Satellites used to gain scientific knowledge. They may be satellites of Earth or of other planets. Satellites used to apply space knowledge and techniques to practical purposes are called applications satellites. Many spacecraft are used for multiple purposes, combining space exploration, science, and applications in various ways. Scientific satellites are often called research satellites, and applications satellites are commonly designated by the application field, for example, navigation satellites. See also Satellite (spacecraft).
The space environment was one of the first areas investigated in scientific satellites. In this environment are cosmic rays, dust, magnetic fields, and various radiations from the Sun and galaxies. The Van Allen radiation belts were discovered by the first United States satellites. Soon thereafter satellites and space probes confirmed the existence of the solar wind. The continued investigation of these phenomena by satellites and space probes has led to the knowledge that a planet with a strong magnetic field, like Earth, is surrounded by a complex region, called a magnetosphere.
Numerous investigations are made of the effect of the space environment on materials and processes, including effects of radiation damage, meteoric erosion processes, and extremely high vacuum for very long periods. The influence of prolonged weightlessness on welding, alloying metals, growing crystals, and biological processes constitutes a promising field for future practical applications.
Scientific satellites, sounding rockets, and space probes are having a profound effect on earth science. The Earth and its atmosphere can now be studied in comparison with the other planets and their atmospheres, providing greater insight into the formation and evolution of the solar system. Satellites also make possible precision measurements of the size and shape of Earth and its gravitational field, and even measurements of the slow drifting of continents relative to each other.
Satellites are also having a great influence on astronomy by making it possible to observe celestial objects in all the wavelengths that reach the vicinity of Earth, whereas the ionosphere and atmosphere prevent most of these radiations from reaching telescopes on the ground. Moreover, even in the visible wavelengths, the small-scale turbulence and continuously varying refraction in the lower atmosphere distorts the image of a stellar object viewed at the ground, so that there is a limit to the improvement that can be achieved by increasing the size of a ground-based telescope. See also Satellite (astronomy).
The applications satellites emphasize the continuing day-to-day practical utilization of the satellite. They are operational in nature, although some of them, either directly or as a by-product of their operational output, contribute significantly to research, for example, with meteorological or Earth survey satellites. An applied research program generally precedes establishment of an applications satellite system. Thus, Tiros and Nimbus laid the groundwork for operational meteorological satellites. Likewise, Syncom satellites preceded the Intelsat communications system. See also Communications satellite; Meteorological satellites; Military satellites; Satellite navigation systems; Space flight.
