space-time, central concept in the theory of relativity that replaces the earlier concepts of space and time as separate absolute entities. In relativity one cannot uniquely distinguish space and time as elements in descriptions of events. Space and time are joined together in an intimate combination in which time becomes the "fourth dimension." The mathematical formulation of the theory by H. Lorentz (see Lorentz contraction) preceded the interpretation by A. Einstein that space and time are not absolute. The abstract description of space-time was made by H. Minkowski. In space-time, events in the universe are described in terms of a four-dimensional continuum in which each observer locates an event by three spacelike coordinates (position) and one timelike coordinate. The choice of the timelike coordinate in space-time is not unique; hence, time is not absolute but is relative to the observer. A striking consequence is that simultaneity is no longer an intrinsic relation between two events; it exists only as a relation between two events and a particular observer. In general, events at different locations that are simultaneous for one observer will not be simultaneous for another observer. Other relativistic effects, such as the Lorentz contraction and time dilation, are due to the structure of space-time.

space travel: see space exploration; space science.

space 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 platforms. The first space station was the Soviet Salyut 1, launched in Apr., 1971. The Soyuz 10 spacecraft docked with this station, but the crew did not enter it; two months later the cosmonauts aboard the Soyuz 11 spacecraft docked and entered Salyut 1, remaining aboard for 22 days. By 1982 five more Salyut space stations had been orbited successfully, two of them for military purposes. By rotating the crews regularly, the Soviets were able to staff the stations for extended periods. All the Salyut space stations have decayed and are no longer in orbit.

During this period the United States launched its only true space station. Called Skylab, it was placed in orbit in May, 1973. Skylab housed three three-person crews, the last remaining aboard for 84 days, which at that time was a record for continuous residency in space. Among the tasks accomplished by the Skylab astronauts were biomedical studies to evaluate the effects of weightlessness, photographing the earth to monitor volcanoes and earthquake faults, astronomical observations of optical sources (including extensive studies of Comet Kohoutek), and materials-processing activities such as brazing and welding (to see how they were affected by the lack of gravity). Skylab fell to earth in July, 1974, showering debris over uninhabited parts of Australia and the Indian Ocean.

The Soviet Union launched the core module of the Mir space station in Feb., 1986. It was enlarged several times so that it could accommodate a crew of up to six cosmonauts. The Mir program was enhanced by having international teams conduct experiments at the station; Afghanistan, Bulgaria, France, Germany, Great Britain, India, Japan, Syria and the United States, in addition to Russia and other nations that were formerly part of the Soviet Union, participated. In 1995, Mir cosmonaut Valery Polyakov set an endurance record of nearly 439 days in space, eclipsing the previous record of 326 days set in 1987 by Yuri Romanenko (also while on Mir). In Aug., 1999, its extended 13-year mission concluded, Mir was abandoned. During its lifetime, it orbited the earth 86,331 times and was home to 104 people, including 42 Russian cosmonauts and 7 American astronauts. In Mar., 2001, Mir fell to earth, the largest spacecraft (143 tons/130 metric tons) ever to decay, showering an estimated 1,500 fragments of 44 lb (20 kg) or more over an uninhabited area 120 mi (193 km) wide by 3,600 mi (5795 km) long in the South Pacific.

The United States, Japan, Canada, Brazil, Russia, and 11 members of the European Space Agency (ESA)—Belgium, Denmark, France, Germany, Italy, the Netherlands, Norway, Spain, Sweden, Switzerland, and the United Kingdom—are developing a space station that is being assembled in space. Each partner is contributing a portion of the complex, called the International Space Station (ISS). For example, of the six laboratories that will be included, three will be provided by Russia, one by the United States, one by Japan, and one by ESA. The first element, Zarya (the control module), was orbited by a Russian Proton rocket in Nov., 1998. A month later the U.S. space shuttle Endeavour initiated the first assembly sequence of the ISS, linking the Unity module, a passageway that connects living and work areas of the station, to Zarya. In July, 2000, the Russian-built Zvezda service module was mated with the two existing components. The first permanent crew—two Russian cosmonauts and an American astronaut—began living aboard the ISS in Nov., 2000, marking the beginning of what is anticipated to be a continuous human presence. When planned construction is completed, the ISS will be 290 ft (88.5 m) long, have a wingspan of 396 ft (120.7 m), and be 143 ft (43.5 m) tall. Its mass will be nearly a million pounds (454,000 kg), and it will support a crew of seven. The plan involves more than 40 space flights by at least three space vehicles (the space shuttle, the Russian Soyuz rocket, and the Russian Proton rocket) to deliver the various ISS components to earth orbit. Assembly of the more than 100 components has utilized a combination of human spacewalks and robot technologies. Components added since 2000 include the Destiny Laboratory (2001, United States), Pirs Docking Compartment (2001, Russia), and Columbus laboratory (2008, ESA).

space shuttle, reusable U.S. space vehicle. Developed by the National Aeronautics and Space Administration (NASA), it consists of a winged orbiter, two solid-rocket boosters, and an external tank. As with previous spacecraft, the shuttle is launched from a vertical position. Liftoff thrust is derived from the orbiter's three main liquid-propellant engines and the boosters. After 2 min the boosters use up their fuel, separate from the spacecraft, and—after deployment of parachutes—are recovered following splashdown. After about 8 min of flight, the orbiter's main engines shut down; the external tank is then jettisoned and burns up as it reenters the atmosphere. The orbiter meanwhile enters orbit after a short burn of its two small Orbiting Maneuvering System (OMS) engines. To return to earth, the orbiter turns around, fires its OMS engines to reduce speed, and, after descending through the atmosphere, lands like a glider. Five different orbiters—Columbia, Challenger, Atlantis, Discovery, and Endeavour—have seen service; two have been lost in accidents.

Following four orbital test flights (1981-82) of the space shuttle Columbia, operational flights began in Nov., 1982. On Jan. 28, 1986, the Challenger exploded shortly after takeoff, killing all seven astronauts. The commission that investigated the disaster determined that the failure of the O-ring seal in one of the solid fuel rockets was responsible. Shuttle flights were halted until Sept., 1988, while design problems were corrected, and then resumed on a more conservative schedule. NASA was forced to reemphasize expendable rockets to reduce the cost of placing payloads in space.

A second disaster struck the shuttle program on Feb. 1, 2003, when the Columbia broke up during reentry, killing the seven astronauts on board. NASA again halted shuttle launches, and a special commission was appointed to investigate the accident. It is believed that damage to the left wing, which could have been caused by insulation that separated from the external fuel tank during launch, ultimately permitted superheated gas to flow into the wing, weaken it, and cause its failure. Modifications were made to external fuel tank and other parts of the shuttle, and shuttle flights resumed in July, 2005. Further problems with fuel tank insulation that developed during that launch led to the suspension of additional flights for a year while the problems were corrected.

Missions of the space shuttle have included the transport of the Spacelab scientific workshop (see space exploration) and the insertion into orbit of the Hubble Space Telescope (1990), the Galileo space probe (1989), the Chandra X-Ray Observatory (1999), and a wide variety of communications, weather, scientific, and defense-related satellites. Other notable achievements of the shuttle program include the rescue and repair of disabled satellites (including the Hubble Space Telescope in 1993 and 1999) and the first three-person spacewalk (1992). In 1995 the Endeavour's mission of Mar. 2-18 set the record for the longest shuttle flight. It was also in 1995 that the crew of Atlantis accomplished the first of nine shuttle-Mir (Russian space station) docking maneuvers and crew transfers, which were designed to pave the way for the assembly of the International Space Station (ISS). The crew of Discovery made the ninth and final docking in 1998, five months before the Russians orbited Zarya, the first ISS module. A month later the astronauts aboard Endeavour initiated the first assembly sequence of the ISS, linking the Unity module, a passageway that connects living and work areas of the station, to Zarya. In 1999 the Discovery crew accomplished the first docking of a shuttle with the ISS during a mission to supply the two modules with tools and cranes. Shuttle flights since have continued to bring supplies and components to the station, including the Destiny (2001, United States) and Columbus (2008, ESA) laboratories.

space 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. The particular disciplines that are relevant depend on the type of mission being planned. There are four basic categories of space mission. The sounding rocket is restricted to suborbital flights with maximum altitude between 35 and 1,300 mi (55-2,100 km). Artificial satellites orbit the earth at altitudes between one hundred and several thousand miles. Space probes travel to the moon and planets. The final and most complex category is human spaceflight, of which the Apollo moon landings, the space shuttle, and the Skylab, Mir, and International space stations are the outstanding examples. The problems that space science must deal with include prediction and control of trajectories and orbits, telecommunications between spacecraft and earth, spacecraft design and fabrication, and life-support systems for human spaceflight.

Trajectories and Orbits

The key contribution of physics is celestial mechanics, the laws that govern the motions of bodies moving under the influence of gravitation. By combining Newton's law of universal gravitation and his laws of motion, the path of a rocket in the earth's vicinity can be calculated. This path, known as the trajectory, is strictly determined by the initial thrust imparted to the rocket, the gravitational field of the earth, and the atmospheric drag encountered. Although the manner in which these factors interact is highly complex, it is possible to determine accurately in advance the trajectory of any rocket and even to alter its course by remote control. If a satellite or unpowered spacecraft is close to the earth, the effects of other heavenly bodies can be ignored and its orbit will be a conic section: circular or elliptical for a satellite that remains in a closed orbit around the earth, and parabolic or hyperbolic for a spacecraft or space probe that escapes the earth's gravitational field into an open orbit.

The criterion that separates the closed and open orbits is the escape velocity, which for the earth is 7 mi (11.3 km) per sec. If the initial thrust provided by a rocket gives the object a speed greater than the escape velocity, it will move away from the earth in an open orbit; if the final velocity is smaller than the escape velocity, it will remain at finite distance from the earth in a closed orbit; if the final velocity is less than 5 mi (8 km) per sec, the flight will be suborbital and the object will follow an arc that returns it to earth.

A satellite in orbit around the earth typically travels at a height of several hundred miles with a velocity of about 5 mi (8 km) per sec and a period of revolution of 90 min. For certain satellites, however—such as communications satellites—synchronous orbits are desirable; at a distance of 22,300 mi (35,900 km), a satellite's period is exactly 24 hours, so it appears to hover over the same point on the earth's surface. Circular orbits are usually the most desirable but are the hardest to achieve. If a satellite is launched eastward near the equator, it receives a boost from the earth's rotation, but the resulting orbit necessarily lies in the earth's equatorial plane. For some applications, polar orbits, which pass near both of the earth's poles, are preferred. In a polar orbit, a satellite will periodically pass directly over every point on the earth's surface. Translunar and interplanetary trajectories are highly complex, because no simplifying assumptions can be made; the gravitational influences of the sun, moon, and other planets must be considered. Such gravitational forces can be exploited advantageously; for example, in the slingshot effect, a space probe is accelerated as it swings past a planet on the correct trajectory.

Telecommunications

Control over unmanned space probes and artificial satellites is maintained from the ground at control centers, where huge electronic computers analyze data and determine the exact moment when a change should be made. These instructions are relayed to the spacecraft by signals carried on certain radio frequencies. Instruments inside the craft also use radio signals to send data back to earth. Radio contact with spacecraft divides naturally into three categories: tracking, telemetry, and control. Tracking is the continuous reporting of a satellite's or space probe's position in space. Telemetry is the transmission of data back to earth by an on-board instrument (e.g., camera, Geiger counter, or magnetometer). Control includes the overall direction of a spacecraft to achieve the intended trajectory. Commands are specific control signals that order execution of a specific maneuver, such as turning on a camera or firing a retro-rocket

Spacecraft Design and Fabrication

Spacecraft employ booster rockets for propulsion and small adjustable retro-rockets for changing the orientation of the craft. Rocket propulsion systems vary from the tiny Aerobee sounding rocket to the giant Saturn V used in the Apollo project. For interplanetary flights, propulsion by nuclear or solar energy may be possible. Also being considered are ion and photon engines, which very efficiently provide low thrust that can build up very high velocity during a long flight. Landing on the earth or any planet with a significant atmosphere raises the problem of atmospheric friction, which can instantly burn up any spacecraft. In the manned space program, shielding that comes apart is used to absorb the frictional energy as the material of the shielding vaporizes. Also, a spacecraft enters the atmosphere at a shallow angle to avoid the friction produced by excessively high velocities.

Without the development of modern electronics based on miniaturized transistor circuitry, space exploration would have been practically impossible. Unmanned space probes and satellites carry on-board computers of varying degrees of sophistication, and even on manned missions, maneuvering the spacecraft requires the rapid calculation and response available only through computerized devices. The instruments carried on spacecraft measure almost every conceivable physical parameter. Devices for measuring micrometeorite density, cosmic rays, magnetic fields, and solar wind were aboard even the early artificial satellites. Television cameras for both visible and infrared light are carried by most space probes. In addition, many spacecraft carry telescopes for different wavelengths of the spectrum, ranging from infrared to X rays and gamma rays. An important technique in space science is called multispectral scanning. Images are formed using only certain selected wavelengths; the data can be used to compile a single, detailed color photograph, or can be studied separately. Certain space probes carry more specialized devices, such as ultraviolet spectrographs for studying stars, and coronographs and spectroheliographs for studying the sun.

Life Support for Human Spaceflight

Long-range life support must be provided in manned spaceflight. This includes oxygen, food, and recycling of waste material. Shielding is also provided against encounters with micrometeorites and cosmic radiation that could damage the spacecraft or be a health hazard for its occupants. The spacesuit is a miniature life-support system for the individual astronaut; it provides sufficient oxygen at the correct pressure to sustain normal body functioning. In more advanced projects like Apollo, the space shuttle, Skylab, Mir, and the International Space Station, a "shirt-sleeve" environment, in which the astronauts do not have to wear any life-support equipment, is provided in a large capsule. Space biology (or exobiology) and space medicine study the reactions of human, animal, and plant life to the physical stresses encountered in space, such as weightlessness and radiation exposure. Attention is also given to the psychological effects on a group of people working together in confined quarters under demanding conditions.

space 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 launched with enough energy to escape the gravitational field of the earth and navigate among the planets. Radio-transmitted commands and on-board computers provide the means for midcourse corrections in the space probe's trajectory; some advanced craft have executed complex maneuvers on command from earth when many millions of miles away in space. Radio contact between the control station on earth and the space probe also provides a channel for transmitting data recorded by on-board instruments back to earth. Instruments carried by space probes include radiometers, magnetometers, and television cameras sensitive to infrared, visible, and ultraviolet light; there also may be special detectors for micrometeors, cosmic rays, gamma rays, and solar wind. A probe may be directed to orbit a planet, to soft-land instrument packages on a planetary surface, or to fly by as close as a few thousand miles from one or more planets. The particulars of trajectory and instrumentation of each space probe are tailored around the mission's scientific and technological objectives; the data provided by a single space probe may require months or even years of analysis. Much has been learned from probes about the origins, composition, and structure of various bodies in the solar system. Scientists trying to understand the earth's weather by constructing theoretical models of global weather systems make use of the knowledge that is gained concerning the atmospheres and meteorology of the planets. Because conditions on other planets are simpler than on earth, scientists can check each of their hypotheses separately in isolation from complicating factors.

The earliest space probes in the U.S. space program were the Mariner series, which investigated Mars, Venus, and Mercury, and the Pioneer series, which explored the outer planets. Pioneer 10 was the first human-made object to entirely escape the solar system. Several Viking space probes voyaged to Mars in the late 1970s, mapping the planet and searching for life. The Voyager probes, launched in 1977, returned spectacular photos and data from brushes by Jupiter, Saturn, Uranus, Neptune, and their moons. The Magellan spacecraft succeeded in orbiting Venus in 1990, returning a radar map of the planet's hidden surface. The Japanese probes Sakigake and Suisei and the European Space Agency's (ESA) probe Giotto both rendezvoused with Halley's comet 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 orbited in 1995. Launched in 1989, the Galileo spacecraft followed a circuitous route that returned data about Venus (1990), the moon (1992), and the asteroids 951 Gaspra (1991) and 243 Ida (1993) before it reached Jupiter in 1995 and sent a small probe into the Jovian atmosphere to study its composition. Over the next eight years it orbited Jupiter 35 times, returning data about the planet's atmosphere and also about Jupiter's largest moons, Io, Ganymede, Europa, and Callisto. The joint U.S.-ESA mission Cassini, launched in 1997, began exploring Saturn and some of its moons in 2004 and deployed the lander Huygens on the surface of Saturn's moom Titan. The Mars Pathfinder and Mars Global Surveyor, both of which arrived at the red planet in 1997, were highly successful, the former in analyzing the Martian surface and the latter in mapping it. Both the Mars Climate Orbiter and Mars Polar Lander, however, were lost upon their arrival at Mars in 1999, setting NASA's Mars exploration program back by at least two years. The NEAR (for Near Earth Asteroid Rendezvous)-Shoemaker probe returned data about the asteroid Mathilde as it flew by in 1997 and the asteroid Eros as it orbited it in 1999 and 2000 and then landed on its surface in 2001, returning unparalleled data about a minor planet. In 2003 the ESA's Mars Express achieved orbit around Mars, and although its Beagle 2 lander proved unsuccessful, the orbiter returned data on the planet. NASA's Spirit and Opportunity rovers landed on the planet shortly afterward in early 2004.

space medicine, study of the medical and biological effects of space travel on living organisms. The principal aim is to discover how well and for how long humans can withstand the extreme conditions encountered in space, as well as how well they can readapt to the earth's environment after a space voyage. The medically significant aspects of space travel include weightlessness, strong inertial forces experienced during liftoff and reentry, radiation exposure, absence of the earth's day-and-night cycle, and existence in a closed ecological environment. Less critical factors are the noise, vibration, and heat produced within the spacecraft. On longer space flights, the psychological effects of isolation and living in close quarters have been a concern, especially among multinational crews with inherent differences in language and culture.

A large body of useful medical data on the effects of a prolonged U.S. space flight was obtained during the Skylab program of the early 1970s and from several medical missions of the space shuttles Challenger and Columbia. The Soviet Union's Soyuz program began Russia's experience with long stays in space; the current record of nearly 439 days was set by Russian cosmonaut Valery Polyakov (Jan. 8, 1994-Mar. 22, 1995) on the space station Mir. With the change in the international political climate in the 1990s, the two countries began to cooperate in life-science research that combined the more sophisticated diagnostic and monitoring equipment of the NASA missions with the greater long-term-stay experience of the Russians. In May, 1995, the Spektr module, containing U.S. medical and research equipment, was added to the Mir. A few months later, American physician-astronaut Norman E. Thagard broke the former U.S. record of 84 continuous days in space when he spent 111 days on the Russian space station.

There have been many indirect benefits to medicine from space science. The need to maintain close watch over the physiological conditions of astronauts has spurred the development of improved means for electronically monitoring essential body functions. The development of programmable heart pacemakers, implantable drug administration systems, magnetic resonance imaging (MRI), and computerized axial tomography (CAT) all depended to some extent on knowledge gained from the space program. Studies of how astronauts would walk in the moon's weak gravitational field led to a deeper understanding of human locomotion.

See also aviation medicine; space science.

Medically Significant Aspects of Space Flight

Weightlessness

Of all the medically significant conditions experienced in space flight, weightlessness has the most drastic effects; moreover, it will be impossible to eliminate this aspect of space travel unless large space stations can be constructed that produce artificial gravity, as by rotating. Because life evolved under the constant influence of gravity, the effects of weightlessness even on the cellular level have been a concern. It was at first feared that a human being in space might lose all coordination and become completely incapacitated. While the human body does appear to adjust fairly quickly in a state of weightlessness, associated problems do occur, often causing difficulties only upon return to earth. Problems include space adaptation syndrome (nausea, motion sickness, and sensory disorientation during the first few days), weakened immune defenses, loss of bone mass, loss of muscle mass (including loss of heart muscle), and space anemia, which results as the number of red cells decreases. Russian astronauts undergo strenuous exercise routines twice daily to try and maintain bone and large muscle mass. Nevertheless, some have had to be carried on stretchers when they first return to earth.

Inertial Forces

Inertial forces due to acceleration are experienced only during liftoff and reentry, but the consequences can be traumatic. The circulatory system is most strongly affected; deprivation of blood to the brain causes dimming of vision and sometimes loss of consciousness. However, lying on a body-contoured couch, astronauts have survived inertial forces eight times stronger than normal gravity.

Ionizing Radiation

In space the astronauts are exposed to ionizing radiation from particles trapped in the earth's magnetic field, from solar flares, and from the onboard nuclear reactors that help power the spacecraft. This radiation can produce deleterious effects, ranging from nausea and lowered blood count to genetic mutations and leukemia. Protective shielding, shielding chemicals, and careful monitoring of the doses of radiation received by each astronaut have been used to reduce radiation exposure to acceptable levels.

Absence of Day and Night

The absence of the earthly cycle of day and night during space travel produces subtle effects, both physiological and psychological. The period from sunrise to sunset in a quickly orbiting space shuttle may be as little as 11∕2 hours long. All body rhythms, such as heartbeat, respiration, and changes in body temperature, are regulated by biological clocks (see biorhythm). These rhythms are related to human patterns of sleep and wakefulness, which in turn are based on the alternation of day and night. On most flights, adherence to "home" schedules maintains normal human cycles.

A Closed Environment

In the closed environment of the spacecraft care must be taken to prevent the buildup of toxic material to dangerous levels; this is accomplished by recycling waste material. The nature of the artificial atmosphere astronauts breathe is an important biomedical consideration. Ideally, this atmosphere would be identical in composition and pressure to the earth's atmosphere. Any alteration involves the risk of decompression sickness. The space shuttle uses a pure oxygen atmosphere or a mixture of oxygen and nitrogen.

space law, agreements governing the exploration and use of outer space, developed since the first launching (1957) by humans of a satellite into space. Space law, an aspect of international law, has grown under the aegis of the United Nations. A 1963 UN declaration stated that the exploration and use of outer space would be for the benefit and in the interest of all people; that no sovereignty could be claimed in space; that objects and persons launched into space would be returned promptly and safely if they landed in a foreign country; and that nations launching objects would be responsible for damages caused by them. In 1967, a general treaty embodying these principles and adding a prohibition on the military use of space and a provision for the inspection of installations on celestial bodies went into effect. A UN treaty on use of the moon's resources was drafted in 1979. The boundary between airspace (see air, law of the), which is subject to sovereignty, and outer space remains an object of discussion. Some favor definitions based on the composition of the atmosphere. Others favor a functional approach; thus, if commercial airlines use a particular layer of the atmosphere, it is to be considered airspace.

space exploration, the investigation of physical conditions in space and on stars, planets, and other celestial bodies through the use of artificial satellites (spacecraft that orbit the earth), space probes (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 telescopes had accumulated much data on the nature of celestial bodies, it was not until after World War II that the development of powerful rockets 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 moon. 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. 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.

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 comet 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.

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 escape the solar system. 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. Launched in 1989, the 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). The joint U.S.-ESA Cassini mission, launched in 1997, began exploring Saturn, its rings, and some of its moons upon arriving in 2004. It deployed Huygens, which landed on the surface of Saturn's moom Titan in early 2005.

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 Gagarin, in Vostok 1 on Apr. 12, 1961. The American Mercury program had its first orbital success in Feb., 1962, when John Glenn 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. Armstrong and Edwin E. ("Buzz") Aldrin, 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 cosmonauts to and from Salyut and Mir.

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 spectrum (e.g., visible, ultraviolet, X-ray, or infrared light). Particular attention was given to the study of solar flares (see sun). The earth applications, which involved remote sensing of natural resources, relied on visible and infrared light in a technique called multispectral scanning (see space science). 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 station) 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 shuttle, capable of ferrying people and equipment into orbit and back to earth, was launched. The shuttle itself is a hypersonic delta-wing airplane about the size of a DC-9. Takeoff is powered by three liquid-fuel engines fed from an external tank and two solid-fuel engines; the last are recovered by parachute. The shuttle itself returns to earth in a controlled glide, landing either in California or in Florida.

The shuttle can put a payload of 20 tons (18,000 kg) in earth orbit below 600 mi (970 km); the payload is 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 have 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. 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. Prior to the Columbia disaster, the shuttle fleet operated on approximately a bimonthly schedule. Shuttle flights resumed in July, 2005, but new problems with fuel tank insulation led NASA to suspend shuttle launches for a year. 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 is carried aloft by a reusable jet mothership (White Knight) to 46,000 ft (13.8 km), where it is released and fires its rocket engine. The spacecraft was designed by Bert Rutan and built by his company, Scaled Composites. The vehicle's 90-minute flight was the first successful nongovernmental spaceflight.

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.

space biology: see exobiology.

outer space: see space exploration.

Spitzer Space Telescope: see infrared astronomy; observatory, orbiting.

Space Infrared Telescope Facility: see observatory, orbiting.

National Aeronautics and Space Administration (NASA), civilian agency of the U.S. federal government with the mission of conducting research and developing operational programs in the areas of space exploration, artificial satellites (see satellite, artificial), rocketry, and space telescopes (see Hubble Space Telescope) and observatories. It is also responsible for international cooperation in space matters. NASA came into existence on Oct. 1, 1958, superseding the National Advisory Committee on Aeronautics (NACA), an agency that had been oriented primarily toward laboratory research. While the NACA budget never went higher than $5 million and its staff never exceeded 500, the NASA annual budget reached $14.2 billion in 1995, and its staff reached a maximum size of 34,000 in 1966 (21,000 in 1995), with some 400,000 contract employees working directly on agency programs.

The creation of NASA was spurred by American unpreparedness at the time the Soviet Union launched (Oct. 4, 1957) the first artificial satellite (Sputnik 1). NASA took over the Langley, Ames, and Lewis research centers from NACA. Soon after its creation, NASA acquired from the U.S. army the Jet Propulsion Laboratory (operated by the California Institute of Technology). Later, the Army Ballistic Missile Arsenal (now the Marshall Space Flight Center) at Huntsville, Ala., was placed under NASA control.

The best-known NASA field installations are the Lyndon B. Johnson Space Center near Houston, Tex., where flights are coordinated, and the John F. Kennedy Space Center at Cape Canaveral, Fla., where all space shuttle launches take place. Other facilities include the Dryden, Glenn, Goddard, and Stennis centers and NASA headquarters, in Washington, D.C. Operationally, NASA is headed by a civilian appointed by the president and has four divisions: the offices of Space Flight, Space Science Programs, Aeronautics Exploration and Technology, and Tracking and Data Acquisition. Despite some highly publicized failures, NASA has in many cases successfully completed its missions within their projected budgets; the total cost of the Apollo project, for example, wound up very close to the original $20-billion estimate. Currently, NASA oversees all space science projects, operates the space shuttle, and launches approximately half of all military space missions.

Mercury space program: see space exploration.

Mariner space program: see space exploration.

Infrared Space Observatory: see infrared astronomy.

Hubble Space Telescope (HST), the first large optical orbiting observatory. Built from 1978 to 1990 at a cost of $1.5 billion, the HST (named for astronomer E. P. Hubble) was expected to provide the clearest view yet obtained of the universe. Using a Ritchey-Chrétien design that affords wider and flatter fields of view than traditional Cassegrain systems, the telescope has a 7.9-ft (2.4-m) primary mirror that can observe 24 hours a day (but usually observes less than 20% of the time) in a sky that is always clear and always has perfect seeing. Among the instruments are two high-resolution cameras and two spectrographs. The HST was launched from shuttle Atlantis in 1990. Initial tests taken after its launch showed that the primary mirror was astigmatic, and it was discovered that the mirror had been mistakenly ground to the wrong figure. The telescope was repaired by astronauts of the space shuttle Endeavour in Dec., 1993, who replaced critical instruments and added corrective optics while in orbit. Subsequent servicing missions in 1997 and 1999 added capabilities to HST, which observes the universe at ultraviolet, near-ultraviolet, visible, and near-infrared wavelengths. In Mar., 2002, astronauts from the space shuttle Columbia made repairs and improvements designed to enable the observatory to function for another decade, but in 2004 the power supply for the ultraviolet spectrograph failed.

Gemini space program: see space exploration.

European Space Research and Technology Center (ESTEC): see European Space Agency.

European Space Research Organization (ESRO): see European Space Agency.

European Space Research Institute (ESRIN): see European Space Agency.

European Space Operations Center (ESOC): see European Space Agency.

European Space Agency (ESA), multinational agency dedicated to the promotion, for exclusively peaceful purposes, of cooperation among European states in space research and technology. Member states include Austria, Belgium, Denmark, Finland, France, Germany, Greece, Ireland, Italy, Luxembourg, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland, and Great Britain; Canada, the Czech Republic, and Hungary participate in selected ESA programs. The financial contribution of each member is determined by the projects it wishes to support, and no member may undertake a project without inviting ESA's participation.

The headquarters of ESA are in Paris, with four major ESA facilities in other countries. The European Space Research and Technology Center (ESTEC), located at Noordwijk, the Netherlands, is the primary research center and manages the satellite projects. The European Space Operations Center (ESOC), located at Darmstadt, Germany, is responsible for satellite control, monitoring, and data retrieval. The European Space Research Institute (ESRIN), located at Frascati, Italy, supports the ESA documentation service and manages the data obtained from remote sensing satellites. The European Astronaut Center (EAC), located at Cologne, Germany, is responsible for the selection and training of astronauts for space station missions. In addition to the major centers, ESA operates sounding-rocket launch stations in Norway and Sweden, a meteorological program office at Toulon, France, and satellite tracking stations in Belgium, Germany, Italy, and Spain.

Major ESA programs include the development of the Ariane rockets used to launch most ESA satellites from a pad at Kourou, French Guiana. ESA developed the Spacelab scientific workshop, which has been transported into space several times by the U.S. National Aeronautics and Space Administration's (NASA) space shuttle; the Giotto space probe, which in a 1986 flyby examined the nucleus of Halley's comet; and the SOHO (Solar and Heliospheric Observatory) and ISO (Infrared Space Observatory) orbiting observatories, launched in 1995. A system of meteorological satellites, called Meteosat, has also been established. Eleven member nations of ESA are also participating with Brazil, Canada, Japan, Russia, and the United States in the International Space Station project (see space station). Arianespace, the first commercial space transportation company and a division of ESA, now conducts more than half of all commercial satellite launches.

The foundation of ESA was laid with the formation of the European Space Research Organization (ESRO) in 1962 and of the European Launcher Development Organization (ELDO) in 1964. ESRO consisted of ten European countries and Australia, which placed its rocket-firing range at Woomera at the organization's disposal; between 1968 and 1972 seven ESRO satellites—Iris (ESRO-2B), Aurorae (ESRO-1A), HEOS-1, BOREAS, HEOS-2, TD-1A, and ESRO-4—were launched on NASA rockets. ELDO, which consisted of seven European countries, developed Kourou's Equatorial Space Range. Intending to build the Europa 1 multistage launch vehicle—combining a British first stage, a French second stage, and a German third stage—to orbit an Italian satellite, ELDO was unsuccessful primarily because of organizational problems. By 1975 it was obvious that a new approach was required, and ESRO and ELDO were merged to form ESA.

Apollo space program: see space exploration.

In mathematics, a collection of objects called vectors, together with a field of objects (see field theory), known as scalars, that satisfy certain properties. The properties that must be satisfied are: (1) the set of vectors is closed under vector addition; (2) multiplication of a vector by a scalar produces a vector in the set; (3) the associative law holds for vector addition, u + (v + w) = (u + v) + w; (4) the commutative law holds for vector addition, u + v = v + u; (5) there is a 0 vector such that v + 0 = v; (6) every vector has an additive inverse (see inverse function), v + (−v) = 0; (7) the distributive law holds for scalar multiplication over vector addition, math.n(u + v) = math.nu + math.nv; (8) the distributive law also holds for vector multiplication over scalar addition, (math.m + math.n)v = math.mv + math.nv; (9) the associative law holds for scalar multiplication with a vector, (math.mmath.n)v = math.m(math.nv); and (10) there exists a unit vector 1 such that 1v = v. The set of all polynomials in one variable with real coefficients is an example of a vector space.

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Single entity that relates space and time in a four-dimensional structure, postulated by Albert Einstein in his theories of relativity. In the Newtonian universe it was supposed that there was no connection between space and time. Space was thought to be a flat, three-dimensional arrangement of all possible point locations, which could be expressed by Cartesian coordinates; time was viewed as an independent one-dimensional concept. Einstein showed that a complete description of relative motion requires equations that include time as well as the three spatial dimensions. He also showed that space-time is curved, which allowed him to account for gravitation in his general theory of relativity.

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Investigation of the universe beyond Earth's atmosphere by means of manned and unmanned spacecraft. Study of the use of rockets for spaceflight began early in the 20th century. Germany's research on rocket propulsion in the 1930s led to development of the V-2 missile. After World War II, the U.S. and the Soviet Union, with the aid of relocated German scientists, competed in the “space race,” making substantial progress in high-altitude rocket technology (see staged rocket). Both launched their first satellites (see Sputnik; Explorer) in the late 1950s (followed by other satellites and unmanned lunar probes) and their first manned space vehicles (see Vostok; Mercury) in 1961. A succession of longer and more complex manned space missions followed, most notably the U.S. Apollo program, including the first manned lunar landing in 1969, and the Soviet Soyuz and Salyut missions. Beginning in the 1960s, U.S. and Soviet scientists also launched unmanned deep-space probes for studies of the planets and other solar system objects (see Pioneer; Venera; Viking; Voyager; Galileo), and Earth-orbiting astronomical observatories (see, for example, Hubble Space Telescope), which permitted observation of cosmic objects from above the filtering and distorting effects of Earth's atmosphere. In the 1970s and '80s the Soviet Union concentrated on the development of space stations for scientific research and military reconnaissance (see Salyut; Mir). After the dissolution of the Soviet Union in 1991, Russia continued its space program, but on a reduced basis owing to economic constraints. In 1973 the U.S. launched its own space station (see Skylab), and since the mid 1970s it has devoted much of its manned space efforts to the space shuttle program and, more recently, to developing the International Space Station in collaboration with Russia and other countries.

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Manned artificial structure designed to revolve in a fixed orbit as a long-term base for astronomical observations, study of Earth's resources and environment, military reconnaissance, and investigations of materials and biological systems in weightless conditions. As of 2001, nine space stations have been placed in a low Earth orbit and occupied for varying lengths of time. The Soviet Union orbited the world's first space station, Salyut 1, designed for scientific studies, in 1971. From 1974 to 1982 five more Salyut stations—two outfitted for military reconnaissance—were successfully placed in orbit and occupied. In 1986 the U.S.S.R. launched the core module of Mir, a scientific station that was expanded with five additional modules over the next decade. The U.S. orbited its first space station in 1973; called Skylab, it was equipped as a solar observatory and medical laboratory. In 1998 the U.S. and Russia began the in-orbit construction of the International Space Station (ISS), a complex of laboratory and habitat modules that would ultimately involve contributions from at least 16 countries. In 2000 the ISS received its first resident crew.

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Sickness caused by contradiction between external data from the eyes and internal cues from the balance centre in the inner ear. For example, in seasickness the inner ear senses the ship's motion, but the eyes see the still cabin. This stimulates stress hormones and accelerates stomach muscle contraction, leading to dizziness, pallor, cold sweat, and nausea and vomiting. Minimizing changes of speed and direction may help, as may reclining, not turning the head, closing the eyes, or focusing on distant objects. Drugs can prevent or relieve motion sickness but may have side effects. Pressing an acupuncture point on the wrist helps some people.

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formally Space Transportation System (STS)

Partially reusable rocket-launched vehicle developed by NASA to go into Earth orbit, transport people and cargo between Earth and orbiting spacecraft, and glide to a runway landing on Earth. The first flight of a space shuttle into orbit took place in 1981. The shuttle consists of a winged orbiter that carries crew and cargo; an expendable external tank of liquid fuel and oxidizer for the orbiter's three main rocket engines; and two large, reusable solid-propellant booster rockets. The orbiter lifts off vertically like an expendable launch vehicle but makes an unpowered descent similar to a glider. Each orbiter was designed to be reused up to 100 times. For manipulating cargo and other materials outside the orbiter, astronauts use a remotely controlled robot arm or exit the orbiter wearing spacesuits. On some missions, the shuttle carries a European-built pressurized research facility called Spacelab in its cargo bay. Between 1981 and 1985, four shuttle orbiters were put into service: Columbia (the first in orbit), Challenger, Discovery, and Atlantis. Challenger exploded in 1986 during launch, killing all seven astronauts aboard; it was replaced in 1992 by Endeavour. From 1995 to 1998, NASA conducted shuttle missions to the Russian space station Mir to prepare for the construction of the International Space Station (ISS). Beginning in 1998, the shuttle was used extensively to ferry components, supplies, and crews to the ISS. In 2003 Columbia disintegrated while returning from a space mission, claiming the lives of its seven-person crew.

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Branch of medicine, pioneered by Paul Bert, dealing with atmospheric flight (aviation medicine) and space flight (space medicine). Intensive preflight simulator training and attention to design of equipment and spacecraft promote the safety and effectiveness of humans exposed to the stresses of flight and can prevent some problems. The world's first unit for space research was established in the U.S. in 1948. Physicians trained in aerospace medicine are known as flight surgeons.

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Rocket system that boosts a spacecraft into Earth orbit or beyond Earth's gravitational pull. A wide variety of launch vehicles have been used to lift payloads ranging from satellites weighing a few pounds (or kilograms) to large modular components of space stations. Most launch vehicles are expendable (one-use) systems; many early ones were derived from intercontinental ballistic missiles (see ICBM). The Saturn V, which launched the spacecraft that carried humans to the Moon (see Apollo), had three stages (see staged rocket). The U.S. space shuttle system (from 1981) represents a significant departure from expendable launch vehicles in that it is partially reusable—its manned orbiting component is designed for numerous flights, and its solid rocket boosters can be recovered and refurbished.

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or space contraction

In relativity physics, the shortening of an object along the direction of its motion relative to an observer. Dimensions in other directions are not contracted. This concept was proposed by the Irish physicist George F. FitzGerald (1851–1901) in 1889 and later independently developed by Hendrik Antoon Lorentz. Significant at speeds approaching that of light, the contraction results from the properties of space and time, not from compression, cooling, or any similar physical disturbance. Seealso time dilation.

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In mathematics, a set of objects equipped with a concept of distance. The objects can be thought of as points in space, with the distance between points given by a distance formula, such that: (1) the distance from point A to point B is zero if and only if A and B are identical, (2) the distance from A to B is the same as from B to A, and (3) the distance from A to B plus that from B to C is greater than or equal to the distance from A to C (the triangle inequality). Two- and three-dimensional Euclidean spaces are metric spaces, as are inner product spaces, vector spaces, and certain topological spaces (see topology).

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in full National Aeronautics and Space Administration

Independent U.S. government agency established in 1958 for research and development of vehicles and activities for aeronautics and space exploration. Its goals include improving human understanding of the universe, the solar system, and Earth and establishing a permanent human presence in space. NASA, previously the National Advisory Committee for Aeronautics (NACA), was created largely in response to the Soviet Union's launch of Sputnik in 1957. Its organization was well under way in 1961, when Pres. John F. Kennedy proposed that the U.S. put a man on the Moon by the end of the 1960s (see Apollo). Later unmanned programs (e.g., Viking, Mariner, Voyager, Galileo) explored other planets and interplanetary space, and orbiting observatories (e.g., the Hubble Space Telescope) have studied the cosmos. NASA also developed and launched various satellites with Earth applications, such as Landsat and communications and weather satellites. It planned and developed the space shuttle and led the development and construction of the International Space Station.

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Most sophisticated optical observatory ever placed into orbit around Earth. Because it is above Earth's obscuring atmosphere, it can obtain images much brighter, clearer, and more detailed than ground-based telescopes can. Named for Edwin Hubble, it was built under NASA supervision and deployed on a 1990 space-shuttle mission. The reflector telescope's mirror optics gather light from celestial objects and direct it to an array of cameras and spectrographs (see spectroscopy). A defect in the primary mirror initially caused it to produce fuzzy images; in 1993 another shuttle mission corrected this and other problems. Subsequent missions to the HST have been for maintenance, repairs, and instrument upgrades.

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French Agence Spatiale Européenne

Western European space and space-technology research organization headquartered in Paris. It was founded in 1975 from the merger of the European Launcher Development Organisation (ELDO) and the European Space Research Organisation (ESRO), both established in 1964. Members are Austria, Belgium, Britain, Denmark, Finland, France, Germany, Ireland, Italy, The Netherlands, Norway, Portugal, Spain, Sweden, and Switzerland. Canada, through a special cooperative agreement, participates in some projects. The ESA developed the Ariane series of space launch vehicles, and it supports a launch facility in French Guiana. It has launched a system of meteorological satellites (Meteosat) as well as the Giotto space probe, which examined the nucleus of Halley's Comet, and Hipparcos, a satellite that measured the parallaxes, positions, and proper motions of more than 100,000 stars. It is also a participant in the construction of the International Space Station.

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