Includes bibliographical references (p. 98-103).

published_or_final_version / abstract / toc / Applied Geosciences / Master / Master of Science

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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.

This research aims to improve our knowledge of debris flow occurrence and behaviour in New Zealand. Detailed field data collected in four debris flow prone areas in New Zealand are presented and compared. The travel distance of these events is then modelled with an empiricalstatistical model, UBCDflow, and an analytical, “equivalent fluid” continuum model DAN-W. While field studies are useful, they are often not linked to the underlying mechanics of debris flow motion or compared with the behavior of small scale flows due to the inherent complexity and unknown boundary conditions in field scale flows. Physical modelling simplifies the situation and allows boundary conditions to be controlled. The second part of this research uses physical modelling, including a series of novel debris flow tests in a geotechnical centrifuge, to compare and contrast flow behaviour and mechanics of laboratory and field scale flows. The debris flows events investigated in the field were categorized into hillslope, torrent, or intermediate-type events. Hillslope events were less channelized and progressively deposited on high slope angles. Consequently, high friction coefficients were needed to model their mobility. Torrent flows entrained more material than hillslope flows and deposited on lower angle slopes in response to unconfinement on the debris flow fan. Friction coefficients back-calculated for torrent events were lower than for the hillslope flows, but still larger than most of the friction coefficients given for large, channelized, debris flow events in the literature. Intermediate events were similar to hillslope events in terms of deposition angle and best-fit friction coefficients, but were very confined. Both UBCDflow and DAN-W were found to be useful decision support tools, but the capability of each model was limited. Greater modelling capability was gained by using the volume change behaviour predicted by UBCDflow in DAN-W, as DAN-W simulates flow heights and velocities, but does not predict the depth of erosion. In the second part of the research, a geotechnical centrifuge is used to model debris flow processes in a larger acceleration field than earth’s gravity. While centrifuges have been used to model a variety of processes in other geotechnical problems, debris flows are a relatively new phenomenon to be tested on a centrifuge. The centrifuge was successful in increasing the frictional properties of flow, but viscous forces were still the dominant form of shear stress with the materials used. Markedly different flow behaviour of tests using different pore-fluid rheologies suggested that the dominant mechanism of shear resistance may have changed between confined, downslope movement and unconfined runout. The results also showed that in geotechnical centrifuge testing, the viscosity of the pore fluid scales with the g-level, N. This research is an important step in developing centrifuge testing as an accepted method of modelling debris flow processes. Finally, a brief comparison of friction slopes between small-scale 1-g flume tests and field scale flows suggests that 1-g flume experiments are able to model the mobility of field scale flows if the soil used is well-graded and the pore-fluid is not too viscous. This research shows that the the ability of laboratory scale flows to model large scale processes may not be as limited as previously suggested by some investigators.

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Circumstellar debris disks are the extrasolar analogues of the asteroid belt and the Kuiper belt. These disks consist of comets and leftover planetesimals that continuously collide to produce copious amounts of circumstellar dust that can be observed as infrared excess or in resolved imaging. As an obvious outcome of the planet formation process, debris disks can help us constrain planet formation theories and learn about the history of our own solar system. Structures in the disks such as gaps or warps can hint at the presence of planets. Thus, the study of debris disks is an important branch of exoplanetary science. In this thesis, some aspects of debris disks are considered in detail. A handful of debris disks show observable amounts of gas besides the dust. One such case is the edge-on debris disk around the young A-type star β Pictoris, where the gas is thought to be of secondary origin, i.e. derived from the dust itself. By observing this gas, we can thus learn something about the dust, and therefore about the building blocks of planets. In paper I, spectrally resolved observations of C II emission with Herschel/HIFI are presented. The line profile is used to constrain the spatial distribution of carbon gas in the disk, which helps understanding the gas producing mechanism. In paper II, we analyse C II and O I emission detected with Herschel/PACS and find that the oxygen must be located in a relatively dense region, possibly similar to the CO clump seen by ALMA. An upcoming analysis of our ALMA C I observations will give us a clearer picture of the system. Another famous debris disk is found around the nearby, 440 Myr old A-star Fomalhaut. Its morphology is that of an eccentric debris belt with sharp edges, suggesting shaping by a planet. However, gas-dust interactions may result in a similar morphology without the need to invoke planets. We test this possibility in paper III by analysing non-detections of C II and O I emission by Herschel/PACS. We find that there is not enough gas present to efficiently sustain gas-dust interactions, implying that the morphology of the Fomalhaut belt is due to a yet unseen planet or alternatively stellar encounters. One of the biggest challenges in exoplanetary research is to answer the question whether there are inhabited worlds other than the Earth. With the number of known rocky exoplanets in the habitable zone increasing rapidly, we might actually be able to answer this question in the coming decades. Different approaches exist to detect the presence of life remotely, for example by studying exoplanetary atmospheres or by analysing light reflected off the surface of an exoplanet. In paper IV, we study whether biosignatures (for example, certain minerals or microorganisms) ejected into a circumstellar debris disk by an impact event could be detected. We consider an impact similar to the Chicxulub event and model the collisional evolution of the ejected debris. Dust from such an event can potentially be detected by current telescopes, but analysis of the debris composition has to wait for future, advanced instruments. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 4: Submitted.</p>

Thesis (M.S.)--West Virginia University, 2002. / Title from document title page. Document formatted into pages; contains xiii, 143 p. : ill. (some col.), maps (some col.). Includes abstract. Includes bibliographical references (p. 113-120).