Why the ISS Doesn't Fall Out of the Sky
🛰️ Orbital mechanics
Question
If gravity pulls everything down, why does the ISS stay up 420 km overhead?
It IS falling - constantly. At 420 km altitude Earth's gravity is still 88% as strong as at the surface (gravity weakens with the square of the distance). The ISS moves sideways at 7.66 km/s, so as it falls, the ground curves away underneath it at exactly the same rate. It never runs out of ground. That continuous free-fall is what 'weightlessness' really is: astronauts float because they and the station are falling together, not because there is no gravity up there.
The math
g(r) = GM/r². At r = 6,791 km: g = 9.81 × (6371/6791)² = 8.63 m/s². In 1 second the ISS falls 0.5 × 8.63 × 1² = 4.3 m - and the Earth's surface curves away by d²/(2r) = 7660²/(2 × 6,791,000) = 4.3 m. Same number: it stays up.
Why it matters
'There is no gravity in space' is false. The ISS feels 88% of the gravity you feel on the ground - attach a scale to the floor and it reads nearly your full weight. Every satellite, the Moon, and the ISS are falling around the Earth right now.