How a satellite stays over one spot in the sky

HostIf you look at the side of a house, you'll often see a little round TV dish. It's always pointed at the same patch of empty blue sky, never moving an inch. It feels a bit strange because we know the Earth is spinning at a thousand miles an hour and everything in space is usually zooming around. How do we get a hunk of metal to just sit there without falling or drifting away?

GuestIt's a bit of a trick of the eye. Those satellites aren't sitting still at all. They're actually screaming through space at thousands of miles an hour. They just happen to be moving at the exact same pace as the ground beneath them. Think of it like a track meet where two runners are locked side-by-side. From their point of view, the other runner isn't moving away. They look frozen in place even though their legs are pumping as fast as they can. To us on the ground, that satellite looks like a fixed star, but it's actually keeping pace with our own spin.

HostBut if I throw a ball into the air, it falls back down. If a satellite is heavy and made of metal, why doesn't the Earth just pull it straight down to the ground?

GuestWell, it's falling. That's the secret to every satellite. They're in a state of constant falling, but they're moving sideways so fast that they keep missing the Earth. Think of it like this. If you throw a rock, it curves down to the ground. If you throw it harder, it goes further before it hits. If you could throw that rock fast enough, the curve of its fall would match the curve of the Earth. It would keep falling and falling, but the ground would keep curving away beneath it. It would just go round and round in a circle. Now, the speed you need to do that depends on how high up you are. The closer you're to the Earth, the harder the Earth tugs on you. So, satellites that are close to us, like the ones that take pictures of the ground, have to zoom around the whole world in about ninety minutes just to stay up.

HostSo if the low ones are zooming around the world that fast, they definitely aren't staying over one house. Do we just put the TV satellites further out so they slow down?

GuestThat's the big idea. As you move further away from Earth, its tug gets weaker. Because the tug is weaker, the satellite can move slower and still stay in its path. There's a very specific height where the math works out perfectly. It's about twenty-two thousand miles up. At that height, the speed a satellite needs to stay in its path is the same speed the Earth takes to do one full spin. It takes twenty-four hours to go around once. Since the Earth also takes twenty-four hours to spin once, the satellite and the person on the ground stay lined up perfectly.

HostTwenty-two thousand miles seems like a very specific number. What happens if a company gets it wrong and parks their satellite a few miles too low?

GuestThen the whole thing falls apart. If it's too low, the Earth pulls a bit harder, so the satellite has to move faster to stay up. It would start to race ahead of the Earth's spin. From your backyard, you would see the satellite slowly drifting toward the east. If it was too high, it would move slower and start to lag behind, drifting toward the west. There's only this one thin ring around the middle of the Earth where this trick works. We call it a graveyard sometimes, or a parking lot, because it's the only piece of real estate in space where you can stay put over one city.

HostA parking lot is a good word for it, but parking lots get crowded. If every country wants their satellites in that one thin ring, don't they bump into each other?

GuestIt's getting crowded, for sure. And there's another problem. The Earth isn't a perfect smooth ball, and the moon and sun are also tugging on these satellites. Over time, that satellite starts to wobble. It wants to tilt or drift out of its spot. To fix this, these satellites carry small fuel tanks and little thrusters. Every now and then, the controllers on the ground have to give the satellite a little nudge to push it back into its exact parking space. It's like a car that keeps rolling out of its spot, and you have to keep hopping in to pull the handbrake.

HostSo what happens when the gas tank runs out? If we can't nudge them back into place, do they just become wandering junk that hits the new satellites?

GuestWe actually have a plan for that because we don't want a crash in such a valuable spot. When a satellite is almost out of fuel, we don't let it just sit there and wobble. We use the last bit of gas to push it even further away from Earth, into a higher path where no one is working. We call that the graveyard orbit. It's a lonely place where thousands of dead satellites will spin for millions of years. They stay up there like ghosts, long after the people who built them are gone.

HostThe TV dish on a roof is really just looking at a tiny window into a crowded parking lot that hangs over our heads.

GuestThose old machines will stay in their silent ring long after our cities have changed and our languages are forgotten.

HostThe metal ring in the sky stays steady while the world turns underneath.