The Plant Water Management Experiments on ISS: Soil

A simple means of watering plants in the low-g environment aboard orbiting spacecraft is not obvious. Since the beginning of spaceflight, numerous approaches have been pursued to water plants that seek to maximize plant viability and system reliability, while minimizing crew time and system complexity. We are not there yet. The Plant Water Management (PWM) Soil experiments seek to apply recent advances in low-g capillary fluidics phenomena to the challenges faced by plant growth operations aboard spacecraft. The primary challenge is to establish earth-like flows minimizing low-g specific adaptations required of the plants. This is difficult due to the ever-present fluid physics challenges associated with poorly-wetting multiphase inertial-visco-capillary flows in geometrically complex conduits and containers�which change dramatically as the plants grow. In this paper, we present recent flight results for the simple visual PWM-Soil experiments where arcillite �soil reservoirs� are arranged in a non-wetting host soil that serves as an O2-breathing wetting barrier. In this way, a largely terrestrial water-soil environment might be mimicked where, as liquid is evapotranspired through the growing plant foliage, the effective water table passively �falls� reducing viscous lengths and increasing oxygenated water uptake for the plant. We present data from 6 days of 24-7 experiments on the ISS testing 3 different plant root models geometries. Not everything goes as planned as the �widely� varying plant models eventually perform similarly regarding total transport rate. We explain why via reference to a capillary flow model developed to capture the primary features of the flow. We summarize that successful passive capillary soil delivery systems can be designed at will, but the time/growth-dependent water requirements for plants in low-g environments may prove an elusive requirement.