An artificial
satellite is an object placed into orbit around the Earth for the purpose of scientific research, Earth applications,
or military reconnaissance. Orbit is achieved when the object is given a horizontal velocity of approximately 28,500 km/h (17,500 mph) at sea level. At this velocity the Earth's surface curves away from the horizontal as fast as gravity pulls the object downward. As the altitude of the
satellite increases, its velocity decreases, and its periodÑthe time the satellite takes to circle the EarthÑincreases. A satellite in a circular orbit 275 km (170 mi) above the Earth's surface will have a period of 1.5 hours and a velocity of 27,860 km/h (17,300 mph), but the same object in a circular orbit 35,840 km (22,300 mi) above the Earth would have a period of 24 hours (the same as the Earth's rotation) and a velocity of only 11,050 km/h (6,850 mph). A satellite in the latter orbit is called a synchronous satellite; if such a satellite orbits in the equatorial plane, it is termed geostationary because it will remain at the same point above the Earth's surface (see astronautics).HistoryThe theoretical possibility of establishing an artificial satellite of Earth had been mentioned in 1687 by the English mathematician Isaac Newton as a consequence of his work on the theory of gravitation. Only in the early 20th century, however, did the theoretical work of the Russian Konstantin Tsiolkovsky and the experimental work of the American Robert Goddard confirm that a satellite might be launched by means of a rocket.During the period from 1943 to 1946 several studies indicated that available rockets would be unable to place a satellite into orbit. Work on rockets for missiles and upper-atmosphere research was so extensive after World War II, however, that by 1954 the feasibility of launching a satellite was no longer in serious doubt (see rockets, missiles, and space launch vehicles). In October 1954 the Committee for the International Geophysical Year (IGY) recommended to member countries that they consider launching small satellite vehicles for scientific space exploration. In April and July 1955 the USSR and the United States, respectively, announced plans to launch such
satellites for the IGY. Accordingly, the USSR launched Sputnik 1 on Oct. 4, 1957, and the United States launched Explorer 1 on Jan. 31, 1958 (see Explorer; Sputnik). These two satellites provided an enormous stimulus for further work on artificial satellites, especially with the discovery of the Van Allen radiation belts made possible by Explorer 1.Emphasis was first placed on satellites to measure the characteristics of the new space environment and to lay a foundation for the design of communications, navigation, reconnaissance, scientific, and weather satellites and eventually for manned spaceflight. Experimental satellites for these applications quickly followed. Since 1957 several thousand satellites have been placed in orbit, and satellites are now an accepted part of daily life. The vast majority of these satellites were built by the United States and the USSR, but the European Space Agency, comprising the countries of Western Europe, is becoming actively engaged in space exploration through satellites. Canada, China, India, Italy, and Japan are among other countries that have built satellites for subsequent launch.