Groundbased and onboard video images of a sunlit Shuttle Orbiter water dump are interpreted as showing that the continuous ~1 mm diameter liquid stream quickly breaks up in near-vacuum to form ice/snow particles of two characteristic sizes. Discrete large droplets (shown from laboratory measurements, to have diameters comparable with that of the initial coherent quasicylinder) are most evident in the close-in photographs, and unresolved submicron ''fog'' from recondensation of overexpanded evaporated water appears to dominate the ground-telescope photographs of the ~21/2 km long optically detectable trail. We estimated the mean diameter of the smaller particles from the spatial distribution of visible radiance using a model of their energy balance, (small) surface roughening as they sublime, and Mie scattering of pre-dawn sunlight. The results are consistent with those from recent space-tank simulations. ¿ American Geophysical Union 1990