Simulation and experiment of gas diffusion in a granular bed

The diffusion of gas through porous material is important to understand the physical processes underlying cometary activity. We study the diffusion of a rarefied gas (Knudsen regime) through a packed bed of monodisperse spheres via experiments and numerical modelling, providing an absolute value of the diffusion coefficient and compare it to published analytical models. The experiments are designed to be directly comparable to numerical simulations, by using precision steel beads, simple geometries, and a trade-off of the sample size between small boundary effects and efficient computation. For direct comparison, the diffusion coefficient is determined in Direct Simulation Monte Carlo (DSMC) simulations, yielding a good match with experiments. This model is further-on used on a microscopic scale, which cannot be studied in experiments, to determine the mean path of gas molecules and its distribution, and compare it against an analytical model. Scaling with sample properties (particle size, porosity) and gas properties (molecular mass, temperature) is consistent with analytical models. As predicted by these, results are very sensitive on sample porosity and we find that a tortuosity $q(\varepsilon)$ depending linearly on the porosity $\varepsilon$ can well reconcile the analytical model with experiments and simulations. Mean paths of molecules are close to those described in the literature, but their distribution deviates from the expectation for small path lengths. The provided diffusion coefficients and scaling laws are directly applicable to thermophysical models of idealised cometary material.Comment: accepted by MNRA

Observatories of the Solar Corona and Active Regions (OSCAR)

Coronal Mass Ejections (CMEs) and Corotating Interaction Regions (CIRs) are major sources of magnetic storms on Earth and are therefore considered to be the most dangerous space weather events. The Observatories of Solar Corona and Active Regions (OSCAR) mission is designed to identify the 3D structure of coronal loops and to study the trigger mechanisms of CMEs in solar Active Regions (ARs) as well as their evolution and propagation processes in the inner heliosphere. It also aims to provide monitoring and forecasting of geo- effective CMEs and CIRs. OSCAR would contribute to significant advancements in the field of solar physics, improvements of the current CME prediction models, and provide data for reliable space weather forecasting. These objectives are achieved by utilising two spacecraft with identical instrumentation, located at a heliocentric orbital distance of 1 AU from the Sun. The spacecraft will be separated by an angle of 68° to provide optimum stereoscopic view of the solar corona. We study the feasibility of such a mission and propose a preliminary design for OSCAR

Erratum: A space weather mission concept: Observatories of the solar corona and active regions (oscar) (Journal of Space Weather and Space Climate (2015) 5 (A4) DOI: 10.1051/swsc/2015003)

In this erratum we acknowledge EASCO as one of the inspirational mission concepts that helped the development of our original mission concept OSCAR

Initial results from the InSight mission on Mars

NASA’s InSight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed in Elysium Planitia on Mars on 26 November 2018. It aims to determine the interior structure, composition and thermal state of Mars, as well as constrain present-day seismicity and impact cratering rates. Such information is key to understanding the differentiation and subsequent thermal evolution of Mars, and thus the forces that shape the planet’s surface geology and volatile processes. Here we report an overview of the first ten months of geophysical observations by InSight. As of 30 September 2019, 174 seismic events have been recorded by the lander’s seismometer, including over 20 events of moment magnitude Mw = 3–4. The detections thus far are consistent with tectonic origins, with no impact-induced seismicity yet observed, and indi- cate a seismically active planet. An assessment of these detections suggests that the frequency of global seismic events below approximately Mw = 3 is similar to that of terrestrial intraplate seismic activity, but there are fewer larger quakes; no quakes exceeding Mw = 4 have been observed. The lander’s other instruments—two cameras, atmospheric pressure, temperature and wind sensors, a magnetometer and a radiometer—have yielded much more than the intended supporting data for seismometer noise characterization: magnetic field measurements indicate a local magnetic field that is ten-times stronger than orbital estimates and meteorological measurements reveal a more dynamic atmosphere than expected, hosting baroclinic and gravity waves and convective vortices. With the mission due to last for an entire Martian year or longer, these results will be built on by further measurements by the InSight lander

Semantic Information Retrieval in the COMPASS Location System

Abstract. In our previous work, we have described the COMPASS location system that uses multiple information sources to determine the current position of a node. The raw output of this process is a location in geo-coordinates, which is not suitable for many applications. In this paper we present an extension to COMPASS, the so called Translator, that can provide facts about the location like city name, address, room number, etc. to the application. These facts are represented in the Semantic Web RDF/XML language and stored on distributed Geo RDF Servers. The main focus of this paper is a location-based service discovery mechanism which allows a node to find all services that can provide facts about its current location. This discovery service is built upon a structured Peer-to-Peer system implementing a distributed hash table.

Impact penetrometry on a comet nucleus — interpretation of laboratory data using penetration models

The first — and possibly deepest — in situ science measurements on the 46P/Wirtanen nucleus will be made by two sensors of the Rosetta Lander's MUPUS experiment. A piezoelectric shock accelerometer (ANC-M) and a resistance temperature sensor (ANC-T) will be mounted in the Lander's harpoon anchor. This will be shot into the surface at about 60 ms−1 on touchdown, reaching a final depth of between a few centimetres and about 2.5 m, depending on the hardness of the ground and the maximum available cable length. Early indications of the strength of the surface material and any distinct layers should prove valuable to subsequent depth-sensitive investigations, including the MUPUS thermal probe, seismic sounding experiments, the sampling drill and composition analyses of the extracted material. Interpretation of the ANC-M data will help to constrain models of the formation and evolution of the material found at the landing site and document the mechanical and structural context of nearby sampled material. We report on the results of recent test shots performed with a prototype anchor into several porous materials: two types of glass foam, H2O ice and CO2 ice. With the help of data from direct shear tests and quasi-static penetration tests, we interpret the processed deceleration data using a cavity-expansion penetration model. Layers of distinctly different strengths can be detected and located, and the deceleration profiles are in reasonable agreement with the profiles obtained by quasi-static tests. The anchor projectile's long sharp tip tends to smear out the boundaries, however. In applying the penetration model we found that the coefficient of sliding friction and the target's volumetric strain have a much stronger influence on the deceleration profile than the initial target density and angle of internal friction. Very small values of volumetric strain (corresponding to high ‘drag coefficient’) were required to fit deceleration profiles to the measured data for the glass foam, contrary to what we initially expected by inspecting the thin layer of crushed material around the walls of the penetrated channel. We interpret this to mean that such brittle, porous materials as the glass foam (and perhaps highly porous, brittle, cryogenic ice) do not exhibit plastic deformation before failing completely by the crushing of cell walls. The decelerating forces are thus thought to be dominated by momentum transfer to the crushed material and by the crushing strength of the cellular microstructure, rather than by the force required to deform the target plastically. The cavity-expansion model seems to be well-suited to the ice shots, but for the brittle, cellular glass foam, alternative approaches, taking into account the material's microstructure, are needed. As a first step in this direction, a microstructural model linking textural properties of the material (pore and grain size, and relative contact area between grains) is applied to the glass foam data, obtained from quasi-static penetration tests and from direct shear strength tests. It is demonstrated that the dependence of strength on porosity can be well represented by the model suggested. A microstructural model for sintered ices, relating strength properties to porosity and thermal properties, would be useful for interpretation of MUPUS ANC-M data in the context of other physical properties measurements. The work presented here may also have some relevance to the design of future comet landers or penetrators. The harpoon anchor/penetrometer approach could be employed on other minor body landing missions, while the modelling technique is similar in many ways to that appropriate for other penetrometers/penetrators