Many hundreds of human-made machines orbit Earth to perform specific tasks that are broadly categorized as civil, commercial, or military. They are generically called satellites in the same way that the moons that orbit planets in this and other solar systems are called satellites. Still other space machines are engaged in the exploration of the solar system and have ranged virtually all over it on science missions that help us to understand the nature of where we live and how it came to be a part of the greater universe. Collectively, the Earth satellites and explorers are called spacecraft. The swarm of satellites around Earth—computer simulations make the planet look like a virtual beehive—has steadily progressed from being relatively exotic to pervasive. The servants ...

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