Why Don't Satellites Fall Down?
They are falling — constantly. The trick is that they are also moving sideways fast enough to keep missing the Earth.
Gravity does not switch off in space. A satellite 400 kilometres up still feels almost as much of Earth's pull as you do standing on the ground. So why does it not simply drop?
Newton's cannonball
Isaac Newton imagined a cannon on top of an impossibly tall mountain. Fire the ball gently and it arcs and lands nearby. Fire it harder and it lands further away, because the ball is falling and travelling forward at the same time.
Now fire it hard enough and something odd happens. The ball falls — but the Earth curves away beneath it at exactly the same rate. The ball never gets any closer to the ground. It has gone into orbit.
An orbit is not the absence of falling. It is falling, forever, and always missing.
Even in low orbit there are a few stray molecules of atmosphere, so satellites there slowly lose speed and must occasionally fire small thrusters to stay up.