1,721,132 research outputs found
Water ice in primitive asteroids?
No full textA short review of the literature dealing with possible signatures of ice in some asteroids is presented. The subject is poorly developed, even if in the last decade several authors started a spectroscopic survey of asteroids in the visible and near-infrared wavelengths in order to find ice features on asteroid spectra. Aqueous alteration of the mineral assemblages has been claimed for a number of B-, F- and for more than 50% of the observed C-type asteroids. The compositional and, consequently, the thermal structure of the asteroid belt supports the hypothesis that ices of volatile elements should be more abundant in the external region of the belt, where pristine materials have not undergone drastic modification processes after the accumulation in the planetesimal swarms between those of Mars and of Jupiter. As of today, there is no evidence of any water, water ice or aqueous alteration materials on the D-type asteroids, which are considered the least altered objects
Are Some Asteroids Dead Comets?
No full textThe relation comet-asteroids is investigated by the comparison between the spectra of primitive asteroids and cometary nuclei
Huygens atmospheric structure instrument of Huygens probe on Cassini mission
No full textIn the framework of the Cassini-Huygens mission towards the Saturnian system, the Huygens Atmospheric Structure Instrument (HASI) is a multisensor package which has been designed to measure the physical quantities characterizing the atmosphere of Titan, Saturn's largest satellite. HASI sensors are devoted to the study of Titan's atmospheric structure and electric properties, and also to provide information on the surface, whatever its phase, solid or liquid. A description of the experiment, its subsystems and sensors package is reported together with their performances. © 2002 International Astronautical Federation. Published by Elsevier Science Ltd. All rights reserved. © 2002 International Astronautical Federation. Published by Elsevier Science Ltd. All rights resreved
Comparing methane and temperature profiles on Titan in 1980 and 2005
No full textThe Huygens Probe data provided a direct measurement of CH4, temperature, and pressure in Titan's atmosphere. This data can be used to compare to the Voyager data in which the effects of CH4, temperature, and pressure were mixed together. Comparison with Huygens data indicates that values of the surface relative humidity of CH4 at the Voyager ingress and egress were between 20% and 45%, and values above 60% are inconsistent with this comparison. The most parsimonious explanation for the Voyager data is that the temperature and CH4 surface humidity at the Voyager ingress and egress profiles were identical to the Huygens values; a surface temperature of 93.65±0.25 K, and a surface relative humidity of 43%. Thus, it is likely that these values have characterized the equatorial region of Titan from 1980 until 2005. The small reduction of 1 K, between the tropopause temperatures of the Voyager profiles and the tropopause temperature of the Huygens profile is explainable by a change in the antigreenhouse flux from the stratosphere from 0.13 to 0.1 of the total average solar flux. This could result from a small seasonal change in the optical properties of the stratospheric haze
Turbulent flow around the Huygens probe: A comparison of algebraic Reynolds stress models
No full textThe turbulent flow around the Huygens probe is characterized by effects such as flows curvature and separation that introduce changes in the turbulence structure, thus invalidating many of the turbulence models widely used for "simple shear flows." In this work the predictions of the standard k-epsilon model, and the Shih et al. (1993) model are compared with an algebraic model developed by the present authors. This algebraic stress model is a way of accounting for the anisotropy of Reynolds stresses without going to the full length of solving the Reynolds stress transport equations. The aim is to establish which models better evaluate the flow around the Huygens probe during the descent phase. The predictions of the axial force coefficient supplied by the model presented below correlate with the experimental data better than the standard k-epsilon model and the Shih et al. model
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A planetary atmosphere simulator: Application to Titan
Full text linkA planetary environment simulation system developed in the framework of the activities connected with the preparation of the Cassini-Huygens mission is presented. In particular, the simulator was designed to recreate Titan's atmospheric conditions specifically to test the HASI instrument, part of the Huygens probe payload. A review of the actual knowledge on Titan's environment is reported. The experimental apparatus and its technical specifications are described together with some applications. Thermal cycles have been done to test the system performance. It has been possible to reproduce the pressure versus temperature profile of the upper atmosphere of Titan for pressures lower than 20 mbar and temperatures higher than 110 K. The thermal insulation ensures better performance regarding conditions of lower temperatures (between 70 and 110 K), improving the stability and internal temperature homogeneity. An application of the simulator as test chamber for nine sets of pressure sensors of the HASI instrument is described together with the results obtained. © 1997 Published by Elsevier Science Ltd. All rights reserved
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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