1,721,216 research outputs found
Geo-engineering using dust grains in heliotropic elliptical orbits
No full textThis paper examines the concept of a Saturn-like Earth ring comprised of dust grains to offset global warming. A new family of non-Keplerian periodic orbits, under the effects of solar radiation pressure and the Earth’s oblateness J2 perturbation, is selected to increase the lifetime of the passive cloud of particles and, thus, increase the efficiency of this geo-engineering strategy. An analytical model is used to predict the evolution of the dust due to solar-radiation pressure and the J2 effect. The attenuation of the solar radiation can then be calculated from the ring model. In comparison to circular orbits, eccentric orbits yield a more stable environment for small grain sizes and therefore achieve higher efficiencies when the orbital decay of the material is considered. Moreover, the special orbital dynamics experienced by high area-to-mass ratio objects, influenced by solar radiation pressure and the J2 effect, ensure the ring will maintain a permanent heliotropic shape, with dust spending the largest portion of time on the Sun facing side. It is envisaged that small dust grains can be released with an initial Δv to enter an eccentric orbit with Sun-facing apogee. Finally, an estimate of 5.94x1011 kg is computed as the total mass required to offset the effects of global warming
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
On testing laser ablation processes for asteroid deflection
No full textAsteroids represent both an opportunity and a risk. Their pristine environment captures the early formation of the solar system; while their impact potential could result in the mass extinction of life. Amongst the many possibilities to deflect Near Earth Asteroids, laser ablation has been shown to be theoretically one of the most effective cases. However, to have full confidence in this approach current assumptions must be verified and fundamental questions answered. Current models assume that the asteroid’s body is a dense, non porous, homogenous structure. Forsterite is typically used to represent asteroids. However asteroids exist over an extended range of material compositions, initial rotational rates and surface features. Models must therefore be advanced to represent the diversity within the asteroid population. The nature, composition and geometry of the ejecta plume also requires accurate modelling. The affect of porosity on the expansion of the ejecta gas remains unknown. Existing models of contamination, particularly on the optical elements are limited to the rocket-engine equivalent model. Therefore to successfully assess the effectiveness and efficiency of the laser ablation technique a detailed understanding of each of these parameters is required. Supported by the Planetary Society and the Institute of Photonics, a series of self contained experiments were conducted that assessed the formation of the ejecta plume - gas, dust and other particles - and the rate of optical contamination for several different laser ablation events. This was a product of asteroid analogue target material(s) –dense, porous, and inhomogeneous -, surface geometry and laser beam characteristics. Each test occurred within a vacuum and was supported by a complementary in-situ monitoring system. High resolution, high speed cameras enabled reconstruction of the ejecta plume and flow field. Coupled with a thermal camera this enabled plume specific issues - geometry, ejection velocities, temperature profiles and the contamination flow to be examined. Highly polished mirrors were also used to collect the contaminated particles and assess the rate of degradation. This paper will therefore present the design and methodology, experimental considerations and results of the laser ablation experiments. All collected data has been compared against the theoretical prediction. This permitted the calibration of the current analytical modelling technique. Ultimately the experiment provided a detailed insight into the effectiveness of the laser system, and the laser ablation potential as a viable method of deflecting Near Earth Asteroids
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
Heliotropic dust rings for Earth climate engineering
No full textThis paper examines the concept of a Sun-pointing elliptical Earth ring comprised of dust grains to offset global warming. A new family of non-Keplerian periodic orbits, under the effects of solar radiation pressure and the Earth's J(2) oblateness perturbation, is used to increase the lifetime of the passive cloud of particles and, thus, increase the efficiency of this geoengineering strategy. An analytical model is used to predict the orbit evolution of the dust ring due to solar-radiation pressure and the J(2) effect. The attenuation of the solar radiation can then be calculated from the ring model. In comparison to circular orbits, eccentric orbits yield a more stable environment for small grain sizes and therefore achieve higher efficiencies when the orbit decay of the material is considered. Moreover, the novel orbital dynamics experienced by high area-to-mass ratio objects, influenced by solar radiation pressure and the J(2) effect, ensure the ring will maintain a permanent heliotropic shape, with dust spending the largest portion of time on the Sun facing side of the orbit. It is envisaged that small dust grains can be released from a circular generator orbit with an initial impulse to enter an eccentric orbit with Sun-facing apogee. Finally, a lowest estimate of 1 x 10(12) kg of material is computed as the total mass required to offset the effects of global warming
New periodic orbits in the solar sail three-body problem
Full text linkWe identify displaced periodic orbits in the circular restricted three-body problem, wher the third (small) body is a solar sail. In particular, we consider solar sail orbits in the earth-sun system which are high above the exliptic plane. It is shown that periodic orbits about surfaces of artificial equilibria are naturally present at linear order. Using the method of Lindstedt-Poincare, we construct nth order approximations to periodic solutions of the nonlinear equations of motion. In the second part of the paper we generalize to the solar sail elliptical restricted three-body problem. A numerical continuation, with the eccentricity, e, as the varying parameter, is used to find periodic orbits above the ecliptic, starting from a known orbit at e=0 and continuing to the requied eccentricity of e=0.0167. The stability of these periodic orbits is investigated
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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