We apply the Mean Exponential Growth Factor of Nearby Orbits (MEGNO) technique to the dynamics of Jovian irregular satellites. The MEGNO indicator is a practical numerical tool to distinguish between quasi‐periodic and chaotic structures in phase space of a given dynamical system. The MEGNO indicator is used to generate a mapping of relevant phase‐space regions occupied by observed Jovian irregular satellites. The construction of MEGNO maps of the Jovian phase‐space region within its Hill‐sphere is addressed and the obtained results are compared with previous studies regarding the dynamical stability of irregular satellites. Since this is the first time the MEGNO technique is applied to study the dynamics of irregular satellites, we provide a review of the MEGNO theory and illustrate basic properties. We consider the elliptic restricted three‐body problem in which Jupiter is orbited by a massless test satellite subject to solar gravitational perturbations. The equations of motion of the system are integrated numerically and the MEGNO indicator computed from the system's variational equations. A large set of initial conditions is studied to generate the MEGNO maps. The chaotic nature of initial conditions is demonstrated by studying a quasi‐periodic orbit and a chaotic orbit. As a result, we establish the existence of several high‐order mean‐motion resonances (MMR) detected for retrograde orbits along with other interesting dynamical features related to various dynamical resonances. The computed MEGNO maps allow us to differentiate qualitatively between chaotic and quasi‐periodic regions of the irregular satellite phase space within a relatively short integration time of 60 000 yr for each orbit. By comparing with previous published results, we can establish a correlation between chaotic regions and corresponding regions of orbital instability. Based on our results, we hypothesize on the possibility of gravitational scattering from high‐order MMR as a dynamical cause to explain the observed orbital velocity dispersion for members of the Pasiphae family.

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We test the hypothesis that an annually repeatable Ca emission excess in Mercury's exosphere at a True Anomaly Angle (TAA) of 25 ∘ ± 5 ∘ is due to particles from comet 2P/Encke impacting the surface. By simulating the dynamical evolution of Encke particles under planetary perturbations and Poynting-Robertson drag, we find that millimeter-sized grains ejected 1-2 ×10 4 years ago encounter Mercury at TAA = 350 ∘-30 ∘. The timing of the excess emission is consistent with a major dust release episode ≲20 kyr ago, possibly due to Encke progenitor breakup. The emission mechanism is likely the direct injection of impact-liberated Ca into sunlight rather than nightside surface adsorption for subsequent release at dawn. The timing of dust release from the comet depends on this mechanism; a 10 kyr age is implied by the direct-injection scenario. This comet may be the remnant of a large (∼40 km) cometary nucleus that disintegrated 20-30 kyr ago [

A multi-colour phase-polarization curve of asteroid (3200) Phaethon has been obtained during the December 2017 apparition by merging measurements taken at the observing station of Calern (France) and at the Rhozen observatory (Bulgaria). All the observations were obtained in the positive polarization branch, the phase angle ranging from 36 • to 116 • . The measured values of linear polarization are among the highest ever observed for a Solar system body. The covered interval of phase angle was not sufficiently extended to derive a firm determination of the P max parameter, but this appears to occur at a phase angle around 130 • and reaches more than 45% of linear polarization. Phaethon is the parent body of the Geminid meteor shower, and the real physical nature of this object (asteroid or comet) has been a long-debated subject. Our polarimetric measurements seem to support the asteroid hypothesis with a phasepolarization curve similar to the asteroid (2) Pallas, but further observations at smaller phase angles are needed to draw definitive conclusions.

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The "Ice Giants" Uranus and Neptune are a different class of planet compared to Jupiter and Saturn. Studying these objects is important for furthering our understanding of the formation and evolution of the planets, and unravelling the fundamental physical and chemical processes in the Solar System. The importance of filling these gaps in our knowledge of the Solar System is particularly acute when trying to apply our understanding to the numerous planetary systems that have been discovered around other stars. The Uranus Pathfinder (UP) mission thus represents the quintessential aspects of the objectives of the European planetary community as expressed in ESA's Cosmic Vision 2015-2025. UP was proposed to the European Space Agency's M3 call for medium-class missions in 2010 and proposed to be the first orbiter of an Ice Giant planet. As the most accessible Ice Giant within the M-class mission envelope Uranus was identified as the mission target. Although not selected for this call the UP mission concept provides a baseline framework for the exploration of Uranus with existing low-cost platforms and underlines

We present a dynamical investigation of a newly found asteroid, 2010 SO16, and the discovery that it is a horseshoe companion of the Earth. The object's absolute magnitude (H=20.7) makes this the largest object of its type known to-date. By carrying out numerical integrations of dynamical clones, we find that (a) its status as a horseshoe is secure given the current accuracy of its ephemeris, and (b) the time spent in horseshoe libration with the Earth is several times 10^5 yr, two orders of magnitude longer than determined for other horseshoe asteroids of the Earth. Further, using a model based on Hill's approximation to the three-body problem, we show that, apart from the low eccentricity which prevents close encounters with other planets or the Earth itself, its stability can be attributed to the value of its Jacobi constant far from the regime that allows transitions into other coorbital modes or escape from the resonance altogether. We provide evidence that the eventual escape of the asteroid from horseshoe libration is caused by the action of planetary secular perturbations and the stochastic evolution of the eccentricity. The questions of its origin and the existence of as-yet-undiscovered co-orbital companions of the Earth are discussed.Comment: Accepted in MNRAS; 6 pages, 3 figures, 2 table