Satellites, Artificial and Natural

A satellite is a small or secondary body that revolves around a larger one in the solar system. There are both natural and artificial satellites, which revolve and rotate at different rates of speed. Consequently, they may be used to measure time on Earth and to determine extraterrestrial occurrences. In the future, time-measuring satellites will be used to aid space travel and to predict cosmic events.

Natural planetary satellites are classified according to either their composition or their type of orbit. Nearly 100 have thus far been observed and identified in our solar system, and a great deal is known about their size, composition, and other physical properties. Our moon is Earth's only natural satellite; some planets have many such [Page 1134]objects circling them. Jupiter, for example, has 49 officially named satellites. The rotation of most natural satellites about their respective planets is west to east, the same as the rotation of their planets. Some outer satellites, however, rotate in the opposite direction.

Artificial satellites are human-made objects that typically orbit the Earth, gathering data that are used for a multitude of purposes. There are six main types: (1) scientific research, (2) weather, (3) communications, (4) navigation, (5) Earth observing, and (6) military. Because their design is dictated by their intended use, satellites come in a variety of shapes and sizes. Some artificial satellites also orbit the moon, the sun, asteroids, and the planets Venus, Mars, and Jupiter, gathering information about the bodies they orbit. All artificial satellites are subject to the same physical laws as their natural counterparts. The first artificial satellite, Sputnik I, was launched in 1957 by the Soviet Union; today, according to estimates by the National Aeronautics and Space Administration (NASA), more than 8,000 satellites are in orbit. All the major industrialized nations use satellite technology.

Satellite observations provide a unique vantage point from which to study Earth, and they have provided researchers with a critical means for studying the solar system. This is the only way to make in situ measurements of Earth's atmosphere, without having to contend with the affects of the atmosphere. Previously, such measurements could only be made on the ground or on ships or airplanes.

Figure: The International Space Station (ISS), the largest human-made object ever to orbit Earth, is shown here after separation from Space Shuttle Atlantis, June 19,2007

Source: NASA.

Most satellites are launched from rockets at fixed sites; in the United States many are launched from Cape Canaveral, Florida. Such sites provide logistical support for the assembly, check-out, and launching of satellites, and these sites possess the technology necessary to support these activities, including radar, optical tracking equipment, and meteorological equipment.

The launching of a satellite is achieved by propelling the object at a high velocity that allows enough energy to be imparted so that the object can remain in orbit without the need for additional energy. Orbits exist in a variety of shapes, from circular to highly elliptical and from high altitude to low altitude. A higher-altitude orbit leads to longer orbital periods (i.e., time required to complete one orbit). Satellites with a low-altitude orbit, on the other hand, orbit more quickly but are more likely to decay and enter Earth's atmosphere sooner than those orbiting at a higher altitude.

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