SPACE EXPLORATION

the investigation, by means of both manned and unmanned spacecraft, of the reaches of the universe beyond the atmosphere of the Earth. Spacecraft, vehicles that operate above the Earth's atmosphere, include sounding rockets, Earth satellites, and lunar, planetary, and deep space probes. Astronaut Edwin Aldrin, Jr., walking near the Lunar Module during the Apollo 11 extravehicular Lift-off of the first U.S. Space Shuttle, on April 12, 1981, from John F. Kennedy Space Center, On October 4, 1957, the Soviet Union launched the world's first artificial satellite, Sputnik 1, and set in motion a series of programs of space exploration by the United States and the Soviet Union. The first U.S. satellite, Explorer 1, was launched on January 31, 1958, not quite four months after Sputnik 1. Both nations participated during the next decades in a space race, with more than 5,000 successful launches of satellites and space probes of all varieties: scientific research, communications, meteorological, photographic reconnaissance, and navigation satellites, lunar and planetary probes, and manned space flights. The Soviet Union launched the first man into orbit around the Earth on April 12, 1961. On July 20, 1969, the United States landed two men on the surface of the Moon. On April 12, 1981, the 20th anniversary of manned space flight, the United States launched the first reusable manned vehicle, the Space Shuttle. Many of the spacecraft, such as manned and reconnaissance vehicles, are designed for recovery. Most operational satellites become inert after a few months or years of operation. The North American Air Defense Command (Norad) keeps a constant watch on the thousands of objects of human origin circling the Earth in a variety of orbital paths. Both radar and optical telescopes are used. In addition to satellite payloads is a much larger number of objects classified as debrisspent upper stages of launch vehicles, tether, cables, etc., that go into orbit along with functioning satellites, as well as fragments that result from in-space explosions. Eventually low-altitude debris reenters the Earth's atmosphere and usually burns up. Discoveries about the Earth Before the advent of artificial satellites, it was accepted that the shape of the Earth was, except for some high mountains and deep valleys, an ellipsoidi.e., a sphere, slightly flattened. Accurate observation of variations of the orbital path of Vanguard 1 proved that, on the contrary, the Earth is slightly pearshaped, the distance from the Earth's centre to the North Pole being greater than the distance to the South Pole. Further observations of anomalies in the orbits of satellites have shown that the Equator is elliptical and that the surface gravity of the Earth has several hills and valleys varying as much as 30 to 90 metres from the average. Originally it was thought that cloud patterns and weather systems were relatively small, perhaps a few hundred kilometres across. Photographs from space have shown that the Earth's cloud cover is completely organized globally. Coherent cloud-cover systems have been found to extend for thousands of kilometres and to be related to other systems of similar dimensions. It was discovered that weather systems could be directly identified by their cloud structure, and thus it was possible immediately to locate important atmospheric phenomena such as fronts and storms and to chart their courses daily with accuracy. Atmospheric density and variation with altitude above the Earth had been studied for many years by balloons and to some extent by sounding rockets. Observations of atmospheric drag on satellites and resultant lowering of orbits, however, revealed that variations in atmospheric density are related directly to solar flares and the impact of electrically charged particles about 26 hours after marked solar activity. Further, solar flares have been found to increase the temperature of the atmosphere by interaction of short-wavelength radiation atoms and molecules in the atmosphere. Recombination of the ionized particles is accompanied by the liberation of heat. Although it had been postulated that some charged particles might be trapped in the Earth's magnetic field, Explorers 1 and 4 revealed a major phenomenon in geophysics: the existence of two permanent doughnut-shaped belts of high-energy radiation caused by trapped charged particles, 3,00016,000 kilometres above the Earth. The earlier view of the ionosphere was that from 60 to 600 kilometres distinct layers of ionized atoms and molecules, known as the D, E, and F layers, existed and were necessary for radio communications. Evidence from space exploration is that ionization in the atmosphere is actually continuous but that peaks of concentration of electrons occur at altitudes where the existence of layers had formerly been postulated. Still higher, ionization is practically complete and extends to great altitudes. It was once believed that the Earth's magnetic field was similar in shape and symmetry to a short bar magnet, with the magnetic field extending outward into space. It has been learned from satellites that the Earth's magnetic field on the side toward the Sun is sharply bounded because of the solar wind (a constant outpouring of high-speed particles from the Sun's corona). On the dark side of the Earth, in the direction away from the Sun, the field streams out in a long, thin tail beyond the Moon's orbit. The Dynamics Explorers and the U.S. Air Force's High Latitude (HiLat) satellite produced images of the entire auroral oval seen from space during night and day and revealed data on how it is pumped by events in the magnetotail. The Dynamics Explorers also discovered fountainlike jets of nitrogen and oxygen ions propelled upward from the magnetic poles along the field lines. Data on solar phenomena Since ultraviolet rays and X rays are absorbed in the atmosphere, measurements of these radiations from the Sun were made by sounding rockets in some of their first flights. Solar spectra have been refined by spectrographic cameras carried aboard many satellites. Variation in measurements correlated with observed solar activity is providing new knowledge of the processes occurring within the Sun. Although the solar radiation in the visible portion of the spectrum is constant, the ultraviolet and X-radiation are variable. From the data gathered by space exploration it also has been learned that the Sun radiates in the manner of an incandescent gas, that ultraviolet and X-radiation are generated not in the region of the solar disk but higher in the solar atmosphere, in the chromosphere and the corona. Skylab's solar telescopes discovered holes (i.e., cool regions) in the corona, which are now believed to be the source of the solar wind. The Solar Maximum Mission satellite showed variations in the Sun's total energy outputas much as 0.2 percent in two daysthough these averaged out to produce a much slighter downward trend that could have implications for climate over the centuries. the investigation of all the reaches of the universe beyond the Earth's atmosphere by means of manned and unmanned spacecraft. Scientific studies on the use of rockets for space flight appeared early in the 20th century. Most notable were the works of Konstantin E. Tsiolkovsky (1903), Robert H. Goddard (1919), and Hermann Oberth (1923). By the early 1930s Germany was conducting extensive research on rocket propulsion, the results of which led to the development of the V-2 guided missile. After World War II the United States and the Soviet Union, with the aid of immigrant German scientists, made substantial progress in high-altitude rocket technology. On Oct. 4, 1957, the Soviets launched the first artificial satellite, Sputnik 1, placing it into orbit around the Earth. Nearly four months later, on Jan. 31, 1958, the United States sent up its first Earth satellite, Explorer 1. Over the next few years both nations launched a number of unmanned spacecraft of various kinds, ranging from meteorological and communications satellites to lunar probes. The next major step in space exploration was the launching of a manned space vehicle. Soviet scientists were the first to accomplish this feat. On April 12, 1961, they launched the Vostok 1 space capsule carrying cosmonaut Yury A. Gagarin in a single orbit around the Earth. Less than a month later, on May 5, the United States launched its first manned spacecraft, a Mercury capsule in which astronaut Alan B. Shepard, Jr., made a 15-minute suborbital flight. A succession of both U.S. and Soviet manned space missions of longer duration and complexity soon followed. The Vostok and Mercury flights had demonstrated that man could function while weightless and in space, but neither type of spacecraft had true operational maneuverability. The U.S. Gemini and the Soviet Voskhod programs of the mid-1960s marked a great advance in their use of two- or even three-man flights, in their development of in-flight rendezvous and docking with unmanned target vehicles, and in the first ventures of astronauts to float freely in the near-vacuum of space outside their spacecraft (extravehicular activity). The succeeding U.S. Apollo program culminated in man's first lunar landing in 1969. On July 20 of that year, U.S. astronauts Neil A. Armstrong and Edwin E. Aldrin exited the Apollo 11 lunar module to become the first humans to set foot on the Moon. Five more lunar landings were made on subsequent Apollo flights, during which astronauts explored the surface of the Moon, collected rock and soil samples, and performed a variety of scientific experiments. While the Soviet Union did not land men on the Moon, it launched a series of robot lunar probes (Luna and Zond) that returned important data and soil samples. Luna 16, for example, made a soft-landing on the Moon in September 1970, obtained a core sample of soil, and returned it to Earth in a sealed capsule. The Soviets in the 1970s and '80s concentrated their efforts on an extensive series of missions (Soyuz) involving their cosmonauts' repeated docking with and occupation of various Salyut orbiting laboratories, which were equipped for extended missions of scientific experimentation and military reconnaissance while in Earth orbit. During the 1960s and '70s, U.S. and Soviet scientists also undertook ambitious planetary studies with unmanned deep-space probes. Some of the more significant of the missions were the Viking landings on Mars; the Voyager flybys of Jupiter, Saturn, and Uranus; and the Venera explorations of Venus' surface. The Viking landers made successful descents to the Martian surface in 1976, where they transmitted detailed colour images of that planet's landscape back to Earth and conducted in situ analyses of the Martian soil and atmosphere. The Voyager flybys of Jupiter, Saturn, and Uranus during 197986 vastly enriched scientific knowledge of these planets, revealing novel and unsuspected features of their moons, magnetic fields, and ring systems. Crucial, too, in the exploration of space have been orbiting astronomical observatories, which permit observations of distant cosmic objects high above the interference and distorting effects of the Earth's atmosphere. Much has been learned, for example, about infrared sources far beyond the limits of the solar system from the unmanned Infrared Astronomy Satellite (IRAS), launched in 1983 by the United States in collaboration with the United Kingdom and The Netherlands. That same year, the Soviet Union placed in Earth orbit another advanced observational satellite, called Astron. Equipped with an ultraviolet telescope developed by France, Astron has studied cosmic radiation from galactic and extragalactic sources. Manned experimental space stations also have been used to conduct scientific research. In the early 1970s U.S. astronauts aboard the Skylab station not only made valuable observations of solar phenomena but undertook tests designed to study how the human body responds to zero-gravity conditions in space on extended missions. Similar research had been conducted by Soviet cosmonauts on board Salyut stations until the mid-1980s. In 1986 the Soviet Union launched a more advanced type of space station dubbed Mir. This stationthe core of a large, permanent, multimanned orbiting complexis designed to accommodate various expansion modules for crew living quarters and research facilities. Since the mid-1970s, the United States has devoted most of its resources to developing the space shuttle, a reusable space vehicle that lifts off like a rocket and lands like an ordinary airplane. The shuttle craft have been largely utilized to deploy and repair satellites in Earth orbit, but plans call for their eventual use in the construction of a permanent orbiting station in space. After the Soviet Union broke up into an array of independent republics in 1991, its space program was continued by Russia, though on a much-reduced basis owing to economic constraints. Additional reading All aspects of space exploration and discovery are covered in Michael Rycroft (ed.), The Cambridge Encyclopedia of Space (1990), with an excellent bibliography; Anthony R. Curtis, Space Almanac, 2nd ed. (1992); and Kenneth Gatland, The Illustrated Encyclopedia of Space Technology, 2nd ed. (1984). Historical treatments of the conquest of space include Willy Ley, Rockets, Missiles, and Men in Space, newly rev. and expanded ed. (1968); Wernher von Braun and Frederick I. Ordway III, History of Rocketry & Space Travel, 3rd rev. ed. (1975), and Space Travel: A History, 4th ed. rev. in collaboration with Dave Dooling (1985); Homer E. Newell, Beyond the Atmosphere: Early Years of Space Science (1980); David Baker, The History of Manned Space Flight, new ed., updated and enlarged (1985); Phillip Clark, The Soviet Manned Space Program (1988); and Patrick Moore, Mission to the Planets (1990). Current information is available in Interavia Space Directory (annual), which includes detailed, illustrated coverage of programs by country and Earth observations; and Space Year (annual), a chronology of the year's major space events, including manned, unmanned, and planetary missions. Earlier missions are covered in Andrew Wilson, Solar System Log (1987), covering mid-1958 to mid-1985; and Tim Furniss, Manned Spaceflight Log, new ed. (1986), covering early 1961 to early 1986. Other aspects of space exploration are considered by William Sims Bainbridge, The Spaceflight Revolution: A Sociological Study (1976, reprinted with a new preface 1983); and Walter A. McDougall, The Heavens and the Earth: A Political History of the Space Age (1985).Coverage of specific programs can be found in the general works cited above and in the following: Loyd S. Swenson, Jr., James M. Grimwood, and Charles C. Alexander, This New Ocean: A History of Project Mercury (1966); on Skylab, Henry S.F. Cooper, Jr., A House in Space (1976); and W. David Compton and Charles D. Benson, Living and Working in Space (1983); Richard S. Lewis, The Voyages of Columbia (1984); David Shapland and Michael Rycroft, Spacelab (1984), an authoritative account of the project from design to results; Douglas R. Lord, Spacelab (1987), a history of the program's development; Robert W. Smith, The Space Telescope (1989), a history up to the completion of its construction; Eric J. Chaisson, Early Results from the Hubble Space Telescope, Scientific American, 266(6):4451 (June 1992); and, on several of the more recent unmanned flights, Richard O. Fimmel, Lawrence Colin, and Eric Burgess, Pioneer Venus (1983); Richard O. Fimmel, James Van Allen, and Eric Burgess, Pioneer: First to Jupiter, Saturn, and Beyond (1980); and Bradford A. Smith, The Voyager Encounters, in J. Kelly Beatty and Andrew Chaikin (eds.), The New Solar System, 3rd ed. (1990), pp. 107130, a profusely illustrated account. Further books on the space probes may be found in the bibliographies to the articles Mercury; Venus; Mars; Jupiter; Saturn; Uranus; and Neptune. Collections of colour photographs of the Earth taken from spacecraft include Oran W. Nicks (ed.), This Island Earth (1970); Nicholas M. Short et al., Mission to Earth: Landsat Views the World (1976); and Lyndon B. Johnson Space Center, Skylab Explores the Earth (1977).The Apollo lunar landings are treated in considerable detail by Neil Armstrong et al., First on the Moon (1970); Edgar M. Cortright (ed.), Apollo Expeditions to the Moon (1975); and Charles Murray and Catherine Bly Cox, Apollo: The Race to the Moon (1989).Speculative treatments of space exploration include Harry L. Shipman, Humans in Space: 21st Century Frontiers (1989); Joseph F. Baugher, On Civilized Stars: The Search for Intelligent Life in Outer Space (1985); Donald Goldsmith and Tobias Owen, The Search for Life in the Universe, 2nd ed. (1992); Theodore R. Simpson (ed.), The Space Station (1985); Ivan Bekey and Daniel Herman (eds.), Space Stations and Space Platforms (1985); and W.W. Mendell (ed.), Lunar Bases and Space Activities of the 21st Century (1985). Frederick C. Durant III David Dooling, Jr. The Editors of the Encyclopdia Britannica Elements of space flight The environment of space Space, as considered here, may be defined as all the reaches of the universe beyond the atmosphere of the Earth. There is no definitive boundary of the atmosphere of the Earth. For convenience it may be considered to extend to an altitude of 160 kilometres above the Earth's surface. Such a distance is infinitesimal in comparison with the immensity of the universe. Even within the solar system, distances between planets are measured in tens of millions of kilometres. The distance to Pluto, the Sun's outermost planet, is more than 4,830,000,000 kilometres. While the planets and their satellites all travel at different speeds around the Sun, the solar system itself is travelling through our galaxy, the Milky Way. The Earth's nearest neighbouring stars lie more than four light-years (approximately 40,000,000,000,000 kilometres) away. This apparent unimaginable void of space, however, is not empty. Throughout these vast reaches, matter (largely hydrogen) is scattered at the extremely low density of perhaps 100 particles per cubic centimetre in interplanetary space and 10 particles per cubic centimetre in interstellar space. This is, however, a much greater vacuum than has been achieved on Earth. Additionally, space is permeated by gravitational fields and a wide spectrum of electromagnetic radiation, by cosmic rays and magnetic fields of unknown intensities and distributions. Until 1946 all deductions about space had been made from observations through the distorting atmosphere of the Earth. With the advent of high-altitude sounding rockets after World War II and the instrumented satellites and probes of the space age, it has been possible to discover firsthand the great complexities of space phenomena. Basic considerations in spacecraft design Spacecraft is a general term that includes sounding rockets, artificial satellites, and space probes. They are considered separately from the rocket-powered space launch vehicle, which gives escape velocity to the craft. A space probe is a spacecraft that is launched at higher than Earth orbital velocity and escapes the Earth's gravitational attraction. Space probes may be classed as lunar, planetary, or deep-space. Other classifications of spacecraft are manned or unmanned, active or passive. A passive satellite transmits no radio signals. It may be tracked optically or with radar, and radio communications signals may be bounced off its surface. Active satellites send out radio signals to make tracking easier and to transmit data from their instruments to ground stations or other craft. One other general differentiation of satellites is by function: scientific or applications. A scientific satellite carries instruments to obtain scientific data on magnetic fields, space radiation, the Sun or other stars, etc. Applications satellites have utilitarian tasks; examples are Earth survey, communications, and navigation satellites. Spacecraft thus differ greatly in size, shape, complexity, and purpose. Because more than 5,000 spacecraft have been launched since 1957, it is convenient to group them into program familiese.g., the Soviet Sputnik, Vostok, Soyuz, and Venera; and the U.S. Explorer, Intelsat, Apollo, Voyager, and Space Shuttle. Lightness of weight and functional reliability are primary features of spacecraft design. Depending upon their mission, spacecraft may spend minutes, days, months, or years in the environment of space. Mission functions must be performed while exposed to high vacuum, extreme variations in temperature, and radiation. There are nine general categories of subsystems found on most spacecraft. They are (1) power supply; (2) on-board propulsion; (3) communications; (4) attitude control (i.e., maintaining a spacecraft's orientation toward a specific direction and pointing precisely at selected targets); (5) environmental control (e.g., regulation of temperature and pressure and removal of toxic substances); (6) guidance and velocity control; (7) computer and auxiliary hardware; (8) structure (skeleton framework of the spacecraft that physically supports all other subsystems); and (9) engineering instruments that monitor the status of the spacecraft. Space programs Considering the two categories of space programs, manned and unmanned, a few generalizations may be made. Spacecraft without a human being aboard have invariably pioneered explorations. They are smaller, can operate for months or years, and offer no hazard to human life. Experiments and measurements, however, are limited by the need for preplanning. In manned flights, the range of experiments is greater because judgment can be exercised in observations, instrumentation can be adjusted, and, perhaps, repairs can be made and equipment maintained. Manned space flight is much more expensive because of the added weight of vehicle and equipment required to provide a habitable atmosphere and controls. This extra weight requires a much larger launch vehicle. Backup systems are often provided in manned spacecraft to provide for high reliability and safety of personnel. Space launch vehicles Although sounding rockets may reach altitudes above the atmosphere of the Earth, the term space launch vehicle is applied usually to those rocket boosters designed to place satellites in orbit or to impart Earth-escape velocity to spacecraft. By about 1950 the technology of rocket propulsion had reached a level at which consideration of a project to launch an Earth satellite became feasible. Worldwide scientific studies during the IGY of 1957?58 provided the basis for funding. In 1955 both the United States and the Soviet Union announced satellite programs as part of their national effort in the IGY. The Soviet Sputnik 3, the first instrumented multipurpose space laboratory. When Sputnik 1 and 2 were launched in 1957 the Soviet Union released no details of their launch vehicles. In May 1958 Sputnik 3, weighing nearly 1,360 kilograms, was launched. It was not until 1967 that the basic Soviet launch vehicle was displayed. It was a 2 1/2-stage vehicle of the ?A? series (in this case, ?A-1?): two stages with four drop-away booster pods. Each booster pod contained four rocket engines (totalling 16) with propellant tankage, and the central core had four engines. Propellants were conventional liquid oxygen and kerosene. The United States launched its early satellites with two different vehicles, the Jupiter-C and Vanguard. Jupiter-C was a modified Redstone liquid-propellant ballistic weapon of medium range to which were added more tankage length and three upper stages of clustered solid-propellant rockets. The modification was originally designed to achieve a velocity of six kilometres per second to test a nose cone (reentry vehicle). The desired velocity was obtained with two upper stages, one a cluster of four solid-propellant rockets and the other a single rocket. It was obvious that by increasing the final velocity 1.5 kilometres per second to the required 7.5 kilometres per second, satellite velocity could be obtained for a small scientific payload. The additional velocity was obtained by adding another stage with a cluster of solid-propellant rockets so that the upper stages consisted of 11, three, and finally one rocket carrying a payload weighing 8.2 kilograms. In 1954 the Army Ballistic Missile Agency and the Office of Naval Research jointly proposed this scheme, known as Project Orbiter, but a newly designed Vanguard launch vehicle was selected. Failures in early attempts to launch Vanguard, however, resulted in eventual approval of the Project Orbiter approach. Thus the first U.S. satellite, Explorer 1, was launched by a Jupiter-C on January 31, 1958. The Vanguard launch vehicle was a three-stage booster approximately equal in length (about 22 metres) to the Jupiter-C but much lighter in takeoff weight (10,250 kilograms compared to 29,000 kilograms). Vanguard launched its first satellite (1.4 kilograms) into high orbit on March 17, 1958. After a few more flights, the Jupiter-C was retired in 1958 and the Vanguard in 1959. During the 1960s the United States developed a series of standard launch vehicles. The Air Force modified a Titan II intercontinental ballistic missile (ICBM) for space launch purposes by strapping two solid-propellant booster rockets, three metres in diameter, to the liquid-propellant core vehicle. The Titan IIIC was used for large military satellites. Then NASA increased performance of the obsolete Thor intermediate-range ballistic missile (IRBM) by adding solid-propellant boosters. A liquid oxygen/liquid hydrogen upper stage, Centaur, was used on obsolete Atlas ICBM's and Titan III ICBM's to launch large spacecraft. The Saturn series of NASA launch vehicles was developed specifically for the Apollo lunar mission program. The two operational Saturn models were the two-stage Saturn IB and three-stage Saturn V. The Saturn IB was used for Earth orbital developmental missions of Apollo, while the Saturn V was employed for lunar missions. Saturn V stood 110.6 metres high and weighed over 2,700,000 kilograms at launch. It could place 104,000 kilograms in orbit and send 45,000 kilograms to escape velocity. For some years the launching of spacecraft was limited to the United States and the Soviet Union. The reason was that the rocket-powered launch vehicles were based on long-range ballistic missiles, which only these countries had developed. France was the third nation to launch a satellite (1965), followed by Japan (1970), the People's Republic of China (1970), and the United Kingdom (1971). Under the auspices of the European Space Agency (ESA), the nations of western Europe developed the Ariane expendable launcher during the 1970s to assure themselves of independent launch capability. This action was taken in response to the U.S. refusal to guarantee flights for communications satellites that might compete with U.S. telecommunications carriers. A three-stage vehicle that burns stored solid propellants in its first two stages and employs a cryogenic engine in its third, Ariane has become a formidable competitor for the U.S. Space Shuttle. It is capable of launching two satellites of the U.S. Delta class (an Earth-orbit payload of 1,770 kilograms) at one time or one Atlas-Centaur-class satellite (an Earth-orbit payload of 4,670 kilograms). With lengthened stages and the addition of solid boosters, Ariane is approaching payload weights that only the Shuttle can handle.