1) Everything has the same angular velocity.

2) Everything has almost the same inclination.

3) The orbit is really, really big.

4) The orbit is really, really far away from most LEOs and MEOs

You can certainly get collisions, but the center-of-mass energy of those collisions is going to be considerably lower than what you get in LEO. If I did the arithmetic right, geosync orbital velocity is about 3100 mps. If you assume a less than 1 degree inclination, the worst case collision you can have is at about 100 mps. That's enough to trash your spacecraft, but it's hardly enough to create showers of fragments with orbits eccentric enough to cause lots of additional damage. At the very worst, the Kessler Syndrome exponential time constant has to be tiny.

I'd be surprised if debris showers from LEO or MEO could reach geosync with very much energy, and they're unlikely to be stable orbits even if they do.

Is this a real problem for GEO?

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