Microelectromechanical systems (MEMS) techniques offer great potential in satisfying the mission requirements for the next generation of miniaturized spacecraft being designed by NASA and Department of Defense agencies. More commonly referred to as 'nanosats', these spacecraft feature masses in the range of 10-100 kg and therefore have unique propulsion requirements. The propulsion systems must be capable of providing extremely low levels of thrust and impulse while also satisfying stringent demands on size, mass, power consumption and cost. We begin with an overview of micropropulsion requirements and some current MEMS-based strategies being developed to meet these needs. The remainder of the paper focuses on the progress being made at NASA Goddard Space Flight Center toward the development of a prototype monopropellant MEMS thruster which uses the catalyzed chemical decomposition of high-concentration hydrogen peroxide as a propulsion mechanism. The products of decomposition are delivered to a microscale converging/diverging supersonic nozzle, which produces the thrust vector; the targeted thrust level is approximately 500 µN with a specific impulse of 140-180 s. Macroscale hydrogen peroxide thrusters have been used for satellite propulsion for decades; however, the implementation of traditional thruster designs on the MEMS scale has uncovered new challenges in fabrication, materials compatibility, and combustion and hydrodynamic modeling. A summary of the achievements of the project to date is given, as is a discussion of remaining challenges and future prospects.
Comparisons of whole-lichen physiology to the respective photobionts have often been unclear due to inherent differences in isolated photobiont culturing techniques. The use of 13-mm-diameter cellulose-acetate discs allows photobiont cultures access to nutrient agar medium, while improving ease of manipulation and distinct separation from the agar. Adequate culture growth for experimentation is reached in approximately three weeks, a time comparable to standard nutrient agar and liquid cultures. These discs are then available for use in a variety of manipulative techniques. Chlorophyll determination of an entire algal disc culture is obtainable because the discs readily dissolve in dimethylsulphoxide (DMSO), with no interference in the 400–700 nm range. Photosynthesis and respiration may be measured with standard gas exchange equipment. Photobiont discs allow for fumigation in the gas phase with no increase in external ⊂pH reported to occur during gaseous fumigations in liquid media. The disc system is also useful for fluorescence studies. Trebouxia erici cultures exhibited a CO⊂2 gas exchange on a gram dry weight basis similar to whole lichen systems. The ease with which photobionts can be cultured and manipulated using this system allows for expanded experimentation and comparisons.
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