Kessler.

the rules

The teal dots are satellites on tidy circular orbits — co-orbiting traffic with almost no relative velocity, which is why a crowded shell can be perfectly safe. Until it isn’t.

Click a satellite. It fragments into debris with random velocity kicks, and that’s the whole difference: fragments fly on eccentric orbits that cross the shell, meeting satellites at killing speed. Every satellite hit becomes more fragments. Whether the chain reaction dies out or consumes the constellation is not about the first explosion — it’s about how much traffic the fragments can find.

the experiment

The satellites slider is a control parameter and the cascade is a phase transition. Run the same click at 40 satellites and at 250:

• sparse shell — a burst of amber, a few unlucky hits, then the drag claims the debris and it’s over
• dense shell — each generation of fragments finds targets faster than decay removes them, and the constellation unravels in a chain you can watch generation by generation

Somewhere in between is the critical density. Find it — it’s sharper than you’d expect.

notes

Kessler & Cour-Palais predicted this regime in 1978; it’s the reason debris mitigation rules exist. This is a toy: two dimensions, exaggerated collision cross-sections, and atmospheric drag compressed from decades to ~30 seconds (debris dims as it nears burn-up) — the compression is what lets you see the threshold in a minute. Collisions between co-orbiting neighbors are ignored below a relative-speed floor, which is also why the constellation is stable until you interfere. Orbits are leapfrog-integrated; fragments that dip too low burn up, and slingshots out of the shell are gone for good.