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Accurate root zone moisture control in microgravity plant growth systems is problematic. With gravity, excess water drains along a vertical gradient, and water recovery is easily accomplished. In microgravity, the distribution of water is less predictable and can easily lead to flooding, as well as anoxia. Microgravity water delivery systems range from solidified agar, water-saturated foams, soils and hydroponics soil surrogates including matrix-free porous tube delivery systems. Surface tension and wetting along the root substrate provides the means for adequate and uniform water distribution. Reliable active soil moisture sensors for an automated microgravity water delivery system currently do not exist. Surrogate parameters such as water delivery pressure have been less successful.

BioServe Space Technologies has developed and flown a series of miniature habitats to house several different biological specimens and one biochemical experiment. This effort was in support of an educational program, Space Technology and Research Students (STARS), developed by SPACEHAB Inc. The STARS program gives students from around the world a chance to design and conduct their own spaceflight experiments. STARS-Bootes, the payload flown on STS-107, housed a Japanese Medaka fish experiment; a Chinese silkworm experiment; an American Harvester ant experiment; a Carpenter bee experiment from Liechtenstein, an Australian Orb Weaver spider experiment; and a biochemical crystal growth experiment from Israel. Each habitat was custom designed to suit each specimen's individual needs. The habitats provided passive humidity control, lighting, feeding areas, and containment as well as an artificial environment for the specimens to be observed in.

PGBA, a 0.08m2 / 27 liter spaceflight plant chamber payload employs two temperature-controlled liquid coolant loops to control the temperature and humidity of the sealed plant chamber independently. Cabin-air cooled thermoelectric heat pumps control the temperature of the water-alcohol coolant fluid in each loop, which is circulated by small, low-power, magnetically-coupled positive displacement gear pumps, designed to meet NASA safety requirements. Pulse-width-modulated DC current control circuits, controlled by two PI software controllers, maintain temperature and humidity accurately. The coolant loops feature bellows-based expansion vessels to accommodate thermal expansion and pressure fluctuations. Pressure sensors monitor the proper function and performance of the system. Pressure decay tests and unique fill procedures should ensure leak and air bubble-free operation.