Radio-frequency emissions from hypervelocity impacts on charged spacecraft
Keywords: Aeronautics and Astronautics.

The space environment is a complex interactive system that deals with the dynamic processes and effects that can occur above the protective cocoon of Earth's atmosphere. Many extreme conditions found in space can pose significant mechanical and electrical risks to spacecraft systems. Space truly is an area of extremes, where spacecraft in orbit around Earth are immersed in the plasmasphere, where solar weather shapes our magnetosphere, and where meteoroids and asteroids careen past spacecraft at hypervelocity shock inducing speeds. We study these phenomena and others to understand how best to design spacecraft and missions to minimize the risks and hazards they are exposed to while operating in this extreme environment. The specific focus of this research is on the effects and hazards generated by collisions between electrically charged spacecraft and hypervelocity impactors. Spacecraft in orbit around Earth can naturally accrue strong surface charging conditions from the high energy electron populations that exist in regions of Earth's plasmasphere. We focus on the effects that occur when these strong spacecraft charging conditions coincide with hypervelocity impacts from meteoroid and orbital debris collisions with spacecraft. The strong shock waves that are generated from hypervelocity impacts compress, heat, and ionize the affected media and produce a plasma at the site of impact. The plasma formation process is predominately an effect of impact velocity and can persist across impactors of all sizes and masses. This plasma acts as a conduit for many different unwanted electrical phenomena. This thesis strives to understand the potential for hazardous radio-frequency electromagnetic pulses to be emitted from the impact plasma when impacts occur on electrically biased spacecraft surfaces. This research is motivated in part by case studies of spacecraft electrical subsystem failures during peak meteoroid shower activity. The expansion and evolution of a hypervelocity impact plasma and its potential to emit radio-frequency electromagnetic pulses is driven in large part by the initial conditions of the impact event and the boundary conditions imposed by the space environment and the charged spacecraft surfaces. This thesis presents novel research on the high energy shock physics that describe the formation of the initial impact plasma, with a specific focus on how material properties and impact conditions affect the thermodynamic states of the shocked media. Following the formation of the impact plasma we show how the expansion process in the presence of a strong negative external electric field can lead to conditions where the bulk of the plasma's electrons are accelerated away from the spacecraft. This bulk acceleration creates a wide-band radio-frequency emission which scales with the amount of plasma produced, the plasma temperature, and the bias conditions on the spacecraft. We corroborate these findings with experimental campaigns at electrostatic accelerator and light gas gun impact facilities. In these experiments we observe for the first time wide-band RF emissions from hypervelocity impacts with charged surfaces. A scaling analysis of the emission strength shows that spacecraft can be exposed to wide-band emissions in excess of $\SI{1}{\watt}$ effective isotropic radiated power for high energy collisions. These emission strengths, which are many orders of magnitude outside of the expectation on spurious emissions and radio-frequency interference, represent a risk for sensitive spacecraft electronics.