individuals who have gone into space are highly trained astronauts (astronaut) and cosmonauts, the two designations having originated in the United States and the Soviet Union, respectively. (Both taikonaut and yuhangyuan have sometimes been used to describe the astronauts in China's manned space program.) Those governments interested in sending some of their citizens into space select candidates from many applicants, on the basis of their backgrounds and physical and psychological characteristics. The candidates undergo rigorous training before being chosen for an initial spaceflight and then prepare in detail for each mission assigned. Training centres with specialized facilities exist in the United States, at NASA's Johnson Space Center, Houston, Texas; in Russia, at the Yury Gagarin Cosmonauts Training Center (commonly called Star City), outside Moscow; in Germany, at ESA's European Astronaut Centre, Cologne; in Japan, at NASDA's Tsukuba Space Center, near Tokyo; and in China, at Space City, near Beijing.

Although the changes in muscle, bone, and blood production do not pose problems for astronauts in space, they do so on their return to Earth. For example, in normal gravity, a person with decreased bone mass runs a greater risk of breaking a bone during normal strenuous activity. Countermeasures, particularly various forms of exercise while in space, have been developed to prevent these effects from causing health problems later on Earth. Even so, people recovering from long-duration flights require varying amounts of time to readjust to Earth conditions. Light-headedness usually disappears within one or two days; lack of balance and symptoms of motion sickness, in three to five days; anemia, in one to two weeks; muscle atrophy, in three to five weeks; and bone atrophy, in one to three years or more.

The first scientific discovery made with instruments orbiting in space was the existence of the Van Allen radiation belts (Van Allen radiation belt), by Explorer 1 and other spacecraft in 1958. Subsequent space missions investigated Earth's magnetosphere, the surrounding region of space in which the planet's magnetic field exerts a controlling effect (see Earth: The magnetic field and magnetosphere (Earth)). Of particular and ongoing interest has been the interaction of the flux of charged particles emitted by the Sun, called the solar wind, with the magnetosphere. Early space science investigations showed, for example, that luminous atmospheric displays known as auroras (aurora) are the result of this interaction, and scientists came to understand that the magnetosphere is an extremely complex phenomenon.

To carry out the investigations required for addressing these scientific questions, the United States, Europe, the Soviet Union, and Japan developed a variety of space missions, often in a coordinated fashion. In the United States, early studies of the Sun were undertaken by a series of Orbiting Solar Observatory satellites (launched 1962–75) and the astronaut crews of the Skylab space station in 1973–74, using that facility's Apollo Telescope Mount. These were followed by the Solar Maximum Mission satellite (launched 1980). ESA developed the Ulysses mission (1990) to explore the Sun's polar regions. Solar-terrestrial interactions were the focus of many of the Explorer series of spacecraft (1958–75) and the Orbiting Geophysical Observatory satellites (1964–69). In the 1980s NASA, ESA, and Japan's Institute of Space and Astronautical Science undertook a cooperative venture to develop a comprehensive series of space missions, named the International Solar-Terrestrial Physics Program, that would be aimed at full investigation of the Sun-Earth connection. This program was responsible for the U.S. Wind (1994) and Polar (1996) spacecraft, the European Solar and Heliospheric Observatory (SOHO; 1995) and Cluster (2000) missions, and the Japanese Geotail satellite (1992).

From the start of space activity, scientists recognized that spacecraft could gather scientifically valuable data about the various planets, moons, and smaller bodies in the solar system. Both the United States and the U.S.S.R. attempted to send robotic missions to the Moon in the late 1950s. The first four U.S. Pioneer spacecraft, Pioneer 0–3, launched in 1958, were not successful in returning data about the Moon. The fifth mission, Pioneer 4 (1959), was the first U.S. spacecraft to escape Earth's gravitational pull; it flew by the Moon at twice the planned distance but returned some useful data. Three Soviet missions, Luna 1–3, explored the vicinity of the Moon in 1959, confirming that it had no appreciable magnetic field and sending back the first-ever images of its far side. Luna 1 was the first spacecraft to fly past the Moon, beating Pioneer 4 by two months; Luna 2, in making a hard landing on the lunar suface, was the first spacecraft to strike another celestial object. Later Luna spacecraft soft-landed on the Moon, and some gathered soil samples and returned them to Earth.

In the 1960s the United States became the first country to send a spacecraft to the vicinity of other planets; Mariner 2 flew by Venus in December 1962, and Mariner 4 flew past Mars in July 1965. Among significant accomplishments of planetary missions in succeeding decades were the U.S. Viking landings on Mars in 1976 and the Soviet Venera explorations of the atmosphere and surface of Venus from the mid-1960s to the mid-1980s. In the years since, the United States has continued an active program of solar system exploration, as did the Soviet Union until its dissolution in 1991. Japan launched missions to the Moon, Mars, and Halley's Comet. Europe's first independent solar system mission, Giotto, also flew by Halley. After the turn of the 21st century, it sent missions to the Moon and Mars and an orbiter-lander to a comet.

Early on, scientists planned to conduct solar system exploration in three stages: initial reconnaissance from spacecraft flying by a planet, comet, or asteroid; detailed surveillance from a spacecraft orbiting the object; and on-site research after landing on the object or, in the case of the giant gas planets, by sending a probe into its atmosphere. By the start of the 21st century, all three of those stages had been carried out for the Moon, Venus, Mars, Jupiter, and a near-Earth asteroid. Several Soviet and U.S. robotic spacecraft have landed on Venus and the Moon, and the United States has landed spacecraft on the surface of Mars. A long-term, detailed surveillance of Jupiter and its moons began in 1995 when the U.S. Galileo spacecraft took up orbit around the planet, at the same time releasing a probe into the turbulent Jovian atmosphere. In 2001 the U.S. Near Earth Asteroid Rendezvous (Near Earth Asteroid Rendezvous Shoemaker) (NEAR) spacecraft landed on the asteroid Eros and transmitted information from its surface for more than two weeks. Among the rocky inner planets, only Mercury has remained relatively neglected. In the first half century of space exploration, Mercury was visited just once—the U.S. Mariner 10 probe made three flybys of the planet in 1974–75. In 2004 the U.S. Messenger spacecraft was launched to Mercury for a series of flybys beginning in 2008 and establishment of an orbit around the planet in 2011.

The question of whether life has ever existed elsewhere in the solar system continues to intrigue both scientists and the general public. The United States sent two Viking spacecraft to land on the surface of Mars in 1976. Each contained three experiments intended to search for traces of organic material that might indicate the presence of past or present life-forms (extraterrestrial life); none of the experiments produced positive results. Twenty years later, a team of scientists studying a meteorite of Martian origin found in Antarctica announced the discovery of possible microscopic fossils resulting from past organic life. Their claim was not universally accepted, but it led to an accelerated program of Martian exploration focused on the search for evidence of the action of liquid water, thought necessary for life to have evolved. A major goal of this program is to return samples of the Martian surface to Earth for laboratory analysis.

The Galileo mission provided images and other data related to Jupiter's moon Europa that suggest the presence of a liquid water ocean beneath its icy crust. Future missions will seek to confirm the existence of this ocean and search for evidence of organic or biological processes in it.

Beginning in the 1960s, a number of countries launched satellites (satellite observatory) to explore cosmic phenomena in the gamma-ray, X-ray, ultraviolet, visible, and infrared regions. More recently, space-based radio astronomy has been pursued. In the last decades of the 20th century, the United States embarked on the development of a series of long-duration orbital facilities collectively called the Great Observatories. They include the Hubble Space Telescope, launched in 1990 for observations in the visible and ultraviolet regions; the Compton Gamma Ray Observatory, launched in 1991; the Chandra X-ray Observatory, launched in 1999; and the Spitzer Space Telescope, launched in 2003. Europe and Japan have also been active in space-based astronomy and astrophysics.

A spacecraft orbiting Earth is essentially in a continual state of free fall (free-fall). All objects associated with the spacecraft, including any crew and other contents, are accelerating—i.e., falling freely—at the same rate in Earth's gravitational field (see Earth: Basic planetary data (Earth)). As a result, these objects do not “feel” the presence of Earth's gravity but instead experience a state of weightlessness, or zero gravity. True zero gravity, however, is experienced only at the centre of mass of a freely falling object. With increasing distance from the centre of mass, the influence of gravity increases in both directions perpendicular to the object's flight path. These constant but tiny accelerations make necessary the use of the term microgravity to describe the space environment. (It is possible to create a similar absence of gravity's effects only briefly on Earth or in an aircraft.) Human activity or the operation of equipment in a spacecraft causes vibrations that impart additional accelerations and so raise gravity levels, which can make it difficult to carry out highly sensitive experiments under sufficiently low microgravity conditions. Although spacecraft designers cannot totally eliminate gravitational effects, they hope to reduce them in some parts of the International Space Station to one microgravity—one-millionth of Earth's gravity—by isolating those areas from vibrations and other disturbances as much as possible.

The microgravity environment also offers unique conditions for experiments designed to explore the behaviour of materials. Among the areas of inquiry are biotechnology, combustion science, fluid physics, fundamental physics, and materials science. Experiments in the microgravity environment on various materials, including metals, alloys, electronic and photonic materials, composites, colloids, glasses and ceramics, and polymers, have resulted in a greater understanding of the role of gravity in similar laboratory and manufacturing processes on Earth. The microgravity environment offers the potential for producing biological materials, including highly ordered protein crystals for crystallographic analysis and even materials resembling human tissues, that are difficult or impossible to make on Earth. Although microgravity research is still largely at the basic level, scientists and engineers hope that additional work—another major focus for the ISS—will lead to practical knowledge of great usefulness to manufacturing processes on Earth.

Satellites, space stations, and space shuttle missions have provided a new perspective for scientists to collect data about Earth itself. In addition to practical applications (see below Space applications (space exploration)), Earth observation from space has made significant contributions to fundamental knowledge. An early and continuing example is the use of satellites to make various geodetic measurements, which has allowed precise determinations of Earth's shape, internal structure, and rotational motion and the tidal and other periodic motions of the oceans. Fields as diverse as archaeology, seismology, and oceanography likewise have benefited from observations and measurements made from orbit.

Space visionaries in the early 20th century recognized that putting satellites into orbit could furnish direct and tangible benefits to people on Earth. For example, Arthur C. Clarke (Clarke, Sir Arthur C.) in 1945 described a way in which three satellites in orbit about 35,800 km (22,250 miles) above the Equator could relay communications around the globe. In this orbit, called a geostationary orbit, the satellites would have an orbital period equal to Earth's rotational period and thus appear from the ground to be stationary in the sky. (For additional information on satellite orbits, see spaceflight: Earth orbit (spaceflight).) A report for the U.S. Army Air Forces in 1946 by Project RAND (the predecessor of the RAND Corporation) identified the benefits of being able to observe Earth from space, which included distinguishing the impact sites of bombs dropped by U.S. aircraft and improved weather forecasting.

In 1957 scientists tracking the first satellite, Sputnik 1, found that they could plot the satellite's orbit very precisely by analyzing the Doppler shift (see Doppler effect) in the frequency of its transmitted signal with respect to a fixed location on Earth. They understood that if this process could be reversed—i.e., if the orbits of several satellites were precisely known—it would be possible to identify one's location on Earth by using information from those satellites.

In addition to recognizing the value of space systems in warfare, national leaders in the United States and the Soviet Union realized early on that the ability to gather information about surface-based activities such as weapons development and deployment and troop movements would assist them in planning their own national security activities. As a result, both countries deployed a variety of space systems for collecting intelligence. They include reconnaissance satellites that provide high-resolution images of Earth's surface in close to real time for use in identifying threatening activities, planning military operations, and monitoring arms-control agreements. Other satellites collect electronic signals such as telephone, radio, and Internet messages and other emissions, which can be used to determine the type of activities that are taking place in a particular location. Most national-security space activity is carried out in a highly secret manner. As the value to national security of such satellite systems has become evident, other countries, such as France, China, India, and Israel, have developed similar capabilities, and still others have begun planning their own systems.

Although some early space experiments explored the use of large orbiting satellites as passive reflectors of signals from point to point on Earth, most work in the late 1950s and early '60s focused on the technology by which a signal sent from the ground would be received by satellite, electronically processed, and relayed to another ground station. American Telephone and Telegraph, recognizing the commercial potential of satellite communications (satellite communication), in 1962 paid NASA to launch its first Telstar satellite. Because that satellite, which operated in a fairly low orbit, was in range of any one receiving antenna for only a few minutes, a large network of such satellites would have been necessary for an operational system. Engineers from the American firm Hughes Aircraft, led by Harold Rosen, developed a design for a satellite that would operate in geostationary orbit. Aided by research support from NASA, the first successful geostationary satellite, Syncom 2, was launched in 1963; it demonstrated the feasibility of the Hughes concept prior to commercial use.

Because many applications of remote sensing have a public-good character, a commercial remote-sensing industry has been slow to develop. In addition, the secrecy surrounding intelligence-gathering satellites during the Cold War era set stringent limits on the capabilities that could be offered on a commercial basis. The United States launched the first remote-sensing satellite, NASA's Landsat 1 (originally called Earth Resources Technology Satellite), in 1972. The goals of the Landsat program, which by 1999 had included six successful satellites, were to demonstrate the value of multispectral observation and to prepare the system for transfer to private operators. Despite two decades of attempts at such a transfer, Landsat remained a U.S. government program at the start of the 21st century. In 1986 France launched the first of its SPOT remote-sensing satellites and created a marketing organization, Spot Image, to promote use of its imagery. Both Landsat's and SPOT's multispectral images offered a moderate ground resolution of 10–30 metres (about 33–100 feet). Japan and India also launched multispectral remote-sensing satellites.

Resources available on the Moon and other bodies of the solar system represent additional potential objectives for commercial development. For example, over billions of years the solar wind has deposited large amounts of the isotope helium-3 in the soil of the lunar surface. Scientists and engineers have suggested that helium-3 could be extracted and transported to Earth, where it is rare, for use in nuclear fusion reactors. In addition, there is evidence to suggest that the Moon's polar regions contain ice, which could supply a manned lunar outpost with drinking water, breathable oxygen, and hydrogen for spacecraft fuel. Significant quantities of potentially valuable resources such as water, carbon, and nitrogen may also exist on some asteroids (asteroid), and space mining of those resources has been proposed.

Broad coverage of space activities can be found in Fernand Verger, Isabelle Sourbès-Verger, and Raymond Ghirardi, The Cambridge Encyclopedia of Space: Missions, Applications, and Exploration (2003). An overall history of space exploration is William E. Burrows, This New Ocean: The Story of the First Space Age (1998). Walter A. McDougall, The Heavens and the Earth: A Political History of the Space Age (1985, reissued 1997), traces the U.S.-Soviet rivalry that led to the space race and comments on its impact on the two countries' societies. Earlier historical discussions include Willy Ley, Rockets, Missiles, and Men in Space, newly rev. and expanded ed. (1968); and Wernher von Braun, Frederick I. Ordway III, and David Dooling, Space Travel: A History, 4th ed. (1985). Frank H. Winter, Rockets into Space (1990), provides an account of the development of rocketry.Speculative discussions of the promises of space exploration include Arthur C. Clarke (compiler and ed.), The Coming of the Space Age: Famous Accounts of Man's Probing of the Universe (1967, reissued 1970); Harry L. Shipman, Humans in Space: 21st Century Frontiers (1989); Carl Sagan, Pale Blue Dot: A Vision of the Human Future in Space (1994, reissued 1997); and Robert Zubrin and Richard Wagner, The Case for Mars: The Plan to Settle the Red Planet and Why We Must (1996). Carl Sagan, Cosmos (1980, reissued 1995), based on Sagan's television series of the same name (1980), discusses the universe and the place of life within it.

Many early American astronauts have written of their experiences. The best of these works is Michael Collins, Carrying the Fire: An Astronaut's Journeys (1974, reissued 2001). An account of the Apollo program that is focused on astronauts is Andrew Chaikin, A Man on the Moon: The Voyages of the Apollo Astronauts (1994, reissued in 3 vol., 1999); this book served as the basis for a video series, From the Earth to the Moon (1998), produced by Tom Hanks. A failed Apollo mission is the subject of the theatrical film Apollo 13 (1995). The best account of Apollo from the perspective of its managers and engineers is Charles Murray and Catherine Bly Cox, Apollo: The Race to the Moon (1989). A noted author offers his impressions of Apollo in Norman Mailer, Of a Fire on the Moon (1970, reissued 1985; also published as A Fire on the Moon, 1970). Tom Wolfe, The Right Stuff, new ed. (1983, reissued 1997), provides an account of the early days of U.S. human spaceflight; the book was turned into a 1983 motion picture of the same name. John M. Logsdon, The Decision to Go to the Moon: Project Apollo and the National Interest (1970, reissued 1976), traces the political underpinnings of the Apollo program. An extensive account of the Soviet space program during the race to the Moon is Asif A. Siddiqi, Challenge to Apollo: The Soviet Union and the Space Race, 1945–1974 (2000).The origins of U.S. post-Apollo spaceflight programs are discussed in T.A. Heppenheimer, The Space Shuttle Decision: NASA's Search for a Reusable Space Vehicle (1999); and Howard E. McCurdy, The Space Station Decision: Incremental Politics and Technological Choice (1990). Events leading to the 1986 Challenger accident are detailed in Joseph J. Trento, Prescription for Disaster (1987). A selective view of U.S.-Russian cooperation in human spaceflight is found in Bryan Burrough, Dragonfly: NASA and the Crisis Aboard Mir (1998, reissued 2000).Available in addition to the works cited above are published studies, sponsored by the NASA History Program, of almost every one of the agency's space programs. Original documents tracing the history of the U.S. space program are reprinted in John M. Logsdon et al. (eds.), Exploring the Unknown: Selected Documents in the History of the U.S. Civil Space Program (1995– ).A comprehensive discussion of European space activities up to 1987 is provided in J. Krige, A. Russo, and L. Sebesta, A History of the European Space Agency 1958–1987, 2 vol. (2000). Roger M. Bonnet and Vittorio Manno, International Cooperation in Space: The Example of the European Space Agency (1994), elaborates on international space activities from a European perspective. Other national space efforts are described in Brian Harvey, The Chinese Space Programme: From Conception to Future Capabilities (1998), The Japanese and Indian Space Programmes: Two Roads into Space (2000), and Russia in Space: The Failed Frontier? (2001).

Discussions of various space science efforts include Homer E. Newell, Beyond the Atmosphere: Early Years of Space Science (1980); Bruce Murray, Journey into Space: The First Three Decades of Space Exploration (1989); and William E. Burrows, Exploring Space: Voyages in the Solar System and Beyond (1990); the last two works deal with U.S. missions to explore the solar system. Also pertinent is Robert W. Smith, The Space Telescope: A Study of NASA, Science, Technology, and Politics (1989, reissued 1993).The origins of reconnaissance satellite programs are covered in Dwayne A. Day, John M. Logsdon, and Brian Latell (eds.), Eye in the Sky: The Story of the Corona Spy Satellites (1998). Subsequent spy satellite programs are discussed in Jeffrey T. Richelson, America's Secret Eyes in Space: The U.S. Keyhole Spy Satellite Program (1990); and William E. Burrows, Deep Black: Space Espionage and National Security (1986, reissued 1988). Early debates over the military use of space are described in Paul B. Stares, The Militarization of Space: U.S. Policy, 1945–1984 (1985); and more recent debates on this issue are summarized in Peter L. Hays et al., Spacepower for a New Millennium: Space and U.S. National Security (2000).Heather E. Hudson, Communication Satellites: Their Development and Impact (1990), gives a synopsis of progress in communications satellites. Controversies surrounding the development of Earth observation satellites are followed in Pamela E. Mack, Viewing the Earth: The Social Construction of the Landsat Satellite System (1990).

Periodicals providing extensive coverage of space issues include the weekly publications Aviation Week & Space Technology and Space News. Jane's Space Directory (annual) also provides up-to-date information on various space activities.John M. Logsdon