Report of the working group on Satellites for the period 1978-1981 (Y. Kozai) a) DISCOVERIES Several small satellites of Jupiter and Saturn were discovered during the triennium, the encounters of the Pioneer 11 spacecraft with Saturn in 1979 September and of the Voyagers with Jupiter in 1979 March and July and with Saturn in 1980 November and 1981 August, as well as the passage of the Earth through the Saturn ring plane in 1979-80 all affording unusual observational opportunities. The three Jovian satellites 1979 J 1, 1979 J 2 and 1979 J 3, discovered from the Voyager spacecraft (26.099.177; 28.099.118; IAU Circ 3470, 3507, 3575) have orbital periods and semimajor axes in terms of the equatorial radius of Jupiter of 7h 09m.5, 1.79 RJ 16h 11.35m, 3.105RJ; and 7h40.5m, 1.89RJ respectively. Discovery of another satellite , 1981 J 1, was reported by D. Pascu and P.K. Seidelmann ( IAU Circ 3603); the suggested identity 1981 J 1 = 1979 J 2 has not yet been definitely confirmed. Discovery of a Jovian ring extending to 1.81RJ was reported by the Voyager 1 team (E C Stone and A L Lane 25.099.059; IAU Circ 3338). Saturnian satellites 1979 S 1 through 1979 S 7, 1980 S 1 through 1980 S 33, and 1981 S 1 and 1981 S 2 had been reported through 1981 June 30 on the basis of observations from Pioneer 11 (IAU Circ 3417, 3454), by J. D. Mulholland (IAU Circ 3430), by Pascu (IAU Circ 3454), by B. A. Smith, H.J. Reitsema, and S. M .Larson (IAU Circ 3456, 3457, 3466, 3496), by D. P. Cruikshank (IAU Circ 3457), by A.W. Harris and J. Gibson (IAU Circ 3463, 3466), by J. Lecacheux et al. (IAU Circ 3457, 3463, 3484), by A. Dollfus (IAU Circ 3474), by C. Veillet (IAU Circ 3470), by P. Lamy and N. Mauron (IAU Circ 3466, 3574), by R. Suggs (IAU Circ 3484), by the Space Telescope Wide Field/Planetary Camera Instrument Definition Team, which included Seidelmann, W.A. Baum and D.G. Currie (IAU Circ 3496), with the 1.54 -m Danish telescope at ESO (IAU Circ 3593), and from Voyager 1 (IAU Circ 3532, 3539, 3602, 3603). Many identities exist among the separately designated Saturnian satellites, but details are obscured by limited intervals of observation and complications introduced by dynamical interactions. For example, the pair of co-orbiting satellites satellites 1980 S 1 and 1980 S 3 are to be associated with the two satellites observed at the time of the 1966 ring plane passage (J W Fountain and Larson 22.100.012; K. Aksnes and F.A. Franklin 22.100.505), but until the mutual interactions are better understood, the 1966 and 1980 observations cannot be linked. Larson et al. (Icarus 46, 175, 1981) have re-examined the 1966 observations in the light of data obtained in 1979-81. R.S. Harrington and Seidelmann (Icarus 47, 97, 1981) studied the dynamics of the satellites 1980 S 1 and 1980 S 3, concluding that the difference in longitude librates. Thus the satellites never approach extreme proximity, and the orbits appear to be stable for extended periods of time. The two satellites 1980 S 26 and 1980 S 27, discovered from Voyager 1 (see S.P. Synnott et al. 29.100.057) are the outer and inner shepherding satellites of the F ring, first detected from Pioneer 11 (T Gehrels et al. 27.100.018). The satellite 1980 S 6, discovered by P. Laques and Lecacheux (IAU Circ 3457; 28.100.020), librates about the leading triangular point of Dione, and 1980 S 13 and 1980 S 25 seem to be associated with the triangular points of Tethys (Harrington et al. IAU Circ 3583; see also Seidelmann et al. Icarus 47, 282, 1981). The orbit of 1980 S 28, also discovered from Voyager 1, lies close to the outer boundary of the A ring. Measurements from Voyager confirmed the existence interior to the C ring of a tenuous ring with numerous narrow features but indicated that this D ring (P. Guerin 3.100.001) was too faint to have been detected from the Earth. The E ring (W.A. Feibelman and D.A. Klinglesmith 28.100.005) was the object of an intensive ground-based observational effort during the 1980 ring plane passages. The new observations (Dollfus and S. Brunier 27.100.030; A. Brahic et al. 28.100.061; Baum et al. 28.100.062; Reitsema et al. 28.100.063; Larson et al. Icarus 47, 288, 1981; Lamy and Mauron 28.100.070 and Icarus 46, 181, 1981) showed an extension to at least 8RS, with a density peak near the orbit of Enceladus. Also discovered from Voyager 1 was the faint, narrow G ring at 2. 8RS. Several groups looked for evidence of a Neptunian ring at the times of appulses to two stars in 1981 May. No evidence of a ring was found (W.B. Hubbard et al. Bull AAS 13, 728, 1981; J.L. Elliot et al. ibid. 13, 729, 1981; Reitsema et al. ibid. 13, 721,1981), but Reitsema (IAU Circ 3608) reported a brief occultation event detected at two stations as evidence for a previously unsuspected satellite, 1981 N 1. b) ASTROMETRIC OBSERVATIONS Photographic astrometric observations of satellites have been made at a number of observatories during the triennium. The Galilean satellites as well as the Saturnian satellites S I - S VIII habe been observed by Pascu, with the collaboration of R.E. Schmidt, with the 0.66-m refractor of the U S Naval Observatory. Some 250 observations were sent to the Jet Propulsion Laboratory for use in orbit improvements. The group led by Mulholland continued the astrometric program on the faint satellites of Jupiter, Saturn and Uranus with the 2.1-m reflector of the McDonald Observatory until impacted by loss of funding in late 1981. Results were published for Jupiter V and the Galilean satellites in 1976-78 (25.099.044), for Jupiter VI-XII in 1975-77 (25.099.077; 26.099.124) and for Saturn I-IX in 1975-76 (28.100.002). P. A. Ianna, F. Levinson and P. Seitzer (25.041.017; 25.100.042; 26.099.047; 28.100.003) used the ˆ.67-m refractor of the McCormick Observatory to observe the Galilean satellites of Jupiter and the eight bright satellites of Saturn. Ianna, in collaboration with L.C. Stayton and J.R. Rohde, is continuing the series of observations with the McCormick refractor and with the O.66-m refractor at the Mt Stromlo Observatory. E. Bowell (IAU Circ 3602, 3603) reported observations of J VI, S VIII and S IX with the 0.33-m astrograph at the Lowell Observatory. H. Debehogne et al. (26.099.064; 27.097.012; 27.097.060; 27.099.086; 27.099.087; 28.099.006; 29.099.054; 29.099.055) published observations of the Galilean and some other bright satellites made with the GPO 0.40-m astrograph at ESO and with the 0.4-m astrograph of the Royal Observatory, Uccle. Observations of Phoebe made at ESO under Debehogne's direction also were published (IAU Circ. 3612). At Pulkovo Observatory 189 plates for the Galilean satellites and 156 plates for Saturnian satellites I-VIII were taken between 1978 January and 1981 May with the 0.65-m refractor. In the same interval 93 plates for the Galilean satellites and 85 plates for the Saturnian satellites S II - S VI and S VIII were taken with the normal astrograph. Some 41 plates for the Galilean satellites were taken with the short-focus double astrograph. At Nikolaev 44 plates for the Galilean satellites and 60 plates for the Saturnian satellites S III - S VI were taken with the zone astrograph. At Tashkent 107 plates for the Saturnian satellites S I - S VI, 144 plates for the Uranian satellites U I - U IV, and 27 plates for N I, Triton, were taken in the interval 1978-81. At Engelhardt Observatory, Kazan, routine observations of the Saturnian satellites have been made with the 0.40-m astrograph. Observations made in the USSR have been published during the triennium as follows : for satellites of Mars made at Kiev in 1973 (Ref Zh Astr 1.51.115, 1981) for Galilean satellites made at the Pulkovo Observatory with the normal astrograph in 1974-76 (27.099.017) and in 1977-78 (Izv GAO N¡ 199, 1981), at Nikolaev in 1975 (Ref Zh Astr. 3.51.199, 1979), and for Saturnian satellites made at Nikolaev in 1973-76 (Ref Zh Astr. 3.51.200, 1979) and at Engelhardt Observatory in 1973-75, 1978, and 1980 (Ref Zh Astr. 12.51.137, 1980; 12.51.138, 1980; 3.51.82, 1981). Astrometric observations of the satellites of Uranus were made at Pic du Midi (Veillet and G. Ratier 28.101.003) and with the 1.88-m reflector of the Tokyo Observatory (K. Tomita and M. S™ma 26.101.035). The satellite of Pluto was observed with the 1.55m-reflector of the U.S. Naval Observatory, Flagstaff, the 4.0-m telescope at Cerro Tololo, the 2.1-m reflector of the McDonald Observatory , the 1.60m-Megantic reflector, and the C-F-H 3.6-m reflector on Mauna Kea equipped with a speckle interferometer. The new results have been discussed by Harrington and J W Christy (27.101.010; 29.101.011) and by D. Bonneau and R. Foy (28.101.032). The best value of the orbital radius (19 700 ± 300 km) implies an inverse mass of the system of (1.34 ± 0.07) x 108. Mutual phenomena may begin with the 1983 apparition, and should certainly be observable in 1984. Astrometric plates are now measured with microdensitometers, instead of coordinate comparators, at a number of observatories. J.-E. Arlot (27.099.080) explained how the positions of Galilean satellites were measured at the CDCA in Nice with the PDS 1010A microdensitometer controlled by a PDP 11/40 computer. c) MOTIONS OF THE SATELLITES OF JUPITER Analysis of the motions of the Galilean satellites remains an important problem in celestial mechanics, and was the subject of a book by S. Ferraz-Mello (27.003.054). J.L. Sagnier treated the problem by an analytical method in his doctoral dissertation (1981). The great inequality terms, together with libration terms of the first three satellites were derived by D.T. Vu (29.099.005) following Sagnier's formulation. J.H. Lieske (27.099.018) provided improved ephemerides based on his new theory (1977) together with analysis of over 4800 Earth-based observations. The estimated error is less than 200 km. Lieske (25.099.045) also derived simplified expressions for the poles of the satellites with an error of <0¡.01, and published (29.099.065) a catalog of eclipses of the galilean satellites for 1610-2000. M. Tsuchida, Ferrza- Mello and R. Biancale compared photographic observations of the Galilean satellites with Sampson's theory as improved by Lieske. For modern observations, 1968-77, the geocentric differences are of the order 0".08 while for older ones, in 1913-28, they are 0".14. P. Nacozy, R. McKenzie, Ferraz-Mello and M. Sato (26.099.202) made numerical integrations after removing short-periodic terms by numerical averaging to obtain long-term motions of the Galilean satellites. W. Thuillot (26.099.062) determined parameters for the satellites of Jupiter by computing satellite orbits by numerical integrations for 88 days in 1975 and comparing them with Pascu's observations. W. Wiesel (27.099.024; 29.099.064) discussed secular effects of orbital and solar resonances. Vu explained the new ephemerides published by the Bureau des Longitudes for Jupiter's satellites, including the Galilean ones, in Supplements to Connaissance des Temps for 1980 and 1981. Positions for the Galilean satellites are expressed by Chebyshev polynomials computed directly from Sampson's theory, rather than from his tables. Positions for satellites relative to Saturn, also expressed by Chebyshev polynomials, are included in the Supplements. J.-F. Lestrade (28.042.065) applied Laplace's idea, using the true longitude as the variable, to derive the motion of Jupiter VI. L.E. Bykova (27.099.016) determined the mass of Jupiter from the motion of its outer satellites. T.S. Boronenko (Astr. Geod. 8, 97, Tomsk 1980) developed analytical theories of the motions of J VI, J VII and J X up to the sixth order with respect to the eccentricities and the inclinations; the theories represent available observations within 2". T.V. Bordovitsyna and Bykova (25.003.025) published a monograph presenting theories of the motion of J VI and J VII as well as ephemerides for 1979-2000. d) MOTIONS OF SATELLITES OF OTHER PLANETS A. T. Sinclair (22.097.504) gave new orbits and J.A. Burns (22.097.508) discussed the dynamical evolution and origin of the satellites of Mars at the colloquium marking the centennial of the discoveries at the U S Naval Observatory. Other studies of the tidal evolution and origin of the Martian satellites include those of D M Hunten (25.097.007), T.C. Van Flandern (26.097.180), K. Lambeck (26.097.031), A. Cazenave et al. (27.097.005; 29.097.035) and F. Mignard (29.097.001), J. Veverka and Burns (27.097.185) reviewed dynamics and origin, as well as physical properties, in their excellent review chapter devoted to the satellites of Mars. A new analytical theory of the motion of Mimas and Tethys was developed by W.H. Jefferys and L.M. Ries (26.100.033; 29.100.901) using the algebraic manipulation language TRIGMAN. They expect that the theory can provide positions with 10 km accuracy. L.E. Rose (26.100.003) derived a new orbit of Saturn IX by numerical integrations, fitting 133 observations for 1904-1969 with a mean residual of 1".52 and determining the mass of Saturn. A. Bec- Borsenberger developed an analytical theory of Phoebe based on her literal series derived for the main problem of the lunar theory. She also made numerical integrations using constants by Rose, compared the results with observations in 1898-1976, and improved the constants for use in the analytical theory. She then extended the integrations up to 1990 and computed an ephemeris in polynomial form for use by the Bureau des Longitudes. I.G. Chugunov (Ref Zh Astr. 6.51.211, 1980; 1.51.77, 1981) developed theories of the motion of Saturn's satellites that take into account the non-sphericity of the planet, effects of the ring, the Sun, and the mutual actions of the satellites. All terms exceeding 10-6 are retained in the solution. Orbital elements of the satellites were improved by using 10 000 observations. A group of scientists at Tomsk State University also has been developing numerical and analytical theories for the Saturnian satellites. A theory for Phoebe has been completed. An improvement of the orbital elements of Hyperion was reported by Y. Hatanaka (25.100.039), who used his own observations made with the 0.65-m refractor at Tokyo. J.D. Anderson et al. (27.100.023) and G.W. Null et al. (29.100.014) derived the mass of Saturn, the values of J2, J4 and J6, and the masses of Rhea, Titan and Iapetus from Pioneer 11 tracking data. Veillet (29.101.018) determined a new orbit of Miranda by using all the observations since its discovery. Terms with period 12.2 yr, very close to the circulation period of the near commensurability between Miranda, Ariel and Umbriel, had to be included to represent the observations. Pluto and the irregular satellite system of Neptune was the subject of a study by Harrington and Van Flandern (25.101.025), who performed numerical experiments to set limits on encounter circumstances of a massive body with Neptune. In addition to the new observations and orbital calculations for the Pluto satellite referenced in section IV.b. above, a discussion of possible origin of Charon was published by Mignard (29.101.016). e) RINGS The Jovian ring discovered by Voyager 1 was found by Voyager 2 to consist of three components. Amazing complexities of the Saturnian rings, including hundreds of ringlets, braided structure of the F ring, transient spokes crossing the B ring, and a very faint and narrow G ring were detected during the two Voyager encounters in 1980 November and 1981 August (Smith et al. 29.100.056; see also J.K. Beatty S & T 62, 430, 1981). Besides observational data referenced above in section IV.a., overviews, comparisons and speculations concerning origin of the ring structures surrounding Jupiter, Saturn and Uranus have been published by R. Smoluchowski (25.100.010; 26.099.002), S.F. Dermott et al. (27.099.006), and Burns et al. (29.099.077). Ip (27.091.018; 27.091.019) reviewed physical studies and orbital dynamics of planetary rings, first on the basis of ground-based observations and then incorporating the new spacecraft data. Burns et al. (26.100.034) examined the effects of satellite and solar perturbations and planetary precession on the 'thickness' of the Saturnian rings, concluding that such effects are inadequate to explain the many-particle- thick nature of the rings commonly accepted as required by optical and radio observations. New observations of the Uranian rings were obtained by a number of groups during occultation events on 1979 June 10, 1980 March 20 and August 15, and 1981 April 26. Papers on the structure and dynamics of the Uranian rings were published by P. Goldreich and S. Tremaine (25.101.001; 26.101.024; 29.101.014), Dermott et al. (26.101.001), Dermott and C.D. Murray (28.101.035), P.D. Nicholson et al. (29.101.025), G.A. Steigmann (25.101.002), and J.C. Bhattacharyya et al. (26.101.034). According to Elliot et al. (29.101.009; 29.101.012) the Uranian rings seem to be coplanar ellipses precessing uniformly according to the harmonics of the Uranian gravitational potential. The ellipticity of Uranus, e = 0.022 ± 0.003, is consistent with the reanalysis of the Stratoscope II images by Franklin et al. (27.101.012); J2 corresponds to a planetary rotation period of 15h5.