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Phenomenology of a Water Venting in Low Earth Orbit

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Venting of excess water from spacecraft, besides leading to physical and optical contamination, has application to studies of the transport of outgas, the interaction of the vehicle with the ionospheric plasma, the energy balance of cometary material, and the uses of liquid streams in space operations. Analysis of intensified video images of a twilight venting of fuel- cell product water from Shuttle Orbiter shows that the initially coherent stream forms within about 0.1s into a approx. 10 deg quasiconical cloud of irregular, polydisperse icewater droplets the product of cavitational rupturing and submicron ice spherules from partial recondensation of overexpanded vacuum- evaporated water gas. The retrograde velocities of both particle components are experimentally indistinguishable from that calculated for the dumped liquid. The visible radiance distribution of the approx. 2 12 km of wake trail detectable in projections to a precisely-tracked groundbased telescope becomes consistent with predictions from the energy balance of the submicron particles when a sublimation rate-enhancing correction to their emissivities and thus temperature, arising from the progressive roughening of their surfaces, is applied. A similar calculation of the thermal radiation, scattering and absorption of earthshine and sunlight, and sublimation of the larger approx. mm- radius particles prominent in the onboard photographs shows that their lifetimes are several orbital periods. The relative discrete and spatially- continuous irradiances in images from the two camera location impose a rough joint constraint on the fraction of water vapor that recondenses and the mean geometric scattering cross-section of the stream-fragmentation droplets.

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  • Atmospheric Physics
  • Fluid Mechanics

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