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Well, if you could go high enough, you could be in a geosynchronous orbit without moving horizontally at all. I think that's how space elevators would work. But

Well, if you could go high enough, you could be in a geosynchronous orbit without moving horizontally at all. I think that's how space elevators would work. But then to move to a lower orbit you would actually need to speed up.

jvanderbot 5y ago
If you "fell" from geosync, wouldn't you speed up? Angular momentum needs to be conserved, right? Edit:my terrible wording created answers to a different question.
- jameshart 5y ago
  If you ’fall’ from geosync orbit, you fall in a circle and wind up where you started. Orbiting is falling.
- blacksmith_tb 5y ago
  That's my understanding, geosync is the point at which you continually fall toward the Earth and miss, closer and you'd eventually hit, farther and you would move away.
  - jameshart 5y ago
    That’s not really correct. Geosync is the altitude at which a circular orbit takes 24 hours - meaning it takes as long to complete one orbit as it takes the earth to complete one revolution. But there’s an orbital velocity at EVERY altitude, and therefore there’s an orbital period at every altitude too. If you are at a particular altitude and you are going at a different speed than the one needed for a circular orbit, you are either going too fast (in which case you are in an elliptical orbit and you will gradually increase altitude and lose speed until you reach the top of that ellipse) or you are going too slow to stay in a circular orbit (in which case you will follow an ellipse down to a lower altitude and gain speed). In that ‘too slow’ case, if it’s much too slow then the ellipse you follow will take you low enough to hit the surface or atmosphere of whatever you’re orbiting (ie you will crash into it)
    - tialaramex 5y ago
      I think you knew this but it wasn't obvious from how you wrote it. Your orbit is not required to be a circle. The diagrams we draw for children are nice circles, but most things that we know are orbiting something do not travel in a circle, e.g. the Earth -- if the orbit was circular our seasons would be slightly different† and the insight that the orbits can be non-circular ellipses was critical to the reasoning that eventually got us a heliocentric model of our solar system. Geostationary communications satellites do have a basically circular orbit, as you said, but many other birds do not. Russia has a bunch of stuff in orbits that "linger" very high over Russian territory for much of their orbital period then shoot right around close to the back side of the planet quickly and linger again, these are called Molniya orbits. The US has a bunch of secret (presumably spy) satellites that fly less obvious orbits like this too, for presumably similar reasons. † Edited: This post originally said "very" different, but in fact the difference would be modest since our orbit just isn't that elliptical. I wasn't able to find out how modest, but certainly if you think summer in New Zealand (in December) is pretty similar to summer in England (in June) then it's reasonable to say at least that similar.
    - frosted-flakes 5y ago
      I think GP means that if you are stationary relative to the ground (such as on a space elevator or a rocket going straight up), 35786 km up is the only height which is a stable orbit. If you let go of the elevator too early, you'll fall to the ground; too high and you'll fly off into space.
- deleted 5y ago
  [deleted]
- kempbellt 5y ago
  Theoretical geosync means zero velocity in any direction relative to ground. Zero up, down, forward, back, left, or right. You're stuck above a single point of dirt/water, over the equator, at a fixed altitude. The only way to "fall" (lower your altitude) from here would be via some sort of acceleration force towards ground (like a burn). So yes, your speed would have to increase.
  - jvanderbot 5y ago
    Well fine. I meant that orbital speed (the "along track part") would increase if you lowered the altitude of a geosync object with rockets, string, or any other force, I think.
    - lazide 5y ago
      It doesn’t. If you pulled ‘down’, it would just make the orbit eccentric.

jvanderbot 5y ago

If you "fell" from geosync, wouldn't you speed up? Angular momentum needs to be conserved, right? Edit:my terrible wording created answers to a different question.

jameshart 5y ago
If you ’fall’ from geosync orbit, you fall in a circle and wind up where you started. Orbiting is falling.
blacksmith_tb 5y ago
That's my understanding, geosync is the point at which you continually fall toward the Earth and miss, closer and you'd eventually hit, farther and you would move away.
- jameshart 5y ago
  That’s not really correct. Geosync is the altitude at which a circular orbit takes 24 hours - meaning it takes as long to complete one orbit as it takes the earth to complete one revolution. But there’s an orbital velocity at EVERY altitude, and therefore there’s an orbital period at every altitude too. If you are at a particular altitude and you are going at a different speed than the one needed for a circular orbit, you are either going too fast (in which case you are in an elliptical orbit and you will gradually increase altitude and lose speed until you reach the top of that ellipse) or you are going too slow to stay in a circular orbit (in which case you will follow an ellipse down to a lower altitude and gain speed). In that ‘too slow’ case, if it’s much too slow then the ellipse you follow will take you low enough to hit the surface or atmosphere of whatever you’re orbiting (ie you will crash into it)
  - tialaramex 5y ago
    I think you knew this but it wasn't obvious from how you wrote it. Your orbit is not required to be a circle. The diagrams we draw for children are nice circles, but most things that we know are orbiting something do not travel in a circle, e.g. the Earth -- if the orbit was circular our seasons would be slightly different† and the insight that the orbits can be non-circular ellipses was critical to the reasoning that eventually got us a heliocentric model of our solar system. Geostationary communications satellites do have a basically circular orbit, as you said, but many other birds do not. Russia has a bunch of stuff in orbits that "linger" very high over Russian territory for much of their orbital period then shoot right around close to the back side of the planet quickly and linger again, these are called Molniya orbits. The US has a bunch of secret (presumably spy) satellites that fly less obvious orbits like this too, for presumably similar reasons. † Edited: This post originally said "very" different, but in fact the difference would be modest since our orbit just isn't that elliptical. I wasn't able to find out how modest, but certainly if you think summer in New Zealand (in December) is pretty similar to summer in England (in June) then it's reasonable to say at least that similar.
  - frosted-flakes 5y ago
    I think GP means that if you are stationary relative to the ground (such as on a space elevator or a rocket going straight up), 35786 km up is the only height which is a stable orbit. If you let go of the elevator too early, you'll fall to the ground; too high and you'll fly off into space.
deleted 5y ago
[deleted]
kempbellt 5y ago
Theoretical geosync means zero velocity in any direction relative to ground. Zero up, down, forward, back, left, or right. You're stuck above a single point of dirt/water, over the equator, at a fixed altitude. The only way to "fall" (lower your altitude) from here would be via some sort of acceleration force towards ground (like a burn). So yes, your speed would have to increase.
- jvanderbot 5y ago
  Well fine. I meant that orbital speed (the "along track part") would increase if you lowered the altitude of a geosync object with rockets, string, or any other force, I think.
  - lazide 5y ago
    It doesn’t. If you pulled ‘down’, it would just make the orbit eccentric.