Every tracked object orbits Earth on its own inclined path. Collision probability grows with the square of object count — twice as many objects means roughly four times the chance of any pair meeting per unit time. Each collision doesn't just remove two objects: at orbital velocities it shatters both, spawning several new fragments that raise the count further and make the next collision more likely still — a self-reinforcing feedback loop.
Working against that, objects slowly decay out of orbit (atmospheric drag) at a rate proportional to how many are up there. Whether the shell settles or runs away depends on which effect wins at the starting density: collision-driven growth scales with N², decay only scales with N, so there is a critical density where they exactly balance. Start below it and decay dominates — the population drifts back down and stays low. Start above it and fragmentation dominates — growth accelerates on its own, and pushing the density back down afterwards is far harder than staying below the line in the first place.
- Satellites (colored) — functioning spacecraft; a hit destroys them and scatters debris.
- Debris (gray) — every collision produces 3-7 new fragments.
- Green flash — an object naturally deorbiting (leaving the shell without a collision).
- Reset with a new density to compare against the previous run — its trace stays on the chart as a dim ghost line.