Shortly after 15s is a pretty funny and unexpected joke.

The paper in question is here: https://link.springer.com/article/10.1007/s40295-024-00458-3

It adds to a pretty large body of literature around this subject, the gist of which is "risk is going up, but we don't really have a good way of estimating what that means in terms of actual collision rates".

As long as it is militarily and commercially viable then the number of satellites will continue increasing, regardless of what academics have to say about collision rates. As per usual this is a coordination problem and in case people have not noticed nations are becoming less coordinated and more insular.

The current trajectory is that SpaceX proved the commercial and military viability of an LEO megaconstellation, repeatedly lowering their target altitudes and raising their satellite count because of debris and cell size concerns...

And now the rest of the world is trying to catch up in a sort of arms race, and not taking any care about debris concerns. The most tempting orbits are the ones in upper LEO that permit them to launch fewer satellites.

SpaceX are going to end up well under 500km (orbital lifespan: a decade) before things are finished, and they switched to very low orbit staging with SEP spiral out to reach final orbit a ways back.

China's newest constellation Thousand Sails is at an altitude of 800km (orbital lifespan: thousands of years), with a thousand satellites in the works over the next year or so and 14,000 planned, and they're launching them using chemical upper stages designed to explode into a thousand pieces at the target altitude. This is sufficient for Kessler Syndrome all on its own, without counting interactions with anything else up there. A catastropic debris cascade at 800km percolates down to lower altitudes over time and impacts.

We need viable treaties limiting development beyond 400 or 500km and we need them ten years ago.

I don't know how to sell the urgency of this predicament. You can have as many satellites as you want, a million uncoordinated bodies, at 400km because direct collision potential scales with (satellite count / orbital lifespan) ^2 . At 1000km, satellites decay so slowly we are already too crowded; we have already overused the space. We are speed-running the end of the space age and we are doing it to save a small number of dollars and to avoid a small amount of diplomacy.

This is not something we get a do-over on. There is no practical way to collect ton-scale debris at present, no way to track kilogram-scale debris, no practical way to shield pressure vessels against gram-scale debris, and even milligram-scale debris can hit with the force of a bullet. After collisions start occurring at a rapid clip, the mass of potential impactors quickly forms a long tailed lognormal distribution that denies us space for centuries.

We need a Mega-Maid like in Spaceballs to clean up the mess: https://spaceballs.fandom.com/wiki/Mega_Maid