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Ephemerides of planetary satellites require regular updates to take into account new observations of the satellites. Such revision has been all the more necessary in the case of outer planetary satellites, since a number of new moons have been discovered recently. Thus, we present updated versions of the ephemerides of the outer planetary satellites. The problem and the methodology for estimating ephemeris accuracy are discussed. Comparison with the Jet Propulsion Laboratory (JPL) ephemerides proves that the accuracy depends largely on the distribution of the observations. We give examples where, for a few satellites, the O−C residuals increase sharply at time intervals lying significantly beyond the time interval of observations used to generate the ephemerides. This fact alone indicates that there is an urgent need for new observations. Besides the ephemerides of moons, which can be accessed online via the MULTI-SAT server, we provide orbital parameters for the recently discovered faint satellites of Jupiter and Saturn. The problems discussed in this work are important for planning space observations of the outer satellites by future space missions like the European Space Agency (ESA) JUpiter ICy moons Explorer (JUICE) and National Aeronautics and Space Administration (NASA) Europa Clipper missions. [ABSTRACT FROM AUTHOR]

In the caviar software package, a standard tool for astrometry of images from the Cassini imaging science subsystem (ISS), Gaussian fitting is used to measure the centre of point-like objects, achieving a typical precision of about 0.2 pixels. In this work, we consider how alternative methods may improve on this. We compare three traditional centroiding methods: two-dimensional Gaussian fitting, median, and modified moment. Results using 56 selected images show that the centroiding precision of the modified moment method is significantly better than the other two methods, with standard deviations for all residuals in sample and line of 0.065 and 0.063 pixels, respectively, representing a factor of over 2 improvement compared to Gaussian fitting. Secondly, a comparison of observations using Cassini ISS images of Anthe is performed. Anthe results show a similar improvement. The modified moment method is then used to reduce all ISS images of Anthe during the period 2008–2017. The observed-minus-calculated residuals relative to the JPL SAT393 ephemeris are calculated. In terms of α × cos(δ) and δ in the Cassini-centred international celestial reference frame, mean values of all residuals are close to 0 km, and their standard deviations are less than 1 km for narrow angle camera images, and about 4 km for wide angle camera images. [ABSTRACT FROM AUTHOR]

This study is a companion paper to Baillié et al. in which we showed that a past episode of inward migration of Mimas could have created the Cassini Division. We here investigate the possible causes of this inward migration. We suggest two scenarios: one based on a past intense heating of Mimas, and another one on a past intense heating of Enceladus, which would have itself driven an inward migration of Mimas due to a mean-motion resonance. These two scenarios are challenged with numerical modelling of the orbital motion of the satellites, and energy budget, which are confronted to our present knowledge of the interior of Mimas and Enceladus. We show that a past hot Mimas requires an eccentricity of 0.22, while a past hot Enceladus would have needed an eccentricity of 0.25. While the scenario of a past hot Mimas preserves the stability of the mid-sized satellites of Saturn, it threatens Janus and Epimetheus and is inconsistent with the observations of impact basins at the surface of Mimas. However, a past hot Enceladus which would have almost fully melted could be consistent with its differentiated interior, but would probably not have preserved the stability of Tethys, given the high eccentricity required. Both of these scenarios would have challenged the stability of Aegaeon, Methone, Pallene, and Anthe, and implied that the Cassini Division would be younger than 10 Myr. [ABSTRACT FROM AUTHOR]

The technique of mutual approximations accurately gives the central instant of the maximum apparent approximation of two moving natural satellites in the plane of the sky. This can be used in ephemeris fitting to infer the relative positions of satellites with high precision. Only mutual phenomena – occultations and eclipses – can achieve better results. However, mutual phenomena only occur every six years in the case of Jupiter. Mutual approximations do not have this restriction and can be observed at any time in the year as long as the satellites are visible. In this work, we present 104 central instants determined from the observations of 66 mutual approximations between the Galilean moons carried out at different sites in Brazil and France during the period 2016–2018. For 28 events, we have at least two independent observations. All telescopes were equipped with a narrow-band filter centred at 889 nm with a width of 15 nm to eliminate the scattered light from Jupiter. The telescope apertures ranged between 25 and 120 cm. For comparison, the precision of the positions obtained with classical CCD astrometry is about 100 mas, for mutual phenomena it can be 10 mas or less, and the average internal precision obtained with mutual approximations is 11.3 mas. This new type of simple, yet accurate, observations can significantly improve the orbits and ephemeris of Galilean satellites and thus it can be very useful for the planning of future space missions to the Jovian system. [ABSTRACT FROM AUTHOR]

We provide new astrometric observations of Enceladus from images taken using the Cassini Imaging Science Subsystem (ISS) Narrow-Angle Camera. Astrometric reduction was performed using a new fully automatic camera repointing technique built within the Caviar software package. The new automatic procedure is based on the k-d tree technique and has been developed as an alternative to the manual technique currently available in Caviar. We describe the new procedure and demonstrate its effectiveness compared with manual processing. Repointing corrections were computed using reference stars from the UCAC4 and Gaia DR1 star catalogues. We compare the observed-minus-computed (O−C) residuals of Enceladus based on UCAC4 with those based on Gaia DR1, relative to the JPL SAT375 ephemeris and the IMCCE NOE ephemeris, respectively. We reduced all available Cassini ISS images of Enceladus during the period 2013–2017, producing a total of 1109 Cassini -centred astrometric observations. These are provided in right ascension (α) and declination (δ) in the International Celestial Reference Frame (ICRF). Estimated mean O−C residuals relative to JPL's SAT375 ephemeris were 0.7 km in α  × cos(δ) and 1.8 km in δ, with standard deviations of 2.4 and 3.2 km, respectively. No improvement in the precision of the observations was obtained when using the GAIA DR1 star catalogue compared with UCAC4. [ABSTRACT FROM AUTHOR]

ABSTRACT In this paper, we present 526 precise CCD positions of Galilean satellite-pairs of Jupiter, which have been extracted from 441 CCD frames captured by a 1-m telescope at the Yunnan Observatory from 2002 to 2010. The four Galilean satellites (Io, Europa, Ganymede and Callisto) are used to calibrate the CCD field of view by comparing their pixel positions with their theoretical positions computed from two modern ephemerides of the Galilean satellites, L2 and JUP230, which have been developed by the Institut de Méchanique Céleste et de Calcul des Éphémérides and the Jet Propulsion Laboratory, respectively. In this paper, we focus on the relative position of a pair of satellites with short separation (less than 85 arcsec) for good internal precision. The mean (O − C) (observed minus computed) values of all these satellite-pairs in right ascension and declination are found to be no larger than 6 mas and 2 mas, respectively, for each ephemeris. The estimated precision for one single observation is better than 30 mas in each direction. [ABSTRACT FROM AUTHOR]

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