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The Saturnian satellite Tethys exhibits geological and spectral properties, whose appearance, nature and spatial distribution partly mirror those identified on the neighboring satellites Dione and Rhea or fit to the picture how spectral surface properties are expected to change from one satellite to the other within the inner Saturnian system. However, we also identified spectral variations that are unique in the Saturnian system. Whereas geologically young surface features are characterized by pure H2O-ice composition with relatively large particles, which match the particle sizes measured for fresh surface features also on Dione and Rhea, geologically old weathered regions are dominated by submicron-sized ice particles. Our investigations confirm that the Odysseus impact event did not cause the formation of Tethys' extended graben system Ithaca Chasma. On the contrary, Odysseus might be responsible for the N-S trending 'icy' bands that mark Tethys' surface in the center of its leading and trailing hemisphere.

We investigate the origin of the steep compositional gradient inferred for the Galilean satellites. We analyze N-body simulations of satellite accretion (Ogihara, M., Ida, S. [2012]. Astrophys. J. 753, 60) to: (1) determine the extent to which individual satellites accrete material from different semi-major axes (‘radial mixing’); and (2) calculate the change in rock:ice ratios due to vapor production or physical erosion during collisions. Because of inwards migration, satellites experience enough radial mixing that any initial compositional gradient is efficiently smoothed out. Mean-motion resonances generally prevent large proto-satellites from colliding with each other; as a result, neither vapor production nor physical erosion are capable of explaining the steep compositional gradient. According to the models presented here, even combining an initial compositional gradient with impact-related mass loss cannot reproduce the observed gradient. Some other physical process must have been responsible, perhaps tidally-driven volatile loss at Io and Europa. Impact-related mass loss was probably more important in the saturnian system, and may explain some of the observed satellite diversity there.

The Cassini Imaging Science Subsystem (ISS) acquired many high-resolution images (

It is widely accepted that Titan and the mid-sized regular satellites around Saturn were formed in the circum-Saturn disk. Thus, if these mid-sized satellites were simply accreted by collisions of similar ice-rock satellitesimals in the disk, the observed wide diversity in density (i.e., the rock fraction) of the Saturnian mid-sized satellites is enigmatic. A recent circumplanetary disk model suggests satellite growth in an actively supplied circumplanetary disk, in which Titan-sized satellites migrate inward by interaction with the gas and are eventually lost to the gas planet. Here we report numerical simulations of giant impacts between Titan-sized migrating satellites and smaller satellites in the inner region of the Saturnian disk. Our results suggest that in a giant impact with impact velocity > 1.4 times the escape velocity and impact angle of ~45 degree, a smaller satellite is destroyed, forming multiple mid-sized satellites with a very wide diversity in satellite density (the rock fraction = 0-92 wt%). Our results of the relationship between the mass and rock fraction of the satellites resulting from giant impacts reproduce the observations of the Saturnian mid-sized satellites. Giant impacts also lead to internal melting of the formed mid-sized satellites, which would initiate strong tidal dissipation and geological activity, such as those observed on Enceladus today and Tethys in the past. Our findings also imply that giant impacts might have affected the fundamental physical property of the Saturnian mid-sized satellites as well as those of the terrestrial planets in the solar system and beyond., 18 pages, 3 figures, Planetary and Space Science, in press

The Cassini Imaging Science Subsystem (ISS) acquired 370 high-resolution images ( http://ciclops.org/maps 〉 and the Planetary Data System 〈 http://pds.jpl.nasa.gov 〉. This atlas completes the series of the atlases of the Saturnian medium-sized satellites Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus.

The Cassini imaging science subsystem (ISS) acquired 449 high-resolution images ( http://ciclops.org/maps ].

The Cassini Imaging Science Subsystem (ISS) acquired 377 high-resolution images ( http://ciclops.org/maps ).

Images of the icy Saturnian satellites Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus, and Phoebe, derived by the Voyager and Cassini cameras are used to produce new local high-resolution image mosaics as well as global mosaics [http://ciclops.org, http://photojournal.jpl.nasa.gov]. These global mosaics are valuable both for scientific interpretation and for the planning of future flybys later in the ongoing Cassini orbital tour. Furthermore, these global mosaics can be extended to standard cartographic products.

During its 2005 January opposition, the saturnian system could be viewed at an unusually low phase angle. We surveyed a subset of Saturn's irregular satellites to obtain their true opposition magnitudes, or nearly so, down to phase angle values of 0.01 deg. Combining our data taken at the Palomar 200-inch and Cerro Tololo Inter-American Observatory's 4-m Blanco telescope with those in the literature, we present the first phase curves for nearly half the irregular satellites originally reported by Gladman et al. [2001. Nature 412, 163-166], including Paaliaq (SXX), Siarnaq (SXXIX), Tarvos (SXXI), Ijiraq (SXXII), Albiorix (SXVI), and additionally Phoebe's narrowest angle brightness measured to date. We find centaur-like steepness in the phase curves or opposition surges in most cases with the notable exception of three, Albiorix and Tarvos, which are suspected to be of similar origin based on dynamical arguments, and Siarnaq.During its 2005 January opposition, the saturnian system could be viewed at an unusually low phase angle. We surveyed a subset of Saturn's irregular satellites to obtain their true opposition magnitudes, or nearly so, down to phase angle values of 0.01 deg. Combining our data taken at the Palomar 200-inch and Cerro Tololo Inter-American Observatory's 4-m Blanco telescope with those in the literature, we present the first phase curves for nearly half the irregular satellites originally reported by Gladman et al. [2001. Nature 412, 163-166], including Paaliaq (SXX), Siarnaq (SXXIX), Tarvos (SXXI), Ijiraq (SXXII), Albiorix (SXVI), and additionally Phoebe's narrowest angle brightness measured to date. We find centaur-like steepness in the phase curves or opposition surges in most cases with the notable exception of three, Albiorix and Tarvos, which are suspected to be of similar origin based on dynamical arguments, and Siarnaq.

The Cassin/Huygens Project is a joint undertaking between NASA, the European Space Agency, and the Italian Space Agency to conduct an in-depth exploration of the Saturnian system. The spacecraft consists of an orbiter vehicle and an atmospheric probe which has completed its mission in the atmosphere and on the surface of Titan, the largest moon of Saturn. The spacecraft was launched on October 15, 1997, has completed its nearly seven years of interplanetary flight, and by the time of the 56th IAC, it will have completed 17 of its planned 75 orbits during its four-year prime orbital mission. This paper gives an overview of the mission, and describes in detail the accomplishments and events over the past year, including the spectacularly successful descent of the .European Space Agency's Huygens probe to the surface of Titan. Initial scientific results from both the Huygens mission as well as from the first one-and-a-quarter years of orbiting Saturn are summarized. The plans for the remainder of the orbiter's tour of the Saturn system and the many flybys of Titan and the smaller icy satellites are described.

The Cassini/Huygens spacecraft arrived in July 2004 at the planet Saturn and landed on 14. January 2005 the atmospheric descent probe Huygens successfully on the surface of Titan, Saturn's largest moon. This paper reviews the related technical challenges and the solutions with emphasis on control engineering aspects. Addressed is the design of a trajectory to bring a spacecraft of almost 6 t by gravity assisted fly-bys to Saturn, despite this is beyond the direct capacity of available launchers. Remote failure diagnosis is highlighted at the example of the telecommunication link problem detected at the distance of Jupiter. In the course of the 10 years long development process different approaches to the descent control system had been discussed for the Huygens probe to enable a safe landing on Titan, satisfying given schedule constraints. These approaches will be compared and discussed in relation to the finally realized solution.

We have reanalyzed the Voyager 1 UVS solar occultations by Titan to expand upon previous analyses and to resolve inconsistencies that have been noted in the scientific literature. To do so, we have developed a detailed model of the UVS detector and improved both the data reduction methods and retrieval techniques. In comparison to the values previously determined by Smith et al. (1982, J. Geophys. Res. 87, 1351–1359) we find N2 densities that are 25–60% higher, CH4 densities that are smaller by a factor of 3–7, and C2H2 densities that are roughly two orders of magnitude smaller. Our values for the thermospheric temperature are 153–158 K, which are approximately 20–40 K colder than previous estimates. We also report the first-ever determination from Voyager UVS data of density profile information for C2H4, HCN, and HC3N. Finally, we present a simple engineering model that is consistent with our new results in the upper atmosphere and merges smoothly with the model of Yelle et al. (1997, in: HUYGENS Science, Payload and Mission, in: ESA SP, vol. 1177, pp. 243–256) in the lower atmosphere. Our results provide improved constraints for photochemical models and offer scientists a better understanding of Titan's upper atmosphere as we head into the Cassini era in the exploration of the saturnian system.

In the framework of the Cassini-Huygens mission towards the Saturnian system, the Huygens Atmospheric Structure Instrument (HASI) is a multisensor package which has been designed to measure the physical quantities characterizing the atmosphere of Titan, Saturn's largest satellite. HASI sensors are devoted to the study of Titan's atmospheric structure and electric properties, and also to provide information on the surface, whatever its phase, solid or liquid. A description of the experiment, its subsystems and sensors package is reported together with their performances.

I report preliminary measurements of the complex permittivity of frozen aqueous ammonia solutions at liquid nitrogen temperatures, representative of those in the saturnian system. The real part of the dielectric constant of 30% ammonia ice is around 4.5 at near-DC frequencies and at ∼1 MHz, compared with around 3.1 for pure water ice. The loss tangents of ammonia-rich ices seem somewhat (∼50%) higher than those for water ice, for which the few low-temperature experiments to date indicate values comparable with predictions by Thompson and Squyres (1990,Icarus86, 336–354) and Maetzler (1998, inSolar System Ices(B. Schmitt, C. DeBergh, and M. Festou, Eds.), pp. 241–257, Kluwer Academic, Dordrecht), but considerably higher than models by Chybaet al. (1998,Icarus, in press). Ammonia-rich ice may reconcile the radar and optical appearance of Titan's surface: the detectability of water–ammonia ice on Titan by the Cassini mission and the implications for Titan's origin and evolution are discussed.

An evolutionary model of the G Ring incorporating theoretical results from R. M. Canup and L. W. Esposito (1995,Icarus113, 331–352) yields a complete particle size distribution that is consistent with existing observations. Results from numerical modeling demonstrate that a G Ring origin from the disruption of a 1.5–3 km progenitor satellite can match all known properties of the ring. In addition, we estimate the population of unseen macroscopic material from both observational upper limits and our theoretical model for the region surrounding the G Ring, where the Cassini spacecraft will likely make its innermost passes in the saturnian system. For models that fit all available data, the probability of Cassini striking a hazardous ring particle is less than 1%.

In October 1997 a unique mission to the Saturnian system will be launched by NASA, the CASSINI mission. One goal of this mission is to study the Saturnian dust environment, and for this task, the COSMIC DUST ANALYZER (CDA) has been developed and is currently under test. Impact ionization is used to determine the speed (1 – 100 km/s) and the mass (1×10 −15 – 1×10 −9 g) of impinging particles. furthermore, the electric charge (1×10 −15 – 1×10 −12 C) of the particles can be measured via the induction principle, whilst an integrated time-of-flight mass spectrometer will analyze the chemical composition of the dust. In order to achieve sufficient sensitivity for dust fluxes as low as 1 particle/month, the sensor has a large sensitive area of 0.1 m 2 . This paper will describe the measurement signals of the engineering model of the CDA and their interpretation.

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.

The Cassini mission to Saturn employs a Saturn orbiter and a Titan probe to conduct an intensive investigation of the Saturnian system. The Cassini orbiter flies series of orbits incorporating flybys of the Saturnian satellites called the “satellite tour”. During the tour, the gravitational fields of the satellites are used to modify and control the orbit, targeting from one satellite flyby to the next. The tour trajectory must also be designed to maximize opportunites for science observations, subject to mission-imposed constraints. Tour design studies have been conducted for Cassini to identify trades and strategies for achieving these sometimes conflicting goals. Concepts, strategies, and techniques previously developed for the Galileo mission to Jupiter have been modified, and new ones have been developed, to meet the requirements of the Cassini mission.

The Cassini mission to Saturn will start a second phase in the exploration of the Saturnian system, after the historical Voyager flybys of Saturn in 1980 and 1981. The Cassini primary mission is scheduled to be launched in October 1997 by the Titan IV/Centaur. Cassini uses four planetary gravity-assist flybys to gain the energy necessary to reach Saturn in July 2004. This arrival date at Saturn provides a unique opportunity for a flyby of Saturn's outer satellite Phoebe on the final approach. This paper provides an overview of the processes involved in the interplanetary trajectory design and analysis of the Cassini mission to Saturn.

The ESA Huygens probe is part of the NASA/ESA Cassini mission to the Saturnian system, and will make a 2–2.5 h exploratory descent to the surface of Titan. Huygens is designed to enter and brake in Titan's atmosphere in order to parachute a fully instrumented robotic laboratory. The main scientific measurements will be performed during the parachute descent, but some will also be performed during atmospheric entry; in addition, the probe resources are sized to allow measurements after it has impacted Titan's surface for at least a few minutes. This paper describes the control design and optimisation of the Huygens mission, based on an open-loop concept which is one of the keys to minimizing the mission risks induced by the uncertainties in the external environment.

If the solar radiation pressure effect is taken into consideration, the structure of the atomic hydrogen cloud emitted from Titan would be significantly modified. Instead of a circular torus characterized by azimuthal symmetry and with density peak near the orbit of Titan, the atomic hydrogen cloud will take on a pear-shaped structure with the long-lived H atoms distributed through the whole Saturnian system.

Considerable and rapid progress has recently been achieved in Titan science, as a result of Voyager data analyses and ground-based observations leading to more complete and precise modeling of the satellite's atmosphere. Valuable scientific information has thus been added to our previous knowledge of Titan. In particular, data sets unexploited up to our days were investigated, new models developed to account for these observations, theories extended and controversed or sustained, and measurements re-analysed. The aim of this paper is to give a brief overview of our current understanding of the satellite's nature and composition. In the interest that has risen in view of future space missions to the saturnian system, such a synthesis could be useful in better making apparent future assignements. After a brief survey of the pre-Voyager rather rudimentary view of Titan's atmosphere and of the post-encounter first-years period —which submerged the scientific community with large amounts of data-, this study will concentrate on the Titan science developments that have occurred in the past five years or so.

The aim of the CASSINI mission is the exploration of the Saturnian system. The spacecraft is composed of an Orbiter and a Probe. The Probe is planned to land on Titan, Saturn's largest moon. While the Probe descends through Titan's atmosphere the Orbiter flying by serves as a relay for the transmission of scientific data towards Earth. Due to the large Earth-Saturn distance, ground control of the Probe's descent is impossible. Further, the knowledge about Titan's atmosphere still includes major uncertainties, preventing an accurate prediction of the planetary entry characteristics and of the subsequent descent via parachute. The descent profile has to provide appropriate conditions for the operation of the scientific instruments and to guarantee the transmission of all gathered scientific data. Thus, according to in-situ measurements adaptive control actions, related to the timing of parachute deployment and parachute exchange, have to be initiated. By these means it seems possible to satisfy the requirements despite uncertainties.