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.
Linked from
GRACE: Weighing the Earth with Two Chasing Satellites Gravity Is a One-Way Street: Pumped-Storage Hydro Gravity is fake - it's just density and buoyancy How a Spacecraft Steals Speed from a Planet How Do You Reach a Higher Orbit? How Fast Is 'Orbit'? Ingenuity (Mars Helicopter) Johannes Kepler
+14 more
Kepler's Third Law Newton's Cannon: the Thought Experiment That Did 270 Years of Field Work Otto Lilienthal Satellite Altimetry: Measuring Sea Level to the Centimetre Sir George Cayley The $10,000 Gravity Challenge 2.0 The Hammer and the Feather: A Drop on the Moon We would be flung off by the Earth's rotation Where in Space Can an Object Sit 'Still'? Why Do Some Orbits Lock Together? Why Jupiter Has Two Asteroid Camps Why Orbits Are Ellipses Why Planets Are Spheres Why the Moon Pulls the Oceans