Influence of the coorbital resonance on the rotation of the Trojan satellites of Saturn

The Cassini spacecraft collects high resolution images of the saturnian satellites and reveals the surface of these new worlds. The shape and rotation of the satellites can be determined from the Cassini Imaging Science Subsystem data, employing limb coordinates and stereogrammetric control points. This is the case for Epimetheus (Tiscareno et al. 2009) that opens elaboration of new rotational models (Tiscareno et al. 2009; Noyelles 2010; Robutel et al. 2011). Especially, Epimetheus is characterized by its horseshoe shape orbit and the presence of the swap is essential to introduce explicitly into rotational models. During its journey in the saturnian system, Cassini spacecraft accumulates the observational data of the other satellites and it will be possible to determine the rotational parameters of several of them. To prepare these future observations, we built rotational models of the coorbital (also called Trojan) satellites Telesto, Calypso, Helene, and Polydeuces, in addition to Janus and Epimetheus. Indeed, Telesto and Calypso orbit around the L_4 and L_5 Lagrange points of Saturn-Tethys while Helene and Polydeuces are coorbital of Dione. The goal of this study is to understand how the departure from the Keplerian motion induced by the perturbations of the coorbital body, influences the rotation of these satellites. To this aim, we introduce explicitly the perturbation in the rotational equations by using the formalism developed by Erdi (1977) to represent the coorbital motions, and so we describe the rotational motion of the coorbitals, Janus and Epimetheus included, in compact form

SATURN'S INNER SATELLITES: ORBITS, MASSES, AND THE CHAOTIC MOTION OF ATLAS FROM NEW CASSINI IMAGING OBSERVATIONS

We present numerically-derived orbits and mass estimates for the inner Saturnian satellites, Atlas, Prometheus, Pandora, Janus and Epimetheus from a fit to 2580 new Cassini ISS astrometric observations spanning February 2004 to August 2013. The observations are provided in a supplementary table. We estimate GM_ Atlas=0.384+/-0.001 x 10^(-3)km^3s^(-2), a value 13% smaller than the previously published estimate but with an order of magnitude reduction in the uncertainty. We also find GM_ Prometheus=10.677+/-0.006x10(-3)km^3s^(-2), GM_Pandora=9.133+/-0.009x10^(-3)km^3s^(-2), GM_Janus=126.51+/-0.03x10^(-3)km^3s^(-2) and GM_Epimetheus=35.110+/-0.009x10^(-3)km^3s^(-2), consistent with previously published values, but also with significant reductions in uncertainties. We show that Atlas is currently librating in both the 54:53 co-rotation-eccentricity resonance (CER) and the 54:53 inner Lindblad (ILR) resonance with Prometheus, making it the latest example of a coupled CER-ILR system, in common with the Saturnian satellites Anthe, Aegaeon and Methone, and possibly Neptune's ring arcs. We further demonstrate that Atlas's orbit is chaotic, with a Lyapunov time of ~10 years, and show that its chaotic behaviour is a direct consequence of the coupled resonant interaction with Prometheus, rather than being an indirect effect of the known chaotic interaction between Prometheus and Pandora. We provide an updated analysis of the second-order resonant perturbations involving Prometheus, Pandora and Epimetheus based on the new observations, showing that these resonant arguments are librating only when Epimetheus is the innermost of the co-orbital pair, Janus and Epimetheus. We also find evidence that the known chaotic changes in the orbits of Prometheus and Pandora are not confined to times of apse anti-alignement.Comment: 23 pages, 16 figures. Accepted for publication in The Astronomical Journal 23 September 2014 (corrected Fig. 11

Analytical description of physical librations of Saturnian coorbital satellites Janus and Epimetheus

Janus and Epimetheus are famously known for their distinctive horseshoe-shaped orbits resulting from a 1:1 orbital resonance. Every four years these two satellites swap their orbits by a few tens of kilometers as a result of their close encounter. Recently Tiscareno et al. (2009) have proposed a model of rotation based on images from the Cassini orbiter. These authors inferred the amplitude of rotational librational motion in longitude at the orbital period by fitting a shape model to the recent Cassini ISS images. By a quasiperiodic approximation of the orbital motion, we describe how the orbital swap impacts the rotation of the satellites. To that purpose, we have developed a formalism based on quasi-periodic series with long and short-period librations. In this framework, the amplitude of the libration at the orbital period is found proportional to a term accounting for the orbital swap. We checked the analytical quasi-periodic development by performing a numerical simulation and find both results in good agreement. To complete this study, the results regarding the short-period librations are studied with the help of an adiabatic-like approach

The reality of real time

According to Bernard Stiegler the technological contrivances that rule our world have set in motion dangerous developments. Although Stiegler is not a technophobe, he believes that the technological devices that are constructed around real time (live broadcasting, mobile phone communications, digital photography, etc.) introduce a new relation to time that jeopardises the cohesion of society. They erase the delay of time that is essential to it and thereby wipe away the singular, which is a crucial element in the construction of the social. This essay examines the nature of this argument and queries its factual basis. It does this by first exploring the technological or prosthetic nature of Dasein by referring to Heidegger’s definition of Dasein as ec-static time. After this short exposition, the essay shows how Stiegler strengthens this pre-prosthetic nature of time that can be found in Heidegger into a time which is fully fledged prosthetic or fundamentally constituted by the technological devices that exteriorise it. The second part of the essay focuses on Stiegler’s hesitations and even contradictory statements regarding the contemporary production of time. Sometimes he presents real time as a factual accomplishment, sometimes he is more careful and characterises it to be merely a tendency; there are passages in which he proclaims the end of history, and other ones in which he presents that end as a fiction and a warning. The comments on the rather dramatic and evocative pages Stiegler inserts in La technique et le temps 1, together with ideas and comments of Maurice Blanchot and Richard Beardsworth, serve as a bridge to discuss the philosophical importance of an ambiguity in the actuality of real time

Cinema as mnemotechnics

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Technics, individuation and tertiary memory: Bernard Stiegler's challenge to media theory

Media studies as a field has traditionally been wary of the question of technology. Discussion of technology has often been restricted to relatively sterile debates about technological determinism. In recent times there has been renewed interest, however, in the technological dimension of media. In part this is doubtless due to rapid changes in media technology, such as the rise of the internet and the digital convergence of media technologies. But there are also an increasing number of writers who seem to believe that media theory, and more widely social science and the humanities, needs to rethink the question of technology and its relationship to society, culture and cultural production. Andrew Feenberg has characterized the two most dominant positions as, on the one hand, the social constructivist or 'technology studies' approach to technology, and, on the other, 'substantivist' theories of technology. The social constructivist approach, aims to counter technological determinism by showing how the development of technology is shaped not by technical and scientific progress but by contingent social, cultural and economic forces. The limitation of this view, however, is that it tends to see technology as no different from any other social process and it may lose the ability to distinguish between the technical object and any other social formation or cultural artefact. Doubtless constructivism would like to see technology as a subset of the cultural artefact and not vice versa, therefore explaining technology in terms of culture and society. But there are powerful arguments for arguing something like the opposite: in other words for understanding culture and society in terms of or as technical objects. In recent years this argument has been put most forcefully by the French philosopher Bernard Stiegler and it is to his work and its implications for media theory that this essay is dedicated. In particular I will show how Stiegler might defend his ideas around technics from the charge of technological determinism. Firstly, technics is not understood in the narrow sense of techno-scientific technology but in the wider sense of all the ways in which the human is exteriorised into artefacts or organized inorganic matter. Technics in this sense is therefore inseparable from culture and society and it makes no sense either to talk of technics determining culture and society or vice versa. Culture and society are not constituted by technics as if by a cause but rather constituted through it. Secondly, a technics in Stiegler's sense does not represent scientific progress or a deterministic evolution; rather, however strange this may seem, technics is a kind of pure accidentality or contingency. For Stiegler it is because of the exteriorisation of the human into artefacts or inorganic organized matter that culture and society constitute themselves contingently. Stiegler's work therefore offers a way for us to rethink the relationship between culture and technology in ways which can productively reconfigure the concerns of critical theory