space exploration

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space exploration,

the investigation of physical conditions in space and on stars, planets, and other celestial bodies through the use of artificial satellitessatellite, artificial,
object constructed by humans and placed in orbit around the earth or other celestial body (see also space probe). The satellite is lifted from the earth's surface by a rocket and, once placed in orbit, maintains its motion without further rocket propulsion.
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 (spacecraft that orbit the earth), space probesspace probe,
space vehicle carrying sophisticated instrumentation but no crew, designed to explore various aspects of the solar system (see space exploration). Unlike an artificial satellite, which is placed in more or less permanent orbit around the earth, a space probe is
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 (spacecraft that pass through the solar system and that may or may not orbit another celestial body), and spacecraft with human crews.

Satellites and Probes

Although studies from earth using optical and radio telescopestelescope,
traditionally, a system of lenses, mirrors, or both, used to gather light from a distant object and form an image of it. Traditional optical telescopes, which are the subject of this article, also are used to magnify objects on earth and in astronomy; other types of
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 had accumulated much data on the nature of celestial bodies, it was not until after World War II that the development of powerful rocketsrocket,
any vehicle propelled by ejection of the gases produced by combustion of self-contained propellants. Rockets are used in fireworks, as military weapons, and in scientific applications such as space exploration.
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 made direct space exploration a technological possibility. The first artificial satellite, Sputnik I, was launched by the USSR (now Russia) on Oct. 4, 1957, and spurred the dormant U.S. program into action, leading to an international competition popularly known as the "space race." Explorer I, the first American satellite, was launched on Jan. 31, 1958. Although earth-orbiting satellites have by far accounted for the great majority of launches in the space program, even more information on the moon, other planets, and the sun has been acquired by space probes.

Lunar Probes

In the decade following Sputnik I, the United States and the USSR between them launched about 50 space probes to explore the moonmoon,
natural satellite of a planet (see satellite, natural) or dwarf planet, in particular, the single natural satellite of the earth. The Earth-Moon System

The moon is the earth's nearest neighbor in space.
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. The first probes were intended either to pass very close to the moon (flyby) or to crash into it (hard landing). Later probes made soft landings with instruments intact and achieved stable orbits around the moon. Each of these four objectives required increasingly greater rocket power and more precise maneuvering; successive launches in the Soviet Luna series were the first to accomplish each objective. Luna 2 made a hard lunar landing in Sept., 1959, and Luna 3 took pictures of the moon's far side as the probe flew by in Nov., 1959. Luna 9 soft-landed in Feb., 1966, and Luna 10 orbited the moon in Apr., 1966; both sent back many television pictures to earth. Beginning with Luna 16, which was launched in Sept., 1970, the USSR sent a several probes to the moon that either returned lunar soil samples to earth or deployed Lunokhod rovers. In addition to the 24 lunar probes in the Luna program, the Soviets also launched five circumlunar probes in its Zond program.

Early American successes generally lagged behind Soviet accomplishments by several months but provided more detailed scientific information. The U.S. program did not bear fruit until 1964, when Rangers 7, 8, and 9 transmitted thousands of pictures, many taken at altitudes of less than 1 mi (1.6 km) just before impact and showing craters only a few feet in diameter. Two years later, the Surveyor series began a program of soft landings on the moon. Surveyor 1 touched down in June, 1966; in addition to television cameras, it carried instruments to measure soil strength and composition. The Surveyor program established that the moon's surface was solid enough to support a spacecraft carrying astronauts.

In Aug., 1966, the United States successfully launched the first Lunar Orbiter, which took pictures of both sides of the moon as well as the first pictures of the earth from the moon's vicinity. The Orbiter's primary mission was to locate suitable landing sites for the Apollo Lunar Module, but in the process it also discovered the lunar mascons, regions of large concentration of mass on the moon's surface. Between May, 1966, and Nov., 1968, the United States launched seven Surveyors and five Lunar Orbiters. Clementine, launched in 1994, engaged in a systematic mapping of the lunar surface. In 1998, Lunar Prospector orbited the moon in a low polar orbit investigating possible polar ice deposits, but a controlled crash near the south pole detected no water. The U.S. Lunar Reconnaissance Orbiter, launched in 2009, was designed to collect data that can be used to prepare for future missions to the moon; information from it has been used to produce a relatively detailed, nearly complete topographic map of the moon.

China became the third nation to send a spacecraft to the moon when Chang'e 1, which was launched in 2007, orbited and mapped the moon until it was crash-landed on the lunar surface in 2009. Chang'e 2 also orbited and mapped the moon (2010–11) and later conducted a flyby of an asteroid (2012). In Dec., 2013, Chang'e 3 landed on the moon and deployed a rover, Yutu, or Jade Rabbit (2013–16), and in Jan., 2018, Chang'e 4 put the first lander on the far side of the moon.

Interplanetary Probes

While the bulk of space exploration initially was directed at the earth-moon system, the focus gradually shifted to other members of the solar system. The U.S. Mariner program studied Venus and Mars, the two planets closest to the earth; the Soviet Venera series also studied Venus. From 1962 to 1971, these probes confirmed the high surface temperature and thick atmosphere of Venus, discovered signs of recent volcanism and possible water erosion on Mars, and investigated Mercury. Between 1971 and 1973 the Soviet Union launched six successful probes as part of its Mars program. Exploration of Mars continued with the U.S. Viking landings on the Martian surface. Two Viking spacecraft arrived on Mars in 1976. Their mechanical arms scooped up soil samples for automated tests that searched for photosynthesis, respiration, and metabolism by any microorganisms that might be present; one test suggested at least the possibility of organic activity. The Soviet Phobos 1 and 2 missions were unsuccessful in 1988. The U.S. Magellan spacecraft succeeded in orbiting Venus in 1990, returning a complete radar map of the planet's hidden surface. The Japanese probes Sakigake and Suisei and the European Space Agency's probe Giotto both rendezvoused with Halley's cometHalley's comet
or Comet Halley
, periodic comet named for Edmond Halley, who observed it in 1682 and identified it as the one observed in 1531 and 1607. Halley did not live to see its return in 1758, close to the time he predicted.
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 in 1986, and Giotto also came within 125 mi (200 km) of the nucleus of the comet Grigg-Skjellerup in 1992. The U.S. probe Ulysses returned data about the poles of the sun in 1994, and the ESA Solar and Heliospheric Observatory (SOHO) was put into orbit in 1995. Launched in 1996 to study asteroids and comets, the Near Earth Asteroid Rendezvous (NEAR) probe made flybys of the asteroids Mathilde (1997) and Eros (1999) and began orbiting the latter in 2000. The Mars Pathfinder and Mars Global Surveyor, both of which reached Mars in 1997, were highly successful, the former in analyzing the Martian surface and the latter in mapping it. The ESA Mars Express, launched in 2003, began orbiting Mars later that year, and although its Beagle 2 lander failed to establish contact, the orbiter has sent back data. Spirit and Opportunity, NASA rovers, landed successfully on Mars in 2004, as did the NASA rover Curiosity in 2012. Messenger, also launched by NASA, became the first space probe to orbit Mercury in 2011; its mission ended in 2015. In 2014 the ESA's Rosetta became the first probe to orbit a comet (Comet 67P, which it then studied for two years); prior to that rendezvous the space probe had made flybys of Mars and two asteroids.

Space probes have also been aimed at the outer planets, with spectacular results. One such probe, Pioneer 10, passed through the asteroid belt in 1973, then became the first object made by human beings to move beyond the orbits of the planets. In 1974, Pioneer 11 photographed Jupiter's equatorial latitudes and its moons, and in 1979 it made the first direct observations of Saturn. Voyagers 1 and 2, which were launched in 1977, took advantage of a rare alignment of Jupiter, Saturn, Uranus, and Neptune to explore all four planets. Passing as close as 3,000 mi (4,800 km) to each planet's surface, the Voyagers discovered new rings, explored complex magnetic fields, and returned detailed photographs of the outer planets and their unique moons. They subsequently moved toward the heliopause, the boundary between the influence of the sun's magnetic field and the interstellar magnetic field, and in 2013 NASA reported that Voyager 1 most likely crossed the heliopause in 2012 and entered interstellar space, becoming the first spacecraft to do so.

Launched in 1989, NASA's Galileo spacecraft followed a circuitous route that enabled it to return data about Venus (1990), the moon (1992), and the asteroids 951 Gaspra (1991) and 243 Ida (1993) before it orbited Jupiter (1995–2003); it also returned data about the Jupiter's atmosphere and its largest moons (Io, Ganymede, Europa, and Callisto). NASA returned to Jupiter in 2016 with Juno (launched 2011); its instruments are designed to examine the nature and properties of the planet. The joint U.S.-ESA Cassini mission, launched in 1997, explored Saturn, its rings, and some of its moons upon from its arrival in 2004 until it was crashed into the planet in 2017. It also deployed Huygens, which landed on the surface of Saturn's moon Titan in early 2005. Pluto was visited for the first time in 2015 by NASA's New Horizons (launched 2006); the space probe then flew by the Kuiper belt's Arrokoth, the most distant object to be visited by a space probe.

Human Space Exploration

Human spaceflight has progressed from the simple to the complex, starting with suborbital flights; subsequent highlights included the launching of a single astronaut in orbit, the launching of several astronauts in a single capsule, the rendezvous and docking of two spacecraft, the attainment of lunar orbit, and the televised landing of an astronaut on the moon. The first person in earth orbit was a Soviet cosmonaut, Yuri GagarinGagarin, Yuri Alekseyevich
, 1934–68, Russian astronaut (cosmonaut), b. near Gzhatsk, RSFSR. He was the first in history to be rocketed into orbital space flight. His flight on Apr. 12, 1961, lasted 1 hr. 48 min. and circled the earth once.
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, in Vostok 1 on Apr. 12, 1961. The American Mercury program had its first orbital success in Feb., 1962, when John GlennGlenn, John Herschel, Jr.,
1921–2016, American astronaut and politician, b. Cambridge, Ohio. On Feb. 20, 1962, he became the first American and the third person to orbit the earth, circling the globe three times in Friendship 7,
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 circled the earth three times; a flight of 22 orbits was achieved by Mercury in May, 1963. In Oct., 1964, three Soviet cosmonauts were launched in a Voskhod spacecraft. During the second Voskhod flight in Mar., 1965, a cosmonaut left the capsule to make the first "walk in space."

The first launch of the Gemini program, carrying two American astronauts, occurred a few days after the Soviet spacewalk. The United States made its first spacewalk during Gemini 4, and subsequent flights established techniques for rendezvous and docking in space. The first actual docking of two craft in space was achieved in Mar., 1966, when Gemini 8 docked with a crewless vehicle. In Oct., 1967, two Soviet Cosmos spacecraft performed the first automatic crewless rendezvous and docking. Gemini and Voskhod were followed by the American Apollo and the Soviet Soyuz programs, respectively.

The Apollo Program

In 1961, President Kennedy had committed the United States to the goal of landing astronauts on the moon and bringing them safely back to earth by the end of the decade. The resulting Apollo program was the largest scientific and technological undertaking in history. Apollo 8 was the first craft to orbit both the earth and the moon (Dec., 1968); on July 20, 1969, astronauts Neil A. ArmstrongArmstrong, Neil Alden,
1930–2012, American astronaut, b. Wapakoneta, Ohio, grad. Purdue Univ. (B.S., 1955), Univ. of Southern California (M.S., 1970). A U.S. Navy fighter pilot during the Korean War, Armstrong became a test pilot for what was then the National Advisory
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 and Edwin E. ("Buzz") AldrinAldrin, Buzz
(Edwin Eugene Aldrin, Jr.), 1930–, American astronaut, b. Montclair, N.J. After graduating from West Point (1951), Aldrin joined the U.S. air force and flew 66 combat missions during the Korean War. His doctoral thesis at the Massachusetts Inst.
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, Jr., stepped out onto the moon, while a third astronaut, Michael Collins, orbited the moon in the command ship. In all, there were 17 Apollo missions and 6 lunar landings (1969–72). Apollo 15 marked the first use of the Lunar Rover, a jeeplike vehicle. The scientific mission of Apollo centered around an automated geophysical laboratory, ALSEP (Apollo Lunar Surface Experimental Package). Much was learned about the physical constitution and early history of the moon, including information about magnetic fields, heat flow, volcanism, and seismic activity. The total lunar rock sample returned to earth weighed nearly 900 lb (400 kg).

Apollo moon flights were launched by the three-stage Saturn V rocket, which developed 7.5 million lb (3.4 million kg) of thrust at liftoff. At launch, the total assembly stood 363 ft (110 m) high and weighed more than 3,000 tons. The Apollo spacecraft itself weighed 44 tons and stood nearly 60 ft (20 m) high. It was composed of three sections: the command, service, and lunar modules. In earth orbit, the lunar module (LM) was freed from its protective compartment and docked to the nose of the command module. Once in lunar orbit, two astronauts transferred to the LM, which then detached from the command module and descended to the lunar surface. After lunar exploration, the descent stage of the LM remained on the moon, while the ascent stage was jettisoned after returning the astronauts to the command module. The service module was jettisoned just before reentering the earth's atmosphere. Thus, of the huge craft that left the earth, only the cone-shaped command module returned.

The Soyuz Program

Until late 1969 it appeared that the USSR was also working toward landing cosmonauts on the moon. In Nov., 1968, a Soviet cosmonaut in Soyuz 3 participated in an automated rendezvous and manual approach sequence with the crewless Soyuz 2. Soyuz 4 and 5 docked in space in Jan., 1969, and two cosmonauts transferred from Soyuz 5 to Soyuz 4; it was the first transfer of crew members in space from separately launched vehicles. But in July, 1969, the rocket that was to power the lunar mission exploded, destroying an entire launch complex, and the USSR abandoned the goal of human lunar exploration to concentrate on orbital flights. The program suffered a further setback in June, 1971, when Soyuz 11 accidentally depressurized during reentry, killing all three cosmonauts. In July, 1975, the United States and the USSR carried out the first internationally crewed spaceflight, when an Apollo and a Soyuz spacecraft docked while in earth orbit. Later Soyuz spacecraft have been used to ferry crew members to and from Salyut, Mir, and the International Space Station.

Space Stations

After the geophysical exploration of the moon via the Apollo program was completed, the United States continued human space exploration with Skylab, an earth-orbiting space station that served as workshop and living quarters for three astronauts. The main capsule was launched by a booster; the crews arrived later in an Apollo-type craft that docked to the main capsule. Skylab had an operational lifetime of eight months, during which three three-astronaut crews remained in the space station for periods of about one month, two months, and three months. The first crew reached Skylab in May, 1972.

Skylab's scientific mission alternated between predominantly solar astrophysical research and study of the earth's natural resources; in addition, the crews evaluated their response to prolonged conditions of weightlessness. The solar observatory contained eight high-resolution telescopes, each designed to study a different part of the spectrumspectrum,
arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer (see mass spectrograph).
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 (e.g., visible, ultraviolet, X-ray, or infrared light). Particular attention was given to the study of solar flares (see chromospherechromosphere
[Gr.,=color sphere], layer of rarefied, transparent gases in the solar atmosphere; it measures 6,000 mi (9,700 km) in thickness and lies between the photosphere (the sun's visible surface) and the corona (its outer atmosphere).
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). The earth applications, which involved remote sensing of natural resources, relied on visible and infrared light in a technique called multispectral scanning (see space sciencespace science,
body of scientific knowledge as it relates to space exploration; it is sometimes also called astronautics. Space science draws on the conventional sciences of physics, chemistry, biology, and engineering, as well as requiring specific research of its own.
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). The data collected helped scientists to forecast crop and timber yields, locate potentially productive land, detect insect infestation, map deserts, measure snow and ice cover, locate mineral deposits, trace marine and wildlife migrations, and detect the dispersal patterns of air and water pollution. In addition, radar studies yielded information about the surface roughness and electrical properties of the sea on a global basis. Skylab fell out of orbit in July, 1979; despite diligent efforts, several large pieces of debris fell on land.

After that time the only continuing presence of humans in earth orbit were the Soviet Salyut and Mir space stations, in which cosmonauts worked for periods ranging to more than 14 months. In addition to conducting remote sensing and gathering medical data, cosmonauts used their microgravity environment to produce electronic and medical artifacts impossible to create on earth. In preparation for the International Space Station (ISS)—a cooperative program of the United States, Russia, Japan, Canada, Brazil, and the ESA—astronauts and cosmonauts from Afghanistan, Austria, Britain, Bulgaria, France, Germany, Japan, Kazakhstan, Syria, and the United States worked on Mir alongside their Russian counterparts. Assembly of the ISS began in Dec., 1998, with the linking of an American and a Russian module (see space stationspace station
or space platform,
artificial earth satellite, usually manned, that is placed in a fixed orbit and can serve as a base for astronomical observations; zero-gravity materials processing; satellite assembly, refueling, and repair; or, possibly, as weapons
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) Once the ISS was manned in 2000, maintaining Mir in orbit was no longer necessary and it was made to decay out of orbit in Mar., 2001.

The Space Shuttle

After the Skylab space station fell out of orbit in 1979, the United States did not resume sending astronauts into space until 1981, when the space shuttlespace shuttle,
reusable U.S. space vehicle (1981–2011). Developed by the National Aeronautics and Space Administration (NASA) and officially known as the Space Transportation System (STS), it was the world's first reusable spacecraft that carried human beings into earth
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, capable of ferrying people and equipment into orbit and back to earth, was launched. The shuttle itself was a hypersonic delta-wing airplane about the size of a DC-9. Takeoff was powered by three liquid-fuel engines fed from an external tank and two solid-fuel engines; the last were recovered by parachute. The shuttle itself returned to earth in a controlled glide, landing either in California or in Florida.

The shuttle put a payload of up to 25 tons (22,700 kg) in earth orbit below 600 mi (970 km); the payload was then boosted into final orbit by its own attached rocket. The Galileo probe, designed to investigate Jupiter's upper atmosphere, was launched from the space shuttle. Astronauts also used the shuttle to retrieve and repair satellites, to experiment with construction techniques needed for a permanent space station, and to conduct scientific experiments during extended periods in space.

At first it was hoped that shuttle flights could operate on a monthly basis, but schedule pressures contributed to the explosion of the Challenger shuttle in 1986, when cold launch conditions led to the failure of a rubber O-ring, and the resulting flame ruptured the main fuel tank. The shuttle program was suspended for three years, while the entire system was redesigned. The shuttle fleet subsequently operated on approximately a bimonthly schedule. A second accident occurred in 2003, when Columbia was lost during reentry because damaged heat shielding on the left wing, which had been damaged by insulation shed from the external fuel tank, failed to prevent superheated gas from entering the wing; the hot gas structurally weakened the wing and caused the shuttle to break up. Shuttle flights resumed in July, 2005, but new problems with fuel tank insulation led NASA to suspend shuttle launches for a year. The last shuttle flight was in July, 2011.

In 2004, President George W. Bush called for a return to the moon by 2020 and the establishment of a base there that would be used to support the human exploration of Mars. The following year NASA unveiled a $104 billion plan for a lunar expedition that resembled that Apollo program in many respects, except that two rockets would be used to launch the crew and lunar lander separately.

In June, 2004, SpaceShipOne, a privately financed spacecraft utilizing a reusable vehicle somewhat similar in concept to the shuttle, was launched into suborbital flight from the Mojave Desert in California. Unlike the shuttle, SpaceShipOne was carried aloft by a reusable jet mothership (White Knight) to 46,000 ft (13.8 km), where it was released and fires its rocket engine. The spacecraft was designed by Bert RutanRutan, Burt
(Elbert Leander Rutan) , 1943–, American aerospace engineer, b. Portland, Oreg., grad. California Polytechnic Univ. (B.S. 1965). From 1965 to 1972 Rutan worked for the U.S.
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 and built by his company, SCALED Composites. The vehicle's 90-minute flight was the first successful nongovernmental spaceflight. SpaceShipTwo, based on SpaceShipOne, is being developed for commercial tourist flights; it made its first powered flight in 2013. Another spacecraft was privately developed by Space Exploration Technologies, or SpaceX, in coordination with NASA. The company's Falcon 9 rocket had its first successful launch, from Cape Canaveral, in June, 2010. In Dec., 2010, SpaceX launched the Dragon space capsule, using a Falcon 9 rocket, and successfully returned the capsule to earth after almost two orbits. In May, 2012, the Dragon made its first resupply trip to the space station, returning with experiments and other items. Orbital Sciences Corp. (OSC) also developed a cargo capsule, Cygnus, in cooperation with NASA. OSC's Antares rocket, which is used to launch Cygnus, had its first test in Apr., 2013, and Cygnus had its first resupply flight later that year. A crew version of the Dragon had its first orbital test flight, without a crew, in Mar., 2019, and in Dec., 2019, the Starliner, a crew capsule developed by Boeing in coordination with NASA, had its first, uncrewed test flight.

The Chinese Space Program

China launched its first satellite in 1970 and then began the Shuguang program to put an astronaut into space, but the program was twice halted, ending in 1980. In the 1990s, however, China began a new program, and launched the crewless Shenzhou 1, based on the Soyuz, in 1999. The Shenzhou, like the Soyuz, is capable of carrying a crew of three. In Oct., 2003, Shenzhou 5 carried a single astronaut, Yang Liwei, on a 21-hr, 14-orbit flight, making China only the third nation to place a person in orbit. A second mission, involving two astronauts, occurred in Oct., 2005. China also launched an unmanned moon mission in Oct., 2007. In June, 2012, the three-person Shenzhou 9, which included China's first woman astronaut, manually docked with the Tiangong 1 prototype space station, which had launched the year before, and the crew of Shenzhou 10 docked with the station in 2013. Tiangong 2, a testing laboratory and precursor to a more permanent space station, was launched in Sept., 2016, and the crew of Shenzhou 11 docked with the station for a monthlong mission the following month.


See T. Wolfe, The Right Stuff (repr. 1983); B. C. Murray, Journey into Space (repr. 1990); V. Neal, Where Next, Columbus?: The Future of Space Exploration (1994); J. Harford, Korolev: How One Man Masterminded the Soviet Drive to Beat America to the Moon (1997); T. A. Heppenheimer, Countdown: A History of Space Flight (1997); F. J. Hale, Introduction to Space Flight (1998); R. D. Launius, Frontiers of Space Exploration (1998); C. Nelson, Rocket Men: The Epic Story of the First Men on the Moon (2009); A. Chaikin with V. Kohl, Voices from the Moon (2009); D. Brinkley, American Moonshoot (2019.

Space Exploration


flights into space; the aggregate of the branches of science and technology used in the exploration of space and extraterrestrial objects by means of various types of spacecraft. Space exploration includes subjects of astronautics, such as problems of the theory of space flight (calculation of trajectories) and scientific and technical problems (the design of space rockets, engines, on-board control systems, launch complexes, unmanned probes, piloted craft, scientific instruments, ground-based flight control systems, and tracking and telemetry services, as well as the organization and supply of orbital stations). Also studied are medical problems (the development of on-board life-support systems; compensation for unfavorable effects of acceleration, weightlessness, and radiation on the human body) and problems of international law connected with the regulation of the use of space and the planets.

Historical survey. Mankind has long dreamed of penetrating the cosmos, as is reflected in the dreams contained in fairy tales, legends, and science-fiction novels. The numerous and usually unrealizable inventions of the past attest to this. Stories of flight are encountered as early as Assyrian-Babylonian epics, as well as in ancient Chinese and Iranian legends. The Sanskrit epic poem Mahabharata contains instructions for a flight to the moon. The Greek myth about Icarus’ flight to the sun on wings fastened together with wax is widely known. In the second century B.C., Lucian described a flight to the moon on wings.

In the late 19th century the Russian scientist K. E. Tsiolkov-skii was the first to provide a theoretical basis for space flight. In his Investigation of Interplanetary Space by Means of Jet Devices (1903) and in subsequent works, Tsiolkovskii solved a number of basic problems of space flight and demonstrated its technical feasibility. In addition to Tsiolkovskii’s works, those of I. V. Meshcherskii (beginning in 1897), lu. V. Kondratiuk (1919-29), F. A. Tsander (1924-32), N. A. Rynin (1928-32), and other Russian scientists were devoted to the problems of space flight. Outside the USSR, early works in the field were published by R. Esnault-Pelterie (France, 1913), R. Goddard (USA, 1919), and H. Oberth (Gemany, 1923). The first space exploration societies were founded in the 1920’s—in the USSR in 1924, Austria in 1926, Germany in 1927, and Great Britain and the USA in 1930. They worked to promote the ideas of space exploration, as well as to assist in solving practical problems.

Work in rocket technology in the USSR began in 1921; the Gas Dynamics Laboratory (GDL) was organized during that period. Flight tests of rockets using smokeless grainy powder began in 1928, under the direction of N. I. Tikhomirov (the founder of the Gas Dynamics Laboratory). In 1929, V. P. Glushko developed rockets with electric and liquid-propellant motors. The first tests of electric and liquid-propellant rocket motors took place in 1929 and 1931, respectively. In 1932 the Moscow Group for the Study of Jet Propulsion (GIRD) was formed. In 1933, S. P. Korolev directed the first launches of Soviet liquid-propellant rockets designed by M. K. Tikhonravov and F. A. Tsander. In late 1933 the GDL and GIRD were merged to form the Jet Scientific Research Institute. These three organizations made the major initial contribution to the development of Soviet rocketry. The subsequent development of rocket and space technology in the USSR came from the experimental design bureau for the development of liquid-propellant rocket motors, that had grown out of the GDL, as well as from other experimental design bureaus, institutes, and factories.

Experimental work on liquid-propellant rocket motors in the USA was began in 1921 by R. Goddard, and launches of liquid-propellant rockets began in 1926. Bench tests of liquid-propellant motors were begun in Germany by H. Oberth in 1929 and flight tests by J. Winkler in 1931. Germany used liquid-propellant rockets with a range of 250–300 km (the V-2 rocket, designed by W. von Braun) during World War II (1939–45). The potential of the new weapon caused many countries to intensify their efforts in rocket technology after the war, resulting in the development of intercontinental and other ballistic missiles armed with nuclear warheads. This work indirectly furthered the development of the technological basis for space flight.

The space age. Oct. 4, 1957, the date on which the USSR launched the first artifical earth satellite, is considered the dawn of the space age. A second important date is Apr. 12, 1961, the date of the first manned space flight, by Iu. A. Gagarin, the start of man’s direct penetration into space. The third historical event is the first lunar expedition, by N. Armstrong, E. Aldrin, and M. Collins (USA), July 16–24, 1969.

A number of countries have developed and are using spacecraft: the USSR since 1957, and USA since 1958, France since 1965, Japan and the People’s Republic of China since 1970, and Great Britain since 1971. The scale of space research may be judged from the number of satellites of the earth, the sun, the moon, Mars, and Venus launched by the USSR (about 900 as of Jan. 1,1976); earth escape velocity has been imparted to 41 space probes, with a total mass of 110 tons (167 tons, including the final stage of the launch vehicle). Space research is conducted on a similar scale in the USA. As of Jan. 1, 1976, 34 soviet cosmonauts had made space flights in 26 spacecraft and three orbital stations of the Salyut series, and 43 American astronauts had flown in 31 spacecraft and one orbital station. The number of satellites orbited by other countires is as follows: 10 by France, six by Japan, and three by the People’s Republic of China (as of Dec. 1, 1975).

S. P. Korolev was the founder of practical astronautics. By 1957 he had directed the construction of a space center, which made possible the launching of the first artificial earth satellite; later, a number of unmanned spacecraft were successfully orbited. By 1961 the Vostok spacecraft, on which Iu. A. Gagarin made the first flight, had been developed and launched. Korolev directed the development of unmanned interplanetary probes for the study of the moon (up to Luna 9, which made the first soft lunar landing), the first spacecraft of the Zond and Venera series, and the Voskhod spacecraft (the first multiseat spacecraft, from which the first space walk was made). Not limiting his work to the development of launch vehicles and spacecraft, Korolev exercised overall technical supervision of work that created the basis for the first space programs. Important contributions to the development of Soviet rocket technology were also made by design bureaus headed by M. K. Iangel’, G. N. Babakin, A. M. Isaev, and S. A. Kosberg. V. P. Glushko, the founder and head of the experimental design bureau affiliated with the GDL, supervised the development of powerful liquid-propellant rocket motors used by all Soviet launch vehicles (1957-73).

Modern space flight theory is based on celestial mechanics and the theory of vehicle control. In contrast to classical celestial mechanics, this new field is called astrodynamics. Space flight gave rise to the need to develop optimum spacecraft trajectories (selection of the launch time and type of trajectory in order to minimize fuel consumption by the launch vehicle). Changes in trajectory caused by perturbational forces—particularly gravitational fields and the aerodynamic braking effect, which results from interaction of the spacecraft with the rarefied upper layers of the atmosphere (for artificial satellites of planets)—and under the influence of solar radiation pressure (for interplanetary flights) are taken into account. The optimality requirement sometimes results in fairly complex trajectories, with extended interruptions in launch vehicle engine operation (for example, for a lunar, Mars, or Venus launch the spacecraft is first inserted into the earth parking orbit and then injected into a planetary trajectory) and with the use of the gravitational field of a planet (for example, for a lunar mission, which requires an arched trajectory for earth return without firing the rocket motor).

The theory of trajectory corrections is an important branch of astrodynamics. Deviation of the actual trajectory from the calculated trajectory is the result of two factors: distortion of the trajectory by perturbational forces that cannot be predicted (slowing of a satellite by the atmosphere, whose density varies unevenly) and the technically unavoidable minor errors in speed and direction of the spacecraft at the moment of shutdown of the launch vehicle engine (in interplanetary flights, the effect of errors gradually increases). Trajectory correction is accomplished by a short burn of the rocket motor. The theory of correction involves questions of optimizing the correction maneuver (the most advantageous number of such maneuvers and the location of correction points along the trajectory). Knowledge of a spacecraft’s actual trajectory is needed in order to make corrections and perform maneuvers. If the actual orbit is determined on board the spacecraft, such a determination is an integral part of the self-contained navigational system and involves the measurement of angles between stars and planets, the distances of planets, the times of setting and rising of the sun and stars relative to the edge of planets, and the processing of the measurements by an on-board computer, using methods of celestial mechanics.

The development of space centers is a complex scientific and technical problem. Large launch vehicles may have a launch mass of as much as 3,000 tons and may be more than 100 m tall. In order to carry the necessary fuel reserves (90 percent of the total mass), a rocket’s structure must be extremely light; this is achieved through appropriate design and judicious reduction of rigidity and durability requirements. As fuel is consumed in flight, the empty parts of the fuel tanks become superfluous, and their further acceleration results in unwarranted expenditure of fuel. Therefore, multistage launch vehicle configurations (usually two to four stages) are advisable; the stages are jettisoned successively as their fuel tanks become empty.

A modern launch vehicle is a complex package of systems, of which the power plant and the control system are the most important. Chemical liquid-propellant rocket motors are normally used; solid-propellant motors are used less frequently. Motors that use nuclear power are still in the experimental stage (1973); however, there is no doubt that nuclear power will actually be used on future space expeditions. Manned flights to Mars (including a Mars landing) and other similar space programs require tremendous amounts of energy, which can be supplied only by nuclear power sources in combination with chemical sources. Launch vehicle power plants are rated at tens of millions of kilowatts. In the development of powerful and economical liquid-propellant rocket motors for launch vehicles, scientists are concentrating their efforts on the selection of optimum energy-producing fuels and the provision of sufficiently complete combustion in the combustion chamber under high pressures and temperatures. In connection with this, solutions must be found to difficult problems, such as in-flight engine cooling and the achievement of stability of fuel combustion.

Launch vehicle power plants usually consist of several engines, whose operation is synchronized by the guidance system. Guidance systems are normally self-contained—that is, they operate independently of ground stations. They consist of gyroscopic and other primary information sensors, which continuously measure the attitude of the launch vehicle and the accelerations acting on it. Using this information, a computer determines the actual trajectory and controls the vehicle in order to achieve the required combination of coordinates and velocity vector of the rocket at the moment of engine shutdown. The control of a launch vehicle’s attitude is complicated by the vehicle’s low structural stiffness and the large proportion of its mass that is liquid. Therefore, the flexural oscillations of the rocket’s hull and the movement of liquids in the fuel tanks must be taken into account.

The flight readiness of a launch vehicle is checked in the field assembly area of the cosmodrome (in the assembly and testing building), and then the vehicle is transported to the launching pad, where it is erected on the launch pedestal, undergoes pre-launch tests, and is fueled and launched. A spacecraft is considered to have been inserted into orbit if it has exceeded orbital velocity (about 7.91 km/sec) for earth satellites or has attained velocity of the order of earth escape velocity (11.19 km/sec) for spacecraft on missions to the moon, Mars, or Venus. (For flights to the remote planets or the sun a speed considerably greater than earth escape velocity is needed.) During orbital insertion the launch vehicle separates from the spacecraft, which continues its orbital flight mainly by inertia, according to the laws of celestial mechanics.

Spacecraft inserted into orbit may be divided into two groups, earth satellites and space probes for flight to the moon or planets. If significant changes in speed are planned, such spacecraft may be equipped with rocket stages of various degrees of power for braking during planet approach (if the flight plan calls for planetary satellite orbit), for soft landing on a planet lacking an atmosphere, for blastoff from the planet, and for accelerating the spacecraft to the velocity required for its return to earth. It is presumed that in the future economical electric rocket motors will accelerate spacecraft from orbital to higher velocities. A shortcoming of the electric motor is its small thrust, as a result of which acceleration from orbital to escape velocity (or braking from escape to orbital velocity) may last several months. High-capacity sources of nuclear-derived electric power are needed to produce the necessary thrust; this causes additional difficulties connected with the need to protect instruments and crew from harmful radiation.

Spacecraft must be capable of long-term independent operation in space. A number of systems are needed for this purpose: a system for maintaining a specified temperature range; a power-supply system using solar radiation (solar batteries), fuel (electrochemical current generators), or nuclear energy to produce electric power; a system for communicating with earth and other spacecraft; and a guidance system. In addition, a spacecraft carries a wide variety of scientific equipment, from small instruments for studying the properties of space to large telescopes.

The on-board control system integrates the operation of these instruments and systems.

Guidance involves solutions to a number of problems such as attitude control and control of engine burns for trajectory correction during ascent and landing, as well as during rendezvous and other maneuvers performed by two spacecraft. Special control is required for descent to the surface of a planet with an atmosphere. A distinction is made between two kinds of atmospheric descent in which the atmosphere itself is used to slow the spacecraft—controlled descent and uncontrolled (ballistic) descent. The former is characterized by a high degree of touchdown accuracy and a smaller g-load during atmospheric braking. Heat shields are used to protect the descent vehicle from the heat generated by atmospheric braking.

In addition, a number of medical problems arise in the case of a manned spacecraft. It must provide protection of the crew from the space environment (vacuum, harmful radiation, and so on) and must be equipped with a life-support system, which maintains the necessary atmospheric composition and the correct temperature, humidity, and pressure inside the spacecraft. Food and water reserves are provided on short-term flights; for long-term flights, food production—as well as water and oxygen regeneration—must take place on board. Space flight places increased demands on the human body (the effect of weightlessness and of the g-load during the launch and landing phases). Therefore, medical criteria must be used in the cosmonaut selection process. The problem of long-term manned flight under conditions of weightlessness has not yet been resolved.

Descent to the surface of heavenly bodies involves the solution of a number of problems: setting up scientific equipment, conducting experiments using stationary and mobile automatic robots, and—in the future—manned expeditions requiring the construction of temporary or permanent bases.

Space flights usually require a broad network of ground-based control services. Space communications centers are located all over the globe, and where this is not feasible, as in the ocean, ships equipped with communications gear (for example, the Iurii Gagarin and Kosmonavt Vladimir Komarov) are used.

After a spacecraft has returned to earth, the recovery team goes into operation. Its mission is to find and recover the descent vehicle and, in the case of manned flights, to recover the crew, render medical assistance if needed, and take quarantine measures (for crews returning from other planets). To facilitate location of the descent vehicle, it is equipped with a radio beacon whose signals are used for homing by the ships, airplanes, and helicopters of the recovery team. Control of a flight from launch to touchdown involves the efforts of a large number of various services. The job of the administrative technical personnel in charge of a flight is to organize and integrate on-board spacecraft control systems with the numerous ground-based services.

The goals of space exploration may be divided into two groups: scientific studies and practical applications. In addition to the indirect influence of space research on the practical sphere through fundamental scientific discoveries, space exploration makes possible the direct use of spacecraft for practical applications. Artificial satellites circling the globe in high orbits and equipped with repeaters receive signals from a ground station and, after appropriate amplification, return it to earth, where it is received by a station located thousands of kilometers away from the transmitting station. Such communications satellites relay television programs and handle telephone and telegraph communications. Satellites are used in meteorology to produce cloud distribution and earth heat radiation maps, as well as to track cyclones. The information is continuously fed to world meteorological centers and is used for weather forecasting. Satellites whose orbital paths have been determined with great accuracy are used for marine and aviation navigation services; they transmit their precise coordinates to ships and aircraft during periods of radio contact. Any object can establish its own coordinates by determining its position relative to the navigation satellite.

Satellites are playing an ever-increasing role in surveying the earth’s natural resources and in continuous checks on their condition. Photography of the earth’s surface through various light filters, as well as other research methods, makes it possible to see the distribution of vegetation and changes in the snow cover, as well as river flooding and the condition of crops and forests; to observe the progress of work in the fields; to estimate the expected harvest; and to record the occurrence of forest fires. Oceanographic and hydrological studies may also be conducted using satellites. The use of satellites is of particular value in geodesy and topography—for precise fixing of points located at great distances from each other and the fast updating of topographic maps through photographs from space, as well as for setting up geodetic reference networks by tracking satellites (whose coordinates are known precisely at every moment) from various ground stations. The particular features of space flight, such as weightlessness and vacuum, may be used for certain particularly delicate industrial processes. For this purpose appropriate industrial equipment will be placed on satellites, and space shuttles will supply them with raw materials and transport the manufactured products to earth.

A considerable number of specialized unmanned satellites (astronomical, solar, geophysical, geodetic, weather, communications, and others), as well as long-term multipurpose manned orbital stations, are needed to solve the problems associated with the exploration of near-earth space. Crew transfer will be carried out as required, by space shuttles making regular trips between an orbital station and ground space centers.

The immediate goal of lunar and planetary exploration is to produce new scientific data. Plans include the continued study of the moon by both manned and unmanned spacecraft, followed by the establishment of a scientific base on the lunar surface. Flights to Mercury, Venus, Mars, Jupiter, and other planets of the solar system are being made by unmanned crafts, and manned landing missions to Mars (with an expedition time of about three years) are seen as possible in the 1980’s and 1990’s. The study of remote planets and flights out of the solar system and to the sun will long remain feasible only for unmanned spacecraft. The very long duration of such flights requires a new step forward in technological progress in order to develop equipment of very high reliability. In the future, space exploration will make it possible for man to harness the material and energy resources of the universe.

Space exploration, by its very nature, is a field involving the efforts of all mankind, and even if carried out within the framework of national interests, it affects the interests of many counties. (See Table 1 for the major events of the space age.)


Table 1. Major events of the space age
Date of launchType of mission
Oct. 4, 1957First artificial satellite, Sputnik (USSR)
Nov. 3, 1957Biological satellite Sputnik 2 with the dog Laika on board (USSR)
Feb. 1, 1958First satellite of the American Explorer series
May 15, 1958Sputnik 3 satellite (geophysical laboratory) (USSR)
Jan. 2, 1959Lunar fly-by of unmanned Luna 1 probe; first artificial satellite of the sun (USSR)
Mar. 3, 1959Pioneer 4, first American artificial satellite of the sun
Sept. 12, 1959Impact of Luna 2 probe on the lunar surface, Sept. 14, 1959 (USSR)
Oct. 4, 1959Circumlunar flight of Luna 3 probe; transmission to earth of first pictures of the far side of the moon (USSR)
Apr. 1, 1960Weather satellite of the Tiros series (USA)
Apr. 13, 1960Navigation satellite of the Transit series (USA)
Feb. 12, 1961Fly-by of Venus by Venera 1 probe; May 19–20, 1961 (USSR)
Apr. 12, 1961First manned orbital flight, by lu. A. Gagarin aboard the Vostok spacecraft (USSR)
May 5, 1961First suborbital flight, by A. Shepard aboard the Mercury spacecraft (USA)
Aug. 6, 1961Day-long orbital flight, by cosmonaut G. S. Titov aboard the Vostok 2 spacecraft (USSR)
Feb. 20, 1962First orbital flight, by astronaut J. Glenn aboard the Mercury spacecraft (USA)
Mar. 7, 1962First orbiting solar observatory (USA)
Mar. 16, 1962First satellite of the Cosmos series (USSR)
Apr. 23, 1962First lunar probe of the Ranger series; photographs of lunar surface before impact (USA)
Aug. 11–12, 1962First group flight, by cosmonauts A. G. Nikolaev and P. R. Popovich aboard the Vostok 3 and Vostok 4 spacecraft (USSR)
Aug. 27, 1962Venus fly-by of first probe of the Mariner series, Dec. 14, 1962 (USA)
Oct. 31, 1962Anna 1B geodetic satellite (USA)
Nov. 1, 1962Mars fly-by of the Mars 1 probe, June 19, 1963 (USSR)
June 16, 1963Orbital flight by the first woman cosmonaut, V. V. Tereshkova, aboard the Vostok 6 spacecraft (USSR)
Nov. 1, 1963First maneuvering unmanned satellite of the Polet series (USSR)
Aug. 19, 1964Insertion of Syncom 3 communications satellite into stationary orbit (USA)
Oct. 12, 1964Orbital flight by cosmonauts V. M. Komarov, K. P. Feoktistov, and B. B. Egorov aboard the three-place Voskhod spacecraft (USSR)
Nov. 28, 1964Mars fly-by and study by the Mariner 4 probe, July 15, 1965 (USA)
Mar. 18, 1965Space walk by A. A. Leonov outside the Voskhod 2 spacecraft, commanded by P. I. Beliaev (USSR)
Mar. 23, 1965First orbital maneuvers by the Gemini 3 spacecraft, with astronauts V. Grissom and J. Young aboard (USA)
Apr. 23, 1965First communications satellite of the Molnia 1 series in synchronous orbit (USSR)
July 16, 1965First heavy research satellite of the Proton series (USSR)
July 18, 1965Zond 3 probe photographs far side of moon and transmits pictures to earth (USSR)
Nov. 16, 1965Venera 3 probe lands on Venus, Mar. 1, 1966 (USSR)
Nov. 26, 1965First French A-1 satellite
Dec. 4–15, 1965Rendezvous of Gemini 7 and Gemini 6 spacecraft with astronauts F. Borman, J. Lovell, W. Schirra, and T. Stafford aboard (USA).
Jan. 31, 1966First soft lunar landing, by Luna 9 probe, Feb. 3, 1966; pictures of lunar landscape transmitted to earth (USSR)
Mar. 16, 1966Manual docking of Gemini 8 spacecraft, piloted by astronauts N. Armstrong and D. Scott, with an Agena rocket (USA)
Mar. 31, 1966Luna 10 probe, first lunar satellite (USSR)
May 30, 1966First soft lunar landing by a probe of the Surveyor series (USA)
Aug. 10, 1966Insertion of first probe of the Lunar Orbiter series into lunar orbit (USA)
Jan. 27, 1967During testing of the Apollo spacecraft on the launching pad, fire broke out in the craft’s cabin, killing astronauts V. Grissom, E. White, and R. Chaffee (USA)
Apr. 23, 1967Flight of Soyuz 1 spacecraft, with cosmonaut V. M. Komarov aboard. Parachute system failure caused the death of the cosmonaut during reentry (USSR)
June 12, 1967Descent of unmanned Venera 4 probe into Venusian atmosphere, Oct. 18, 1967, accompanied by transmission of scientific data (USSR)
June 14, 1967Fly-by of Venus (Oct. 19, 1967) by Mariner 5 space probe (USA)
Sept. 15 and Nov. 10, 1968Circumlunar navigation and return to earth by Zond 5 and Zond 6 spacecraft, using ballistic and controlled descent (USSR)
Dec. 7, 1968First orbiting astronomical observatory (USA)
Dec. 19, 1968Stationary communications satellite of the Intelsat 3B series (USA)
Dec. 21, 1968Circumlunar flight, including lunar orbit insertion (Dec. 24, 1968) and return to earth, by Apollo 8 spacecraft, piloted by astronauts F. Borman, J. Lovell, and W. Anders (USA)
Jan. 5 and 10, 1969Studies of atmosphere of Venus by Venera 5 (May 16, 1969) and Venera 6 (May 17, 1969) unmanned probes (USSR)
Jan. 14–15, 1969First orbital docking of Soyuz 4 and Soyuz 5 spacecraft, with cosmonauts V. A. Shatalov, B. V. Volynov, A. S. Eliseev, and E. V. Khrunov aboard. The last two cosmonauts exited their craft and transferred to the other spacecraft (USSR).
Feb. 24 and Mar. 27, 1969Fly-by of Mars by Mariner 6, July 31, 1969, and Mariner 7, Aug. 5, 1969 (USA)
May 18, 1969Apollo 10 circumlunar flight, with astronauts T. Stafford, J. Young, and E. Cernan aboard; selenocentric orbit insertion, May 21, 1969; orbital maneuvers and return to earth (USA)
July 16, 1969First lunar landing, by Apollo 11 spacecraft. Astronauts N. Armstrong and E. Aldrin spent 21 hr 36 min on the lunar surface in the Sea of Tranquillity (July 20–21, 1969). M. Collins remained in selenocentric orbit in the command module. Having accomplished the flight program, the astronauts returned to earth (USA)
Aug. 8, 1969Circumlunar flight and return to earth by Zond 7 probe using controlled descent (USSR)
Oct. 11, 12, and 13, 1969Group flight with maneuvers of the Soyuz 6, Soyuz 7, and Soyuz 8 spacecraft, with cosmonauts G. S. Shonin and V. N. Kubasov; A. V. Filipchenko, V. N. Volkov, and V. V. Gorbatko; and V. A. Shatalov and A. S. Eliseev (USSR)
Oct. 14, 1969First research satellite of the Intercosmos series, carrying scientific instruments from socialist countries (USSR)
Nov. 14, 1969Apollo 12 lunar landing in Oceanus Procellarum (Ocean of Storms). Astronauts C. Conrad and A. Bean spent 31 hr 31 min on the lunar surface (Nov. 19–20, 1969). R. Gordon remained in selenocentric orbit (USA)
Feb. 11, 1970Ohsumi, the first Japanese satellite
Apr. 11, 1970Apollo 13 circumlunar flight by astronauts J. Lovell, J. Swigert, and F. Haise. Planned lunar landing cancelled as a result of an emergency aboard the spacecraft (USA)
Apr. 24, 1970First Chinese satellite
June 1, 1970Soyuz 9 flight, lasting 425 hr, by cosmonauts A. G. Nikolaev and V. I. Sevast’ianov (USSR)
Aug. 17, 1970Soft landing on surface of Venus by Venera 7 unmanned probe (USSR)
Sept. 12, 1970Soft lunar landing by Luna 16 unmanned probe in Mare Fecunditatis (Sea of Fertility), Sept. 20, 1970; spacecraft drilled into the surface, took soil samples, and successfully launched payload to earth (USSR)
Oct. 20, 1970Circumlunar flight and return to earth on northern hemisphere side by Zond 8 (USSR)
Nov. 10, 1970Unmanned Luna 17 probe landed remote-control Lunokhod 1 roving vehicle, with scientific instrumentation. During 11 lunar days it traveled 10.5 km, studying an area in Mare Imbrium (Sea of Rains) (USSR)
Jan. 31, 1971Apollo 14 lunar landing in the vicinity of the Fra Mauro crater. Astronauts A. Shepard and E. Mitchell spent 33 hr 30 min on the lunar surface (Feb. 5–6, 1971). S. Roosa remained in selenocentric orbit (USA)
Apr. 19, 1971First Salyut long-term piloted orbital station (USSR)
May 19, 1971First Mars landing. Mars 1 probe orbited the planet on Nov. 27, 1971 (USSR)
May 28, 1971Mars 3 probe orbited the planet Dec. 2, 1971; ejected first soft-landing Mars lander
May 30, 1971Mariner 9, first artificial satellite of Mars, inserted into Mars orbit Nov. 13, 1971 (USA)
June 6, 1971570-hr flight by cosmonauts G. T. Dobrovol’skii, V. N. Volkov, and V. I. Patsaev in the Soyuz 11 spacecraft and the Salyut orbital station. During descent to earth, the cosmonauts perished as a result of depressurization of the cabin (USSR)
July 26, 1971Apollo 15 lunar landing. Astronauts D. Scott and J. Irwin spent 66 hr 55 min on the lunar surface (July 30-Aug. 2, 1971). A. Worden remained in lunar orbit (USA)
Oct. 28, 1971Prospero, first British satellite, launched by British launch vehicle
Feb. 14, 1972Luna 20 probe brought to earth lunar soil samples from area adjoining Mare Fecunditatis (USSR)
Mar. 3, 1972Pioneer 10 probe explored asteroid belt (July 1972-February 1973), flew by Jupiter (Dec. 4, 1973), and left the solar system (USA)
Mar. 27, 1972Venera 8 probe made soft landing on Venus, Jul. 22, 1972. Planet’s atmosphere and surface studied (USSR)
Apr. 16, 1972Apollo 16 lunar landing. Astronauts J. Young and C. Duke spent 71 hr 02 min on the lunar surface (Apr. 21–24, 1972). T. Mattingly remained in selenocentric orbit (USA)
Dec. 7, 1972Apollo 17 lunar landing. Astronauts E. Cernan and H. Schmitt spent 75 hr 00 min on the lunar surface (Dec. 11–15, 1972). R. Evans remained in selenocentric orbit (USA)
Jan. 8, 1973Luna 21 probe delivered Lunokhod 2 to the moon, Jan. 16, 1973. Roving vehicle traveled 37 km during five lunar days (USSR)
Apr. 6, 1973Fly-by of Jupiter by Pioneer 11 probe on Dec. 3, 1974, at a distance of 43,000 km, or three times closer than Pioneer 10. The probe then left the solar system (USA)
May 14, 1973Skylab long-term orbital station. Astronauts C. Conrad, P. Weitz, and J. Kerwin spent 28 days on board beginning May 25. Another crew, consisting of A. Bean, O. Garriot, and J. Lousma, boarded the station on July 28 and spent 59.5 days there, and the third crew (astronauts G. Carr, E. Gibson, and W. Pogue) spent 84 days on board, beginning November 16 (USA)
Nov. 5, 1973First fly-by of Mercury by Mariner 10 probe (Mar. 29, 1974); the gravitational field of Venus, an intermediate planet, was used for the first time on Feb. 5, 1974, to reach another planet (USA)
Apr. 13, 1974The first regional system communications satellite in the USA and the third in the world (after the USSR and Canada) was put into operation after the insertion of Westar 1 satellite into synchronous orbit (USA)
May 17, 1974First synchronous weather satellite, SMS-1, providing weather data for a hemisphere every 30 min (USA)
May 30, 1974Multipurpose ATS-6 synchronous satellite. On July 2 the satellite began relaying educational programs directly to commercial TV sets in remote areas in the USA and India. ATS-6 was successfully used during the experimental flight of the Soyuz and Apollo spacecraft (EPAS) in July 1975
Aug. 30, 1974First Dutch satellite, ANS, placed in orbit by an American launch vehicle
Nov. 15, 1974First Spanish satellite, lntasat-1, placed in orbit along with two American satellites by an American launch vehicle
Dec. 10, 1974Helios 1, first space probe in Western Europe (Federal Republic of Germany), placed into orbit by an American launch vehicle
Dec. 26, 1974Salyut 4 long-term piloted orbital station. Cosmonauts A. A. Gubarev and G. M. Grechko spent 29.5 days on board, beginning Jan. 12, 1975. P. I. Klimuk and V. I. Sevast’ianov boarded the station on May 26 and spent about 63 days there (the longest flight in Soviet space exploration). On November 19, automatic docking of Soyuz 20 unmanned spacecraft (launched on November 17) with Salyut 4 orbital station took place (USSR)
Feb. 6, 1975Second synchronous weather satellite, SMS-2. Combined with the SMS-1, it provides round-the-world weather monitoring every 30 min (USA)
Apr. 19, 1975First Indian satellite, Aryabhata, placed in orbit by a Soviet launch vehicle
June 8, 1975, and June 14, 1975Venera 9 and Venera 10, first artificial satellites of Venus (inserted into Venus orbit on Oct. 22 and 25, 1975, respectively). Soft landing of Venus landers ejected from the spacecraft; first pictures of the planet at the landing site (USSR)
July 15, 1975First international space experimental program (EPAS) involving manned spacecraft Soyuz 19 (USSR) and Apollo (USA)
Aug. 25, 1975, and Sept. 5, 1975Flight to Mars of Viking 1 and Viking 2 unmanned probes for the purpose of detecting any signs of life on the planet (USA)
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