Currently, the Low Earth Orbit (LEO) space environment contains a growing number of orbital debris objects. This growing orbital debris population increases collision probabilities between both orbital debris and functioning satellites. A phenomenon known as Kessler Syndrome can be induced if these collisions occur. Kessler Syndrome states that these collisions can lead to an exponential increase in the orbital debris population, which could dangerously impede future space missions. Current literature outlines the necessity of stabilizing the near-Earth environment debris population and introduces the concept of active debris removal (ADR). The use of ADR on five orbital debris objects per year was found to be a requirement to achieve stability within the orbital debris population. A viable mission architecture is henceforth explored to utilize ADR for near-future execution to further develop research for orbital debris mitigation missions. The larger orbital debris objects are found in many different orbital regimes and are primarily composed of spent rocket bodies and retired satellites. Different orbital debris ranking schemes have been developed based on the population density in these different regimes, which are linked to higher collision probabilities. Using these ranking schemes, a set of target objects are selected to be investigated for this mission design that was composed of target objects with similar orbital characteristics that were not launched by the Commonwealth of Independent States (CIS) to minimize legal barriers. Different ADR capture and removal methods are inspected to find the optimal methods for this mission. An Analytical Hierarchy Process (AHP) has been used to assess these different methods, which utilizes comparisons of the different methods among a set of weighted criteria. A net capture method with a low thrust chemical engine for removal is identified as the optimal ADR method. The use of a laser detumbling system is also selected to stabilize target objects with a high rotation rate. A rendezvous and deorbit orbital analysis are conducted using both a low fidelity tool (for preliminary results) and a high fidelity tool (for more precise results). The rendezvous analysis is used to select a mission architecture that was composed of two different chaser satellites which rendezvous with the five different target objects by taking advantage of nodal precession. The deorbit analysis investigates different decay timelines and found the delta-v estimates that would be required to deorbit the target objects within the same year that they were captured in. These two orbital analyses provide valuable insight to the mission timeline, delta-v estimates, and approximate mass requirement for the chaser satellite and deorbit kits. The results of the target selection process, ADR selection process, and the rendezvous and deorbit analyses are meant to provide an initial concept and analysis for a near-future ADR mission. These approximate results provide insight and information to further develop orbital debris mitigation research to help solve the orbital debris population growth challenge for future space missions. / Master of Science / Currently, the near Earth space environment contains a growing number of space debris. This growth in the orbital debris population increases the likelihood of collisions with orbital debris, functioning satellites, and launch vehicles. These collisions can generate a chain of events that could exponentially increase the population of orbital debris, which at some scale could become a major obstacle for future space missions. Researchers have introduced the concept of active debris removal (ADR), which in simulations has been shown to help stabilize the growth of orbital debris. The use of ADR to remove as low as five orbital debris objects per year has been found to be sufficient to stabilize debris growth. A viable mission architecture using ADR technologies that can be implemented in the near future is henceforth explored to further develop research for orbital debris missions. The larger orbital debris objects are found in many different areas in space and are primarily made up of used rocket bodies and retired satellites. Different ranking schemes have been developed by researchers for these larger orbital debris objects based on the population density within these areas in space, which are linked to the chance of a collision. Using these ranking schemes, a set of orbital debris objects are selected to be targeted for this mission design. This set of selected target objects have similar orbital characteristics and the political/legal barriers that could be present during removal are minimal. An ADR mission is composed of two primary components, a capture method and a removal method, which are inspected to find the optimal methods for this mission. A decision-making technique, called an Analytical Hierarchy Process (AHP), has been used to assess these different methods. The AHP compares different capture and removal methods using a set of weighted criteria. A net capture method with small thrusters for removal is identified as the optimal ADR method. Additionally, the use of a laser system is selected to stabilize target objects that may be rotating too quickly for capture. An analysis on different mission architectures is conducted using both a low fidelity tool (for preliminary results) and a high fidelity tool (for more precise results). A mission architecture composed of two different "chaser" satellites which rendezvous with and deorbit the five different target objects is selected. The analysis used on the selected mission architecture provides valuable insight to the mission timeline, fuel estimates, and approximate mass requirements. The results of the target selection process, ADR selection process, and the mission architecture analysis are meant to provide an initial concept and introduce possible requirements for a nearfuture ADR mission. These approximate results provide information to further develop research that can help us solve the orbital debris population growth challenge for future space missions.

The growth in orbital debris has been predicted since the dawn of the space age. Now the debris fields cascade through orbits and the risk of collision is on an infinite upward trajectory. This thesis will examine what impact a wider concept of space security can have our understanding of orbital debris and the space security agenda. The space security agenda is in a state a flux as it seeks the most effective way to deal with the threat posed by orbital debris. A traditionally narrow approach of security would see debris discarded as a security threat due to its limited threat to a state. However, a broader approach would see aspects of environmental security emerge, allowing both public and private sectors to act to solve this crisis. There is a sizeable void in the literature that links policy and science when analysing orbital debris. Therefore, when applying the theory, it is best to find consensus and collaboration. The Copenhagen and Welsh Schools of International Security offer opposing views initially. Nonetheless, when examined closely they reveal similarities that allow for a 'hybrid' theory to emerge. The international challenges to legal and policy changes are diverse and complex. Consequently, the significance of transparency and confidence- building measures to lead space policy and...

Spacecraft and satellites in orbit are under an increasing threat of impact from orbital debris and naturally occurring meteoroids. While objects larger than 10 cm are routinely tracked and avoided, collisions inevitably occur with smaller objects at relative velocities exceeding 10 km/s. Such hypervelocity impacts (HVI) create immense shock pressures and can melt or vaporize aerospace materials, even inducing brief plasmas at higher speeds. Sacrificial shields have been developed to protect critical components from damage under these conditions, but the response of many materials in such an extreme event is still poorly understood. This work presents the summary of computational analysis methods to quantify the relevant physical mechanisms at play in a hypervelocity impact. Strain rate-dependent behavior was investigated using several models, and fluid material descriptions were used to draw parallels under high shear rate loading. The production and expansion of impact plasmas were modeled and compared to experimental evidence. Additionally, a parametric study was performed on a multitude of possible material candidates for sacrificial shield design, and new shielding configurations were proposed. A comparison of material models indicated that the Johnson-Cook and Steinberg-Cochran-Guinan-Lund metallic formulations yielded the most consistent results with the lowest deviation from experimental measures in the strain rate regime of interest. Both meshless Lagrangian and quasi-Eulerian meshed schemes approximated the qualitative and quantitative characteristics of HVI debris clouds with average measurable errors under 5%. While the meshless methods showed better resolution of interfaces and small details, the meshed methods were shown to converge faster under several metrics with fewer regions of spurious instability. Additionally, a new technique was introduced using hypothetical viscous fluids to approximate debris cloud behavior, which showed good correlation to experimental results when such models were constructed using the shear rates seen in hypervelocity impacts. Formulations using non-Newtonian fluids showed additional capability in approximating solid behavior, both quantitatively and qualitatively. Such fluid models are significant, in that they reproduced the qualitative and quantitative characteristics of evolving debris clouds with better fidelity than purely hydrodynamic models using inviscid fluids. This indicates that while inertial effects can dominate overdriven shock phenomena, neglecting shear forces invariably introduces errors; such forces can instead be simplistically approximated via viscous models. The viscous approximation also allowed for a successful scaling analysis using dimensionless Pi terms, which was unfeasible using solid constitutive relations. Attempts to model plasma dynamics saw success in the simulation of a laser ablation-driven flyer plate by using a hot gas with solid initial conditions; similar strategies were used to analyze plasma production in hypervelocity impacts with reasonable correlation to experimental measurements. Lastly, the analysis of bumper material candidates showed that metals with a low density such as beryllium and magnesium yield a higher specific energy and momentum reduction of incident projectiles with lower weight requirements than a similarly constructed bumper using aluminum. Investigations of bumpers using a combination of materials and variations in microstructure showed promise in increasing weight-normalized efficacy. Through these computational models, the parameters which influence damage and debris in hypervelocity impacts are more critically understood. / PHD

Orbital debris is a current and growing threat to reliable space operations and new space vehicle traffic. As space traffic increases, so does the economic impact of orbital debris on the sustainability of systems that increasingly support national security and international commerce. Much of the debris collision risk is concentrated in specific high-density debris clusters in key regions of Low Earth Orbit (LEO). A potential long-term solution is to employ a constellation of observation satellites within these debris clusters to improve monitoring and characterization efforts, and engage in Laser Debris Removal (LDR) as means of collision mitigation. Here we adapted and improved a previous methodology for evaluating such designs. Further, we performed an analysis on the observer constellations' effectiveness over a range of circular, elliptical, and self-maneuvering designs. Our results show that increasingly complex designs result in improved performance of various criteria and that the proposed method of observation could significantly reduce the threat orbital debris poses to space operations and economic growth. / Master of Science / Orbital debris is defined as all non-operational, man-made objects currently in space. US national space regulations require every new satellite to have a de-orbit plan to prevent the creation of new debris, but fails to address the thousands of derelict objects currently hindering space operations. As space traffic increases, so does the economic impact of orbital debris on the sustainability of systems that increasingly support national security and commercial growth. While orbital debris is usually assessed by looking at the full volume of space, most massive debris objects are concentrated in high-density clusters with a higher than normal probability for collision. A potential solution to the growing orbital debris problem is to place a group of observation satellites within these debris clusters to both improve monitoring capabilities and provide a means for preventing potential collisions by engaging with debris via Laser Debris Removal (LDR). This research presents a methodology for comparing and contrasting different observer satellite constellation designs. Our results show that increasingly complex orbit designs improve various performance criteria, but ultimately orbits that more closely match those of the debris objects provide the best coverage. The proposed method of observation and engagement could significantly reduce the threat orbital debris poses to space operations and economic growth.

Magister Legum - LLM / Environmental Law is at the forefront of the global community and environmental protection and conservation is regarded as of the utmost importance.1 Outer Space is a unique, limited, and valuable resource. Outer space allows states to utilise thousands of satellites for research, national defence, and communications. At the inception of space law, only a few states dominated space activities and all human space activities were so challenging that nearly any method seemed acceptable for placing objects in outer space, currently more countries have space industries and launch capabilities

Once a satellite has completed its operational period, it must be removed responsibly in order to reduce the risk of impacting other missions. Geostationary Transfer Orbits (GTOs) offer unique challenges when considering disposal of spacecraft, as high eccentricity and orbital energy give rise to unique challenges for spacecraft designers. By leveraging small satellite research and integration techniques, a deployable drag sail module was analyzed that can shorten the expected orbit time of launch vehicle stages in GTO. A tool was developed to efficiently model spacecraft trajectories over long periods of time, which allowed for analysis of an object’s expected lifetime after its operational period had concluded. Material limitations on drag sail sizing and performance were also analyzed in order to conclude whether or not a system with the required orbital performance is feasible. It was determined that the sail materials and configuration is capable of surviving the expected GTO environment, and that a 49 m2 drag sail is capable of sufficiently shortening the amount of time that the space vehicles will remain in space.

There is an increased need for spacecraft to quickly and efficiently de-orbit themselves as the amount of debris in orbit around Earth grows. Defunct spacecraft pose a significant threat to the LEO environment due to their risk of fragmentation. If these spacecraft are de-orbited at the end of their useful life their risk to future spacecraft is greatly lessened. A proposed method of efficiently de-orbiting spacecraft is to use an inflatable thin-film envelope to increase the body's area to mass ratio and thusly shortening its orbital lifetime. The system and analysis presented in this project is sized for use on a CubeSat as they are an effective utility as a technology demonstration platform. Analysis has been performed to characterize the orbital dynamics of high area to mass ratio spacecraft as well as the leak rate of such an inflatable device in a vacuum environment. Results show that a 1U CubeSat can be de-orbited using a 1.7 meter diameter spherical device in just under one year while using 0.7 grams of inflating gas, this is compared to over 25 years without any method of post-mission disposal.

This thesis will present the effects of the orbital debris evolution in two key areas: the geosynchronous disposal orbit regime known as “graveyard” and the two geopotential wells found in 105◦ W and 75◦ E longitude bins. After developing a GEO specific orbit propagator for NASA Johnson Space Center’s Orbital Debris Of- fice, collisions were simulated throughout these regimes using a low velocity breakup model. This model considered the effects of perturbations particularly non-spherical Earth effects (specifically sectorial and zonal harmonics), lunar effects, third body effects and solar radiation pressure effects. The results show that CDPROP does well in simulating the presence of the Eastern and Western geopotential wells, as well as catching drifting GEO objects. It does not do as well in catching East-West trapped objects. Three collision test cases were then simulated in graveyard and the East and West geopotential wells.

With the increase in space traffic, proliferation of inexpensive launch opportunities, and interest from many countries in utilizing the space domain, threats to existing space assets are likely to increase dramatically in the coming years. The development of a system that can identify potential threats and alert space operators is vital to maintaining asset resiliency and security. The focus of this thesis is the design and evaluation of such a system. The design is comprised of the development of a classification hierarchy and the selection of machine learning models that will enable the identification of anomalous object behavior. The hierarchy is based on previous examples applied to object classification while reconsidering the assumption that a satellite may perform only one mission. The selected machine learning models perform both supervised classification of actively maneuvering objects and unsupervised identification of anomalous behavior within large satellite constellations. The evaluation process considers the independent adjustment of model hyperparameters to achieve optimal model settings. The optimal models perform both classification functions and return moderate accuracy. The system is applied to several case studies examining edge cases and what factors constitute a threatening object and what factors do not. Suggestions for improvement of the system in the future are presented. / Master of Science / The increase in space traffic, proliferation of inexpensive launch opportunities, and interest from many countries in utilizing the space domain represent existential threats to existing spacecraft and operations in low-Earth orbit. Threats to the safe operation of spacecraft are likely to increase dramatically in the coming years. The development of a system that can identify potential threats and alert space operators is vital to maintaining asset resiliency and security. The focus of this thesis is the design and evaluation of such a system. This is accomplished by identifying a system architecture through evaluating current assumptions of what missions satellites are capable of performing. Following the system-level design, modules are proposed that utilize machine learning to identify satellite behavior that is abnormal. These modules are tested and tuned with optimal parameters to deliver improved identification performance. The system is applied to several case studies examining edge cases and what factors constitute a threatening object and what factors do not. Suggestions for improvement of the system in the future are presented.

Approved for public release; distribution in unlimited. / The International Space Station (ISS) must be able to withstand the hypervelocity impacts of micrometeoroids and orbital debris that strike its many surfaces. In order to design and implement shielding which will prevent hull penetration or other operational losses, NASA must first model the orbital debris and micrometeoroid environment. Based upon this environment, special multi-stage shields called Whipple and Enhanced Stuffed Whipple Shields are developed and implemented to protect ISS surfaces. Ballistic limit curves that establish shield failure criteria are determined via ground testing. These curves are functions of material strength, shield spacing, projectile size, shape and density, as well as a number of other variables. The combination of debris model and ballistic limit equations allows NASA to model risk to ISS using a hydro-code called BUMPER. This thesis modifies and refines existing ballistic limit equations for U.S. Laboratory Module shields to account for the effects of projectile (debris/ micro-meteoroid) densities. Using these refined ballistic limit equations this thesis also examines alternative shielding materials and configurations to optimize shield design for minimum mass and maximum stopping potential, proposing alternate shield designs for future NASA ground testing. A final goal of this thesis is to provide the Department of Defense a background in satellite shield theory and design in order to improve protection against micrometeoroid and orbital debris impacts on future spacebased national systems. / Lieutenant, United States Navy