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   6030篇
  免费   8篇
  国内免费   24篇
航空   2670篇
航天技术   1956篇
综合类   28篇
航天   1408篇
  2021年   47篇
  2019年   33篇
  2018年   213篇
  2017年   168篇
  2016年   136篇
  2015年   52篇
  2014年   151篇
  2013年   180篇
  2012年   178篇
  2011年   283篇
  2010年   240篇
  2009年   336篇
  2008年   359篇
  2007年   240篇
  2006年   129篇
  2005年   179篇
  2004年   169篇
  2003年   176篇
  2002年   127篇
  2001年   184篇
  2000年   84篇
  1999年   113篇
  1998年   134篇
  1997年   86篇
  1996年   100篇
  1995年   149篇
  1994年   131篇
  1993年   81篇
  1992年   100篇
  1991年   40篇
  1990年   47篇
  1989年   106篇
  1988年   36篇
  1987年   43篇
  1986年   43篇
  1985年   176篇
  1984年   120篇
  1983年   98篇
  1982年   97篇
  1981年   177篇
  1980年   55篇
  1979年   39篇
  1978年   37篇
  1977年   42篇
  1976年   31篇
  1975年   43篇
  1973年   29篇
  1972年   31篇
  1970年   29篇
  1969年   31篇
排序方式: 共有6062条查询结果,搜索用时 15 毫秒
151.
Geiss  J.  Bühler  F.  Cerutti  H.  Eberhardt  P.  Filleux  Ch.  Meister  J.  Signer  P. 《Space Science Reviews》2004,110(3-4):307-335
Space Science Reviews - The Apollo Solar Wind Composition (SWC) experiment was designed to measure elemental and isotopic abundances of the light noble gases in the solar wind, and to investigate...  相似文献   
152.
Jurewicz  A.J.G.  Burnett  D.S.  Wiens  R.C.  Friedmann  T.A.  Hays  C.C.  Hohlfelder  R.J.  Nishiizumi  K.  Stone  J.A.  Woolum  D.S.  Becker  R.  Butterworth  A.L.  Campbell  A.J.  Ebihara  M.  Franchi  I.A.  Heber  V.  Hohenberg  C.M.  Humayun  M.  McKeegan  K.D.  McNamara  K.  Meshik  A.  Pepin  R.O.  Schlutter  D.  Wieler  R. 《Space Science Reviews》2003,105(3-4):535-560
Genesis (NASA Discovery Mission #5) is a sample return mission. Collectors comprised of ultra-high purity materials will be exposed to the solar wind and then returned to Earth for laboratory analysis. There is a suite of fifteen types of ultra-pure materials distributed among several locations. Most of the materials are mounted on deployable panels (‘collector arrays’), with some as targets in the focal spot of an electrostatic mirror (the ‘concentrator’). Other materials are strategically placed on the spacecraft as additional targets of opportunity to maximize the area for solar-wind collection. Most of the collection area consists of hexagonal collectors in the arrays; approximately half are silicon, the rest are for solar-wind components not retained and/or not easily measured in silicon. There are a variety of materials both in collector arrays and elsewhere targeted for the analyses of specific solar-wind components. Engineering and science factors drove the selection process. Engineering required testing of physical properties such as the ability to withstand shaking on launch and thermal cycling during deployment. Science constraints included bulk purity, surface and interface cleanliness, retentiveness with respect to individual solar-wind components, and availability. A detailed report of material parameters planned as a resource for choosing materials for study will be published on a Genesis website, and will be updated as additional information is obtained. Some material is already linked to the Genesis plasma data website (genesis.lanl.gov). Genesis should provide a reservoir of materials for allocation to the scientific community throughout the 21st Century. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
153.
Schunker  H.  Donea  A. -C. 《Space Science Reviews》2003,107(1-2):99-102
We present preliminary results from high resolution observations obtained with the Michelson Doppler Imager (MDI) instrument on the SOHO of two large solar flares of 14 July 2000 and 24 November 2000. We show that rapid variations of the line-of-sight magnetic field occured on a time scale of a few minutes during the flare explosions. The reversibility/irreversibility of the magnetic field of both active regions is a very good tool for understanding how the magnetic energy is released in these flares. The observed sharp increase of the magnetic energy density at the time of maximum of the solar flare could involve an unknown component which deposited supplementary energy into the system. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
154.
Israel  G.  Cabane  M.  Brun  J-F.  Niemann  H.  Way  S.  Riedler  W.  Steller  M.  Raulin  F.  Coscia  D. 《Space Science Reviews》2002,104(1-4):433-468
ACP's main objective is the chemical analysis of the aerosols in Titan's atmosphere. For this purpose, it will sample the aerosols during descent and prepare the collected matter (by evaporation, pyrolysis and gas products transfer) for analysis by the Huygens Gas Chromatograph Mass Spectrometer (GCMS). A sampling system is required for sampling the aerosols in the 135'32 km and 22'17 km altitude regions of Titan's atmosphere. A pump unit is used to force the gas flow through a filter. In its sampling position, the filter front face extends a few mm beyond the inlet tube. The oven is a pyrolysis furnace where a heating element can heat the filter and hence the sampled aerosols to 250 °C or 600 °C. The oven contains the filter, which has a thimble-like shape (height 28 mm). For transferring effluent gas and pyrolysis products to GCMS, the carrier gas is a labeled nitrogen 15N2, to avoid unwanted secondary reactions with Titan's atmospheric nitrogen. Aeraulic tests under cold temperature conditions were conducted by using a cold gas test system developed by ONERA. The objective of the test was to demonstrate the functional ability of the instrument during the descent of the probe and to understand its thermal behavior, that is to test the performance of all its components, pump unit and mechanisms. In order to validate ACP's scientific performance, pyrolysis tests were conducted at LISA on solid phase material synthesized from experimental simulation. The chromatogram obtained by GCMS analysis shows many organic compounds. Some GC peaks appear clearly from the total mass spectra, with specific ions well identified thanks to the very high sensitivity of the mass spectrometer. The program selected for calibrating the flight model is directly linked to the GCMS calibration plan. In order not to pollute the two flight models with products of solid samples such as tholins, we excluded any direct pyrolysis tests through the ACP oven during the first phase of the calibration. Post probe descent simulation of flight results are planned, using the much representative GCMS and ACP spare models. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
155.
Eruptive prominences as sources of magnetic clouds in the solar wind   总被引:2,自引:0,他引:2  
Large amounts of coronal material are propelled outward into interplanetary space by Coronal Mass Ejections (CMEs). Thus one might expect to find evidence for expanding flux ropes in the solar wind as well. To prove this assumption magnetic clouds were analyzed and correlated with H-observations of disappearing filaments. When clouds were found to be directly associated with a disappearing filament, the magnetic structure of the cloud was compared with that of the associated filament. Additionally the expansion of magnetic clouds was examined over a wide range of the heliosphere and compared with the expansion observed for erupting prominences.  相似文献   
156.
Geomagnetic micropulsations and diagnostics of the magnetosphere   总被引:3,自引:0,他引:3  
Plasma oscillations in a wide range of spectrum exist in the magnetosphere. Part of them penetrate the ionosphere and are recorded on the earth's surface. In the range of frequencies from millihertz to several hertz, the so-called micropulsations (ULF) are observed. In the range from hundred of hertz to several kilohertz the low-frequency emissions (VLF) are registered. Both types of emissions contain interesting and important information on the physical parameters of the magnetosphere and on the processes developing in it. The following paper describes the main problems of the diagnostics of the magnetosphere, which are based on the surface observations of micropulsations.In the first part of the paper, a short summary of theoretical conceptions on micropulsations is given. The main part of the paper describes the methods of diagnostics of the location of the boundary of the magnetosphere, of cold-plasma concentration in the outer regions of the magnetosphere, as well as of the energies and fluxes of fast charged particles in the geomagnetic trap. Some experimental results of the diagnostics of the parameters of the magnetosphere are given. Advantages and deficiencies of the existing methods of surface diagnostics are discussed, and the directions of further investigations are traced.  相似文献   
157.
An optimum filter for direct A/D conversion   总被引:1,自引:0,他引:1  
The author describes a design procedure to find the optimum finite-impulse-response (FIR) filter coefficients when the characteristics of the bandpass filter preceding the analog-to-digital (A/D) converter are known. A simulation of direct A/D conversion performance shows the improvement over the filter coefficients designed to minimize the maximum error over a specified band of frequencies. DC cancellation may be added to the process with very little change in the coefficients of the optimum FIR filter or its performance.<>  相似文献   
158.
The control of a linear system with random coefficients is studied here. The cost function is of a quadratic form and the random coefficients are assumed to be partially observable by the controller. By means of the stochastic Bellman equation, the optimal control of stochastic dynamic models with partially observable coefficients is derived. The optimal control is shown to be a linear function of the observable states and a nonlinear function of random parameters. The theory is applied to an optimal control design of an aircraft landing in wind gust.  相似文献   
159.
On-Line Computer for Transient Turbine Cascade Instrumentation   总被引:1,自引:0,他引:1  
A 32-channel computer based data acquisition and processing system em has been developed for use with the new type of transient cascade facility at Oxford. This is used for testing turbine blades and nozzle guide vanes at full-scale engine Reynolds and Mach numbers ers with correct wallto-flow temperature ratios. A novel technique for processing transient heat transfer data from thin film surface resistance thermometers has been developed. Measurements of surface ace pressure around blades, and of the upstream turbulence level have been made. The cascade and instrumentation are shown to have advantages both in cost and effectiveness over continuous running cascades.  相似文献   
160.
The current state of knowledge of the chemistry, dynamics and energetics of the upper atmosphere and ionosphere of Venus is reviewed together with the nature of the solar wind-Venus interaction. Because of the weak, though perhaps not negligible, intrinsic magnetic field of Venus, the mutual effects between these regions are probably strong and unique in the solar system. The ability of the Pioneer Venus Bus and Orbiter experiments to provide the required data to answer the questions outstanding is discussed in detail.  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