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   18899篇
  免费   74篇
  国内免费   139篇
航空   10062篇
航天技术   5750篇
综合类   318篇
航天   2982篇
  2021年   163篇
  2018年   219篇
  2016年   156篇
  2014年   447篇
  2013年   538篇
  2012年   439篇
  2011年   624篇
  2010年   440篇
  2009年   771篇
  2008年   824篇
  2007年   407篇
  2006年   468篇
  2005年   445篇
  2004年   449篇
  2003年   556篇
  2002年   495篇
  2001年   597篇
  2000年   401篇
  1999年   475篇
  1998年   458篇
  1997年   348篇
  1996年   418篇
  1995年   486篇
  1994年   481篇
  1993年   361篇
  1992年   352篇
  1991年   252篇
  1990年   247篇
  1989年   431篇
  1988年   206篇
  1987年   240篇
  1986年   245篇
  1985年   650篇
  1984年   532篇
  1983年   419篇
  1982年   489篇
  1981年   616篇
  1980年   248篇
  1979年   188篇
  1978年   189篇
  1977年   146篇
  1976年   155篇
  1975年   197篇
  1974年   180篇
  1973年   161篇
  1972年   188篇
  1971年   148篇
  1970年   144篇
  1969年   148篇
  1967年   142篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The imaging capabilities of the Exosat and Einstein satellites at soft X-ray wavelengths have begun to show that suitable Galactic X-ray sources have extended ( 10 arcmin) haloes due to scattering of soft X-rays by interstellar dust. A simple argument suggests that similar haloes, due to scattering by intergalactic dust, should exist around distant (z 1) quasars and detailed analysis confirms this conclusion. A search for such haloes around suitable X-ray quasars could provide valuable, model-independent, constraints on the amount and origin of intergalactic dust.  相似文献   
992.
The radar cross section of a flat plate has a sin x/x lobe structure. If a synthetic-array radar flies past the plate the resulting image is dependent upon what part of this lobe structure is sampled. When the plate is parallel to the flight path, the image is a line as wide as the plate. If the plate is not parallel, only the discontinuities at the ends of the plate are visible in the image.  相似文献   
993.
This paper presents an overview of the identification and selection process of experiments and payloads for manned space flight missions, emphasizing the scope and magnitude of the problem of doing activity planning and the need for a methodology to assure timely flight and appropriate spacecraft design. Conclusions and results derived from the past several years are presented together with an analysis of the current procedure for defining activity for the space station.  相似文献   
994.
The techniques of recursive estimation and Kalman filtering are applied to the problem of estimation of space vehicle orbits and trajectories using only measurements of the Doppler shifts in radio signals transmitted by or reflected from the target. Two approaches are described. One is a global fit technique in which the parameters of an assumed trajectory model are estimated. The second is a pointwise mapping technique in which the trajectory estimate is extrapolated as the target tracking data is received. The paper provides a tutorial illustration of the direct application of recursive optimization techniques to a class of engineering problems.  相似文献   
995.
996.
Position-Location Solutions by Taylor-Series Estimation   总被引:12,自引:0,他引:12  
Taylor-series estimation gives a least-sum-squared-error solution to a set of simultaneous linearized algebraic equations. This method is useful in solving multimeasurement mixed-mode position-location problems typical of many navigational applications. While convergence is not proved, examples show that most problems do converge to the correct solution from reasonable initial guesses. The method also provides the statistical spread of the solution errors.  相似文献   
997.
998.
Results of the study of the influence of Martian soil analogues, both as described by American scientists and as prepared by us, and of hydrogen peroxide on the viability of microorganisms are presented. The experiments were carried out using mixtures of soil analogues with desert soil and black earth (chernozem) samples, and pure cultures of microorganism. Microorganisms capable of withstanding a concentration of hydrogen peroxide in the medium as high as 1.5–2.0% were isolated. None of the 40 strains of microorganisms studied, all belonging to different systematic and physiological groups, exhibited growth inhibition on solid media in the presence of Martian soil analogues. In view of the fact that Martian soil cannot contain microorganisms in great quantities, we suggest using electroadsorption for their concentration, to make detection reliable. A device was designed for this purpose, using the principle of electroadsorption on a polarisable carrier (sterile cotton wool or cheesecloth). The concentrated suspension of microorganisms thus obtained was then characterized by various physicochemical methods.  相似文献   
999.
This paper describes two rocket experiments “Aelita” with high power lithium plasma injection. The results of onboard magnetometer, massspectrometer, photometer, plasma, corpuscular and ground radar measurements are given. Dynamics and structure of plasma formation are discussed.  相似文献   
1000.
"I can't get that stupid kite in the air! I can't! I can't!" "Oh, come on now, Charlie Brown ... that's no way to talk. The whole trouble with you is that you don't believe in yourself! You don't believe in your own abilities. You've got to say to yourself, 'I believe that I can fly this kite.' Now, go ahead, say to yourself, ' I believe that I can fly this kite.'" "I believe that I can fly this kite." "All right, now say it out loud. Say it over and over." "I believe that I can fly this kite! I believe that I can fly this kite! I actually believe I can fly this kite!" "You do? I'll bet you ten-to-one you're wrong!"  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