As electromagnetic radiation leaves its source, it spreads out, traveling in straight lines, as if it were covering the surface of an ever-expanding sphere. This area increases proportionally to the square of the distance the radiation has traveled. In other words, the area of this expanding sphere is calculated as 4pi R 2, where R is the distance the radiation has travelled, that is, the radius of the expanding sphere. This relationship is known as the inverse-square law of (electro-magnetic) propagation. It accounts for loss of signal strength over space, called space loss. For example, Saturn is approximately 10 times farther from the sun than is Earth. (Earth to sun distance is defined as one astronomical unit, AU). By the time the sun’s radiation reaches Saturn, it is spread over 100 times the area it covers at one AU. Thus, Saturn receives only 1/100th the solar energy flux (that is, energy per unit area) that Earth receives.
The inverse-square law is significant to the exploration of the universe. It means that the concentration of electromagnetic radiation decreases very rapidly with increasing distance from the emitter. Whether the emitter is a spacecraft with a low-power transmitter, an extremely powerful star, or a radio galaxy, because of the great distances and the small area that Earth covers on the huge imaginary sphere formed by the radius of the expanding energy, it will deliver only a small amount of energy to a detector on Earth.1
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