十万个为什么·科学启蒙30篇（2）

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卫星如何绕地球运行而不坠落？

Satellites don’t defy gravity — instead, they fall continuously toward Earth while moving sideways so fast that the planet curves away beneath them.
This balance between forward velocity and gravitational pull creates a stable orbit, similar to how the Moon circles Earth.
Low Earth orbit satellites travel at about 28,000 km/h — completing one lap every 90 minutes — while geostationary ones orbit slower at 36,000 km altitude.
At higher altitudes, gravity weakens slightly, so satellites need less speed to stay in orbit, but take longer to circle Earth.
Orbital decay happens when atmospheric drag, however thin, gradually slows a satellite down until it re-enters the atmosphere.
Engineers carefully calculate launch angles and speeds so rockets place satellites precisely into desired orbits with minimal fuel use.
GPS satellites orbit at 20,200 km and send timed radio signals that receivers use to triangulate position within meters.
Without orbital mechanics — based on Newton’s laws and later refined by Einstein — modern navigation and weather forecasting wouldn’t exist.