Eli5: How do satellites stay in orbit, and don’t gravity pull them to the surface?


Eli5: How do satellites stay in orbit, and don’t gravity pull them to the surface?

In: Earth Science

Gravity does pull them back down to the surface but they’re just going too fast and end up missing the ground and have to keep going around

The area that they orbit in has gravity that is strong enough to hold a moving body in orbit, but not enough to pull it down. Like Earth is far enough from the sun that we don’t get sucked in, but the gravity is strong enough to hold us in orbit.

Satellites are always being pulled to the ground. They just keep missing it.

When you throw a ball, it travels in an arc toward the ground. Throw it harder and it goes further before hitting the ground.

A rocket is able to move fast enough sideways that even though it is always falling down, it is always going past the earth on its way down. Because there is barely any air at orbital elevations, things can stay in orbit for decades in low earth orbit, or essentially permanently further out.

For a way to really get a feel for it, play Kerbal Space Program. It’s a great way to get used to orbital mechanics and it’s fun as hell

They’re in “free fall” but they’re also moving so fast, that they’re constantly falling with the curve of the earth. It’s nifty

Gravity does pull them towards the surface, they’re just going fast enough sideways to miss the planet on the way down. .

Imagine a ball attached to the end of a rubber band. Now swing that ball around. Keep it moving fast enough and it will spin in a circle. The rubber band is always trying to pull the ball to your hand, but the ball is moving too fast for it to fall. It keeps “missing” your hand instead of getting pulled right back to it, but it’s still tethered so it’s not flying away. Slow that spin down and the ball is pulled back to your hand. Let go of the rubber band and it flies off in the last direction you were spinning it.

Same with a satellite. Gravity is like the rubber band, and the satellite is the ball, and your hand is the Earth. The satellite is moving fast enough that even though gravity is trying to pull it back to Earth, out keeps missing and spinning merrily along. If the satellite stopped moving, gravity would pull it back towards the Earth. If the Earth didn’t affect the satellite (ie there’s nobody holding onto the rubber band), it would fly off into space.

Until I found a random interest in space and rocket science, I thought rockets just went straight up, stopped, and used the baby thrusters to move themselves around. I found that wasn’t the case, there’s two points I want to make here. And I’m no professional so if I get anything wrong anyone feel free to correct me.

1. What goes up must come down
If a rocket is launched straight up, unless it goes fast enough to break itself away from the gravity of the earth [escape velocity] it will come straight down.

2. That obviously led me to discover that the only way a rocket can get in to earths orbit is by going up and at some point before leaving the atmosphere, side ways (go right or suffer). When you kill the throttle and coast to the top of that arc (apogee) , you will eventually slow down and start falling back to earth. But if you coast to the top of the arc and throttle up, you start going faster forward than you are down towards earth which causes the endpoint of that arc to change and eventually your arc will have widened so much that the ending point meets the beginning point. Now you’re going forward fast enough that earth is still pulling you down but, instead of falling back into the atmosphere and getting M3RK3D by drag, you fall to the other end of the circular (there are very few space craft in perfectly circular orbits) path you’re on around the planet.


So the way space craft change their orbits is by either 1. moving forward (prograde) faster and making the side directly across from it go higher
2. Apply thrust the backwards (retrograde) and you will slow your forward speed and make the side directly across from it go lower until the other side of the circle touches earth and becomes two points bringing your path back to an arc. If you apply enough thrust in one direction it kills the movement in the other direct which makes the arc get skinnier. If you kill all forward momentum you fall back down and bump your head.

Hope my autistic writing skills made sense, it’s been a long day 😩

Imagine you build a paper plane and you give a throw. It travels quite the distance before gravity pulls it to the ground.

If you were to build the perfect paper plane and were able to throw it hard enough and from a higher orbit, it still would be pulled towards the ground, but it’d keep ‘missing’ the ground as it travels over the horizon.

Because they move fast, its kinda (but not exactly) like when you have a rock on an elastic string and spin it around your head, the elastic pulls the rock In but the centrifugal force pushes the rock away, “orbit” would be if the rock was spun at just the right speed that the centrifugal force and the force from the elastic are the same.

In orbit the rock is a satelite, the elastic rope Gravity and the centrifugal force is caused by the high velocity.

In addition to the others, it might help to visualize that a low earth orbit satellite is not very high up. Look at [this graphic](https://i0.wp.com/www.cybera.ca/wp-content/uploads/2016/02/orbit-1.jpeg?w=800&ssl=1). Relatively, they are barely skimming the Earth, just above the atmosphere to avoid drag, but they’re going about 17,000 mph. You see the videos of rockets going straight up, but they only go straight up long enough to get out of the thicker part of the atmosphere. Then they start going mostly sideways to accelerate the satellites up to that velocity before releasing them. If a rocket went straight up and dropped off a satellite, it would quickly fall back to Earth because it doesn’t have the sideways velocity to keep “missing” Earth as it was pulled in by gravity.

Orbiting an object is when you fall towards it but move sideways so fast you just keep missing. When you missed for a full circle you are in orbit. As long as your sideways momentum doesn’t decrease you’ll keep missing. Luckily high up there aren’t many things that can kill your speed.

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