Context. The ESA Rosetta spacecraft has been orbiting the nucleus of comet 67P/Churyumov-Gerasimenko since August 2014. The rotation axis of the irregularly shaped nucleus has a large obliquity (52°) ...and is oriented such that the southern hemisphere is insolated during perihelion. Aims. We calculate the change in the rotation period as a function of the cometary orbital position due to forces exerted by cometary activity. Methods. We used a detailed shape model of 67P with >105 facets. We calculated the efficiency of the facets to exert a torque based on their radial distance from the center of mass and their orientation. We applied our thermal model to calculate the diurnal water-ice sublimation rate from each facet. The reaction force per facet combined with its torque efficiency creates a torque and changes the angular momentum. The component of the torque parallel to the spin axis changes the rotation period. Results. Our model shows that the rotation period increases slightly during the approach of the comet to the Sun. It reaches a maximum shortly before equinox and drops rapidly during perihelion passage. The magnitude of the change depends on the actual sublimation rates. The change in sign mainly depends on the shape of the nucleus and not much on the sublimation variation. The roughness of the nucleus has little influence. Conclusions. For the given geometry of the rotation axis, the change in the rotation period is mainly influenced by the sublimation activity of the irregular shape of the nucleus. The rotation period increases until shortly before equinox in early May 2015, in good agreement with observations, and will then become shorter rapidly.
Zeolite Rho is able to successfully separate CO(2) from CH(4) with the highest selectivity ever observed on the basis of pore diameter and surface polarity. The adsorption of CO(2) provokes ...structural changes in the zeolite Rho.
The formation of mesopores in microporous zeolites is generally performed by postsynthesis acid, basic, and steam treatments. The hierarchical pore systems thus formed allow better adsorption, ...diffusion, and reactivity of these materials. By combining organic and inorganic structure-directing agents and high-throughput methodologies, we were able to synthesize a zeolite with a hierarchical system of micropores and mesopores, with channel openings delimited by 28 tetrahedral atoms. Its complex crystalline structure was solved with the use of automated diffraction tomography.
Rationnel La prise en charge diététique constitue la pierre angulaire du traitement du diabète gestationnel (DG), mais est parfois mise en défaut en raison de méconnaissances et/ou de fausses ...croyances. L’objectif de notre étude était de déterminer les connaissances diététiques de patientes présentant un DG préalablement à leur entrée et à distance de leur prise en charge dans un programme d’éducation thérapeutique (ETP) spécifique au DG. Patients et méthodes Nous avons conduit une étude prospective auprès d’un échantillon représentatif de notre programme (n = 50 ; 31 ans ; IMC préconceptionnel 28 kg/m2 ). Les connaissances diététiques étaient évaluées par un questionnaire remis à l’entrée de la journée d’ETP (J1) au terme moyen de 28 semaines d’aménorrhée, puis quinze jours après (J15) lors de la consultation de suivi avec le médecin. Résultats À J1, seulement 38 % des patientes sont capables d’identifier un repas équilibré et 2 sur 3 des aliments glucidiques et les boissons autorisées pendant la grossesse. Le programme ETP améliore significativement ces connaissances (respectivement 84 %, 84 % et 85,8 %). Seulement 14 % connaissent le nombre de produits laitiers à consommer quotidiennement pour couvrir leurs besoins calciques. Si elles savent globalement qu’il est déconseillé de grignoter (92 %), elles savent peu qu’il est possible de réaliser des collations équilibrées (70 %) et que les édulcorants sont utilisables (38 %). Ces fausses-croyances n’existent plus à J15 (respectivement 88 % et 92 %). À J15, 30 % des patientes avouent avoir rencontré des difficultés à mettre en place les conseils prodigués et 56 % avoir ressenti une sensation de faim. Conclusion Les messages diététiques délivrés dans notre programme ETP DG semblent adaptés et assimilés puisque les connaissances globales sont améliorées de 32 % à J15. Leur mise en pratique reste parfois difficile et une réflexion est à engager dans ce sens, en travaillant notamment sur la perception de la sensation de faim et en incluant des conseils spécifiques sur les apports calciques quotidiens recommandés.
This paper is focused on qualification procedures for metal parts manufactured using new additive manufacturing (AM) techniques in the aerospace industry. The main aim is to understand the ...interaction between these technologies and the stringent regulatory framework of this industry in order to develop correct quality assurance and quality control procedures in accordance with the certification process for the technology and spare parts. These include all the testing and validation necessary to implement them, as well as to maintain their capability throughout their life-cycle, specific procedures to manufacture or repair parts, work-flows and records, amongst others. An entire qualification procedure for electron beam melting (EBM) to reproduce and repair an aerospace part has been developed and it is presented in this paper. These will be part of the future quality assurance and quality management systems of those aerospace companies that implement AM in their supply chain.
In this work a new ultrafast data collection strategy for electron diffraction tomography is presented that allows reducing data acquisition time by one order of magnitude. This methodology minimizes ...the radiation damage of beam-sensitive materials, such as microporous materials. This method, combined with the precession of the electron beam, provides high quality data enabling the determination of very complex structures. Most importantly, the implementation of this new electron diffraction methodology is easily affordable in any modern electron microscope. As a proof of concept, we have solved a new highly complex zeolitic structure named ITQ-58, with a very low symmetry (triclinic) and a large unit cell volume (1874.6 Å3), containing 16 silicon and 32 oxygen atoms in its asymmetric unit, which would be very difficult to solve with the state of the art techniques.
The structure and composition of cometary constituents, down to their microscopic scale, are critical witnesses of the processes and ingredients that drove the formation and evolution of planetary ...bodies toward their present diversity. On board Rosetta's lander Philae, the Comet Infrared and Visible Analyser (CIVA) experiment took a series of images to characterize the surface materials surrounding the lander on comet 67P/Churyumov-Gerasimenko. Images were collected twice: just after touchdown, and after Philae finally came to rest, where it acquired a full panorama. These images reveal a fractured surface with complex structure and a variety of grain scales and albedos, possibly constituting pristine cometary material.
Cometary nuclei consist mostly of dust and water ice. Previous observations have found nuclei to be low-density and highly porous bodies, but have only moderately constrained the range of allowed ...densities because of the measurement uncertainties. Here we report the precise mass, bulk density, porosity and internal structure of the nucleus of comet 67P/Churyumov-Gerasimenko on the basis of its gravity field. The mass and gravity field are derived from measured spacecraft velocity perturbations at fly-by distances between 10 and 100 kilometres. The gravitational point mass is GM = 666.2 ± 0.2 cubic metres per second squared, giving a mass M = (9,982 ± 3) × 10(9) kilograms. Together with the current estimate of the volume of the nucleus, the average bulk density of the nucleus is 533 ± 6 kilograms per cubic metre. The nucleus appears to be a low-density, highly porous (72-74 per cent) dusty body, similar to that of comet 9P/Tempel 1. The most likely composition mix has approximately four times more dust than ice by mass and two times more dust than ice by volume. We conclude that the interior of the nucleus is homogeneous and constant in density on a global scale without large voids. The high porosity seems to be an inherent property of the nucleus material.
Aims. Using data from the Rosetta mission to comet 67P/Churyumov–Gerasimenko, we evaluate the physical properties of the surface and subsurface of the nucleus and derive estimates for the thermal ...inertia (TI) and roughness in several regions on the largest lobe of the nucleus. Methods. We have developed a thermal model to compute the temperature on the surface and in the uppermost subsurface layers of the nucleus. The model takes heat conduction, self-heating, and shadowing effects into account. To reproduce the brightness temperatures measured by the MIRO instrument, the thermal model is coupled to a radiative transfer model to derive the TI. To reproduce the spatially resolved infrared measurements of the VIRTIS instrument, the thermal model is coupled to a radiance model to derive the TI and surface roughness. These methods are applied to Rosetta data from September 2014. Results. The resulting TI values from both instruments are broadly consistent with each other. From the millimetre channel on MIRO, we determine the TI in the subsurface to be <80 JK−1 m−2 s−0.5 for the Seth, Ash, and Aten regions. The submillimetre channel implies similar results but also suggests that higher values could be possible. A low TI is consistent with other MIRO measurements and in situ data from the MUPUS instrument at the final landing site of Philae. The VIRTIS results give a best-fitting value of 80 JK−1 m−2 s−0.5 and values in the range 40–160 JK−1 m−2 s−0.5 in the same areas. These observations also allow the subpixel scale surface roughness to be estimated and compared to images from the OSIRIS camera. The VIRTIS data imply that there is significant roughness on the infrared scale below the resolution of the available shape model and that, counter-intuitively, visually smooth terrain (centimetre scale) can be rough at small (micrometre–millimetre) scales, and visually rough terrain can be smooth at small scales.
Adsorption of CO2 and CH4 has been measured on the Na-, K-, and Cs-forms of zeolite Rho (0–9 bar; 283–333 K). Although CH4 is excluded, CO2 is readily taken up, although the uptake at low pressures ...decreases strongly, in the order Na+ > K+ > Cs+. Structural studies by powder X-ray diffraction (PXRD) suggest that cations in intercage window sites block CH4 adsorption; however, in the presence of CO2, the cations can move enough to permit adsorption (several angstroms). Determination of time-averaged cation positions during CO2 adsorption at 298 K by Rietveld refinement against PXRD data shows that (i) in Na-Rho, there is a small relaxation of Na+ cations within single eight-ring (S8R) sites, (ii) in Cs-Rho, D8R cations move to S8R sites (remaining within windows) and two phases of Cs-Rho (I4̅3m, Im3̅m) are present over a wide pressure range, and (iii) in K-Rho, there is relocation of some K+ cations from window sites to cage sites and two phases coexist, each with I4̅3m symmetry, over the pressure range of 0–1 bar. The final cation distributions at high P CO2 are similar for Na-, K-, and Cs-Rho, and adsorption in each case is only possible by “trapdoor”-type cation gating. Complementary studies on K-chabazite (Si/Al = 3) also show changes in time-averaged cation location during CO2 adsorption.
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