Microgravity effects on water flow and distribution in unsaturated porous media: Analyses of flight experiments

Plants grown in porous media are part of a bioregenerative life support sys tem designed for long-duration space missions. Reduced gravity conditions o f orbiting spacecraft (microgravity) alter several aspects of liquid flow a nd distribution within partially saturated porous media. The objectives of this study were to evaluate the suitability of conventional capillary flow theory in simulating water distribution in porous media measured in a micro gravity environment. Data from experiments aboard the Russian space station Mir and a U.S, space shuttle were simulated by elimination of the gravitat ional term from the Richards equation. Qualitative comparisons with media h ydraulic parameters measured on Earth suggest narrower pore size distributi ons and inactive or nonparticipating large pores in microgravity. Evidence of accentuated hysteresis, altered soil-water characteristic, and reduced u nsaturated hydraulic conductivity from microgravity simulations may be attr ibutable to a number of proposed secondary mechanisms. These are likely spa wned by enhanced and modified paths of interfacial flows and an altered for ce ratio of capillary to body forces in microgravity.