Franz T. Geyling _was born in Tientsin, China, and received a B.S. in 1950, an M.S. in 1951, and a Ph.D. in 1954 from Stanford University. He joined Bell Telephone Laboratories in 1954, and has been engaged in celestial mechanics studies of rockets and satellites, as well as stress analysis of submarine cables._ CASE HISTORY NO. 2 What Color Should a Satellite Be? Peter Hrycak _Mechanical Engineer—Member of Staff, Electron Device Laboratory_ THE PROBLEM _It is important for a satellite to stay at the proper temperature while it is orbiting in space. The instruments aboard it must continue to operate properly, and one way of insuring this is to keep them from being exposed to extreme heat or cold. We can, of course, regulate a satellite’s temperature somewhat with various kinds of devices, and we can see that one of its ends does not point towards the sun for too long. But in designing the Telstar satellite we also wanted to control temperature in an easier way: by covering the satellite’s external surface with material with the best properties—including the right color—for maintaining its over-all temperature at the right level._ The Radiation of Heat A satellite’s temperature is determined by the balance between the heat that enters the satellite and the heat that leaves it. This means that we must be concerned with how heat is transferred. Heat can be transferred in three ways: by _conduction_, when two bodies are in direct contact and their molecules collide; by _convection_, which utilizes the movement of warm currents in a fluid; and by _radiation_, in which heat energy travels as electromagnetic waves at the speed of light. With a satellite, we are concerned only with the last of these, since the only way energy can be gained or lost in space is by radiation. In the transfer of heat by radiation, the surface of the heated body—such as a satellite—is very important. All energy gained must be absorbed at the surface; all energy leaving must be emitted at the surface. So the physical properties of this surface control how energy is absorbed and how it is emitted. The origin of the radiant energy is vitally important; most surfaces, for instance, will behave differently when exposed to solar radiation from the sun’s temperature of 10,000° Fahrenheit than when exposed to radiation from nearby objects at room temperature. Absorptivity and Emissivity The physical property of a material that controls the way it absorbs radiant energy is called its _absorptivity_, and the property that controls its emission of energy is its _emissivity_. For absorptivity we use the symbol α; for emissivity we use the symbol ε.