How does a geostationary satellite remain over the same spot on Earth?
In: Planetary Science
Because this is ELI5, I’m going to use imprecise language that won’t *directly* translate to the underlying physics and math. I’m doing this so that we don’t actually need to use the formulas for centripetal force or get into “reference frames” and centrifugal force. The principles should be correct enough.
Being in orbit can be thought of as “continuously falling towards Earth, but always missing because you’re traveling too fast”. Basically, your “sideways” momentum, caused by moving in a direction *not* towards Earth, “cancels out” Earth’s gravity trying to pull you in.
To be in a stable orbit (i.e. not moving towards or away from Earth), these values need to *exactly* cancel out. The “force” of your momentum that prevents you falling towards Earth is dependent on your speed; if you’re moving faster, you’re more likely to break Earth’s gravity and go flying off. If you’re moving slower, Earth’s gravity will win out and you’ll fall to Earth.
The force of Earth’s gravity at a given distance away is fairly straightforward to calculate. When you combine all of this, you end up finding that, at every distance from Earth, there is one specific speed you need to maintain in order to stay in a stable orbit.
So, we want a “geostationary” satellite to always hover over the exact same spot on Earth. What this means is, we need that satellite’s (rotational) speed to exactly match that of the Earth’s rotation – it will complete 1 full orbit every 24 hours, which is when Earth completes 1 full rotation. You calculate that speed, and then, using our above approach, you figure out the distance from Earth at which that speed results in a stable orbit.
The result looks [like this.](https://i.stack.imgur.com/nxIh2.jpg). The satellites close to Earth in that image are all moving much faster than Earth’s rotation – you can occasionally see them moving across the night sky like airplanes or shooting stars. The ones in those “rings” *way* out there are geosynchronous/geostationary. They need to be moving much slower, so they have to be placed way out where Earth’s gravity is weaker (and the radius of their orbit is bigger).
In practice, nearly all satellites have stabilizers that will fire off to make tiny adjustments to keep the satellite on its orbit, because we’ll never get everything *perfectly* right (and there are too many variables to make it worthwhile over these simple adjustments every so often).
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