 The small total mass of the dynamically cold population presents some problems for models of the Solar System's formation because a sizable mass is required for accretion of KBOs larger than in diameter.If the cold classical Kuiper belt had always had its current low density, these large objects simply could not have formed by the collision and mergers of smaller planetesimals.Moreover, the eccentricity and inclination of current orbits makes the encounters quite "violent" resulting in destruction rather than accretion.