1,720,957 research outputs found
Constant orbit elements under the third body effect
No full textAn analysis to determine solutions with constant orbit elements has been carried out through a vectorial formulation of the perturbation equations, under the long-term influence due to the attraction of a disturbing body moving over an inclined elliptical orbit. After having gained a frozen orbital plane by assuming an orbital pole parallel or perpendicular to the perturbing body pole, the feasibility to get a frozen condition also on eccentricity or argument of pericentre has been demonstrated and several solutions have been proposed. Moreover, when the orbital pole is perpendicular to the perturbing body pole, a prime integral of motion, linking orbit eccentricity and argument of pericentre, has been retrieved. This prime integral has permitted the identification of solutions characterised by slow variations of eccentricity. A study to obtain orbits at constant eccentricity or argument of pericentre has also been carried out, regardless of the orbital plane evolution. This has highlighted how, while the solutions with a frozen apsidal line have to be determined by means of numerical methods, not pursued in this paper, the ones characterised by a null variation of eccentricity can be retrieved analytically. Examples, for a probe orbiting Mercury, have also been presented
Moon’s influence on the plane variation of circular orbits
No full textThe long-term orbital plane motion of a satellite in circular orbit has been investigated, taking into account the influence of the Earth’s oblateness and luni-solar perturbations, to determine solutions characterised by initial conditions (in terms of Right Ascension of the Ascending Node and orbit inclination) which guarantee quasi-frozen orbital planes. The Moon’s influence has been investigated considering the real motion of the lunar pole around the ecliptic pole, also evaluating the differences with respect to the simplified case in which these poles are considered as coincident at 23.445 deg of inclination. After having studied in detail the case of geosynchronous orbits, where at low inclination differences in the order of ±1.5 deg have been found for the optimal initial inclination with respect to the above-mentioned simplified case, the analysis has been extended to the range of altitude 20,000–60,000 km. The study has highlighted how the optimal initial conditions are strictly correlated to the launch date and how the differences between the real and the simplified cases increase with satellite altitude
Frozen orbits with equatorial perturbing bodies: the case of Ganymede, Callisto, and Titan
No full textFollowing an approach based on the Milankovitch elements, equations able to describe the long-term evolution of the trajectory of a probe orbiting a celestial body (primary body) have been analytically determined, taking into account the perturbative effects deriving from the planetary oblateness and from one or more perturbing bodies lying on the equatorial plane of the primary body. Using these equations, three families of orbits, characterized by constant orbit elements on average, have been obtained. Whereas the first family, composed of elliptical polar orbits, ensures frozen conditions on all the orbit elements, the other two lead to null variations of semimajor axis, eccentricity, inclination, and argument of pericenter. For the latter two families, the concept of critical inclination, which, in the case of only planetary oblateness, assumes the well-known values 63.43 and 116.57 deg, has been generalized adding the effect of several attracting bodies. Interesting solutions have been found for the observation of Jupiter’s moons Ganymede and Callisto and for Saturn’s moon Tita
A vectorial approach to determine frozen orbital conditions
No full textTaking into consideration a probe moving in an elliptical orbit around a celestial body, the possibility of determining conditions which lead to constant values on average of all the orbit elements has been investigated here, considering the influence of the planetary oblateness and the long-term effects deriving from the attraction of several perturbing bodies. To this end, three equations describing the variation of orbit eccentricity, apsidal line and angular momentum unit vector have been first retrieved, starting from a vectorial expression of the Lagrange planetary equations and considering for the third-body perturbation the gravity-gradient approximation, and then exploited to demonstrate the feasibility of achieving the above-mentioned goal. The study has led to the determination of two families of solutions at constant mean orbit elements, both characterised by a co-planarity condition between the eccentricity vector, the angular momentum and a vector resulting from the combination of the orbital poles of the perturbing bodies. As a practical case, the problem of a probe orbiting the Moon has been faced, taking into account the temporal evolution of the perturbing poles of the Sun and Earth, and frozen solutions at argument of pericentre 0 or 180 have been found
Coverage of Mars by probes slightly off the Areostationary Orbit
No full textThis paper aims at investigating the feasibility of gaining complete and continuous coverage of an extended equatorial belt of Mars, under the effects of orbital perturbations. The problem could be solved by considering a minimum number of three probes, which move slightly off the Martian Areostationary Orbit. The proposed configurations allow the achievement of the goal without entailing corrective manoeuvres and presenting only a longitudinal motion with respect to the surface of Mars
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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