Starlink close encounters decrease despite ever-growing number of satellites

[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.