R. A. Jacobson

THE GRAVITY FIELD OF THE SATURNIAN SYSTEM FROM SATELLITE OBSERVATIONS AND SPACECRAFT TRACKING DATA

We present values for the masses of Saturn and its major satellites, the zonal harmonics in the spherical harmonic expansion of Saturns gravitational potential, and the orientation of the pole of Saturn. We determined these values using an extensive data set: satellite astrometry from Earth-based observatories and the Hubble Space Telescope; Earth-based, Voyager 1, and Voyager 2 ring occultation measurements; Doppler tracking data from Pioneer 11; and Doppler tracking, radiometric range, and imaging data from Voyager 1, Voyager 2, and Cassini.

The Gravity Field and Interior Structure of Enceladus

Inside Enceladus Saturns moon Enceladus has often been the focus of flybys of the Cassini spacecraft. Although small—Enceladus is roughly 10 times smaller than Saturns largest moon, Titan—Enceladus has shown hints of having a complex internal structure rich in liquid water. Iess et al. (p. 78) used long-range data collected by the Cassini spacecraft to construct a gravity model of Enceladus. The resulting gravity field indicates the presence of a large mass anomaly at its south pole. Calculations of the moment of inertia and hydrostatic equilibrium from the gravity data suggest the presence of a large, regional subsurface ocean 30 to 40 km deep. The saturnian moon is differentiated and likely hosts a regional subsurface sea at its southern pole. The small and active Saturnian moon Enceladus is one of the primary targets of the Cassini mission. We determined the quadrupole gravity field of Enceladus and its hemispherical asymmetry using Doppler data from three spacecraft flybys. Our results indicate the presence of a negative mass anomaly in the south-polar region, largely compensated by a positive subsurface anomaly compatible with the presence of a regional subsurface sea at depths of 30 to 40 kilometers and extending up to south latitudes of about 50°. The estimated values for the largest quadrupole harmonic coefficients (106J2 = 5435.2 ± 34.9, 106C22 = 1549.8 ± 15.6, 1σ) and their ratio (J2/C22 = 3.51 ± 0.05) indicate that the body deviates mildly from hydrostatic equilibrium. The moment of inertia is around 0.335MR2, where M is the mass and R is the radius, suggesting a differentiated body with a low-density core.

The Orbits and Masses of the Martian Satellites and the Libration of Phobos

This paper reports on an update to the orbits and masses of the Martian satellites Phobos and Deimos. We obtained the orbits by fitting a numerical integration to all available Earth-based astrometry through the opposition of 2003, spacecraft imaging observations through 2007, and the Doppler tracking of the Viking and Phobos 2 spacecraft; the Doppler data provide information on the satellite masses. Our dynamical model included the figure acceleration due to a librating Phobos; we determined the amplitude of the forced libration. We also took into account the secular acceleration of Phobos due to the tide that it raises on Mars and estimated the Martian tidal quality factor Q. We provide an assessment of the accuracy of the orbits and a geometrical description of the orbits in the form of mean elements.

THE ORBITS OF SATURN'S SMALL SATELLITES DERIVED FROM COMBINED HISTORIC AND CASSINI IMAGING OBSERVATIONS

We report on the orbits of the small, inner Saturnian satellites, either recovered or newly discovered in recent Cassini imaging observations. The orbits presented here reflect improvements over our previously published values in that the time base of Cassini observations has been extended, and numerical orbital integrations have been performed in those cases in which simple precessing elliptical, inclined orbit solutions were found to be inadequate. Using combined Cassini and Voyager observations, we obtain an eccentricity for Pan 7 times smaller than previously reported because of the predominance of higher quality Cassini data in the fit. The orbit of the small satellite (S/2005 S1 [Daphnis]) discovered by Cassini in the Keeler gap in the outer A ring appears to be circular and coplanar; no external perturbations are apparent. Refined orbits of Atlas, Prometheus, Pandora, Janus, and Epimetheus are based on Cassini , Voyager, Hubble Space Telescope, and Earth-based data and a numerical integration perturbed by all the massive satellites and each other. Atlas is significantly perturbed by Prometheus, and to a lesser extent by Pandora, through high-wavenumber mean-motion resonances. Orbital integrations involving Atlas yield a mass of GMAtlas = (0.44 ± 0.04) × 10-3 km3 s -2, 3 times larger than reported previously (GM is the product of the Newtonian constant of gravitation G and the satellite mass M). Orbital integrations show that Methone is perturbed by Mimas, Pallene is perturbed by Enceladus, and Polydeuces librates around Diones L5 point with a period of about 791 days. We report on the nature and orbits of bodies sighted in the F ring, two of which may have persisted for a year or more.

The Orbits of the Neptunian Satellites and the Orientation of the Pole of Neptune

This paper reports on an update to the orientation of Neptunes pole and to the orbits of the Neptunian satellites, Triton, Nereid, and Proteus. We determined the new pole and orbits in the International Celestial Reference Frame by fitting them to all available observations through the opposition of 2008. The new data in the fit are high-quality modern astrometry and constitute a 19 year extension of the previous data arc. We assess the accuracy of the orbits and compare them with our earlier orbits. We also provide mean elements as a geometrical description for the orbits.

Revised Orbits of Saturn's Small Inner Satellites

We have updated the orbits of the small inner Saturnian satellites using additional Cassini imaging observations through 2007 March. Statistically significant changes from previously published values appear in the eccentricities and inclinations of Pan and Daphnis, but only small changes have been found in the estimated orbits of the other satellites. We have also improved our knowledge of the masses of Janus and Epimetheus as a result of their close encounter observed in early 2006.

The Orbits of the Inner Uranian Satellites from Hubble Space Telescope and Voyager 2 Observations

This article presents revised orbital elements for the 10 small Uranian satellites discovered by the Voyager 2 spacecraft. The elements have been determined from a fit to astrometric observations made with the Hubble Space Telescope and imaging data acquired by Voyager 2. An assessment of the accuracy of the orbits represented by the elements is provided, as are comparisons with orbits found by previous investigators.

THE ORBITS OF THE INNER NEPTUNIAN SATELLITES FROM VOYAGER, EARTH-BASED, AND HUBBLE SPACE TELESCOPE OBSERVATIONS

We present revised orbital elements for the six small Neptunian satellites discovered by Voyager 2. The elements have been determined from a fit to imaging data acquired by the Voyager 2 spacecraft and astrometric observations made at Earth-based observatories and by the Hubble Space Telescope. An assessment of the accuracy of the elements is provided, as are comparisons with orbits found by previous investigators.

The Orbits of the Uranian Satellites and Rings, the Gravity Field of the Uranian System, and the Orientation of the Pole of Uranus

French et al. determined the orbits of the Uranian rings, the orientation of the pole of Uranus, and the gravity harmonics of Uranus from Earth-based and Voyager ring occultations. Jacobson et al. determined the orbits of the Uranian satellites and the masses of Uranus and its satellites from Earth-based astrometry and observations acquired with the Voyager 2 spacecraft; they used the gravity harmonics and pole from French et al. Jacobson & Rush reconstructed the Voyager 2 trajectory and redetermined the Uranian system gravity parameters, satellite orbits, and ring orbits in a combined analysis of the data used previously augmented with additional Earth-based astrometry. Here we report on an extension of that work that incorporates additional astrometry and ring occultations together with improved data processing techniques.

THE GM VALUES OF MIMAS AND TETHYS AND THE LIBRATION OF METHONE

We have determined the GM of the Saturnian satellite Mimas from an analysis of its resonance with Tethys and with the newly discovered satellite Methone (GM is the product of the Newtonian constant of gravitation G and the satellites mass M). Observations of the latter permit a factor of 5 improvement in our knowledge of Mimass GM; its value is 2.504 ± 0.002 km3 s-2. Tethyss GM was originally found from the Mimas-Tethys resonance. We, however, estimate it using observations of its two Lagrangian satellites, Calypso and Telesto; its value is 41.200 ± 0.007 km3 s-2.