全文获取类型
收费全文 | 6291篇 |
免费 | 6篇 |
国内免费 | 14篇 |
专业分类
航空 | 3311篇 |
航天技术 | 2068篇 |
综合类 | 193篇 |
航天 | 739篇 |
出版年
2018年 | 84篇 |
2017年 | 47篇 |
2016年 | 43篇 |
2014年 | 88篇 |
2013年 | 125篇 |
2012年 | 123篇 |
2011年 | 191篇 |
2010年 | 131篇 |
2009年 | 222篇 |
2008年 | 271篇 |
2007年 | 133篇 |
2006年 | 104篇 |
2005年 | 120篇 |
2004年 | 152篇 |
2003年 | 190篇 |
2002年 | 206篇 |
2001年 | 239篇 |
2000年 | 125篇 |
1999年 | 145篇 |
1998年 | 198篇 |
1997年 | 125篇 |
1996年 | 173篇 |
1995年 | 201篇 |
1994年 | 200篇 |
1993年 | 129篇 |
1992年 | 161篇 |
1991年 | 80篇 |
1990年 | 88篇 |
1989年 | 167篇 |
1988年 | 72篇 |
1987年 | 79篇 |
1986年 | 91篇 |
1985年 | 211篇 |
1984年 | 159篇 |
1983年 | 155篇 |
1982年 | 154篇 |
1981年 | 197篇 |
1980年 | 86篇 |
1979年 | 64篇 |
1978年 | 75篇 |
1977年 | 51篇 |
1976年 | 49篇 |
1975年 | 65篇 |
1974年 | 58篇 |
1973年 | 44篇 |
1972年 | 63篇 |
1971年 | 63篇 |
1970年 | 43篇 |
1969年 | 51篇 |
1967年 | 38篇 |
排序方式: 共有6311条查询结果,搜索用时 5 毫秒
611.
The original basis for the Lorentz transformations, and thus special relativity, was the assumption that the observed velocity of interaction of light with matter represents a unique velocity of the electromagnetic wave. This arbitrary decision is not borne out by Maxwell's theories or by any test that might prove that EM energy actually travels in a continuum of velocities. The second postulate as stated by Einstein does not deserve the status of a postulate, as it is at once overly restrictive and ultimately phenomenological-the nature of c is based on experimental measurement rather than on analysis of first principles. The radiation continuum model's (RCM) modified second postulate, however, says nothing about the actual propagation of EM energy, but only of the relative speed with which it must interact with matter to be detected. Utilizing this modified light principle we obtain an intuitive Galilean form invariance for Maxwell's equations. RCM places no upper limit on attainable velocities, and allows for the possibility of communications between humans or particles at speeds far in excess of c. This precludes many of the compatibility problems between the highly successful quantum mechanics and relativity theory 相似文献
612.
H.G. Mayr I. Harris F.A. Herrero N.W. Spencer A.E. Hedin R.E. Hartle H.A. Taylor L.E. Wharton F. Varosi H. Volland G.R. Carignan 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1985,5(4):283-288
Thermospheric temperature, composition and wind measurements from the Dynamics Explorer satellite (DE-2) are interpreted using a three dimensional, multiconstituent spectral model. The analysis accounts for tides driven by the absorbed solar radiation as well as energy and momentum coupling involving the magnetosphere and lower atmosphere. We discuss phenomena associated with the annual tide, polar circulation, magnetic storms and substorms. 相似文献
613.
J M Klingler R L Mancinelli M R White 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1989,9(6):173-176
Early Earth and early Mars were similar enough such that past geochemical and climatic conditions on Mars may have also been favorable for the origin of life. However, one of the most striking differences between the two planets was the low partial pressure of dinitrogen (pN2) on early Mars (18 mb). On Earth, nitrogen is a key biological element and in many ecosystems the low availability of fixed nitrogen compounds is the main factor limiting growth. Biological fixation of dinitrogen on Earth is a crucial source of fixed nitrogen. Could the low availability of dinitrogen in the primordial Martian atmosphere have prevented the existence, or evolution of Martian microbiota? Azotobacter vinelandii and Azomonas agilis were grown in nitrogen free synthetic medium under various partial pressures of dinitrogen ranging from 780-0 mb (total atmosphere=1 bar). Below 400 mb the biomass, cell number, and growth rate decreased with decreasing pN2. Both microorganisms were capable of growth at a pN2 as low as 5 mb, but no growth was observed at a pN2 < or = 1 mb. The data appear to indicate that biological nitrogen fixation could have occurred on primordial Mars (pN2=18 mb) making it possible for a biotic system to have played a role in the Martian nitrogen cycle. It is possible that nitrogen may have played a key role in the early evolution of life on Mars, and that later a lack of available nitrogen on that planet (currently, pN2=0.2 mb) may have been involved in its subsequent extinction. 相似文献
614.
H. Fuke J.E. Koglin T. Yoshida T. Aramaki W.W. Craig L. Fabris F. Gahbauer C.J. Hailey F.J. Jou N. Madden K. Mori H.T. Yu K.P. Ziock 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(12):2056-2060
We discuss current progress and future plans for the general antiparticle spectrometer experiment (GAPS). GAPS detects antideuterons through the X-rays and pions emitted during the deexcitation of exotic atoms formed when the antideuterons are slowed down and stopped in targets. GAPS provides an exceptionally sensitive means to detect cosmic-ray antideuterons. Cosmic-ray antideuterons can provide indirect evidence for the existence of dark matter in such form as neutralinos or Kaluza–Klein particles. We describe results of accelerator testing of GAPS prototypes, tentative design concepts for a flight GAPS detector, and near-term plans for flying a GAPS prototype on a balloon. 相似文献
615.
David W. Forslund 《Space Science Reviews》1985,42(1-2):3-16
With the increasing size and speed of modern supercomputers, the incredibly complex nonlinear properties of plasmas in the
laboratory and in space are being successfully explored in increasing depth. Of particular importance have been numerical
simulation techniques involving finite size particles on a discrete mesh. After discussing the importance of this means of
understanding a variety of nonlinear plasma phenomena, we describe the basic elements of particle-in-cell simulation and their
limitations and advantages. The differencing techniques, stability and accuracy issues, data management and optimization issues
are discussed by a simple example of a particle-in-cell code. Recent advances in simulation methods allowing large space and
time scales to be treated with minimal sacrifice in physics are reviewed. Various examples of nonlinear processes successfully
studied by plasma simulation are given. 相似文献
616.
R.H. Miller 《Progress in Aerospace Sciences》1985,22(2):113-160
The complex problems associated with the aerodynamics of blade/vortex interactions, the dynamics of a blade displaced out of its plane of rotation, and the position control of a hovering helicopter are discussed. Simplified models help to describe, and appear to be adequate for analysing, many aspects of these problems. Free wake analyses are shown to be of importance in determining blade loads and performance for hovering flight or for wind turbines. Unsteady aerodynamics and careful definition of the wake formation are required when computing vibratory airloads in forward flight. 相似文献
617.
Complex honeycomb space structures (i.e. antennas, solar panels, etc.) must be inspected and accurately tested before flight.The thermography can be employed with success for the detection of the position of defects (delaminations, noneffective bondings, cracks, etc.) and for the evaluation of their size and geometry in all the cases in which the defect acts as a thermal resistance due to the low conductivity of the air filling the defect volume.The basic idea is to create in the specimen a heat flow distribution that is altered by the presence of the defect.The surface temperature distribution is then measured by means of a thermograph and is correlated with the presence of the defect.A numerical analysis and preliminary experiments have been carried out which show the feasibility of the method as applied to honeycomb structures. 相似文献
618.
A study of the effect of observation errors on the best least squares estimate of satellite attitude based on two sets of direction measurements, where one set contains two independent measurements which are corrupted by zero-mean normally distributed additive errors, is summarized. Using Monte-Carlo techniques, the statistics of the estimated rotation of the satellite, from a known reference, were obtained. It was found that one of the parameterizations of the rotation, namely, the vector z, is best suited for estimation, since ?z is normally distributed with zero mean and its standard deviation is independent of the two direction measurements. 相似文献
619.
620.