十万个为什么·科学启蒙30篇（3）
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Why Do Astronauts Feel Weightless?
为什么宇航员会失重
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Astronauts feel weightless not because there’s no gravity in space—but because they’re in constant free fall.
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The International Space Station orbits Earth at about 28,000 km/h, falling toward the planet while moving forward fast enough to miss it.
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Everything inside—including astronauts and floating tools—falls at the exact same rate, so nothing presses against anything else.
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This creates the sensation of zero gravity, even though Earth’s pull is still about 90% as strong up there.
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Gravity weakens with distance, but astronauts only experience true microgravity because they’re falling continuously.
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Parabolic airplane flights mimic this feeling for about 20 seconds by following the same curved path as a thrown object.
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Long-term weightlessness affects muscles, bones, and fluid distribution, requiring daily exercise aboard the ISS.
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Without regular resistance training, astronauts can lose 1–2% of bone density per month in orbit.
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Returning to Earth feels heavy at first, as gravity suddenly pushes blood downward and muscles readjust.
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Weightlessness is physics in action—not magic, but motion shaped by gravity itself.