187 1 Analyzing Miniature Electrodynamic Tether Propulsion Capabilities and the Interaction with the Low Earth Orbit–Plasma Environment Abstract— The sub-kilogram, “smartphone”-sized satellite is the next frontier in satellite miniaturization. Previous studies have shown that short (few meters), electrodynamic tethers can provide 10-g to 1-kg satellites with complete drag cancellation and the ability to change orbit. The miniaturized ED tethers considered here connect a pair of nearly identical pico- or femtosatellites that work together as a unit. Electron emitting and collecting plasma contactors are critical components of this propulsion concept because they close the tether circuit in the ambient plasma, allowing conduction of current in the tether and generation of thrust. One approach for electron collection is to collect current on the positively biased surface of one of the satellites in the tethered pair. This paper presents progress made on ground-based plasma experiments that capture key characteristics of the low Earth orbit (LEO)–tether system interaction, like the current collector’s geometry and the ratio of the Debye length to the collector’s characteristic dimensions. This paper describes the characterization of the laboratory plasma, compares the environment with LEO, and presents the current–voltage (I–V) characteristics of planar and cubic probes in the plasma, each probe approximating a small satellite in LEO. The tests showed that the current for planar and cubic probes increased nearly linearly with voltage at large potentials relative to the local plasma potential. The planar and cubic probes also collected less current on a per-area basis than a similarly-sized sphere. In the voltage range tested, theory used in previous trade-studies to predict the collection current provides a rough, order-of-magnitude estimate. The paper also describes an in-space experiment that will demonstrate miniature tether technology (MiTEE-Miniature Tether Electrodynamic Experiment). Keywords—Electrodynamic tether; anode; electron sheath; current collection; picosatellite; femtosatellite. I. INTRODUCTION A new class of small, sub-kilogram, “smartphone”-sized satellites are emerging as the next frontier of miniaturized satellites. The concept represents an evolution beyond the nanosatellite (1–10 kg) platform, made possible because of advances in electronics miniaturization and power reduction. These spacecrafts, known as picosatellites (100 g–1 kg) and femtosatellites (<100 g), have longest dimensions that range from about ten centimeters down to a few centimeters in length [1]–[4]. Due to their small size and mass, pico- and femtosatellites show potential to be dramatically less expensive (on a per unit basis) to boost into orbit. As a result, it may be possible to launch large numbers of these satellites, unlocking the potential for low cost constellations. The potential to deploy large numbers of pico- and femtosats in a controlled, coordinated formation suggests that propulsion and maneuverability would significantly enable the concept. Propulsion is also important because these small satellites will likely have high area-to-mass ratios, causing short orbital lifetimes. Previous trade studies have shown that a short (few meters) electrodynamic tether shows potential to provide propellantless propulsion for pico- and femtosatellites [5]. Fig. 1 shows an illustration of the concept, where pairs of femtosatellites are connected by short, semi-rigid ED tethers. To generate current flow in the tether, the system concept emits electrons from one of the tethered satellites and collects electrons from the ionosphere on the positively-biased, exposed conducting surfaces of the opposite satellites. Thrust estimates were based on a set of simplifying assumptions made to facilitate estimating the current collection. In this paper, we revisit these assumptions and describe a ground-based laboratory experiment in which we simulate characteristics of the orbital environment in order to refine previous estimates. We also discuss the Miniature Tether Electrodynamics Experiment (MiTEE) mission being planned to demonstrate the miniature tether concept. II. BACKGROUND A. Electrodynamic Tether Background An electrodynamic (ED) tether is a conducting wire connecting a pair of satellites. ED tethers are used in the ionosphere, which is a plasma. If both ends of the tether are able to exchange charge with the plasma, the ionosphere can provide a path to “complete the circuit,” enabling current flow Iverson C. Bell, III, Brian E. Gilchrist, Jesse K. McTernan, Sven G. Bilén I. Bell is with the Radiation Laboratory, Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, 48109, USA (e-mail: icbell@umich.edu) B. Gilchrist is with the Radiation Laboratory, Department of Electrical Engineering and Computer Science and the Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, 48109, USA (e-mail: brian.gilchrist@umich.edu) J. McTernan is with the Department of Electrical Engineering, Pennsylvania State University, State College, 16801, USA S. Bilén is with the School of Engineering Design, the Department of Electrical Engineering, and the Department of Aerospace Engineering, Pennsylvania State University, State College, 16801, USA The authors gratefully acknowledge support from AFOSR grant FA9550- 09-1-0646, the National Science Foundation Graduate Student Research Fellowship under Grant No. DGE. 1256260, and the Michigan Space Grant Consortium.