Harnessing energy in the space environment for spacecraft operations
Keywords: Aeronautics and Astronautics.

Exploration of the outer solar system is an expensive endeavor. Although these missions typically involve large monolithic spacecraft, interplanetary small spacecraft — especially CubeSats — have the potential to reduce hardware costs while increasing the science return. Spacecraft in the outer solar system generally have access to far less power than their counterparts in the inner solar system. The extreme distance from the sun necessitates the use of large deployable solar photovoltaic arrays to capture useful amounts of power, but threats from trapped radiation and microscopic particulate environments decrease their effectiveness. Radioisotope thermoelectric generators (RTGs), the typical solution in the outer solar system, are tremendously more expensive and do not scale well to small spacecraft. A different paradigm is required to enable these missions. This thesis proposes concepts for harvesting energy from the space environment, surface charging due to space plasmas and radio emissions from hypervelocity impacts in particular. A survey of the plasma and debris environments throughout the solar system is presented with estimates of the energy content. This survey is used to identify the most favorable environments in the solar system for energy harvesting. Using experimental data and theoretical models, more sophisticated energy harvesting estimates are constructed. Measurements of emission polarization and frequency spectrum from a hypervelocity impact experimental campaign conducted at the NASA Ames Vertical Gun Range (AVGR) are compared with theories of electromagnetic pulse emission and used to place an upper bound on the electromagnetic energy emitted in usable bands. A general theory for harvesting from differential charging is derived and presented. An Orbit-Motion-Limited (OML) charging model is used to determine how the harvested power scales with plasma properties and spacecraft design parameters. Surfaces with highly disparate secondary electron yields charge differentially without the use of electron emitters and can be used as anodes and cathodes to supply current to an instrument. The model suggests that load impedances and anode-to-cathode area ratios can be varied to optimize the power intake from the ambient plasma. Optimal loads are simply impedance matches to the background plasma characteristics, while large anodes are favored to capture the most electrons. These theories are used to estimate the energy harvested in two outer solar system environments: Jupiter and Saturn. The former has warm dense plasma (n ~ 1e4/cm3, T ~ 50 eV) ideal for harvesting from surface charging, while the latter has a well-studied, dense (n ~ 3.5e-8/cm3) dust environment in its ring system. Results from the OML model agree well with predictions from the Spacecraft Plasma Interaction Software (SPIS). Harvesting from hypervelocity impact electromagnetic pulses is found to be inefficient and impractical for powering spacecraft. On the other hand, differential charging generates areal power densities on the order of 0.1--10 mW/m2 at Jupiter but may require deployable surfaces to maximize the system efficiently. Although neither power source improves upon the performance of solar panels or RTGs, they may be more robust in the face of radiation and hypervelocity impacts