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A relativistic equation for the study of nuclear systems is proposed. The effects due to the isovector nature of the pion field are taken into account by an additional contribution to the Green's function.

We analyse the polarizability shifts in exotic atoms in the same theoretical framework that is commonly adopted to study the sum rules and the photon scattering amplitude. Both the orbiting particle and the nucleus are treated as fermion composite systems. The Coulomb polarizabilities are derived for all the multipole operators. The penetration contributions are briefly studied and the relativistic corrections, up to order 1/m2, are obtained in the dipole limit.

Q/V band frequencies are expected to play a key role in enabling challenging and unproven broadband communications missions for a wide range of applications. In the perspective to exploit these frequencies, several satellite missions have been proposed to explore the physics of the satellite channel. The strategic importance of probing higher and higher frequencies has been recognized by the Italian Space Agency through the ITALSAT mission that up to date has been the major contributor to the advancement of the Q/V band propagation knowledge. In 2004 ASI funded a feasibility study (phase A study), called TRANSPONDERS, Italian acronym for "research, analysis and study of Q/V payloads for telecommunications", whose main objectives were the improvement of the knowledge of propagation impairments at Q/V band and the evaluation of the effectiveness of PIMTs (Propagation Impairment Mitigation Techniques) in very high frequency band operation. In this paper, after an overview of the potential applications in Q/V band and the top level mission architecture relative to the selected applicative mission at these frequencies, the applicability of three selected PIMTs, such as site diversity, power control and Adaptive Coding and Modulation (ACM), in a Q/V band scenario is discussed and main challenges and key issues are identified.

Requirements and preliminary specifications to design a micro-satellite flower constellation, named FLORAD, of millimeter-wave (MMW) radiometers for atmospheric observations are here introduced. The FLORAD small-mission scientific objectives are aimed at the retrieval of thermo-dynamical and hydrological properties of the atmosphere. Radiometer specifications and inversion algorithms are briefly discussed, pointing out the trade-off between performances and complexity for low-cost low-weight micro-satellite platforms. In order to fulfill the goal of a short revisit time for meteorological monitoring and now casting purposes, a flower constellation (FC) of micro-satellites is proposed due to its design flexibility.

The development of a technology demonstration payload for experimental broadband communications at Q/V bands has been funded by the Italian Space Agency. This payload will be embarked on the Alphasat satellite for testing fixed broadband ground-satellite communication services. However, the interest in the further experimentation of aeronautical mobile satellite communication services has been recognized. Therefore, in this work the feasibility of an experiment for aeronautical-satellite broadband communications in Q-band is analyzed through the definition of a novel simulation method of an aeronautical satellite multipath channel. Simulation results regarding the performance of DVB-S2 communication links demonstrate the feasibility of the experiment for the provision of broadband communication services.

Flower constellations are a particular set of satellite constellations where every satellite covers the same repeating space track. When the flower constellations are visualized on an Earth centred earth fixed reference frame, the relative orbits shows flower-shaped figures centered on the Earth. In this paper the shape and the position of a particular flower constellations has been designed for the provision of telemedicine services. Once that performance metrics of the constellation have been defined and the service targets have been identified, the performance of the flower constellation have been compared with the well known polar and Walker constellations. The particular properties of the flower constellations allow an optimized coverage of a list of targets. It was found that the flower constellations provide better performance in terms access availability and mean access time.

The spectrum of the nonstrange baryons in the constituent quark model is critically analysed. The electromagnetic properties of the nucleon are calculated with particular emphasis on the electric and magnetic polarizabilities. Relativistic corrections up to orderm−2 are consistently considered.

Flower Constellations are a particular set of satellite constellations where every satellite covers the same repeating space track. When the Flower Constellations are visualized on an Earth centered Earth fixed reference frame, the relative orbits show flower-shaped figures centered on the Earth. This innovative type of constellation presents features useful to be used in several applications, such as telecommunications, navigation, Earth science and interferometric radar. Several missions are foreseen to explore Mars in the next years to collect data in order to enhance our knowledge of the red planet. This effort requires the development of a reliable orbital infrastructure to support telecommunications with orbiters, landers and rovers. In this paper, a novel telecommunication architecture is presented, based on the previously introduced Flower Constellations. We designed an optimized Flower Constellation for the coverage of sites/regions of interest of the Mars surface. We proved that our proposed constellation provides better performance with respect to a reference constellation called 4retro 111 in terms of access duration and average gap time.

The initial exploration of any planetary object requires a careful mission design guided by our knowledge of that object as gained by terrestrial observers. This process is very evident in the development of the Dawn mission to the minor planets 1 Ceres and 4 Vesta. This mission was designed to verify the basaltic nature of Vesta inferred both from its reflectance spectrum and from the composition of the howardite, eucrite and diogenite meteorites believed to have originated on Vesta. Hubble Space Telescope observations have determined Vesta’s size and shape, which, together with masses inferred from gravitational perturbations, have provided estimates of its density. These investigations have enabled the Dawn team to choose the appropriate instrumentation and to design its orbital operations at Vesta. Until recently Ceres has remained more of an enigma. Adaptive-optics and HST observations now have provided data from which we can begin to confidently plan the mission. These observations reveal a rotationally symmetric body with little surface relief, an ultraviolet bright point that can be used as a control point for determining the pole and anchoring a geographic coordinate system. They also reveal albedo and color variations that provide tantalizing hints of surface processes.

Since ancient times explaining the motion of celestial bodies has been a must and a challenge for scientists. Many people in history, from Ptolomaeus to Copernicus, from Brahe to Kepler, from Galileo to Newton up to Lagrange and Einstein, have investigated the motion of planets, giving birth to celestial mechanics. Orbital mechanics is based on the same laws applied to artificial satellites and is pervasively employed in space missions to design orbits according to specific requirements and to predict the movement of satellites over long periods. This has a significant importance in satellite constellations design where the presence of many satellites results in a growing complexity in terms of deployment, dynamics, reconfigurability and control, up to the disposal at the end of life. Today the theory on orbit dynamics is developed, but the problem is to predict the movement of many satellites, choosing the suitable propagator, considering all the perturbations, taking into account the station keeping manoeuvres, tracking and observability. In this paper, an overview of a new kind of orbit propagator is presented, together with an analysis of perturbation corrections using both impulsive and low-thrust manoeuvres (electric engine); some application examples will be presented as well. The paper will deal with the following issues: satellites constellation propagator (from 2 to n satellites), orbital perturbations, Sun visibility, Earth coverage, inter satellites links, station keeping (impulsive and continuous firing).

This paper addresses the issue of energy efficiency for error control mechanisms over wireless personal area networks. The study of the trade-off between link layer performance and energy consumption for error control schemes based on ARQ leads to the definition of an optimum value of the SNR to be used by the inner loop power control in MC-SS-based air interface. In the classical implementation of the power control, the value of the SNR depends on performance metrics such as BER/FER. The proposed implementation is an example of explicit physical/link layer cross-layer optimization with top-down approach

The Advanced Moon micro-Imager Experiment (AMIE), on-board SMART-1, the first European mission to the Moon, is an imaging system with scientific, technical and public outreach objectives. The science objectives are to image the lunar South Pole, permanent shadow areas (ice deposit), eternal light (crater rims), ancient lunar non-mare volcanism, local spectrophotometry and physical state of the lunar surface, and to map high latitudes regions (south) mainly at far side (South Pole Aitken basin). The technical objectives are to perform a Laserlink experiment (detection of laser beam emitted by ESA/Tenerife ground station), flight demonstration of new technologies and on-board autonomy navigation. The public outreach and educational objectives are to promote planetary exploration and space. We present here the first results obtained during the cruise phase. � 2005 Published by Elsevier Ltd on behalf of COSPAR.

This paper deals with the study of low-cost orbital positioning for the overall class of small satellites with orbital constraints. The paper proposes an integrated approach for satellite subsystems design in order to provide a greater number of possibilities for low cost launch. Low cost launches can be generally achieved with piggyback or shared launch approaches. In both cases, the small satellite has to correct its orbit from the release to the final one. The correction of the orbit can be performed by using the multiple burns technique. The paper addresses the above issue and provides computation of the propellant mass needed for the orbital transfer.

Udgivelsesdato: DEC Thus far, high-altitude platform (HAP)-based systems have been mainly conceived as an alternative to satellites for complementing the terrestrial network. This article aims to show that HAP should no longer be seen as a competitor technology by investors of satellites, but as a key element for an efficient hybrid terrestrial-satellite communication system. Two integrated HAP-satellite scenarios are presented, in which the HAP is used to overcome some of the shortcomings of satellite- based communications. Moreover, it is shown that the integration of HAPs with satellite systems can be used to provide more efficient fleet-management and traffic-control services and more powerful data-relay systems.

In HAP systems, the on-board power is a limited resource and each HAP-based communication system should use it as effectively as possible. In this paper we address the issue of the energy efficiency of a HAP-based communication system at link layer. An analytical formulation is carried out in order to investigate the trade-off between performance and energy consumption of different error recovery mechanisms, namely, pure-ARQ and type II hybrid ARQ protocols. The results are then used to optimize physical layer parameters, such as the transmission power. A Multicarrier Code Division Multiple Access based air interface is considered. After the definitions of energy efficiency of link level error control schemes, the optimum value of the transmission power in terms of energy efficiency will be computed

In this paper, some HAP-satellite integrated architectures for the provision of broadband connectivity to homes are presented. In these architectures, the HAP is exploited as an intermediate node between the ground terminal and the satellite. By splitting the connection, several advantages can be achieved. In particular, the lower transmission delay between user and HAP renders more effective link layer error recovery mechanisms based on retransmissions (ARQ) to make the channel more reliable and to improve TCP/IP performance. Simulations of the described architecture have shown a gain in the order of 30% in terms of TCP throughput when the HAP is used to split the user terminal-satellite link.

In this work, several aspects on IP routing for LEO satellite networks are investigated. First, a proper downlink routing procedure is defined, which has the advantage to allow the identification of the egress satellite, without the need of information from GPS systems to determine the position of the destination mobile terminal. Furthermore, the performance of the routing algorithm that minimizes the number of hops are evaluated through simulations, for an Iridium-like and a Teldesic-like constellation in order to show the sensitivity of the routing algorithms to the availability of cross-seam ISLs. A modification of the "minimization of number of hops" algorithm is proposed, which allows to reduce the total path length, and hence, the end-to-end delay. The idea is to exploit as much as possible inter-plane ISLs at higher latitudes. The comparison between the two IP routing algorithms shows that the proposed algorithm can significantly reduce the end-to-end delay when the source interface and the destination interface are largely spaced in latitude and longitude.

By successively orbiting both 4 Vesta and I Ceres the Dawn mission directly addresses the long-standing goals of understanding the origin and evolution of the solar system. Ceres and Vesta are two complementary terrestrial protoplanets (one apparently "wet" and the other "dry"), whose accretion was probably terminated by the formation of Jupiter. They provide a bridge in our understanding between the rocky bodies of the inner solar system and the icy bodies of the outer solar system. Ceres appears to be undifferentiated while Vesta has experienced significant heating and likely differentiation. Both formed very early in the history of the solar system and while suffering many impacts have remained intact, thereby retaining a record of events and processes from the time of planet formation. Detailed study of the geophysics and geochemistry of these two bodies provides critical benchmarks for early solar system conditions and processes that shaped its subsequent evolution. Dawn provides the missing context for both primitive and evolved meteoritic data, thus playing a central role in understanding terrestrial planet formation and the evolution of the asteroid belt. Dawn is to be launched in May 2006 arriving at Vesta in 2010 and Ceres in 2014, stopping at each to make I I months of orbital measurements. The spacecraft uses solar electric propulsion, both in cruise and in orbit, to make most efficient use of its xenon propellant. The spacecraft carries a framing camera, visible and infrared mapping spectrometer, gamma ray/neutron spectrometer, magnetometer, and radio science. (C) 2003 Elsevier Ltd. All rights reserved.

IPSE is a micro-laboratory for Mars soil and environment analysis. It provides the capability to serve and handle scientific miniaturised instruments accommodated inside its envelope. The instruments have the goal to perform in situ study of the collected martian samples, thus quantitatively characterizing the mineralogy, the composition, the microphysical structure of the materials of the Martian soils down to the depth available to the sampling mechanism. Given the complex structure of the surface material it will be essential to perform in-situ science, both at the surface and at different depths. This is done in order to validate remote sensing observations through specific measurements, identify local characteristics of the selected landing areas, document sample collection both for in situ and sample return. IPSE is an example of a small and flexible lab, that can be integrated on different Landers and Megarovers. IPSE contains: • Scientific instruments • A small robotic arm - with five degrees of freedom - to provide samples to the IPSE instruments. • Power conditioning. • Electronics for system and thermal control, communications and instrument data handling. The Phase A report for all the IPSE instruments has been already provided to the Italian Space Agency. The experiments MAGO, IRMA, MA_FLUX are inherited from previous space qualified instruments and breadboards of them already exist. DOSE is a new experiment, however, a breadboard of the detector and of the photomultiplier is under development.

In-situ experiments on cometary dust on board GIOTTO and VEGA missions demonstrated that in comet P/Halley's refractory component it is possible to evidentiate a refractory organic phase, the so called CHON, and a Mg-rich silicate phase. The two phases were interspersed in the individual grains. Each of these phases has different average physical properties. Laboratory experiments demonstrated that the characteristics of a crust mainly composed of silicatic particles would be different from that of a crust composed by silicatic and organic particles: thermal conductivities, for example, differ by orders of magnitude. In this paper we want to show how the simulation, in our numerical model of cometary nucleus thermal evolution, of two different dust distributions with different and independent physical properties gives us a much better modeling of the complexity of real phenomena. We attributed to one of these distributions the characteristics, taken mainly from laboratory experiments, of silicatic/organic material, and to the other one the characteristics of purely silicatic material. We applied the model to improve our simulation of comet P/Wirtanen, the target of Rosetta mission. Local temperature and evolution of the crust are different depending on which of the two distribution dominates: the maximum temperature reached by a crust with a strong organic component is lower than that reached by a purely silicatic crust.

The visible infrared thermal imaging spectrometer (VIRTIS) is one of the principal payloads to be launched in 2003 on ESA's Rosetta spacecraft. Its primary scientific objective s are to map the surface of the comet Wirtanen, monitor its temperature, and identify the solids and gaseous species on the nucleus and in the coma. VIRTIS will also collet data on two asteroids, one of which has been identified as Mimistrobell. The data is collected remotely using a mapping spectrometer co-boresighted with a high spectral resolution spectrometer. The mapper consists of a Shafer telescope matched to an Offner grating spectrometer capable of gathering high spatial, medium spectral resolution image cubes in the 0.25 to 5 micrometers waveband. The high spectral resolution spectrometer uses an echelle grating and a cross dispersing prism to achieve resolving powers of 1200 to 300 in the 1.9 to 5 micrometers band. Both sub-systems are passively cooled to 130 K and use two Sterling cycle coolers to enable two HgCdTe detector arrays to operate at 70 K. The mapper also uses a silicon back-side illuminated detector array to cover the ultra-violet to near-infrared optical band.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The baseline model payload of Rosetta includes a visual infrared spectral and thermal mapper among the instruments on board the spacecraft orbiting around the comet. This instrument will allow the following scientific goals to be achieved: to characterise the nucleus surface composition (concentration of the ices, mineralogical composition of dust, characteristics of organic compounds); to determine the surface temperature distribution and the dust and gas distribution in the inner cometary coma; to map the asteroid's surface mineralogical composition. In order to achieve these scientific objectives the authors are projecting an instrument composed of two channels: an imaging spectrometer (VIRTIS-M, where M means Mapping mode) and a high resolution spectrometer (VIRTIS-H).

Water plays a key role in the evolution of terrestrial planets, and notably in the occurrence of Earth's oceans. However, the mechanism by which water has been incorporated into these bodies—including Earth—is still extensively debated. Here we report the detection of widespread 2.8 μm OH absorption bands on the surface of the asteroid Vesta by the VIR imaging spectrometer on board Dawn. These observations are surprising as Vesta is fully differentiated with a basaltic surface. The 2.8 μm OH absorption is distributed across Vesta's surface and shows areas enriched and depleted in hydrated materials. The uneven distribution of hydrated mineral phases is unexpected and indicates ancient processes that differ from those believed to be responsible for OH on other airless bodies, like the Moon. The origin of Vestan OH provides new insight into the delivery of hydrous materials in the main belt and may offer new scenarios on the delivery of hydrous minerals in the inner solar system, suggesting processes that may have played a role in the formation of terrestrial planets.

The Dawn spacecraft orbited Asteroid (4) Vesta for a year, and returned disk-resolved images and spectra covering visible and near-infrared wavelengths at scales as high as 20 m/pix. The visible geometric albedo of Vesta is ~ 0.36. The disk-integrated phase function of Vesta in the visible wavelengths derived from Dawn approach data, previous ground-based observations, and Rosetta OSIRIS observations is consistent with an IAU H-G phase law with H=3.2 mag and G=0.28. Hapke's modeling yields a disk-averaged single-scattering albedo of 0.50, an asymmetry factor of -0.25, and a roughness parameter of ~20 deg at 700 nm wavelength. Vesta's surface displays the largest albedo variations observed so far on asteroids, ranging from ~0.10 to ~0.76 in geometric albedo in the visible wavelengths. The phase function of Vesta displays obvious systematic variations with respect to wavelength, with steeper slopes within the 1- and 2-micron pyroxene bands, consistent with previous ground-based observations and laboratory measurement of HED meteorites showing deeper bands at higher phase angles. The relatively high albedo of Vesta suggests significant contribution of multiple scattering. The non-linear effect of multiple scattering and the possible systematic variations of phase function with albedo across the surface of Vesta may invalidate the traditional algorithm of applying photometric correction on airless planetary surfaces.

Main Belt comets represent a recently discovered class of objects. They are quite intriguing because, while having a Tisserand invariant value higher than 3, are showing cometary activity. We study the activity of the Main Belt comets making the assumption that they are icy-bodies and that the activity has been triggered by an impact. We determine the characteristics of this activity and if the nowadays impact rate in the Main Asteroid Belt is compatible with the hypothesis of an activity triggered by a recent impact. Due to the fact that the Main Belt comets can be considered as a kind of comets, we apply a thermal evolution model developed for icy bodies in order to simulate their activity. We also apply a model to derive the impact rate, with respect to the size of the impactor, in the Main Belt. We demonstrate that a stable activity can result from a recent impact, able to expose ice-rich layers, and that the impact rate in the Main Belt is compatible with this explanation., Comment: 9 pages, 7 figures

Aims. On October 24, 2007 the comet 17P/ Holmes underwent an outburst that was extraordinary in both duration and size. We attempted to determine the cause of the outburst by acquiring and analyzing high resolution spectra of the comet. Methods. On two nights, October 25 and December 2, we acquired four high resolution spectra in the visible range at the Telescopio Nazionale Galileo in La Palma. The oxygen lines intensity ratio was estimated for the observation performed on October 25, and computed for the observation performed on December 2. Results. Many emission lines were detected in the spectra, most of them attributable to C2 ,N H 2, CN and CH. The atomic O green line and one of the red doublet lines were detected in a spectrum acquired on October 25. All the three atomic O lines were detected in a spectrum acquired on December 2. The value obtained for the first night, 0.1 ± 0.11, implies that the H2O molecule was the main parent of the emission lines, while the value obtained for the second night, 0.3 ± 0.1, implies that the main parent was CO or CO2 instead than H2O. Conclusions. Our results imply that in the days around the outburst the water production was exceptionally high for that heliocentric distance. By December, however, the behavior of 17P/ Holmes had returned to that of a comet orbiting at 2.6 AU from the Sun, in which state the activity is probably sustained mostly by gases more volatile than water, such as CO and CO2.

A phenomenological model for the evaluation of the relativistic corrections and other nonstandard effects in the3He photodisintegration sum rules is presented.

After having considered the spectral and geometrical performances of the Rosetta/VIRTIS-M experiment, we complete here the analysis by evaluating quantitatively the flat-field and radiometric responses. The purpose of this work is to retrieve the flat-field matrix necessary to homogenize the focal plane response. Moreover, the most important result is the determination of the instrument transfer function that allows to convert digital numbers in physical units of spectral radiance (Wm−2μm−1sterad−1). The strategy adopted to organize measurement sequence, a basic description of the on-ground experimental setups and the analysis of the collected data, is included in this article. An analysis of the instrumental stability has been performed as well by examining how the internal calibration data are affected by environmental conditions. These data allow to evaluate the cumulative effects of thermal and vibrational stresses on the instrumental performances: up to now we have verified that this effect is neglig...

The complete characterization of complex imaging spectrometers, such as VIRTIS-M (visual infrared thermal imaging spectrometer) aboard the Rosetta mission, requires a detailed and prolonged activity starting with the instrument integration and continuing during the entire operational life of the experiment. In this article we report the main experimental activities realized during the on-ground characterizations to evaluate the spectral and geometric performances in order to check the conformance with the technical requirements derived from the scientific goals of the experiment. Spectral calibrations allow to confirm instrumental spectral range, resolution, and sampling; geometric calibrations are necessary to estimate the pixel and slit functions, field of view extension, and possible optical aberrations. Two separate sections are dedicated to each one of these subjects, including the strategy followed to prepare measurements, a basic description of the on-ground experimental setups, and the analysis of the collected data.

Inclusive coherent proton dissociation on helium has been measured for a missing mass near the threshold of one-pion production, in the | t | range 0.05–1.6 (GeV/ c ) 2 and at an alpha incident momentum of 7.0 GeV/ c . A strong t -slope dependence of the cross section at | t | ⩽ 0.2 (GeV/ c ) 2 is followed at larger | t | by a less structured distribution. Data were analyzed using the Glauber-Margolis formula and by assuming a peripheral elementary diffraction amplitude for the production process.

We study photon scattering polarizabilities including m −2 relativistic corrections. Particular emphasis is put on the significant cancellations obtained by extensive use of gauge conditions. Low-energy theorems are derived for both elastic and inelastic parity non-changing transitions.

We investigate the Poincare covariance of the Hamiltonian of aN-fermion system, in which the two-body interaction is produced by the exchange of a massless vector boson with a nonconstant form factor at vertices. The relativistic separation of center-of-mass and internal-motion variables is performed by exploiting an approach developed some years ago by Krajcik and Foldy. It is shown that, in order to ensure that theN-fermion Hamiltonian (expanded up to the order 1/c2) is the time-translation generator of the Poincare algebra, one requires a correct treatment of the form factor at the order considered, and, for noncommuting particle «charges», the inclusion in the potential of three-body terms, whose expression is explicitly derived. A preliminary analysis of the effect of such terms is given for baryons and deuterium in the framework of the constituent quark model.

A complete expansion up to order 1/m2 for one- and two-photon matrix elements is derived keeping also into account the behaviour of the wave functions under Lorentz transformations. Lowest-order binding-energy corrections are inserted in the verification of the low-energy theorem for Compton scattering. Gauge invariance is ensured by extensive use of the minimal substitution.

Vesta to the Core Vesta is one of the largest bodies in the main asteroid belt. Unlike most other asteroids, which are fragments of once larger bodies, Vesta is thought to have survived as a protoplanet since its formation at the beginning of the solar system (see the Perspective by Binzel , published online 20 September). Based on data obtained with the Gamma Ray and Neutron Detector aboard the Dawn spacecraft, Prettyman et al. (p. 242 , published online 20 September) show that Vesta's reputed volatile-poor regolith contains substantial amounts of hydrogen delivered by carbonaceous chondrite impactors. Observations of pitted terrain on Vesta obtained by Dawn's Framing Camera and analyzed by Denevi et al. (p. 246 , published online 20 September), provide evidence for degassing of volatiles and hence the presence of hydrated materials. Finally, paleomagnetic studies by Fu et al. (p. 238 ) on a meteorite originating from Vesta suggest that magnetic fields existed on the surface of the asteroid 3.7 billion years ago, supporting the past existence of a magnetic core dynamo.

This work compares different solutions using Extremely High Frequencies (EHF) for broadband satellite communications. Advantages and drawbacks of each solution are discussed accounting for the most important features needed to achieve very high capacity systems (i.e. multibeam capabilities, bandwidth, signal attenuations and impairments) and the most important features needed for commercial success (i.e. use cases, cost per bit, cost of the terminal).

The successful accomplishment of the ROSETTA mission to a cometary nucleus requires some preliminary knowledge of comet status and activity, such as surface temperature, percentage of active surface, intensity of gas and dust fluxes and so on. It would be impossible to answer these questions only on the basis of ground-based observations, so it is necessary to use a predictive model of the thermal evolution and differentiation of a cometary nucleus. The nucleus of the model built by the authors' group is a sphere, initially homogeneous, composed of a highly porous predefined mixture of ices of water (the dominant constituent), CO/sub 2/, CO and silicatic dust grains embedded in it. In this work the authors applied the model, with different choices of parameters, to a comet on the orbit of P/Wirtanen, the target of ROSETTA mission, with the aim of obtaining information necessary to the planning of the mission. The results of this simulation seem to indicate that: The range of expected comet surface temperatures is: at the equator-aphelion 130 K; at the equator-perihelion 210 K; at latitude 80/sup deg/-aphelion 90 K; at latitude 80/sup deg/-perihelion 160: K; The circumpolar regions are probably inactive and covered by a dusty crust, while the equatorial regions are probably characterized by active areas. The depth of the volatile-depleted ice layers varies from 5 m at the equator to 10 m at the pole.

From the current understanding we know that comet nuclei have heterogeneous compositions and complex structures. It is believed that cometary activity is the result of a combination of physical processes in the nucleus, like sublimation and recondensation of volatile ices, dust grains release, phase transition of water ice, depletion of the most volatile components in the outer layers and interior differentiation. The evolution of the comet depends on the sublimation of ices and the release of different gases and dust grains: the formation of a dust crust, the surface erosion and the development of the coma are related to the gas fluxes escaping from the nucleus. New observations, laboratory experiments and numerical simulations suggest that the gas and dust emissions are locally generated, in the so-called active regions. This localized activity is probably superimposed to the global nucleus activity. The differences between active and inactive regions can be attributed to differences in texture and refractory material content of the different areas. In this paper we present the results of numerical models of cometary nucleus evolution, developed in order to understand which are the processes leading to the formation of active and non-active regions on the cometary surface. The used numerical code solves the equations of heat transport and gas diffusion within a porous nucleus composed of different ices—such as water (the dominant constituent), CO 2 , CO- and of dust grains embedded in the ice matrix. By varying the set of physical parameters describing the initial properties of comet P/Wirtanen, the different behaviour of the icy and dusty areas can be followed. Comet P/Wirtanen is the target of the international ROSETTA mission, the cornerstone ESA mission to a cometary nucleus. The successful design of ROSETTA requires some knowledge of comet status and activity: surface temperatures, amount of active and inactive surface areas, gas production rate and dust flux.

in press; International audience; Hydrated sodium sulfates have been suggested to be present in variable amounts in Solar System objects such as Mars and Europa, among the possible others. The presence of these hydrated species is related to current/past aqueous environments, thus has an importance regarding the potential habitability of planetary objects. In this study, we analyzed anhydrous sodium sulfate (thénardite) and the hydrated sodium sulfate (mirabilite) by means of visible-infrared reflectance spectroscopy in the 0.4–5 μm spectral range, at different low temperatures between 80 and 298 K. Each mineral has been analyzed in three different grain sizes, between 36 and 150 μm. The anhydrous compound, thénardite, is characterized by a nearly flat spectrum in the visible and near IR up to 2.6 μm, while in the 3–4 μm region, the spectrum shows a few weak features due to H2O and SO_4^2- overtones/combinations. The first strong SO_4^2- overtone is visible at 4.6 μm. Spectra of mirabilite are substantially characterized by H2O absorption features in the 1–3 μm region, and by sulfate overtone/combination bands occurring at 3.8 and 4.7 μm. A weak feature appearing at 2.18 μm is also putatively attributed to the sulfate ion. The bands show changes as a function of temperature. The hydration absorption features in mirabilite show the strongest dependence with temperature, both in terms of shift in position and change of spectral shape. Bands at 3.1–3.24 μm in th´enardite, as well as absorption features located at 1.78 and 2.47 μm in mirabilite, could be used as diagnostic proxies for the detection of these two minerals on planetary bodies.