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A bit of a tangent, but is there any information about how many constellations are physically possible? Obviously there's a lot of space in LEO, but it's still

A bit of a tangent, but is there any information about how many constellations are physically possible? Obviously there's a lot of space in LEO, but it's still limited. Given that each provider would need their own very dense constellation of satellites in different orbits, things would eventually start getting crowded. I don't think we'll get there any time soon (or ever), but kind of an interesting thought experiment.

lazide 5y ago
You’re underestimating the amount of space in space (sorry, couldn’t resist). Volume increases exponentially as diameter increases. I don’t think anyone would worry about 1800 or even 180000 things spread over the entire surface of the earth. We have billions of cars, and they mostly avoid running into each other. Even 100 miles up? Far, far more space - and a lot less ‘stuff’ to worry about too. They could pack a million satellites up there and basic traffic management would be more than enough to avoid collisions.
- itzprime 5y ago
  But it is not the same thing, due to the speed that satelites travel.
  - BenjiWiebe 5y ago
    Satellites aren't going to swerve, though.
- rini17 5y ago
  We will see. But it's definitely don't comparable to cars. Say you want safety margin of mere 1 second, that means reserving 8 kilometers free corridor in front of every satellite. At all times. That isn't a small volume.
  - relativ575 5y ago
    Why one second and 8km? Satellites are not manually maneuvered. Airplanes are only required to keep 1,000 to 2,000 feet vertical separation. They travel at much slower speed than satellites (550 mph vs. 17,200 mph), but they are also much bigger. They tend to travel in narrow corridors, as opposed to being spread out like satellites. NASA and SpaceX have started working together to address the collision issues with their fleet: https://www.nasa.gov/press-release/nasa-spacex-sign-joint-spaceflight-safety-agreement https://www.nasa.gov/press-release/nasa-spacex-sign-joint-sp...
    - rini17 5y ago
      They are not manually maneuvered but there's still unpredictable inteplay of solar wind and tenuous atmosphere to correct. And some sats will fail, the environment is harsh. So I believe some safety separation will be a must, used 1 second as a rule of thumb and the orbital speed is 7.8 km/s at the altitude. I suspect more safety time will be required. Say a micrometeoroid impact happens causing one satellite to go off track. Now the controller computer needs some time to recompute whole swarm trajectories to minimize debris impacts. Even sats that are currently on the other side of planet might need to start thrusters immediately, as it takes only 10 minutes on average to cover the distance.
      - lazide 5y ago
        Let’s do some math and see. If we assume the earth is a sphere (I know it isn’t, but it’s mostly close), and using 4(pi)r^2, it’s about 511 million km2 in area on the surface (counting oceans, etc). Double checking, it’s actually about 507 million km2, so decently close. 4 * 3.1459 * (6378^2) Starlink is orbiting at 550km altitude (approx). At that altitude, we’re looking at… 4 * 3.1459 * (6928^2) = 603,977,365 km2 The thing to keep in mind about orbits is the defining factor is your eccentricity and velocity. Faster velocity == higher orbit, assuming it’s a round/circular orbit, that velocity will also be consistent. As long as everyone is going in the same direction and trying to maintain a circular orbit, you can stack orbits ‘on top’ of each other or put satellites quite close to each other just like planes do with flight levels. So even that number is misleading, as you could 10x that if you had decent traffic control. Even without that, you’re talking a density of 1 satellite per 600 square kilometers of space with a million satellites for a given ‘flight level’. The velocity matters a lot less than the relative velocity to other things you might be nearby - think of it like two cars going in lanes next to each other on the same ‘direction’ of a freeway, vs a free for all demolition derby with no traffic control, lanes, etc. If everyone is co-ordinated you can have problem free high density. If random stuff is happening, including people potentially trying to hit you - it’s going to be a mess. If everyone just does whatever and never co-ordinates, yeah someone is eventually going to run into someone else - but unless there are a LOT of satellites, the odds are low. With it being LEO, debris and dead satellites are also going to be a aerobraked pretty quickly.
- vl 5y ago
  >Volume increases exponentially as diameter increases. Cubically.
  - lazide 5y ago
    Thank you for the correction! That is indeed correct.
- dnadler 5y ago
  Yeah, that's certainly a good point. The analogy with cars is a bit tricky though as we have well defined rules that help us pack them very closely together on the surface. For example: roads, lanes, controlled intersections. Those things aren't quite the same when talking about orbits because cars and satellites aren't spread out the same way. The other commenter made the point about some safety margin, which obviously has to be a part of the equation. The speed and error of orbits would play into whatever that number comes out to be. Regardless of there being lots of space, there's still some limit, even if it's 1,000 constellations. Some variables I don't quite know: 1. Acceptable altitude range for constellations 2. Typical positioning error of a satellite 3. Average failure rate and failure mode of a satellite - ex. does a dead satellite automatically deorbit itself or do we need to wait for the orbit to decay? I'd imagine decay at that altitude is fairly quick. Would a decaying satellite in a higher orbit need to be accounted for? 4. Required satellite density in orbit and the number of unique orbits, their arrangement, etc. I'm sure there are other factors, too...
ncallaway 5y ago
I haven't done the math and I'm certainly not an expert, but my lay understanding is that the number of constellations is much more limited by spectrum frequency availability, rather than the space constraints in LEO. That is, if we had an very large number of companies wanting to create satellite internet constellations, we would run out of useful EM spectrum before we would run out of LEO orbits. I'm not positive about that, though!
- dnadler 5y ago
  Yeah, that occurred to me as well, though I don't know much about how the satellites communicate with eachother. I imagine each constellation would need its own band, but not sure how wide it needs to be? Intra-constellation communication could feasibly be done with lasers, in which case, all they may need to worry about is the Space-to-Ground leg. So maybe it's not too bad? I don't really know, but certainly interested in learning more...
rcxdude 5y ago
The limit is far more how tight you can beamform the signals and how much usable spectrum there is: this is the fundamental thing limiting the density and hence number of users starlink can have.

lazide 5y ago

You’re underestimating the amount of space in space (sorry, couldn’t resist). Volume increases exponentially as diameter increases. I don’t think anyone would worry about 1800 or even 180000 things spread over the entire surface of the earth. We have billions of cars, and they mostly avoid running into each other. Even 100 miles up? Far, far more space - and a lot less ‘stuff’ to worry about too. They could pack a million satellites up there and basic traffic management would be more than enough to avoid collisions.

itzprime 5y ago
But it is not the same thing, due to the speed that satelites travel.
- BenjiWiebe 5y ago
  Satellites aren't going to swerve, though.
rini17 5y ago
We will see. But it's definitely don't comparable to cars. Say you want safety margin of mere 1 second, that means reserving 8 kilometers free corridor in front of every satellite. At all times. That isn't a small volume.
- relativ575 5y ago
  Why one second and 8km? Satellites are not manually maneuvered. Airplanes are only required to keep 1,000 to 2,000 feet vertical separation. They travel at much slower speed than satellites (550 mph vs. 17,200 mph), but they are also much bigger. They tend to travel in narrow corridors, as opposed to being spread out like satellites. NASA and SpaceX have started working together to address the collision issues with their fleet: https://www.nasa.gov/press-release/nasa-spacex-sign-joint-spaceflight-safety-agreement https://www.nasa.gov/press-release/nasa-spacex-sign-joint-sp...
  - rini17 5y ago
    They are not manually maneuvered but there's still unpredictable inteplay of solar wind and tenuous atmosphere to correct. And some sats will fail, the environment is harsh. So I believe some safety separation will be a must, used 1 second as a rule of thumb and the orbital speed is 7.8 km/s at the altitude. I suspect more safety time will be required. Say a micrometeoroid impact happens causing one satellite to go off track. Now the controller computer needs some time to recompute whole swarm trajectories to minimize debris impacts. Even sats that are currently on the other side of planet might need to start thrusters immediately, as it takes only 10 minutes on average to cover the distance.
    - lazide 5y ago
      Let’s do some math and see. If we assume the earth is a sphere (I know it isn’t, but it’s mostly close), and using 4(pi)r^2, it’s about 511 million km2 in area on the surface (counting oceans, etc). Double checking, it’s actually about 507 million km2, so decently close. 4 * 3.1459 * (6378^2) Starlink is orbiting at 550km altitude (approx). At that altitude, we’re looking at… 4 * 3.1459 * (6928^2) = 603,977,365 km2 The thing to keep in mind about orbits is the defining factor is your eccentricity and velocity. Faster velocity == higher orbit, assuming it’s a round/circular orbit, that velocity will also be consistent. As long as everyone is going in the same direction and trying to maintain a circular orbit, you can stack orbits ‘on top’ of each other or put satellites quite close to each other just like planes do with flight levels. So even that number is misleading, as you could 10x that if you had decent traffic control. Even without that, you’re talking a density of 1 satellite per 600 square kilometers of space with a million satellites for a given ‘flight level’. The velocity matters a lot less than the relative velocity to other things you might be nearby - think of it like two cars going in lanes next to each other on the same ‘direction’ of a freeway, vs a free for all demolition derby with no traffic control, lanes, etc. If everyone is co-ordinated you can have problem free high density. If random stuff is happening, including people potentially trying to hit you - it’s going to be a mess. If everyone just does whatever and never co-ordinates, yeah someone is eventually going to run into someone else - but unless there are a LOT of satellites, the odds are low. With it being LEO, debris and dead satellites are also going to be a aerobraked pretty quickly.
vl 5y ago
>Volume increases exponentially as diameter increases. Cubically.
- lazide 5y ago
  Thank you for the correction! That is indeed correct.
dnadler 5y ago
Yeah, that's certainly a good point. The analogy with cars is a bit tricky though as we have well defined rules that help us pack them very closely together on the surface. For example: roads, lanes, controlled intersections. Those things aren't quite the same when talking about orbits because cars and satellites aren't spread out the same way. The other commenter made the point about some safety margin, which obviously has to be a part of the equation. The speed and error of orbits would play into whatever that number comes out to be. Regardless of there being lots of space, there's still some limit, even if it's 1,000 constellations. Some variables I don't quite know: 1. Acceptable altitude range for constellations 2. Typical positioning error of a satellite 3. Average failure rate and failure mode of a satellite - ex. does a dead satellite automatically deorbit itself or do we need to wait for the orbit to decay? I'd imagine decay at that altitude is fairly quick. Would a decaying satellite in a higher orbit need to be accounted for? 4. Required satellite density in orbit and the number of unique orbits, their arrangement, etc. I'm sure there are other factors, too...

ncallaway 5y ago

I haven't done the math and I'm certainly not an expert, but my lay understanding is that the number of constellations is much more limited by spectrum frequency availability, rather than the space constraints in LEO. That is, if we had an very large number of companies wanting to create satellite internet constellations, we would run out of useful EM spectrum before we would run out of LEO orbits. I'm not positive about that, though!

dnadler 5y ago
Yeah, that occurred to me as well, though I don't know much about how the satellites communicate with eachother. I imagine each constellation would need its own band, but not sure how wide it needs to be? Intra-constellation communication could feasibly be done with lasers, in which case, all they may need to worry about is the Space-to-Ground leg. So maybe it's not too bad? I don't really know, but certainly interested in learning more...

rcxdude 5y ago

The limit is far more how tight you can beamform the signals and how much usable spectrum there is: this is the fundamental thing limiting the density and hence number of users starlink can have.