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It is an object made by man and placed in orbit around a celestial body.
The word artificial satellite became a reality on October 4, 1957, with the placement in Earth orbit of Sputnik 1. Thereafter thousands of artificial bodies with very diverse functions, scientific, military, meteorological, communications, etc., they have been placed in orbit around the Earth, as well as other planets and natural satellites of other planets.
A satellite remains in orbit around the Earth (or another celestial body) when the gravitational attraction force is balanced with the centrifugal force. As the force of gravity exerted by a celestial body decreases in inverse proportion to the square of the distance, the higher the satellite is located, the lower the gravitational attraction force will be and the lower its orbital velocity.
160 km away from Earth, a satellite needs, to remain in orbit, a speed of approximately 28,000 km / h .; 500 km. a speed of about 27,000 km / h is sufficient; 5,000 km. away, the speed drops to 21,000 km / h.
Naturally, the higher the orbit, the greater the time taken by the satellite to make a turn around the Earth (period). The orbital periods of the three cases taken into consideration are, respectively, 1h 28m, lh 34m and 3h 17m.
A particularly special orbit is the one at 36,000 km. from Earth, where the satellite takes exactly 24 hours to complete a full turn. This means that, with respect to a certain geographical point of our planet, the satellite remains motionless because its orbital period coincides with that of Earth's rotation. An orbit of this type is called synchronous or geostationary.
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