_Before we learned about the Van Allen belts, we expected that the solar cells used to power satellites would last for many years in space. We thought they would be damaged only by cosmic rays, micrometeorites, and occasional bursts of particles from the sun. But when the solar plants of some American satellites went out of action after only a few weeks in orbit, we realized that in the future solar cell power units would need better protection from radiation damage. We had learned that satellites—and particularly medium altitude communications satellites—must spend a lot of time in regions where they will be struck by thousands or even millions of high-speed radiation particles each second._ _This fact forced us to change almost all our thinking about solar power plants for satellites. To make sure they would last for several years, we had to design new types of solar cells and devise new ways of mounting them. We also had to revise our estimates of how much power we could expect to get from our cells._ _If a communications satellite is to go into regular commercial service, it must continue working for several years in space. The Telstar satellite, however, was designed as an experimental project, and we decided that two years would be a reasonable lifetime to plan for. When Project Telstar began, our problem was to develop solar cells that would operate in an environment subject to strong radiation effects—and keep on operating there for two years._ Organizing the Work Our work on the solar cells for Telstar I began in October, 1960. With just a little more than a year to go before the satellite had to be ready, there was no time to lose. So we decided to break down the over-all problem into three parts: —Finding out how radiation would affect various kinds of solar cells; —Making experimental cells and, when the best had been picked, determining the best ways to make them in the large quantities we would need; and —Developing ways to mount the cells on the Telstar satellite so that they would withstand the stresses of being launched, the effects of radiation particles, and extreme changes in temperature. A different group of people began work simultaneously on each of these three parts of the problem, with each of them going ahead under the assumption that the others would be successful. Each group had to find the answers to many very interesting questions, but since our space is limited we can only discuss some of them here. Before doing so, however, we must say something about what a solar cell is and how it works. Technical Background on Solar Cells