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   6188篇
  免费   37篇
  国内免费   10篇
航空   2969篇
航天技术   2128篇
综合类   192篇
航天   946篇
  2021年   50篇
  2019年   45篇
  2018年   108篇
  2017年   70篇
  2016年   67篇
  2015年   38篇
  2014年   124篇
  2013年   151篇
  2012年   149篇
  2011年   215篇
  2010年   142篇
  2009年   245篇
  2008年   297篇
  2007年   162篇
  2006年   145篇
  2005年   166篇
  2004年   155篇
  2003年   193篇
  2002年   222篇
  2001年   258篇
  2000年   111篇
  1999年   150篇
  1998年   180篇
  1997年   131篇
  1996年   178篇
  1995年   225篇
  1994年   171篇
  1993年   101篇
  1992年   145篇
  1991年   55篇
  1990年   61篇
  1989年   142篇
  1988年   60篇
  1987年   57篇
  1986年   59篇
  1985年   185篇
  1984年   162篇
  1983年   129篇
  1982年   153篇
  1981年   178篇
  1980年   51篇
  1979年   43篇
  1978年   41篇
  1977年   46篇
  1975年   50篇
  1974年   49篇
  1973年   32篇
  1972年   53篇
  1971年   36篇
  1969年   34篇
排序方式: 共有6235条查询结果,搜索用时 0 毫秒
151.
We summarize the high-resolution science that has been done on high redshift galaxies with Adaptive Optics (AO) on the world’s largest ground-based facilities and with the Hubble Space Telescope (HST). These facilities complement each other. Ground-based AO provides better light gathering power and in principle better resolution than HST, giving it the edge in high spatial resolution imaging and high resolution spectroscopy. HST produces higher quality, more stable PSF’s over larger field-of-views in a much darker sky-background than ground-based AO, and yields deeper wide-field images and low-resolution spectra than the ground. Faint galaxies have steadily decreasing sizes at fainter fluxes and higher redshifts, reflecting the hierarchical formation of galaxies over cosmic time. HST has imaged this process in great structural detail to z  6, and ground-based AO and spectroscopy has provided measurements of their masses and other physical properties with cosmic time. Last, we review how the 6.5 m James Webb Space Telescope (JWST) will measure First Light, reionization, and galaxy assembly in the near–mid-IR after 2013.  相似文献   
152.
Stapley P  Pozzo T 《Acta Astronautica》1998,43(3-6):163-179
In normal gravity conditions the execution of voluntary movement involves the displacement of body segments as well as the maintenance of a stable reference value for equilibrium control. It has been suggested that centre of mass (CM) projection within the supporting base (BS) is the stabilised reference for voluntary action, and is conserved in weightlessness. The purpose of this study was to determine if the CM is stabilised during whole body reaching movements executed in weightlessness. The reaching task was conducted by two cosmonauts aboard the Russian orbital station MIR, during the Franco-Russian mission ALTAIR, 1993. Movements of reflective markers were recorded using a videocamera, successive images being reconstructed by computer every 40ms. The position of the CM, ankle joint torques and shank and thigh angles were computed for each subject pre- in- and post-flight using a 7-link mathematical model. Results showed that both cosmonauts adopted a backward leaning posture prior to reaching movements. Inflight, the CM was displaced throughout values in the horizontal axis three times those of pre-flight measures. In addition, ankle dorsi flexor torques inflight increased to values double those of pre- and post-flight tests. This study concluded that CM displacements do not remain stable during complex postural equilibrium tasks executed in weightlessness. Furthermore, in the absence of gravity, subjects changed their strategy for producing ankle torque during spaceflight from a forward to a backward leaning posture.  相似文献   
153.
三旋流器加装外套环对燃烧性能的影响   总被引:2,自引:2,他引:2       下载免费PDF全文
对带有两种不同头部结构(基准型为方案1,头部加装外套环为方案2)的三旋流单头部燃烧室进行了研究,对有两种头部结构的燃烧室进行了流场和燃烧场的数值模拟,在相同的进口试验条件下,用燃气分析法和15点测温耙分别测量了两种头部的燃烧效率、出口冒烟数和出口温度场.结果表明:三旋流器的燃烧效率超过了99.7%,加装外套环后其流场、温度场分布均较基准型有所改善,回流区更加饱满,油气混合和燃油雾化效果增强,出口温度分布系数(OTDF)和出口冒烟数分别降低了36%和25%.   相似文献   
154.
155.
The influence of various control systems of the orbital motion of a technological spacecraft on the level of microacceleration of its internal environment is simulated. Conclusions are drawn about the effectiveness of control systems with different actuators for realization of certain gravitationally sensitive processes onboard a spacecraft.  相似文献   
156.
    
Solid CH(3)CN and solid H(2)O + CH(3)CN were ion irradiated near 10 K to initiate chemical reactions thought to occur in extraterrestrial ices. The infrared spectra of these samples after irradiation revealed the synthesis of new molecules. After the irradiated ices were warmed to remove volatiles, the resulting residual material was extracted and analyzed. Both unhydrolyzed and acid-hydrolyzed residues were examined by both liquid and gas chromatographic-mass spectral methods and found to contain a rich mixture of products. The unhydrolyzed samples showed HCN, NH(3), acetaldehyde (formed by reaction with background and atmospheric H(2)O), alkyamines, and numerous other compounds, but no amino acids. However, reaction products in hydrolyzed residues contained a suite of amino acids that included some found in carbonaceous chondrite meteorites. Equal amounts of D- and L-enantiomers were found for each chiral amino acid detected. Extensive use was made of (13)C-labeled CH(3)CN to confirm amino acid identifications and discriminate against possible terrestrial contaminants. The results reported here show that ices exposed to cosmic rays can yield products that, after hydrolysis, form a set of primary amino acids equal in richness to those made by other methods, such as photochemistry.  相似文献   
157.
Le Traon  P.Y.  Hernandez  F.  Rio  M.H.  Davidson  F. 《Space Science Reviews》2003,108(1-2):239-249
With a precise geoid, GOCE will allow an estimation of absolute dynamic topography from altimetry. The projected benefits to operational oceanography and its applications are analyzed herein. After a brief overview of operational oceanography, we explain how the new geoids will be used in the future to improve real time altimeter products and to better constrain modelling and data assimilation systems. A significant impact is expected both for mesoscale (e.g. better estimations and forecasts of currents for pollution monitoring, marine safety, offshore industry) and climate (better initialization of coupled ocean/atmosphere models) applications. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
158.
  总被引:2,自引:0,他引:2  
An extended Kalman filter is used to process line-of-sight measurements to stars and known landmarks providing a statistical indication of performance in estimating spacecraft attitude, orbital ephemeris, and the bias drift of a set of three strapdown gyros. The landmark measurements were assumed to have been taken from the imagery of an Earth-observing multispectral scanner. It is shown that filtering of these noisy measurements results in highly accurate estimates of the above parameters. Results are given showing the sensitivity of performance to various system parameters such as star tracker accuracy, errors in the knowledge of landmark position, and number of stars and landmarks processed.  相似文献   
159.
组合式三相逆变器负载特性的分析与研究   总被引:4,自引:0,他引:4  
分析了组合式三相逆变器,该逆变器三相分别独立控制,易实现功率器件的软开关、无输出变压器的三相四线制输出和模块化的结构;具体分析了影响该三相逆变器输出电压不平衡度的因素,讨论了其带对称与不对称负载的特性,着重分析了不对称负载对每个单相逆变器的影响,得到了组合式三相逆变器中单模块设计与普通单相逆变器的不同点,并给出了实验数据和相应波形。实验结果表明,组合式三相逆变器具有良好的带不平衡负载的能力和优良的电气性能。  相似文献   
160.
The presence and movement of plasma density fluctuations in the F-region of the ionosphere are studied by monitoring phase and amplitude of radio waves propagating through the region. In this paper, we have used weak scattering theory and assumed the plasma density fluctuations to behave like phase changing diffraction screen. Appropriate relations for scintillation index S4, and phase variance δ? are derived and computed for different parameters of the plasma density irregularities of the ionosphere. SROSS-C2 satellite in situ measurements of plasma density fluctuations, which provide direct information about the structure and morphology of irregularities that are responsible for scintillation of radio waves, were used first time to develop a scintillation model for low latitude. It is observed that the scintillation index S4 and phase variance δ? depends on the strength of the plasma turbulence. Finally, the results obtained from modeling are compared and discussed with the available recent results.  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