REPORT OF THE WORKING GROUP ON SATELLITES - Period July 1990- June 1993 (J.E. Arlot) 1.Observations for astrometric puposes - Photometry At USNO, photographic observations of the Galilean satellites of Jupiter, satellites I-VIII of Saturn and the Martian satellites were continued by D. Pascu with the 26-inch refractor at Washington. This brings to 20 the number of apparitions obsserved for ghe Galilean moons, 19 for Saturn's system and 12 for the moons of Mars. Positions for Saturn's moons are available only from 1974 to 1980. Positions for the Galilean moons are available for 1967/68 with the McCormick refractor and 1973 - 1993 with the USNO 26-inch. Positions for the Martian moons are available for 1967, some for 1969, 1971 and all from 1982 - 1988. Some of the Martian satellite observations were made with the 61-inch Astrometric Reflector. The Galilean moon observations are sent to JPL in support of the GALILEO project, and to the Bureau des Longitudes in Paris in support of ephemeris development.In addition to the photographic program for the brighter moons, CCD observations are made of seven faint inner moons: JV, JXIV, SXII, SXIII, SXIV, UV and NII (Rohde et al. BAAS 25, 1993 ; Pascu et al. BAAS 24, 1059, 1992). These observations have been continued for 12 years with the 61-inch Astrometric reflector at Flagstaff by Pascu, P.K. Seidelman and J.R. Rohde. These observations provide both astrometric and photometric data (Pascu et al. BAAS 22, 1117, 1990 ; Pascu et al. Icarus 98, 38, 1992). Astrometric positions of satellites VI and VII of Jupiter were obtained on plates taken at Table Mountain Observatory in 1968 and are published by J.D. Mulholland (PASP, 102, 1328, 1990). Shelus et al. obtained positions of JVI to JXIII at McDonald Observatory (53.099.006) as well as Whipple et al. (55.099.015,105). At JPL, the activities of R.A. Jacobson was the post-encounter analysis of observations acquired from the Voyager missions to Uranus and Neptune. The results of the analyses were revised sets of dynamical constants, e.g., planet and satellites masses, for both the Uranian and Neptunian systems and numerically integrated ephemerides for the satellites. In addition, he converted the Voyager satellites observations to an equivalent astrometric form for general use and published them (Jacobson et al., 54.101.007 ; Jacobson, 54.101.003 ; Jacobson, A&ASS, 96, 549,1992). From Voyager images, Showalter detected 1981S13 (SXVIII) and studied its role in Enke gap (53.100.015). T. Nakamura reports the observations of close approaches between Galilean Satellites (Nakamura et al., Proceeding of IAU Colloquium 127, "Reference Systems" p319, 1991) and the publication of astrometric observations of the center satellites of Jupiter during 1986-1990 (Nakamura et al., 53.99.001). D. B. Taylor from RGO reports that CCD observations of the major satellites of Saturn, Uranus and Neptune were made using the 1m Jacobus Kapteyn Telescope on La Palma, Canary Islands during the 1990 and 1991 oppositions. Williams et al (MNRAS, 250,1, 1991) used the 1990 CCD data for Nereid together with comparisons stars to estimate the period of rotation of Nereid. Beurle et al (1993) have made a preliminary reduction of the Saturn satellite data for the 1991 opposition. They have shown these observations to be comparable in quality to the best photographic observations. R. Viera-Martins from Brazil reports that, using the Brazilian 1.6m reflector, 668 CCD images of Uranus system and 622 CCD images of Neptune system were made and are now in reduction. A new process for the position reduction of natural satellites was also developed at the Observatorio Nacional (Rio de Janeiro, Brazil) as a photometric reduction method for images near a bright planet. These methods were applied to 173 photographic plates of the Uranium system. The results are presented in the Ph. D. of C.H. Veiga. At Bordeaux Observatory (France) a CCD camera was built and used for the observation of the mutual events of the Galilean Satellites as well as for the astrometric measurements of positions of the satellites of Saturn. At Bureau des Longitudes (France), a campaign of CCD astrometric observations was continued and some data were published concerning the satellites of Mars (Colas and Arlot, 54.097.037 ; Colas, A&ASS 96, 485, 1992). and the satellites of Neptune (Colas and Buil, A&A 262, L13,1992). In the states of the former SSSR, observations were made: positions of the martian satellites were made photographically at Maidanak Observatory in 1988 by Bugaenko et al (52.097.126) and at Abastumani Observatory by Kalinichenko et al. (52.097.126). Positions of JVI were obtained by Izhakevich et al. at the Observatory of the Ukrainian Academy of Sciences (53.099.017, 54.099.039). Nicholson et al. (52.100.031) and Hamilton et al. (54.100.037) reported the observations of co-orbital satellites of Saturn (52.100.031). In 1991, an important campaign of observation of the mutual events of the Galilean satellites took place. Predictions were made by Aksnes and Franklin (Icarus 84, 542, 1990), by the British Astron. Ass. in London (52.099.042) and by Arlot (A&A 237, 259, 1990). Some observations were published (Souchay et al. A&A 264, 314, 1992 ; Le Campion et al. A&A 266, 568, 1992 ; Mallama Icarus 97, 298, 1992 and Icarus 95, 309, 1992 ; Oprescu et al. Rom. Astr. J. 2, 61, 1992 ; Allen et al. Earth Moon and Planets 58, 105, 1992 ; Froeschle et al. A&A 1992). Descamps et al. (Icarus 100, 235, 1992) put also into evidence the positions of hot spots on Io, the first satellites of Jupiter. At the same time, some results of the former campaign made in 1985 were also published 5Arlot et al. A&ASS 80, 1, 1990 ; Arlot et al. 55.099.027 ; Franklin et al. A.J. 102, 806, 1991 ; Vasundhara J. Astrophys. Astr. 12, 69, 1991). Blanco et al. (53.101.015) and Buie et al. (55.101.090) reported the obseervation of Pluto/Charon mutual events. 2 - Comparison of observations with theories - Determination of elements At JPL, using the recent Voyager data, the masses of Uranus and its satellites were determined (Jacobson et al., Meeting of the Am. Ast. Soc. Div. on Dynamical Astronomy, Key Biscayne, Fla, 1991; Jacobson et al., paper G 32A-3, Spring Meeting of the Am. Geo. Union, Montreal, Canada, 1992 ; Jacobson et al., 55.101.016). At RGO, in cooperation with Bordeau Observatory, Taylor et al. (A&A 249, 569, 1991) have analysed meridian circle observations of Titan and Iapetus made with the Carlsberg Automatic Meridian Circle on La Palma and the Meridian Circle at Bordeaux. From their analysis the corrections to the postion of Saturn from the DE200 ephemeris agree well with the latest ephemeris DE202. The only significant correction to the elements of the satellites is to the mean longitudes of Iapetus. Harper and Taylor (A&A 268, 326, 1993) have determinded new improved orbital elements for the major satellites of Saturn from an analysis of observations spanning the period 1874 to 1988. New masses for the satellites Mimas, Enceladus, Tethys and Dione were derived. At USNO, an analysis of photographic astrometric observations of the Galilean Satellites was presented (55.101.091). At Bordeaux Observatory (France) Dourneau (A&A 267, 292, 1993) presented a theoretical work and fit the orbital elements of the eight major satellites of Saturn on 100 years of observations. At Bureau des Longitudes, M. Chapront-Touze calculated the orbits of the martian satellites from her theories (A&A 240, 159, 1990). >From Voyager 2, Owen et al. (53.101.004) prsented the orbits of the 6 new satellites of Neptune. An analysis of the Pluto/Charon mutual events was made by Marcialis (52.101.043) and by Tholen and Buie (52.101.111). 3 - Theoretical studies Concerning the Galilean Satellites, the orbital resonance among JI,II,III was studied by Changyin and Lin (54.099.056). At Lille University (France), Duriez and Vienne have built a new theory of the eight major satellites of Saturn where all the dynamical parameters are taken into account at the same time (Duriez and Vienne, 53.100.006 ; Vienne and Duriez 53.100.014 ; Vienne and Duriez 55.100.009). More precisely, a study of the secular acceleration of Mimas was presented (Vienne et al, 55.042.043) and a synthetic theory of the motion for Titan-Hyperion was studied (Duriez, 55.042.041). Gerasimov and Mushailov studied the evolution of the orbit of Hyperion (53.100.004,020) and an analytical theory of the motion of Iapetus was presented by Gasanov (54.100.017). Sessin made theoretical studies of the Mimas-Tethys system (55.042.044) and the influence of Enceladus in the motion of Helene was studied by Bevilacqua and Sessin (55.042.045). M. Chapront-Touze completed a theory of the motion of Phobos and Deimos (A&A 235, 447, 1990), Ivanov et al. (52.097.031) published a new theory of the motion of Phobos and Vashkov'yak and Emel'yamov (52.097.027) improved Phobos'orbit. Wnuk and Breiter also published a study on the motion of satellites in Mars' gravity field (53.097.200). Krasil Nikov (52.042.027) studied the spatial rotations of a satellite about the mass center in the circular 3-body problem in the case of a main resonance. Secular perturbations of fictitious satellites of Uranus were studied by Kinoshita and Nakai (54.042.092) and a semi-analytical theory for the motion of Uranus' satellites was presented by Lazzaro (54.101.012). At OCA (France) Oberti published an analytic modelling for an accurate solution for Nereid's motion (52.101.013). 4 - Ephemerides - Numerical Integrations - Predictions of events At JPL, R.A. Jacobson began the preliminary development of numerically integrated Galilean and Saturnian satellite ephemerides in support of JPL's Galileo, and Cassini missions, respectively (Jacobson et al., Meeting of he Am; Ast. Soc, Div. on Dynamical Ast., Santa Barbara, Ca, 1993). Other activities included maintaining and updating satellite observations database and providing ephemerides for a number of JPL and non-JPL users. Among the former were the JPL Mars Observer project (Phobos and Deimos) and the Pluto Fast Fly-by pre-project (Charon). Among the latter were the Space Telescope Science Institute (all natural satellites). At RGO, using the numerical integration of Hyperion by Sinclair and Taylor (1985), Taylor (A&A 265, 825, 1992) has developed a synthetic theory for the motion of this satellite and fitted it to the modern photographic observations. The coefficients of the synthetic long-period theory were used by Message (1993) as starting conditions in his differential correction procedure (based on a new theory of the long- period motion) to determine a set of dynamically consistent coefficients. A study on the calculation of the ephemeris of the satellites of Saturn was presented by Rongchuan and Kaixian (54.100.035). Mutual events and eclipses by Saturn will imply the Saturnian satellites starting 1993 and preliminary predictions were made by Sema (A&A 265, L21, 1992) for eclipses of Iapetus and by Thuillot et al. (BAAS 24, 938, 1992) for the eclipses and mutual events. 5 - Dynamics of the Ring Systems Goldreich (55.042.024) outlined some of the main processes that shape the planetary rings and Three-dimensional perturbations in a narrow planetary ring were studied by Kolvoord and Burns (55.091.005). A theory of the Neptunian ring arcs was made by Gor'kavyj et al. (54.101.126,127) and an explanation of those arcs was also given by Porco (54.101.005). Concerning the rings of Uranus, papers "Shepherding Satellites and Dynamical Structure of the rings of Uranus" were published by Kozai (PASJ, 44, 135, 1992 and PASJ, 45, 263, 1993). Colwell and Esposito a numerical model of of Uranian rings (52.101.003). A paper on the orbits of shepherd satellites deduced from the structure of the rings of Uranus was also published by Murray and Thomson (52.101.025). French et al. presented a work on the dynamics and structure of uranian rings (54.101.104). Concerning the rings of Saturn, Kolvoord and Burns published a collisional modelling of Saturn's F rings (52.100.030) and the dynamics of Saturn's E ring particles were also published (55.100.069). Concerning observations, several works were made : Turtle et al. (52.100.026) deduced the kinematics of Saturn major narrow rings from Voyager and ground-based observations and Nicholson et al. (52.101.011) reported on the observation of Neptune's rings arc by stellar occultation as well as Sicardy et al. for the 1983-1989 period (53.101.006). 6 - Radar astrometry of Natural Satellites In addition to the existing radar astrometric data for several asteroids and comets, Ostro et al. (JGR. 97, no. E11. 18227) presented Goldstone and Arecibo Doppler observations of Europa, Ganymede and Callisto over the interval from June 1987 through FeB. 1991. Doppler observations were published for Europa (18 observations), Ganymede (23) and Callisto (35). The largest discrepancies between the predicted and observed Doppler observations implied that Callisto may have been lagging its ephemeris by an average of about 200 km. Arecibo observations from Feb.-March 1992 have yielded the first successful radar ranging measurements to the Galilean satellites of Jupiter. In 1992, Harmon et al. (AJ, submitted) made range measurements to Ganymede on March 9 and Feb. 20 and to Callisto on 1992 March 3 and March 7. These observations suggest that the required along track position corrections to the ephemeris predictions in that time frame were approximately +122 km for Ganymede, -307 km for Callisto and about +11 km for Jupiter itself. J.H. Lieske is currently making the necessary program modifications to allow the processing of radar data in his Galilean satellite ephemeris development software at JPL.