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   18432篇
  免费   56篇
  国内免费   124篇
航空   10189篇
航天技术   5248篇
综合类   249篇
航天   2926篇
  2021年   160篇
  2018年   263篇
  2017年   154篇
  2016年   176篇
  2014年   435篇
  2013年   515篇
  2012年   429篇
  2011年   651篇
  2010年   475篇
  2009年   835篇
  2008年   834篇
  2007年   437篇
  2006年   425篇
  2005年   407篇
  2004年   450篇
  2003年   515篇
  2002年   480篇
  2001年   573篇
  2000年   354篇
  1999年   460篇
  1998年   424篇
  1997年   316篇
  1996年   362篇
  1995年   441篇
  1994年   391篇
  1993年   354篇
  1992年   306篇
  1991年   247篇
  1990年   235篇
  1989年   378篇
  1988年   200篇
  1987年   232篇
  1986年   225篇
  1985年   636篇
  1984年   514篇
  1983年   404篇
  1982年   484篇
  1981年   609篇
  1980年   246篇
  1979年   186篇
  1978年   189篇
  1977年   145篇
  1976年   156篇
  1975年   184篇
  1974年   180篇
  1973年   161篇
  1972年   188篇
  1971年   148篇
  1970年   143篇
  1969年   147篇
排序方式: 共有10000条查询结果,搜索用时 409 毫秒
991.
The paper has presented a study of the dependence of the H+ ions concentration in the plasmasphere on geographic longitude. A vast database of measurements of the cold plasma density by the Alpha-3 instrument on board the INTERBALL-1 satellite has been used for the study. Based on these measurements, a dependence of the H+ ions concentration in the filled magnetic flux tube in the plasmasphere in the equatorial plane under quiet geomagnetic conditions has been obtained as a function of geographic longitude. Studies have been performed for two seasons, summer and winter. It has been shown that, during the summer in the near-midnight sector, the minimum in the H+ concentration falls within geographic longitudes of 270°–315°. The ratio of the concentration of H+ ions at various longitudes could reach a factor of three. During the winter, in the near-noon sector, the maximum of the H+ ions concentration falls within longitudes of 180°–225°, whereas the concentration ratio could reach a factor of 2.2.  相似文献   
992.
Lauretta  D. S.  Balram-Knutson  S. S.  Beshore  E.  Boynton  W. V.  Drouet d’Aubigny  C.  DellaGiustina  D. N.  Enos  H. L.  Golish  D. R.  Hergenrother  C. W.  Howell  E. S.  Bennett  C. A.  Morton  E. T.  Nolan  M. C.  Rizk  B.  Roper  H. L.  Bartels  A. E.  Bos  B. J.  Dworkin  J. P.  Highsmith  D. E.  Lorenz  D. A.  Lim  L. F.  Mink  R.  Moreau  M. C.  Nuth  J. A.  Reuter  D. C.  Simon  A. A.  Bierhaus  E. B.  Bryan  B. H.  Ballouz  R.  Barnouin  O. S.  Binzel  R. P.  Bottke  W. F.  Hamilton  V. E.  Walsh  K. J.  Chesley  S. R.  Christensen  P. R.  Clark  B. E.  Connolly  H. C.  Crombie  M. K.  Daly  M. G.  Emery  J. P.  McCoy  T. J.  McMahon  J. W.  Scheeres  D. J.  Messenger  S.  Nakamura-Messenger  K.  Righter  K.  Sandford  S. A. 《Space Science Reviews》2017,212(1-2):925-984

In May of 2011, NASA selected the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) asteroid sample return mission as the third mission in the New Frontiers program. The other two New Frontiers missions are New Horizons, which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on January 1, 2019, and Juno, an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The spacecraft departed for near-Earth asteroid (101955) Bennu aboard an United Launch Alliance Atlas V 411 evolved expendable launch vehicle at 7:05 p.m. EDT on September 8, 2016, on a seven-year journey to return samples from Bennu. The spacecraft is on an outbound-cruise trajectory that will result in a rendezvous with Bennu in November 2018. The science instruments on the spacecraft will survey Bennu to measure its physical, geological, and chemical properties, and the team will use these data to select a site on the surface to collect at least 60 g of asteroid regolith. The team will also analyze the remote-sensing data to perform a detailed study of the sample site for context, assess Bennu’s resource potential, refine estimates of its impact probability with Earth, and provide ground-truth data for the extensive astronomical data set collected on this asteroid. The spacecraft will leave Bennu in 2021 and return the sample to the Utah Test and Training Range (UTTR) on September 24, 2023.

  相似文献   
993.
One of global processes in ionosphere–thermosphere–magnetosphere system is the geomagnetic storms. It is of great importance to develop an algorithm that auto-detects sudden commencement because it could be an indicator of onset of the geomagnetic storm. Automatic detection of geomagnetic sudden commencement is based on time–frequency clusters generated by spectrogram. Proposed algorithm is tested on data set collected from stations belong to the international real-time magnetic observatory network (INTERMAGNET). Maximum standard deviation of algorithm detection times is observed to be one minute of the corresponding arrival times published by National Geophysical Data Center (NGDC).  相似文献   
994.
By using a Doppler Weather Radar (DWR) at Shriharikota (13.66°N & 80.23°E), an Artificial Neural Network (ANN) based technique is proposed to improve the accuracy of rain intensity estimation. Three spectral moments of a Doppler spectra are utilized as an input data to an ANN. Rain intensity, as measured by the tipping bucket rain gauges around the DWR station, are considered as a target values for the given inputs. Rain intensity as estimated by the developed ANN model is validated by the rain gauges measurements. With the help of a developed technique, reasonable improvement in the estimation of rain intensity is observed. By using the developed technique, root mean square error and bias are reduced in the range of 34–18% and 17–3% respectively, compared to ZR approach.  相似文献   
995.

A Time-Delay Integration (TDI) image acquisition and processing system has been developed to capture ICON’s Far Ultraviolet (FUV) Spectrographic Imager data. The TDI system is designed to provide variable-range motion-compensated imaging of Earth’s nightside ionospheric limb and sub-limb scenes viewed from Low Earth Orbit in the 135.6 nm emission of oxygen with an integration time of 12 seconds. As a pre-requisite of the motion compensation the TDI system is also designed to provide corrections for optical distortions generated by the FUV Imager’s optical assembly. On the dayside the TDI system is used to process 135.6 nm and 157.0 nm wavelength altitude profiles simultaneously. We present the TDI system’s design methodology and implementation as an FPGA module with an emphasis on minimization of on-board data throughput and telemetry. We also present the methods and results of testing the TDI system in simulation and with Engineering Ground Support Equipment (EGSE) to validate its performance.

  相似文献   
996.
The requirements for skid landing gear static strength ensuring are examined based on the comparative analysis of the Airworthiness Standards requirements. Probabilistic principle of determining the safety factor of the skid landing gear is presented. The analysis of the experimental data on bench and flight tests is performed for calculating the actual value of this factor.  相似文献   
997.
998.
The discretization of the boundary value problem for laminated composite shells is based on the finite difference approach using the regular mesh with the constant grid step and the difference operators of the second order of accuracy. The dynamic relaxation method is proposed for the solution of the nonlinear problem. The evolutionary equations of the dynamic relaxation are constructed, and the optimum parameters of the converging linear iterative process are estimated.  相似文献   
999.
The dried, fleshy stems of Cistanche deserticola (Orobanchaceae) are popular tonics in Traditional Chinese Medicine (TCM) to treat the inability of kidney in expelling extra fluid in the body, causing fluid retention, and reform reproductive system. However, the wild plants of C. deserticola have become endangered due to habitat downsizing and over-harvesting for its medicinal usages. The present research was carried out for the following purposes: (1) promoting the space-breeding research; (2) providing molecular evidence for agricultural selective breeding; and (3) protecting this endangered herbal medicine and conserving its genetic resources.  相似文献   
1000.
This paper presents a brief review of activities in laminar flow control being performed at the Central Aerohydrodynamic Institute named after Prof. N.E. Zhukovsky (TsAGI). These efforts are focused on the improvement of the existing laminar flow control methods and on the development of new ones. The investigations have demonstrated the effectiveness of aircraft surface laminarization applications with the aim of friction drag reduction. The opportunity of considerable delaying of laminar-turbulent transition due to special wing profile geometry and using boundary layer suction and surface cooling has been verified at sub- and supersonic speeds through various wind tunnel testing at TsAGI and during flying laboratory experiments at the Flight Research Institute (LII). The investigations on using hybrid laminar flow control systems for friction drag reduction were also carried out. New techniques of laminar flow control were proposed, in particular, the method of local heating of the wing leading edge, boundary layer laminarization by means of receptivity control, and electrohydrodynamic methods of boundary layer stability control.  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