Artificial Earth Satellites
Artificial Earth Satellites
spacecraft that are inserted into earth orbit and are intended for scientific and practical purposes. The first artificial earth satellite, which became the first man-made heavenly body, was launched by the USSR on Oct. 4, 1957; the launch was the result of achievements in rocketry, electronics, automatic control, computer technology, celestial mechanics, and other areas of science and technology. This satellite made possible for the first time the measurement of the density of the upper atmosphere (by changes in the satellite’s orbit), the study of the characteristics of radio-wave propagation in the ionosphere, and the verification of the theoretical calculations and basic engineering principles involved in orbiting a satellite. The first American satellite, Explorer 1, was orbited on Feb. 1, 1958. Other countries launched their own artificial earth satellites somewhat later: France on Nov. 26, 1965 (A-l), Australia on Nov. 29, 1967 (WRESAT 1), Japan on Feb. 11, 1970 (Ohsumi), The People’s Republic of China on Apr. 24, 1970 (China 1), and Great Britain on Oct. 28, 1971 (Prospero). Since 1962, some satellites built in Canada, France, Italy, and Great Britain have been launched by American launch vehicles. There is considerable international cooperation in space research. A number of satellites have been launched within the framework of scientific and technical cooperation among socialist countries. The first of them, Intercosmos 1, was launched on Oct. 14, 1969. By 1973, 1, 300 artificial earth satellites of various types had been launched, including about 600 Soviet satellites and more than 700 American and other satellites, including manned spacecraft and manned orbital space stations.
General information. By international agreement a spacecraft is called a satellite if it has made at least one revolution around the earth. Otherwise, it is considered a rocket probe carrying out measurements along a ballistic trajectory and is not recorded as a satellite. Depending on the purposes for which artificial earth satellites are used, they are divided into research and applications technology satellites. If a satellite is equipped with radio transmitters, some type of measuring instrumentation, or flash lamps for producing light signals, it is called an active satellite. Passive satellites are usually intended to be observed from the earth’s surface for the solution of scientific problems (such satellites include balloon-satellites up to several dozen meters in diameter). Research satellites are used in studies of the earth, heavenly bodies, and space. They include, in particular, geophysical satellites, geodetic satellites, and orbital astronomical observatories. Applications technology satellites include communications satellites, weather satellites, earth resource technology satellites, navigation satellites, and satellites designed to solve engineering problems (for studying the effects of the space environment on various materials and for the development and testing of on-board systems). Satellites intended to carry people are called manned satellites. Satellites in equatorial orbit lying near the plane of the equator are called equatorial satellites; those in polar, or circumpolar, orbit are called polar satellites. Satellites inserted into circular equatorial orbit at a distance of 35,860 km from the earth’s surface and moving in the direction of the earth’s rotation “hang” motionless over a single point on the planet’s surface and are called stationary satellites. The last stages of launch vehicles, nose cones, and some other components that separate from satellites during orbital insertion are secondary orbital objects; they are normally not called satellites, even though they revolve around the globe in circumterrestrial orbits and, in some cases, are objects of scientific investigation.
According to the international system of registration of space vehicles (artificial earth satellites, space probes, and so on) within the framework of the international organization COSPAR, space vehicles in the period from 1957 to 1962 were designated by the year of launch, plus a letter of the Greek alphabet corresponding to the order of launch in the given year and an Arabic numeral that is the number of the vehicle depending on its brightness or level of scientific significance. Thus, 1957α2 is the designation for the first Soviet satellite, which was launched in 1957, and 1957α1 is the designation for the last stage of the satellite’s launch vehicle (the launch vehicle was brighter). Since the number of satellite launchings was increasing, as of Jan. 1, 1963, space vehicles began to be designated by the year of launch, the number of the launch in the given year, and a capital letter from the Latin alphabet (sometimes also replaced by a number). Thus, the Intercosmos 1 satellite has the designation 1969 88A or 1969 088 01. Within the various national space research programs satellite series often also have their own names: Cosmos (USSR), Explorer (USA), Diademe (France), and so on. Until 1969, the word “sputnik” was used outside the USSR only for Soviet satellites. In 1968–69, during the preparation of an international multilingual space dictionary, an agreement was reached according to which the term “sputnik” is to apply to satellites launched by any country.
Because of the diversity of the scientific and practical purposes for which artificial earth satellites are used, they may be of various shapes, weights, and configurations and may carry various on-board equipment. For example, the weight of the smallest satellites (of the ERS series) is only 0.7 kg; the Soviet Proton 4 satellite weighed about 17 tons. The weight of the Salyut orbital space station and the Soyuz spacecraft docked with it was more than 25 tons. The weight of the largest payload injected into earth orbit was about 135 tons (the American Apollo spacecraft together with the last stage of the launch vehicle).
Three types of artificial earth satellites are distinguished: unmanned satellites (research and applications technology satellites), on which all instruments and systems are controlled by commands either from the ground or by an on-board programming device; manned satellites; and manned orbital stations.
For the solution of certain scientific and practical problems, satellites must be oriented in space in a fixed position; the type of orientation is determined primarily by the satellite’s mission or the special features of the on-board equipment. Satellites intended for observing objects on the earth’s surface and in the atmosphere are oriented in such a way that one of their axes is constantly directed along the earth’s vertical, and satellites intended for astronomical research are oriented toward celestial objects (the stars and sun). The orientation may be changed by command from earth or by a preset program. In some instances only certain parts, rather than the entire satellite (for example, highly directional antennas), are directed toward ground receiving stations; solar batteries are directed toward the sun. To maintain a constant direction of one axis of the satellite, rotation around that axis is imparted to it. Gravitational, aerodynamic, and magnetic systems (the so-called passive attitude control systems), as well as systems using reaction and inertial controls (usually on sophisticated satellites and spacecraft), which are called active attitude control systems, also exist. Satellites with reaction engines for maneuvering, trajectory corrections, or de-orbiting are equipped with in-flight guidance systems, an integral part of which is the attitude control system.
The power supply for the on-board equipment of most satellites is solar batteries, whose panels are oriented perpendicular to the direction of solar rays or are arranged in such a way that some part of them is illuminated by the sun, whatever the sun’s position relative to the satellite (so-called omnidirectional solar batteries). Solar batteries ensure the long-term operation of equipment (up to several years). Electrochemical current sources—storage batteries and fuel cells—are used on satellites designed for limited periods of operation (as long as two to three weeks). Some satellites are equipped with radioisotope batteries for the generation of electric power. Temperature control systems maintain the temperature range required for the operation of equipment. Liquid heat-transfer loops are used on spacecraft and satellites whose equipment generates a considerable quantity of heat; for satellites whose equipment generates little heat, the systems are in a number of cases limited to passive means of temperature control (selection of an external surface with a suitable optical coefficient; thermal insulation of individual elements).
Scientific data are transmitted from the satellite to the ground using radiotelemetry systems, which are often equipped with on-board memory units for recording data when the satellite is outside the areas of radio visibility of ground stations.
Manned spaceships and some automatic satellites have reentry vehicles for returning the crew, some instruments, film, and experimental animals to earth.
Motion. Artificial earth satellites are injected into orbit with the aid of automatic, controlled multistage launch vehicles, which move from liftoff to some predetermined point in space as a result of the thrust developed by their rocket engines. This path, called the satellite’s orbital injection trajectory or the powered phase of the rocket’s movement, is usually several hundred to 2,000–3,000 km long. The rocket lifts off vertically and passes through the dense layers of the atmosphere at relatively low speed, which reduces the energy consumption needed to overcome atmospheric drag. During ascent the rocket turns gradually, and the direction of its movement becomes nearly horizontal. During this almost horizontal portion of the rocket’s flight its thrust is used mainly to increase its speed rather than to overcome the braking effect of the earth’s gravity and atmospheric drag. At the end of the powered phase, after the rocket has achieved a predetermined speed (and direction), the rocket engines shut down; this is the point of orbital injection. The spacecraft, which is carried by the last stage of the rocket, automatically separates from it and begins to move along an orbit relative to the earth, thus becoming an artificial heavenly body. Its movement is subject to passive forces (the earth’s gravity, as well as that of the moon, sun, and other planets; atmospheric drag; and so on) and active, or controlling, forces (if the spacecraft is equipped with special rocket motors). The type of initial orbit achieved by the satellite depends entirely on its position and speed at the end of the powered phase (at the moment of orbital injection) and is calculated mathematically with the aid of the methods of celestial mechanics. If this speed is equal to or not more than 1.4 times greater than orbital velocity (about 8 km/sec measured at the earth’s surface) and its direction does not deviate greatly from the horizontal, the spacecraft achieves earth orbit. The point of orbital injection in this case lies close to the orbit’s perigee. Orbital injection is possible at other points along the orbit (for example, near the apogee), but since the satellite’s orbit lies at a lower altitude than the point of orbital injection, the point itself must be located at a sufficiently high altitude, and the spacecraft’s speed at the end of the powered phase must be somewhat less than circular velocity.
In the first approximation, the orbit of an artificial earth satellite is an ellipse whose focus is at the center of the earth (in a particular case, a circle) and which maintains an unchanged position in space. Movement along such an orbit is called undisturbed motion and proceeds on the assumption that the earth attracts, according to Newton’s law, like a sphere with spherical density distribution and that the satellite is affected only by the earth’s gravitational attraction.
Such factors as atmospheric drag, compression of the earth, solar radiation pressure, and the attraction of the moon and sun are the reasons for deviations from undisturbed motion. The study of these deviations makes it possible to obtain new data on the properties of the atmosphere and the earth’s gravitational field. Because of atmospheric drag, satellites in orbits at altitudes of several hundred kilometers gradually descend and, entering the dense layers of the atmosphere at an altitude of 120–130 km and lower, disintegrate and burn up; thus, they exist for a limited period. For example, the first Soviet satellite at the moment of orbital injection was at an altitude of 228 km above the earth’s surface and was moving at an almost horizontal velocity of about 7.97 km/sec. The semimajor axis of its elliptical orbit (that is, the average distance from the center of the earth) was about 6,950 km, the orbital period was 96.17 min, and the perigee and apogee were about 228 and 947 km, respectively. The satellite existed until Jan. 4, 1958, when, as a result of perturbational variations in its orbit, it reentered the dense layers of the atmosphere.
The orbit into which a satellite is injected immediately after the launch vehicle’s boost phase is sometimes only an intermediate orbit. In this case the satellite is equipped with rocket motors that fire (on command from the ground) at specific moments for short periods of time, thus imparting additional speed to the satellite. As a result the satellite shifts to a different orbit. Unmanned interplanetary probes are usually first injected into earth orbit and then are moved directly to a lunar or interplanetary transfer trajectory.
Tracking. The movement of artificial earth satellites and secondary orbital objects is monitored by tracking them from special ground stations. Using the results of this tracking, the satellites’ orbital elements are determined more precisely and the ephemeris are calculated for subsequent observations, including those made for scientific and practical purposes. According to the equipment used, tracking is divided into optical, radio, and laser categories; according to the ultimate goal, it may be divided into positional tracking (determination of the direction to the satellite) and distance-measurement tracking, which is accompanied by measurements of the satellite’s actual speed and angular velocity.
Among the simplest positional tracking observations are visual (optical) observations, which are carried out with the aid of optical instruments and make possible the determination of the celestial coordinates of a satellite with an accuracy of up to several minutes of arc. Photographic observations are made for scientific purposes with the aid of satellite cameras, which ensure accuracy up to 1–2” in position and 0.001 sec in time. Optical observations are possible only if the satellite is illuminated by the sun’s rays (geodetic satellites equipped with pulse light sources are an exception; they may be observed while in the earth’s shadow), if the sky over the ground station is sufficiently dark, and if the weather is favorable for such observations. These conditions considerably limit the opportunities for optical observation. Radio tracking methods, which are the primary methods for tracking satellites while the special radio systems on them are functioning, are less dependent on such conditions. Such tracking includes the reception and analysis of radio signals that are either generated by the satellite’s on-board transmitters or are sent from the ground and relayed back by the satellite. A comparison of the phases of the signals being received by several widely separated antennas (a minimum of three) makes it possible to determine the satellite’s position on the celestial sphere. The accuracy of such tracking is about 3’ in position and 0.001 sec in time. Measurement of the Doppler shift of the signal frequency makes possible the determination of the satellite’s relative speed, the minimum distance to it during the given orbital pass, and the moment when the satellite was at that distance; observations made simultaneously from three ground stations make it possible to calculate the angular velocities of the satellite.
Distance-measurement tracking is accomplished by measuring the time interval between the transmission of a radio signal from the ground and its reception after being relayed back by the satellite’s radio transponder. Laser range finders ensure the most accurate measurement of the distances to satellites (accuracy to within 1–2 m and better). Radar systems are used for radio tracking of passive space objects.
Research satellites. On-board instrumentation, as well as ground-based tracking, makes possible the conduct of various types of geophysical, astronomical, and geodetic research. The orbits of such satellites vary from the nearly circular at altitudes of 200–300 km to the bulging elliptical with apogees of up to 500,000 km. Examples of research satellites are the Soviet Elek-tron, Proton, and Cosmos series; the American Avant-garde, Explorer, OGO, OSO, and OAO (Orbiting Geophysical, Solar, and Astronomical observatories) series; the British Ariel satellite; and the French Diademe. Research satellites account for about half of all satellites launched.
Satellite-based instrumentation is used in studying the neutral and ionic composition of the upper atmosphere and its pressure and temperature, as well as changes in these parameters. The electron concentration in the atmosphere and variations in the concentration are studied both with on-board instruments and by observation of the passage of the signals of satellite radio beacons through the atmosphere. Ionosphere probes have aided in the detailed study of the structure of the upper part of the ionosphere (above the area of maximum electron concentration) and changes in electron concentration depending on the geomagnetic latitude and time of day. All the results of atmospheric research obtained with the help of satellites are important and reliable experimental material for understanding the mechanisms governing atmospheric processes and for resolving such practical problems as the forecasting of radio communications conditions and of the state of the upper atmosphere.
Artificial earth satellites are being used in the detection and study of the earth’s radiation belts. Along with space probes, satellites have made possible the study of the structure of the earth’s magnetosphere and the nature of the solar wind’s flow past the earth, as well as the characteristics of the solar wind itself (flow density and particle energy, the magnitude and nature of an “infused” magnetic field) and other solar emissions that cannot be observed from the ground (ultraviolet and X-ray emissions, the study of which is of considerable interest from the point of view of understanding sun-earth relationships). Valuable scientific data are also provided by some applied-technology satellites. For example, the results of meteorological satellite observations are widely used for various geophysical studies.
The results of satellite tracking make possible the highly accurate determination of perturbations in their orbits, changes in the density of the upper atmosphere associated with various manifestations of solar activity, the laws governing atmospheric circulation, and the structure of the earth’s gravitational field. Synchronous position-fixing and distance-measurement satellite observations taken simultaneously from several ground stations make possible by methods of satellite geodesy the geodetic correlation of points thousands of kilometers apart, as well as the study of the movement of continents.
Applications technology satellites. Satellites launched for scientific, economic, or military purposes are classified as applications technology satellites.
Communications satellites provide television transmissions and radiotelephone and telegraph communications between ground stations separated by distances of up to 10,000–15,000 km. The on-board radio equipment of such satellites receives signals from ground stations, amplifies them, and rebroadcasts them to other ground stations. Communications satellites are placed in high orbits (up to 40,000 km). Examples of satellites of this type are the Soviet Molniia and the American Syncom and Intelsat satellites. Communications satellites inserted into stationary orbits remain over specific areas of the earth’s surface.
Weather satellites provide regular transmission to ground stations of television pictures of the earth’s cloud, snow, and ice covers, as well as data on the thermal radiation emitted by the earth’s surface and clouds. Satellites of this type are launched into nearly circular orbits at altitudes from 500–600 km to 1,200–1,500 km; their scan path may be as wide as 2,0003,000 km. Examples of weather satellites include some Soviet satellites of the Cosmos series, the Meteor satellites, and the American Tiros, ESSA, and Nimbus satellites. Experiments are being conducted on global observations from altitudes of up to 40,000 km (the Soviet Molniia 1 satellite and the American ATS satellite).
Earth resource technology satellites are particularly promising from the standpoint of their use in the economy. In addition to meteorological, oceanographic, and hydrological observations, such artificial earth satellites make it possible to obtain the current data necessary in geology, agriculture, fishing, and forestry and for the monitoring of environmental pollution. The results obtained with the aid of artificial earth satellites and manned spaceships on the one hand and the control measurements made by balloons and aircraft on the other indicate a promising future for this area of research.
Navigation satellites supported by a special ground-based system provide for ship navigation, including that of underwater craft. Ships fix their positions by receiving radio signals and determining the ship’s location relative to the satellite, whose orbital coordinates are known at every second with great accuracy. The American Transit and Navsat satellites are examples of navigation satellites.
Manned spacecraft. Manned spacecraft and orbital stations represent the most complex and sophisticated type of satellites. As a rule, they are designed for a broad spectrum of tasks, above all interdisciplinary scientific research, the testing of space hardware, and the study of the earth’s natural resources. The first manned space flight took place on Apr. 12, 1961: Cosmonaut Iu. A. Gagarin flew around the earth in the Soviet Vostok space craft along an orbit having an apogee of 327 km. On Feb. 20, 1962, the first American spaceship, piloted by J. Glenn, orbited. The flight of the Soviet Salyut orbital station represented a new stage in space research with the aid of manned satellites. Its crew, consisting of G. T. Dobrovol’skii, V. N. Volkov, and V. I. Patsaev, carried out an extensive program of scientific, medical, and other research.
REFERENCESAleksandrov, S. G., and R. E. Fedorov. Sovetskie sputniki i kosmicheskie korabli, 2nd ed. Moscow, 1961.
El’iasberg, P. E. Vvedenie v teoriiu poleta iskusstvennykh sputnikov Zemli. Moscow, 1965.
Ruppe, H. O. Vvedenie v astronavtiku, vol. 1. Moscow, 1970. (Translated from English.)
Levantovskii, V. I. Mekhanika kosmicheskogo poleta v elementarnom izlozhenii. Moscow, 1970.
King-Hele, D. Teoriia orbit iskusstvennykh sputnikov v atmosfere. Moscow, 1966. (Translated from English.)
Riabov, Iu. A. Dvizhenie nebesnykh tel. Moscow, 1962.
Mueller, I. Vvedeniev sputnikovuiu geodeziiu. Moscow, 1967. (Translated from English.)
N. P. ERPYLEV, M. T. KROSHKIN, IU. A. RIABOV, and E. F. RIAZANOV