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Saturn's Meteoroid Crash During Saturn's equinox in 2009, when the Sun illuminated its rings edge-on, images taken by the Cassini spacecraft showed dust clouds appearing as bright streaks above the rings. Similar streaks were detected in 2005 and 2012 when Cassini observed the C ring at close range. Tiscareno et al. (p. 460 ) suggest that the cause of each observed feature is likely to be the impact of a stream of recently disrupted material originating from a meteoroid impact onto the ring and derive the influx rate of meteoroids at Saturn.

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

Using Cassini images, we examine the faint material along the orbits of Methone, Anthe and Pallene, three small moons that reside between the orbits of Mimas and Enceladus. A continuous ring of material covers the orbit of Pallene; it is visible at extremely high phase angles and appears to be localized vertically to within ±25 km of Pallene's inclined orbit. By contrast, the material associated with Anthe and Methone appears to lie in longitudinally confined arcs. The Methone arc extends over ∼10° in longitude around the satellite's position, while the Anthe arc reaches ∼20° in length. The extents of these arcs are consistent with their confinement by nearby corotation eccentricity resonances with Mimas. Anthe has even been observed to shift in longitude relative to its arc in the expected manner given the predicted librations of the moon.

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.

The Cassini Imaging Science Subsystem (ISS) began observing Saturn in early February 2004. From analysis of cloud motions through early October 2004, we report vertical wind shear in Saturn's equatorial jet and a maximum wind speed of ∼375 meters per second, a value that differs from both Hubble Space Telescope and Voyager values. We also report a particularly active narrow southern mid-latitude region in which dark ovals are observed both to merge with each other and to arise from the eruptions of large, bright storms. Bright storm eruptions are correlated with Saturn's electrostatic discharges, which are thought to originate from lightning.

In order to assess the possibility of meteoroid streams detectable from the surface of Mars as meteor showers we have derived minimum distances and associated velocities for a large sample of small body orbits relative to the orbits of Mars and the Earth. The population ratio for objects approaching to within 0.2 AU of these two planets is found to be approximately 2:1. The smaller relative velocities in the case of Mars appears to be the main impediment to the detection of meteors in the upper atmosphere of that planet. We identify five bodies, including the unusual object (5335) Damocles and periodic comet 1P/Halley, with relative orbital parameters most suitable to produce prominent meteor showers. We identify specific epochs at which showers related to these bodies are expected to occur. An overview of possible detection methods taking into account the unique characteristics of the Martian environment is presented. We pay particular attention on the effects of such streams on the dust rings believed to be present around Mars.

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.

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.

We report the discovery of several large "propeller" moons in the outer part of Saturn's A ring, objects large enough to be followed over the 5-year duration of the Cassini mission. These are the first objects ever discovered that can be tracked as individual moons, but do not orbit in empty space. We infer sizes up to 1--2 km for the unseen moonlets at the center of the propeller-shaped structures, though many structural and photometric properties of propeller structures remain unclear. Finally, we demonstrate that some propellers undergo sustained non-keplerian orbit motion. (Note: This arXiv version of the paper contains supplementary tables that were left out of the ApJL version due to lack of space)., Comment: 9 pages, 4 figures; Published in ApJL

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.

"The narrow rings are thought to be stabilised by small shepherd moons that orbit in or near the rings and stabilize them by gravitational in uences. The narrow F ring of Saturn which lies just outside the spectacular main rings is tended by two small shepherds. Prometheus (100 km in diameter) orbits just inside the F ring, while Pandora (85 km in diameter) moves around Saturn just outside the F ring. Images from the Saturn F ring region obtained by the Cassini Imaging Science Subsystem (ISS) cameras have revealed structure never seen before in a planetary ring. Periodic structures such as azimuthal gaps, channels of low optical depth and streamers have been discovered. We reported these features previously in Murray et al. (2005). Updated orbits are used here. The orbit of Prometheus has been taken from Spitale et al. (2006), the orbit of the F ring core from Bosh et al. (2002). The dimensions and mass of Prometheus have been taken from Porco et al. (2006). We made a direct comparison between the previous numerical results using the orbits reported in Murray et al. (2005) and our model for the same conguration, and we nd an excellent agreement."

The Cassini Imaging Science Subsystem acquired high-resolution imaging data on the outer Saturnian moon, Phoebe, during Cassini's close flyby on 11 June 2004 and on Iapetus during a flyby on 31 December 2004. Phoebe has a heavily cratered and ancient surface, shows evidence of ice near the surface, has distinct layering of different materials, and has a mean density that is indicative of an ice-rock mixture. Iapetus's dark leading side (Cassini Regio) is ancient, heavily cratered terrain bisected by an equatorial ridge system that reaches 20 kilometers relief. Local albedo variations within and bordering Cassini Regio suggest mass wasting of ballistically deposited material, the origin of which remains unknown.

Images acquired of Saturn's rings and small moons by the Cassini Imaging Science Subsystem (ISS) during the first 9 months of Cassini operations at Saturn have produced many new findings. These include new saturnian moons; refined orbits of new and previously known moons; narrow diffuse rings in the F-ring region and embedded in gaps within the main rings; exceptionally fine-scale ring structure in moderate– to high–optical depth regions; new estimates for the masses of ring-region moons, as well as ring particle properties in the Cassini division, derived from the analysis of linear density waves; ring particle albedos in select ring regions; and never-before-seen phenomena within the rings.

The Cassini Imaging Science Subsystem (ISS) is the highest-resolution two-dimensional imaging device on the Cassini Orbiter and has been designed for investigations of the bodies and phenomena found within the Saturnian planetary system. It consists of two framing cameras: a narrow angle, reflecting telescope with a 2-m focal length and a square field of view (FOV) 0.35∘ across, and a wide-angle refractor with a 0.2-m focal length and a FOV 3.5∘ across. At the heart of each camera is a charged coupled device (CCD) detector consisting of a 1024 square array of pixels, each 12 μ on a side. The data system allows many options for data collection, including choices for on-chip summing, rapid imaging and data compression. Each camera is outfitted with a large number of spectral filters which, taken together, span the electromagnetic spectrum from 200 to 1100 nm. These were chosen to address a multitude of Saturn-system scientific objectives: sounding the three-dimensional cloud structure and meteorology of the Saturn and Titan atmospheres, capturing lightning on both bodies, imaging the surfaces of Saturn’s many icy satellites, determining the structure of its enormous ring system, searching for previously undiscovered Saturnian moons (within and exterior to the rings), peering through the hazy Titan atmosphere to its yet-unexplored surface, and in general searching for temporal variability throughout the system on a variety of time scales. The ISS is also the optical navigation instrument for the Cassini mission. We describe here the capabilities and characteristics of the Cassini ISS, determined from both ground calibration data and in-flight data taken during cruise, and the Saturn-system investigations that will be conducted with it. At the time of writing, Cassini is approaching Saturn and the images returned to Earth thus far are both breathtaking and promising.

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.