Forces acting on artificial satellites
 
Since October 4th, 1957, many hundreds of artificial satellites have been placed in orbit about the Earth. We have seen that Newton himself showed that if the projectile was given a sufficient velocity outside the Earth’s atmosphere, it would become a satellite of the Earth. But it was only by the development of the rocket during and after the Second World War that a means was provided of imparting to a payload of instruments the velocity necessary to keep it in orbit.
Artificial satellites are subject to Newton’s laws of motion and the law of gravitation. They usually obey Kepler’s laws very closely. If the Earth were a point-mass and no other force acted upon the satellite, a satellite would obey Kepler’s laws exactly and remain in orbit for ever. Many forces, however, may act on the satellite. Among these forces are:
(1) the Earth’s gravitational field,
(2) the gravitational fields of the Sun, Moon and the planets,
(3) the Earth’s atmosphere and
(4) the Sun’s radiation pressure.
In almost every case, the orbital changes produced by the Sun, Moon and the planets are so small that they can be neglected. Only in the case of those artificial satellite orbits that take the satellite many thousands of kilometres away from the Earth does the disturbing effect of the Moon have to be considered. Even then, it is still small.
As a result of the momentum associated with photons within any flow of radiation, the flux produces a pressure or force on any surface which intercepts the radiation. For a satellite whose size is large (for example a balloon satellite) and whose mass is small, the Sun’s radiation pressure can produce large changes in the satellite orbit over many months. For all other satellites, the orbital changes due to solar radiation pressure are negligible unless orbital positions are required to very high precision.
The two main causes of change in a satellite orbit are, therefore, the departure of the Earth’s shape from that of a perfect sphere and the drag due to the Earth’s atmosphere on those satellites low enough to experience it. We examine each in turn.