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   7685篇
  免费   25篇
  国内免费   16篇
航空   3786篇
航天技术   2691篇
综合类   203篇
航天   1046篇
  2019年   50篇
  2018年   103篇
  2017年   53篇
  2016年   54篇
  2014年   136篇
  2013年   191篇
  2012年   157篇
  2011年   244篇
  2010年   160篇
  2009年   272篇
  2008年   350篇
  2007年   197篇
  2006年   187篇
  2005年   205篇
  2004年   175篇
  2003年   231篇
  2002年   240篇
  2001年   301篇
  2000年   140篇
  1999年   199篇
  1998年   230篇
  1997年   171篇
  1996年   232篇
  1995年   277篇
  1994年   230篇
  1993年   142篇
  1992年   175篇
  1991年   91篇
  1990年   84篇
  1989年   178篇
  1988年   79篇
  1987年   83篇
  1986年   80篇
  1985年   263篇
  1984年   217篇
  1983年   166篇
  1982年   189篇
  1981年   244篇
  1980年   76篇
  1979年   57篇
  1978年   64篇
  1977年   67篇
  1976年   48篇
  1975年   70篇
  1974年   58篇
  1973年   51篇
  1972年   63篇
  1971年   56篇
  1970年   62篇
  1969年   59篇
排序方式: 共有7726条查询结果,搜索用时 15 毫秒
941.
The metaphor of the unique and strictly bifurcating tree of life, suggested by Charles Darwin, needs to be replaced (or at least amended) to reflect and include processes that lead to the merging of and communication between independent lines of descent. Gene histories include and reflect processes such as gene transfer, symbioses and lineage fusion. No single molecule can serve as a proxy for the tree of life. Individual gene histories can be reconstructed from the growing molecular databases containing sequence and structural information. With some simplifications these gene histories can be represented by furcating trees; however, merging these gene histories into web-like organismal histories, including the transfer of metabolic pathways and cell biological innovations from now-extinct lineages, has yet to be accomplished. Because of these difficulties in interpreting the record retained in molecular sequences, correlations with biochemical fossils and with the geological record need to be interpreted with caution. Advances to detect and pinpoint transfer events promise to untangle at least a few of the intertwined histories of individual genes within organisms and trace them to the organismal ancestors. Furthermore, analysis of the shape of molecular phylogenetic trees may point towards organismal radiations that might reflect early mass extinction events that occurred on a planetary scale.  相似文献   
942.
Editorial     
  相似文献   
943.
A Nonlinear Tracker Using Attitude Measurements   总被引:1,自引:0,他引:1  
The subject of this paper involves tracking the present position of a maneuvering aircraft as well as predicting its future position. A tracking filter is developed that uses aircraft attitude angles (yaw, pitch, roll) in addition to the usual radar measurements. Computer simulation of tracker performance when tracking violently maneuvering aircraft indicates that a dramatic improvement is obtained by using attitude information. The approach taken is to develop a 12-or 15-state extended Kalman filter that models both translational and rotational degrees of freedom. By measuring and estimating attitude it is possible to approximately determine the magnitude and direction of the force system acting on the vehicle and therefore determine vehicle linear acceleration. Knowledge of acceleration is then used to improve the estimate of present and future position of the vehicle being tracked. Simulation of a T-38 aircraft performing a 5 g turn indicates that the new tracker produces maximum trajectory prediction errors that are 36 percent of the errors experienced by more conventional trackers.  相似文献   
944.
    
Energy flow in various large-scale processes of the Earth's magnetosphere is examined. This energy comes from the solar wind, via the dawn-to-dusk convection electric field, a field established primarily by magnetic merging but with viscous-like boundary interaction as a possible contributor. The convection field passes about 5 × 1011 W to the near-Earth part of the plasma sheet, and also moves the plasma earthward. In addition, 1–3 × 1011 W are given to the complex system of the Birkeland currents: about 4 × 1010 of this, on the average, goes to parallel acceleration, chiefly of auroral electrons, about 2–3 times that amount to joule heating of the ionosphere, and the rest heats the ring current. The ring current stores energy (mainly as kinetic energy of particles) of the order of 2 × 1015 J, and this value rises and decays during magnetic storms, on time scales ranging from a fraction of a day to several days. The tail can store comparable amounts as magnetic energy, and appreciable fractions of its energy may be released in substorms, on time scales of tens of minutes. The sporadic power level of such events reaches the order of 3 × 1012 W. The role of magnetic merging in such releases of magnetic energy is briefly discussed, as is the correlation between properties of the solar wind and magnetospheric power levels.  相似文献   
945.
Earth's subsurface offers one of the best possible sites to search for microbial life and the characteristic lithologies that life leaves behind. The subterrain may be equally valuable for astrobiology. Where surface conditions are particularly hostile, like on Mars, the subsurface may offer the only habitat for extant lifeforms and access to recognizable biosignatures. We have identified numerous unequivocally biogenic macroscopic, microscopic, and chemical/geochemical cave biosignatures. However, to be especially useful for astrobiology, we are looking for suites of characteristics. Ideally, "biosignature suites" should be both macroscopically and microscopically detectable, independently verifiable by nonmorphological means, and as independent as possible of specific details of life chemistries--demanding (and sometimes conflicting) criteria. Working in fragile, legally protected environments, we developed noninvasive and minimal impact techniques for life and biosignature detection/characterization analogous to Planetary Protection Protocols. Our difficult field conditions have shared limitations common to extraterrestrial robotic and human missions. Thus, the cave/subsurface astrobiology model addresses the most important goals from both scientific and operational points of view. We present details of cave biosignature suites involving manganese and iron oxides, calcite, and sulfur minerals. Suites include morphological fossils, mineral-coated filaments, living microbial mats and preserved biofabrics, 13C and 34S values consistent with microbial metabolism, genetic data, unusual elemental abundances and ratios, and crystallographic mineral forms.  相似文献   
946.
  总被引:2,自引:0,他引:2  
Complementary to the already well-studied microorganisms, lichens, symbiotic organisms of the mycobiont (fungi) and the photobiont (algae), were used as \"model systems\" in which to examine the ecological potential to resist to extreme environments of outer space. Ascospores (sexual propagules of the mycobiont) of the lichens Fulgensia bracteata, Xanthoria elegans and Xanthoria parietina were exposed to selected space-simulating conditions (up to 16 h of space vacuum at 10(-3) Pa and UV radiation at 160 nm < or = lambda < or = 400 nm), while embedded in the lichen fruiting bodies. After exposure, the ascospores were discharged and their viability was tested as germination capacity on different culture media including those containing Mars regolith simulant. It was found that (i) the germination rate on media containing Mars regolith simulant was as high as on other mineral-containing media, (ii) if enclosed in the ascocarps, the ascospores survived the vacuum exposure, the UV-irradiation as well as the combined treatment of vacuum and UV to a high degree. In general, 50 % or more viable spores were recovered, with ascospores of X. elegans showing the highest survival. It is suggested that ascospores inside the ascocarps are well protected by the anatomical structure, the gelatinous layer and the pigments (parietin and carotene) against the space parameters tested.  相似文献   
947.
    
Detailed derivation of the transfer function for a multigimbal, elastically supported, tuned gyro is presented and comparison made between its characteristics and those of a classical two-axis, free-rotor gyro. Knowledge of the gyro transfer function is necessary for the purpose of servo analysis of the system in which the gyro is used; also, the transfer function is a basis of evaluation of errors caused by angular inputs that occur at twice spin frequency.  相似文献   
948.
In this work a study is performed on the correlation between fast forward interplanetary shock parameters at 1 Astronomical Unit and sudden impulse (SI) amplitudes in the H-component of the geomagnetic field, for periods of solar activity maximum (year 2000) and minimum (year 1995–1996). Solar wind temperature, density and speed, and total magnetic field, were taken to calculate the static pressures (thermal and magnetic) both in the upstream and downstream sides of the shocks. The variations of the solar wind parameters and pressures were then correlated with SI amplitudes. The solar wind speed variations presented good correlations with sudden impulses, with correlation coefficients larger than 0.70 both in solar maximum and solar minimum, whereas the solar wind density presented very low correlation. The parameter better correlated with SI was the square root dynamic pressure variation, showing a larger correlation during solar maximum (r = 0.82) than during solar minimum (r = 0.77). The correlations of SI with square root thermal and magnetic pressure were smaller than with the dynamic pressure, but they also present a good correlation, with r > 0.70 during both solar maximum and minimum. Multiple linear correlation analysis of SI in terms of the three pressure terms have shown that 78% and 85% of the variance in SI during solar maximum and minimum, respectively, are explained by the three pressure variations. Average sudden impulse amplitude was 25 nT during solar maximum and 21 nT during solar minimum, while average square root dynamic pressure variation is 1.20 and 0.86 nPa1/2 during solar maximum and minimum, respectively. Thus on average, fast forward interplanetary shocks are 33% stronger during solar maximum than during solar minimum, and the magnetospheric SI response has amplitude 20% higher during solar maximum than during solar minimum. A comparison with theoretical predictions (Tsyganenko’s model corrected by Earth’s induced currents) of the coefficient of sudden impulse change with solar wind dynamic pressure variation showed excellent agreement, with values around 17 nT/nPa1/2.  相似文献   
949.
950.
The Yohkoh soft X-ray telescope obtains several images every 90 minutes. Data from the declining phase of the solar cycle have been used to compare the X-ray signal with other indicators of activity and to study coronal heating. X-ray emission from a north polar coronal hole is found broadly consistent with results of previous EUV observations. In diffuse emission regions, temperature rises to around 2.2 MK and levels off in the height range 1.5 – 1.9 RO. Such emission underlies streamers and may be the source of the low-speed solar wind. X-ray signatures for Coronal Mass Ejection (CME) events which involve the detection of reduced X-ray intensities in the corona, have been developed with Yohkoh data. CME observations are described  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