the scientific investigation of physical conditions in space and in celestial bodies (including stars, and planets and their moons) by means of spacecraft and space probes. In a large sense, space exploration, or astronautics, is interdisciplinary in that it draws upon the findings of such fields as physics, astronomy, mathematics, chemistry, biology, medicine, electronics, and meteorology.

Automated space probes and human spaceflight have provided a wealth of scientific data on the nature and origin of the solar system and the universe (see Cosmology). Earth-orbiting satellites have improved global communications, weather forecasting, navigational aids, and reconnaissance of the earth's surface for the location of mineral resources and for military purposes.

The space age and practical astronautics commenced with the launching of Sputnik 1 by the Soviet Union in October 1957 and of Explorer 1 by the U.S. in January 1958. In October 1958 the National Aeronautics and Space Administration (NASA) was created in the U.S. During the following decades, thousands of spacecraft of all varieties were launched, mostly in earth orbit. The overwhelming majority of these were launched by the Soviet Union (or, after the Soviet Union's dissolution, Russia) and by the U.S., but other countries also carried out successful launches, as did international organizations, notably the European Space Agency (ESA), and private companies. By 2005, well over 400 individuals had flown in space, and 12 men had walked on the moon's surface and returned to earth. Today, many thousands of objects—mostly spent, upper stages of space-launch vehicles and inert spacecraft—as well as countless smaller pieces of debris are circling the earth.

THE PHYSICS OF SPACE

The boundary between the atmosphere of the earth and space is diffuse rather than sharp. Because the density of air diminishes gradually with increasing altitude, the air in the upper atmosphere is so thin that it merges almost imperceptibly with space. The barometric pressure (see Barometer), which is a measure of atmospheric density, is 760 torrs at sea level. (One torr is defined as the pressure caused by the weight of a column of mercury 1 mm/0.039 in. high at sea level.) At 30 km (19 mi) above sea level, the barometric pressure is 9.5 torrs; at 60 km (37 mi), 0.21 torr; at 90 km (56 mi), 0.0019 torr. Even at an altitude of 200 km (124 mi), sufficient residual atmosphere remains to slow down artificial satellites by aerodynamic drag; thus, long-duration satellites must have a higher orbital altitude.

Radiation in Space.

By ordinary standards, space is a vacuum. Space, however, does contain very minute quantities of gases such as hydrogen and small quantities of meteorites and meteoric dust (see Meteor; Meteorite). X rays, ultraviolet radiation, visible light, and infrared radiation from the sun all traverse space. Cosmic Rays, consisting mainly of protons, alpha particles, and heavy nuclei, are also present. See also Astronomy.

Gravitation.

The law of universal gravitation states that every particle of matter in the universe attracts every other particle with a force directly proportional to the products of their masses and inversely proportional to the square of the distance between them. Consequently, the gravitational pull exerted by the earth upon all other bodies (including spacecraft) diminishes with distance from the earth. The gravitational field, however, extends to an infinite distance; gravity does not cease to act at any altitude. A spacecraft is said to be weightless when it is in orbit around the earth (or around any other celestial body) because the centrifugal effect (which acts away from the center) is then equal and opposite to the force of gravity. Under these conditions, objects in a spacecraft seem to float in space. In the same way, the moon does not fall toward the earth because of the centrifugal effect that balances the force of gravity.

Aerodynamic forces on the lifting surfaces (for example, the wings) of an aircraft keep it aloft against the force of gravity, but a space vehicle cannot stay aloft in this way because of the absence of air in space. The spacecraft, therefore, must orbit if it is to remain in space. Aircraft flying in the earth's atmosphere can use propellers and winged surfaces for propulsion and maneuvering, but spacecraft cannot do so because of the lack of air. A space vehicle must rely primarily on the reaction of rockets or other technologies for propulsion and maneuvers, based on Newton's laws of motion (see Mechanics). When a spacecraft fires a rocket blast in one direction, reaction against the rocket exhaust imparts momentum to the spacecraft in the opposite direction.

Humans in Space.

Space is a hostile environment for humans in a number of ways. It contains neither air nor oxygen, so human beings are unable to breathe. The vacuum of space can destroy an unprotected human body in a few seconds by explosive decompression. Temperatures in space in the shadow of a planet approach absolute zero; on the other hand, temperatures can become fatally high under direct solar radiation. Energetic solar and cosmic radiations in space also may be fatal to an unshielded person who is not protected by the atmosphere of the earth. These environmental conditions may also affect the instruments and devices used in spacecraft, so the design and construction of these materials are dictated by the space environment. Experiments in weightlessness for long periods of time have been studied intensively to discover what adverse effects this condition will have on humans in space (see Aerospace Medicine).

Humans are protected against the space environment in several ways. They are enclosed inside a hermetically sealed cabin or space suit, with a supply of pressurized air or oxygen to approximate conditions on earth. Air conditioning controls the temperature and humidity inside the cabin or space suit. Absorbing and reflecting surfaces on the outside of the spacecraft regulate the amount of heat radiation affecting the craft. Furthermore, space journeys are carefully planned to avoid the intense radiation belts around the earth. On long interplanetary voyages of the future, heavy shielding may be necessary to protect against solar radiation storms, or crews might be sheltered in a central position within the spacecraft with supplies and equipment to surround and shield them. For lengthy space journeys, or for prolonged stays in an earth-orbiting satellite, the effects of weightlessness can be reduced by spinning the craft so that the centrifugal effect provides artificial gravity.

HISTORY

People dreamed of spaceflight for millennia before it became reality. Evidence of the dream exists in myth and fiction as far back as Babylonian texts of 4000 bc. The ancient Greek myths of Daedalus and Icarus also reflect the universal desire to fly. As early as the 2d century ad the Greek satirist Lucian wrote about an imaginary voyage to the moon. In the early 17th century the German astronomer Johannes Kepler wrote Somnium (Sleep), which might be called a scientific satire of a journey to the moon. A story by the British prelate Francis Godwin (1562–1633) about a voyage to the moon was published in 1638, and a couple of decades later the French writer Cyrano de Bergerac wrote fantasies describing trips to the moon and the sun. The French writer and philosopher Voltaire, in Micromégas (1752), told of the travels of certain inhabitants of Sirius and Saturn. In 1865 the French author Jules Verne depicted space travel in his popular novel From the Earth to the Moon. The dream of flight into space continued unabated into the 20th century, notably in the works of the British writer H. G. Wells, who published The War of the Worlds in 1898 and The First Men in the Moon in 1901. Fantasies of spaceflight continue to be nourished by science fiction.

Early Developments.

During the centuries when space travel was only a fantasy, researchers in the sciences of astronomy, chemistry, mathematics, meteorology, and physics developed an understanding of the solar system, the stellar universe, the atmosphere of the earth, and the probable environment in space. In the 7th and 6th centuries bc, the Greek philosophers Thales and Pythagoras noted that the earth is a sphere; in the 3d century bc the astronomer Aristarchus of Samos asserted that the earth moved around the sun. Hipparchus, another Greek, prepared information about stars and the motions of the moon in the 2d century bc. In the 2d century ad Ptolemy of Alexandria placed the earth at the center of the solar system in the Ptolemaic system.

Scientific Discoveries.

Not until some 1400 years later did the Polish astronomer Nicolaus Copernicus systematically explain that the planets, including the earth, revolve about the sun (see Copernican System). Later in the 16th century the observations of the Danish astronomer Tycho Brahe greatly influenced the laws of planetary motion set forth by Kepler (see Orbit). Galileo, Edmund Halley, and William Herschel were other astronomers of the period who made contributions pertinent to astronautics.

Physicists and mathematicians also helped to lay the foundations of astronautics. In 1654 the German physicist Otto von Guericke proved that a vacuum could be maintained, refuting the old theory that nature “abhors” a vacuum. In the late 17th century Isaac Newton formulated the laws of universal gravitation and motion. Newton's laws of motion established the basic principles governing the propulsion and orbital motion of modern spacecraft.

Despite the scientific foundations laid in earlier ages, however, space travel did not become possible until the advances of the 20th century provided the actual means of rocket propulsion, guidance, and control for space vehicles.

Rocket Propulsion.

The techniques of rocket propulsion originated long ago. Ancient rockets used gunpowder as fuel, very much as in fireworks today. In ad 1232 in China the city of Kaifeng was reportedly defended against the Mongols by the use of rockets. From the Renaissance onward, references were made to the proposed or actual military use of rockets in European warfare. As early as 1804 the British army established a rocket corps equipped with rockets that had a range of about 1830 m (about 6000 ft).

In the U.S. the foremost pioneer in rocket propulsion was Robert Goddard, a professor of physics at Clark College (now Clark University). He began experimenting with liquid fuels for rocketry in the early 1920s. He launched the first successful liquid-propelled rocket on March 16, 1926. During the same general period, studies on spaceships and rocket propulsion were being conducted in several parts of the world. About 1890 Herman Ganswindt (1856–1934), a German law student, conceived of a solid-propellant spaceship that demonstrated a marked awareness of the stability problem. Konstantin Tsiolkovsky, a Russian schoolteacher, published in 1903 A Rocket into Cosmic Space, which proposed the use of liquid propellants for spaceships. In 1923 a German mathematician and physicist, Hermann Oberth (1894–1989), published his prophetic work Die Rakete zu den Planetenräumen (The Rocket into Interplanetary Space). The book was supplemented by Walter Hohmann (1880–1941), a German architect, who published in 1925 Die Erreichbarkeit der Himmelskörper (The Possibility of Reaching Celestial Bodies), which contained the first detailed calculation of interplanetary orbits.

World War II provided the impetus and motivation for the development of long-range suborbital rockets. The U.S., the Soviet Union, Great Britain, and Germany simultaneously developed rockets for military purposes. The most successful were the Germans, who developed the V-2 (a liquid-propellant rocket used in the bombardment of London) at Peenemünde, a village near the Baltic coast. At the close of the war, the U.S. Army brought back a number of the V-2s, which were then used in the U.S. for experimental research in vertical flights. Some German engineers went to the USSR after the war, but the leading rocket experts went to the U.S., including Walter Dornberger (1895–1980) and Wernher von Braun. See Guided Missiles.

SPACECRAFT

Spacecraft that do not have to carry humans may take a great variety of sizes, from a few centimeters to several meters in diameter, and many shapes, depending on the purposes for which they are designed. Automated spacecraft rely on two-way radio communication systems, both to relay information back to earth and to receive guidance from the control center.

A space vehicle designed for humans must provide air, food and water, navigation and guidance equipment, seating and sleeping accommodations, and communication equipment for sending and receiving information from the control center on earth. Another distinctive feature of human spaceflights is the heat shield that protects the vehicle as it reenters the atmosphere (see below).

The rocket engines that launch and propel spacecraft are of two main types: solid-propellant rockets, which use chemicals that burn in a fashion similar to gunpowder, and liquid-propellant rockets, which use liquid fuels and oxidizers carried in separate tanks (see Rocket). Most of the rockets that have launched American spacecraft have consisted of several separate rocket stages; each stage is separately powered with its own fuel. After the fuel in each stage is consumed, the empty stage drops away from the spacecraft. Efforts to develop a single-stage-to-orbit reusable launch vehicle have included the X-33 project initiated by NASA in 1996; funding for the X-33 was halted in 2001, after NASA had invested more than $900 million on the program. As of 2004, two U.S. rocket planes had ventured into space, defined by the Fédération Aéronautique Internationale as an altitude exceeding 100 km (62 mi); these reusable craft were launched from an airplane and did not enter orbit. The experimental X-15 made two such suborbital flights in 1963, and SpaceShipOne, the first privately developed manned craft, built by the firm Scaled Composites, performed three flights in 2004.

Because the technology to build launch vehicles is closely akin to that for long-range ballistic missiles, the U.S. and the USSR were the only two countries that had the capability to launch satellites from 1957 to 1965. In subsequent years France, Japan, China, Britain, India, and Israel launched earth satellites, and in May 1984 the ESA began its own launch program from a space center at Kourou in French Guiana. In 1999 the international consortium Sea Launch began placing commercial satellites into orbit using a mobile launch platform near the equator in the Pacific Ocean. As of the close of the 1990s, the U.S. and Russia were the only nations with launch vehicles capable of placing in orbit payloads (passengers, supplies, equipment, or cargo essential to the flight) of many tons—the prerequisite for human spaceflight. In October 2003, China became the third nation to launch humans into space; after the spacecraft, Shenzhou 5, made 14 orbits of earth, the single astronaut aboard returned to earth in the reentry module.

A space vehicle is launched from a specially constructed launchpad, where the space vehicle and the rocket that propels it are set up and carefully inspected before launching. The operation is supervised by engineers and technicians in the nearby control center. When all preparations are complete, the rocket engines are fired and the rocket and spacecraft lift off.

Reentry is the name applied to the problem of slowing down a returning spacecraft so that it lands on earth without being destroyed by aerodynamic heating. The U.S. Mercury, Gemini, and Apollo programs overcame the problem of reentry by protecting the leading surface of the returning capsule with a specially developed heat shield, made of metals, plastics, and ceramic materials that melt and vaporize during reentry, thereby carrying off or dissipating the heat without damage to the capsule or its astronaut occupants. The heat shield developed to protect the space shuttle during reentry consists of a covering of ceramic tiles individually cemented to the shuttle's hull. Prior to the development of the space shuttle, which lands on a runway (see Space Shuttle below), all American spacecraft designed to carry humans used the ocean to cushion the impact of landing; the astronauts and the capsules were retrieved quickly by helicopter and taken aboard waiting naval vessels. In the space programs of the Soviet Union and the Commonwealth of Independent States (including Russia), cosmonauts have landed on solid ground in various sites in Siberia.

The orbit of a spacecraft around the earth may be in the shape of a circle or an ellipse. A satellite in a circular orbit travels at a constant speed. The higher the altitude, however, the lower the speed relative to the surface of the earth. Maintaining an altitude of 35,800 km (22,300 mi) over the equator, a satellite is geostationary. It moves in geosynchronous orbit, at exactly the same speed as the earth, so it remains in a fixed position over some particular spot on the equator. Most communications satellites are placed in such orbits.

In an elliptical orbit, the speed varies and is greatest at perigee (minimum altitude) and least at apogee (maximum altitude). Elliptical orbits can lie in any plane that passes through the earth's center. A polar orbit lies in a plane passing through the North and South poles; in other words, it passes through the axis of rotation of the earth. An equatorial orbit is one that lies in a plane passing through the equator. The angle between the orbital plane and the equator plane is called the inclination of the orbit.

The earth rotates once every 24 hr under a satellite in a polar orbit. A polar-orbit weather satellite (see (PUT XREF HERE) Satellite, Artificial), carrying television and infrared cameras, can thus observe meteorological conditions over the entire globe from pole to pole in a single day. An orbit at another inclination covers a smaller portion of the earth, omitting areas around the poles.

As long as the orbit of an object keeps it in the vacuum of space, the object will continue to orbit without propulsive power because no frictional force slows it down. If part or all of the orbit passes through the atmosphere of the earth, however, the body is slowed by aerodynamic friction with the air. This causes the orbit to decay gradually to lower and lower altitudes until the object has fully reentered the atmosphere and burns up, like a meteor.

ARTIFICIAL SATELLITES AND SPACE PROBES

The long history of myths, dreams, fiction, science, and technology surrounding space travel culminated in the dramatic launching of the first artificial orbiting earth satellite, Sputnik 1, by the USSR on Oct. 4, 1957. iskustvennyi sputnik zemli, meaning “artificial earth satellite,” is the Russian term for an artificial satellite orbiting the earth. In the U.S. the name Sputnik was used.

Early Artificial Satellites.

Sputnik 1 was an aluminum sphere, 58 cm (23 in) in diameter, weighing 83 kg (184 lb). It orbited the earth in 96.2 min. The elliptic orbit of the satellite carried it to an apogee of 946 km (588 mi) and a perigee of 227 km (141 mi). The sphere contained instruments which, for 21 days, radioed data concerning cosmic rays, meteoroids, and the density and temperature of the upper atmosphere. At the end of 57 days the satellite reentered the atmosphere of the earth and was destroyed by aerodynamic frictional heat.

The second artificial earth satellite was also a Soviet space vehicle, called Sputnik 2. It was sent aloft on Nov. 3, 1957, with a dog named Laika aboard, and it relayed the first biomedical measurements in space. Sputnik 2 reentered the atmosphere of the earth and was destroyed after 162 days aloft.

While Sputnik 2 was still in orbit, the U.S. successfully launched its first earth satellite, Explorer 1, from Cape Canaveral (named Cape Kennedy 1963–73), Fla., on Jan. 31, 1958. The 14-kg (31-lb) cylindrical spacecraft, 15 cm (6 in) in diameter and 203 cm (80 in) long, transmitted measurements of cosmic rays and micrometeorites for 112 days and gave the first satellite-derived data leading to the discovery of the Van Allen radiation belts.

On March 17, 1958, the U.S. launched its second satellite, Vanguard 2; a precise study of variations of its orbit showed that the earth is slightly pear-shaped. Using solar power, the satellite transmitted signals for more than six years. Vanguard 2 was followed by the American satellite Explorer 3, launched on March 26, 1958, and by the Soviet satellite Sputnik 3, launched on May 15. The 1327-kg (2925-lb) Soviet spacecraft measured solar radiation, cosmic rays, magnetic fields, and other space phenomena until the craft's orbit decayed in April 1960.

Lunar Probes.

As the closest neighbor of the earth, the moon has been the objective of many space missions. In 1958 the first attempts by the U.S. and the USSR at lunar probes failed. The Soviet Luna 1, launched Jan. 2, 1959, flew by the moon at a distance of 5998 km (mi); it was the first spacecraft to achieve escape velocity and the first to reach the vicinity of the moon. Luna 2, launched Sept. 12, 1959, hit the moon 36 hr later. Since that date many moon shots have been made by both countries, with mixed results. The first photographs of the far side of the moon were taken by Luna 3, which was launched by the USSR on Oct. 4, 1959. One of the most dramatically successful moon shots was the mission accomplished by Ranger 7, launched by the U.S. on July 28, 1964. Just before hitting the side of the moon that faces the earth, it transmitted 4308 television pictures of the lunar surface beginning at an altitude of about 2110 km (1310 mi) and ending, with a final partial-scan picture, less than half a second before impact. The images gave earth-bound humans their first close-up view of the moon.

On Jan. 31, 1966, the USSR launched Luna 9, which made the first soft landing on the moon; that is, it landed without being destroyed. The craft also sent back to earth the first photographs taken from the surface of another planetary body. The U.S. followed with Surveyor 1 on May 30, which also made a soft landing on the lunar surface. It sent back to earth 11,150 close-up photographs of the moon.

Aside from the scientific information that was gathered, much of the interest of the lunar missions centered on the American program to land an astronaut on the moon. To this end a number of further automated moon flights were undertaken, among which were two soft landings made by Surveyor 3 and 5 in 1967. Both craft, after taking about two days for their journeys, sent back to earth a large number of television pictures of the lunar surface. Surveyor 3 picked up samples of lunar soil and examined them by television camera. Surveyor 5 chemically analyzed the lunar surface, using an alpha-particle scattering technique; this was the first on-site analysis of an extraterrestrial body.

Another spacecraft that contributed to future lunar landings was the Lunar Orbiter. In 1966 and 1967 five Lunar Orbiters circled the moon, relaying thousands of photographs to earth. From these photographs, landing sites were selected for the Apollo moon-landing program.

Two other automated lunar projects by the USSR are noteworthy. The Luna 16 spacecraft, launched Sept. 12, 1970, landed on the moon and placed about 101 gr (3.6 oz) of lunar soil in a sealed container that was then launched from the moon and recovered in the USSR. Luna 20 in 1972 and Luna 24 in 1976 also brought back to earth small amounts of lunar surface material. Luna 17, launched Nov. 10, 1970, softlanded an automated lunar-roving vehicle, Lunokhod 1, equipped with a television camera and solar batteries. During ten lunar days the vehicle, controlled from the earth, traveled 10.5 km (6.5 mi) on the moon, relaying television pictures and scientific data. Luna 21 in 1973 repeated this performance, placing Lunokhod 2 on the moon.

Luna 24 in 1976 was the last lunar mission until January 1990, when Japan became the third country to become involved in exploration of the moon. Its space agency launched the earth-orbiting satellite Hiten, which made several swings by the moon, where it was finally made to crash in 1993. Hagoromo, a small orbiter, was placed in lunar orbit by Hiten in March 1990, but its transmitter failed to function. Meanwhile, the U.S. Jupiter-bound spacecraft Galileo observed the moon during flybys in 1990 and 1992. The U.S. spacecraft Lunar Prospector, launched in January 1998 on a mission to orbit the moon for at least a year, was designed to map the composition of the moon's surface. NASA's first lunar mission in 25 years, the probe detected evidence suggestive of water ice at the lunar poles, lending support to an indication by Clementine, a joint NASA/U.S. Defense Department probe, in 1994. In an attempt to generate evidence confirming the presence of water, Lunar Prospector was made to impact near the moon's south pole in 1999, but the crash failed to produce detectable signs of water. The ESA spacecraft SMART-1 (“Small Missions for Advanced Research in Technology–1”) went into orbit around the moon in November 2004 on a mission scheduled to end with impact on the lunar surface in the latter part of 2006. The craft performed imaging and X-ray and infrared studies of the moon. Unmanned missions to the moon were expected to be carried out in subsequent years by such nations as China, India, Japan, and the U.S., which planned to launch Lunar Reconnaissance Orbiter in 2008.

Scientific Satellites.

As space-launch vehicles (rocket boosters) and scientific measuring devices became more reliable, numerous types of satellites were developed to enable scientists to obtain data and make accurate studies of the sun, other stars, the earth, and space itself. While powerful ground-based telescopes can yield enormous amounts of information, in some segments of the electromagnetic spectrum the enveloping atmosphere of the earth prevents data from being obtained at the earth's surface except in a limited way through the use of high-altitude rockets, planes, or balloons (see Balloon).

The U.S. and other countries have deployed many satellites that make scientific observations from space. Between 1962 and 1978, for example, a series of eight Orbiting Solar Observatories (OSO) launched by the U.S. studied the sun's ultraviolet, X-ray, and gamma radiation. Perhaps the first full-fledged orbiting astronomical observatory was the Soviet satellite Cosmos 215, which was launched in April 1968 and operated for several weeks; it carried instruments for visible, ultraviolet, and X-ray wavelengths, and was largely devoted to solar studies. The two successful U.S. Orbiting Astronomical Observatories (OAO)—OAO 2, launched in December 1968 and operational for more than four years, and OAO 3 (better known as Copernicus), launched in August 1972 and operational through late 1980—observed stellar radiation. Six Orbiting Geophysical Observatories (OGO), launched by NASA between 1964 and 1969, studied the earth's magnetosphere and other aspects of the relationships between the sun, the earth, and their space environment. The Infrared Astronomical Satellite (IRAS), an Anglo-Dutch-American project that operated in 1983, probed the hidden reaches of our galaxy.

Even richer scientific rewards were returned from a series of major space-based observatories launched by NASA that it dubbed the Great Observatory Program. The series began with the Hubble Space Telescope; cosponsored by the ESA and launched by the space shuttle Discovery in 1990, it makes observations in the visible, near-infrared, and ultra-violet, and X-ray ranges of the spectrum. Other spacecraft in NASA's program were the Compton Gamma Ray Observatory (in orbit 1991–2000); the Chandra X-ray Observatory (launched 1999); and the Spitzer Space Telescope (launched 2003), devoted to infrared observation.

In addition to its role with the Hubble, the ESA helps operate the Solar and Heliospheric Observatory (SOHO), which was launched by NASA in December 1995 and travels in an orbit around the sun in step with the earth, The ESA has also placed in orbit such space telescopes as Hipparcos (operational 1989–93), devoted to astrometry; the Infrared Space Observatory (operational 1995–98); and INTEGRAL (“INTErnational Gamma-Ray Astrophysics Laboratory”; launched 2002). Other countries with orbiting space telescopes include Canada, whose “microsatellite” MOST (“Microvariability and Oscillations of STars”) was launched in 2003, and Japan, whose infrared telescope Akari entered orbit in 2006.

Applications Satellites.

Automatic communications, earth-survey, and navigation satellites are all classified as applications satellites. Earth-survey satellites observe the earth and transmit, or return to deliver, photographs for a variety of purposes. A weather satellite provides daily transmissions of temperatures and cloud patterns. Early examples were the three Synchronous Meteorological Satellites (SMS) deployed by the U.S. in the mid-1970s. The first two played developmental roles, and the third became the first in the continuing Geostationary Operational Environmental Satellite (GOES) series of weather satellites. From a geosynchronous orbit, weather satellites can readily image worldwide weather patterns around the clock.

Since 1972 the U.S. Landsats (originally called Earth Resources Technology Satellites)), have observed the earth through multispectral optical filters, transmitting the data to ground stations. Processed into color photographs, these pictures reveal data of great range and value. Information concerning specific soil characteristics, water and ice quantities, coastal-water pollution, salinity, and insect blights of crops and forests is obtained. Even forest fires can be detected from earth orbit. Study of folds and fractures in the earth's crust helps geologists to identify deposits of oil and minerals. SPOT (Système Probatoire d'Observation de la Terre), a French satellite system first launched in 1986, transmits images that show the earth in greater detail than Landsats can. See also Remote Sensing.

Earth observation satellites are used by the U.S. and other countries to obtain photographs of military value, such as detection of nuclear explosions in the atmosphere and in space, ballistic-missile launch sites, and ship and troop movements. In the 1980s controversy was aroused by a U.S. proposal to develop a satellite antiballistic missile defense system making use of laser technology (see Strategic Defense Initiative).

Navigation satellites provide an observation point orbiting the earth that, when observed by ships and submarines, can fix the vessel's position within a few yards. The U.S. Navy began deploying an early system of navigational satellites known as Transit in the early 1960s. The system was superseded in 1996 by the complex Global Positioning System (GPS) of Navstar satellites, which also provide GPS services for civilian use.

Planetary Studies.

Beyond the moon, spacecraft have landed on Mars and Venus, as well as two asteroid and Saturn's moon Titan. In addition, they have flown by Mercury, Jupiter, Saturn, Uranus, and Neptune. They have also examined comets; in 2005 an “impactor” released by a NASA spacecraft crashed into Comet Tempel 1. Other research carried out by spacecraft includes solar studies that involve the collecting of solar wind particles (see Sun). In the U.S. space program, radio communications with interplanetary spacecraft are handled through NASA's Deep Space Network.

Mars.

The USSR launched Mars 2 and 3 in May 1971; both went into orbit around Mars late that year, and successfully sent images and data back to earth. Each carried a lander, the Mars 2 descent module crashed on Mars in late November, becoming the first artificial object to reach the planet's surface; the Mars 3 lander made a successful soft landing in December, but communication with it was quickly lost, before completion of transmission of its first image. In July and August 1973 the USSR launched Mars 4, 5, 6, and 7, but various technical malfunctions plagued all of the missions, which returned only small amounts of data and images. In 1988 the USSR sent two probes, Phobos 1 and 2, to land on the Martian moon Phobos; the first was lost through human error, and the second dropped out of radio contact after relaying back some data and photographs.

In the U.S. program, Mariner 9 was launched in May 1971, orbited Mars from November 1971 to October 1972, and transmitted enough photographs for a nearly complete map of the planet. In August and September 1975 Viking 1 and 2 began an 11-month journey to Mars. Each spacecraft carried a lander equipped with life-detecting and chemical laboratories, two color television cameras, weather and seismographic instruments, and a 3-m (10-ft) retractable claw designed to be manipulated from the earth. Both landers functioned well for several years.

In September 1992, NASA launched Mars Observer, the first U.S. mission to Mars since the Viking program. Mission controllers lost contact with the spacecraft in August 1993 as it prepared to enter Mars orbit. Both the U.S. and Russia launched Mars probes in late 1996. Launched in November, the Russian spacecraft Mars 96, consisting of an orbiter, two landers, and two surface penetrators, spun out of control and fell apart over the Pacific Ocean when a fourth-stage engine failed.

The U.S. inaugurated a planned 10-year Mars exploration program with the successful launching in November 1996 of Mars Global Surveyor, which entered planetary orbit in September 1997; and of Mars Pathfinder in December 1996, which descended to the surface in July 1997 (the first spacecraft to reach the surface of Mars without orbiting prior to landing) and released a robot rover to explore the Martian terrain. Pathfinder and its robotic explorer, a small vehicle named Sojourner that used laser technology to navigate, gathered data that provided unprecedented details on the climate, atmosphere, and geology of Mars, including three-dimensional panoramic photographs. NASA released images of Mars's rocky terrain, and announced the results of the first chemical analysis of a rock carried out on Mars by Sojourner. The Mars program suffered a major setback in 1999 when two spacecraft, the Mars Climate Orbiter and the Mars Polar Lander, malfunctioned and were lost as they approached the planet. The project was restructured, and the Mars Odyssey, launched in April 2001, was successfully inserted into orbit around the planet in October. Using a gamma ray spectrometer, the Odyssey reported extensive quantities of water ice just below the Martian surface.

In June and July 2003, NASA launched two solar-powered rovers—Spirit and Opportunity—each much larger than Sojourner and capable of navigating by means of computer software, stereoscopic vision, and six steerable wheels. The twin rovers were deployed on opposite sides of the planet in January 2004; the Opportunity rover found evidence that a region near its landing site had formerly been covered by a shallow, salty sea. The Mars Reconnaissance Orbiter was launched by NASA in August 2005; its mission was to conduct detailed, low-orbit observation of the planet's atmospheric, surface, and subsurface features and to assist future Mars landers

Besides Russia and the U.S., missions to Mars have also been launched by Japan and the ESA. The orbiter Nozomi was Japan's first planetary space probe; it was launched in July 1998, but the spacecraft, plagued by malfunctions, failed to keep to its planned trajectory and was abandoned in December 2003. The ESA's Mars Express, launched in June 2003, arrived at Mars before the end of the year and began orbiting the planet in order to carry out an observational program. It carried a British-built lander, Beagle 2, but communication with the module was lost after it was deployed.

Venus.

The first spacecraft to fly by Venus was the Soviet Union's Venera 1, in May 1961; communication with the craft, however, had been lost several days after its February launch.

The first flyby by a U.S. spacecraft was in 1962 by Mariner 2. It was followed by Mariner 5 in 1967 and Mariner 10 in 1974. The USSR's Venera 3, launched in November 1965, crash-landed on Venus in March 1966, becoming the first spacecraft to impact on the surface of another planet, although it lost contact with earth before it could transmit any planetary data. The Soviet program to penetrate the dense, cloud-covered atmosphere of Venus, however, in general met with success. Venera 4 (launched in 1967, 5 (1969), and 6 (1969), returned data from within the atmosphere. Venera 7, launched in August 1970, deployed a lander that survived long enough on the surface to transmit 23 min of temperature data, thereby becoming the first probe to send back data after landing on another planet. Venera 8, launched in 1972, transmitted surface data that included soil analysis. In October 1975 Venera 9 and 10 placed landers on the surface; both survived for an hour and relayed the first photographs of the Venusian surface. In 1978 Venera 11 and 12 released probes that landed on Venus on December 25 and 21, respectively. Both probes recorded a pressure of 88 atmospheres and a surface temperature of 460° C (860° F). On March 1 and 5, 1982, Venera 13 and 14 landed on Venus. The craft relayed photographs of the planet's surface and analyzed the chemical composition of the atmosphere and soil. On Oct. 10 and 14, 1983, Venera 15 and 16 entered orbit around Venus and returned radar images; and in June 1985, Vega 1 and 2, en route to Halley's comet, each released a lander and a balloon aerostat into the Venusian atmosphere.

The U.S. Pioneer Venus 1, an orbiter, and 2, consisting of five atmospheric probes, were launched on May 20 and Aug. 8, 1978, and reached Venus on Dec. 5 and 9, 1978. The orbiter mapped nearly the entire surface of Venus, and the probes analyzed the composition and movement of the atmosphere and its interaction with the solar wind. The Magellan probe was launched toward Venus from a space shuttle in 1989 and began transmitting pictures of the planet in August 1990. The craft orbited Venus more than 15,000 times and used radar to map about 98 percent of the planet's surface before it made a fiery descent into the planet's atmosphere in October 1994. Observations of Venus were also made by the U.S. spacecraft Galileo in 1990, while en route to Jupiter and by the Saturn-bound spacecraft Cassini during flybys in 1998 and 1999.

The ESA launched Venus Express in November 2005. The spacecraft, equipped with instruments for studying the Venusian surface and atmosphere, began orbiting the planet in April 2006.

Mercury.

The planet nearest the sun came under scrutiny when the U.S. sent Mariner 10 on a journey through the inner solar system in November 1973. The spacecraft passed Venus in February 1974 and used its gravity to enter a solar orbit. In March it flew by Mercury at a distance of 704 (mi (437 mi), providing the first views of the planet's moonlike cratered surface. On its second encounter with Mercury in September, the spacecraft detected a totally unsuspected magnetic field. On its third and final encounter in March 1975 Mariner 10 came as close as 327 km (203 mi) to the planet. In August 2004 the U.S. launched toward Mercury the spacecraft Messenger (“MErcury Surface, Space ENvironment, GEochemistry, and Ranging”), with arrival scheduled for 2011, following Venus flybys in October 2006 and June 2007; if the mission proceeds as planned, Messenger will become the first spacecraft to orbit Mercury.

Jupiter and Saturn.

The U.S. Pioneer 10 and 11 spacecraft, launched in 1972 and 1973, passed safely through the unexplored asteroid belt beyond the orbit of Mars and flew by Jupiter in December 1973 and December 1974. The two 258-kg (570-lb) spacecraft passed the planet at a distance of 130,400 km (81,000 mi) and 46,700 km (29,000 mi), and Pioneer 10 continued on its way out of the solar system, the first spacecraft ever targeted toward interstellar space. Pioneer 10 had traveled 10.28 billion km (6.39 billion mi) when the mission was officially ended, because of weakening transmission, in March 1997; the spacecraft's final, faint signal was received in January 2003. Pioneer 11 traveled by Saturn in September 1979, preparing the way for Voyager 1 and 2.

Launched in 1977, the spectacularly successful Voyager 1 and 2 encountered the Jovian system in March and July 1979 and took a variety of measurements and photographs. The spacecraft then flew by the Saturnian system in November 1980 and August 1981. Voyager 1 surpassed Pioneer 10's distance record in 1998. In mid-2005, NASA reported that Voyager 1, now more than 14 billion km (8.7 billion mi) from the sun, had entered the heliosheath, the region where interstellar gas and the solar wind begin to mix. Voyager 2, following a different path and moving more slowly, was about 10.4 billion km (6.5 billion miles) from the sun. Both spacecraft were still transmitting data.

Galileo, the first spacecraft to orbit one of the outer planets, was launched in October 1989, flew by Venus in February 1990, and on its approach to Jupiter in July 1995 released a probe that reached the planet in December. As it plunged through the Jovian atmosphere, the probe relayed its observations back to earth via the main craft, which then went into orbit around Jupiter. Flybys of Ganymede, Jupiter's largest moon, revealed the presence of a magnetic field, and approaches to two other moons, Europa and Callisto, found evidence suggesting subsurface oceans. During its 34th orbit around Jupiter, in 2002, Galileo made its closest approach to Amalthea, one of the planet's inner moons. NASA controllers then set the aging spacecraft on a collision course with Jupiter. The NASA/ESA spacecraft Ulysses, launched by space shuttle in 1990 into a polar orbit around the sun, made observations of Jupiter during swingbys in 1992 and 2004.

In October 1997 the Cassini spacecraft was launched by NASA, the European Space Agency, and several other partners on a mission to Saturn and its moon Titan. The trajectory of Cassini carried it past Venus (twice) and Jupiter before it entered orbit around Saturn in July 2004; the spacecraft was expected to explore the Saturnian system for at least four years. The Huygens probe, deployed by Cassini in December 2004, landed on the surface of Titan on Jan. 14, 2005.

Uranus, Neptune, and Pluto.

After flying past Saturn, Voyager 2 was directed toward Uranus. It passed about 80,000 km (50,000 mi) from the cloud-covered planet in January 1986, discovering two more rings as well as 11 new moons. The spacecraft came even closer to one of the moons, Miranda, transmitting startling pictures of that icy body. Voyager 2 then headed for Neptune, flying as close as 5000 km (3100 mi) to the planet in August 1989 and discovering six additional Neptunian moons before continuing its journey toward interstellar space.

NASA launched a mission to Pluto and the Kuiper Belt in January 2006. The spacecraft, called New Horizons, was scheduled to fly past Jupiter in 2007, reach Pluto in 2015, and then explore the Kuiper Belt.

HUMAN SPACEFLIGHT

Within a year after the successes of the first small artificial satellites in 1957 and 1958, both the U.S. and the USSR were developing programs to place people in earth orbit. Both countries sent carefully monitored dogs and primates into orbit to study the effects of weightlessness on living creatures.

Vostok and Mercury Programs.

The USSR was the first to put a human into space when cosmonaut Yury A. Gagarin made one orbit of the earth in Vostok 1 on April 12, 1961. During his flight time of 1 hr 48 min he reached an apogee of 327 km (203 mi) and a perigee of 180 km (112 mi). He landed safely in Siberia. In the next two years five more Vostok flights were made. The pilot of Vostok 6 was Valentina V. Tereshkova, the first woman to fly in space. Launched on June 16, 1963, she orbited the earth 48 times.

Meanwhile, a similar U.S. program, called Mercury, was taking shape. On May 5, 1961, Comdr. Alan B. Shepard, Jr., of the U.S. Navy became the first American in space. The Mercury spacecraft, named Freedom 7, flew a ballistic trajectory and made a 15-min suborbital flight. A similar flight followed on July 21, flown by Capt. Virgil I. (Gus) Grissom (1926–67) of the U.S. Air Force. On Feb. 20, 1962, Lt. Col. John H. Glenn, Jr., of the U.S. Marine Corps, became the first U.S. astronaut to orbit the earth, in a flight of three orbits. Three additional Mercury flights were made in 1962 and 1963 by Lt. Col. M. Scott Carpenter (1925–    ) of the navy; Comdr. Walter (Wally) M. Schirra, Jr. (1923–    ), also of the navy; and Maj. Leroy Gordon Cooper, Jr. (1927–2004) of the air force.

Voskhod and Gemini Programs.

The Russian Voskhod was an adaptation of the Vostok spacecraft modified to accommodate two and three cosmonauts. On Oct. 12, 1964, cosmonauts Vladimir M. Komarov (1927–67), Boris B. Yegorov (1937–94), and Konstantin P. Feoktistov (1926–    ) made a 15-orbit flight in Voskhod 1. This was the only piloted flight that year and brought the total cumulative man-hours of Soviet cosmonauts in space to 455. The U.S. astronauts had a total then of 54 man-hours in space. On March 18, 1965, cosmonauts Pavel I. Belyayev (1925–70) and Aleksei A. Leonov (1934–    ) were launched in Voskhod 2. During this 17-orbit flight, Leonov made the first walk in space, or performed extravehicular activity (EVA), leaving the spacecraft and drifting out on an umbilical tether.

The U.S. Gemini program was designed to develop the technology required to go to the moon. In May 1961 U.S. President John F. Kennedy had instituted the Apollo program, designed to land a man on the moon and return him safely to the earth “before the decade is out.” This national commitment resulted in an intensive, large-scale, piloted flight program. The Gemini spacecraft carried two astronauts and was designed to operate for extended periods of time and to develop rendezvous and docking techniques with another orbiting spacecraft. Ten Gemini flights with human passengers were made in 1965–66.

During the Gemini 4 flight Maj. Edward H. White II (1930–67) of the air force became the first U.S. astronaut to perform EVA. Using a pressurized-gas, jet-maneuvering device, he spent 21 min in space. While Gemini 6 and 7 were in orbit together in December 1965, they rendezvoused within a few feet of each other. After orbiting for 20 hr, Gemini 6 with Schirra and Maj. Thomas P. Stafford (1930–    ) of the air force landed, and Gemini 7 with Lt. Col. Frank Borman (1928–    ) of the air force and Comdr. James A. Lovell, Jr. (1928–    ), of the navy went on to spend a total of 334 hr in orbit. This flight of nearly 14 days provided medical data on humans in space that was necessary to assure success of the 10-day Apollo lunar mission. Furthermore, it demonstrated the reliability of systems such as hydrogen-oxygen fuel-cell electric power and reaction controls. On the Gemini 10, 11, and 12 flights, rendezvous and docking were accomplished repeatedly with a target vehicle that had previously been orbited.

By the end of the last Gemini flight in November 1966, U.S. astronauts had accumulated nearly 2000 man-hours in space, which exceeded the Soviet cosmonaut total, and about 12 hr in EVA.

Soyuz and Apollo.

The year 1967 was one of tragedy for both space-faring nations. On January 27, during a ground test of the Apollo spacecraft at Cape Kennedy (now Cape Canaveral), fire broke out in the three-man command module (CM). Because of the pressurized pure-oxygen atmosphere inside the spacecraft, a flash fire engulfed and killed the three astronauts—Grissom, White, and Lt. Comdr. Roger B. Chaffee (1935–67) of the navy. As a result of this tragedy, the Apollo program was delayed more than a year for a major review of vehicle design and materials.

On April 23, 1967, cosmonaut Komarov was launched in the first manned flight of a new Soviet spacecraft, Soyuz. The Soyuz had room for three cosmonauts and a separate working compartment, accessible through a hatch, for experiments. Following reentry into the earth's atmosphere and deployment of landing parachutes, the shroud lines became twisted, and Komarov plunged to his death. The Soviet space program was set back nearly two years.

In October 1968 the first manned Apollo flight was launched by a Saturn 1B booster. Astronauts Schirra, Maj. R. Walter Cunningham (1932–    ) of the U.S. Marine Reserve Corps, and Maj. Donn F. Eisele (1930–87) of the air force circled the earth for 163 orbits, checking spacecraft performance, photographing the earth, and transmitting television pictures. In December 1968 Apollo 8, a landmark flight carrying astronauts Borman, Lovell, and Maj. William A. Anders (1933–    ) of the air force, circled the moon 10 times and returned to earth safely. The Apollo 9 flight, with Maj. James A. McDivitt (1929–    ) and Col. David R. Scott (1932–    ) of the air force and civilian Russell L. Schweickart (1935–    ), checked out undocking, rendezvous, and docking of the Apollo lunar module (LM) landing craft during a 151-orbit mission. The Apollo 10 flight, with astronauts Stafford and Lt. Comdr. John W. Young (1930–    ) and Comdr. Eugene A. Cernan (1934–    ) of the navy, made 31 orbits of the moon in a rehearsal for the lunar landing. As planned, Stafford and Cernan transferred from the Apollo CM to the LM, separated, and descended to within 16 km (10 mi) of the lunar surface while astronaut Young piloted the CM. Subsequently, rendezvous and docking of the ascent stage of the LM was accomplished; the two astronauts then transferred to the CM, discarded the LM, fired the service module rocket for return trajectory to earth, and returned safely. Project Apollo was now ready to land astronauts on the moon (see Human Lunar Exploration below).

Meanwhile, the USSR launched automated Zond spacecraft around the moon, carrying cameras and biological specimens. Col. Georgi T. Beregovoi (1921–95) flew a 60-orbit mission in Soyuz 3 in October 1968. Soyuz 4 and 5 rendezvoused and docked in earth orbit in January 1969. While the spacecraft were linked, cosmonauts Aleksei S. Yeliseyev (1934–    ) and Lt. Col. Yevgeny V. Khrunov (1933–2000), in space suits, transferred by EVA from Soyuz 5 to Soyuz 4, which was piloted by Col. Vladimir A. Shatalov (1927–    ). In October 1969 Soyuz 6, 7, and 8, each launched a day apart, rendezvoused in orbit but did not dock. Soyuz 9, with a two-cosmonaut crew, set a flight duration record of almost 18 days in June 1970.

Human Lunar Exploration.

In 1969 humans achieved the long-awaited goal of actually landing on the moon. Six manned landings were made as part of the U.S. Apollo program, the last coming in 1972. In 2004, U.S. President George W. Bush called for a new manned mission to the moon by 2020.

Apollo 11.

The historic flight of Apollo 11 was launched on July 16. After entering lunar orbit, astronauts Edwin E. (Buzz) Aldrin, Jr., of the air force and Neil A. Armstrong transferred to the lunar module (LM). Armstrong, a civilian, was a navy veteran. Lt. Col. Michael Collins of the air force remained in lunar orbit following the separation, piloting the command and service module. The LM descended to the surface of the moon on July 20, landing at the edge of Mare Tranquilitatis. A few hours later, Armstrong, in his bulky space suit, descended the ladder and, at 10:56 pm (Eastern Daylight Time) stepped onto the surface of the moon. His first words were, “That's one small step for a man, one giant leap for mankind.” He was soon joined by Aldrin, and the two astronauts spent more than two hours walking on the lunar surface. They gathered 21 kg (47 lb) of soil samples, took photographs, and set up a solar wind experiment, a laser-beam reflector (see Laser), and a seismic experiment package (see Seismology). Armstrong and Aldrin also erected an American flag and talked, by satellite communications, with U.S. President Richard M. Nixon in the White House. They found that walking and running at one-sixth the gravity of earth was not difficult. Also by satellite communications, millions of people watched live television broadcasts from the moon. Returning to the LM and discarding their space suits, the two astronauts rested several hours before takeoff. They left the moon in the ascent stage of the LM, using the lower half of the module, which remained on the moon, as a launchpad. The ascent stage was jettisoned after docking with the command and service module and the transfer of the astronauts to the spacecraft. The return flight of Apollo 11 to the earth was completed without mishap, and the vehicle splashed down and was recovered on July 24 in the Pacific Ocean near Hawaii.

Because of the slight possibility of terrestrial contamination by living lunar organisms, the astronauts put on “biological isolation garments” before leaving the spacecraft and were placed under quarantine for three weeks. They remained in good health.

The Apollo 11 flight attracted great interest around the world. The general feeling was that the lunar landing signaled a first step on a new plateau of evolution.

Apollo 12.

The next moon-landing flight began on Nov. 14, 1969, when Apollo 12 was launched with astronauts Charles Conrad, Jr. (1930–99), Richard F. Gordon, Jr. (1929–    ), and Alan L. Bean (1932–    ), all of the navy, aboard. After entering lunar orbit, command pilot Conrad and Bean, the pilot of the LM, transferred to the LM. They landed north of the Riphaeus Mountains, at a spot just 180 m (600 ft) from where the Surveyor 3 spacecraft had landed two years before.

The two astronauts explored their surroundings during two periods, each lasting nearly four hours. They set up scientific experiments, took photographs, collected samples of lunar soil, and removed pieces from Surveyor 3 to be examined on their return to earth. After takeoff from the moon and rendezvous with the CM piloted by Gordon, successful splashdown and recovery took place on November 24. Quarantine procedures were repeated but, as with the Apollo 11 crew, the astronauts emerged in good health on December 10.

Apollo 12 demonstrated many improvements over Apollo 11 techniques, particularly in the accuracy of landing guidance. So successful were these changes that Apollo 13 was intended to land on more rugged terrain on the moon.

Apollo 13.

On April 11, 1970, Apollo 13, carrying veteran astronaut Lovell and the civilian astronauts Fred W. Haise, Jr. (1933–    ), and John L. Swigert, Jr. (1931–82), was launched. The spacecraft encountered difficulties during the flight when an oxygen tank ruptured. As a result the astronauts were obliged to cancel their planned landing on the lunar surface. Instead, using the power and survival systems of the LM, the astronauts swung behind the moon and were then brought back to earth by the navigating technology of the mission control center in Houston, Tex., for a splashdown south of Pago Pago in the South Pacific Ocean on April 17.

Apollo 14 and 15.

The mission of the aborted Apollo 13 was accomplished by the crew of Apollo 14, launched on Jan. 31, 1971, after modifications were carried out in the spacecraft to prevent the malfunctions encountered by Apollo 13. Capt. Shepard, who had been promoted after his successful suborbital flight in 1961 (see Vostok and Mercury Programs above), and Comdr. Edgar D. Mitchell (1930–    ), also of the navy, successfully landed the LM in the rugged Fra Mauro region of the moon, while astronaut Stuart A. Roosa (1933–94) of the air force remained in lunar orbit in the CM. Shepard and Mitchell spent more than 9 hr exploring an area that was believed to contain some of the oldest rocks yet recovered, collecting about 43 kg (95 lb) of geological samples and deploying scientific instruments. The astronauts returned to earth without incident on Feb. 9, 1971.

Apollo 15 was launched on July 26, 1971, with Col. Scott as flight commander, Lt. Col. James B. Irwin (1930–91) as pilot of the LM, and Maj. Alfred M. Worden (1932–    ) as pilot of the CM, all officers of the air force. Scott and Irwin spent 2 days 18 hr on the lunar surface at the edge of Mare Imbrium, close to the 366-m (1200-ft) deep Hadley Rille and the Apennine mountain range, one of the highest on the moon. During their 18 hr 37 min exploration of the lunar surface, the astronauts traversed more than 28.2 km (17.5 mi) in the vicinity of Mount Hadley in an electrically propelled four-wheeled “lunar rover.” They also deployed an elaborate package of scientific instruments and collected about 91 kg (about 200 lb) of rocks, including what was believed to be a sample of anorthosite, a crystalline piece of the original lunar crust, about 4.6 billion years old. A television camera left on the moon photographed Scott and Irwin's departure from the surface, and before the crew left the lunar orbit for their return to earth, they launched into lunar orbit a 35.6-kg (78.5-lb) “subsatellite” designed to transmit data about gravitational, magnetic, and high-energy fields in the lunar environment. On the return journey, Worden made a 16-min walk in deep space while the spacecraft was about 315,400 km (196,000 mi) from the earth, a record distance. The Apollo 15 astronauts splashed down without incident on August 7, about 530 km (about 330 mi) north of Hawaii, and were the first moon-landing crew that was not required to undergo a quarantine.

Apollo 16 and 17.

On April 16, 1972, astronauts Young, Charles Moss Duke, Jr. (1935–    ), and Thomas Kenneth (Ken) Mattingly (1936–    ) were launched on the Apollo 16 mission to the moon, to explore the Descartes Highlands and the Cayley Plains regions. While Mattingly remained in orbit, the two other astronauts landed in the assigned area on April 20. They spent 20 hr 14 min on the moon, setting up a number of experiments powered by a small nuclear station, traveling about 26.6 km (about 16.5 mi) in the lunar rover, and collecting more than 97 kg (214 lb) of rock samples.

The Apollo series missions to the moon concluded with the flight of Apollo 17, Dec. 6–19, 1972. During their smooth 13-day voyage, veteran astronaut Cernan and the American civilian geologist Harrison H. Schmitt (1935–    ) spent 22 hr on the moon, traveling 35 km (22 mi) in the lunar rover and exploring the Taurus-Littrow Valley region, while Comdr. Ronald E. Evans (1933–90) of the navy remained in lunar orbit.

SPACE STATIONS

Salyut and Skylab were the first spacecraft designed as space stations. Orbiting the earth for extended periods, while crews came and went on other vehicles, these space stations made possible many valuable new experiments and astronomical observations. The Soviet station, Mir, which was expected to have a useful life of 5 years, was still manned and in orbit 14 years after its launch. The long-term success of Mir led to joint efforts toward an international space station.

Salyut Program.

The Soviet Salyut 1 space station, weighing 18,600 kg (41,000 lb), was launched on April 19, 1971. Three days later Soyuz 10, with a crew of three cosmonauts, rendezvoused and docked. For some unspecified reason, however, the cosmonauts did not enter the Salyut but undocked and returned to earth. In June Soyuz 11 joined with Salyut 1, and the three-man crew moved into the station to set a human spaceflight duration record of 24 days. Numerous earth-resources and biological experiments were conducted. During the return journey to earth, however, tragedy struck, and upon landing the three cosmonauts—Georgi T. Dobrovolsky (1928–71), Vladislav N. Volkov (1935–71), and Viktor I. Patsayev (1933–71)—were found dead, victims of an air leak. Because they wore no space suits, the cosmonauts had been killed quickly by the sudden depressurization. The Soviet program suffered another setback when the Salyut 2 space station, launched in April 1973, apparently went out of control, shedding various parts in orbit.

Thereafter, however, the Soviet Union sent up Salyut 3 (June 1974–January 1975), 4 (December 1974–February 1977), and 5 (June 1976–August 1977). Salyut 6 (September 1977–July 1982) and 7 (launched April 1982) were visited by a large number of international crews, including Cuban, French, and Indian cosmonauts and the first woman to perform extravehicular activity, Svetlana Savitskaya (1948–    ), during the flight of Soyuz T-12 on July 17–29, 1984. One of the most notable flights of the Salyut/Soyuz series occurred in 1984 when cosmonauts Leonid Kizim (1941–    ), Vladimir Solovyov (1946–    ), and Oleg Atkov (1949–    ), spent 237 days aboard the Salyut 7 before returning to earth, the longest space flight to that date. Unused since 1986, Salyut 7 plunged to earth in February 1991.

Skylab Program.

The U.S. Skylab program was much more extensive and complex than the Soviet Salyut program. Skylab, launched by the first two stages of a Saturn 5 rocket, weighed 88,900 kg (196,000 lb), nearly five times the weight of Salyut. In contrast to the estimated 99-cu m (3500-cu ft) interior space of Salyut, Skylab had 357 cu m (12,600 cu ft), about three and one-half times greater. Skylab served as a laboratory in earth orbit. It was used for solar-astronomical studies, long-duration medical studies of the three-man crew, extensive multispectral observations of the earth, and a variety of other scientific and technological experiments, such as metallic-crystal growth in the weightless state.

Skylab was damaged during launch on May 25, 1973, but the crew, veteran astronaut Conrad, Comdr. Joseph P. Kerwin (1932–    ), and Comdr. Paul J. Weitz (1932–    ), all of the navy, carried out EVA repairs, erected a heat-shielding canopy over the exterior of the spacecraft, and freed a jammed solar panel. Their flight lasted 28 days. A second crew spent 59 days in orbit; the third and final crew, 84 days. The Skylab project was considered completely successful. More than 740 hr were spent in observing the sun by telescopes, and 175,000 solar pictures were returned to earth, as were about 64 km (about 40 mi) of electronic data tape and 46,000 photographs of the earth's surface. On July 11, 1979, during its 34,981st orbit, Skylab plunged to earth, raining fiery debris over sparsely populated western Australia and over the Indian Ocean.

The U.S., in cooperation with Canada, Japan, and the European Space Agency, planned a permanent space station to be assembled in space as a successor to Skylab, but delays and cost overruns led to cancellation of the project.

Mir.

The Mir space station, which the Soviets designed as a successor to the Salyut series, was launched on Feb. 20, 1986. Described by the Soviets as the core of the first permanently occupied space station, it featured six docking ports and could be operated by two cosmonauts. In 1987, Col. Yuri Romanenko (1944–    ) spent 326 days aboard Mir, the longest space flight then on record. On April 12, 1987, the Soviets succeeded in docking Mir with Kvant, an 18,000-kg (40,000-lb) astrophysics module. Carrying four X-ray telescopes, the Kvant was designed to link with Mir and observe a newly discovered supernova. (X rays from the exploding star, blocked by the earth's atmosphere, could not be detected from earth.) In 1987–88, Soviet cosmonauts Vladimir Titov (1947–    ) and Musa Manarov (1951–    ) set a new record for time spent in space—366 days; the record was raised to 439 days by Russian cosmonaut Valery Polyakov (1942–    ) in 1995. By that time, more than 20 Soyuz missions to the station had been accomplished.

International Cooperation.

The U.S. and Russian space station programs were formally joined in 1993. U.S. space shuttle craft began docking regularly at Mir, and U.S. astronauts began spending extended visits in preparation for future space station missions. In 1996 NASA and the Russian, European, Japanese, and Canadian space agencies agreed to cooperate on an International Space Station (ISS), designed to be a multinational research complex.

Construction of the station, based on the existing space shuttle and Mir programs, was originally scheduled to begin in late 1997. In that year, however, serious problems occurred aboard Mir. In February an air-filtering unit caught fire and burned for several minutes, in the worst space fire on record; the following month two oxygen generators malfunctioned. On June 25, in the worst collision in the history of human spaceflight, the station lost between 40 and 50 percent of its power supply in a crash with an unmanned cargo craft during a practice docking maneuver; none of the three Mir crew members (including one U.S. astronaut) was injured. A Russian commission blamed the accident on an overworked crew and inadequate training, among other factors, but some U.S. analysts raised questions about the safety of the aging craft. In June 1998 the U.S. space shuttle Discovery made the last scheduled shuttle mission to Mir. The last Russian crew left the space station in June 2000, and after more than 86,300 trips around the earth, Mir was taken out of orbit and allowed to crash into the South Pacific Ocean in March 2001.

Problems with Mir and other aspects of the Soviet space program delayed the ISS project by a year. In November 1998 Russia launched the first component of the space station, a propulsion and power module called Zarya. Two weeks later the U.S. launched the space shuttle Endeavour, carrying the space station's large core unit, called Unity, which was attached to Zarya; Unity was to serve as the principal connector for future pieces of the station. In-orbit assembly of the ISS was scheduled for completion by 2004, at a total cost of more than $35 billion; however, the catastrophic failure of the space shuttle Columbia in early 2003 (see below) forced postponement of this task. In March 2006, project leaders scaled back plans for the ISS with an eye toward completion by 2010. Meanwhile, a 2005 estimate by the ESA put the overall cost of developing and building the ISS and operating it for at least a decade at 100 billion euros, or more than $120 billion.

THE SPACE SHUTTLE

In the early 1980s, the Space Transportation System (STS), better known as the space shuttle, became the major U.S. space program. The shuttle, a multipurpose, orbital-launch space plane, was designed to carry payloads of up to about 30,000 kg (65,000 lb) and up to seven crew members and passengers. The upper part of the spacecraft, the orbiter stage, had a theoretical lifetime of perhaps 100 missions, and the winged orbiter could make unpowered landings on returning to earth. Because of the shuttle's designed flexibility and its planned use for satellite deployment and the rescue and repair of previously orbited satellites, its proponents saw it as a major advance in the practical exploitation of space. Others, however, worried that NASA was placing too much reliance on the shuttle, to the detriment of automated vehicles and missions. Meanwhile, the Soviet Union developed its own shuttle, which unlike the U.S. shuttle was capable of automated flight and did not employ reusable boosters. One craft, the Buran, was completed; it made an unmanned orbital flight in 1988. The expensive program was discontinued following the 1991 dissolution of the Soviet Union.

Problems with the STS eventually led to resumption of the use of expendable launch vehicles (ELVs) by the U.S. for launching satellites. The U.S. had planned to replace the space shuttle with a new spacecraft, the X-30, in the 1990s. Faced with budgetary constraints, however, the U.S. decided to rely instead on a mixed fleet of ELVs and space shuttles to place payloads into orbit for the remainder of the decade. In July 1996, NASA gave the go-ahead for construction of the X-33, a one-half scale model of a new reusable launch vehicle (RLV); when funding for the program was terminated in 2001, NASA had no choice but to depend on the space shuttle for missions to the Hubble Space Telescope and the International Space Station (see International Cooperation, above). As an alternative to the shuttle, the European Space Agency in the 2000s developed the Automated Transfer Vehicle (ATV) to service the ISS; the initial launch of the craft was expected in 2007.

Flight Testing Programs and Early Shuttle Missions.

In the late 1970s and early 1980s, two types of flight testing programs were organized: the approach and landing test (ALT) program and the orbital flight test program. During the ALT program (February–November 1977) the first space shuttle orbiter, a test vehicle called Enterprise, demonstrated that it could fly in the atmosphere and land like an airplane. Because it was not equipped for space travel, the Enterprise was launched from atop an airborne 747 aircraft.

The orbital flight test program began on April 12, 1981, with the launch into orbit of the space shuttle Columbia from the Kennedy Space Center in Florida; considered the first true spaceship, Columbia maneuvered through space during its 36 orbits and landed on a runway at Edwards Air Force Base in California two days later. The shuttle program's first operational mission took place on Nov. 11, 1982, when Columbia deployed two commercial communications satellites.

Over the next decade, Columbia was joined in the shuttle fleet by Challenger in 1982 (see below), Discovery in 1983, Atlantis in 1985, and Endeavour in 1991. Early memorable flights included mission STS-7, on June 18–24, 1983, whose crew included the first U.S. woman astronaut, Sally K. Ride; mission STS-9, on Nov. 28–Dec. 8, 1983, which carried the first of the European Space Agency's Spacelabs; mission 41-C, on April 6–13, 1984, during which a satellite was retrieved, repaired, and redeployed; and mission 51-A, on Nov. 8–16, 1984, when two expensive malfunctioning satellites were retrieved and returned to earth.

By the mid-1980s, the shuttle launch program was behind schedule. Increasingly the shuttle was being used for U.S. military missions, and it faced stiff competition from the European Space Agency's automated Ariane program for the orbiting of satellites.

Challenger Disaster.

On Jan. 28, 1986, the shuttle Challenger was destroyed about one minute after launch because of the failure of an O-ring, a sealant ring, on one of its solid rocket boosters. The booster nosed into the main propellant tank of liquid hydrogen and oxygen, causing a near-explosive disruption of the entire system. Seven astronauts were killed in the disaster: commander Francis (Dick) R. Scobee (1939–86); pilot Michael J. Smith (1945–86); mission specialists Judith A. Resnik (1949–86), Ellison S. Onizuka (1946–86), and Ronald E. McNair (1950–86); and payload specialists Gregory B. Jarvis (1944–86) and Christa McAuliffe (1948–86). McAuliffe had been selected the preceding year as the first “teacher in space,” a civilian spokesperson for the shuttle program.

The tragedy brought an immediate halt to shuttle flights until systems could be analyzed and redesigned. A presidential commission headed by former secretary of state William Rogers and former astronaut Neil Armstrong placed much of the blame on NASA's administrative system and its failure to maintain an effective system of quality control.

The Shuttle after Challenger.

In the aftermath of the Challenger disaster, the O-ring seals on the solid rocket booster were redesigned, a replacement shuttle was ordered, and NASA's goal of flying two dozen shuttle missions a year was scaled back. The shuttle launch program resumed on Sept. 29, 1988, with the flight of Discovery and its crew of five astronauts. On this mission (STS-26) a NASA communications satellite, TDRS-3, was placed in orbit and a variety of experiments were carried out. The success of this mission encouraged the U.S. to resume an active launch schedule. By the end of 1997 another 62 shuttle missions had been flown, including the deployment in 1990 of the $1.5 billion Hubble Space Telescope, a daring mission to repair the malfunctioning telescope in 1993, and a servicing mission for the telescope in 1997. Notable in 1998 were the last two scheduled shuttle missions to Mir, John Glenn's highly publicized return to space aboard Discovery, and the deployment of the first two pieces of the International Space Station.

The Columbia Disaster and Its Aftermath.

After a series of successful missions in the late 1990s and early 2000s, mostly to support the construction and servicing of the International Space Station, the U.S. space program experienced its worst disaster since the loss of the Challenger. On Feb. 1, 2003, at the end of a 16-day scientific research mission, the space shuttle Columbia disintegrated while reentering the earth's atmosphere, 16 min before its scheduled landing; the fiery explosion left a huge trail of debris, mostly in E Texas. All seven astronauts on board Columbia were killed: commander Rick Husband (1957–2003); pilot William (Willie) McCool (1961–2003); mission specialists Michael Anderson (1959–2003), David Brown (1956–2003), Kalpana Chawla (1961–2003), and Laurel Clark (1961–2003); and payload specialist Ilan Ramon (1954–2003), the first Israeli astronaut to fly in space. The flight was Columbia's 28th mission and the 113th mission of the shuttle program overall.

The Columbia Accident Investigation Board (CAIB), headed by a retired admiral, Harold W. Gehman, Jr. (1942–    ), and including former astronaut Ride, issued its final report on the disaster in August 2003. The CAIB traced the physical cause of the accident to a breach in the shuttle's thermal protection system, which occurred when a piece of insulating foam detached from an external fuel tank at 81.7 seconds after launch and struck the leading edge of Columbia's left wing; at reentry, superheated gases, or plasma, penetrated the wing, causing it to melt and the spacecraft to break apart. The panel recommended numerous changes in the thermal protection system and, like the Challenger commission, urged improvements in NASA's administrative and safety procedures. All other shuttle flights were suspended until the CAIB's key recommendations could be implemented.

The shuttle program returned to space with the launch of Discovery on July 26, 2005. The primary goals of the mission were to assist in assembling and supplying the International Space Station and to evaluate the modified external fuel tank and new safety procedures introduced since the Columbia disaster. In a succession of spacewalks, the crew replaced a malfunctioning gyroscope on the space station, tested methods of repairing the shuttle's protective tiles, and, in what was an unscheduled spacewalk, removed excess ceramic-coated “gap filler” material protruding from the heat shield on the shuttle's underbelly. Although Discovery made a successful reentry and landing on Aug. 9, the mission showed that redesign of the external fuel tank had not succeeded in eliminating the shedding of foam chunks like the one that had doomed Columbia, and shuttle flights were once again postponed while NASA made another attempt to fix the problem.

The next mission, flown by Discovery, was finally launched on July 4, 2006, after a crack that had been discovered in the insulating foam was deemed not sufficiently serious to merit further delay. In addition to resupply and maintenance of the ISS, the mission's objectives included testing of new procedures designed to enhance shuttle safety. Astronauts performed three spacewalks, during which they carried out maintenance on the space station's mobile transporter, installed a spare thermal control system pump outside an air lock, tested the shuttle's robotic arm boom extension as a possible platform for heat shield repairs in subsequent shuttle flights, and demonstrated repair techniques for the heat shield. The heat shield was painstakingly inspected both after the launch and before the return to earth, which took place successfully on July 17.

F.C.D., FREDERICK C. DURANT III, B.S.

For further information on this topic, see the Bibliography, sections 446. Exobiology, 579. Rocket – 582. Astronaut.