A satellite is an object that orbits another object. Gravity pulls the satellite closer to the primary object it orbits, but the satellite is moving perpendicular to that pull so quickly that the satellite continually avoids colliding with the primary object.
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 similar 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 the other object.
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 (As opposed to natural satellites).
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.
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 atmosphericpressure, 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."
