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New images from the Cassini spacecraft reveal optically thick clumps, capable of casting shadows, and associated structures in regions of Saturn's F ring that have recently experienced close passage by the adjacent moon Prometheus. Using these images and modeling, we show that Prometheus' perturbations create regions of enhanced density and low relative velocity that are susceptible to gravitational instability and the formation of distended, yet long-lived, gravitationally coherent clumps. Subsequent collisional damping of these low-density clumps may facilitate their collapse into ~10-20 km contiguous moonlets. The observed behavior of the F ring is analogous to the case of a marginally stable gas disk being driven to instability and collapse via perturbations from an embedded gas giant planet.

Aegaeon (Saturn LIII, S/2008 S1) is a small satellite of Saturn that orbits within a bright arc of material near the inner edge of Saturn's G ring. This object was observed in 21 images with Cassini's Narrow-Angle Camera between June 15 (DOY 166), 2007 and February 20 (DOY 51), 2009. If Aegaeon has similar surface scattering properties as other nearby small Saturnian satellites (Pallene, Methone and Anthe), then its diameter is approximately 500 m. Orbit models based on numerical integrations of the full equations of motion show that Aegaeon's orbital motion is strongly influenced by multiple resonances with Mimas. In particular, like the G-ring arc it inhabits, Aegaeon is trapped in the 7:6 corotation eccentricity resonance with Mimas. Aegaeon, Anthe and Methone therefore form a distinctive class of objects in the Saturn system: small moons in co-rotation eccentricity resonances with Mimas associated with arcs of debris. Comparisons among these different ring-arc systems reveal that Aegaeon's orbit is closer to the exact resonance than Anthe's and Methone's orbits are. This could indicate that Aegaeon has undergone significant orbital evolution via its interactions with the other objects in its arc, which would be consistent with the evidence that Aegaeon's mass is much smaller relative to the total mass in its arc than Anthe's and Methone's masses are., 35 pages, 12 figures, accepted for publication in Icarus. Some typographical errors fixed

We describe the astrometry and dynamics of Anthe (S/2007 S 4), a new satellite of Saturn discovered in images obtained using the Imaging Science Subsystem (ISS) of the Cassini spacecraft. Included are details of 63 observations, of which 28 were obtained with Cassini's narrow-angle camera (NAC) and 35 using its wide-angle camera (WAC), covering an observation time-span of approximately 3 years. We estimate the diameter of Anthe to be ∼1.8 km. Orbit modeling based on a numerical integration of the full equations of motion fitted to the observations show that Anthe is in a first-order 11:10 mean motion resonance with Mimas. Two resonant arguments are librating: ϕ 1 = 11 λ ′ − 10 λ − ϖ and ϕ 2 = 11 λ ′ − 10 λ − ϖ ′ − Ω ′ + Ω , where λ, ϖ and Ω refer to the mean longitude, longitude of pericenter and longitude of ascending node of Mimas and Anthe, with the primed quantities corresponding to Anthe. These resonances cause periodic variations in the orbital elements. The semi-major axis varies by ±26 km over a 913-day period. Anthe is also close to a second-order eccentricity-type mean motion resonant relationship of the form 77:75 with Methone. Since Methone is also in a first-order resonance with Mimas [Spitale, J.N., Jacobson, R.A., Porco, C.C., Owen, W.M., 2006. Astron. J. 132, 692–710], an additional indirect perturbation exists between Methone and Anthe via Mimas. Neither effect is detectable in the orbit fitting and the short-term dynamical evolution of Anthe is dominated by the Mimas–Anthe resonances alone. The expected modulation effect from the Mimas–Tethys 4:2 inclination resonance is also insignificant over this time period. By including Cassini ISS observations of Mimas in the numerical integration fit, we estimate the GM of Mimas to be 2.509 ± 0.004 km 3 s −2 , consistent with Jacobson et al. [Jacobson, R.A., Spitale, J., Porco, C.C., Owen, W.M., 2006. Astron. J. 132, 711–713].

We describe the astrometry and dynamics of S/2004 S 5 (provisionally named Polydeuces), a new satellite of Saturn, discovered in images recorded using the Imaging Science Subsystem (ISS) of the Cassini spacecraft. A total of 119 observations are presented, 101 of which were obtained with Cassini's narrow-angle camera and 18 using the wide-angle camera. Orbit determinations, based on a uniformly precessing ellipse model clearly showed S/2004 S 5 to be a possible co-orbital of Dione. By fitting a numerical solution of the full equations of motion to the available observations, we show that S/2004 S 5 is executing tadpole motion around Dione's L 5 Lagrangian point, with a libration amplitude varying between + 21.16 ± 0.03 ° and − 31.68 ± 0.06 ° over a period of 791.3 ± 0.1 days.

The rings around Jupiter, Saturn, Uranus and Neptune are stabilized by small ‘shepherd moons’ that orbit in or near the rings and stabilize them by gravitational influences. The narrow F-ring of Saturn is tended by two shepherds, Prometheus and Pandora. Images of Saturn's F ring from the Cassini spacecraft have revealed structures never before seen in a planetary ring. The perturbing effect of Prometheus, the inner shepherding satellite, opens up channels through the F ring and forms a stream of particles (a ‘streamer’) linking the ring to the satellite. Images of Saturn's narrow and contorted F ring returned by the Cassini spacecraft1 have revealed phenomena not previously detected in any planetary ring system. The perturbing effect of the inner shepherding satellite, Prometheus, seems to introduce channels through the F ring and a ‘streamer’—a line of particles that link the ring to the satellite. The detailed mechanism for the formation of these features has been lacking an explanation. Here we show that these phenomena can be understood in terms of a simple gravitational interaction as Prometheus approaches and recedes from the F ring every 14.7 hours. Our numerical models show that as Prometheus recedes from its closest approach to the F ring, it draws out ring material; one orbital period later, this affected region has undergone keplerian shear and is visible as a channel, in excellent agreement with structures seen in the Cassini images. Prometheus' periodic disruption of the F ring will become more pronounced as the two orbits approach their minimum separation in 2009. The model predicts that the appearance of streamers and the associated channels will vary in a regular fashion on a timescale of one orbital period.

There is substantial evidence for the existence of small satellites in the saturnian system in addition to those with well-determined orbits derived from ground-based and Voyager observations. Charged particle data from Pioneer 11 and Voyager 2 provide specific evidence for the existence of material coorbital with Mimas. This work is an attempt to identify small satellites coorbital with Mimas which may be responsible for the charged particle results. The research uses a combination of the NAIF/SPICE Toolkit, IRAF, and locally developed software to analyze images in the Voyager archive. We also develop a new approach to the problem of blemish removal. Four possible new satellites are detected in Voyager 2 images after the identification and elimination of blemishes and background stars. Details of the search criteria and probable orbits are discussed for each of the candidate objects. Three of the candidates may be coorbital with Mimas. If there is such a collection of small satellites in the orbit of Mimas it could explain the depletions seen in the charged particle data. One of the candidates, if it is in the ring plane, would be on an orbit at the outer edge of the faint G ring. If so, it may provide a source for the ring material. Due to the limited coverage of Voyager images in the search region it was not possible to find additional images of the candidates. As a consequence the features identified in this work must remain candidate objects until confirmed either by ground-based or spacecraft observations.

International audience; Saturn's narrow F ring exhibits several unusual features that vary on timescales of hours to years. These include transient clumps, a central core surrounded by a multistranded structure and a regular series of longitudinal channels associated with Prometheus, one of the ring's two 'shepherding' satellites. Several smaller moonlets and clumps have been detected in the ring's immediate vicinity, and a population of embedded objects has been inferred. Here we report direct evidence of moonlets embedded in the ring's bright core, and show that most of the F ring's morphology results from the continual gravitational and collisional effects of small satellites, often combined with the perturbing effect of Prometheus. The F-ring region is perhaps the only location in the Solar System where large-scale collisional processes are occurring on an almost daily basis.

Hard X-ray balloon altitude measurements with a 1600 cm 2 phoswich array are described. Data from observations on Sco X-1, GX1+4, GX5−1, Nova Oph. 1977, SMC X-1, SS433, IC 4329A and MR 2251-178 are presented. The role of Comptonisation in X-ray production for Sco X-1 and GX1+4 is discussed.

Two flights from Alice Springs, Australia, were achieved in November 1977 and November 1978 with a plastic scintillator γ-burst detector, effective area 6.3 m2, thickness 5 cm, energy response in the range 50 keV to 2 MeV. In 33 hr of good, high altitude data, two bursts were detected, yielding a rate corrected to an isotropic flux of\(7.03 \times 10^3 {\text{y}}^{ - 1} \left\{ {\begin{array}{*{20}c} { + 10 \times 10^3 } \\ { - 6 \times 10^3 } \\ \end{array} } \right\}\) at a size of 8.5×10−9 erg cm−2. One event, seen at 22.14 on 15 Nov 1978, was confirmed by spacecraft measurements. The second, too small to be detected by spacecraft, arrived from 0 hr RA, −13.2° Decl. ±12° and possibly comes from a confirmed γ-burst source location. A galactic origin with a source distribution originating from a relatively thick disk, is favoured by these results.

To obtain more data at the low flux end of the size spectrum and in particular to both lower the size limit of burst detection and look for a possible excess flux from the galactic centre region, large-area detector flights were undertaken from Australia. The apparatus consisted of plastic scintillators, 5 cm thick and 6.3 m2 in effective area, arranged in six trays, two of which were horizontal and four of which were inclined at 30? to the horizontal so as to make up the four sloping sides of a box. Laboratory calibration gave a mean efficiency for the detection of 50 keV to 2 MeV photons of 0.404 when translated to 4 g cm-2 atmospheric depth and for all bursts arriving up to a cut-off zenith angle of 60?. Thirty-three hours of good data were obtained in total during flights in November 1977 and November 1978. Gamma-burst searches were made by demanding three successive 2o increases in at least three of the six trays with a sampling interval of 1/16s. Two bursts were detected, one at 1978, 15 November, 22h14, in coincidence with Venera spacecraft observations of an event, and the second at 1978, 16 November, 13h40 at a flux level of 9.2 x 10-8 erg cm-2. Directional determination for this second burst with the inclined tray rates gave I- 82.5?, b = - 71.4?, with an error circle of 12'. A Venera event occurred on 19 November 1978, 24' away from this position. Correction of the flight results to an isotropic flux, missed short bursts, the flat detector response and missing energy being taken into account, on the assumption of the Apollo 16 (Mezger et al. I974) spectrum, yielded a rate (7+10) x 103 year-1 at sizes greater than or equal to 8.47 x 10-9 erg cm-2. This point is plotted

In this paper, we publish 1206 measurements of positions of the major satellites of Saturn made in 1990, 1991, 1993 and 1994 using CCD detectors attached to the 1-metre Jacobus Kapteyn Telescope on the island of La Palma. Analysis of the data as inter-satellite posi- tions shows that the observations of Tethys, Dione, Rhea and Titan have root-mean-square residuals of 0: 08, corre- sponding to 500 km at the distance of Saturn.

In this paper, we publish measurements of 1514 positions of the major satellites of Saturn made in 1995 and 1997 using CCD detectors attached to the 1-metre Jacobus Kapteyn Telescope on the island of La Palma. Analysis of the data as inter-satellite positions shows that the observations of Tethys, Dione, Rhea and Titan have root-mean-square residuals of 0: 08 in 1995 and 0: 10 in 1997.