Measurement requirements for a Near-Earth Asteroid impact mitigation demonstration mission   总被引：1，自引：0，他引：1  

Stephen D. Wolters  Andrew J. Ball  Nigel Wells  Christopher Saunders  Neil McBride 《Planetary and Space Science》2011,59(13):1506-1515

A concept for an Impact Mitigation Preparation Mission, called Don Quijote, is to send two spacecrafts to a Near-Earth Asteroid (NEA): an Orbiter and an Impactor. The Impactor collides with the asteroid while the Orbiter measures the resulting change in the asteroid's orbit, by means of a Radio Science Experiment (RSE) carried out before and after the impact. Three parallel Phase A studies on Don Quijote were carried out for the European Space Agency: the research presented here reflects the outcomes of the study by QinetiQ. We discuss the mission objectives with regard to the prioritisation of payload instruments, with emphasis on the interpretation of the impact. The Radio Science Experiment is described and it is examined how solar radiation pressure may increase the uncertainty in measuring the orbit of the target asteroid. It is determined that to measure the change in orbit accurately a thermal IR spectrometer is mandatory, to measure the Yarkovsky effect. The advantages of having a laser altimeter are discussed. The advantages of a dedicated wide-angle impact camera are discussed and the field-of-view is initially sized through a simple model of the impact.  相似文献   

Solitons and other solutions to the quantum Zakharov-Kuznetsov equation     

Ghodrat Ebadi  Aida Mojaver  Daniela Milovic  Stephen Johnson  Anjan Biswas 《Astrophysics and Space Science》2012,341(2):507-513

In this paper, we present G′/G-expansion method, exp-function method, modified F-expansion method as well as the traveling wave hypothesis for finding the exact traveling wave solutions of the quantum Zakharov-Kuznetsov equation which arises in quantum magneto-plasmas. By these methods, rich families of exact solutions have been obtained, including soliton solutions. This work continues to reinforce the idea that the proposed methods, with the help of symbolic computation, provide a powerful mathematical tool for solving nonlinear partial differential equations.  相似文献   

PheniX: a new vision for the hard X-ray sky     

Jean-Pierre Roques  Elisabeth Jourdain  Loredana Bassani  Angela Bazzano  Renaud Belmont  A. J. Bird  E. Caroli  M. Chauvin  D. Clark  N. Gehrels  U. Goerlach  F. Harrisson  P. Laurent  J. Malzac  P. Medina  A. Merloni  S. Paltani  J. Stephen  P. Ubertini  J. Wilms 《Experimental Astronomy》2012,34(2):489-517

We are proposing a mission devoted to high energy X-ray astronomy that is based on a focusing telescope operating in the 1?C200?keV energy range but optimized for the hard X-ray range. The main scientific topics concern: Physics of compact objects: The proximity of compact objects provides a unique laboratory to study matter and radiation in extreme conditions of temperature and density in strong gravitational environment. The emission of high energy photons from these objects is far from being understood. The unprecedented sensitivity in the high energy domain will allow a precise determination of the non-thermal processes at work in the vicinity of compact objects. The full 1?C200?keV energy coverage will be ideal to disentangle the emission processes produced in the spacetime regions most affected by strong-gravity, as well as the physical links: disk?Cthermal emission?Ciron line?Ccomptonisation?Creflection?Cnon-thermal emission?Cjets. Neutron stars?Cmagnetic field?Ccyclotron lines: Time resolved spectroscopy (and polarimetry) at ultra-high sensitivity of AXP, milliseconds pulsars and magnetars will give new tools to study the role of the synchrotron processes at work in these objects. Cyclotron lines?Cdirect measurement of magnetic filed?Cequation of state constraints?Cshort bursts?Cgiant flares could all be studied with great details. AGN: The large sensitivity improvement will provide detailed spectral properties of the high energy emission of AGN??s. This will give a fresh look to the connection between accretion and jet emission and will provide a new understanding of the physical processes at work. Detection of high-redshift active nuclei in this energy range will allow to introduce an evolutionary aspect to high-energy studies of AGN, probing directly the origin of the Cosmic X-ray Background also in the non-thermal range (> 20?keV). Element formation?CSupernovae: The energy resolution achievable for this mission (<0.5?keV) and a large high energy effective area are ideally suited for the 44Ti line study (68 and 78?keV). This radioactive nuclei emission will give an estimate of their quantities and speed in their environment. In addition the study of the spatial structure and spectral emission of SNR will advance our knowledge of the dynamics of supernovae explosions, of particles acceleration mechanisms and how the elements are released in the interstellar medium. Instrumental design: The progress of X-ray focusing optics techniques allows a major step in the instrumental design: the collecting area becomes independent of the detection area. This drastically reduces the instrumental background and will open a new era. The optics will be based on depth-graded multi-layer mirrors in a Wolter I configuration. To obtain a significant effective area in the hundred of keV range a focal length in the 40?C50 meters range (attainable with a deployable mast) is needed. In addition such a mission could benefit from recent progress made on mirror coating. We propose to cover the 1?C200?keV energy range with a single detector, a double-sided Germanium strip detector operating at 80?K. The main features will be: (a) good energy resolution (.150?keV at 5?keV and <.5?keV at 100?keV), (b) 3 dimensional event localization with a low number of electronic chains, (c) background rejection by the 3D localization, (d) polarisation capabilities in the Compton regime.  相似文献   

Sub-electron readout noise in a Skipper CCD fabricated on high resistivity silicon     

Guillermo Fernández Moroni  Juan Estrada  Gustavo Cancelo  Stephen E. Holland  Eduardo E. Paolini  H. Thomas Diehl 《Experimental Astronomy》2012,34(1):43-64

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An Assessment of Snow Avalanche Paths and Forest Dynamics Using Ikonos Satellite Data     

Stephen J. Walsh  Daniel J. Weiss  David R. Butler  George P. Malanson 《国际地球制图》2013,28(2):85-93

Abstract

Ikonos panchromatic and multispectral satellite data were acquired in October 2000 and August 2002 for a test area along US Highway 2, the southern border of Glacier National Park (GNP), Montana, USA. The research goals were to map snow avalanche paths and to characterize vegetation patterns in selected paths for longitudinal (i.e., source, track, and runout) and transverse (i.e., inner, flanking, outer) zones as part of a study of forest dynamics and nutrient flux from paths into terrestrial and aquatic systems. In some valleys, as much as 50 percent of the area may be covered by snow avalanche paths, and as such, serve as an important carbon source servicing terrestrial and aquatic ecosystems. Snow avalanches move woody debris down‐slope by snapping, tipping, trimming, and excavating branches, limbs, and trees, and by injuring and scaring trees that remain in‐place. Further, snow avalanches alter the vegetation structure on paths through secondary plant succession of disturbed areas. Contrast and edge enhancements, Normalized Difference Vegetation Index (NDVI), and the Tasseled Cap greenness and wetness transformations were used to examine vegetation patterns in selected paths that were affected by high magnitude snow avalanches during the winter of 2001-2002. Using image transects organized in longitudinal patterns in paths and in forests, and transects arranged in transverse patterns across the sampled paths, the Tasseled Cap transforms (and NDVI values) were plotted and assessed. Preliminary results suggest that NDVI patterns are different for paths and forests, and Tasseled Cap greenness and wetness patterns are different for longitudinal and transverse zones that describe the morphology of snow avalanche paths. The differentiation of paths from the background forest and the characterization of paths by morphometric zones through remote sensing has implications for mapping forest disturbances and dynamics over time and for large geographic areas and for modeling nutrient flux in terrestrial and aquatic systems.  相似文献   

A quasi-geostrophic numerical model of a rotating internally heated fluid     

Stephen R. Lewis 《地球物理与天体物理流体动力学》2013,107(1-4):31-55

Abstract

A quasi-geostrophic numerical model of flow in a rotating channel is integrated under conditions typical of laboratory experiments with an internally heated annulus system. Compared to a laboratory experiment, or a full Navier-Stokes simulation, the quasi geostrophic numerical model is a simple system. It includes nonlinear interactions, dissipation via conventional parameterizations of Ekman layers and internal diffusion, and a steady forcing term which represents heating near the centre of the channel and cooling near both sides. Explicit boundary layers, cylindrical geometry effects, horizontal variations in static stability and variations in conductivity and diffusivity with temperature are all absent, and ageostrophic advection is incompletely represented. Nevertheless, over a range of parameters, flows are produced which strongly resemble those seen in the laboratory thus suggesting that the most important physical processes are represented. The numerical model is used to map out a regime diagram which includes examples of steady flows, flows with periodic time dependence (wavenumber vacillations) and flows which are irregularly time dependent.  相似文献   

The status of wetlands, threats and the predicted effect of global climate change: the situation in Sub-Saharan Africa     

Stephen Anthony Mitchell 《Aquatic Sciences - Research Across Boundaries》2013,75(1):95-112

The larger wetlands of Sub-Saharan Africa, cover 2,072,775 km² (9.01 %) of the landmass. This paper reviews the major threats, including climate change, to these wetlands, a number of which lie in semi-arid regions. Climate change predictions are that the arid or semi-arid regions of Africa in the latitudes around the Tropics of Capricorn and Cancer will become drier. The future of wetlands is allied to human well-being, and the effects of climate change cannot be de-linked from human activities occurring in and around wetlands. The high productivity of wetlands supports substantial populations of poor people dependent on ecosystem services for at least part of their livelihood. This is particularly so in the semi-arid Sahel in the North and equivalent latitudes in the south, which are seen as vulnerable to climate change largely due to high levels of poverty and low adaptive capacity. While sustainable ecosystem management is a long-term goal, survival is more immediate to poor people depending on ecosystem services for their livelihoods. Population increase and a decrease in the resource base due to predicted decreased rainfall will lead to over exploitation of the resource base. Certain engineering interventions redistribute ecosystem services to the benefit of those upstream or away from the river system. Governance systems play a key role in the sustainable management of resources. Breakdown of governance systems through civil war is seen as a driver of poverty and a major cause of breakdown in resource conservation, increasing the dependence of poor people on ecosystems.  相似文献   

Current state of knowledge regarding the world’s wetlands and their future under global climate change: a synthesis   总被引：1，自引：0，他引：1  

Wolfgang J. Junk  Shuqing An  C. M. Finlayson  Brij Gopal  Jan Květ  Stephen A. Mitchell  William J. Mitsch  Richard D. Robarts 《Aquatic Sciences - Research Across Boundaries》2013,75(1):151-167

Wetlands cover at least 6 % of the Earth’s surface. They play a key role in hydrological and biogeochemical cycles, harbour a large part of the world’s biodiversity, and provide multiple services to humankind. However, pressure in the form of land reclamation, intense resource exploitation, changes in hydrology, and pollution threaten wetlands on all continents. Depending on the region, 30–90 % of the world’s wetlands have already been destroyed or strongly modified in many countries with no sign of abatement. Climate change scenarios predict additional stresses on wetlands, mainly because of changes in hydrology, temperature increases, and a rise in sea level. Yet, intact wetlands play a key role as buffers in the hydrological cycle and as sinks for organic carbon, counteracting the effects of the increase in atmospheric CO₂. Eight chapters comprising this volume of Aquatic Sciences analyze the current ecological situation and the use of the wetlands in major regions of the world in the context of global climate change. This final chapter provides a synthesis of the findings and recommendations for the sustainable use and protection of these important ecosystems.  相似文献   

COMPUTER-ASSISTED TERRAIN STRATIFICATION     

Michael J. Medler  Stephen R. Yool 《自然地理学》2013,34(5):433-443

Terrain influences many ecological processes. Therefore, ecologists must consider terrain effects in sampling designs. Incomplete information on the ecological role of terrain, however, makes it difficult to account for terrain variability in sample designs. As a partial solution, we introduce a protocol that automatically stratifies complex terrain. This stratification method uses image-processing techniques to cluster digital images of slope, elevation, and aspect into homogeneous groupings. The technique is demonstrated for a stratification of complex terrain disturbed by a large forest fire. By identifying terrain complexity at several spatial resolutions, we were able to place transects that sample terrain variability and fire severity representatively across a controlled range of scales. [Key words: terrain, stratification, image processing, sample design.]  相似文献   

GIS data modelling-lessons from the analysis of DTMs     

Stephen Wise 《International journal of geographical information science》2013,27(4):313-318

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