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Context. The European Space Agency (ESA) Rosetta mission was the most comprehensive study of a comet ever performed. In particular, the Rosetta orbiter, which carried many instruments for monitoring the evolution of the dusty gas emitted by the cometary nucleus, returned an enormous volume of observational data collected from the close vicinity of the nucleus of comet 67P/Churyumov-Gerasimenko. Aims. Such data are expected to yield unique information on the physical processes of gas and dust emission, using current physical model fits to the data. We present such a model (the RZC model) and our procedure of adjustment of this model to the data. Methods. The RZC model consists of two components: (1) a numerical three-dimensional time-dependent code solving the Eulerian/Navier-Stokes equations governing the gas outflow, and a direct simulation Monte Carlo (DSMC) gaskinetic code with the same objective; and (2) an iterative procedure to adjust the assumed model parameters to best-fit the observational data at all times. Results. We demonstrate that our model is able to reproduce the overall features of the local neutral number density and composition measurements of Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) Comet Pressure Sensor (COPS) and Double Focusing Mass Spectrometer (DFMS) instruments in the period August 1–November 30, 2014. The results of numerical simulations show that illumination conditions on the nucleus are the main driver for the gas activity of the comet. We present the distribution of surface inhomogeneity best-fitted to the ROSINA COPS and DFMS in situ measurements.

International audience; In the context of an increasing number of complex multiparametric dust coma models it was found convenient to construct an elementary model with a minimum number of parameters selected to represent the key processes acting on the dust. The models outputs can be used as a reference evaluation of these processes with rough estimates of the resulting dust properties e.g. velocity.The present work introduces three, universal, dimensionless parameters which characterize the dust motion in an expanding flow, and computes as a function of these parameters the dust terminal velocity, the time it takes to acquire it, and the distance at which it is acquired.The motion of dust grains is presented as a system of dimensionless ordinary differential equations the solution of which depends upon the above mentioned three parameters. The numerical integration of this system was performed over a wide range of parameter space covering the whole range of physically possible conditions.Precomputed results of dust terminal velocity, time and distance where it is reached are presented in dimensionless form. To obtain dimensional values for a particular case it is sufficient to perform algebraic operations.

International audience; In-situ measurements of individual dust grain parameters in the immediate vicinity of a cometary nucleus are being carried by the Rosetta spacecraft at comet 67P/Churyumov-Gerasimenko. For the interpretations of these observational data, a model of dust grain motion as realistic as possible is requested. In particular, the results of the Stardust mission and analysis of samples of interplanetary dust have shown that these particles are highly aspherical, which should be taken into account in any credible model. The aim of the present work is to study the dynamics of ellipsoidal shape particles with various aspect ratios introduced in a spherically symmetric expanding gas flow and to reveal the possible differences in dynamics between spherical and aspherical particles. Their translational and rotational motion under influence of the gravity and of the aerodynamic force and torque is numerically integrated in a wide range of physical parameters values including those of comet 67P/Churyumov-Gerasimenko. The main distinctions of the dynamics of spherical and ellipsoidal particles are discussed. The aerodynamic characteristics of the ellipsoidal particles, and examples of their translational and rotational motion in the postulated gas flow are presented

GIADA (Grain Impact Analyzer and Dust Accumulator) on-board the Rosetta space probe is designed to measure the momentum, mass and speed of individual dust particles escaping the nucleus of comet 67P/Churyumov-Gerasimenko (hereafter 67P). From 2014 August to 2016 June, Rosetta escorted comet 67P during its journey around the Sun. Here, we focus on GIADA data taken between 2015 January and 2016 February which included 67P's perihelion passage. To better understand cometary activity and more specifically the presence of dust structures in cometary comae, we mapped the spatial distribution of dust density in 67P's coma. In this manner, we could track the evolution of high-density regions of coma dust and their connections with nucleus illumination conditions, namely tracking 67P's seasons. We also studied the link between dust particle speeds and their masses with respect to heliocentric distance, i.e. the level of cometary activity. This allowed us to derive a global and a local correlation of the dust particles' speed distribution with respect to the H2O production rate. © 2016 The Authors.

International audience; One of the objectives of the ESA ”ROSETTA” mission to the comet 67P was to insert, in August 2014, an orbiter probe around the so-called nucleus of the comet, and to deposit the ”PHILAE” lander at the surface of the nucleus in November 2014. The selection of the landing site and the definition of the release point and initial descent velocity vector were made in the period August to October 2014 on the basis of simulations of the descent trajectory. This requested an assessment of the gravitational and aerodynamic forces on PHILAE. We here describe the so-called RZC model developed to predict the gas environment of 67P in November 2014 and compute the aerodynamic force. We first outline the unusual diffculties resulting from (1) the complexity of the nucleus surface on all scales, (2) the absence of direct measurements of the gas flux at the surface itself, (3) the time-dependence of the gas production induced by the fast nucleus rotation, (4) the need to perform the whole program within less than three months. Then we outline the physical approach adopted to overcome these diffculties, and describe the RZC model which included three differing tools: (1) a set of gasdynamic/gaskinetic codes to compute the vacuum outflow of a rarefied gas mixture from a highly aspherical rotating solid source; (2) an heuristic approach to deal with the solid/gas initial boundary conditions, and (3) an iterative procedure to derive the gas production parameters on the nucleus surface from the observational data acquired from the orbiter probe. The satisfactory operation of the RZC code in the weeks preceding the November 2014 PHILAE descent is shown, and the forecasted aerodynamic force during the PHILAE descent is compared to the gravitational force.

The Rosetta probe, orbiting Jupiter-family comet 67P/Churyumov–Gerasimenko, has been detecting individual dust particles of mass larger than 10^(−10) kg by means of the GIADA dust collector and the OSIRIS Wide Angle Camera and Narrow Angle Camera since 2014 August and will continue until 2016 September. Detections of single dust particles allow us to estimate the anisotropic dust flux from 67P, infer the dust loss rate and size\ud distribution at the surface of the sunlit nucleus, and see whether the dust size distribution of 67P evolves in time.\ud The velocity of the Rosetta orbiter, relative to 67P, is much lower than the dust velocity measured by GIADA, thus\ud dust counts when GIADA is nadir-pointing will directly provide the dust flux. In OSIRIS observations, the dust\ud flux is derived from the measurement of the dust space density close to the spacecraft. Under the assumption of radial expansion of the dust, observations in the nadir direction provide the distance of the particles by measuring\ud their trail length, with a parallax baseline determined by the motion of the spacecraft. The dust size distribution at\ud sizes >1 mm observed by OSIRIS is consistent with a differential power index of −4, which was derived from\ud models of 67P’s trail. At sizes

International audience; On the 12th of November 2014, The Rosetta Lander Philae became the first spacecraft to softly land on a comet nucleus. Due to the double failure of the cold gas hold-down thruster and the anchoring harpoons that should have fixed Philae to the surface, it spent approximately two hours bouncing over the comet surface to finally come at rest one km away from its target site. Nevertheless it was operated during the 57 hours of its First Science Sequence. The FSS, performed with the two batteries, should have been followed by the Long Term Science Sequence but Philae was in a place not well illuminated and fell into hibernation. Yet, thanks to reducing distance to the Sun and to seasonal effect, it woke up at end of April and on 13th of June it contacted Rosetta again. To achieve this successful landing, an intense preparation work had been carried out mainly between August and November 2014 to select the targeted landing site and define the final landing trajectory. After the landing, the data collected during on-comet operations have been used to assess the final position and orientation of Philae, and to prepare the wake-up. This paper addresses the Flight Dynamics studies done in the scope of this landing preparation from Lander side, in close cooperation with the team at ESA, responsible for Rosetta, as well as for the reconstruction of the bouncing trajectory and orientation of the Lander after touchdown.

We pursue our program of comparative simulations of the cometary gas coma by the two most advanced techniques available: (1) numerical solution of Navier–Stokes equations coupled to the Boltzman equation in the surface boundary layer, and (2) direct Monte-Carlo simulation. Here, we consider two different spherical but compositionally inhomogeneous nuclei, at three very different levels of gas production. The results show the same excellent agreement between the two methods in a domain adjacent to the surface as found precedingly, practically down to free-molecular conditions. A wealth of coma density patterns with non-intuitive structure is obtained. Some of these structures appear even under free-molecular effusion from the surface. The physical origin of all structures is discussed, and their evolution with changing gas production is studied. The computed comae are compared to those computed by various authors precedingly. Intercomparison of the present results demonstrates that differing inhomogeneity patterns may lead to similar structures in the gas coma. Comparison between these structures and those created by homogeneous, aspherical surfaces shows that it is not possible to guess from empirical rules which one of the two processes is responsible for the creation of a given structure. The implications for the interpretation of future high resolution images, or of future in situ mass spectrometric samplings of the near-nucleus gas coma are discussed.

G.A. Lukyanova, , J.F. Crifob, , V.V. Zakharova, c, , , and A.V. Rodionovd aCenter for Advanced Studies, St.-Petersburg State Polytechnical University, Polytechnicheskaya str., 29, 195251, St-Petersburg, Russia bCNRS, Service d'Aeronomie, BP 3, F 91371 Verrieres Cedex, France cObservatoire de Meudon, Place Janssen, 92190 – Meudon, France dCentral Research Institute on Machine Building (TsNIIMASH), Pionerskaya str., 4, Korolev, Moscow Region 141070, Russia Received 30 October 2004; revised 24 May 2006; accepted 26 May 2006. Available online 25 September 2006. Abstract We describe the first step of development of a most general code for the investigation of the dust dynamics inside the gas–dust interaction region surrounding an active cometary nucleus. The code is based on a Monte Carlo approach, which allows (a) consideration of realistic, i.e., extremely complicated assumptions with respect to the dust and nucleus size, shape and composition, (b) the derivation of the dust velocity distribution at each point, an information needed for the analysis of future in situ dust samplings, and (c) allowance for all the forces acting on the grains, i.e., aerodynamical, gravitational, radiative and inertial. In the present first step, the trivial (unrealistic) assumptions of nucleus and dust grain homogeneity and sphericity are still made, and only the nucleus gravity, the aerodynamic force and the radiation pressure force are considered. The code was tested assuming a Halley-like grain mass distribution extending from 10−18 to 10−2 g, and nucleus H2O sublimation rates from 3 × 1026 to 3 × 1028 molecule/s. The results demonstrate that (1) even in this simple case, none of the preceding forces can be a priori neglected; (2) due to the combined action of these forces, a wealth of currently overlooked complex grain trajectories exist. These results are compared to the previously developed dust multi-fluid (DMF) method, thus evidencing the relative capabilities of the two methods.

This paper describes the first computations of dust distributions in the vicinity of an active cometary nucleus, using a multidimensional Direct Simulation Monte Carlo Method (DSMC). The physical model is simplistic: spherical grains of a broad range of sizes are liberated by H 2 O sublimation from a selection of nonrotating sunlit spherical nuclei, and submitted to the nucleus gravity, the gas drag, and the solar radiation pressure. The results are compared to those obtained by the previously described Dust Multi-Fluid Method (DMF) and demonstrate an excellent agreement in the regions where the DMF is usable. Most importantly, the DSMC allows the discovery of hitherto unsuspected dust coma properties in those cases which cannot be treated by the DMF. This leads to a thorough reconsideration of the properties of the near-nucleus dust dynamics. In particular, the results show that (1) none of the three forces considered here can be neglected a priori, in particular not the radiation pressure; (2) hitherto unsuspected new families of grain trajectories exist, for instance trajectories leading from the nightside surface to the dayside coma; (3) a wealth of balistic-like trajectories leading from one point of the surface to another point exist; on the dayside, such trajectories lead to the formation of “mini-volcanoes.” The present model and results are discussed carefully. It is shown that (1) the neglected forces (inertia associated with a nucleus rotation, solar tidal force) are, in general, not negligible everywhere, and (2) when allowing for these additional forces, a time-dependent model will, in general, have to be used. The future steps of development of the model are outlined.

The dayside near-nucleus comae formed by solar-driven sublimation from two different aspherical nuclei made of an homogeneous mixture of ice and dust are computed by (1) solving Navier–Stokes equations and (2) direct Monte Carlo simulations, for different nucleus sizes, heliocentric distances, and dust-to-ice mixing ratios. Excellent agreement between the two methods is found down to surprisingly low production rates; it is found that the limit of validity of the first method is not simply related to the coma rarefaction: a new dimensionless number is tentatively offered to characterize this limit. The present solutions show that the weak shocks always present in the fluid coma persist practically down to truly free-molecular conditions, excluding the observational discovery of a structureless coma. They also show that rarefied flow in the near-nucleus coma can have a quite complicated structure, in particular inside topographic depressions. As an example, coma recondensation on the sunlit flanks of a cavity was found to be possible. We compute, for the first time, a true collisionless coma and show that structures are still present in it but are confined to the immediate vicinity of the surface. Finally, we describe in detail the kinetic conditions in a rarefied water coma, i.e., the velocity distribution asymmetry and the rotational–translational nonequilibrium. The significance of the results for future missions to comets is outlined.

Context. During the period between 15 September 2014 and 4 February 2015, the Rosetta spacecraft accomplished the circular orbit phase around the nucleus of comet 67P/Churyumov-Gerasimenko (67P). The Grain Impact Analyzer and Dust Accumulator (GIADA) onboard Rosetta monitored the 67P coma dust environment for the entire period.\ud Aims. We aim to describe the dust spatial distribution in the coma of comet 67P by means of in situ measurements. We determine dynamical and physical properties of cometary dust particles to support the study of the production process and dust environment modification.\ud Methods. We analyzed GIADA data with respect to the observation geometry and heliocentric distance to describe the coma dust spatial distribution of 67P, to monitor its activity, and to retrieve information on active areas present on its nucleus. We combined GIADA detection information with calibration activity to distinguish different types of particles that populate the coma of 67P: compact particles and fluffy porous aggregates. By means of particle dynamical parameters measured by GIADA, we studied the dust acceleration region.\ud Results. GIADA was able to distinguish different types of particles populating the coma of 67P: compact particles and fluffy porous aggregates. Most of the compact particle detections occurred at latitudes and longitudes where the spacecraft was in view of the comet’s neck region of the nucleus, the so-called Hapi region. This resulted in an oscillation of the compact particle abundance with respect to the spacecraft position and a global increase as the comet moved from 3.36 to 2.43 AU heliocentric distance. The speed of these particles, having masses from 10-10 to 10-7 kg, ranged from 0.3 to 12.2 m s−1. The variation of particle mass and speed distribution with respect to the distance from the nucleus gave indications of the dust acceleration region. The influence of solar radiation pressure on micron and submicron particles was studied. The integrated dust mass flux collected from the Sun direction, that is, particles reflected by solar radiation pressure, was three times higher than the flux coming directly\ud from the comet nucleus. The awakening 67P comet shows a strong dust flux anisotropy, confirming what was suggested by on-ground dust coma observations performed in 2008.

Critical measurements for understanding accretion and the dust/gas ratio in the solar nebula, where planets were forming 4.5 billion years ago, are being obtained by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment on the European Space Agency’s Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko. Between 3.6 and 3.4 astronomical units inbound, GIADA and OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) detected 35 outflowing grains of mass 10 −10 to 10 −7 kilograms, and 48 grains of mass 10 −5 to 10 −2 kilograms, respectively. Combined with gas data from the MIRO (Microwave Instrument for the Rosetta Orbiter) and ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instruments, we find a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface. A cloud of larger grains also encircles the nucleus in bound orbits from the previous perihelion. The largest orbiting clumps are meter-sized, confirming the dust/gas ratio of 3 inferred at perihelion from models of dust comae and trails.

We describe the present state of an advanced physical model aimed at the simulation of the environment of active cometary nuclei. The model can handle the complicated shapes of real cometary nuclei, and is ready to handle alternative assumptions concerning the nucleus composition and structure; its present version is based on the simple, but hitherto highly successful, Whipple (1950) paradigm: at small heliocentric distances, strong sublimation theory is used to compute the surface gas production, taking into account the time-dependent heat flow in the nucleus interior; at large distances, different types of gas molecules may be assumed to dominate the gas production. Any size and shape distribution of dust can be assumed. The nucleus spin motion is modelled with allowance for the outgassing and solar tidal torques. The gas outflow is computed by solving quasistationary flow equations (Euler, or Navier–Stokes), hence the extent of the coma which can be modelled is limited either by the breakdown of the fluid approximation, or by that of the steady-state approximation. The dust outflow is computed by solving quasi-stationary “zero-temperature” multifluid Eulerian equations in the gas–dust interaction region, and from a “Keplerian fountain model” beyond it: the extent of the dust distribution which can be modelled is only limited by computer resources limitations. In addition to the detailed gas and dust coma structure, the resulting net nucleus mass loss, net sublimation recoil force, net sublimation torque, and net thermal emission are computed. We mention the past applications of the model to comets P/Halley and C/Hyakutake, and indicate some of the future steps of development of the model.

The structure of the near-nucleus H 2 O atmosphere formed by sublimation under solar heating of a hypothetical large dusty ice sphere is computed (1) by the direct simulation Monte–Carlo method (DSMC) and (2) by solving Navier–Stokes equations (NSE) combined with a locally plane-parallel solution of the collisional Boltzmann equation for the nonequilibrium near-surface Knudsen layer. For Hale–Bopp-like comets, perfect agreement is obtained between the two methods on the day and night sides. This excellent agreement is maintained on the near-nucleus dayside for less productive comets, even down to production rates as low as those expected for Comet P/Wirtanen near 3 AU. It provides a direct validation of the gasdynamic simulations performed in support of the ESA Rosetta mission lander descent optimization studies (Crifo et al. 2001) and also confirms the similarity between the dayside coma and underexpanded axially symmetric free jets pointed out in Crifo (1986). On the nightside, moderate to high discrepancies appear between the two solutions as the production rate decreases, revealing the limits of the NSE method. The limits of the present study are delineated, and directions for future investigations are indicated.

We revisit the rotation of P/Halley taking into account the most essential observational constraints, as well as the external torques aecting the nucleus, in particular the outgassing torque. We solve the dirty ice sublimation equations at each point of the sunlit surface, using, for the rst time, the surface shape derived from the 1986 flybys imaging data. We assume that the nuclear surface is homogeneous in composition, thus reducing the number of model free parameters to one only: the dust-to-ice ratio on the surface. Our derived rotation model is a short-axis mode; it is consistent both with the 1986 nucleus imaging data, with the estimated non gravitational force, and with the observed time variations of the nucleus production rates. The outgassing torque results in a signicant variation of the angular momentum vector { for the assumed nucleus density of 0.5 g/cm 3 .

The structure of the nightside coma in the vicinity of a strongly active comet nucleus of pure ice is investigated by solving gasdynamic equations for the flow of water vapour sublimated from—or condensed onto—the nucleus surface. To guarantee the physical validity of the solution, both Euler and Navier–Stokes Equations are solved, and the solutions are compared. A spherical nucleus is considered first and then a triaxial ellipsoidal nucleus. The results show that (1) a fluid coma of significant extent and very complicated physical structure is formed; (2) for low heat conduction transfer across the nucleus from the dayside to the nightside surface, a narrow conical weak shock appears near to the antisolar axis; the whole nightside surface acts as a cold trap for the vapor, part of which recondenses onto it; (3) for intermediate heat conduction, part of the nightside surface becomes weakly sublimating, and a different weak shock pattern is formed; and (4) at high heat conduction, the whole nightside surface is weakly sublimating, and the resulting flow pattern becomes similar to that existing in a coma formed by diffusion from the nucleus interior (see Crifo, Rodionov and Bockelee-Morvan, 1999, Icarus 138 , 83–106). The results are compared to related model results by other authors, and a discussion is made of their relevance to the 1996 observation of the near-nucleus nightside coma of Comet C/1996 B2 Hyakutake.

We investigate whether the modelling of the immediate vicinity of an active nucleus—currently unobservable—can, as the modelling of the outer, observable coma, be based on unrealistic simple assumptions such as those of nucleus and dust grains sphericity. We point out the inconsistency of models based on such assumptions, which, to manage compatibility with the observations, have to introduce additional assumptions that conflict with the previous ones, such as the existence of active areas of the nucleus. We argue that, while the outer coma models being phenomenological in nature, can perhaps tolerate such inconsistencies, the circumnuclear coma models must be predictive, having to obviate the lack of observational data, and therefore must exclude implausible and ad hoc assumptions, and advocate only well-understood physical processes and duly validated modelling methods. We describe the first steps of development of a predictive circumnuclear coma model, and present a set of results obtained with parameters fitted to comet P/Wirtanen, the target of the Rosetta mission, but of a quite general significance. Considering, first an inhomogeneous spherical nucleus with spherical dust grains, and then an aspherical homogeneous nucleus with spherical dust grains, we show that, in both cases (1) the surface temperature and initial gas parameters differ considerably from the Hertz–Knudsen values; (2) the near-surface gas and dust flows are not in general vertical, (3) the gas and dust density do not always monotonically decrease outwards, (4) the gas and dust velocity vary strongly from point to point, (5) shock structures are formed, which result in the formation of pseudo-jets of dust grains originating from various points of the surface. No simple method to distinguish between dust structures created by the surface inhomogeneity and by the surface orography is found. We show, for the first time, the deformation of the near-nucleus dust coma during a full rotation of an homogeneous, aspherical nucleus. We also show that identical active regions located at different points of an inhomogeneous spherical nucleus produce very different dust distributions, suggesting that the dust distribution is also strongly deformed during the rotation of such a nucleus. Finally, we consider, for the first time, a spherical homogeneous nucleus emitting aspherical dust grains. We show that, in such a case, the terminal grain velocity depends upon the shape, initial position, and even possibly upon the initial orientation of the grain at the surface, so that there cannot exist a precise relation between terminal velocity and dust grain mass. We conclude that, far from giving an approximate or average representation of the circumnuclear coma, the classical modelling approach yields in this region predictions that are in total conflict with the real behaviour of the gas and dust. As a most dramatic consequence, the use of this classical approach may have obscured completely the significance of the few direct and of the many indirect informations acquired hitherto on the nucleus activity.

We present the first gasdynamic simulations of the coma formed by the diffusion from a comet nucleus interior of a volatile molecule at large heliocentic distance. The method used is a generalization of that described in J. F. Crifo et al. (1995, Icarus 116 , 77–112). The molecule is assumed to be CO. Three homogeneous nuclei with the same mass, but different shapes , are considered. Molecular production rates and nucleus sizes appropriate for Comets 46 P/Wirtanen at 3 AU from the Sun and 29 P Schwassmann–Wachmann I at 6 AU are considered in alternance. The main results are: (1) a fluid region surrounds the whole nucleus (not only the sunward hemisphere) even for production rates in the 10 25 molecule per second range; (2) the densest part of the near-nucleus coma is not necessarily the sunward hemisphere; (3) strong day-to-night asymmetries in the CO velocity are predicted, which are due to the surface temperature distribution, not to differences in surface gas production rate from point to point; (4) even in the case of a near-uniform production of CO over the surface, weak shock structures appear in the coma at the terminator, due to the surface temperature gradient (thus, even a uniform gas production over a spherical nucleus does not produce a uniform coma!); (5) shock structures are also formed in the vicinity of surface concavities, even if the gas production is uniform; (6) the ejection of dust grains up to at least millimeter sizes is possible from both the sunward and the anti-sunward side of the nucleus; (7) the terminal dust grain velocities are comparable to those computed for a H 2 O-dominated coma at a similar gas production rate. The reasons why we believe that CO is the best candidate for forming the distant comas of comets are discussed at length, but we also indicate why the present results are not critically affected if CO is replaced by another candidate volatile molecule (e.g., CO 2 , HCN, H 2 CO…), or by a mixture of such molecules. The CO velocities we derive for 29 P Schwassmann–Wachmann I are shown to be in excellent agreement with those experimentally derived for this comet (J. Crovisier et al. 1995, Icarus 115 , 213–216; N. Biver et al. 1997, Science 275 , 1915–1918); as regards the experimentally derived temperatures, we show that their interpretation is very complicated and will require, in particular, the incorporation in the present model of a gas translational relaxation scheme.

Rosetta is one of the most ambitious missions planned by ESA for the beginning of the next millennium. It will explore from very close a comet nucleus along its trajectory up to perihelion. In the instrument complex forming the scientific payload, the GIADA (Grain Impact Analyser and Dust Accumulator) experiment is devoted to study the cometary dust flux evolution and grain dynamic properties. To achieve the required performances and the expected scientific return, GIADA has been designed as a multi-sensor instrument. It is able to detect grain passage by laser light scattering measurement, particle momentum through piezoelectric transducers and mass flux by means of quartz crystal microbalances. In this paper we describe the technical solutions and performances which have been reached on the development models of GIADA.

GIADA (Grain Impact Analyser and Dust Accumulator) on-board the Rosetta mission to comet 67P/Churyumov-Gerasimenko was designed to study the physical and dynamical properties of dust particles ejected by the comet during the encounter. In this paper we report the results of the analysis of data collected by GIADA during the past seven years of the cruise phase. During this period the GIADA detection subsystems were switched on for periodic in-flight payload checkouts to monitor their state-of-health including potential changes in its performance that could affect its data collection. Only slight variations in sensitivity and dynamical range were identified that will not affect the GIADA measurement capability during the Rosetta comet encounter and rendezvous phase. The GIADA microbalance system detected the presence of low-volatility material over a period of about 169 days when the GIADA cover remained partially opened. It is highly probable that this material originated from the spacecraft itself, as a spacecraft's outgassing was observed by the ROSINA mass spectrometer (on-board Rosetta) during the cruise phase. The identification of the low-volatility mass deposited on the microbalances as self-contamination will allow us to evaluate the mass rate background to be subtracted from the GIADA science data. These results obtained from GIADA cruise data analysis coupled with laboratory calibration data obtained from measurements using the GIADA spare model for selected cometary dust analogs will be the basis for the interpretation of the GIADA scientific data.

GIADA (Grain Impact Analyzer and Dust Accumulator) is an in situ instrument, on board the Rosetta spacecraft, designed to measure the dynamical properties of the dust grains emitted by the comet 67P/Churiumov–Gerasimenko (hereafter 67P/C–G). It consists of three subsystems able to measure the mass and speed of single dust grain and dust mass flux. Once the orbit and the attitude of a spacecraft are defined, it is needed to simulate the performances of an in situ instrument. We present simulated GIADA performances to evaluate its capability in fulfilling its scientific objectives along specific orbits. In order to perform these simulations, because of the lack of real data on near-nucleus cometary environment, it is necessary to use a modeled dust coma along the spacecraft ( S / C ) orbits. We developed GIPSI (GIADA Performance Simulator), a simulation tool conceived to replicate the GIADA capability in detecting coma dust features through the dust abundances, mass and velocity dust distributions measurements. Using state-of-the-art coma modeling, we evaluated three different Rosetta orbit mission scenarios. We outline the optimal S / C orbit for GIADA by means of achievable dust coma evolution description, number of collected particles and grain velocity measurements. The quasi circular orbit with a 5 km peri-center radius and a 10 km apo-center radius, during the pre-landing close observation phase is the best suited for the GIADA instrument.

This work is the continuation of a program aimed at a physically and geometrically realistic modeling of the gas and dust distribution in the circumnuclear coma (CNC) of comets. Here, we consider two differenthomogeneous, but asphericalnuclei. Thus, this is the first time that non-spherical nucleus surfaces are taken into account in a coma model. For definiteness, and as in Crifo and Rodionov (1997Icarus,in press). The parameters of the model are chosen to be applicable to the faint Comet P/Wirtanen. As was the case in Paper I, the results reveal basic properties of the CNC that are artificially concealed when the nucleus is assumed spherical: 1. Standing shock structures appear in the gas CNC when a sufficiently deep concave region of the surface is sunlit. 2. Even a shallow concavity gives birth to sharp dust density enhancements that, on CNC images, might look like pseudo-jet structures. Since similar effects have been obtained forinhomogeneous, sphericalnuclei, it can be concluded that standing shocks and pseudo-jets are inherent characteristics of the CNC of all comets. This fact suggests a re-examination of the previous interpretations of the appearance of P/Halley's CNC in terms of intrinsic “active areas” of the nucleus. 3. Sublimation from a subsolar icy region can occur either sonically or subsonically, depending on its surroundings: thus, instead of being locally determined by the properties of the nucleus surface, as currently assumed, the distribution of the CNC gas and dust areglobalcharacteristics of large areas of the nucleus surface. 4. Nearly horizontal flow of gas can occur near the surface of the day side, even near noon. 5. During the limited nucleus rotation of 45° considered in this study, the gas shock structures were found to co-rotate, but with significant distortion; the narrow dust pseudo-jets were found to rotate by differing amounts for the two nuclei; the background gas and dust distributions remained approximately fixed in a solar-ecliptic frame, but also underwent significant spatial distortion. These results indicate that the current practice of discussing the CNC processes on the basis of spherically symmetric assumptions must be abandoned: the assessment of the basic physical properties of the CNC, the interpretation ofin situor remote-sensing data concerning this region, in terms of nucleus surface properties and/or nucleus rotational state definitely require tridimensional simulations of the kind presented here.

International audience; Millimetre line observations of comet C/1995 O1 (Hale-Bopp) close to perihelion, completed using the IRAM Plateau de Bure Interferometer, have detected temporal variations in the CO J(2–1) 230 GHz line shape, and in the position of its maximum emission brightness within the field-of-view, whose heuristic analysis has suggested the presence in the coma of a slowly rotating spiral-shaped enhancement of the CO density. Aims. Here, we reanalyse these data using a physically consistent model of the coma. Methods. We consider a large, rotating, icy nucleus with an arbitrarily aspherical shape and an adhoc rotation mode, and compute its tridimensional, time-dependent (”3D+t”) mixed CO + H2O coma, using a previously developed tridimensional hydrodynamical code (HDC). The line emission of CO is then computed using a molecular excitation and radiation transfer code (ERC). In the present, pioneering phase, the HDC and ERC both contain crude, and not thoroughly mutually consistent approximations. Several alternative CO surface flux distributions are considered, and the resulting CO 230 GHz line spectra and brightness maps are compared with observations. Results. We find that when an uniform surface flux of CO is assumed, the spiral structures created by the nucleus asphericity in the CO coma are too faint to account for the observational data, whereas we confirm earlier conclusions based on a heuristic approach that the assumption of an area of suitable dimensions and localization with increased CO flux leads to results in agreement with a large subset of (but not all) the data. This suggests that the true CO coma production map may be more complex than the presently assumed rather simple-minded one. Refined and mutually consistent HDC and ERC are needed for a more satisfactory interpretation of the present and any similar future data.

The MCY-type semi-rigid, and BJH- and MCY-type flexible water—water potentials (recently refined with respect to observed dimerization equilibrium constants) are used for evaluation of the water-dimer populations in saturated steam. The study was initiated by the recent need for such data in research into the Earth's, planetary and cometary atmospheres. Populations are studied from 50 K up to the critical point. Effects of higher clusters have been simulated in an estimation based on a geometrical series-type reasoning; however, they are shown to be insignificant. In contrast to some previous reports, the dimer populations are considerably small in low temperature regions, increasing with temperature. This behaviour is a result of the competition between temperature increase of the saturated steam pressure and temperature decrease of the dimerization equilibrium constant. The reported dimer populations represent the most reliable data available.

We use our newly developed Dust Monte-Carlo (DMC) simulation technique [Crifo, J.F., Lukianov, G.A., Rodionov, A.V., Zakharov, V.V., 2005. Icarus 176, 192–219] to study the dynamics of dust grains in the vicinity of some of the benchmark aspherical, homogeneous cometary nuclei and of the benchmark spherical, inhomogeneous nuclei studied by us precedingly. We use the interim unrealistic simplifying assumptions of grain sphericity, negligible nucleus rotation rate, and negligible tidal force, but take accurately into account the nucleus gravitational force, gas coma aerodynamic force, and solar radiation pressure force, and consider the full mass range of ejectable spherical grains. The resulting complicated grain motions are described in detail, as well as the resulting complicated and often counter-intuitive dust coma structure. The results are used to answer several important questions: (1) When computing coma dust distributions, (a) is it acceptable to take into consideration only one or two of the above mentioned forces (as currently done)? (b) to which accuracy must these forces be known, in particular is it acceptable to represent the gravity of an aspherical nucleus by a spherically symmetric gravity? (c) how do the more efficient but less general Dust Multi-Fluid (DMF) computations compare with the DMC results? (2) Are there simple structural relationships between the dust coma of a nucleus at small heliocentric distance r h , and that of the same nucleus at large r h ? (3) Are there similarities between the gas coma structures and the associated dust coma structures? (4) Are there dust coma signatures revealing non-ambiguously a spherical nucleus inhomogeneity or an homogeneous nucleus asphericity? (5) What are the implications of the apparently quite general process of grain fall-backs for the evolution of the nucleus surface, and for the survival of a landed probe?

A radiative hydrodynamic model of comet P/Halley is used to investigate quantitatively the constraints imposed onto the Comet loss rate in solids by both the in-situ data and the remote sensing data. In particular, for the first time, the in-situ data are used to fit composite spectra extending from near-IR wavelengths to the microwave region. Also, for the first time the uncertainty affecting the ejection velocity of large grains is taken into account. The results suggest that only future rendez-vous studies of the very large grain density and velocity in the vicinity of a comet would be able to provide definitely reliable values of the comet loss rate.

The formation of water clusters of all sizes by water recondensation in an expanding cometary atmosphere is investigated. Several competing algorithms are used, and the present uncertainties concerning the physical properties of the clusters are carefully taken into account. Numerical estimates appropriate to Comet P/Halley at the time of the flyby observations are given. The results strongly suggest that about 15% of the water recondenses into clusters of 100 to 1000 water molecules which disappear beyond a cometocentric distance of about 500 nucleus radii. Surprisingly, these predictions are found to be weakly sensitive to the existing uncertainties concerning the mathematical model of the nucleation process and the physical properties of the water clusters. The most probable observable effect of these large clusters is a strong perturbation of the water radial temperature profile at all altitudes including beyond that of cluster disparition. Recondensation also increases the gas outflow velocity in the inner coma: the present results suggest that the gas velocities derived in situ by the Giotto mass spectrometer could be about 30% below the coma-averaged outflow velocities. This study does not support previously published speculations concerning the detectability of chemical or optical effects associated with the formation of small water clusters, particularly of the dimer (H2O)2. It also excludes the detectability of the continuum emissions from the clusters, at any wavelength because of the masking effect of the coma dust. Finally, it indicates that recondensation cannot form a dense halo of macroscopic ice grains: such a halo, if discovered in the future, can only result from surface fragmentation.

We describe here the first tridimensional, time-dependent model of a gas coma, and its first application to observed rotationally induced gas coma structures. The present version of the model uses inviscid flow equations and, because of it, is applicable rigorously only to highly productive comets, such as comet C1995/O1 Hale-Bopp in the inner Solar System. Using parameters suitable for this comet, and an arbitrary aspherical nucleus shape and rotation mode, the model is capable to generate large-scale gas spirals of the kind observed in this comet, thus demonstrating that the existence of such spirals does not require surface heterogeneity (active/inactive areas). This result does not in itself exclude heterogeneity. The question whether, in the absence of direct nucleus observations, any reliable interpretation of such structures is possible is raised.

The published multidimensional physicochemical simulations of the near-nucleus coma (hereafter “NNC”) are reviewed. The goals of these simulations are to understand the physical origin of the observed NNC structure(s), and to relate the latter to properties of the nucleus – an inference which is often done on the basis of subjective impressions only. The NNC simulations are mostly based on ad hoc benchmark model nuclei. However, a systematic program of simulation of the NNC of comet Halley exists, which uses the observed shape of its nucleus with or without filtering of topographic details. The main conclusions to be derived from the simulations of the NNC around idealized benchmark nuclei are that: (1) only fully comprehensive quantitative modelling of the whole NNC can reveal the origin of NNC structures: there does not exist any universal relation between one nucleus feature and the NNC structure adjacent to it, independent from its surrounding; (2) in particular, it is impossible to separate the effect of the nucleus heterogeneity on the NNC, from that of the surface topography, except by global quantitative modelling; (3) these two conclusions follow from the physical laws of gas flow and dust motion, hence are independent from the nucleus gas and dust production mechanism. Regarding comet P/Halley, it was possible, using the nucleus shape derived from the cameras, to show that the observed dust “filament” structures are the signatures of the nucleus shape, and are insensitive to whether the nucleus is assumed compositionally homogeneous or inhomogeneous. Application of similar models to the high quality nucleus and NNC observations made during the recent flybys of comets Borelly, Wild-2, and Tempel-I is badly needed. In the future, the nucleus-orbiting mission Rosetta should provide the ultimate benchmark to achieve our understanding of the NNC.

This note (1) provides a formal derivation of an algorithm proposed precedingly without proof for evaluating the sublimation rate of dusty ice under solar illumination, and (2) illustrates the importance of adopting a correct algorithm for such a purpose, by rediscussing the basic characteristics of P/Halley nucleus activity derived from the 1996 flyby data.

We overview an ongoing program of three-dimensional, time-dependent, physically realistic simulation of the gas and dust coma of the comet, using both fluid equations and Direct Monte-Carlo Simulations.

We briefly describe an advanced 3D gas dynamical model developed for the simulation of the environment of active cometary nuclei. The model can handle realistic nucleus shapes and alternative physical models for the gas and dust production mechanism. The inner gas coma structure is computed by solving self-consistently (a) near to the surface the Boltzman Equation (b) outside of it, Euler or Navier-Stokes equations. The dust distribution is computed from multifluid “zero-temperature” Euler equations, extrapolated with the help of a Keplerian fountain model. The evolution of the coma during the nucleus orbital and spin motion, is computed as a succession of quasi-steady solutions. Earlier versions of the model using simple, “paedagogic” nuclei have demonstrated that the surface orography and the surface inhomogeneity contribute similarly to structuring the near-nucleus gas and dust coma, casting a shadow on the automatic attribution of such structures to “active areas”. The model was recently applied to comet P/Halley, for which the nucleus shape is available. In the companion paper of this volume, we show that most near-nucleus dust structures observed during the 1986 Halley flybys are reproduced, assuming that the nucleus is strictly homogeneous. Here, we investigate the effect of shape perturbations and homogeneity perturbations. We show that the near nucleus gas coma structure is robust vis-a-vis such effects. In particular, a random distribution of active and inactive areas would not affect considerably this structure, suggesting that such areas, even if present, could not be easily identified on images of the coma.

The cameras carried onboard the flyby missions to comet P/Halley in 1986 imaged the near nuclear jet activity from several spatial directions. The observed, very structured near nuclear dust jets were considered at that time as the result of dust emission from well localized active surface regions (without supporting 3-D model computations, however). Based on the first, recently developed 3-D gas dynamical model of P/Halley’s activity, we have been shown that jet features can be reproduced assuming a homogeneous dusty ice nucleus surface. The dust in the collisional near nuclear coma is concentrated along the gas flow discontinuities resulting from the complicated surface orography, creating the visual impression of dust jets. We present here the results of these calculations for the near nucleus dust distributions, and we compare them with the direct observations made during the three Halley flybys (Vega 1, Vega 2, and Giotto).

International audience; We present the discovery and observations throughout the period March 25–April 5, 1996, ofC2andCNarc structures centered on the anti-sunward axis of comet Hyakutake (C/1996 B2). We interpret them as the signatures of the interaction between two supersonic rarefied gas jets: H2O vapor emanating from the comet nucleus on one hand and gas flowing from a secondary source centered on the anti-sunward direction on the other. An upgraded version of the Gasdynamic code described in Crifoet al.(1995, 1997a) is used to model the interaction. To accommodate low gas densities, Navier–Stokes equations are used instead of Euler equations, and photochemical effects are introduced to compute the water-group secondary molecules' distributions. A double-shock H2O structure, characteristic of the interaction between two opposite supersonic flows, is found. Associated with it is an arc-shaped distribution of OH. In view of the dominance of the collisions between and with H2O molecules, the distribution of any other primary molecules with lifetimes comparable to that of H2O (e.g., HCN) will be identical to that of H2O. The spatial distribution of their daughter products (e.g., CN) will be similar to that of OH, if their lifetimes are comparable to that of OH. We show that, given the limitations of the observations and of the modeling method, it is not possible to derive a unique solution in terms of secondary source properties and of near-nucleus night-side production. We show in particular that the solution proposed by Harriset al.(1997) for the companion OH arcs is only one of the possible solutions, and that, in conflict with what those authors find, it does result in a standing shock structure between the nucleus and the source. The successful simulations of the arcs presented here constitute the first observational evidence for the formation of shock waves in neutral cometary atmospheres, originally predicted by Kitamura (1990) and subsequently advocated as an essential process in the formation of the circumnuclear coma by Crifoet al.(1995, 1997a, b).

International audience; We present the description of a design for a proposed Dust Flux Analyser for the Rosetta cometary mission. A concept first developed for the NASA/ESA Tempel II Rendezvous and Halley Intercept Mission /1/, the instrument is able to measure dust particle parameters and fluxes over a velocity range typical of emission from cometary surfaces. It would be mounted on the Rosetta Orbiter and would measure the variation in flux rate throughout all mission phases at the comet. The instrument would measure particle flux, velocity, momentum and density, shape and scattering properties.

International audience; A model for the interaction between a multicomponent Maxwellian atmosphere and a spacecraft is described. Multidimensional, time-dependent gasdynamical simulations of the gas coma around the recently reconstructed aspherical rotating nucleus of comet 67P/C-G is used to analyze the outgassing pressure forces on the ESA spacecraft Rosetta. The forces were in general found to be directed significantly away from the cometocentric position vector of the spacecraft. It was also found that in a maximum outgassing scenario at comet rendezvous, the outgassing pressure force exceeds the gravitational attraction from the nucleus in the cometocentric direction of the Sun. Furthermore, the highly non-spherical pressure field was found to undergo very large changes as the nucleus rotated. Still, it was possible to represent the mean pressure field experienced by Rosetta by a fairly simple model, which can be used for the determination of the comet mass and the so-called oblateness coefficient c20 from the spacecraft Doppler signal. The oblateness coefficient represents a type of asphericity of the gravity field. The determination of the so-called triaxiality coefficient of the gravity field c22 may require using the true pressure field instead of the mean pressure field.

An infrared sounder is being developed in France to observe in 1986 Comet Halley from the Soviet “VEGA” flyby probes. The instrument, called “I.K.S.”, has three measuring channels. Two of these channels will provide the spectrum of the comet emission in the spectral intervals 2.5–5.0 μ and 6–12 μ, at a constant resolution λ/Δλ = 50. The third channel analyzes the comet I.R. image at a spatial frequency of about 1 arc minute −1 ; two I.R. colours are used in this channel: 7–10 μ and 10–14 μ. From the results expected, it is hoped that (1) most primary simple molecules emitted by the nucleus will be identified; (2) the chemical composition and perhaps crystalline structure of the dust grains and ices released by the comet will be derived; and (3) the diameter of the nucleus and its brightness temperatures will be measured.

An overview of recent inner coma models is attempted from the somewhat unconventional point of view of the physics of rarefied inhomogeneous jets : in doing this we try to bridge the gap which separates cometary modelling from the treatments of similar problems which appear in other contexts. We introduce the Navier Stokes Equations as the relevant method by which this class of problem should be dealt with. The characteristics of 19 recent inner coma models are given. The question of their mutual consistency is raised the discussed in detail in one particular but very important case : that of the water temperature field. It is shown that none of the reviewed works can claim to produce a fully satisfactory temperature field. As a consequence, while all works demonstrate remarkable achievements, their relevance for the interpretation of specific experimental results may be smaller than implied by the authors. The main conclusion of this overview is that standardisation of the modelling of the inner coma (parameter values and algorithms by which elementary processes are accounted for) is in urgent need.

We try to evaluate the level of confidence that can be placed in the optical methods available for sizing cometary dust (and, at the same time, deriving all its other characteristics). We use for this purpose the new informations provided by the in-situ sampling of Comet Halley dust. We try to determine whether data from this origin are compatible with visible and I. R. spectra of this comet acquired from the Earth, and with earlier interpretations of such spectra. We find that, as happens frequently when remote sensing and local sampling techniques data can be compared, none of these techniques can claim to be fully satisfactory. In the case of cometary dust, current estimates based on optical sizing only may be much less accurate than precedingly assumed. Improvements in the observational approaches appear needed, as well as new laboratory investigations and theoretical developments to improve the capabilities of the optical studies of cometary dust.

The infrared instrument IKS flown on board the VEGA space probes was designed for the detection of emission bands of parent molecules, and for a measurement of the size and temperature of the thermal emitting nuclear region. The instrument had three channels with cooled detectors: an "imaging channel" designed to modulate the signal of the nucleus and two spectroscopic channels operating at 2.5-5 and 6-12 micrometers, respectively, equipped with circular variable filters of resolving power approximately 50. This paper presents and discusses the results from the spectral channels. On VEGA 1, usable spectra were obtained at distances D from the comet nucleus ranging from 250,000 to 40,000 km corresponding to fields of view 4000 and 700 km in diameter, respectively. The important internal background signal caused by the instrument itself, which could not be cooled, had to be eliminated. Since no sky chopping was performed, we obtain difference spectra between the current spectrum and a reference spectrum with little or no cometary signal taken at the beginning of the observing sequence (D approximately 200,000 km). Final discrimination between cometary signal and instrumental background is achieved using their different time evolution, since the instrumental background is proportional to the slow temperature drift of the instrument, and the cometary signal due to parent molecules or dust grains is expected to vary in first order as D-1. The 2.5-5 micrometers IKS spectra definitely show strong narrow signals at 2.7 and 4.25 micrometers, attributed to the nu 3 vibrational bands of H2O and CO2, respectively, and a broader signal in the region 3.2-3.5 micrometers, which may be attributed to CH-bearing molecules. All these signals present the expected D-1 intensity variation. Weaker emission features at 3.6 and 4.7 micrometers could correspond to the nu 1 and nu 5 bands of H2CO and the (1 - 0) band of CO, respectively. Molecular production rates are derived from the observed emissions, assuming that they are due to resonance fluorescence excited by the Sun's infrared radiation. For the strong bands of H2O and CO2, the rovibrational lines are optically thick, and radiative transfer is taken into account. We derive production rates, at the moment of the VEGA 1 flyby, of approximately 10(30) sec-1 for H2O, approximately 2.7 x 10(28) sec-1 for CO2, approximately 5 x 10(28) sec-1 for CO, and 4 x 10(28) sec-1 for H2CO, if attributions to CO and H2CO are correct. The production rate of carbon atoms in CH-bearing molecules is approximately 9 x 10(29) sec-1 assuming fluorescence of molecules in the gas phase, but could be much less if the 3.2-3.5 micrometers emission is attributed to C-H stretch in polycyclic aromatic hydrocarbons or small organic grains. In addition, marginal features are present at 4.85 and 4.45 micrometers, tentatively attributed to OCS and molecules with the CN group, respectively. Broad absorption at 2.8-3.0 micrometers, as well as a narrow emission at 3.15 micrometers, which follow well the D-1 intensity variation, might be due to water ice. Emission at 2.8 micrometers is also possibly present, and might be due to OH created in vibrationally excited states after water photodissociation. The 6-12 micrometers spectrum does not show any molecular emission, nor emission in the 7.5-micrometers region. The spectrum is dominated by silicate emission showing a double structure with maxima at 9.0 and 11.2 micrometers, which suggests the presence of olivine.

Synthetic spectra of Comet Halley between 2.5 and 15 μm are calculated on the basis of current cometary models. This study shows that molecules which are most likely detectable in the infrared range include H 2 O, CO, CO 2 , CH 4 , NH 3 , N 2 H 4 , and H 2 CO. The flux emitted by these molecules, mostly due to resonant scattering, should be sufficient for detection by a flyby mission.

A rocket borne electrostatic analyzer was flown through the near equatorial ionosphere, up to L ∼ 1.3, at night, during a magnetic storm associated with spread-F. After careful correction of the observed signal, which was highly contaminated by the U.V. night glow, it appears a field aligned, upwards directed flux of soft ions with a power law spectrum, through a restricted altitude range. A comparison with earlier results indicates that it is not inconsistent with previously published upper limits, owing to the limited spatial extent of the phenomenon.

The IKS infrared spectro-photometer will fly on board the VEGA platforms. It is designed to characterize the size, temperature and emissivity of the Comet Halley nucleus, to identify the major gaseous components of the inner coma and to detect the emission of the cometary grains. This paper presents the “calibration” experiments required to reduce the raw data: (i) absolute wavelength calibration of the filter wheels; (ii) modeling of the internal signal, as a function of the temperature of the different sub-systems; (iii) absolute and spectral responsivities of each of the spectrometric and photometric channels, as a function of the wavelength and position of the source in the field of view. Finally, we shall indicate the expected S/N ratios.

In order to prepare infrared sounding of comet Halley from the flyby VEGA probes, we have computed the synthetic spectrum between 2.5 and 15 μ of a typical comet at a heliocentric distance of ∼ 0.8 AU. The present paper is particularly devoted to the contribution from the cometary gases. For a selection of 20 possible parent molecules, the most efficient excitation process is resonant fluorescence by the solar radiation field. The H 2 O, CO, CO 2 , CH 4 , NH 3 and H 2 CO molecules are the best candidates for detection by the IKS infrared spectrometers aboard the VEGA probes. For the water molecule, collisions are too rare to ensure thermal equilibrium in the whole coma ; therefore a limited number of fluorescence lines are expected to be present in the H 2 O vibrational bands.

The IKS instrument carried by the Vega 1 spacecraft performed infrared sounding of the inner coma and nuclear region of comet Halley. An emissive centre, a few kilometres in size, was detected with a temperature significantly in excess of 300 K. In the surrounding central coma, hydrocarbon species or other carbonaceous material seem to be present. The fluorescence signatures of H2O and CO2 were identified.

The purpose of this work is to try to evaluate the level of confidence that can be placed in the present techniques available for sizing the cometary dust (and, at the same time, deriving all its other characteristics). We use for this the new possibilities opened by the recent in-situ sampling of Comet Halley dust. We look whether data from this origin are compatible with visible and I.R. spectra of this comet acquired from the Earth, and we also examine whether they are compatible with the set of observations acquired on another comet considered similar to Halley, i.e. Comet Kohoutek “1973f”. We find that, as happens frequently when remote sensing and local sampling techniques data can be compared, none of these techniques can claim to be fully satisfactory. In the case of cometary dust, current estimates based on optical sizing only may be much less accurate than precedingly assumed. In any case, improvements in the observational approaches appear needed, as well as dedicated laboratory investigations and theoretical developments.

Results of the 2.5–5 micron spectroscopic channel of the IKS instrument on Vega are reported and the data reduction process is described. H2O and CO2 molecules have been detected with production rates of 1030 s−1 and 1.5 1028 s−1 respectively. Emission features between 3.3 and 3.7 microns are tentatively attributed to CH - bearing compounds - CO is marginally detected with a mixing ratio CO/H2O ⩽ 0.2. OH emission and H2O - ice absorption might also be present in the spectra.