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   3945篇
  免费   9篇
  国内免费   11篇
航空   1798篇
航天技术   1428篇
综合类   17篇
航天   722篇
  2021年   30篇
  2019年   24篇
  2018年   64篇
  2017年   52篇
  2016年   49篇
  2015年   23篇
  2014年   65篇
  2013年   114篇
  2012年   92篇
  2011年   159篇
  2010年   111篇
  2009年   154篇
  2008年   199篇
  2007年   118篇
  2006年   86篇
  2005年   123篇
  2004年   130篇
  2003年   123篇
  2002年   81篇
  2001年   122篇
  2000年   68篇
  1999年   93篇
  1998年   107篇
  1997年   77篇
  1996年   72篇
  1995年   109篇
  1994年   128篇
  1993年   64篇
  1992年   79篇
  1991年   33篇
  1990年   42篇
  1989年   69篇
  1988年   30篇
  1987年   26篇
  1986年   38篇
  1985年   134篇
  1984年   117篇
  1983年   91篇
  1982年   79篇
  1981年   161篇
  1980年   31篇
  1979年   34篇
  1978年   37篇
  1977年   34篇
  1976年   29篇
  1975年   36篇
  1974年   29篇
  1973年   32篇
  1972年   42篇
  1969年   22篇
排序方式: 共有3965条查询结果,搜索用时 359 毫秒
971.
Terrestrial planets are accreted in a disk orbiting a central star. The basic challenge of their formation consists of assembling micron-sized or smaller dust grains to bodies with over 104 km in diameter. This formation process, ultimately based on collisions, occurs in three very different physical regimes depending upon the size of the bodies present: 1) Early on, micron- to mm-sized dust grains, chondrules and chondrites are strongly coupled to the gas. 2) As they grow larger, gravity increases the collisional cross section allowing runaway growth to occur. 3) After this runaway phase stops from exhaustion of matter in the immediate surroundings of the protoplanets, further growth occurs on timescales typical of mutual gravitational perturbations. The emphasis of this book is on the timescales corresponding to these formation phases as well as the characteristic chemical and isotopic composition of the bodies involved. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   
972.
The nanosatellite BLITS (Ball Lens In The Space) demonstrates a successful design of the new spherical lens type satellite for Satellite Laser Ranging (SLR). The spin parameters of the satellite were calculated from more than 1000 days of SLR data collected from 6 High Repetition Rate (HRR) systems: Beijing, Changchun, Graz, Herstmonceux, Potsdam, Shanghai.  相似文献   
973.
ENGINE SENSOR FAULT DIAGNOSIS USING MAIN AND DECENTRALIZED NEURAL NET WORKS   总被引:1,自引:1,他引:0  
AnalyticalredundancytechniquessuchasextendedKalmanfilter,componentstrackingfilterandsooncandetect,isolateandacommodatefailur...  相似文献   
974.
We discuss the electromagnetic processes in the ULF range which are important for the coupling between the atmosphere, ionosphere, and magnetosphere (AIM). The main attention is given to the Pc1–2 frequency ranges (f≈0.1–10 Hz) where some natural resonances in the AIM system are located. In particular, we consider the resonant structures in the spectra of the magnetic background noise related to the Alfvén resonances in the ionosphere as a possible diagnostic tool for studies of the ionospheric parameters. We also discuss the self-excitation of Alfvén waves in the ionosphere due to the AIM coupling and the role of such waves in the acceleration of electrons in the upper ionosphere and magnetosphere. Precipitation of magnetospheric ions due to their interaction with the ion-cyclotron waves is analyzed in relation to the ionospheric current systems, formation of partial ring current, and the influence of the ionosphere-magnetosphere feedback on the generation of such waves.  相似文献   
975.
Jokipii  J.R.  Wibberenz  G. 《Space Science Reviews》1998,83(1-2):365-368
Space Science Reviews -  相似文献   
976.
During recent years, defense budgets have been cut across all services while the cost of maintaining current standards has risen. The Aircraft Stores Interface Manual (ASIM) Program Office found that maintaining a staff for paper updates was no longer feasible and was tasked by the North Atlantic Treaty Organization (NATO) Air Armament Working Party (AAWP) to digitize the ASIM manuals. Converting this information from paper to electronic media presented several challenges which are discussed. Adopting Paperless Information Technology and replacing “Snail Mail” with the World Wide Web is presented  相似文献   
977.
The coupling between the ionised plasma and the neutral thermospheric particles plays an important role for the dynamics of the upper atmosphere. Significant progress in understanding the related processes has been achieved thanks to the availability of continuous accurate measurements of thermospheric parameters like mass density and wind by high resolution accelerometers on board the satellites CHAMP and GRACE. Here we present some examples of ionosphere-thermosphere coupling where CHAMP observations contributed considerably to their interpretation. We start with the derived properties of the thermosphere at altitudes around 400 km. A new aspect is the significant control of the geomagnetic field geometry on thermospheric features. Phenomena discussed in some depths are the equatorial mass density anomaly, the cusp-related mass density enhancement and the thermospheric response to magnetospheric substorms. Here we consider both the effect on the density and on the wind. A?long predicted process is the wind-driven ionospheric F region dynamo. The high-resolution magnetic field measurements of CHAMP enabled for the first time a systematic study of that phenomenon considering longitudinal, local time, seasonal and solar flux dependences. Some open issues that require further investigations are mentioned at the end.  相似文献   
978.
For a better understanding of the processes which lead to the formation of planetesimals in the early solar nebula, we performed an extensive series of laboratory experiments. We find that the capture velocities in collisions between spherical grains are more than one order of magnitude higher than predicted by Chokshi et al (1993). In contrast, irregular grains have no capture threshold and can be better described by a sticking probability which is typically a few 10%, even for velocities exceeding 10 m/s. However, adhesion forces between spherical, micron-sized particles match the theoretical predictions very well, although contact areas and deformations are of the order of inter-atomic distances only. Aggregation experiments in rarefied turbulent gases reveal the fractal nature of dust aggregates. Mass distribution functions are bell-shaped. Similar behaviour can be found in aggregation experiments with sedimenting particles. Experiments on collision-induced aggregate compaction and fragmentation match the numerical simulations by Dominik and Tielens (1997) very well if revised experimental values of the break-up energy (from our impact experiments) and the rolling-friction force (from our AFM measurements on particle chains) are used. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   
979.
Coastal salinity is characterised by large and variable salinity contrasts on relatively small scales. Measurements of salinity at a resolution compatible with these coastal regions on a regular basis would provide a rich source of information that could be used for a number of applications that have a fundamental bearing on the world’s lifestyle. Doppler radiometry offers an approach to capture such measurements, as it reduces the number of required antennas needed to form an image, compared with an Interferometer type instrument.  相似文献   
980.
The RADiatiOn Monitor (RADOM) is a miniature dosimeter-spectrometer that flew onboard the Chandrayaan-1 lunar mission in order to monitor the local radiation environment. Primary objective of the RADOM experiment was to measure the total absorbed dose, flux of surrounding energetic particles and spectrum of the deposited energy from high energy particles both en-route and in lunar orbit. RADOM was the first experiment to be switched on after the launch of Chandrayaan-1 and was operational until the end of the mission. This paper summarizes the observations carried out by RADOM during the entire life time (22 October 2008–31 August 2009) of the Chandrayaan-1 mission and compares the measurement by RADOM with the radiation belt models such as AP-8, AE-8 and CRRESS.  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