首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   339篇
  免费   112篇
  国内免费   37篇
航空   179篇
航天技术   147篇
综合类   8篇
航天   154篇
  2025年   3篇
  2024年   14篇
  2023年   16篇
  2022年   23篇
  2021年   31篇
  2020年   25篇
  2019年   24篇
  2018年   22篇
  2017年   18篇
  2016年   20篇
  2015年   18篇
  2014年   38篇
  2013年   22篇
  2012年   16篇
  2011年   28篇
  2010年   18篇
  2009年   18篇
  2008年   25篇
  2007年   18篇
  2006年   17篇
  2005年   18篇
  2004年   10篇
  2003年   14篇
  2002年   5篇
  2001年   4篇
  2000年   7篇
  1999年   2篇
  1998年   4篇
  1997年   3篇
  1996年   2篇
  1995年   1篇
  1992年   2篇
  1990年   1篇
  1981年   1篇
排序方式: 共有488条查询结果,搜索用时 0 毫秒
131.
132.
TEGA, one of several instruments on board of the Phoenix Lander, performed differential scanning calorimetry and evolved gas analysis of soil samples and ice, collected from the surface and subsurface at a northern landing site on Mars. TEGA is a combination of a high temperature furnace and a mass spectrometer (MS) that was used to analyze samples delivered to the instrument via a robotic arm. The samples were heated at a programmed ramp rate up to 1000 °C. The power required for heating can be carefully and continuously monitored (scanning calorimetry). The evolved gases generated during the process can be analyzed with the evolved gas analyzer (a magnetic sector mass spectrometer) in order to determine the composition of gases released as a function of temperature. Our laboratory has developed a sample characterization method using a pyrolyzer integrated to a quadrupole mass spectrometer to support the interpretations of TEGA data. Here we examine the evolved gas properties of six types of hyperarid soils from the Pampas de La Joya in southern Peru (a possible analog to Mars), to which we have added with microorganisms (Salmonella typhimurium, Micrococcus luteus, and Candida albicans) in order to investigate the effect of the soil matrix on the TEGA response. Between 20 and 40 mg of soil, with or without ∼5 mg of lyophilized microorganism biomass (dry weight), were placed in the pyrolyzer and heated from room temperature to 1200 °C in 1 h at a heating rate of 20 °C/min. The volatiles released were transferred to a MS using helium as a carrier gas. The quadrupole MS was ran in scan mode from 10 to 200 m/z. In addition, ∼20 mg of each microorganism without a soil matrix were analyzed. As expected, there were significant differences in the gases released from microorganism samples with or without a soil matrix, under similar heating conditions. Furthermore, samples from the most arid environments had significant differences compared with less arid soils. Organic carbon released in the form of CO2 (ion 44 m/z) from microorganisms evolved at temperatures of ∼326.0 ± 19.5 °C, showing characteristic patterns for each one. Others ions such as 41, 78 and 91 m/z were also found. Interestingly, during the thermal process, the release of CO2 increased and ions previously found disappeared, demonstrating a high-oxidant activity in the soil matrix when it was subjected to high temperature. Finally, samples of soil show CO2 evolved up to 650 °C consistent with thermal decomposition of carbonates. These results indicate that organics mixed with these hyperarid soils are oxidized to CO2. Our results suggest the existence of at least two types of oxidants in these soils, a thermolabile oxidant which is highly oxidative and other thermostable oxidant which has a minor oxidative activity and that survives the heat-treatment. Furthermore, we find that the interaction of biomass added to soil samples gives a different set of breakdown gases than organics resident in the soil. The nature of oxidant(s) present in the soils from Pampas de La Joya is still unknown.  相似文献   
133.
脉冲推力轨道拦截可达性描述及求解方法   总被引:1,自引:1,他引:1       下载免费PDF全文
针对航天器单脉冲轨道拦截可达性分析问题,基于共面变轨、逆轨拦截假设,考虑能量、时间和交会角约束,提出了拦截航天器可拦截区和可发射区的概念。在航天器单脉冲空间可达范围的基础上,进一步考虑了目标轨道的约束,建立了目标轨道命中区的计算策略,对异面轨道交叉点为燃料最省点做出了解释。把拦截可达性相关的问题归纳为8个基本拦截问题,通过这8个问题的组合,描述了考虑能量、时间和交会角约束下拦截问题的可拦截区和可发射区的计算方法。采用圆轨道共面变轨、逆轨拦截场景进行了仿真验证,结果表明该方法能够快速有效地计算出约束条件下航天器的拦截可达范围,能够用于分析特定任务情况下的拦截可达性。  相似文献   
134.
针对纯距离目标跟踪可观测程度较低的问题,提出一种基于角度参数的纯距离目标跟踪改进算法。文中方法将系统的观测区间范围划分为若干个子扇区,每个子扇区采用一个独立的加权扩展卡尔曼滤波器进行纯距离跟踪,利用贝叶斯理论来修正各滤波器的权重,系统的输出为各并行运行的滤波器估计值的加权和,最终获得目标的运动轨迹。系统的仿真结果表明,方法收敛速度快,跟踪精度高于扩展卡尔曼滤波器的跟踪精度。  相似文献   
135.
基于能量法的超声速飞航导弹/冲压发动机一体化研究   总被引:1,自引:0,他引:1  
参考飞机/发动机一体化设计的思路,建立了基于能量法的超声速飞航导弹/冲压发动机一体化设计的约束分析和任务分析模型.简述了超声速飞航导弹任务剖面、约束条件的给定、导弹的质量组成以及冲压发动机安装模型.利用高超声速飞行器的升阻特性,对超声速飞航导弹的升阻特性进行了预测.分析了冲压发动机性能、导弹升阻比对导弹发射总重的影响.约束分析和任务分析的计算结果表明,所建立的一体化约束分析与任务分析模型是合理可行的.  相似文献   
136.
简述了采用3DSTUDIO软件后,给飞机机动飞行科目试飞结果分析带来的重要变化。并从实际应用的角度论述了3DSTUDIO辅助提高飞机设计完善性的原因;展望了将3DSTUDIO软件,应用于辅助完善飞机设计的前景。  相似文献   
137.
Sending man to Mars has been a long-held dream of humankind. NASA plans human planetary explorations using approaches that are technically feasible, have reasonable risks and have relatively low costs. This study presents a novel Multi-Attribute Decision Making (MADM) model for evaluating a range of potential mission scenarios for the human exploration of Mars. The three alternatives identified by the Mission Operations Directorate (MOD) at the Johnson Space Center (JSC) include split mission, combo lander and dual scenarios. The proposed framework subsumes the following key methods: first, the conjunction method is used to minimize the number of alternative mission scenarios; second, the Fuzzy Risk Failure Mode and Effects Analysis (RFMEA) is used to analyze the potential failure of the alternative scenarios; third, the fuzzy group Real Option Analysis (ROA) is used to estimate the expected costs and benefits of the alternative scenarios; and fourth, the fuzzy group permutation approach is used to select the optimal mission scenario. We present the results of a case study at NASA’s Johnson Space center to demonstrate: (1) the complexity of mission scenario selection involving subjective and objective judgments provided by multiple space exploration experts; and (2) a systematic and structured method for aggregating quantitative and qualitative data concerning a large number of competing and conflicting mission events.  相似文献   
138.
    
This paper presents an overview of the analysis performed on the lunar orbit and some of the possible contingencies for the European Student Moon Orbiter (ESMO). Originally scheduled for launch in 2014 –2015 as a piggyback payload, it was the only ESA planned mission to the Moon. By way of a weak stability boundary transfer, ESMO is inserted into an orbit around the Moon. Propellant use is at a premium, so the operational orbit is selected to be highly eccentric. In addition, an optimization is presented to achieve an orbit that is stable for 6 months without requiring orbit maintenance. A parameter study is undertaken to study the sensitivity of the lunar orbit insertion. A database of transfer solutions across 2014 and 2015 is used to study the relation between the robustness of weak capture and the planetary geometry at lunar arrival. A number of example recovery scenarios, where the orbit insertion maneuver partially or completely fails, are also considered.  相似文献   
139.
论当前大学图书馆的使命   总被引:3,自引:0,他引:3  
论述了在当前形势下,中国大学图书馆应重新诠释其使命,即从支持教学改革,支持科学研究,参加大学信息基础建设,保障学生信息平等方面支持高等教育的发展,以及应在支持个人的学习方面发挥自己不可替代的作用。  相似文献   
140.
    
On February 13th 2012, the LARES satellite of the Italian Space Agency (ASI) was launched into orbit with the qualification flight of the new VEGA launcher of the European Space Agency (ESA). The payload was released very accurately in the nominal orbit. The name LARES means LAser RElativity Satellite and summarises the objective of the mission and some characteristics of the satellite. It is, in fact, a mission designed to test Einstein's General Relativity Theory (specifically ‘frame-dragging' and Lense-Thirring effect). The satellite is passive and covered with optical retroreflectors that send back laser pulses to the emitting ground station. This allows accurate positioning of the satellite, which is important for measuring the very small deviations from Galilei–Newton's laws. In 2008, ASI selected the prime industrial contractor for the LARES system with a heavy involvement of the universities in all phases of the programme, from the design to the construction and testing of the satellite and separation system. The data exploitation phase started immediately after the launch under a new contract between ASI and those universities. Tracking of the satellite is provided by the International Laser Ranging Service. Due to its particular design, LARES is the orbiting object with the highest known mean density in the solar system. In this paper, it is shown that this peculiarity makes it the best proof particle ever manufactured. Design aspects, mission objectives and preliminary data analysis will be also presented.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号