From the beginning of the Space Age, people recognized that Earth-orbiting satellites—able to see and communicate across vast distances—promised unique benefits. In the tense years of the Cold War, such spacecraft (known as applications satellites) evolved down two separate paths: one devoted to national security needs, the other to civilian interests.

Today, hundreds of civilian and military applications satellites ring the Earth, often operating side-by-side in orbit. They provide similar services—communications, photography, remote sensing, weather analysis, and navigation—reaching different but occasionally overlapping communities of users. These satellites have become an integral part of contemporary life. We take for granted daily reports on weather as seen from space and television via satellite, and we have come to expect that satellites will be on alert to enhance national security.