1,720,958 research outputs found
Special Orbits for Mercury Observation
No full textChapter 5, by Generoso Aliasi, Giovanni Mengali and Alessandro A. Quarta, deals with an advanced scientific mission concept in which the existence of suitable positions for the observation and the measurement of the Mercury’s magnetotail are investigated. The scientific mission is based on the use of artificial equilibrium points in the elliptic three-body system, constituted by the Sun, Mercury, and a spacecraft, which is modeled as a massless point. The spacecraft motion in the Sun–Mercury system is first discussed under the assumption that the propulsion
system provides a radial continuous thrust with respect to the Sun. In particular, the spacecraft is assumed to have a generalized sail as its primary propulsion system. A generalized sail models the performance of different types of advanced propulsion systems, including a (photonic) solar sail, an electric solar wind sail and an electric thruster, by simply modifying the value of a thrusting parameter. The location of the artificial equilibrium points is derived, and their stability is also investigated. It is shown that that collinear artificial equilibrium points are always
unstable, except for a range of L2-type points which are placed far away from Mercury. A similar result is obtained for triangular equilibrium points. A control strategy is introduced to maintain the spacecraft in the neighborhood of an artificial equilibrium point. In this context, a simple and effective way to actively control the spacecraft dynamics is by means of a Proportional-Integral-Derivative feedback control law. The latter control law is finally employed in the magnetotail mission scenario, whose fundamental idea is to continuously and slowly displacing the
artificial equilibrium point along the Sun–Mercury direction. Numerical simulations show the effectiveness of the proposed mission strategy
Analysis of Smart Dust-Based Frozen Orbits Around Mercury
No full textAccording to the classical two-body Keplerian model, the orbital parameters of a spacecraft are constant during a mission. However, real-life spacecraft motion is different from a classical Keplerian model due to the presence of perturbing forces, whose effects are usually undesirable, especially for observation and communication spacecraft that require accurate pointing capabilities. Therefore, active control systems are usually required to maintain the working orbit. However, an alternative strategy consists of suitably selecting the initial orbital elements to generate a “frozen orbit”, which on average maintains some of the design orbital elements. The utilization of spacecraft with large area-to-mass ratio could extend the flexibility on the initial choice of orbital parameters. In this context, a novel option is represented by smart dusts (SDs), which are femto-satellites with large area-to-mass ratio (or millimeter-scale solar sails). In this chapter, a double-averaging technique is used to determine planetocentric frozen orbits maintained by SDs. In particular, a numerical analysis of frozen orbits is discussed, with a special application focused on orbits around Mercury, which are fit for an SD-based scenario due to their closeness to the Sun and the absence of atmospheric drag
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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Get PDFWe 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
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