Iverson C. Bell

The potential of miniature electrodynamic tethers to enhance capabilities of femtosatellites

Summary form only given. The emerging interest in nanospacecraft (1–10 kg) has generated interest in exploring the potential for even smaller spacecraft, both as stand-alone satellites or as a distributed swarm. Because of advances in integrated circuit and microelectromechanical systems (MEMS) technology, the feasibility of miniaturized spacecraft at the levels of fully monolithic semiconductor integrated circuits (10–100 mg) or hybrid integrated circuits (10–100 g) is being seriously investigated. Effectively, this architecture can be thought of as a small “satellite-on-a-chip,” or ChipSat. ChipSats belong to the picosatellite (100 g–1 kg) and femtosatellite (<100 g) mass categories. However, flat ChipSat wafers can have an inherently high area-to-mass ratio. Although this feature can be exploited for new behaviors, it can result in an undesirably short orbital life in low Earth orbit (LEO) due to atmospheric drag, ranging from a few weeks to a few hours.

Current Collection to and Plasma Interaction with Femtosatellite- and CubeSat-Scale Electrodynamic Tether Subsystems

The efficacy of an electrodynamic tether system in generating a propulsive force or harvesting energy is limited, in part, by its ability to collect current from and emit current to the surrounding ambient plasma. This process is facilitated by active and passive electron emitters. Laboratory experiments were designed and conducted in a ground-based vacuum chamber to investigate the current–voltage characteristics of potential devices within the plume of a plasma source capable of producing ion and electron temperatures and densities similar to that found in low Earth orbit. We achieved active electron emission using a tungsten filament and lanthanum hexaboride crystal (approximately milliamps and microamps, respectively). We achieved a passive plasma contact by biasing a planar surface coated in a thin layer (15 Ω/sq) of indium tin oxide, which yielded current levels comparable to that of alodined aluminum. We also developed laboratory experiments designed to validate certain assumptions made about the behavior of ultra-small electron collecting interfaces for femtosatellites using short electrodynamic tethers.

Enabling Ultra-small Sensor Spacecraft for the Space Environment using Small-Scale Electrodynamic Tethers

In this paper we investigate an approach that appears to scale to the small size needed for femtosatellite (commonly called “ChipSats”) drag make-up and even orbit raising with the added benefit of being propellantless. The approach uses a short, semi-rigid electrodynamic tether (EDT) for propulsion, which keeps the overall ChipSat mass low and provides enough thrust to overcome drag in LEO. We report on our trade studies to assess the feasibility of using the EDT for ChipSat propulsion. We have analyzed the EDT anode’s ability to draw current from the ionosphere and thereby generate thrust. We then traded this performance against the tether mass and material, electron emitter and collector types, and power needed to determine the EDT’s capability of overcoming atmospheric drag forces. The results reveal that an insulated tether only a few meters long and tens of microns in diameter could provide milligram to 100 gram-level ChipSats with complete drag cancellation and even the ability to change orbit. The EDT system described here might be considered inefficient in terms of the power required for thrust. However, the received solar power is sufficient and the EDT is propellantless, so we believe the EDT still provides a viable approach for propulsion. We also explore the assumption that the gravity gradient force aligns the tether along the local vertical and find that this assumption needs further investigation. A more complete systems design and analysis is continuing.

Investigating Miniature Electrodynamic Tethers and Interaction with the Low Earth Orbit Plasma

The sub-kilogram, “smartphone”-sized satellite is a transformative concept, inspired by the success of nanospacecraft (1–10 kg) and millimeter-scale wireless sensor network concepts. These ultra-small satellites, known as picosatellites (100 g–1 kg) and femtosatellites (<100 g), show potential to be less costly to manufacture and boost into orbit. Thus, it may be possible to launch them in large numbers, enabling unique capabilities. Organized “fleets” of picoor femtosatellites, however, will need a high level of coordination and maneuverability capability (i.e., propulsion). Also, many of these satellites can have a high area-to-mass ratio, which results in a short orbital lifetime in low Earth orbit due to atmospheric drag. In this paper, we summarize studies that found that short (few meters), semi-rigid electrodynamic tethers can provide 10-g to 1-kg satellites with complete drag cancellation and the ability to change orbit. We also present progress on the Miniature Tether Electrodynamics Experiment (MiTEE), currently in development. The goal of MiTEE will be to demonstrate miniature electrodynamic tether capabilities in space and study the fundamental dynamics and electrodynamics of the propulsion system.

Development of a Modeling Capability for Energy Harvesting Modules in Electrodynamic Tether Systems

Orbital energy of a spacecraft can be transferred into electrical energy using an electrodynamic tether. The energy can be stored or used immediately for onboard power. We developed an energy storage module for our simulation software, TeMPEST, that models various storage devices such as supercapacitors, lithium{ion batteries, or a generic storage device. The principle of conservation of energy was used to verify our system model as well as our electrodynamic orbital-perturbation model. The energy storage module is also capable of examining other aspects of a spacecraft’s energy budget, such as the in-plane or out-of-plane contributions of the electrodynamic work done on the system. An emphasis was placed on scaling the storage devices to satisfy the requirements of the CubeSat platform.

Experimental Investigation of Electron Collection by Rectangular Cuboid Probes in a High-Speed Plasma

This paper describes ground-based scaled plasma experiments that simulated picosatellite and femtosatellite electron collection in the low earth orbit (LEO) ionospheric plasma, capturing special characteristics of these small spacecraft, including their possible rectangular cuboid shape, size relative to the plasma Debye length and the electron thermal gyroradius, and orientation with respect to plasma flow and magnetic field. It was observed that enhancing magnetic field strength to an appropriately scaled value to approximate LEO decreased the electron saturation current. The current collection characteristics were also impacted by the orientation of the probes relative to the magnetic field and the direction of plasma flow. In the voltage range tested, a model used in previous studies to predict electron saturation current underestimated the measured probe currents by a factor of two or more. Semi-empirical models developed here to calculate electron saturation currents that were within 10% of the measured currents. This experimental analysis enhances our understanding of current collection by rectangular cuboid probes in high-speed plasmas.