_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._