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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 interpretation of these observational data, a model of dust grain motion as realistic as possible is requested. In particular, the results of Stardust mission and analysis of samples of interplanetary dust have shown that these particles are highly non-spherical. In many cases precise simulations of non-spherical grain's dynamics is either impossible or computationally too expensive. In such situation it is proposed to use available experimental or numerical data obtained for other conditions and rescale them considering similarity of the physical process. In the present paper we focus on the derivation of scaling laws of rotational motion applicable for any shape of particles. We use a set of universal, dimensionless parameters characterizing the dust motion in the inner cometary coma. The scaling relations for translational and rotational motion of dust grains in a cometary environment are proposed. The scaled values are compared with numerically computed ones in our previous works. [ABSTRACT FROM AUTHOR]

We related morphological (size/shape) and dynamical properties of the dust ejected from the 67P/Churyumov–Gerasimenko comet by combining data from two instruments onboard the ESA's Rosetta mission, i.e. the MIDAS atomic force microscope and the GIADA dust detector. The two instruments detected dust of different size (10−6–10−5 and 10−4–10−3 m, respectively). MIDAS detected dust in four periods, three during the inbound orbit arc (2014 September–November; 2014 December– 2015 February; 2015 February–March) and one corresponding to a post-perihelion outburst (2016 February 19). For these periods, we analysed the dust particles' spatial distribution on the MIDAS targets to obtain the number of parent particles hitting the instrument by means of an empirical procedure and to measure the corresponding dust flux. For the same periods, we retrieved the dust flux measured by GIADA. The ratio between the two dust fluxes is constant. By coupling this result with activity models, we inferred that the particles detected by MIDAS are fragments of hundreds-micron- to mm-sized particles detected by GIADA. In addition, the similar dust flux ratios between nominal activity and outburst indicates that the outburst did not include micro- and nano-sized dust, differently from other outbursts previously observed. Dust and surface properties were related by applying a traceback algorithm to GIADA data to retrieve the source regions of dust ejected in different periods. We did not detect variations of morphological properties between dust ejected from more and less processed terrains, concluding that compact dust particles (detected by MIDAS) have the same properties across the comet surface. [ABSTRACT FROM AUTHOR]

In this work, we aim to characterize the dust motion in the inner coma of comet 67P/Churyumov–Gerasimenko to provide constraints for theoretical 3D coma models. The OSIRIS camera on-board the Rosetta mission was able for the first time to acquire images of single dust particles from inside the cometary coma, very close to the nucleus. We analyse a large number of particles, performing a significant statistic of their behaviour during the post-perihelion period, when the spacecraft covered distances from the nucleus ranging between 80 and 400 km. We describe the particle trajectories, investigating their orientation and finding highly radial motion with respect to the nucleus. Then, from the particle brightness profiles, we derive a particle rotational frequency of ν < 3.6 Hz, revealing that they are slow rotators and do not undergo fragmentation. We use scattering models to compare the observed spectral radiance of the particles with the simulated ones in order to estimate their size, finding values that range from millimetres up to centimetres. The statistics performed in this paper provide useful parameters to constrain the cometary coma dynamical models. [ABSTRACT FROM AUTHOR]

We monitor the seasonal erosion and accretion of dust deposits in the Imhotep, Hatmehit, and Ma'at regions of comet 67P/Churyumov–Gerasimenko with OSIRIS Narrow Angle Camera images. The vertical accuracy of such measurements is 0.2 m and the spatial scale of the images we used is lower than 0.60 m pixel−1. We calculate the height of 21 boulders by applying a tool that allows to measure the shadow length of a boulder projected on the surrounding dust deposit, assuming that any height variation is not due to boulder intrinsic change. Any boulder height variation provides a direct measurement about the thickness variation of the surrounding dust layer due to the occurring erosion and/or accretion. The analysis concerns the period from 2014 August, inbound to perihelion, to 2016 September, outbound. We measured the erosion in the Ma'at region of 0.6 ± 0.2 m from 2014 September 12 to December 2, and an erosion of 0.4 ± 0.3 m from 2014 December 3 to 2015 February 15. Then, we measured a dust deposition of 0.7 ± 0.3 m during the following perihelion phase, until 2016 May–September. This result confirms the link between the erosion of the Southern hemisphere and the fallout in the northern regions. The Imhotep and Hatmehit regions are characterized by a negligible erosion during the inbound orbit, consistent with pebble-made nucleus models predicting no erosion when the temperature of the nucleus surface is T s < 205 K. [ABSTRACT FROM AUTHOR]

When comets approach the Sun, their surface is heated and the volatile species start to sublimate. Due to the increasing gas pressure, dust is ejected off the surface, which can be observed as cometary coma, dust tail, and trail. However, the underlying physical processes are not fully understood. Using state-of-the-art results for the transport of heat and gas as well as of the mechanical properties of cometary matter, we intend to describe the activity pattern of comets when they approach the Sun. We developed a novel thermophysical model to simulate the dust ejection from comet 67/Churyumov–Gerasimenko's south-pole region at perihelion. Based on the input parameters, this model computes the sub-surface temperature profile, the pressure build-up, and the redistribution of volatiles inside the cometary sub-surface region and provides mass-loss rates of dust and gas as well as typical sizes and ice content of the ejected dust chunks. Our thermophysical model allows for continuous gas and dust ejection from the Southern hemisphere of comet 67/Churyumov–Gerasimenko at perihelion. We find that the model output is in general agreement with the observed Rosetta data. The sublimation of CO2 ice drives the ejection of very large (⁠|$\gtrsim 10\, \mathrm{cm}$|⁠) chunks, which contain |$10\, {{\ \rm per\ cent}}$| to |$90 \, {{\ \rm per\ cent}}$| of the initial water–ice content. In contrast, the outgassing of H2O ice causes the lift-off of small clusters of dust aggregates, which contain no ice. [ABSTRACT FROM AUTHOR]

Abstract: 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 ( ) 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 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. [Copyright &y& Elsevier]

The Comet Interceptor space mission, selected by ESA in June 2019 as the first F-Class mission, will study a dynamically new comet or an interstellar object by a unique multi-point 'snapshot' measurement. The mission design will allow to complement previous single spacecraft's fly-by cometary observations. The Dust Impact Sensor and Counter (DISC), devoted to the dust coma characterization, is part of the payload selected for Comet Interceptor. It will be mounted on-board two of the three spacecraft, as part of the Dust-Fields-Plasma (DFP) suite, dedicated to understand further: 1) dust in the coma; 2) magnetic field; 3) plasma and energetic neutral atoms. DISC architecture originates from the Impact Sensor subsystems, part of the Grain Impact Analyzer and Dust Accumulator (GIADA) that successfully flew on-board the ESA/Rosetta spacecraft. DISC main scientific objectives are: 1) to define the dust mass distribution for particles in the mass range 10−15 − 10−8 kg ejected from the cometary nucleus; 2) to count dust particles with mass > 10−15 kg; 3) to constrain dust particle density/structure. In this paper, we describe DISC design, aims, methods, feasibility and performances evaluations, carried out by real and simulated dust impacts and by retrieving the number of particles, and their corresponding momentum, using the Comet Interceptor's Engineering Dust Coma Model. [ABSTRACT FROM AUTHOR]

Highlights • We report on the performance of GIADA, an instrument onboard Rosetta designed to study the 67P/CG coma dust environment. • Dust contamination of the Grain Detection System (GDS), a GIADA subsystem, did not prevent its nominal operations. • GDS temperature dependence did not add error in the particle cross-section measurements. • During the first 15 months of measurements at comet 67P/CG GIADA operated nominally. Abstract The Grain Impact Analyser and Dust Accumulator (GIADA) instrument onboard Rosetta studied the dust environment of comet 67P/Churyumov–Gerasimenko from 3.7 au inbound, through perihelion, to 3.8 au outbound, measuring the dust flow and the dynamic properties of individual particles. GIADA is composed of three subsystems: (1) Grain Detection System (GDS); (2) Impact Sensor (IS); and (3) Micro-Balances System (MBS). Monitoring the subsystems' performance during operations is an important element for the correct calibration of scientific measurements. In this paper, we analyse the GIADA inflight calibration data obtained by internal calibration devices for the three subsystems during the period from 1 August 2014 to 31 October 2015. The calibration data testify a nominal behaviour of the instrument during these fifteen months of mission; the only exception is a minor loss of sensitivity for one of the two GDS receivers, attributed to dust contamination. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) onboard the European Space Agency's Rosetta spacecraft acquired images of comet 67P/Churyumov–Gerasimenko (67P) and its surrounding dust coma starting from May 2014 until September 2016. In this paper we present methods and results from analysis of OSIRIS images regarding the dust outflow in the innermost coma of 67P. The aim is to determine the global dust outflow behaviour and place constraints on physical processes affecting particles in the inner coma. We study the coma region right above the nucleus surface, spanning from the nucleus centre out to a distance of about 50 km comet centric distance (approximately 25 average comet radii). We primarily adopt an approach used by Thomas and Keller (1990) to study the dust outflow. We present the effects on azimuthally-averaged values of the dust reflectance of non-radial flow and non-point-source geometry, acceleration of dust particles, sublimation of icy dust particles after ejection from the surface, dust particle fragmentation, optical depth effects and the influence of gravitationally bound particles. All of these physical processes could modify the observed distribution of light scattered by the dust coma. In the image analysis, profiles of azimuthally averaged dust brightness as a function of impact parameter b (azimuthal average, “Ā-curve”) were fitted with a simple function that best fits the shape of our profile curves ( f ( b ; u , v , w , z ) = u / b v + w b + z ). The analytical fit parameters ( u, v, w, z ), which hold the key information about the dust outflow behaviour, were saved in a comprehensive database. Through statistical analysis of these information, we show that the spatial distribution of dust follows free-radial outflow behaviour (i.e. force-free radial outflow with constant velocity) beyond distances larger than ∼11.9 km from the comet centre, which corresponds to a relative distance of about 6 average comet radii from the comet centre. Hence, we conclude that beyond this distance, and on average, fragmentation and gravitationally bound particles are negligible processes in determining the optically scattered light distribution in the innermost coma. Closer to the nucleus we observe dust outflow behaviour that deviates from free-radial outflow. A comparison of our result profiles with numerical models using a Direct Simulation Monte Carlo (DSMC) approach with dust particle distributions calculated using a test particle approach has been used to demonstrate the influence of a complex shape and particle acceleration on the azimuthal average profiles. We demonstrate that, while other effects such as fragmentation or sublimation of dust particles cannot be ruled out, acceleration of the dust particles and effects arising from the shape of the irregular nucleus (non-point source geometry) are sufficient to explain the observed dust outflow behaviour from image data analysis. As a by-product of this work, we have calculated “Afρ” values for the 1/r regime. We found a peak in the coma activity in terms of Afρ (normalised to a phase angle of 90°) of ∼210 cm 20 days after perihelion. Furthermore, based on simplified models of particle motion within bound orbits, it is shown that limits on the total cross-sectional area of bound particles might be derived through further analysis. An example is given. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The atmosphere of a comet is formed by the sublimation, due to solar illumination, of its volatile component and the dust particles ejected from its nucleus and entrained by the gas flow. Contemporary dusty-gas coma models take into account numerous physical processes occurring in the coma and a complex geometry of the nucleus. For the description of the dusty-gas flow in the coma, such models introduce a large number of governing parameters characterizing physical properties and processes. The relative role of these processes is not easy to ascribe therefore a relevant inter-comparison of model results becomes difficult. The present work introduces a set of universal, dimensionless parameters, which characterize the dust motion in the inner cometary coma. This approach allows one to: (i) reduce the number of parameters for analysis; (ii) reveal dust flows similarities; (iii) rescale the available numerical solutions. The present work demonstrates application of this approach to a realistic coma model. Description of dust motion with dimensionless parameters allows us to make a parametric study for a broad range of conditions and to find simple analytic approximations (via a polynomial function) of the numerical results suitable for rough estimations of dust density in the coma. • Universal, dimensionless parameters of the dust motion in the inner cometary coma. • The approach which allows reduction of the number of parameters for analysis. • The approach which allows revealing of the dust flow similarities. • The approach which allows re-scaling of the available numerical solutions. • Application to a realistic model of the coma. [ABSTRACT FROM AUTHOR]