title: New Techniques and Methods for In-Situ Orbital Debris Detectors
Creators: name: Cornwell, Luke Travis
Publisher: name: University of Kent
Year: 2024

description: The rapidly increasing population of orbital debris in the near-Earth environment poses a significant hazard for operational spacecraft and future space missions. This has led to an increased need for in-situ detectors capable of observing, and distinguishing between, natural space dust and anthropogenic orbital debris, to both measure their flux and help quantify the threat that they pose. Accordingly, this thesis is concerned with the development of new techniques for in-situ orbital debris detection, with a focus on the development of acoustic thin film time of flight (TOF) detectors, specifically the Debris Resistive Acoustic Grid Orbital NASA-Navy Sensor (DRAGONS) developed by NASA, and my industrial partners AstroAcoustics. TOF detectors are valued for their impactor speed and direction measurement capabilities that allow the distinction between orbital debris and natural space dust particles. The development and successful implementation of thin film TOF detectors, which measure impactor speed via the passage of the impactor through successive thin films, requires that two key questions be answered: Firstly, what is the measurement accuracy of such a detector? Secondly, what effect does passage through the thin film have on the impactor and the resulting speed measurement, i.e. is the impactor decelerated upon passage through the first film? To address these questions prototype detectors based on the DRAGONS concept were constructed, one with two successive 12.5 µm Kapton films and the other with two successive 25 µm Kapton films. These were then impacted with stainless steel projectiles ranging from 0.2 mm to 1 mm in diameter at hypervelocity speeds of ~ 2 km s-1 and ~ 4 km s-1 using the University of Kent's two-stage Light Gas Gun. This range of projectile sizes provided a film thickness to projectile diameter ratio (f/dp) of between 1/80 ≤ f/dp ≤ 1/8. For the largest 1 mm projectiles impacting 12.5 µm Kapton films, no deceleration was observed, and the speed obtained from the detector was found to be accurate to less than 1% error. This confirms that acoustic thin film detectors can measure the speed of 1 mm-sized impactors to a high degree of accuracy and are thus suitable for use in space to measuring this size of orbital debris, which poses the greatest threat to space missions in low Earth orbit (LEO). As f/dp increases, the penetration hole morphology becomes more complex and with-it acoustic signal onset determination decreases in accuracy, resulting in a decrease in speed measurement accuracy. Deceleration was not observed for projectiles ≥ 0.4 mm impacting 12.5 µm Kapton films (f/dp = 1/32), however, as f/dp increased to f/dp = 1/16, deceleration started to occur. Broadly deceleration was found to have size dependent effects, with the absolute film thickness playing a role as well as f/dp. Furthermore, comparison to previous results in the literature would suggest that there is also a material dependence. During the investigation, non-acoustic noise was identified in some of the traces from the polyvinylidene fluoride (PVDF) acoustic sensors. This was observed to coincide with impact light flash produced during projectile impact and is the likely source of this noise. With the space industry moving towards using smaller spacecraft in favour of larger, more traditional spacecraft, a preliminary analysis of the feasibility of using small area detectors applied to small spacecraft (specifically CubeSats), to perform orbital debris flux measurements, was conducted. Traditionally large areas on single spacecraft are required for impact detectors to ensure they provide meaningful statistical data. Thus, the use of small area detectors that can be applied to CubeSats faces an important question: can the accumulation of data from detection areas split over multiple small detectors provide statistically meaningful results? Comparison between the accumulated flux from CubeSat sized surfaces that have previously been exposed to space and predictions from ESA's most up to date space environment modelling software - MASTER 8.0.3 - suggests that the accumulation of detection area does provide statistically meaningful data, with accumulated fluxes within or very close to the estimated minimum uncertainties for the predicted fluxes.