十万个为什么·科学启蒙30篇（3）

7 / 30

为什么宇航员会失重

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