Trajectories of collisional debris in a laminar flow. In this representative example, the headwind velocity is 20 m s^-1, and the axes have units of meters. All dust particles begin with the same location, direction α, and velocity (20 m s^-1), indicated by a dot on the proto-planetesimal surface. The stopping times of the dust particles are, for trajectories 1–4, t_s = 0.1, 1, 8, and 40 s, respectively. See the text for a discussion of the different outcomes.