1,720,978 research outputs found
Dynamical model of binary asteroid systems through patched three-body problems
Full text linkThe paper presents a strategy for trajectory design in the proximity of a binary asteroid pair. A novel patched approach has been used to design trajectories in the binary system, which is modeled by means of two different three-body systems. The model introduces some degrees of freedom with respect to a classical two-body approach and it is intended to model to higher accuracy the peculiar dynamical properties of such irregular and low gravity field bodies, while keeping the advantages of having a full analytical formulation and low computational cost required. The neighborhood of the asteroid couple is split into two regions of influence where two different three-body problems describe the dynamics of the spacecraft. These regions have been identified by introducing the concept of surface of equivalence (SOE), a three-dimensional surface that serves as boundary between the regions of influence of each dynamical model. A case of study is presented, in terms of potential scenario that may benefit of such an approach in solving its mission analysis. Cost-effective solutions to land a vehicle on the surface of a low gravity body are selected by generating Poincaré maps on the SOE, seeking intersections between stable and unstable manifolds of the two patched three-body systems
Periodic Orbit-Attitude Solutions Along Planar Orbits in a Perturbed Circular Restricted Three-Body Problem for the Earth-Moon System
No full textThe Earth-Moon system is nowadays attracting more and more interest as a well suited location for near and far future long term missions based on large structures. As a natural consequence, their design process has to go through a deep understanding of the orbital and attitude coupled motions which, although being significantly complex in a multi-attractors environment, may highlight interesting dynamical structures to be exploited for designing the vehicle passive guidance. Under the adoption of the Circular Restricted Three-Body Problem (CR3BP) model the understanding of either periodic or, at least, bounded behaviors for the rotational motion is still a young research field. The paper presents a systematic approach to search and identify the aforementioned structures, beneficial for naturally coupled stable motions of large structures located in the Earth-Moon system: firstly planar orbit-attitude periodic solutions are obtained and classified for rigid bodies travelling along known CR3BP periodic trajectories, disturbed by the gravity gradient effects only. The torque exerted by Solar Radiation Pressure (SRP) is, next, introduced into the model. The performed analysis highlighted that even with SRP torque included, rotational periodic behaviors still exist for specific resonance ratios between the solar apparent motion and the spacecraft orbit periods. The assumption of the Sun apparent orbit to be coplanar with the Earth-Moon motion is adopted and its effects assessed with respect to the Sun ephemerides exploitation; the reference path is initially kept fixed, and deviations due to SRP are then analyzed leading to nearly-periodic solution structures. A sensitivity analysis incorporating SRP investigates the uncertainties on some spacecraft parameters (e.g. reflectance/absorbance coefficients, position of the center of pressure, irradiated area) and their effects on the orbit-attitude nominal motion. As final step to refine the here presented model, the large structure flexibility has been inserted as perturbation, superimposed to a reference orbit-attitude rigid body motion: selected periodic solutions are assumed as reference and the modal and frequency analysis are performed, to detect excitation of the spacecraft structural modes and assess their effects with respect to the nominal rigid body solution. At the end, the presented model represents an effective tool to address and support the large structures in Earth-Moon system design, to preserve at the most a periodic nature of their coupled attitude-orbital dynamics. As a case study, the effectiveness of the proposed tool is discussed on a simplified extended space vehicle model
Dynamical Evolution About Asteroids with High Fidelity Gravity Field and Perturbations Modeling
No full textOne of the most important aspects when dealing with a Potentially Hazardous Object (PHO) is the accurate determination of its dynamical state. In particular, the determination of orbital and rotational perturbations is important to propagate accurately the heliocentric orbital path of an asteroid or a comet, and to be more precise in the impact risk determination and related uncertainty containment. The paper discusses the analysis and study of the motion of an irregularly-shaped celestial body, with particular attention to its complex three-dimensional rotational dynamics: the rotation state, nutation and precession motions are considered while modelling. All perturbations, relevant to the case of study, are included in the dynamical model, from the classical to the more complex, such as the Solar Radiation Pressure (SRP), the third body gravitational effect (presence of the Sun), the YORP effect and the internal dissipation of energy. In addition, particular attention has been paid to accurately model the shape of the asteroid: simple spherical models demonstrated to possess low accuracy when the asteroid or the comet is not spherically shaped. Irregular shapes represent, indeed, one of the most important aspects to compute the disturbances affecting the dynamics of these objects. The study has been performed by considering different characteristic shapes for typical irregular bodies: from the quasi-spherical, to the dog-bone and the elongated shapes. The perturbations due to external sources are modelled numerically. The sources of disturbances are then ranked and different criteria to propagate rotational motion have been derived depending on the shape of the observed asteroid. Even if the simulation results have been verified on selected asteroids dynamics, the presented methods and approach apply to the dynamical propagation of any kind of asteroid or comet
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
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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