Initially the particles are in roughly similar orbits to the two bodies that initially collided, but not precisely the same. Over time (exactly like billard balls without friction), those minor divergences mean they cover very wide areas.
Look up the "Gabbard diagram", which is a great way of depicting the outcome of a collision. There's a spray of different orbits, with some properties that resemble the original objects, but diverging significantly from them.
Edit: Here's an AMAZING 3d animated Gabbard diagram, that shows not only the altitude and period, but also the right ascension of each object. You can see the debris from the Fengyun booster sweeping around the whole planet, crossing the orbits of other objects: https://www.youtube.com/watch?v=ePuvJDVNJd0
Even if they did stay in the same orbit, a thousand objects in the same orbit presents a thousand times more collision risk than one object in orbit. But because the collision changes the momentum of all the objects involved, it's much worse than that -- the results end up all over, and some of them are too small to track but still large enough to do damage. Those are the ones you really have to worrry about.
Initially the particles are in roughly similar orbits to the two bodies that initially collided, but not precisely the same. Over time (exactly like billard balls without friction), those minor divergences mean they cover very wide areas.
Look up the "Gabbard diagram", which is a great way of depicting the outcome of a collision. There's a spray of different orbits, with some properties that resemble the original objects, but diverging significantly from them.
Edit: Here's an AMAZING 3d animated Gabbard diagram, that shows not only the altitude and period, but also the right ascension of each object. You can see the debris from the Fengyun booster sweeping around the whole planet, crossing the orbits of other objects: https://www.youtube.com/watch?v=ePuvJDVNJd0
Even if they did stay in the same orbit, a thousand objects in the same orbit presents a thousand times more collision risk than one object in orbit. But because the collision changes the momentum of all the objects involved, it's much worse than that -- the results end up all over, and some of them are too small to track but still large enough to do damage. Those are the ones you really have to worrry about.