Title: Obstacles to the Collisional Growth of Planetesimals

Abstract: The debate over whether kilometer-sized solids, or planetesimals, assemble by collision-induced chemical sticking or by gravity-driven unstable modes remains unsettled. In light of recent work showing that gravitational growth can occur despite turbulent stirring, we critically evaluate the collisional hypothesis. Impact speeds in protoplanetary disks reach 50 m/s in a laminar disk and may be larger in turbulent disks. We consider the role of elastic and plastic deformations, restructuring of "rubble piles," and sticky organic matter. Coagulation is possible for dust grains with large surface area-to-volume ratios. Simple energetic arguments show that bouncing, cratering, and fragmentation should dominate collisional dynamics between millimeter and kilometer sizes.