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Comet observation and analysis is an area where amateur observers can make a significant contribution. Their observations allow regular monitoring of comets, alerting the professional community to interesting events, and providing raw data to supplement professional data. The links between the professional and amateur communities are therefore very important. Comet analysis is challenging. Encouraging the community to agree consistent methodologies, and parameters for analysis, will result in a more robust data set for monitoring and analysis. The last comet workshop was held in 2015 and the comet community welcomed the opportunity to meet again. The main aims and objectives of the workshop were agreed, following consultation, to be: To foster stronger working relationships and cooperation within the professional and amateur comet community, based on a shared understanding of the challenges and opportunities. To take stock of where cometary science stands post-Rosetta and how Pro-Am observations fit into potential future research. To draw together the various strands of work currently going on within the community, particularly on coordination, techniques, standards and archiving and agree the way forward. To consider how best to encourage, and equip, more people to become involved in the study of comets, whether directly through observation (including access to the Europlanet Telescope Network), or through analysis of online data sources. To explore how cometary science can be used in outreach and education. It was also agreed that speakers should include a wide range of amateurs, students and professional astronomers, and that allowing ample time for panel-led discussions was very important. The panels should include specialists and society representatives. An accessible location was chosen, the Stefanik Observatory in Prague, along with online access. We will report on the outcomes from the hybrid workshop and the proposed next steps. Acknowledgements The workshop has been organised in cooperation with Europlanet 2024 Research Infrastructure (RI), the British Astronomical Association, Planetum Prague, and the Czech cometary community SMPH. Europlanet 2024 RI has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 871149.

The success of the Rosetta mission to comet 67P/Churyumov–Gerasimenko has revolutionized our view of comets, while opening a plethora of new questions. In order to find the answers to them and harness the full potential of the new data, an international consortium named “Cophylab – Comet Physics Laboratory” (Cophylab.space) was launched in 2018. In this project several experiment campaigns were initialized to study cometary properties in a controlled environment. The idea was to isolate individual properties and processes in dedicated laboratory experiments. One of the experiment campaigns was designed to characterize gas flow properties of dry porous materials in a first step, with the aim of developing a model that improves our understanding of the outgassing of comets.Before the initial model is extended to consider the sublimation of volatile components, it needs to be validated by alternative methods, such as numerical simulations. For this purpose, we chose the finite element method, to test the combination of the Darcy and Knudsen flow model, which was used in the preceding study.Our approach was to use the results of the experiment as input in the simulations and compare the output with the measurements. This comparison confirmed the validity of the model and its assumptions. In particular, the sample is assumed to be homogenous and isotropic on a macroscopic scale, so that it can be described by a set of averaged parameters. While this description is relatively accurate for samples with well-defined grain shapes (e.g. spherical glass beads), significant discrepancies occur for inhomogeneous materials such as lunar, Asteroid or Martian analogues.We investigated various aspects that were initially neglected in the evaluation of the measurements, such as channel building in the sample, boundary effects and non-ideal geometry of the experimental setup, which will be complemented by inhomogeneities that occur naturally in random close packing or ballistic deposition samples. Furthermore, we assessed the models range of applicability through a thorough review of the different flow regimes encountered in the measurements. Our findings indicate that boundary effects, as well as non-ideal geometry have a significant influence particularly in samples with larger grains. For finer grained samples on the other hand, inhomogeneities are the most probable cause for discrepancies. The grain size also plays an important role regarding the flow regime and its corresponding parameters.The work for this study was performed in the framework of a master’s thesis, as part of the Cophylab project, which is funded by the D-A-CH program (DFG GU1620/3-1 and BL 298/26-1 / SNF 200021E 177964 / FWF I 3730-N36)

To validate the physical model used to describe gas flow through porous, granular media in recent experiments [1], we conducted a finite element analysis (FEA). The main parameters of a sample, such as gas permeability, Knudsen diffusion coefficient and porosity were determined from the experiment and used as an input for the simulations, with the aim of replicating the pressure evolution (measured quantity) depending on external gas flow. While the results matched well for samples with large and spherical grains, the deviations were significant for materials with small and irregularly shaped grains. An improvement of the model could be achieved with complementary simulation methods and a more detailed analysis of the samples. 1. Introduction In the past decades, multiple cometary space missions (e.g. Deep Impact and Rosetta) have provided us with better data and understanding of the solar system’s primordial objects. The most recent project is a cooperation of six European institutions, called “Cometary Physics Laboratory (CoPhyLab)” and has already produced a plethora of results (see webpage https://cophylab.space/index.php?id=27). In this work we investigated gas flow through porous media with a simulation approach, in order to compare it with the experiment and validate its physical model. 2. Experiment The basis and motivation of this work was the experiment performed by [1] (see Figure 1), where the gas flux through a set of artificial and natural porous materials was analysed. To describe the flux J [mol∙m-2∙s-1] of dry air in the given pressure range, a combination of viscous flow and free molecular diffusion was chosen as the physical model: The gas permeability B­0 (viscous flow) and Knudsen diffusion coefficient DK (molecular diffusion) are the main parameters which describe a sample’s ability to allow gas to flow through it. They were determined by measuring the steady state pressure difference across the inlet and outlet of the sample for different gas flux levels. The porosity of the materials was measured through packing experiments. 3. Simulation Since the physical model describes the gas flow on a macroscopic level, with the main parameters being determined as average values across the whole sample, a FEA is the ideal method. The simulations were computed with the software “COMSOL Multiphysics®”, in which the sample was modelled as a cylinder (see Figure 2). The built-in physics module “Darcy’s Law” was selected to provide the governing equations and all input parameters were taken from the measurements (sample dimensions, B0, DK, porosity, viscosity etc.). In stationary studies, the pressure at the sample outlet and the inward mass flux of the measurements were defined as boundary conditions for the simulation, with the pressure in the rest of the sample being the free variable. For transient studies, the empty volumes of the vacuum chamber, as well as the pressure dependent performance characteristic of the pump were taken into consideration to be able to compare the dynamic process. Furthermore, variation studies of the main parameters were carried out to investigate their individual influence on the pressure evolution. Boundary effects such as the facilitation of gas flow due to lower porosity at the container wall or opening of channels in the sample, or impediment due to the holding sieve, were also analysed by adapting the material parameters for the respective boundary layers. 4. Results For the comparison of stationary simulation results with those of the experiment, the measured pressure at the inlet face of the sample was taken as a boundary condition and for transient simulations also the outlet face pressure (see Figure 3 for an exemplary comparison of results). The deviations were relatively small for synthetic material with large grain sizes (i.e. glass beads with a diameter above 200 µm). However, for smaller grain sizes and natural materials the discrepancies were quite large (up to 35%) and featured no apparent correlation regarding the amount of gas flow. Regarding parameter variation studies, it is important to note that the creation of physical samples where only one parameter varies is very difficult, because many of the parameters are directly or indirectly coupled with each other. Nevertheless, the results indicated that in regimes of low B0 and DK, as is the case for small grain size samples, a variation of either parameter has a large influence on the pressure evolution. This is particularly important when considering measurement and regression errors. When boundary layers were added for the analysis of boundary effects (higher permeability and diffusion at the cylinder wall or in a channel), most of the gas flowed through it and the pressure difference across the sample was minimal. Deviations between simulations with and without the holding sieve, manifested themselves as temporary spikes when the gas flow was increased, with steady state differences below 2%. 5. Summary and Outlook The results indicate that the applied model is well consistent with the measurements of homogenous samples (e.g. synthetic materials with large, spherical grains). However, in the case of small and irregularly shaped grains, the material’s anisotropy could explain the discrepancies. To improve the existing model, it is necessary to analyse the physical samples on a smaller scale and better understand the interdependencies of the governing parameters. This can be achieved by creating virtual samples and using alternative simulation methods (i.e. Monte Carlo), which work on a microscopic scale. 6. Acknowledgements This work was carried out in the framework of the CoPhyLab project funded by the D-A-CH programme (GU 1620/3-1 and BL298/26-1/SNF 200021E 177964/FWF I 3730-N36). References [1] M. Schweighart, W. Macher, G. Kargl, B. Gundlach, H. L. Capelo, Viscous and Knudsen gas flow through dry porous cometary analogue material, Monthly Notices of the Royal Astronomical Society, 2021;, stab934, https://doi.org/10.1093/mnras/stab934

CoPhyLabLaboratory experiments are of major importance to understand the activity of comets and to support future space missions. However, past comet simulation experiments were performed under the assumption that comets are mainly composed of water ice with only a limited amount of dust. In the past years, however, the Rosetta mission has shown that cometary nuclei consist primarily of dust and less volatile materials are present than previously thought. Hence, it is high time to set up a new series of laboratory experiments with the aim to investigate the physics of realistic cometary analogue materials. This task is currently addressed by the CoPhyLab (Comet Physics Laboratory) which is a joint project among different partner institutions. This laboratory aims at studying the physics of cometary analogue materials. This task is approached by firstinvestigating isolated physical properties in so-called small experiments (S experiments). In a next step, the experiment’s complexity is increased step-by-step by either adding further components to the sample, or by studying several physical properties under different conditions (large experiments, which will be performed in the L chamber). S1: the tensile strength of organic materialsThe knowledge of the tensile strength of the cometary surface is of key importance to better understand the activity of comets. The tensile strength determines the strain required to detach material from the surface. As organic materials are ubiquitous in space, they could have played an important role during the planet formation process and are most likely incorporated into cometary nuclei. This S experiment campaign provides new measurements on the tensile strength of various granular organic materials. These materials are investigated by the Brazilian Disc Test and the measured values are normalised to a grain size of one micrometer and a volume filling factor of 0.5 for better comparability. The experiments show that the tensile strength of organic materials ranges over four orders of magnitude. Graphite and paraffin have much higher tensile strengths values compared to silica, whereas the tensile strength of coals is very low. This work demonstrates that organic materials are not generally stickier than silicates, or water ice. S2: gas permeability of analogue materials The cometary nucleus is made of water ice, organics and silicateous dust and the ice is trapped inside the matrix of non-volatiles. Hence, the evolving gas has to stream away from its originating region inside the surface layers towards the surface. This work package has the aim to investigate the gas transport mechanisms through porous cometary analogue materials. Therefore, gas flow measurements are performed to investigate the permeability of several materials, which are chosen to mimic cometary surface properties. With these measurements, the gas permeability and the Knudsen diffusion coefficient of the sample materials are obtained. These simulants are tested with respect to different filling heights, packing properties and grain shapes. The gas flow experiments show that the grain size distribution and the packing density of the samples are primarily influencing the permeability of the sample. S3: thermal conductivity of analogue materialsMeasurements of the thermal properties of analogue materials are essential in interpreting remote sensing data and the findings of in-situ instruments. The thermal properties of the subsurface layers determine the surface temperature of asteroids and comets. The temperature stratification inside planetary object is a key parameter to understand the processing of their interior. This experiment campaign is dedicated to measure the thermal properties of analogue materials. In preparation for these measurements we have set up a small vacuum chamber equipped with an infrared camera and temperature sensors. The samples are illuminated for a short duration by a laser. We then compare the measured temperature profiles with the predictions of a thermophysical model to determine the thermal conductivity of the samples. S4: ejection of materialWhen comets approach the Sun, the sublimation pressure will be reached inside the material. If the tensile strength is exceeded by the evolving pressure, the particles can be ejected from the cometary surface and are accelerated. However, the details of the dust dynamics close to the surface are not understood in detail. The idea of this S experiment campaign is to develop an experimental routine to track ejected particles from a sample composed of granular water ice. Therefore, we recorded the power of the illumination, the temperature of the sample and measured the particle trajectories with an high speed camera. Furthermore, the experiments are also simulated by a thermophysical model. Our experiments show that samples composed of pure granular water ice can eject water-ice particles by the pressure build up of water vapour in their interior. Compressed samples posses an higher activity level (ejection events per second) compared to uncompressed samples. The ejected particles have a non-zero initial velocity which is most probably caused by a very fast acceleration of the particles before the first data point is recorded by the camera. The L chamberThe core of this project is the realisation of a comet simulation chamber which will be capable to utilise multiple instruments to monitor and measure the sample properties before, during and after the experiment campaigns. This chamber will be used to perform long duration experiments at low temperatures and low pressures. At this stage (end of June, 2020), the chamber is already installed in place and is vacuum tight, the cooling shield is assembled and the sample carrier cart as well as the self-made glove box are ready to use. The next steps comprise the integration of the cooling shield and the main cooling system. We foresee to run the first experiments in approximately six weeks from now. During the EPSC conference we will provide a technical overview of the chamber and we will present the first experiments performed in the L chamber. AcknowledgementsThis work was carried out in the framework of the CoPhyLab project funded by the D-A-CH programme (GU 1620/3-1 and BL 298/26-1 / SNF 200021E 177964 / FWF I 3730-N36). DB and JB thank the Deutsches Zentrum f\"ur Luft- und Raumfahrt for support under grant 50WM1846.

Over the last few decades, our picture of comets has been continuously changing and growing due to several successful space missions, as well as cometary simulation projects in the laboratory (e.g. KOSI 1987-1992, CoPhyLab 2018 - 2021). This work aims for a better understanding of the gas transport through a porous cometary surface layer. Therefore, gas flow measurements have been performed in our laboratory to investigate the permeability of several analogue materials, which have been chosen to mimic cometary surface properties.For the first measurements, which we are reporting here, only dry materials, free of volatiles have been selected, to isolate the gas transport from gas production inside the materials. They include glass beads made of soda lime glass, which are sieved into separate fractions to obtain distinct grain size ranges from 45 µm up to 4.3 mm. The Mars simulant JSC-Mars 1 is used in the experiments, as well as JSC-1 as a lunar soil simulant. Furthermore, an Asteroid analogue material named UCF/DSI-CI-2 from the Exolith Lab in Florida is also used. A quartz sand called UK4 mined at a local quarry in Graz is investigated as well. In a further step, a sample is created by mixing different grain size fractions of the glass beads replicating the grain size distribution of the Asteroid simulant.The materials are also treated on a shaking table in order to obtain the packing properties of the samples. For the gas flow experiments a cylindrical sample container, with 4 cm diameter, is filled with the sample (30 mm in height) and placed inside of the vacuum chamber at the interface of two separate volumes. Four pressure sensors covering different pressure ranges monitor the gas pressure in the two volumes. A vacuum pump in the lower volume removes the gas from the chamber and through a gas inlet a defined flow of the test gas (compressed air) is inserted into the upper volume. Due to this set-up, the gas flow can only pass through the sample material. To avoid particle fluidisation and thus a texture change in the sample the gas flow is intentionally directed downwards through the sample. The gas flow is controlled by regulators from 0.15 mg/s up to 19.2 mg/s. Via the measured pressure difference between the upper and lower volume, in equilibrium flow, the gas permeability and the Knudsen diffusion coefficient of the sample material are obtained. The gas flow experiments show that the grain size distribution and the packing density of the sample play a major role for the permeability of the sample. From the analysis of the permeability measurements it is clearly visible that the larger the grains the bigger the permeability. The measured permeability values range from 10-13 to 10-8 m². This work is part of the CoPhyLab project funded by the D-A-CH programme (DFG GU1620/3-1 and BL 298/26-1 / SNF 200021E 177964 / FWF I 3730-N36).