eli5 how are satellites placed in just the right spot so they dont fall to earth or drift into space?
In: Planetary Science
They aren’t exactly. Most are placed in kind-of the right spot. Then they have to be periodically boosted or adjusted. Most have some sort of on-board propulsion capability for this reason.
There is no “right spot”. A satellite is moving sideways fast enough to miss Earth as it falls and slow enough not to fly away.
The maximum possible speed is about double the minimum, so it’s really really easy to fall within this window.
They aren’t perfect; all objects in orbit require small adjustments from onboard “engines”/boosters to maintain orbit.
The moon is slowly drifting away from earth; and if satellites did nothing they would too or crash back into the atmosphere and crash into earth
So for the most part they’re actually not “just in the right spot” as you put it. A lot of orbits have to be maintained and corrected with smaller rockets on the satellite other wise they will fall to the earth.
But the answer is a lot of math and computer simulations. Orbits are highly predictable and the math that describes them is pretty well understood. Given what we know of the solar system, we can figure out what positions and velocities are required to maintain the orbit. Then when we launch the satellite, we can do Ben more math to figure out rocket burns that move the satellite into the right position and velocity.
Orbit is a condition, not a location
You can only achieve orbit if the right criteria are met, which happens when you achieve a specific horizontal velocity
An orbit is not an altitude and orbit is speed. https://what-if.xkcd.com/58/ .
Low earth orbit is about 7km/sec. This is the cheapest orbit you can get to. The altitude is a couple hundred miles, so that the drag of atmosphere does not slow you down enough to de orbit you.
Geosynchronous orbit is 3km/sec, at 36,000 km. However to get to this altitude you need to be going 10 km/sec in LEO to get there ( https://en.wikipedia.org/wiki/Geostationary_transfer_orbit )
Escape velocity from Earth (drift off into space) is 11km/sec
Because we know how orbits work – it’s just math. All you have to do is put the satellite in the right altitude with the right speed. The satellites themselves has small thrusters to made minor adjustments as needed.
That’s not how orbits work.
Say you have a bottle rocket. If you shoot it up at an angle, it’ll fly for a bit and then fall back down some distance away.
If you add more speed and make the thrust last longer, it’ll fly a bit farther and land a bit farther away in a *parabolic* arc.
Add enough speed and thrust for long enough, you’ll eventually fly so far that you go all the way around the planet. Just a bit more and you’ll miss the ground every time, but you’ll still be flying a circular path around the planet. That’s an orbit.
Add too much speed or thrust and your circular path will grow so large that the sun’s gravity will start overpowering earth’s, and you’ll drift away on a *hyperbolic* orbit.
**Ergo**; Orbit is not just a place, it’s a condition defined by a speed. And speed is easy to control.
Disclaimer: There are obviously other facets to this, like what happens if you just go straight up, or what happens if you stop thrusting for a bit and then start again later. These will change the shape of the orbit, but the general rule remains the same.
Acceleration is the way to leave an object’s orbit (escape velocity). We don’t accelerate a satellite fast enough to reach that velocity, so it remains in earth’s gravitational pull and starts its orbit. Any other adjustments come from some amount of propulsion system built-in beforehand.
At any altitude there’s one specific speed that will put you in a *circular* orbit – just fast enough to fall around the Earth at the same altitude all the time. But other speeds also produce orbits. Go a little slower and your sideways path will approach Earth, and will also speed up because you’re falling “down”. By the time you’ve reached the opposite side of the orbit you’ll be going so fast that you start to climb again, and eventually you end up at the initial position and speed, completing an *elliptical* orbit. Similarly, if you’re a bit too fast, you’ll climb for the first part of the orbit and descend for the second part, making a larger ellipse.
So you only need the precise correct speed and direction if you want your orbit to be a perfect circle. But there’s a wide range of speeds that allow an orbit that’s not circular.
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