Space exploration

Space Exploration, science and engineering of manned and unmanned space travel. 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.
Manned and unmanned space probes have provided a vast new source 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 Union of Soviet Socialist Republics (USSR) in October 1957 and of Explorer 1 by the United States in January 1958. In October 1958 the National Aeronautics and Space Administration (NASA) was created in the United States. During the next two decades, more than 1,600 spacecraft of all varieties were launched, mostly in earth orbit. Twelve men walked on the moon's surface and returned to earth. By 1986 several thousand objects-mostly spent, upper stages of space-launch vehicles and inert spacecraft-were circling the earth. Operational spacecraft numbered about 300.

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. At 30 km (19 mi) above sea level, the barometric pressure is one-eightieth of that at sea level; at 60 km (37 mi), it is 1/3,600; at 90 km (56 mi) it is 1/400,000. 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 meteoroids 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. Objects in a spacecraft are said to be weightless when it is in orbit around the earth (or around any other celestial body) because they do not experience the normal effects of weight, since all are moving in the same way under the influence of gravity. Under these conditions, the objects float freely in the craft.
Aerodynamic forces on the lifting surfaces (for example, the wings) of an aircraft keep it up 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 wings for propulsion and manoeuvring, but spacecraft cannot do so because of the lack of air. A space vehicle must rely on the reaction of rockets for propulsion and manoeuvres, using Newton's laws of motion. When a spacecraft fires a rocket blast in one direction, the reaction imparts momentum to the spacecraft in the opposite direction.

Human Beings in Space

Space is a hostile environment for human beings 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 may also be fatal to an unshielded person who is not protected by the atmosphere of the earth. These environmental conditions can 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 travellers in space (see Aerospace Medicine).
People can be protected against the space environment in several ways. At present, 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. 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 might 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 might be reduced by spinning the craft to provide artificial gravity. For this purpose, the spacecraft might be shaped like a large wheel that spins slowly around its own axis, or like a dumbbell, rotating end over end.

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 2nd 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. The French writer and philosopher Voltaire, in Micromégas (1752), told of the travels of certain inhabitants of Sirius and Saturn; and 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 English writer H. G. Wells, who published The War of the Worlds in 1898 and The First Men in the Moon in 1901. More recently, fantasies of spaceflight have been 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 3rd 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 2nd century BC. In the 2nd century AD Ptolemy of Alexandria placed the earth at the centre of the solar system in his cosmic scheme, now called the Ptolemaic system.

Scientific Discoveries
Not until some 1,400 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. Galileo, Edmund Halley, Sir William Herschel, and Sir James Jeans were other astronomers 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 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 also 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 onwards, 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 1,830 m (6,000 ft).
In the United States, 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, a German law student, conceived of a solid-propellant spaceship that demonstrated his 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, published Die Rakete zu den Planetenräumen (The Rocket into Interplanetary Space). The book was supplemented by Walter Hohmann, 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 United States, the USSR, 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 US Army brought back a number of the V-2s, which were then used in the United States, in vertical flights, for experimental research. Some German engineers went to the USSR after the war, but the leading rocket experts went to the United States, including Walter Dornberger, and Wernher von Braun. See Guided Missiles.

Spacecraft
Spacecraft that do not have to carry human beings can be of a great variety of sizes, from a few centimetres to several metres in diameter, and of many shapes, depending on the purposes for which they are designed. Spacecraft that do not carry a crew have radio-transmitting equipment, both to relay information back to earth and to signal the position of the spacecraft.
Manned spacecraft must fulfil more exacting requirements than unmanned vehicles because of the needs of the human occupants. A manned space vehicle is designed to provide air for the astronauts, food and water, navigation and guidance equipment, seating and sleeping accommodation, and communication equipment, so that the astronauts can send and receive information. A distinctive feature of manned spacecraft is the heat shield that protects the vehicle as it reenters the atmosphere (see "Launching and Reentry" below).

Propulsion
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. Most of the rockets that have launched American spacecraft have had several separate rocket stages; each stage is separately powered with its own fuel. After the fuel in each stage has been consumed, the empty stage drops away from the spacecraft.
Because the technology to build space-launch vehicles is closely akin to that for long-range ballistic missiles, the United States and the USSR were the only two countries that had the ability to launch satellites from 1957 to 1965. In subsequent years France, Japan, India, and China launched earth satellites of ever-increasing sophistication, and in May 1984 the 13-member European Space Agency began its own launch programme from a space centre at Kourou in French Guiana. The United States and the USSR, however, remained the only nations with launch vehicles capable of placing in orbit payloads of many tonnes-the prerequisite for manned spaceflight.

Launching and Reentry
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 centre. When all preparations are complete, the rocket engines are fired and the rocket and spacecraft lift off.
Reentry poses the problem of slowing down a returning spacecraft so that it lands on earth without being destroyed by aerodynamic heating. The spaceflights of the US Mercury, Gemini, and Apollo programmes 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 the astronauts. 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. Before the development of the space shuttle, which lands on a runway (see "Space Shuttle" below), all American manned spacecraft 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. Soviet cosmonauts have landed on solid ground in various sites in Siberia.

Orbiting the Earth
The orbit of a spacecraft around the earth can be in the shape of a circle or an ellipse. An artificial 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 that 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 centre. A polar orbit lies in a plane passing through the North and South Poles-that is, 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 equatorial plane is called the inclination of the orbit.
The earth rotates once every 24 hours under a satellite in a polar orbit. A polar-orbit weather satellite, 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.

Space Programmes-Unmanned
The long history of myths, dreams, fiction, science, and technology culminated in the dramatic launching of the first artificial orbiting earth satellite, Sputnik 1, by the USSR on October 4, 1957. Sputnik Zemli, meaning "travelling companion of the world" is the full Russian name for an artificial satellite, a companion of the earth as it travels around the sun.

Early Artificial Satellites
Sputnik 1 was an aluminium sphere, 58 cm (23 in) in diameter, weighing 83 kg (184 lb). It orbited the earth in 96.2 minutes. The elliptical 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 November 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 United States successfully launched its first earth satellite, Explorer 1, from Cape Canaveral (named Cape Kennedy 1963-1973), Florida, on January 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 micrometeoroids 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 United States 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 1,327-kg (2,925-lb) Soviet spacecraft measured solar radiation, cosmic rays, magnetic fields, and other space phenomena until the craft's orbit decayed in April 1960.

Unmanned Lunar Missions
As the closest neighbour of the earth, the moon has been the objective of many space missions. In 1958 the first attempts by the United States and the USSR at lunar probes failed. The Russian Luna 2, launched on September 12, 1959, hit the moon 36 hours 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 October 4, 1959. One of the most dramatically successful moon shots was the mission accomplished by Ranger 7, launched by the United States on July 28, 1964. Just before hitting the side of the moon that faces the earth, it transmitted 4,316 television pictures of the lunar surface from altitudes of about 1,800 km (1,120 mi) to about 300 m (1,000 ft), giving earth-bound human beings their first close-up view of the moon.
On January 31, 1966, the USSR launched Luna 9, which made the first soft landing on the moon-that is, it landed without being destroyed. The United States 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 centred on the American programme to land a man on the moon. To this end, a number of further unmanned moon flights were undertaken, among which were two soft landings made by Surveyors 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 analysed the lunar surface, using an alpha-particle scattering technique; this was the first on-site analysis of an extraterrestrial body.
Another spacecraft supporting 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 programme.
Two other unmanned, automated lunar projects by the USSR are noteworthy. The Luna 16 spacecraft, launched on September 12, 1970, landed on the moon and placed about 113 g (4 oz) of lunar soil in a sealed container that was then launched from the moon and recovered in the USSR. Luna 17, launched on November 10, 1970, soft-landed 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, travelled 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.

Scientific Satellites
As space-launch vehicles (rocket boosters) and scientific measuring devices became more reliable, a wide variety of satellites was developed. Scientists were eager to obtain data and make accurate studies of the sun, other stars, the earth, and space itself. The enveloping atmosphere of the earth prevents such data from being obtained from the earth's surface, except in a limited way through the use of high-altitude balloons.
In the United States, many astronomical satellites have been launched. Since 1962, for example, the Orbiting Solar Observatories (OSO) have studied the sun's ultraviolet, X-ray, and gamma radiation. Pioneer satellites have studied cosmic radiation, the solar wind, and the electromagnetic characteristics of space. The Orbiting Astronomical Observatories (OAO) have observed stellar radiation, and Orbiting Geophysical Observatories (OGO) have studied the relationships between the sun, the earth, and their space environment. The Infrared Astronomy Satellite (IRAS), an Anglo-American project launched in 1983, has probed the hidden reaches of our galaxy. The Hubble Space Telescope was launched by the space shuttle Discovery in 1990. Although scientists discovered soon after the space telescope began operating that the telescope's main mirror was flawed, astronauts aboard the space shuttle Endeavour repaired the Hubble Space Telescope in December 1993. Even before the repair, however, the telescope was able to transmit valuable images-some of never-before-observed phenomena-back to astronomers on earth.

Applications Satellites
This class of unmanned spacecraft also performs useful functions for the earthbound scientist. The three general classifications of such satellites are communications, environmental, and navigation satellites.
Environmental satellites observe the earth and atmosphere, and transmit images for a variety of purposes. Weather satellites provide daily transmissions of temperatures and cloud patterns. One example is the Synchronous Meteorological Satellite (SMS). From stationary orbit, it sends pictures of a large area of the earth's surface at 30-minute intervals. Two SMS's can cover an entire continent and adjacent ocean areas.
The US Landsats observe the earth with multispectral optical scanners and transmit the data to ground stations. Processed into colour images, these pictures reveal data of great range and great potential value. Information on soil characteristics, water and ice quantities, coastal-water pollution, salinity, and insect blights of crops and forests are 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 (Systeme Probatoire pour Observation de la Terre), a European satellite launched in 1985, transmits images that show the earth in even greater detail than Landsats can. See also Remote Sensing.
Earth observation satellites are used by the United States and other countries to obtain images of military value, as 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 the American proposal to develop a satellite antiballistic missile defence system making use of laser technology (see Strategic Defence Initiative).
Navigation satellites provide a known observation point orbiting the earth that, when observed by ships and submarines, can fix the vessel's position within a few yards. The US Navy Transit Satellite is now available to commercial maritime fleets that can measure the Doppler shift of the satellite's signal, and a complex Navstar family of satellites is being deployed for military and commercial use.

Planetary Studies
Beyond the moon, spacecraft have landed on Mars and Venus, have flown by every planet except Pluto, and have made comet studies.

Mars
The USSR launched Mars 2 and 3 in May 1971, two probes that crash-landed on Mars but transmitted data briefly. In August 1973, the USSR launched Mars 4, 5, 6, and 7; but various technical malfunctions plagued all the missions. 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.
In the US programme, 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 colour television cameras, weather and seismographic instruments, and a 3-m (10-ft) retractable claw designed to be manipulated from the earth. Both functioned well for several years.

Venus
The USSR's programme to penetrate the dense, cloud-covered atmosphere of Venus met with great success. Venera 7 was launched in August 1970 and survived long enough to transmit 23 minutes of temperature data. Venera 8, launched in 1972, transmitted surface data that included soil analysis. In October 1975, Veneras 9 and 10 placed landers on the surface; both survived for an hour and relayed the first photographs of the Venusian surface. In 1978, Veneras 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, Veneras 13 and 14 landed on Venus. The craft relayed photographs of the planet's surface and analysed the chemical composition of the atmosphere and soil. On October 10 and 14, 1983, Veneras 15 and 16 entered orbit around Venus and returned radar images; and in June 1985, Vegas 1 and 2, en route to Halley's comet, released four probes into the Venusian atmosphere.
The US Pioneer Venus 1, an orbiter, and 2, consisting of five atmospheric probes, were launched on May 20 and August 8, 1978, and they reached Venus on December 5 and 9, 1978. The orbiter mapped nearly the entire surface of Venus, and the probes analysed the composition and movement of the atmosphere and its interaction with the solar wind. The Magellan probe was launched towards Venus from a space shuttle in 1989 and began transmitting radar images of the planet's surface in August 1990.

Mercury
The planet nearest the sun came under scrutiny when the United States sent Mariner 10 on a journey through the inner solar system in October 1973, on a journey to Mercury. The spacecraft passed Venus in February 1974 and used the planet's gravity to enter a solar orbit. In March it came within 692 km (430 mi) of Mercury, providing the first views of the planet's moon-like, 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 within 317 km (197 mi).

Jupiter and Saturn
The US 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 craft ever sent into interstellar space; it is expected to encounter its first star in about 80,000 years. Pioneer 11 travelled by Saturn in September 1979, preparing the way for Voyagers 1 and 2.
Launched in 1977, the spectacularly successful Voyagers 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.

Uranus
After flying past Saturn, Voyager 2 was directed towards Uranus. It passed within 80,000 km (50,000 mi) of the cloud-covered planet in January 1986, discovering four more rings as well as ten 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 within 5,000 km (3,100 mi) of the planet in August 1989 and discovering six additional Neptunian moons before it finally left the solar system.

Space Programmes-Manned

Within a year after the successes of the first small artificial satellites in 1957 and 1958, both the United States and the USSR were developing programmes 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 Programmes
The USSR was first into space with a man, cosmonaut Yury A. Gagarin, who made one orbit of the earth in Vostok 1 on April 12, 1961. During his flight time of 1 hour, 48 minutes, 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 Tereshkova, the first woman to fly in space. Launched on June 16, 1963, she orbited the earth 48 times.
Meanwhile, a similar US programme, called Mercury, was taking shape. On May 5, 1961, Commander Alan B. Shepard, Jr., of the US Navy, became the first American in space. The Mercury spacecraft, named Freedom 7, flew a ballistic trajectory and made a 15-minute suborbital flight. A similar flight followed on July 21, flown by Captain Virgil I. Grissom of the US Air Force. On February 20, 1962, Lieutenant Colonel John H. Glenn, Jr., of the US Marine Corps became the first American astronaut to orbit the earth, in a flight of three orbits. Three additional Mercury flights were made in 1962 and 1963 by Lieutenant Colonel M. Scott Carpenter of the navy, Commander Walter M. Schirra, Jr., also of the navy, and Major Leroy Gordon Cooper, Jr., of the air force.

Voskhod and Gemini Programmes
The Voskhod was an adaptation of the Vostok spacecraft, modified to accommodate two or three cosmonauts. On October 12, 1964, cosmonauts Vladimir M. Komarov, Boris B. Yegorov, and Konstantin P. Feoktistov 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 American astronauts then had a total of 54 man-hours in space. On March 18, 1965, cosmonauts Pavel I. Belyayev and Aleksei A. Leonov were launched in Voskhod 2. During this 17-orbit flight, Leonov performed the first walk in space, or extravehicular activity (EVA), leaving the spacecraft and drifting out on an umbilical tether.
The US Gemini programme was designed to develop the technology required to go to the moon. In May 1961, US President John F. Kennedy had instituted the Apollo programme, 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 programme. 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 were made in 1965-1966.
During the Gemini 4 flight, Major Edward H. White II of the air force became the first US astronaut to perform an EVA. Using a pressurized-gas, jet-manoeuvring device, he spent 21 minutes 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 hours with Schirra and Major Thomas P. Stafford of the air force, Gemini 6 landed. Gemini 7, with Lieutenant Colonel Frank Borman of the air force and Commander James A. Lovell, Jr., of the navy, went on to spend a total of 334 hours in orbit. This flight of nearly 14 days provided medical data on human beings in space that was necessary to assure the success of the 10-day Apollo lunar mission. Furthermore, it demonstrated the reliability of systems such as hydrogen-oxygen fuel-cells. 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, US astronauts had accumulated nearly 2,000 man-hours in space, which exceeded the Soviet cosmonaut total, and about 12 hours in EVA.

Soyuz and Apollo
The year 1967 was one of tragedy for both spacefaring nations. On January 27, during a ground test of the Apollo spacecraft at Cape Kennedy, fire broke out in the three-man command module. Because of the pressurized pure-oxygen atmosphere inside the spacecraft, a flash fire engulfed and killed the three astronauts-Grissom, White, and Lieutenant Commander Roger B. Chaffee of the navy. As a result of this tragedy, the Apollo programme was delayed by more than a year while vehicle design and materials underwent a major review.
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 programme was set back by nearly two years.
In October 1968, the first manned Apollo flight was launched by a Saturn 1B booster. Astronauts Schirra, Major R. Walter Cunningham of the US Marine Reserve Corps, and Major Donn F. Eisele 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 Major William A. Anders of the air force, circled the moon ten times and returned to earth safely. The Apollo 9 flight, with Major James A. McDivitt and Colonel David R. Scott of the air force and a civilian Russell L. Schweickart, tested 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 Lieutenant Commander John W. Young and Commander Eugene A. Cernan 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 command module (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 were accomplished; the two astronauts then transferred to the CM, discarded the LM, fired the service module rocket to enter the return trajectory to earth, and returned safely. Project Apollo was now ready to land astronauts on the moon (see "Human Beings on the Moon" below).
Meanwhile, the USSR launched unmanned Zond spacecraft around the moon, carrying cameras and biological specimens. Colonel Georgi T. Beregovoi 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 and Lieutenant Colonel Yevgeny V. Khrunov, in space suits, transferred by EVA from Soyuz 5 to Soyuz 4, which was piloted by Colonel Vladimir A. Shatalov. In October 1969, Soyuz 6, 7, and 8, 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 Beings on the Moon
In 1969, humankind achieved the long-sought goal of landing on the moon. The historic flight of Apollo 11 was launched on July 16. After entering lunar orbit, Colonel Edwin E. Aldrin, Jr., of the air force and Neil A. Armstrong transferred to the LM. Armstrong, a civilian, was a naval veteran. Lieutenant Colonel 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 Tranquillitatis. 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, and a seismic experiment package. Armstrong and Aldrin also erected an American flag and talked, by satellite communications, with President Richard M. Nixon in the White House. They found that walking and running in one-sixth the gravity of earth was not difficult. Also via satellite, millions of people watched live television broadcasts from the moon. Returning to the LM and discarding their space suits, the two astronauts rested for several hours before takeoff. They left the moon in the ascent stage of the LM, using the lower half, 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 was without mishap and the vehicle splashed down on July 24 in the Pacific Ocean near Hawaii and was recovered.
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 in quarantine for three weeks. They remained in good health.

Apollo 12
The next moon-landing flight began on November 14, 1969, when Apollo 12 was launched with astronauts Charles Conrad, Jr., Richard F. Gordon, Jr., and Alan L. Bean, 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 command module piloted by Gordon, successful splashdown and recovery took place on November 24. Quarantine procedures were repeated, but 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 was launched, carrying the veteran astronaut Lovell and the civilian astronauts Fred W. Haise, Jr., and John L. Swigert, Jr. The craft met with disaster during the flight when an oxygen tank ruptured. The astronauts were obliged to cancel their planned landing on the lunar surface. Instead, using the power and survival systems of the LM, they swung behind the moon and were then brought back to earth 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 January 31, 1971, after modifications were carried out in the spacecraft to prevent the malfunctions encountered by Apollo 13. Captain Shepard, who had been promoted after his successful suborbital flight in 1961 (see "Vostok and Mercury Programmes" above), and Commander Edgar D. Mitchell, also of the navy, successfully landed the LM in the rugged Fra Mauro region of the moon, while astronaut Stuart A. Roosa of the air force remained in lunar orbit in the command module. Shepard and Mitchell spent more than 9 hours exploring an area believed to contain some of the oldest rocks yet recovered, collecting about 43 kg (96 lb) of geological samples and deploying scientific instruments. The astronauts returned to earth without incident on February 9, 1971.
Apollo 15 was launched on July 26, 1971, carrying three air force officers: Colonel Scott as flight commander, Lieutenant Colonel James B. Irwin as pilot of the LM, and Major Alfred M. Worden as pilot of the command module. Scott and Irwin spent 2 days, 18 hours on the lunar surface at the edge of Mare Imbrium, close to the 366-m (1,200-ft) deep Hadley Rille and the Apennine mountain range, one of the highest on the moon. During their 18 hours, 37 minutes of 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 (200 lb) of rocks, including what was believed to be 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-minute spacewalk while the spacecraft was about 315,400 km (196,000 mi) from the earth, a record distance for an EVA. The Apollo 15 astronauts splashed down without incident on August 7, about 530 km (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., and Thomas Kenneth (Ken) Mattingly 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 hours, 14 minutes on the moon, setting up a number of experiments powered by a small nuclear station, travelling about 26.6 km (16.5 mi) in the lunar rover, and collecting more than 97 kg (214 lb) of rock samples.
The projected missions to the moon by the United States were concluded with the flight of Apollo 17, December 6-19, 1972. During their smooth 13-day voyage, the veteran astronaut Cernan and the American civilian geologist Harrison H. Schmitt spent 22 hours on the moon, travelling 35 km (22 mi) in the lunar rover and exploring the Taurus-Littrow Valley region, while Commander Ronald E. Evans 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. Many valuable new experiments and astronomical observations could now be performed.

Soviet Stations
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 manned-flight duration record of 24 days. A large number of 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, Vladislav N. Volkov, and Viktor I. Patsayev-were found dead, victims of an air leak in a valve. Because they wore no space suits, the cosmonauts had been killed quickly. The Soviet programme suffered another setback. The Salyut 2 space station was launched in April 1973, but 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 (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, during the flight of Soyuz T12 on July 17-29, 1984. One of the most notable flights of the Salyut/Soyuz series occurred in 1984 when cosmonauts Leonid Kizim, Vladimir Solovyov, and Oleg Atkov, spent 237 days aboard the Salyut 7 before returning to earth, the longest space flight to that date. Salyut 7, now abandoned, remains in orbit.
The Mir space station, which the Soviets designed as a successor to the Salyut series, was launched on February 19, 1986. Described by the Soviets as the core of the first permanently staffed space station, it features six docking ports and can accommodate two cosmonauts. In 1987, Colonel Yuri Romanenko 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 the supernova that had recently exploded in a nearby galaxy, the Large Magellanic Cloud. X-rays from the exploding star, blocked by the earth's atmosphere, could not be detected from the earth. In 1987-1988, Soviet cosmonauts Vladimir Titov and Musa Manarov set a new record for time spent in space-366 days.

US Stations
The US Skylab programme was much more extensive and complex than the Soviet Salyut programme. Skylab, launched by the first two stages of a Saturn 5 rocket, weighed 88,900 kg (196,000 lb) compared with the 18,600-kg (41,000-lb) Salyut. In contrast to the estimated 99-cu m (3,500-cu ft) interior space of Salyut, Skylab had 357 cu m (12,600 cu ft), about 3.5 times greater. Skylab served as a laboratory in earth orbit. It was used to make solar-astronomical studies, long-duration medical studies of the three-person crew, extensive multispectral observations of the earth, and a variety of 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, Commander Joseph P. Kerwin, and Commander Paul J. Weitz, 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 and the third and final crew, 84 days. The Skylab project was considered completely successful. More than 740 hours were spent in observing the sun with telescopes, and 175,000 solar pictures were returned to earth, as were about 64 km (40 mi) of electronic data tape and 46,000 photographs of the earth's surface. On July 11, 1979, during orbit number 34,981, Skylab plunged to earth, raining fiery debris over sparsely populated western Australia and over the Indian Ocean.
The US government, in cooperation with Russia, Canada, Japan, and the 13-member European Space Agency, is planning for a permanent space station that is to be assembled in space. The space station, called Alpha, is projected to be complete around 2002.

Current and Future Programmes
In the early 1980s, the Space Transportation System (STS), better known as the space shuttle, became the major American space programme. Problems with the STS later led to resumption of the use of expendable launch vehicles (ELVs) for launching satellites. The United States had planned to replace the space shuttle with a new spacecraft, the X-30, in the 1990s but, faced with budgetary constraints, decided to rely instead on a mixed fleet of ELVs and space shuttles to place payloads into orbit for the remainder of the decade.

Space Shuttle
The shuttle, a manned, multipurpose, orbital-launch spaceplane, 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 planned lifetime of perhaps 100 missions, and the winged orbiter could make unpowered landings on returning to earth. Because of the shuttle's intended 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 other, unmanned missions.
The first space shuttle mission, piloted by John W. Young and Robert Crippen aboard the orbiter Columbia, was launched on April 12, 1981. It was a test flight flown without a payload in the orbiter's cargo bay. The fifth space shuttle flight was the first operational mission; the astronauts in the Columbia deployed two commercial communications satellites from November 11 to 16, 1982. Later memorable flights included the seventh, whose crew included the first US woman astronaut, Sally K. Ride; the ninth mission, November 28-December 8, 1983, which carried the first of the European Space Agency's Spacelabs; the 11th mission, April 7-13, 1984, during which a satellite was retrieved, repaired, and redeployed; and the 14th mission, November 8-14, 1984, when two expensive malfunctioning satellites were retrieved and returned to earth.
Despite such successes, the shuttle was falling behind in its planned launch programme, was increasingly being used for military tests, and was meeting stiff competition from the European Space Agency's unmanned Ariane programme for the launching of satellites. Then, on January 28, 1986, the shuttle Challenger was destroyed about one minute after launch because of the failure of a sealant ring on one of its solid boosters. The booster nosed into the main propellant tank of liquid hydrogen and oxygen, causing a nearly explosive disruption of the entire system. Seven astronauts were killed in the disaster: commander Francis R. Scobee, pilot Michael J. Smith, mission specialists Judith A. Resnik, Ellison S. Onizuka, and Ronald E. McNair, and payload specialists Gregory B. Jarvis and Christa McAuliffe. McAuliffe had been selected the preceding year as the first "teacher in space", a civilian representative of the shuttle programme. The tragedy brought an immediate halt to shuttle flights until systems could be analysed and redesigned. A presidential commission headed by a former secretary of state, William Rogers, and a former astronaut, Neil Armstrong, placed much of the blame on NASA's administrative system and its failure to maintain an efficient system of quality control.
In the aftermath of the Challenger disaster, the O-ring seals on the solid rocket booster (SRB) were redesigned to prevent recurrence of the January 28 failure. The shuttle launch programme resumed on September 29, 1988, with the flight of Discovery and its crew of five astronauts. On this mission, a NASA communications satellite, TDRS-3, was placed in orbit and a variety of experiments were carried out. The success of this 26th mission encouraged the United States to resume an active launch schedule. The long-delayed $1.5-billion Hubble Space Telescope was deployed by space shuttle in 1990 but, because of an optical defect, failed to provide the degree of resolution it was designed to have until it was repaired in December 1993.

Prospects
With setbacks such as the malfunction of the Hubble Space Telescope and leaks in the hydrogen fuel lines of the space shuttle, the US space programme seemed less likely to attain its goals in the 1990s. In addition to the manned space station, goals for this decade had included construction of the X-30, designed to take off from a conventional runway and boost itself into orbit using powerful ramjet engines. More ambitious programmes, such as the establishment of a lunar base or the exploration of Mars by astronauts, were years from realization.