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Comment by scarecrowbob | Hacker News Reader

I have a dumb Kessler syndrom question, but I don't really have the physics to answer it myself.

I've been told that worrying about the longer-term implications of systems like Starlink aren't so terrifying because these satellites are in LEO. Specifically, the time for that kind of junk to de-orbit is on the order of 5 years, so the worst case scenario for this junk is to wait it out.

However, if we get a good set of bouncing space junk, how does that affect this? The vectors generated from the parts two colliding small object might be in any direction, including outward?

My physics isn't that great, so I am left with this question:

is it worth thinking about the small bits of junk re-orbiting other junk? Would that push out the timeline of being able to simply wait out a incipient Kessler syndrome?

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Kerbal Space Program intuition tells me that the worst case is that you produce a high velocity piece of debris aligned perpendicular to radial vector, e.g. aligned with the existing orbit. I think this is a fairly unlikely direction to transfer energy in a head-on collision (by conservation of momentum arguments). That debris would then traverse a fairly elliptical orbit with frequent passes at the same altitude, likely resulting in relatively rapid de-orbit. The main risk would be inducing chain reaction at higher altitudes during those few passes, which seems unlikely.

Any other debris trajectory induced by collision would hit lower altitudes in less than one orbit time (either going outwards and thus on a much more eccentric orbit, resulting in lower altitude pass in less than one orbit time, or going inwards and immediately passing at lower altitudes). So that debris seems likely to deorbit even more quickly, indicating your intuition is correct.

This all makes sense once you grok why craft need two separate boosts to transfer to a higher geocentric orbit.

Of course there's a transition point where satellites are so dense that even these short-lived trajectories are sufficient to induce a self-maintaining chain reaction of debris. I suspect that's unlikely - the probability of a collision occurring in one or two orbital periods seems negligible.

Here is my intuition: let's say two satellites impact at high relative speed to reach other in LEO. All orbits of the debris have to pass through the impact point (since all debris comes from the point of impact). Any fragments on an orbit that go much higher will have the other end of the orbit intersect the earth's atmosphere. For debris to remain up the longest, it needs to have a very circular orbit (whether retrograde, polar, whatever).

Disclaimer: I don't really know anything about the topic, so if there is something I am missing, then hopefully someone corrects me!

Basically when you modify an orbit with a single impulse - short rocket burn, bumping into something, etc - the resulting orbit will still intersect that point (unless you reach escape velocity). So those pieces should return to that piece of LEO, although they could still be spending large amounts of time far from there.

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