How Gravity Keeps the ISS in Orbit

The delicate balance between Earth's gravity and the ISS's forward motion

The Orbital Paradox

The International Space Station (ISS) orbits Earth at about 400 km altitude, traveling at 7.66 km/s (27,600 km/h). It's in a constant state of freefall, yet never crashes. Here's why:

Newton's Cannonball Thought Experiment

Imagine firing a cannon horizontally from a tall mountain:

  • At low speed - projectile falls to Earth (A)
  • At medium speed - travels farther before falling (B)
  • At orbital velocity - falls at same rate Earth curves away (C)
A B C

The Physics of Orbit

Centripetal Force = Gravity

The ISS stays in orbit because Earth's gravity provides exactly the centripetal force needed for circular motion:

Fgravity = G × (mEarth × mISS) / r²

Fcentripetal = mISS × v² / r

At orbital velocity, these forces balance perfectly, creating stable orbit.

Gravity Velocity

Orbital Velocity Equation

v = √(G × MEarth / r)

Where:

  • v = orbital velocity
  • G = gravitational constant
  • MEarth = Earth's mass
  • r = distance from Earth's center

For ISS at 400km altitude: v ≈ 7.66 km/s

400 km altitude 7.66 km/s

Why Doesn't the ISS Fall Down?

Continuous Freefall

The ISS is constantly falling toward Earth, but its tremendous horizontal speed means it keeps missing the ground:

  • Gravity pulls ISS downward at 8.68 m/s²
  • Earth's surface curves away at same rate
  • Result: stable orbit with no propulsion needed
Initial position Without speed With orbital speed