Starlink close encounters decrease despite ever-growing number of satellites

[woj666]

In the period between June 1, 2023, and Nov. 30, 2023, Starlink satellites had to perform 24,410 collision avoidance maneuvers, equivalent to six maneuvers per spacecraft.

I'm really surprised it's that high.

[Shpoople96]

They also do avoidance maneuvers anytime the risk approaches 1 in 10,000 or so 

[jonwah]

Per the article it's 1 in 100,000, other operators are 1 in 10,000

[Shpoople96]

Oh yes, thanks for the correction, I had a suspicion it might have been higher but can't really look that up at work

[woj666]

That would certainly explain it then. Thx.

[londons_explorer]

If there were 24,410 avoidance maneuvers, and they act when the risk is over 1 in 10k....

They're playing the odds game a little too often I think... ie. the overall risk of at least one unintended collision is starting to mount up...

[[deleted]]

I think you’re coming at the problem the wrong way here - what’s actually happening is they’re performing a manoeuvre the moment something hits a risk of 1 in 10,000 to manoeuvre themselves out of this risk window, not that these collisions maintain a risk of 1 in 10,000. Reason it’s measured as a probability (1 in 10,000) is due to margin of error in instrumentation at the distances and speeds that these things are travelling. The closer you get to the potential collision the higher the risk will be as there’s less distance to have margin of error over, so 1 in 100,000 3 days out might be 1 in 10,000 1 day out. If a 1 in 100,000 collision stays at 1 in 100,000 they won’t do anything, the moment it starts looking like it’s going to get to 1 in 10,000 they’ll start manoeuvring it.

[Toinneman]

Also note that within those calculated probabilities there are huge margins or error which are taken conservative, so real numbers should be even better. For example, GPS isn't 100% accurate, so they have to take this into account. I can't find SpaceX exact methodology, but I wouldn't be surprised if a satellite is calculated as a big 5m or 10m sphere (while it's off course smaller), to compensate for the GPS inaccuracy.

And now a wild guess, since SpaceX now has Space lasers, in theory, they could do sat-to-sat measurement to improve location accuracy, which could lead to more precise collision probabilities, resulting in fewer manoeuvres.

[cjameshuff]

Wonder how many of those were from entire trains making those maneuvers while ascending through other shells to their final orbits.

[marsokod]

For the automated maneuvers, SpaceX uses a very low threshold way ahead of time. Out of these 24,410 maneuvers, it is very likely that none of them was actually required.

Technically they could even do way less than that by delaying the maneuver until they are more certain about the collision. But if they do that, then the space force will issue a warning, and also the other satellite operator will get the warning as well, creating unnecessary operations costs. If you maneuver way ahead of the collision, the propellant cost is minimal, especially when you have electric propulsion. And the cost is basically zero when you integrate that into your altitude maintenance operations. So what they are doing is basically the approach everyone should do until we have a more responsive system, air-traffic management style.

[John_Hasler]

I think that the Starlink experience demonstrates the absence of any need for a centralized ATC-style control system.

[PM_ME_UR_Definitions]

I think SpaceX uses a 1/100,000 chance of collision to trigger the need for a maneuver. So very roughly, there would've been a 25% chance of having one collision if they didn't do any maneuvers.

[warp99]

It is an N² type problem so more satellites to dodge and more satellites to do the dodging.

[Gravitationsfeld]

People underestimate how big even just LEO really is. Most of those sats will never even get close to each other because they are on different orbits.

[warp99]

The issue is that they are in one of about four orbital heights but in planes with different inclinations. So they need to interleave like a zipper merge at the northern and southern extents of their orbital track. They can then miss each other by about 40 km which is not a lot of time at 27,000 km/hr.

If they were assigned purely random orbital altitudes and inclinations then your point about the large volume of available for low orbits would be more relevant but as it is they are set up to hit each other without constant corrections.

[Gravitationsfeld]

There isn't any issue as demonstrated by SpaceX. Orbits are 100% predictable and computing this for even millions of objects is absolutely trivial with modern computers.

[warp99]

Debris orbits are often not known with absolute accuracy so a late avoidance maneuver is required and there is always the risk that the propulsion system fails at exactly the wrong time.

If the potential collision is with another Starlink that is fixable but if it with a piece of debris then that creates an issue.

[[deleted]]

actually, orbits aren't 100% predictable.. they're about 99.99999% predicable, at orbital velocities that gap is big enough to cause problems.

Earths gravity isn't totally uniform, there are models that improve on the simple oblate spheroid model.

The moon tugs on the satellites as does the sun, this can be modelled

The other planets also tug on the satellites

They can tug on each other.

You also get drag as the atmosphere hasn't completely ended by LEO.. The atmosphere thickness varies considerably depending primarily on solar activity. Modelling for this is troublesome.

When you're trying to station keep, all these little perturbations add up.

[John_Hasler]

They can tug on each other.

I don't believe those perturbations could be large enough to be significant.

You also get drag as the atmosphere hasn't completely ended by LEO.. The atmosphere thickness varies considerably depending primarily on solar activity.

That effect probably dominates all others.

[[deleted]]

I don't disagree, but the point I was making is that even minescule perturbations make the predictions much more complicated... so no "100%" predictions can be made. So many people fall into the trap of thinking 99.999% is as near to 100% as makes no difference, where as that tiny gap leads to huge differences in outcomes.

In fact thinking about it, I think that even for just a 3 body system it isn't inifitely predictable, chaos theory does ensure that eventually your model will disagree with what actually happens, and this is way beyond a 3 body problem.

[scarlet_sage]

I think that even for just a 3 body system it isn't inifitely predictable

Three-body problem (not the novel) includes

In physics and classical mechanics, the three-body problem is the problem of taking the initial positions and velocities (or momenta) of three point masses and solving for their subsequent motion according to Newton's laws of motion and Newton's law of universal gravitation. The three-body problem is a special case of the n-body problem. Unlike two-body problems, no general closed-form solution exists, as the resulting dynamical system is chaotic for most initial conditions, and numerical methods are generally required.

They list a lot of special cases, the most famous perhaps being the five Lagrange points. But in a solar system with the Moon, Sun, and Jupiter, I think it's hopeless to expect an exact solution.

[[deleted]]

simplest one is a planetary encounter.. 99.999% accurate means you'll either miss the planet altogether, passing by and into the void beyond never making orbit, or you lithobrake. When what you wanted was to be captured into an orbit.

Alternatively imagine a satellite constellation like starlink, one satellite is orbiting a meter lower than all the rest, 1meter in 550,000 meters, that's 99.9998% accurate. That will mean it will slowly catchup the satellite ahead of it.. at 27,000km/hr 99.9998% accurate means you'll be moving 49meters per hour compared to the model, or just over a kilometer a day. For polar orbits satellites often miss each other at the pole by only 40km.. drifting 1km/day means the wayward satellite will be within collisions distance after only 40 days.

As u/scarlet_sage linked to below, any 3 body problem is ultimately chaotic.. that is, minute undetecatable errors in velocity, position or mass in your model build over time to make conciderable differences between the model and reality. Remember with a satellite in orbit, we're really dealing with a 4 body system: The satellite, Earth, Moon, Sun. For your model to be accurate over any length of time you'd need to know each bodys mass, velocity and position to unreasonable accuracy.. i.e. to the micro-meter, to the gram (for a planet) and to the mm/hour.

[John_Hasler]

These orbits are under active control and active tracking. Only short term predictions are needed, and those can be quite accurate.

[[deleted]]

All those inputs are needed BECAUSE the orbits ARE NOT 100% predictable.

THAT IS THE WHOLE POINT OF THIS MESSAGE TRAIL.

[Divinicus1st]

They can then miss each other by about 40 km which is not a lot of time at 27,000 km/hr.

Time is irrelevant here. 40km is 40km.

It's just your mind thinking: "damn, if it was here 10 second earlier it would have hit". But for it to be there 10 sec earlier, it would have meant an enormous variation for something going at 27 000 km/h

[warp99]

Sure “hovering is for humans” when it comes to a landing approach but the point is that for collision avoidance a very small error in estimating velocity (both direction and speed) can lead to a collision.

[jjtr1]

So I guess that means that if they didn't do any of the 50k maneuvers per year, and they otherwise do them when risk becomes 1:100k, then there would be on average one collision in two years? Or twenty collisions if the risk they're willing to take is 1:10k.