A satellite or artificial satellite is an object intentionally placed
into orbit around a celestial body. Satellites have a variety of uses,
including communication relay, weather forecasting, navigation (GPS),
broadcasting, scientific research, and Earth observation. Additional military
uses are reconnaissance, early warning, signals intelligence and, potentially,
weapon delivery. Other satellites include the final rocket stages that place
satellites in orbit and formerly useful satellites that later become defunct.
Except for passive satellites, most satellites have an electricity generation
system for equipment on board, such as solar panels or radioisotope
thermoelectric generators (RTGs).