1,720,957 research outputs found
Effects of Orbit Raising and Deorbiting in Source-Sink Evolutionary Models
Full text linkThe sustainability of the low-Earth-orbit (LEO) environment is threatened by the growing number of anthropogenic space objects planned to be launched in the coming years. This paper investigates the evolution of objects residing in LEO through the MIT Orbital Capacity Assessment Tool (MOCAT), an evolutionary multishell, multispecies source-sink model. The proposed novelty considers the flow of objects crossing multiple shells during orbit raising and deorbiting maneuvers, modeled through the secular variation of the semimajor axis under a low-thrust continuous applied control. To this aim, a higher-fidelity MOCAT version, including active satellites, derelicts, debris, and rocket bodies, has been developed and used. The results demonstrate that incorporating orbit transfer fluxes into the model results in a higher number of collisions, which leads to a greater quantity of debris and poses a greater threat to the safety of LEO
Effects of Orbit Raising and Deorbiting in Source-Sink Evolutionary Models
Full text linkThe sustainability of the Low Earth Orbit (LEO) environment is threatened by the growing number of Anthropogenic Space Objects planned to be launched in the coming years. This paper investigates the evolution of objects residing in LEO through the MIT Orbital Capacity Assessment Tool (MOCAT), an evolutionary multi-shell multi-species source-sink model. The proposed novelty considers the flow of objects crossing multiple shells during orbit raising and deorbiting maneuvers, modeled through the secular variation of the semi-major axis under a low-thrust continuous applied control. To this aim, a higher fidelity MOCAT version including active satellites, derelicts, debris, and rocket bodies has been developed and used. The results demonstrate that incorporating orbit transfer fluxes into the model results in a higher number of collisions, which leads to a greater quantity of debris and poses a greater threat to the safety of low Earth orbit.This is a manuscript of the article Published as Gusmini, Davide, Andrea D’Ambrosio, Simone Servadio, Peng M. Siew, Pierluigi Di Lizia, and Richard Linares. "Effects of Orbit Raising and Deorbiting in Source-Sink Evolutionary Models." Journal of Spacecraft and Rockets 61, no. 3 (2024): 784-797. doi: https://doi.org/10.2514/1.A35849
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
A new source-sink space environment evolution model including the effects of orbit transfer maneuvers
Full text linkLAUREA MAGISTRALEIl crescente numero di Oggetti Spaziali Antropogenici (ASO) previsti per essere lanciati, in particolare mega-costellazioni, rappresenta una minaccia per la sostenibilità dell'orbita terrestre bassa (Low Earth Orbit, LEO). Questa tesi utilizza il Massachusetts Institute of Technology Orbital Capacity Assessment Tool (MOCAT), un modello evolutivo multi-guscio multi-specie fonti-pozzi, per analizzare l'evoluzione degli oggetti in LEO. Viene proposto un nuovo approccio che tiene conto del flusso di oggetti attraverso più gusci durante le manovre di trasferimento orbitale, il che aumenta il rischio di collisioni e creazione di detriti.
Una versione ad alta fedeltà del MOCAT è stata sviluppata per includere satelliti attivi, derelitti, detriti e lanciatori, oltre a dividere i satelliti attivi in tre sottocategorie che rappresentano diverse fasi della loro vita: satelliti in fase di salita, satelliti operativi e satelliti in fase di discesa. I satelliti in fase di salita sono quelli che manovrano dall'orbita di immissione all'orbita operativa, i satelliti operativi sono quelli che svolgono i compiti di missione e i satelliti in fase di discesa o deorbitanti manovrano per rientrare nell'atmosfera alla fine della loro vita.
Il modello MOCAT propaga gli ASO come specie usando un sistema di equazioni differenziali ordinarie, dividendo l'ambiente LEO in gusci sferici concentrici per orbite quasi circolari. Gli oggetti possono essere aggiunti tramite nuovi lanci o collisioni, indicati come fonti, mentre gli oggetti vengono rimossi attraverso lo smaltimento post-missione e la resistenza atmosferica, indicati come pozzi. Il flusso di oggetti viene modellato utilizzando una legge di controllo a bassa spinta applicata continuamente, risultando in una traiettoria di trasferimento a spirale, che aumenta la densità orbitale di un guscio specifico.
Lo studio si propone di indagare l'evoluzione dell'ambiente spaziale confrontando diversi modelli MOCAT con o senza manovre di trasferimento orbitale. I risultati mostrano che l'inclusione di tali manovre porta a un numero maggiore di collisioni e detriti in orbita. Lo studio presenta anche un confronto tra il modello atmosferico esponenziale statico e il più complesso modello Jacchia-Bowman 2008. Il primo ha costi computazionali inferiori ma non considera i cicli solari e gli eventi geomagnetici, che possono comportare stime imprecise della popolazione. Inoltre, la capacità orbitale viene valutata utilizzando tre diverse metriche: il prodotto numero-tempo, l'indice di criticità cumulativa del veicolo spaziale e l'indice di rischio.
Infine, lo studio conduce un'analisi di sensibilità sulla magnitudo dell'accelerazione a bassa spinta e sul numero di satelliti lanciati, seguita da un algoritmo di ottimizzazione che determina l'altitudine ottimale dell'orbita di immisione per massimizzare il numero di satelliti in orbita riducendo al minimo i detriti. I risultati hanno dimostrato l'importanza di includere manovre di trasferimento orbitale nel modello per non sottostimare l'attuale popolazione in LEO e sottolinea l'importanza di selezionare un'altitudine orbitale di immissione adatta per evitare gusci con alta densità orbitale.
Nel complesso, questa tesi presenta un prezioso contributo al campo della modellazione dell'ambiente spaziale, fornendo un nuovo approccio che consente una migliore comprensione delle sfide affrontate dal crescente numero di oggetti spaziali antropogenici in LEO, evidenziando anche il potenziale per la ricerca e lo sviluppo futuri.The growing number of Anthropogenic Space Objects (ASO) planned for launch, particularly mega-constellations, poses a threat to the sustainability of Low Earth Orbit (LEO). This thesis uses the Massachusetts Institute of Technology Orbital Capacity Assessment Tool (MOCAT), an evolutionary multi-shell multi-species source-sink model, to analyze the evolution of objects in LEO. A novel approach is proposed that accounts for the flow of objects across multiple shells during orbit transfer maneuvers, which increases the risk of collisions and debris creation.
A higher fidelity version of MOCAT is developed to include active satellites, derelicts, debris, and rocket bodies, as well as dividing active satellites into three subcategories representing different phases of their lifetime: raising satellites, operative satellites, and deorbiting satellites. Raising satellites are those maneuvering from the injection to the operative orbit, operative satellites perform mission tasks, and deorbiting satellites maneuver to re-enter the atmosphere at the end of their lifetime.
The MOCAT model propagates ASOs as species using a system of ordinary differential equations, dividing the LEO environment into concentric spherical shells for near-circular orbits. Objects can be added via new launches or collisions, referred to as sources, while objects are removed through post-mission disposal and atmospheric drag, referred to as sinks. The flux of objects is modeled using a low-thrust continuously applied control law, resulting in a spiraling transfer trajectory, which increases the orbital density of a specific shell.
The study aims to investigate the evolution of the space environment by comparing different MOCAT models with or without orbit transfer maneuvers. Results show that including such maneuvers leads to a higher number of collisions and debris in orbit. The study also presents a comparison between the static exponential atmospheric model and the more complex Jacchia-Bowman 2008 model. The former has lower computational costs but does not consider solar cycles and geomagnetic events, which may result in inaccurate estimations of the ASO population. Additionally, the orbital capacity is evaluated using three different metrics: the number-time product, the cumulative criticality of spacecraft index, and the risk index.
Lastly, the study conducts a sensitivity analysis on the low-thrust acceleration magnitude and the number of satellites launched, followed by an optimization algorithm that determines the optimal injection orbit altitude to maximize the number of satellites in orbit while minimizing debris. Results demonstrated the importance of including orbit transfer maneuvers in the model not to underestimate the actual LEO population and highlights the importance of selecting a suitable injection orbit altitude to avoid dense orbital shells.
Overall, this thesis presents a valuable contribution to the field of space environment modeling, providing a novel approach that allows for a better understanding of the challenges faced by the increasing number of anthropogenic space objects in low Earth orbit, while also highlighting the potential for future research and development
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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