1,720,966 research outputs found
The space debris environment: future evolution
Get PDFSpace debris represents a significant risk to satellite operations, due to the possibility of damaging or catastrophic collisions. Consequently, many satellite operators screen the orbiting population for close approaches with their on-orbit assets and a public conjunction assessment service, Satellite Orbital Conjunction Reports Assessing Threatening Encounters in Space (SOCRATES), generates close approach predictions on a daily basis for all satellite payloads in the catalogue. These screening capabilities are used to inform operational decisions relating to risk mitigation but it is anticipated that the demands placed on these services will increase as debris becomes more prolific. This hypothesis is explored in a preliminary analysis of conjunction data for the years 2004 to 2009 and a new ‘Business As Usual’ study using the Debris Analysis and Monitoring Architecture for the Geosynchronous Environment (DAMAGE) model. The results suggest a 50% increase in the number of close approaches reported by SOCRATES (or its equivalent) within the next ten years. By 2059, daily conjunction reports could contain over 50,000 close approaches below 5 km, affecting the demands placed on tracking facilities and satellite resources
Self-induced collision hazard in high and moderate inclination satellite constellations
No full textThe assessment of the hazard posed by space debris to constellations of satellites in low Earth orbit is of growing importance, with the proliferation of proposed and implemented constellation systems with a variety of mission objectives. This applies to current constellation-based commercial communication systems, in particular, since these are typically deployed at altitudes where there is a peak in the space debris environment. An impact risk analysis is performed over a period of up to 1 month after a breakup event using two examples of constellation configurations. The first is similar to the IRIDIUM system, containing around 70 satellites in near-polar orbits at approximately 800 km altitude, and the second is a Globalstar-like configuration with 56 satellites at around 1400 km altitude, distributed in orbit planes inclined at 52°. The analysis is performed using the SDS software, which applies the probabilistic continuum dynamics technique. This has the benefit of being a self-contained and rigorous method. However, it is found to be not well-suited to ‘long-term’ analysis, due to the computational effort required. The risk analysis for the chosen examples is presented, as well as an investigation of the robustness of the method when applied to complex and ‘long-period’ simulations
A sensitivity analysis of breakup models
No full textFour of the leading breakup models, IDES, SDM, MASTER and EVOLVE 4 were compared in terms of their distributions of the cumulative number of fragments, fragment delta-vs and mass-to-area ratios with respect to fragment mass. These comparisons were made for a number of fragmentation scenarios including high and low intensity explosions and catastrophic and non-catastrophic collisions. It has been found that the IDES, SDM and MASTER models are generally in fairly good agreement for all these distributions in all scenarios, although there are slight differences to be found in certain cases. the EVOLVE 4 model produces generally different results for all scenarios, most notably in its delta-v and mass-tp-area distributions. the major differences between all the models in terms of their equations are discussed herein. An example scenario of a low intensity explosion in GEO was chosen and the resulting debris cloud for all models was propagated using the same high-fidelity orbital propagator. The differences in the propagated clouds for each model were compared. Generally the debris clouds produced by the IDES, SDM and MASTER models were in fairly good agreement for most scenarios, whilst the debris cloud produced by EVOLVE 4 shows some significant differences
A new analysis of debris mitigation and removal using networks
No full textModelling studies have shown that the implementation of mitigation guidelines, which aim to reduce the amount of new debris generated on-orbit, is an important requirement of future space activities but may be insufficient to stabilise the near-Earth debris environment. The role of a variety of mitigation practices in stabilising the environment has been investigated over the last decade, as has the potential of active debris removal (ADR) methods in recent work. We present a theoretical approach to the analysis of the debris environment that is based on the study of networks, composed of vertices and edges, which describe the dynamic relationships between Earth satellites in the debris system. Future projections of the 10 cm and larger satellite population in a non-mitigation scenario, conducted with the DAMAGE model, are used to illustrate key aspects of this approach. Information from the DAMAGE projections are used to reconstruct a network in which vertices represent satellites and edges encapsulate conjunctions between collision pairs. The network structure is then quantified using statistical measures, providing a numerical baseline for this future projection scenario. Finally, the impact of mitigation strategies and active debris removal, which can be mapped onto the network by altering or removing edges and vertices, can be assessed in terms of the changes from this baseline. The paper introduces the network methodology, highlights the ways in which this approach can be used to formalise criteria for debris mitigation and removal. It then summarises changes to the adopted network that correspond to an increasing stability and changes that represent a decreasing stability of the future debris environment
Investigating the long-term evolution of the debris environment in high Earth orbit using the DAMAGE model
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Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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