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91.
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93.
磁扰动和磁静时近地等离子体片中脉冲电场的分布 总被引:1,自引:1,他引:0
Geotail卫星的电场数据被用于分析近地磁尾等离子体片中电场在磁扰动(Dst<-25nT)和磁静时(Dst>-25 nT的统计分布.结果表明,伴随着地向高速离子流,在X>-16Re以内区域出现强电场(高达 5-8 mV/m).磁扰动期间强电场的幅值较磁静时大,并且出现在更靠近地球的位置.较强和较靠近地球的强电场与磁扰动时更薄的等离子体片和更接近地球的等离子体片内边界相联系.观测结果意味着磁扰动期间的亚暴可能更有效地将高能粒子注射到环电流中.这对磁暴和亚暴的关系问题的解决有重要意义. 相似文献
94.
The current approach to the study of the origin of life and to the search for life elsewhere is based on two assumptions.
First, life is a purely physical phenomenon closely linked to specific environmental conditions. From this, we hypothesise
that when these environmental conditions are met, life will arise and evolve. If these assumptions are valid, the search for
life elsewhere should be a matter of mapping what we know about the range of environments in which life can exist, and then
simply trying to find these environments elsewhere. Second, life can be clearly distinguished from the non-living world. While
a single feature of a living organism left in the rock record is not always sufficient to determine unequivocally whether
life was present, life often leaves multiple structural, mineralogical and chemical biomarkers that, in sum, support a conclusion
that life was present. Our understanding of the habitats that can sustain or have sustained life has grown tremendously with
the characterisation of extremophiles. In this chapter, we highlight the range of environments that are known to harbour life
on Earth, describe the environments that existed during the period of time when life originated on Earth, and compare these
habitats to the suitable environments that are found elsewhere in our solar system, where life could have arisen and evolved. 相似文献
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明确了卫星综合组网和区域组网的概念 ,研究在对地观测卫星区域组网中卫星网的综合设计原则 ,提出确定区域组网中卫星轨道平面与卫星数量的一般方法 ,初步建立卫星网性能分析的指标体系和评估模型。 相似文献
97.
With a maximum time of 12 days out of ground contact and a round-trip light time as high as 56 minutes, The Near Earth Asteroid Rendezvous (NEAR) spacecraft requires a moderate degree of onboard autonomy to react to faults and safe the spacecraft. Beyond the basic safing requirements, additional functions are carried out onboard. For example, on-board calculation of the Sun, Earth, asteroid, and spacecraft positions allow the spacecraft to autonomously orient itself for science and downlink operations. On-board autonomous momentum management during cruise relieves Mission Operations from planning, scheduling, and carrying out many manual momentum dumps. During development, additional operations, such as center-of-mass management during propulsive maneuvers and optical navigation were also considered for onboard autonomy on the NEAR spacecraft, but were not selected. The allocation of functions to onboard software or to ground operations involved tradeoffs such as development time for onboard software versus ground software, resource management, life cycle costs, and spacecraft safety.After two years of cruise operations, considerable experience with the NEAR autonomy system has accrued. The utility of some autonomous capabilities is greater than expected, others less so. Software uploads increased spacecraft autonomy in some cases, and the impact on Mission Operations can be assessed. Allocation of functions between spacecraft autonomy and ground operation during development of future missions can be improved by applying the lessons learned from the NEAR experience. 相似文献
98.
本文研究了在图象模式识别过程中引入人工智能的知识学习、知识表达和知识推理的理论和技术,设计并实现了一个应用于遥感图片解译的计算机视觉专家系统。 为使系统具有较高的智能化程度,本文提出了一种新的推理方法——多重推理法,它具有处理某些不完全知识和错误知识的能力,并且推理过程所运用的部分知识(规则)是从已知样本中自动归纳学习生成的。 相似文献
99.
Many concepts of future space systems involve the use of parabolic mirrors for optical applications. The need for a highly reflective finish means that performance of such systems will be particularly vulnerable to space debris and micrometeoroids. A case study was performed to examine the micrometeoroid and debris hazard posed to an orbiting parabolic mirror. The mirror considered was nominally Earth-pointed in a circular orbit with two candidate altitudes in low Earth orbit (LEO), well within the region inhabited by man-made debris. The timeframes of interest for the two missions were 2002–2004 and 2005–2015. Microgram and larger particles were considered.To perform this study, it was necessary to determine the debris and meteoroid flux across the parabolic surface. To assess sensitivity of results to uncertainity in available data, two approaches were taken. The first approach was an analytical procedure based on use of long duration exposure facility (LDEF) data and published theoretical results. The second approach used two readily available computer models: the ESA MASTER model and NASA's ORDEM96. In addition, an in-house implementation of the Grün meteoroid model was used. While multiple results were available for the total flux and flux distributed over azimuth, only the MASTER model was available for generating the desired elevation data to obtain the flux distribution over the parabolic mirror. In an attempt to bound the uncertainty in the knowledge of the elevation distribution, the results from both the MASTER and ORDEM96 models were processed together to form a separate, hybrid prediction. In addition, results were used in the preliminary design of a protective skirt.This case study elucidated the practical obstacles and considerations in performing a sufficiently accurate debris and meteoroid analysis using data and tools that are readily available to the broad space sector. The resulting procedures are useful in the assessment of the risk posed to optics by the meteoroid and debris environment and in the design of protection. 相似文献
100.
K.C. Arun Kumar G.P. Obi Reddy P. Masilamani Satish Y. Turkar P. Sandeep 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2021,67(1):298-315
In this study, integrated drought monitoring index (IDMI) was proposed as a tool to assess and monitor the spatio-temporal dynamics of agricultural drought during the northeast monsoon season for the period from 2000 to 2016 in Tamil Nadu state, south-eastern part of Indian peninsula. The IDMI is characterized as the principal component of precipitation condition index (PCI), soil moisture condition index (SMCI), temperature condition index (TCI), and vegetation condition index (VCI) derived from time-series satellite observations of climate hazards group infra-red precipitation with stations (CHIRPS), European space agency climate change initiative (ESA-CCI) and moderate resolution imaging spectroradiometer (MODIS). The study shows that in the year 2016, about 44.4 and 17.8% of Tamil Nadu state was under extreme and severe drought conditions, respectively. Sensitivity analysis of the study shows that PCI is the most influential parameter to IDMI, followed by VCI and TCI. The validation of IDMI with 3-month standardized precipitation index (SPI) by using Pearson correlation test shows a strong positive correlation between IDMI and 3-month SPI with correlation coefficient (r) value of 0.73 and 0.77 for the wet (2005) and dry year (2016), respectively. The study clearly demonstrates the potential of IDMI derived from time-series datasets of earth observation satellites as a tool in assessment and monitoring of spatio-temporal dynamics of agricultural drought. The proposed IDMI could be effectively used as a reliable tool to monitor agricultural drought and develop its mitigation strategies to minimise the adverse effects of drought on agriculture, water resources, and livelihoods of the people. 相似文献