十万个为什么·科学启蒙30篇（2）
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How Do Satellites Stay in Orbit Without Falling?
卫星如何绕地球运行而不坠落？
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Satellites don’t defy gravity — instead, they fall continuously toward Earth while moving sideways so fast that the planet curves away beneath them.
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This balance between forward velocity and gravitational pull creates a stable orbit, similar to how the Moon circles Earth.
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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.
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At higher altitudes, gravity weakens slightly, so satellites need less speed to stay in orbit, but take longer to circle Earth.
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Orbital decay happens when atmospheric drag, however thin, gradually slows a satellite down until it re-enters the atmosphere.
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Engineers carefully calculate launch angles and speeds so rockets place satellites precisely into desired orbits with minimal fuel use.
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GPS satellites orbit at 20,200 km and send timed radio signals that receivers use to triangulate position within meters.
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Without orbital mechanics — based on Newton’s laws and later refined by Einstein — modern navigation and weather forecasting wouldn’t exist.