Protoplanet formation & Kessler Syndrome In the early stages of the solarsystem rocks clumped together randomly until their gravity drew in more material which led to protoplanet formation. But how does the dust and do the rocks form bigger pieces? Wouldn't the Kessler Syndrome cause these rocks to split up in a collision and after that act as projectiles that destroy other rocks? Wouldn't the natural tendency for the rocks be to decrease in size rather than increase?
 A: I can't give a truly mathematical answer, but consider Saturn's rings.   There's 2 key differences between proto-planet dust and Saturn's rings and that's that Saturn's rings are inside the planet's Roche limit so they won't ever have the cohesion to clump, and Saturn's rings are remarkably flat.   
So, what happens with proto-planet dust is that it would obey the laws of angular momentum, on average maintaining orbital velocity, while forming into a plane or ring like structure over time - similar to Saturn's rings, and that would increase density as well as reduce a lot of the velocity of collisions.    In simple terms, first thing to happen is that the proto-planet dust flattens out.   After that, the planet begins to form.
http://www.universetoday.com/108355/why-is-the-solar-system-flat/
The mass of Saturn's rings are about 1/2000th the mass of the moon, roughly 1/200,000th the mass of the earth.    Now the size of the ring - the outer diameter is about 270,000 KM with a width of about 70,000 KM.   Now, if we assume the earth's orbit hasn't changed too much, the earth's orbit is 550 times the size of the outer edge of Saturn's main rings, so 550 squared - 300,000 times the area (the width would be greater too), but generally speaking, the material that makes up the earth wouldn't be much more than 1, maybe 2 at the most, orders of magnitude less dense (ignoring thickness) than Saturn's rings - that's actually pretty dense as far as dust in space is concerned, so there would be enough low-velocity collisions that it's not hard to see that it would clump, though the initial clumping would be more like electromagnetic stickiness than gravity, but over time, as larger objects grew, gravity would take over, but I absolutely think that some debris scattering would occur as well, but we don't see Saturn's rings scattering from debris on debris collisions, so that wouldn't be the driving factor.
