1,721,148 research outputs found
Hypervelocity impact in low earth orbit: finding subtle impactor signatures on the Hubble Space Telescope
Full text linkpublisher: Elsevier articletitle: Hypervelocity impact in low earth orbit: finding subtle impactor signatures on the Hubble Space Telescope journaltitle: Procedia Engineering articlelink: http://dx.doi.org/10.1016/j.proeng.2017.09.746 content_type: article copyright: © 2017 The Author(s). Published by Elsevier Ltd.© 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 14th Hypervelocity Impact Symposium 2017. The file attached is the Published/publisher’s pdf version of the article.NHM Repositor
Laboratory investigation of oblique hypervelocity impacts with relevance to in situ meteoroid and space debris detectors
Full text linkInferring the properties of the near-Earth meteoroid and space debris environments as sampled by in situ detectors and retrieved spacecraft surfaces requires understanding of the hypervelocity impact process. To infer the nature of an impact after the event, we need to establish relationships between the relevant impact parameters and the resulting impact features. This task is performed in the laboratory by controlled hypervelocity impact experiments using various acceleration techniques. A facet of understanding impact processes in space is the investigation of behaviour under impact from an oblique angle. Despite the fact that impacts normal to the target surface are the exception under real conditions, impact angle studies are often given low priority in investigation of a material's response to hypervelocity impact, with other parameters such as projectile size, velocity and density being initially studied at normal incidence. The author has identified, in his analysis of space-flown surfaces and previous applications of empirical relationships to space data, two areas where oblique impact studies are lacking leading to an uncertain interpretation of the near-Earth environment. The first of these areas is oblique penetration of thin aluminium targets as observed on capture cell detectors such as the EURECA-TICCE experiment. An experimental programme was performed using the University of Kent's light-gas gun to fire steel and aluminium ball bearings through aluminium plates, covering more angles and target thicknesses than similar previous studies. It is found that the method by which an empirical equation derived from normal impact studies has been modified for application to space data using an assumed angle dependence does not predict the laboratory data well. An alternative method of applying the same laboratory-derived equation is presented that more closely reproduces the oblique experimental data. This new method is shown to give a significantly different estimation of the size distribution of meteoroids and debris when applied to the EURECA-TICCE penetration record. The second area is oblique impacts on solar cells as observed on the EURECA and HST solar arrays. A second experimental programme using the light-gas gun was performed firing 50 f.lm soda-lime glass beads at solar cell samples over a range of impact angles from 0-75° from normal. It is found that, of impact crater features previously used as a guide to impact angle, only the pit circularity is primarily related to impact angle. It is also found that the conchoidal diameter, previously believed to have a power law dependence on impact angle, is insensitive to impact angle for angles less than 45° from normal and decreases in size linearly with the cosine of the impact angle for angles greater than 45° from normal. This experimental programme was extended to determine the survivability of the glass beads to launch in the light-gas gun and it was found that although it is likely that soda-lime glass beads reach the target intact, other commonly used small projectiles such as meteorite-analogue mineral powders do not
Can tardigrades survive Hypervelocity Impacts?
Full text linkThere are many objects in space. It may be unlikely that life evolved there, but if material was removed from a planetary surface as impact ejecta it could contain biological materials and then distribute it across space along with the rocks themselves (e.g. Martian meteorites found on Earth). For this idea, Lithopanspermia, to work, the biological materials or even life itself, needs to be able to survive the shock pressures associated with impacts. So, could tardigrades survive this?
This thesis therefore aims to show whether or not microorganisms like tardigrades can survive hypervelocity impacts. This could demonstrate that not only organic compounds, sugars and spores could survive the phenomenon known as panspermia, but also, complex organisms like tardigrades.To achieve this, we used the Light Gas Gun at University of Kent. A shot program was developed and followed to shoot frozen projectiles containing tardigrades. This was done to simulate a possible asteroid or comet which impacts a planetary body, and contains tardigrades frozen in its ice. Optical microscopy was used to detect any survival and to perform different controls on the tardigrade's development throughout this project.The results are discussed in Chapter 4. They show a survival rate of 100% up to ~1GPa. It seems 825 ms-1 is the threshold for tardigrades survival to these high speed impacts. The targets used were sand, and a new target type of water was tested, with preliminary experiments also reported. A set of experiments should be carried out to confirm these results in the future (this is explained in Chapter 5 as future work)
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
New Techniques and Methods for In-Situ Orbital Debris Detectors
Full text linkThe rapidly increasing population of orbital debris in the near-Earth environment poses a significant hazard for operational spacecraft and future space missions. This has led to an increased need for in-situ detectors capable of observing, and distinguishing between, natural space dust and anthropogenic orbital debris, to both measure their flux and help quantify the threat that they pose. Accordingly, this thesis is concerned with the development of new techniques for in-situ orbital debris detection, with a focus on the development of acoustic thin film time of flight (TOF) detectors, specifically the Debris Resistive Acoustic Grid Orbital NASA-Navy Sensor (DRAGONS) developed by NASA, and my industrial partners AstroAcoustics. TOF detectors are valued for their impactor speed and direction measurement capabilities that allow the distinction between orbital debris and natural space dust particles. The development and successful implementation of thin film TOF detectors, which measure impactor speed via the passage of the impactor through successive thin films, requires that two key questions be answered: Firstly, what is the measurement accuracy of such a detector? Secondly, what effect does passage through the thin film have on the impactor and the resulting speed measurement, i.e. is the impactor decelerated upon passage through the first film? To address these questions prototype detectors based on the DRAGONS concept were constructed, one with two successive 12.5 µm Kapton films and the other with two successive 25 µm Kapton films. These were then impacted with stainless steel projectiles ranging from 0.2 mm to 1 mm in diameter at hypervelocity speeds of ~ 2 km s-1 and ~ 4 km s-1 using the University of Kent's two-stage Light Gas Gun. This range of projectile sizes provided a film thickness to projectile diameter ratio (f/dp) of between 1/80 ≤ f/dp ≤ 1/8. For the largest 1 mm projectiles impacting 12.5 µm Kapton films, no deceleration was observed, and the speed obtained from the detector was found to be accurate to less than 1% error. This confirms that acoustic thin film detectors can measure the speed of 1 mm-sized impactors to a high degree of accuracy and are thus suitable for use in space to measuring this size of orbital debris, which poses the greatest threat to space missions in low Earth orbit (LEO). As f/dp increases, the penetration hole morphology becomes more complex and with-it acoustic signal onset determination decreases in accuracy, resulting in a decrease in speed measurement accuracy. Deceleration was not observed for projectiles ≥ 0.4 mm impacting 12.5 µm Kapton films (f/dp = 1/32), however, as f/dp increased to f/dp = 1/16, deceleration started to occur. Broadly deceleration was found to have size dependent effects, with the absolute film thickness playing a role as well as f/dp. Furthermore, comparison to previous results in the literature would suggest that there is also a material dependence. During the investigation, non-acoustic noise was identified in some of the traces from the polyvinylidene fluoride (PVDF) acoustic sensors. This was observed to coincide with impact light flash produced during projectile impact and is the likely source of this noise. With the space industry moving towards using smaller spacecraft in favour of larger, more traditional spacecraft, a preliminary analysis of the feasibility of using small area detectors applied to small spacecraft (specifically CubeSats), to perform orbital debris flux measurements, was conducted. Traditionally large areas on single spacecraft are required for impact detectors to ensure they provide meaningful statistical data. Thus, the use of small area detectors that can be applied to CubeSats faces an important question: can the accumulation of data from detection areas split over multiple small detectors provide statistically meaningful results? Comparison between the accumulated flux from CubeSat sized surfaces that have previously been exposed to space and predictions from ESA's most up to date space environment modelling software - MASTER 8.0.3 - suggests that the accumulation of detection area does provide statistically meaningful data, with accumulated fluxes within or very close to the estimated minimum uncertainties for the predicted fluxes
Cratering in Marine Environments and on ice
No full textDespite their global importance, little is known about the few existing examples of impacts into marine environments and icy targets. They are among the least understood and studied parts of impact crater geology. The icy impacts are also of great importance in understanding the developments of the outer planets and their satellites such as Mars or Europa. Furthermore, the impact mechanisms, crater formation and collapse, melt production and the ejecta distribution are scarcely known for impact on targets other than the "classical" solid silicates of the continental crust. The reaction of water and ice to impacts clearly deserves a more thorough study. The understanding of impact effects and consequences in the case of aqueous hits, soft sediments and icy targets has not been thoroughly explored and comprises the main focus of this book.
A number of papers in the field of hypervelocity impacts on ice are included. These cover a review of available literature in the field of laboratory studies of such impacts, large impact structures on Titan, predicting impact cratering on a comet nucleus, and a novel report on the survival of bacteria fired at hypervelocity into icy surfaces. This latter paper is concerned with astrobiology and in particular Panspermia (natural migration of life through space)
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
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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