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I took a screenshot of the default view on my web browser. The satellites appear to be around 100km in diameter, but they are in reality 7m wide, so they are 14

I took a screenshot of the default view on my web browser. The satellites appear to be around 100km in diameter, but they are in reality 7m wide, so they are 14,000x larger on the map than in reality. The entire constellation would fit within 0.2% of one of the white dots as they appear in the map. Then you need to remember that the space is 3D, unlike road surfaces (ignoring bridges etc.)

gibolt 4y ago
Put another way, it is like there are 10,000 fish equally spaced around the ocean. One could swim 100 lifetimes and never see another fish.
- walrus01 4y ago
  As the surface area of a theoretical 3 dimensional spherical shell at earth diameter plus 550 km is considerably larger than the oceans, more like having 10,000 tiny anchovy sized fish, swimming a hundred lifetimes and never seeing one.
Veedrac 4y ago
Space is not really 3d in the intuitive sense from a packing perspective, because satellites take up orbits rather than stationary points and orbits don't pack densely. It's more like an embedded 2d space, the intuition for that being that a ring, think Saturn's, is 2d and intersects any orbit.
- godelski 4y ago
  Bullshit. Space is definitely 3D and has 3D packing. Starlink is at about 550km. That has an orbital period of 1.5917hrs. Satellites are 7m (as suggested above, but this sounds large). So let's space the next group by 10km. Now the orbital period is 1.5952hrs. We have to go to 580km (+30km) to get to 1.6hrs, which is a difference of 37 seconds. So you got 60km (520km-580km) where you have an error rate of under 1% (0.65%). That drift per day is easily accounted for and you're not going to have a destructive resonance event during the lifetime of the satellites. For comparison, the ISS (~420km orbit) has a natural decay of 2km per year. It's safe to say that you can pack these satellites <1km in orbital height without realistic risk of them running into one another. The real problem is tracking everything and we often have large margins of safety. Sure, you won't pack them like you would oranges, but even several kms apart that can create a pretty dense coverage map. Gravitational orbits aren't electron orbits and don't need to be quantized (at least on the tens of km level).
  - Veedrac 4y ago
    I'm not considering such concerns at all; my comment holds even considering perfect orbits followed by frictionless point objects of zero mass. You can of course pack into the height-dimension trivially when using circular orbits; the issue then is how many orbits you can pack into an orbital plane. I contend it scales with radius, not radius squared.
    - about3fitty 4y ago
      Also, it is kind of like an extreme version of the risks of flying cars. One hubcap/collision can mean catastrophic downstream effects.
    - deleted 4y ago
      [deleted]
  - Retric 4y ago
    The problem with your model is all orbits at a given altitude intersect. You can orbit at the equator, or orbit above and below the equator thus crossing the equator. There’s nothing special about the equator here. All polar orbits meet at the poles, and any non polar obit is forced to cross all polar orbits etc. In practice this is not be a significant problem because you can pack a great deal of satellites at a given altitude and moving to a slightly lower or higher orbit works just fine.
    - godelski 4y ago
      > The problem with your model is all orbits at a given altitude intersect. > In practice this is not be a significant problem Which is really why I didn't bring it up. We didn't even talk about eccentricity. My model is simple but this is HN, so I added magnitudes of error.
  - chaboud 4y ago
    Huh. I wasn’t worried before reading this thread. Now I’m a little more worried. 1. We don’t need to worry about Kessler syndrome, as the orbits of Starlink will decay in five years (from 550km). 2. ISS is at 420km. Has anyone else ever worked with something dangerous dangling over their heads? It’s kind of nerve wracking, even if you know that there’s very little chance of it falling and hitting you.
- deleted 4y ago
  [deleted]
- lutorm 4y ago
  Yes, the key is that all circular orbits at the same altitude must cross, and if the orbits cross there is a potential collision. Satellites at the same altitude can only avoid collisions by controlling the position in the orbit. Also, mean free path in a particle cloud depends on number density, cross sectional area and speed. The cross sectional area is small, but relative speed is also very high, which makes collisions more likely than one would think just based on their number density.
  - mlindner 4y ago
    Though note: SpaceX does exactly that. They passively position the positions of the satellites along their orbits so that Starlink satellites don't need to avoid other Starlink satellites in orbital operation.
samstave 4y ago
I found this helpful, to illustrate how massive the orbital 'spheres' are https://i.imgur.com/zGaLWDg.jpg https://i.imgur.com/zGaLWDg.jpg

gibolt 4y ago

Put another way, it is like there are 10,000 fish equally spaced around the ocean. One could swim 100 lifetimes and never see another fish.

walrus01 4y ago
As the surface area of a theoretical 3 dimensional spherical shell at earth diameter plus 550 km is considerably larger than the oceans, more like having 10,000 tiny anchovy sized fish, swimming a hundred lifetimes and never seeing one.

Veedrac 4y ago

Space is not really 3d in the intuitive sense from a packing perspective, because satellites take up orbits rather than stationary points and orbits don't pack densely. It's more like an embedded 2d space, the intuition for that being that a ring, think Saturn's, is 2d and intersects any orbit.

godelski 4y ago
Bullshit. Space is definitely 3D and has 3D packing. Starlink is at about 550km. That has an orbital period of 1.5917hrs. Satellites are 7m (as suggested above, but this sounds large). So let's space the next group by 10km. Now the orbital period is 1.5952hrs. We have to go to 580km (+30km) to get to 1.6hrs, which is a difference of 37 seconds. So you got 60km (520km-580km) where you have an error rate of under 1% (0.65%). That drift per day is easily accounted for and you're not going to have a destructive resonance event during the lifetime of the satellites. For comparison, the ISS (~420km orbit) has a natural decay of 2km per year. It's safe to say that you can pack these satellites <1km in orbital height without realistic risk of them running into one another. The real problem is tracking everything and we often have large margins of safety. Sure, you won't pack them like you would oranges, but even several kms apart that can create a pretty dense coverage map. Gravitational orbits aren't electron orbits and don't need to be quantized (at least on the tens of km level).
- Veedrac 4y ago
  I'm not considering such concerns at all; my comment holds even considering perfect orbits followed by frictionless point objects of zero mass. You can of course pack into the height-dimension trivially when using circular orbits; the issue then is how many orbits you can pack into an orbital plane. I contend it scales with radius, not radius squared.
  - about3fitty 4y ago
    Also, it is kind of like an extreme version of the risks of flying cars. One hubcap/collision can mean catastrophic downstream effects.
  - deleted 4y ago
    [deleted]
- Retric 4y ago
  The problem with your model is all orbits at a given altitude intersect. You can orbit at the equator, or orbit above and below the equator thus crossing the equator. There’s nothing special about the equator here. All polar orbits meet at the poles, and any non polar obit is forced to cross all polar orbits etc. In practice this is not be a significant problem because you can pack a great deal of satellites at a given altitude and moving to a slightly lower or higher orbit works just fine.
  - godelski 4y ago
    > The problem with your model is all orbits at a given altitude intersect. > In practice this is not be a significant problem Which is really why I didn't bring it up. We didn't even talk about eccentricity. My model is simple but this is HN, so I added magnitudes of error.
- chaboud 4y ago
  Huh. I wasn’t worried before reading this thread. Now I’m a little more worried. 1. We don’t need to worry about Kessler syndrome, as the orbits of Starlink will decay in five years (from 550km). 2. ISS is at 420km. Has anyone else ever worked with something dangerous dangling over their heads? It’s kind of nerve wracking, even if you know that there’s very little chance of it falling and hitting you.
deleted 4y ago
[deleted]
lutorm 4y ago
Yes, the key is that all circular orbits at the same altitude must cross, and if the orbits cross there is a potential collision. Satellites at the same altitude can only avoid collisions by controlling the position in the orbit. Also, mean free path in a particle cloud depends on number density, cross sectional area and speed. The cross sectional area is small, but relative speed is also very high, which makes collisions more likely than one would think just based on their number density.
- mlindner 4y ago
  Though note: SpaceX does exactly that. They passively position the positions of the satellites along their orbits so that Starlink satellites don't need to avoid other Starlink satellites in orbital operation.

samstave 4y ago

I found this helpful, to illustrate how massive the orbital 'spheres' are https://i.imgur.com/zGaLWDg.jpg https://i.imgur.com/zGaLWDg.jpg