The determination of the rotational state of celestial bodies

Determination of the rotation of a celestial body using the imaging from orbit; here the applied rotational models and the developed technique are presented. The applications for the determination of the rotational state of Titan using data from Cassini and the simulation of the rotation experiment for the BepiColombo Mission to Mercury are also reported

The determination of the rotational state of celestial bodies

Determination of the rotation of a celestial body using the imaging from orbit; here the applied rotational models and the developed technique are presented. The applications for the determination of the rotational state of Titan using data from Cassini and the simulation of the rotation experiment for the BepiColombo Mission to Mercury are also reported

Determination of the planetary rotation by imaging from orbit

The knowledge of the rotational state of planetary bodies provides crucial information on their interior structure. Evolution models of the orbital dynamics use the obliquity as a constraint, together with the eccentricity. When the quadrupole gravity field is known, the obliquity may provide also the moment of inertia, one of the most important quantities to constrain the body's density profile. The spin rate and the physical librations provides a strong indication on the internal differentiation of a body, as well as information on the possible orbital resonances. Here we present a technique for the estimation of the rotational state of a body from orbit, with applications to Titan and Mercury. For Titan we have used existing SAR images from the Cassini mission, while for Mercury we relied on simulations of the optical observations from ESA's BepiColombo high resolution camera. Georeferenced images of the same area, taken at different times, are compared by pattern matching algorithms in order to determine the registration error. Different pattern matching procedures can be applied, as such as cross-correlation, mutual information technique, and SIFT/SURF algorithms. The mismatching is mainly due to errors in the rotational model, with smaller contributions from the spacecraft ephemerides and attitude, camera or radar calibration, and image processing. The image correlation is followed by a weighted least-squares fit to update the rotational model and minimize the mismatch between the features. The apparent misregistration of tiepoints is used to estimate the rotational parameters, such as the spin pole location, the spin rate and precession and nutation coefficients. We report on the results and the methods obtained for Cassini and BepiColombo, providing new estimates of the obliquity and spin rate of Titan and expected accuracies the obliquity and physical libration amplitude of Mercury. We report on the applied methods, the error budgets relative to each experiment and the obtained results. Copyright © (2012) by the International Astronautical Federation

Observations planning optimization for BepiColombo's Mercury rotation experiment

The achievement of the desired accuracies in estimating Mercury's rotational parameters in the frame of the radio science experiment hosted on board ESA's BepiColombo mission strongly depends upon a scrupulous analysis of the most suitable observations. In this case, observables are constituted by images captured by the High Resolution Imaging Channel (HRIC) depicting the same portion of the surface at two different epochs which, opportunely georeferenced and transposed in an inertial reference frame, provide information on the displacement of the features identified in the images and thus on the rotation of the planet. The critical part in the accomplishment of the experiment is represented by the fact that observations will be limited by mission constraints and therefore selection criteria need to be applied to filter the database and choose the ones characterized by the highest information content. Hence, screening parameters could be illumination conditions or altitude variations between images to be compared in order to favor their matching, rather than a temporal gap allowing to observe a consistent change in libration. All these aspects are encompassed in a global simulator of the experiment, deemed to be the most effective way to analyze the problem. The final aim of the software is to converge toward a solution ensuring the maximum scientific output with the minimum number of observations. In order to do so, the simulator first generates a database of the spacecraft ground tracks, selects the dataset of observations and implements an optimization algorithm. In parallel to the analyses performed using the optimizator, other independent simulations have been run so as to investigate on the minimum number of observations needed to obtain the desired accuracies. Hence, while the last ones provides an hint on the quantity of measurements needed, the end-to-end simulator is aimed at individuating the most favorable strategy of observation, in terms of epochs and surface features. © 2013 2013 California Institute of Technology

The rotational dynamics of titan from Cassini radar images

Between 2004 and 2009 the RADAR instrument of the Cassini mission provided 31 SAR images of Titan. We tracked the position of 160 surface landmarks as a function of time in order to monitor the rotational dynamics of Titan. We generated and processed RADAR observables using a least squares fit to determine the updated values of the rotational parameters. We provide a new rotational model of Titan, which includes updated values for spin pole location, spin rate, precession and nutation terms. The estimated pole location is compatible with the occupancy of a Cassini state 1. We found a synchronous value of the spin rate (22.57693 deg/day), compatible at a 3- σlevel with IAU predictions. The estimated obliquity is equal to 0.31 °, incompatible with the assumption of a rigid body with fully-damped pole and a moment of inertia factor of 0.34, as determined by gravity measurements

Shape, topography, gravity anomalies and tidal deformation of Titan

International audienc

The exploration of Titan with an orbiter and a lake probe

International audienceFundamental questions involving the origin, evolution, and history of both Titan and the broader Saturnian system can be answered by exploring this satellite from an orbiter and also in situ. We present the science case for an exploration of Titan and one of its lakes from a dedicated orbiter and a lake probe. Observations from an orbit-platform can improve our understanding of Titan׳s geological processes, surface composition and atmospheric properties. Further, combined measurements of the gravity field, rotational dynamics and electromagnetic field can expand our understanding of the interior and evolution of Titan. An in situ exploration of Titan׳s lakes provides an unprecedented opportunity to understand the hydrocarbon cycle, investigate a natural laboratory for prebiotic chemistry and habitability potential, and study meteorological and marine processes in an exotic environment. We briefly discuss possible mission scenarios for a future exploration of Titan with an orbiter and a lake probe. •We present the science case for Titan׳s exploration.•We present mission scenarios for a future exploration of Titan.•This paper is based on a white paper presented for 2013 ESA׳s Call for Large missions