A satellite is a human-made object placed into orbit around a celestial body. The Moon is a natural satellite. The satellites we launch are artificial. They orbit Earth, Mars, or other bodies. They relay communications, observe weather, navigate, and spy. Thousands are active. Thousands more are defunct debris.

Satellites come in different orbits. Low Earth orbit is a few hundred kilometers up. It is used for imaging and internet constellations. Geostationary orbit is 35,786 kilometers up. It matches Earth's rotation, so the satellite stays over one spot. It is used for TV and weather. Medium Earth orbit hosts navigation satellites like GPS.

Types of satellites

• Communication: TV, internet, phone
• Navigation: GPS, Galileo, GLONASS
• Weather: forecasting and climate
• Earth observation: imaging and mapping
• Scientific: telescopes and research

Launch costs have fallen. SpaceX and others reuse rockets. Small satellites, called CubeSats, can be built by universities. The space around Earth is getting crowded. Collisions create debris that threatens other satellites. Space traffic management is a growing problem.