Theory of the rotation of Janus and Epimetheus

The Saturnian coorbital satellites Janus and Epimetheus present a unique dynamical configuration in the Solar System, because of high-amplitude horseshoe orbits, due to a mass ratio of order unity. As a consequence, they swap their orbits every 4 years, while their orbital periods is about 0.695 days. Recently, Tiscareno et al.(2009) got observational informations on the shapes and the rotational states of these satellites. In particular, they detected an offset in the expected equilibrium position of Janus, and a large libration of Epimetheus. We here propose to give a 3-dimensional theory of the rotation of these satellites in using these observed data, and to compare it to the observed rotations. We consider the two satellites as triaxial rigid bodies, and we perform numerical integrations of the system in assuming the free librations as damped. The periods of the three free librations we get, associated with the 3 dimensions, are respectively 1.267, 2.179 and 2.098 days for Janus, and 0.747, 1.804 and 5.542 days for Epimetheus. The proximity of 0.747 days to the orbital period causes a high sensitivity of the librations of Epimetheus to the moments of inertia. Our theory explains the amplitude of the librations of Janus and the error bars of the librations of Epimetheus, but not an observed offset in the orientation of Janus.Comment: Accepted for publication in Icaru

Corrigendum to “Interpreting the librations of a synchronous satellite – How their phase assesses Mimas’ global ocean” [Icarus 282 (2017) 276–289]

A mistake appeared in the original paper, which propagated. This affects the phase of the diurnal libration. The conclusions are unchanged

Interior properties of the inner saturnian moons from space astrometry data

International audienc

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

The Phemu03 campaign of observation of the mutual events of the Galilean satellites of Jupiter in 2003 at the Bordeaux Observatory

This workshop is mainly dedicated to astrometry and photometry of Solar System objects events and to the preparation of future campaigns of observation. Several astronomical studies require a large network of observers in order to acquire enough data of rare phenomena or to observe objects on alert or for a follow-up

Observation of 13 mutual events of Jovian satellites performed at Lille Observatory

We have observed the four Galilean satellites of Jupiter during their mutual occultations and eclipses from February to April 2003 using a CCD camera attached to the 32.5 cm refractor of the observatory of Lille. We have recorded 13 lightcurves of these events. We have performed a first astrometric reduction based on the method developed in Noyelles et al. (2003). This analysis of the results and comparison with theory show that the observations are good; the residuals are about 0.03 arcsec. The observations are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5

Atalante research facility implementation of a rule of fractions for the management of reflecting materials in mass-limited units

International audienceATALANTE,located in Marcoule,is one of the main Nuclear Facilities of the French CEA.In terms of criticality risk prevention, the facility is divided into work units mostly managed through a mass control mode.The limit authorized is 350 g of fissile materialwith 239Pu-H2O as reference fissile medium. This mass limit was determined considering a reflectionby 20 cm of water.Under these conditions, some neutronic reflectors, which are more efficient than watercan be authorized only in limited quantities. In ATALANTEthe reflecting materials identified as requiring a specific management are lead, uranium (235U/Utotal = 1%), graphite, heavy water and beryllium.Initiallya maximum permissible mass was determined for each of these materials taken separately. However, this method requires that when different reflectors are present simultaneouslythe sum of the masses of all these reflectors must be less than the limit specified for the most penalizing of them.This rule has proved to impose too many re-strictions on the operator.A new rule has therefore been implemented:the rule of fractions.To demonstrate that this rule is acceptable, criticality calculations have been performed. The geometric configurations studied are a sphere of 350 g of 239Pu moderated by water and re-flected by various masses of 2 to 6 reflectors (successions of concentric shells) followed by 20 cm of water.It has been concluded that compliance with the new rule makes it possible to ensurecriticality safety in the case of the simultaneous presence of different reflectors