—_What color should a satellite be?_ —_How can we make optical measurements on a satellite?_ —_How do we keep solar cell power plants working in space?_ —_Would time delay be a problem in using a synchronous satellite?_ —_How can we repair an orbiting satellite?_ As you can see, we have picked problems that offer a good deal of variety. Some of them have been satisfactorily solved; for others the solutions are not yet complete. Some deal with basic scientific research; others are much more concerned with the engineering applications of technical knowledge. Some were solved by careful, logical thinking; others were solved almost by accident. Some deal with a particular immediate task (in this case, Project Telstar); others are more concerned with general planning for satellite communications. At the Foundation: Basic Physical Principles Despite these many important differences, there is one common thread running through the solving of all the problems we have chosen. The men who have been working on them had to know some basic principles of classical physics—principles that most of them first learned in their high school physics classes. You can’t, for example, calculate a satellite’s orbit without knowing Newton’s Laws of Motion. You can’t make optical measurements on a satellite without knowing the law of reflection of light. You can’t decide what color a satellite should be without knowing the law of heat exchange. To emphasize the importance of a solid grounding in basic physical principles, we have tried to have our problems touch on most of the general areas of physics: mechanics, heat, sound, light, electricity and magnetism, electronics, the properties of matter, atomic physics, physics of the solid state. But most of them, of course, are not limited to just one of these—they cross the lines of a number of areas. For instance, the problem of keeping solar cell power plants working in space involves laws of heat, mechanics, and atomic physics, as well as physics of the solid state. And, in studying the perception of time delay, we even branch out into experimental psychology. Problem-Solving Techniques When you start to solve a problem in science or engineering you can go about it in several ways. In some cases you have no choice: There may be only one practical method of doing the job. Other times, there may be several ways to attack the problem. You may try one, find it to be unfruitful, and then work on another approach. You will see both these methods of attack in the case histories we present in the next chapter. Here are some of the techniques of scientific problem solving that we will be discussing: —_Applying basic principles directly._ In answering the questions “How do we calculate a satellite’s orbit?” and “What color should a satellite be?” the successful procedure was to begin with basic known concepts and use them in a new field.