There were several substances that met our requirements. After testing a number of them, we decided to use aluminum panels coated with a thin layer of aluminum oxide (Al₂O₃). This coating is very pure, hard, and stable, and we left it rough to minimize changes due to meteoroid abrasion. Its α/ε ratio is 0.24. The aluminum oxide coating can be applied by means of the plasma jet process—particles of aluminum oxide are heated to a partially molten state, mixed with gases, and then sprayed onto the cleaned, pre-coated aluminum panels (_see illustration above_). Using this carefully selected outer surface has helped solve the temperature-control problem. Since Telstar has been in orbit its internal and skin temperatures have kept well within the ranges we wanted them to. Thus you can see how some basic formulas from classical physics helped us choose the right material for the satellite’s surface—and even what color it should be. The blue-and-white checkered appearance that Telstar I finally took on was no accident—it was the result of carefully combining various colors and materials in just the right amounts to obtain the temperature balance we needed. Peter Hrycak _was born in Przemysl, Western Ukraine, and received a B.S. in 1954, an M.S. in 1955, and a Ph.D. in 1960 from the University of Minnesota. He joined Bell Telephone Laboratories in 1960, and has worked on low temperature refrigeration problems and thermal design and thermal testing of the Telstar satellite._ CASE HISTORY NO. 3 How Can We Make Optical Measurements on a Satellite? Jeofry S. Courtney-Pratt _Physicist—Head, Mechanics Research Department_ THE PROBLEM _Since the first Telstar satellite went into orbit, we have tried to trace its path through space precisely. But we also have had to keep a constant check on the position, or attitude, that the satellite takes as it travels. We are particularly interested in the direction of the spin axis about which it revolves, and we also want to know its spin rate, which is the number revolutions the satellite makes each minute. Although these might seem relatively simple jobs, they actually turned out to be rather complicated. And only at virtually the last minute, just before the satellite’s design was finally set, did we think of a new way of using reflected flashes of sunlight to report on its spin axis and spin rate._ Why We Want to Know About the Spin Axis