Orbital Mechanics
Definition
The physics of satellites and planets — orbits are continuous falling, where forward speed exactly balances the fall toward the planet.
Technical definition
Bound orbits follow Kepler's laws; circular orbital speed is v = √(GM/r) and total mechanical energy is E = −GMm/2r.
Example
The ISS at 400 km altitude circles at 7.66 km/s — about 92 minutes per orbit — forever falling toward Earth but moving too fast sideways to land.
Common confusion
Astronauts are not in zero gravity — at 400 km Earth's gravity is still ~90% of surface strength. They float because they are in free fall, not because gravity vanished.