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.
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-sp...
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.
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.
2) Many of these incidents are from people are also putting things in Lagrange points and Geosynchronous orbits, and similar 'crowded' areas of space - unlike these constellations. Think jostling for space in downtown Manhattan, vs being fine having a place in Kansas somewhere.
3) Right now it's mostly a demolition derby out there, with only minimal traffic handling/control and the occasional 'haha, I got you' intentional satellite explosion/missile test.
As noted, even with all those factors, it is still incredibly rare. Only a tiny handful of problems have been tracked to orbital collisions with anything man made, and that is with us putting all sorts of things (including 'off the books' military top secret satellites, nuclear reactors, random bits of micro satellites, etc.) up there since the 50's.
Months is pretty quick for this kind of thing - and remember, unless it gets energy from some kind of serious-delta-v impact (or other delta-V adding type of event like a high energy explosion), it will lose velocity and DROP in altitude if the orbit remains circular. If the orbit doesn't remain circular, especially at LEO, it will have to dip significantly into the atmosphere to get to a higher altitude at any point in it's orbit, meaning even faster degradation and burn-up.
The reality is that it's huge up there, and if we do even basic thinking and planning on this to not do something obviously dumb (like tangential, perpendicular, or opposite direction orbits all over the place at the same level), it isn't likely to be a significant problem; just like it hasn't been a real (as in significant probability event) problem so far.
Cosmic rays, high potential static build-up, magnetic storms, micrometeorites, etc. are just as big or more of a problem. They rarely cancel a mission or damage anything notable, but it isn't zero risk there either.
Satellites are actually manually maneuvered generally. Only the large constellations have somewhat automated systems (SpaceX's system is especially automated because there's too many for humans to monitor).
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?
I'm sure there are other factors, too...
Cubically.
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.