1,721,016 research outputs found
Relazioni compatte per il progetto di costellazioni satellitari
Get PDFIn questo lavoro saranno individuate equazioni originali in grado di descrivere gli effetti
perturbativi di lungo periodo, sugli elementi orbitali classici, di un satellite posto sotto l’influenza di
un terzo corpo, nel caso più generale in cui quest’ultimo descriva un’orbita ellittica ed inclinata
rispetto all’equatore del corpo primario del satellite.
Tali equazioni, unitamente a quelle in grado di stimare gli effetti perturbativi dovuti all’asimmetria
del geopotenziale, saranno utilizzate per giungere- analiticamente- a polinomi risolutivi per orbite
dedite all’osservazione ambientale (orbite periodiche sincrone e multi-sincrone con il moto
apparente del Sole).
Saranno dunque fornite relazioni compatte originali per il disegno rapido di costellazioni basato
sulla disposizione delle tracce a terra di satelliti posti su una comune orbita periodica, valutata
mediante l’uso dei polinomi risolutivi che tengono conto degli effetti perturbativi dell’asimmetria
del geopotenziale e del terzo corpo, della quale sono state accuratamente scelte le caratteristiche di
osservazione. Data l’estrema semplicità di queste relazioni, ne è possibile l’implementazione su
appositi codici che consentiranno di disegnare le costellazioni e valutarne le prestazioni nell’ambito
dell’osservazione planetaria, con estrema facilità ed accuratezza.
Sono proposti esempi applicativi atti a validare sia le equazioni funzionali alla stima degli effetti
perturbativi di terzo corpo che le relazioni compatte per il disegno rapido di costellazioni di satelliti
per l’osservazione ambientale
An analytical approach to retrieve the effects of a non-coplanar disturbing body
No full textThe determination of analytical expressions which, including the main perturbative effects, allow the retrieval of the orbit elements of a probe represents an important requirement in designing science trajectories. One of these perturbations is given by the third body attraction. The case in which the perturbing body moves on a plane coincident with the equatorial plane of the primary body has been investigated in previous studies and equations able to provide the temporal evolution of the orbit elements have been determined and applied to the main moons of the Solar System. In this paper an extension of this topic has been carried out and equations which allow the determination of the orbit evolution have been analytically retrieved in the general case in which one or more perturbing bodies describe elliptical and inclined orbits with respect to the equatorial plane of the primary. Then, introducing these equations into the periodicity condition for the probe ground track, and considering the J2 and J4 effects coming from the primary body, an equation able to provide repeating ground track orbits has been determine
Optimal orbits around Ganymede for the JUICE mission
No full textThe JUpiter ICy moons Explorer mission of the European Space Agency, that will be launched in the year 2022, will study the geodesy and physical characteristics of Jupiter and its icy moons Europa, Callisto and Ganymede. The final phase of this mission, planned for the year 2032, is still under investigation and foresees phases in circular orbit around Ganymede. In this paper two types of science orbits around Ganymede, that optimize the observational performances and the probe life-time, are proposed. While the first type allows, at “zero cost”, the obtaining of repeated observations of a given point of the surface, the second type, with a limited cost, adds to the periodicity of observation, the condition of synchronism with the Sun. This synchronism permits both the achievement of a condition of permanent sunlight for the probe, thus leading to an extension of the mission duration, and a minimization of the variations of spectral signature due to the Sun’s position
Polynomial equations for science orbits around Europa
No full textIn this paper, the design of science orbits for the observation of a celestial body has been carried out using polynomial equations. The effects related to the main zonal harmonics of the celestial body and the perturbation deriving from the presence of a third celestial body have been taken into account. The third body describes a circular and equatorial orbit with respect to the primary body and, for its disturbing potential, an expansion in Legendre polynomials up to the second order has been considered. These polynomial equations allow the determination of science orbits around Jupiter’s satellite Europa, where the third body gravitational attraction represents one of the main forces influencing the motion of an orbiting probe. Thus, the retrieved relationships have been applied to this moon and periodic sunsynchronous and multi-sun-synchronous orbits have been determined. Finally, numerical simulations have been carried out to validate the analytical results
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
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
- …
