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International audience; As a result of the 13-year exploration of the Saturn system by the Cassini-Huygens mission (2004–17), there has been a revolution in our understanding of Titan, Saturn's largest satellite. Titan's atmosphere shows the most complex chemistry in the solar system. A chain of chemical reactions is being initiated in the upper atmosphere by ionization and dissociation of the major species, nitrogen and methane, primarily by solar UV photons. The produced photochemical molecules and aerosols have a strong impact on the radiative budget of Titan's atmosphere, and hence on its temperature profile. Furthermore, transport by global dynamics, which reverses each half Titan year, greatly affects the distribution of these compounds. These complex couplings between chemistry, radiation, and dynamics make Titan's atmosphere an ideal laboratory to understand physical and chemical processes at play in atmospheres in general, particularly those showing the presence of photochemical haze such as Pluto or the increasing number of detected exoplanets showing hazy atmospheres. Finally, the study of Titan's chemistry has astrobiological implications as it naturally produces complex organic nitrogen species that settle to the surface to produce an organic soil. This material is postulated to be exposed to aqueous melt pools produced through meteoritic impact or cryovolcanism, or even to be transported to an underground ocean of liquid water. Therefore, we may entertain the idea that, in certain regions at the (sub-)surface of Titan, the results of chemistry leading to life as we know it may be present.

1 Introduction: Titan viewed from Earth The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man . - George Bernard Shaw The exploration of the solar system has become an important part of the glory and dreams of our civilization. The study of the Saturnian system and Titan itself can be traced back more than 400 years to when Galileo first pointed his telescope to the night sky to seek the mysteries of the universe. The Cassini-Huygens mission represents the first time in human history that a spacecraft was sent to spend time in the Saturnian system for close-up observations, bringing humanity to the surface of one of Saturn's satellites. We believe that this extraordinary achievement will be remembered as a major milestone in planetary exploration 400 years from now. More than that, it is also the first planetary science project of truly global scale, with scientists from three continents joining the effort. How did it come about? To begin, let us first summarize what we knew about Titan before the birth of the Cassini-Huygens mission. Comprehensive descriptions of Titan studies during the period from its discovery in 1655 to the landing of the Huygens probe in 2001 have been published by Fortes (1997) and Coustenis et al. (2009). Here we simply present a few highlights in chronological order.

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.

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

著者人数：156名, Accepted: 2008-05-27, 資料番号: SA1000998000

The Cassini-Huygens arrival into the Saturnian system brought a large amount of data about the satellites and rings. Two diffuse rings were found in the region between the A ring and Prometheus. R/2004 S1 is coorbital to Atlas and R/2004 S2 is close to Prometheus. In this work we analysed the closest approach between Prometheus and both rings. As a result we found that the satellite removes particles from R/2004 S2 ring. Long-term numerical simulations showed that some particles can cross the F ring region . The well known region of the F ring, where small satellites are present and particles are being taking from the ring, gains a new insight with the presence of particles from R/2004 S2 ring. The computation of the Lyapunov Characteristic Exponent reveled that the R/2004 S2 ring lies in a chaotic region while R/2004 S1 ring and Atlas are in a stable region. Atlas is responsible for the formation of three regimes in the R/2004 S1 ring, as expected for a satellite embedded in a ring.

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.

The Cassini mission to Saturn employs a Saturn orbiter and a Titan probe to conduct an intensive investigation of the Saturnian system. The orbiter flies a series of orbits, incorporating flybys of the Saturnian satellites, called the ‘satellite tour.’ During the tour, the gravitational fields of the satellites (mainly Titan) 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 opportunities for a diverse set of science observations, subject to mission-imposed constraints. Tour design studies have been conducted for Cassini over a period of several years 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. A sample tour is presented illustrating the application of tour design strategies developed for Cassini.

International audience; Fundamental questions involving the origin, evolution, and history of both Titan and the broader Saturnian system can be answered by exploring this satellite from an orbiter and also in situ. We present the science case for an exploration of Titan and one of its lakes from a dedicated orbiter and a lake probe. Observations from an orbit-platform can improve our understanding of Titan׳s geological processes, surface composition and atmospheric properties. Further, combined measurements of the gravity field, rotational dynamics and electromagnetic field can expand our understanding of the interior and evolution of Titan. An in situ exploration of Titan׳s lakes provides an unprecedented opportunity to understand the hydrocarbon cycle, investigate a natural laboratory for prebiotic chemistry and habitability potential, and study meteorological and marine processes in an exotic environment. We briefly discuss possible mission scenarios for a future exploration of Titan with an orbiter and a lake probe. •We present the science case for Titan׳s exploration.•We present mission scenarios for a future exploration of Titan.•This paper is based on a white paper presented for 2013 ESA׳s Call for Large missions.

The Cassini mission has been in orbit for eight years, returning a wealth of scientific data from Titan and the Saturnian system. The mission, a cooperative undertaking between NASA, ESA and ASI, is currently in its second extension of the prime mission. The Cassini Solstice Mission (CSM) extends the mission's lifetime until Saturn's northern summer solstice in 2017. The Titan Orbital Science Team (TOST) has the task of integrating the science observations for all 56 targeted Titan flybys in the CSM. In order to balance Titan science across the entire set of flybys during the CSM, to optimize and influence the Titan flyby altitudes, and to decrease the future workload, TOST went through a "jumpstart" process before the start of the CSM. The "jumpstart" produced Master Timelines for each flyby, identifying prime science observations and allocating control of the spacecraft attitude to specific instrument teams. Three years after completing this long-range plan, TOST now faces a new challenge: incorporating changes into the Titan Science Plan without undoing the balance achieved during the jumpstart.

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.

Spectra from Cassini's Visual and Infrared Mapping Spectrometer reveal the presence of a vast tropospheric cloud on Titan at latitudes 51 degrees to 68 degrees north and all longitudes observed (10 degrees to 190 degrees west). The derived characteristics indicate that this cloud is composed of ethane and forms as a result of stratospheric subsidence and the particularly cool conditions near the moon's north pole. Preferential condensation of ethane, perhaps as ice, at Titan's poles during the winters may partially explain the lack of liquid ethane oceans on Titan's surface at middle and lower latitudes.

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.

[1] Dust grains originating from the Edgeworth-Kuiper Belt (EKB) are believed to be the dominant species of dust in the outer solar system. These grains, evolving inward from the EKB under the influence of a variety of forces, will encounter the giant planets or their ring and moon systems. At Saturn, this influx drives several physical processes including the generation of tenuous dusty exospheres and rings, the spatial and compositional evolution of Saturn's main planetary ring system, and the generation of ionospheric and neutral gas layers in the atmospheres of Saturn and Titan. Recent comparisons between in-situ dust density measurements in the outer solar system and a dynamical dust grain tracing model have placed experimental limits on the mass production rate and power-law exponent of EKB-generated grains. Using this model and the experimental constraints, we make predictions for the influx of micron-sized, EKB-generated grains into the saturnian system, where the Cosmic Dust Analyzer onboard the Cassini mission is currently making measurements of both endogenous and exogenous dust populations.

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.

This chapter sets the scene for the current investigation of Titan with Cassini—Huygens, by reviewing the steps that took us there, from the first glimpses through a small telescope to the satellite observations passing by the first hints of an atmosphere about a 100 years ago.

Based on the overwhelming success of the Cassini/Huygens 4-year tour of Saturn from July 2004 to June 2008, NASA Headquarters approved at least two years of extended mission for continued study of the target-rich Saturnian system. After a rigorous phase of science objective definition and trajectory design and analysis, the Cassini project initiated an efficient, scientifically intense and operationally challenging mission phase, including 60 orbits around Saturn, 26 close Titan flybys, and 10 close icy satellite flybys — including seven more flybys of Enceladus. At the conclusion of the 2-year extended mission, substantial operating margins should be present with some fascinating options for further extensions

The sizes of the Saturnian icy satellites range from ~ 1;500 km in diameter (Rhea) to ~20km (Calypso), and even smaller ‘rocks’ of only a kilometer in diameter are common in the system. All these bodies exhibit remarkable, unique features and unexpected diversity. In this chapter, we will mostly focus on the ‘medium-sized icy objects’ Mimas, Tethys, Dione, Rhea, Iapetus, Phoebe and Hyperion, and consider small objects only where appropriate, whereas Titan and Enceladus will be described in separate chapters. Mimas and Tethys show impact craters caused by bodies that were almost large enough to break them apart. Iapetus is unique in the Saturnian system because of its extreme global brightness dichotomy. Tectonic activity varies widely — from inactive Mimas through extensional terrains on Rhea and Dione to the current cryovolcanic eruptions on Enceladus — and is not necessarily correlated with predicted tidal stresses. Likely sources of stress include impacts, despinning, reorientation and volume changes. Accretion of dark material originating from outside the Saturnian system may explain the surface contamination that prevails in the whole satellite system, while coating by Saturn's E-ring particles brightens the inner satellites.

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.

After the forty-fifth flyby of Titan, the Cassini spacecraft has successfully completed the planned four-year prime mission tour of the Saturnian system. This paper reports on the orbit determination performance of the Cassini spacecraft over two years spanning 2006 - 2008. In this time span, Cassini's orbit progressed through the magnetotail and pi-transfer phases of the mission. Thirty-four accurate close encounters of Titan, one close flyby of Iapetus and one 50 km flyby of Enceladus were performed during this period. The Iapetus and Enceladus flybys were especially challenging and so the orbit determination supporting these encounters will be discussed in more detail. This paper will show that in most cases orbit determination has exceeded the navigation requirements for targeting flybys and predicting science instrument pointing during these encounters.

The Cassini Mission to Saturn, a joint undertaking by NASA, the European Space Agency, and the Italian Space Agency, is now at about the mid-point of its planned four-year tour of the Saturnian system. Close encounters with Titan and the smaller icy satellites are occurring on schedule, and observations of these bodies along with Saturn's atmosphere, its rings, and magnetosphere are being made and scientific data returned on a daily basis. The icy satellites are turning out to be more distinct and unique than had been anticipated, the dynamics of the ring particles have been shown to be even more extensive than expected, and Titan's atmosphere had surprises for both the scientists as well as the engineers. A denser than expected upper atmosphere of Titan has led to the requirement to modify the planned orbital tour about Saturn to increase the flyby altitudes for about 20 of the Titan flybys to values above the originally planned 950 km level in order to avoid the risk of atmospherically induced torques causing the spacecraft to be unable to maintain its commanded attitude. Navigation of the spacecraft on the tour continues to go exceptionally well, with optical navigation enabling delivery accuracies to some of the satellites down to the 1 or 2 km level, and propellant consumption is running slightly below budget. Science observation sequence development and execution have been highly successful in acquiring virtually all of the originally designed observations. This paper summarizes the supporting engineering activities over the past year in support of operating the mission, as well as provides a high-level summary of the scientific results of the Cassini mission to date.

International audience; In the context of the Cassini/Huygens mission, we performed supporting ground-based observations to complement the results from the NASA/ESA/ASI space mission to the Saturnian system with particular focus on Titan. On the nights of 18 and 19 December 2004, we conducted adaptive optics observations with VLT/NACO to search for and map the distribution of CO2 ice deposits on the spatially resolved surface of Titan (65 mas resolution). This experiment became possible because (1) solid CO2 possesses two narrow and strong absorption lines at 2012 nm and 2070 nm that fall into the 2.05 mum window of Titan's atmosphere and (2) the width of these bands matches the band pass of the Fabry-Perot instrument installed in NACO. We do not detect this chemical compound, but we can derive firm limits on the abundance of CO2 ice on the surface of Titan at sub-Earth longitudes 284°W and 307°W. With a spatial sampling of 65 mas, we conclude that a partial surface coverage of segregated CO2 ice does not exceed 7% or 14% for bright or dark surface regions, respectively.

The Cassini-Huygens Mission consists of a large interplanetary spacecraft that has traveled about 3.5 billion kilometers (2.2 billion miles) during the cruise phase of its mission to Saturn. It is an international cooperative effort involving NASA, the European Space Agency, the Italian Space Agency (Agenzia Spaziale Italiana) as well as several separate European academic and industrial contributors. The Jet Propulsion Laboratory (JPL) manages the U.S. portion of the mission for NASA. The primary objectives of the mission include the ongoing survey the complex Saturnian system and the successful release and relay of data from the ESA-Huygens probe at Titan. The success of the Cassini Mission has been largely due its many simulation test beds and its rigorous test program. The Integrated Test Laboratory (ITL) at the JPL is an integral part of the Cassini Program. It provides hardware in the loop simulation for both the Attitude Articulation and Control Subsystem (AACS) and the Command Data Subsystem (CDS). This paper describes the methodology used to verify and validate the Saturn Orbit Insertion (SOI) Critical Sequence in the Cassini ITL. It details the global metric approach of using deterministic maneuver execution error to quantify the success of SOI critical sequence tests.

International audience; The Cassini visual and infrared mapping spectrometer (VIMS) investigation is a multidisciplinary study of the Saturnian system. Visual and near-infrared imaging spectroscopy and high-speed spectrophotometry are the observational techniques. The scope of the investigation includes the rings, the surfaces of the icy satellites and Titan, and the atmospheres of Saturn and Titan. In this paper, we will elucidate the major scientific and measurement goals of the investigation, the major characteristics of the Cassini VIMS instrument, the instrument calibration, and operation, and the results of the recent Cassini flybys of Venus and the Earth Moon system.

The Cassini orbital mission is to explore the Saturnian system are much greater depth than was possible by the Voyager flyby missions. To do this, the Cassini spacecraft is comprised of a Saturn orbiter and a Titan probe. The Command and Data Subsystem (CDS) on Cassini is responsible for uplink command processing, spacecraft intercommunications and control, and downlink telemetry formatting. The 10.7 year mission life, 160 minute round-trip light time, and extended periods of operation without continuous ground communications drive the CDS design in directions of redundancy, autonomy, and fault protection to assure the mission objectives. >

Galileo Galilei left a rich scientific legacy that enabled Cassini, Huygens, and other scientists to continue to open the modern era of planetary exploration. The legacy relationship also applies to the spacecraft and missions named after these men. The fundamental philosophy of the Cassini/Huygens mission and its investigative approach were adopted from the Galileo mission. The Cassini spacecraft even uses spare hardware that the Galileo Project did not use. Thus the parallels between the two missions run very deep. This thesis is reinforced in the following description of the Cassini and Huygens missions. Many things are seen to resemble their counterparts on the Galileo mission at Jupiter.

Cassini is the name given to an interplanetary spacecraft and also to the mission which it is designed to fly. The Cassini spacecraft carries a smaller spacecraft, the Huygens Titan atmospheric probe, which will go to Titan, Saturn's largest moon. Scheduled for launch in October 1997, Cassini will take seven years to reach Saturn. In November of 2004, about six months after the Cassini spacecraft begins its first orbit about Saturn, Huygens will be dropped into Titan's atmosphere. The Huygens probe will descend slowly, by parachute, through Titan's thick, obscuring atmosphere. On the way down it will observe and measure the atmosphere and finally the surface. With the completion of Huygens' mission, the Cassini spacecraft will then embark upon an orbital tour of the Saturnian system, including an extended series of flybys of Titan, and close encounters with several of the smaller, icy satellites. Some instruments will observe by remote sensing; others will make in situ measurements. All of the elements of the systems (rings, magnetosphere, satellites, and Saturn itself) will be studied.

A system has been built at the University of Kent at Canterbury to calibrate devices that will be carried on the Huygens probe to the surface of Titan as part of the Cassini mission to the Saturnian system. This system can simulate Titan's atmosphere to an effective altitude of around 30 km, and it can condense, store, and sample quantities of liquid hydrocarbons. The tested devices form the Huygen's surface science package (SSP) which measures a wide variety of parameters, making it a critical tool for exploring Titan's troposphere and surface. Preliminary results from the calibration of SSP instruments are presented and an assessment is made of the accuracy with which a medium's composition can be determined by sensors acting alone, and in concert with other SSP instruments.

Cassini/Huygens is a joint project of NASA and the European Space Agency designed to explore the Saturnian system in depth during its four-year mission. Cassini, the orbiter spacecraft, will carry twelve hardware investigations while Huygens, the Titan atmospheric probe, will carry an additional six. The Cassini Plasma Spectrometer (CAPS), one of 12 orbiter investigations, includes 3 plasma sensors designed to cover the broadest possible range of plasma energy, composition, and temporal variation. It is conservatively estimated that CAPS will provide a factor of ten or more improvement in measurement capabilities over those of the Voyager spacecraft at Saturn.

The Cassini spacecraft will perform a detailed examination of the Saturnian system, including the release of a probe to study Saturn's largest satellite, Titan. The star tracker for the Cassini mission must provide accurate data during the entire flight including four years of measurement in a harsh radiation environment.

On the occasion of the fifth anniversary of the European Space Agency's Huygens probe landing on Saturn's moon Titan (14 January 2005), approximately a hundred international scientists and engineers met at the CosmoCaixa Science Museum, in Spain, to assess what was discovered and what remains to be done by future spacecraft missions to Titan and the Saturnian system. The Huygens probe was deployed from NASA's Cassini spacecraft. Speakers at the meeting pointed out that discoveries from the Cassini-Huygens mission have shown that Titan is much more interesting than even their wildest expectations more than 20 years ago when the mission was conceived, in the 1980s, several years after the Voyager flybys. Perhaps most stunning are the many similarities between Titan and the Earth. Though there are important differences, when one views the lakes and seas, the fluvial terrain, the sand dunes, and other features through the hazy, nitrogen atmosphere, it calls to mind the geological processes that created analogous features on the Earth. On Titan it has been discovered that frozen water plays the geological role of rock, and liquid methane takes the role of terrestrial water. As Saturn proceeds along its orbit about the Sun, the seasons change on Titan. The seasonal effects on the transport of methane are starting to be seen. A large lake in the south polar region seems to be getting smaller as winter sets in.

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.

As Voyager 1 sailed through Saturn’s system of moons and rings last November 1980 it revealed new worlds not seen by man before. For centuries, since Galileo’s first telescopic observations in 1610, the satellites of Saturn had been no more than pin points of light, whilst the structure of the rings was barely resolved beyond 3 principal bands. Yet, within the space of a few hours, that picture changed dramatically as the images of these objects grew through Voyager’s cameras from mere specks into full and wondrous worlds. These pictures contained features that were not only intricate and astonishing in detail but which were, in many cases, unfamiliar and unexpected. A composite view of the Saturnian system as seen by Voyager 1 appears in Figure 1. Saturn’s rings, once thought to be broad belts of particles spread uniformly thin through billions of years of evolution and interparticle collisions, were found to be divided into hundreds of individual ringlets (Figure 2). And Cassini’s Division, a region which had been previously thought to be empty because of a ‘sweeping’ influence of Mimas, was found to contain many ringlets itself! The appearance of light and dark radial spokes in the B ring, which rotated with a velocity contrary to the law expected of Keplerian orbits, was baffling. And the F ring (Figure 3) was found to contain knots, kinks and braids which probably indicated the presence of electro-magnetic forces as well as gravitational forces (Smith et al. 1981).

An overview of the Voyager 2 encounter with Saturn is presented, including a brief discussion of the trajectory, the planned observations, and highlights of the results described in the subsequent reports.

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.

A theory concerning the orbital spacing patterns in the solar system, more intriguing than many of the traditional topics to our general studies astronomy students and useful pedagogically since it is accessible to such students in quantitative as well as qualitative terms, is presented. The orbital distributions of the planets and major satellite systems are fitted with a Bode‐type form. The Saturnian system requires vacant orbitals if the best fit is to have an accuracy comparable to that obtained for the other systems. One pattern of vacant orbitals is shown to be consistent with what would be expected if Titan produced resonant perturbations on the forming satellite system.

An overview of the Voyager 1 encounter with Saturn is presented, including a brief discussion of the flight, trajectory, science plan formulation, and highlights of the results described in the subsequent reports.

Since the Voyager 1 close-pass observations of Titan, scientific interest in the nature and composition of the atmosphere and surface of Titan has increased greatly. This enhanced interest has led to the study of mission options to the Saturnian system with a principal goal of depositing a probe into the atmosphere of Titan. Such a probe could carry instrumentation to perform in situ measurements of the structure and composition of the atmosphere and clouds and of the morphology and composition of the surface. Previous studies have examined the possibility of modifying, in a minimal way, the Galileo probe currently being prepared for an atmospheric sampling mission to Jupiter. The purpose of this paper is to report the results of a study of probe-system concepts optimized for the Titan mission. The scientific rationale for such a mission to Titan is discussed and instruments to accomplish the scientific goals are indicated. Probe systems compatible with those goals and instruments are described, and the associated development costs for each are discussed.

We searched for H2O 6(1,6)-5(2,3) maser emission at 22.235 GHz from several Saturnian satellites with the Nobeyama 45m radio telescope in May 2009. Observations were made for Titan, Hyperion, Enceladus and Atlas, for which Pogrebenko et al. (2009) had reported detections of water masers at 22.235 GHz, and in addition for Iapetus and other inner satellites. We detected no emission of the water maser line for all the satellites observed, although sensitivities of our observations were comparable or even better than those of Pogrebenko et al.. We infer that the water maser emission from the Saturnian system is extremely weak, or sporadic in nature. Monitoring over a long period and obtaining statistical results must be made for the further understanding of the water maser emission in the Saturnian system., Comment: 8 pages, 2 figures, accepted for publication in PASJ (Letter)

In July 2004 the Cassini-Huygens mission reached the Saturnian system and started its orbital tour. A total of 75 orbits will be carried out during the primary mission until August 2008. In these four years Cassini crosses the ring plane 150 times and spends approx. 400 h within Titan's orbit. The Cosmic Dust Analyser (CDA) onboard Cassini characterises the dust environment with its extended E ring and embedded moons. Here, we focus on the CDA results of the first year and we present the Dust Analyser (DA) data within Titan's orbit. This paper does investigate High Rate Detector data and dust composition measurements. The authors focus on the analysis of impact rates, which were strongly variable primarily due to changes of the spacecraft pointing. An overview is given about the ring plane crossings and the DA counter measurements. The DA dust impact rates are compared with the DA boresight configuration around all ring plane crossings between June 2004 and July 2005. Dust impacts were registered at altitudes as high as 100 000 km above the ring plane at distances from Saturn between 4 and 10 Saturn radii. In those regions the dust density of particles bigger than 0.5 mu m can reach values of 0.001 m(-3). (c) 2006 Published by Elsevier Ltd.