The generation of either propulsive thrust or electrical power with an electro-dynamic tether necessarily depends on driving a return current through the system's electrically conducting space plasma environment. An electrical connection is, therefore, required between the plasma and each end of the tether. The voltage required to drive the current (derived from either an external power source or the orbital motion of the conducting tether through the geomagnetic field) produces a positive bias between one end of the tether and the plasma. Electrons can be effectively collected at this positive pole with a simple, passive electrode. The proposed Grid-Sphere electrode concept (shown in Exhibit 1) is particularly interesting because of its unique capability to decouple current collection from tether length and to achieve a high area-to-mass ratio, a low drag coefficient, and operational simplicity. Phase I will include the identification of photolizable thin film materials and grid encapsulation techniques, a preliminary investigation of grid electrode current collection physics, and a top-level applications study. Phase II will include the development of a high fidelity current collection model and the design, fabrication, and functional testing of a prototype Grid-Sphere system. POTENTIAL COMMERCIAL APPLICATIONS The simplicity, robustness, and efficiency of the Grid-Sphere make it an attractive alternative current collection electrode for electrodynamic space tethers. In some applications, it may even be "enabling." Potential applications include electrodynamic tether power generation, orbit control, and capture at outer planets such as Jupiter.
