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   3906篇
  免费   15篇
  国内免费   8篇
航空   1748篇
航天技术   1451篇
综合类   11篇
航天   719篇
  2021年   31篇
  2018年   63篇
  2017年   53篇
  2016年   51篇
  2015年   22篇
  2014年   74篇
  2013年   109篇
  2012年   93篇
  2011年   164篇
  2010年   113篇
  2009年   156篇
  2008年   202篇
  2007年   115篇
  2006年   85篇
  2005年   113篇
  2004年   127篇
  2003年   127篇
  2002年   81篇
  2001年   119篇
  2000年   61篇
  1999年   86篇
  1998年   107篇
  1997年   77篇
  1996年   72篇
  1995年   109篇
  1994年   125篇
  1993年   65篇
  1992年   76篇
  1991年   32篇
  1990年   42篇
  1989年   68篇
  1988年   32篇
  1987年   27篇
  1986年   38篇
  1985年   125篇
  1984年   115篇
  1983年   94篇
  1982年   79篇
  1981年   157篇
  1980年   32篇
  1979年   35篇
  1978年   37篇
  1977年   34篇
  1976年   29篇
  1975年   34篇
  1974年   29篇
  1973年   31篇
  1972年   41篇
  1971年   22篇
  1970年   23篇
排序方式: 共有3929条查询结果,搜索用时 0 毫秒
971.
972.
We used the ugr magnitudes of 1437467 F-G type main-sequence stars with metal abundance -2?[Fe/H]?+0.2 dex and estimated radial and vertical metallicity gradients for high Galactic-latitude fields, 50°<b?90° and 0°<l?360°, of the Milky Way Galaxy. The radial metallicity gradient d[Fe/H]/dR=-0.042±0.011 dex kpc?1 estimated for the stars with 1.31<z1.74 kpc is attributed to the thin-disc population. While, the radial gradients evaluated for stars at higher vertical distances are close to zero indicating that the thick disc and halo have not undergone a radial collapse phase at least at high Galactic latitudes. The vertical metallicity gradients estimated for stars with three different Galactic latitudes, 50°<b?65°,65°<b?80° and 80°<b?90° do not show a strong indication for Galactic latitude dependence of our gradients. The thin disc, 0.5<z?2 kpc, with a vertical metallicity gradient dFe/H/dz=-0.308±0.018 dex kpc?1, is dominant only in galactocentric distance interval 6<R?10 kpc, while the thick disc (2<z?5 kpc) could be observed in the intervals 6<R?10 and 10<R?15 kpc with compatible vertical metallicity gradients, i.e. dFe/H/dz=-0.164±0.014 dex kpc?1 and dFe/H/dz=-0.172±0.016 dex kpc?1. Five vertical metallicity gradients are estimated for the halo (z>5 kpc) in three galactocentric distance intervals, 6<R?10,10<R?15 and 15<R?20 kpc. The first one corresponding to the interval 6<R?10 kpc is equal to dFe/H/dz=-0.023±0.006 dex kpc?1, while the others at larger galactocentric distances are close to zero. We derived synthetic vertical metallicity gradients for 2,230,167 stars and compared them with the observed ones. There is a good agreement between the two sets of vertical metallicity gradients for the thin disc, while they are different for the thick disc. For the halo, the conspicuous difference corresponds to the galactocentric distance interval 6<R?10 kpc, while they are compatible at higher galactocentric distance intervals.  相似文献   
973.
Observations of comet Halley through CCD and Schmidt plates have been performed at Catania Observatory (Italy) from October 1985 through April 1986. Preliminary results are presented concerning the spatial structures for different spectral ranges of cometary light.  相似文献   
974.
A review is given of recent developments in the theory of emission from a magnetized plasma for accreting neutron star conditions. Some observational data on X-ray pulsars are discussed, and present problems are indicated.  相似文献   
975.
A method and the results of calculating the temperature field distribution for a three-focus plasma formation are presented; the calculation is made by the numerical method with the use of the theoretical Glumb and Krier model. It is proposed that multi-phase plasma formations consisting of solid laser sparks should be used in the absorption chamber of a laser rocket engine (LRE).  相似文献   
976.
977.
    
Specific features of propagation of a wideband rectangular pulse along the route spacecraft-Martian surface-spacecraft caused by the influence of the planetary ionosphere are considered as applied to the problem of radio pulse sounding of the subsurface Martian soil from the Mars-Express satellite. The night Martian ionosphere considerably reduces the energy of the pulse, but does not lead to degradation of its envelope or uncertainty function. When sounding a two-layer surface, the influence of the ionosphere is also manifested in limitation from below of the thickness of the upper layer accessible for measurement, which is more essential than when sounding with the use of a wideband Gaussian pulse. It is demonstrated that the surface sounding is possible through the dayside planet ionosphere at the parameters of operating pulse of the MARSIS radar.  相似文献   
978.
    
This paper describes an approach to the realization of a VLF loop-type antenna system. The theoretical analysis is applied to a four-loop system for Loran-C reception.  相似文献   
979.
980.
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   
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