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A satellite is an object that orbits another object. With sufficient tangential velocity, the object does not collide with the primary object it orbits, but maintains a distance from that object as the rate at which it falls towards that object is similar to the rate that it travels away, thus the object orbits the primary object and becomes a satellite. In other words: gravitational force serves as the centripetal force needed to make the object circle the primary object. The motion of the satellite around its primary gravitational source is known as freefall.

Because all objects exert gravity, the motion of the primary object is also affected by the satellite. (This observation allows for the discovery of extrasolar planets) If two objects are sufficiently close in mass, they are generally referred to as a binary system[?] rather than a primary object and satellite. The general criterion for an object to be a satellite is that the center of mass of the two objects is inside one of the objects.

All masses that are part of the solar system, including the Earth, are satellites of the Sun, or satellites of those objects, such as the Moon.

In common usage, the term is usually used to describe an artificial satellite.

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History of Artificial Satellites

In May, 1946, the Preliminary Design of an Experimental World-Circling Spaceship stated, "A satellite vehicle with appropriate instrumentation can be expected to be one of the most potent scientific tools of the Twentieth Century. The achievement of a satellite craft would produce repercussions comparable to the explosion of the atomic bomb..." (see: Project RAND)

The space age began in 19461, as scientists began using captured German V-2 rockets to make measurements in the upper atmosphere. Before this period, scientists could use balloons up to 30 km and radio waves to study the ionosphere, rockets changed that. From 1946 to 1952, upper-atmosphere[?] research was conducted using V-2s and Aerobee rockets. This allowed measurements of atmospheric pressure, density, and temperature up to 200 km. (see also: magnetosphere, Van Allen radiation belt)

The US had been considering launching orbital satellites since 1945 under the Bureau of Aeronautics[?] of the United States Navy. The Air Force's Project RAND eventually released the above report, but did not believe that the satellite was a potential military weapon, rather they considered it to be a tool for science, politics, and propaganda. In 1954, the Secretary of Defense stated, "I know of no American satellite program."

Following, pressure by the American Rocket Society, the National Science Foundation, and the International Geophysical Year; military interest picked up and in early 1955 the Air Force and Navy were working on Project Orbiter[?], which involved using a Jupiter C rocket to launch a small satellite called Explorer 1 on January 31st 1958.

On July 29, 1955, the White House announced that the US intended to launch satellites by the spring of 1958. This became known as Project Vanguard. On July 31, the Soviets announced that they intended to launch a satellite by the fall of 1957 and on October 4, 1957 Sputnik I was launched into orbit.

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