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>> ISS constantly falls down to earth. It is also moving so fast that it constantly misses earth. I'm gonna have to roll up some herb while you explain this co

>> ISS constantly falls down to earth. It is also moving so fast that it constantly misses earth. I'm gonna have to roll up some herb while you explain this comment further.

Dylan16807 8y ago
It is always accelerating straight toward the center of the planet at ~1G.
superice 8y ago
Think of firing a canon sideways: it takes a while for the cannon ball to drop and hit the earth. Now imagine firing it faster and faster, till eventually it's dropping at the same rate as the earth curves. That is essentially what the ISS is doing at a height of about 400km above the surface.
- aurailious 8y ago
  And the important part about orbits is that wind resistance won't slow it down significantly. At a low orbit like the ISS it touches the atmosphere ever so slightly and needs a boost every now and then.
- deleted 8y ago
  [deleted]
- oliveshell 8y ago
  It’s worth noting that if you shoot a cannonball dead horizontally, it hits the ground at the same time as it would if dropped from the same height [1]. It just has more horizontal speed, so it travels sideways a decent amount during its fall. Anyways, yes, the ISS is scooting sideways so fast that, in the time it would have taken to fall straight down, the ground beneath it has dropped away by an amount equal to its initial height, so the thing stays at the same altitude. [1]: *This hurt my brain when I first heard it, but I promise it’s true: it’s just that cannons are normally fired at an upward angle, giving them upward velocity and thus “hang time.”
  - ethbro 8y ago
    > It’s worth noting that if you shoot a cannonball dead horizontally, it hits the ground at the same time as it would if dropped from the same height Do you have a citation for this? I'd expect, as with all physics questions, there's a caveat. In this case, "... over sufficiently small distances." If you fire, say, the main guns of a battleship "over the horizon" (from your initial vantage point), I'd strongly suspect this doesn't hold. In order for it to, the gravitational force vector, integrated over flight path, would have to perfectly counter the curvature of the Earth... which doesn't seem like it would line up so neatly. In reality, air friction and maximum muzzle velocities probably render most of these concerns moot for practical purposes.
    - JumpCrisscross 8y ago
      > there's a caveat Air.
    - sonnyblarney 8y ago
      "Do you have a citation for this?" This is Newtonian physics. Obviously, air, other things, friction mess it up. But essentially that's it. Horizontal motion of a body won't affect gravitations vertical pull.
      - plaguuuuuu 8y ago
        in a completely Newtonian universe (ignoring relativity), if you shoot the ball at high enough speed it will never hit the earth. horizontal motion doesn't affect the vertical pull, but the direction of the vertical pull changes by the time the ball's traveled well over the horizon.
        dragonwriter 8y ago
        > in a completely Newtonian universe (ignoring relativity), if you shoot the ball at high enough speed it will never hit the earth. Right. Assuming a perfectly spherical earth, at below orbital velocity, it hits the ground somewhere; at orbital velocity up to (but excluding) escape velocity it (assuming the cannon gets out of the way) orbits with the low point at (and opposite) the firing position, and beyond escape velocity it takes a curving path getting ever farther away.
thousandautumns 8y ago
What they mean is by "moving so fast" is, it's moving so fast horizontally. Imagine being at the top of a steep mountain and throwing a rock directly out from you. The rock will continue to move horizontally, due to the force behind your throw, as well as vertically, as it drops due to gravity. If you don't throw it hard enough, then the trajectory of the rock won't clear the mountain slope. If you throw it too hard, it will clear the base of the mountain by a wide margin. But if you throw it just right, the rock could hypothetically have a trajectory that is perfectly parallel to the mountain slope (in reality, due to things like air resistance, this isn't all that feasible). Anything in orbit it literally the exact same. You get something moving fast enough horizontally that even though the Earth's gravity is still pulling on it, its trajectory towards earth is perfectly parallel (actually often not perfectly, but in principle) with the curvature of the planet. But by doing it high enough in the sky, you can escape the atmosphere of the earth, to where there is no wind resistance, meaning once you get up to a high horizontal speed, you can turn off the engines and coast perpetually without slowing down. So everything in orbit, from the ISS to satellites, to the moon, are all in Earth's gravity well and are falling towards the planet, but they have a horizontal speed as well that keeps them from colliding with the surface. The Earth and the rest of the planets in the solar system do the same thing in terms of their orbit with the Sun. It's also why sometimes satellites or space junk that have been in the sky for years will come crashing down to Earth. Sometimes the calculations for how fast you need to be going are off, or something will throw off it's horizontal momentum, and that will cause it's trajectory to dip just enough that it is no longer orbiting the earth perfectly, but instead is spiraling ever so slightly towards the surface, and will over the course of months or years or decades dip closer and closer until it enters the atmosphere, at which time air resistance becomes a factor again and it breaks up and really plummets.
- fmihaila 8y ago
  > But if you throw it just right, the rock could hypothetically have a trajectory that is perfectly parallel to the mountain slope (in reality, due to things like air resistance, this isn't all that feasible). Not quite. Ignoring air resistance, any throw you could possibly produce will have a parabolic trajectory, which means it could be parallel with the mountain slope only if the slope itself is parabolic.
quickthrower2 8y ago
https://xkcd.com/2011/ https://xkcd.com/2011/ - so ignoring the sarcasm in the comic, the two red lines are "throwing a ball" like you are used to - and the black line is the ball that never quite reaches the ground - i.e. the ISS.

>> ISS constantly falls down to earth. It is also moving so fast that it constantly misses earth. I'm gonna have to roll up some herb while you explain this comment further.

Dylan16807 8y ago

It is always accelerating straight toward the center of the planet at ~1G.

superice 8y ago

Think of firing a canon sideways: it takes a while for the cannon ball to drop and hit the earth. Now imagine firing it faster and faster, till eventually it's dropping at the same rate as the earth curves. That is essentially what the ISS is doing at a height of about 400km above the surface.

aurailious 8y ago
And the important part about orbits is that wind resistance won't slow it down significantly. At a low orbit like the ISS it touches the atmosphere ever so slightly and needs a boost every now and then.
deleted 8y ago
[deleted]
oliveshell 8y ago
It’s worth noting that if you shoot a cannonball dead horizontally, it hits the ground at the same time as it would if dropped from the same height [1]. It just has more horizontal speed, so it travels sideways a decent amount during its fall. Anyways, yes, the ISS is scooting sideways so fast that, in the time it would have taken to fall straight down, the ground beneath it has dropped away by an amount equal to its initial height, so the thing stays at the same altitude. [1]: *This hurt my brain when I first heard it, but I promise it’s true: it’s just that cannons are normally fired at an upward angle, giving them upward velocity and thus “hang time.”
- ethbro 8y ago
  > It’s worth noting that if you shoot a cannonball dead horizontally, it hits the ground at the same time as it would if dropped from the same height Do you have a citation for this? I'd expect, as with all physics questions, there's a caveat. In this case, "... over sufficiently small distances." If you fire, say, the main guns of a battleship "over the horizon" (from your initial vantage point), I'd strongly suspect this doesn't hold. In order for it to, the gravitational force vector, integrated over flight path, would have to perfectly counter the curvature of the Earth... which doesn't seem like it would line up so neatly. In reality, air friction and maximum muzzle velocities probably render most of these concerns moot for practical purposes.
  - JumpCrisscross 8y ago
    > there's a caveat Air.
  - sonnyblarney 8y ago
    "Do you have a citation for this?" This is Newtonian physics. Obviously, air, other things, friction mess it up. But essentially that's it. Horizontal motion of a body won't affect gravitations vertical pull.
    - plaguuuuuu 8y ago
      in a completely Newtonian universe (ignoring relativity), if you shoot the ball at high enough speed it will never hit the earth. horizontal motion doesn't affect the vertical pull, but the direction of the vertical pull changes by the time the ball's traveled well over the horizon.
      - dragonwriter 8y ago
        > in a completely Newtonian universe (ignoring relativity), if you shoot the ball at high enough speed it will never hit the earth. Right. Assuming a perfectly spherical earth, at below orbital velocity, it hits the ground somewhere; at orbital velocity up to (but excluding) escape velocity it (assuming the cannon gets out of the way) orbits with the low point at (and opposite) the firing position, and beyond escape velocity it takes a curving path getting ever farther away.

thousandautumns 8y ago

What they mean is by "moving so fast" is, it's moving so fast horizontally. Imagine being at the top of a steep mountain and throwing a rock directly out from you. The rock will continue to move horizontally, due to the force behind your throw, as well as vertically, as it drops due to gravity. If you don't throw it hard enough, then the trajectory of the rock won't clear the mountain slope. If you throw it too hard, it will clear the base of the mountain by a wide margin. But if you throw it just right, the rock could hypothetically have a trajectory that is perfectly parallel to the mountain slope (in reality, due to things like air resistance, this isn't all that feasible). Anything in orbit it literally the exact same. You get something moving fast enough horizontally that even though the Earth's gravity is still pulling on it, its trajectory towards earth is perfectly parallel (actually often not perfectly, but in principle) with the curvature of the planet. But by doing it high enough in the sky, you can escape the atmosphere of the earth, to where there is no wind resistance, meaning once you get up to a high horizontal speed, you can turn off the engines and coast perpetually without slowing down. So everything in orbit, from the ISS to satellites, to the moon, are all in Earth's gravity well and are falling towards the planet, but they have a horizontal speed as well that keeps them from colliding with the surface. The Earth and the rest of the planets in the solar system do the same thing in terms of their orbit with the Sun. It's also why sometimes satellites or space junk that have been in the sky for years will come crashing down to Earth. Sometimes the calculations for how fast you need to be going are off, or something will throw off it's horizontal momentum, and that will cause it's trajectory to dip just enough that it is no longer orbiting the earth perfectly, but instead is spiraling ever so slightly towards the surface, and will over the course of months or years or decades dip closer and closer until it enters the atmosphere, at which time air resistance becomes a factor again and it breaks up and really plummets.

fmihaila 8y ago
> But if you throw it just right, the rock could hypothetically have a trajectory that is perfectly parallel to the mountain slope (in reality, due to things like air resistance, this isn't all that feasible). Not quite. Ignoring air resistance, any throw you could possibly produce will have a parabolic trajectory, which means it could be parallel with the mountain slope only if the slope itself is parabolic.

quickthrower2 8y ago

https://xkcd.com/2011/ https://xkcd.com/2011/ - so ignoring the sarcasm in the comic, the two red lines are "throwing a ball" like you are used to - and the black line is the ball that never quite reaches the ground - i.e. the ISS.