全文获取类型
收费全文 | 6981篇 |
免费 | 14篇 |
国内免费 | 22篇 |
专业分类
航空 | 3533篇 |
航天技术 | 2472篇 |
综合类 | 29篇 |
航天 | 983篇 |
出版年
2019年 | 43篇 |
2018年 | 96篇 |
2017年 | 52篇 |
2016年 | 49篇 |
2014年 | 120篇 |
2013年 | 187篇 |
2012年 | 143篇 |
2011年 | 212篇 |
2010年 | 162篇 |
2009年 | 259篇 |
2008年 | 325篇 |
2007年 | 166篇 |
2006年 | 177篇 |
2005年 | 179篇 |
2004年 | 155篇 |
2003年 | 215篇 |
2002年 | 126篇 |
2001年 | 240篇 |
2000年 | 136篇 |
1999年 | 176篇 |
1998年 | 207篇 |
1997年 | 157篇 |
1996年 | 209篇 |
1995年 | 251篇 |
1994年 | 235篇 |
1993年 | 146篇 |
1992年 | 154篇 |
1991年 | 86篇 |
1990年 | 78篇 |
1989年 | 163篇 |
1988年 | 82篇 |
1987年 | 79篇 |
1986年 | 83篇 |
1985年 | 245篇 |
1984年 | 197篇 |
1983年 | 157篇 |
1982年 | 181篇 |
1981年 | 218篇 |
1980年 | 69篇 |
1979年 | 49篇 |
1978年 | 69篇 |
1977年 | 58篇 |
1976年 | 43篇 |
1975年 | 76篇 |
1974年 | 50篇 |
1973年 | 51篇 |
1972年 | 62篇 |
1971年 | 50篇 |
1970年 | 52篇 |
1969年 | 44篇 |
排序方式: 共有7017条查询结果,搜索用时 15 毫秒
441.
B. I. Rabinovich 《Cosmic Research》2007,45(5):393-407
This paper continues the series of papers [1–5] and generalizes the previous results to a proto-ring of magnetized plasma whose density decreases in the radial direction. The problem of quantization of the sector and orbital velocities, and of the radii and periods of revolution of elite plasma rings is considered. A new concept of super-elite rings is introduced. Their isomorphism with the orbits of the planets and planetary satellites in the Solar System is proved. This isomorphism also extends to the orbits of electrons in the Bohr’s model of the hydrogen atom. 相似文献
442.
443.
Billingham J 《Acta Astronautica》1990,21(2):69-72
This paper attempts to apply a traditional risk and value analysis to the Search for Extraterrestrial Intelligence--SETI. In view of the difficulties of assessing the probability of success, a comparison is made between SETI and a previous search for extraterrestrial life, the biological component of Project Viking. Our application of simple Utility Theory, given some reasonable assumptions, suggests that SETI is at least as worthwhile as the biological experiment on Viking. 相似文献
444.
While the microbial diversity of a spacecraft assembly facility at the Jet Propulsion Laboratory (Pasadena, CA) was being monitored, H2O2-resistant bacterial strains were repeatedly isolated from various surface locations. H2O2 is a possible sterilant for spacecraft hardware because it is a low-temperature process and compatible with various modern-day spacecraft materials, electronics, and components. Both conventional biochemical testing and molecular analyses identified these strains as Bacillus pumilus. This Bacillus species was found in both unclassified (entrance floors, anteroom, and air-lock) and classified (floors, cabinet tops, and air) locations. Both vegetative cells and spores of several B. pumilus isolates were exposed to 5% liquid H2O2 for 60 min. Spores of each strain exhibited higher resistance than their respective vegetative cells to liquid H2O2. Results indicate that the H2O2 resistance observed in both vegetative cells and spores is strain-specific, as certain B. pumilus strains were two to three times more resistant than a standard Bacillus subtilis dosimetry strain. An example of this trend was observed when the type strain of B. pumilus, ATCC 7061, proved sensitive, whereas several environmental strains exhibited varying degrees of resistance, to H2O2. Repeated isolation of H2O2-resistant strains of B. pumilus in a clean-room is a concern because their persistence might potentially compromise life-detection missions, which have very strict cleanliness and sterility requirements for spacecraft hardware. 相似文献
445.
It is thought that the chiral molecules of living material can induce circular polarization in light at levels much higher than expected from abiotic processes. We therefore obtained high quality imaging circular polarimetry of the martian surface during the favorable opposition of 2003 to seek evidence of anomalous optical activity. We used two narrow-band filters covering 43% of the martian surface, 15% of it in-depth. With polarization noise levels <0.1% (4.3 upper limits 0.2-0.3%) and spatial resolution 210 km, we did not find any regions of circular polarization. When data were averaged over the observed face of the planet, we did see a small non-zero circular polarization 0.02%, which may be due to effects associated with the opposition configuration though it is at the limit of the instrumental capability. Our observations covered only a small fraction of parameter space, so although we obtained a null result, we cannot exclude the presence of optical activity at other wavelengths, in other locations, or at higher spatial resolution. 相似文献
446.
A. V. Zyuzgin A. I. Ivanov V. I. Polezhaev G. F. Putin E. B. Soboleva 《Cosmic Research》2001,39(2):175-186
The results of processing and interpreting the data of joint Russian–French experiments for studying the heat and mass transfer in near-critical fluids are presented. The experiments were carried out with the ALICE-1 instrument during an orbital flight of the Mirstation from September 30 to October 2, 1995 [1]. For such fluids with a point-like source of heat, when they are placed in the field of uncontrolled inertial accelerations of the spacecraft, the influence of thermovibrational and thermogravitational mechanisms of convection on the propagation of the region of optical irregularity is investigated. It is shown that, near the thermodynamic critical point, local heating of the medium leads to generation of either intense thermogravitational convection or averaged vibroconvective flow, depending on the frequency of variations of the microaccelerations. The structure and characteristics of discovered motions are studied. The results of numerical simulations are presented that confirm the conclusion about a possibility of generation of an averaged convective flow of a vibrational type by the high-frequency component of microaccelerations. 相似文献
447.
In a model experiment plants were grown in sealed chambers on expanded clay aggregate under the luminance of 150 W/m2 PAR and the temperature of 24 degrees C. Seven bacterial strains under investigation, replicated on nutrient medium surface in Petri dishes, were grown in the atmosphere of cultivated plants. Microbial response was evaluated by the difference between colony size in experiment and in control. In control, bacteria grew in the atmosphere of clean air. To study the effects of volatile metabolites of various plant on microbial growth, the experimental data were compared with the background values defined for each individual experiment. Expanded clay aggregate, luminance, temperature, and sealed chamber (without plants) for the background were the same. Volatile metabolites from 28-days old radish plants have been reliably established to have no effect on the growth of microbes under investigation. Metabolites of 30-days old dill and 50-days old garlic have been established to have reliable bacteriostatic effect on the growth of three bacterial strains. Dill and garlic have been found to have different range of effects of volatile substances on bacterial growth. Volatile metabolites of dill and garlic differed in their effect on the sensitivity spectrum of bacteria. An attempt has been made to describe the obtained data mathematically. 相似文献
448.
Prolonged exposure to microgravity in space flight missions (days) impairs the mechanisms responsible for defense of arterial blood pressure (ABP) and cardiac output (CO) against orthostatic stress in the post-flight period. The mechanisms responsible for the observed orthostatic intolerance are not yet completely understood. Additionally, effective counter measures to attenuate this pathophysiological response are not available. The aim of this study was to investigate the ability of our proposed system identification method to predict closed-loop dynamic changes in TPR induced by changes in mean arterial pressure (MAP) and right atrial pressure (RAP). For this purpose we designed and employed a novel experimental animal model for the examination of arterial and cardiopulmonary baroreceptors in the dynamic closed-loop control of total peripheral resistance (TPR), and applied system identification to the analysis of beat-to-beat fluctuations in the measured signals. Grant numbers: NAG5-4989. 相似文献
449.
450.
Do large craters on Mars represent sites that contain aqueous and hydrothermal deposits that provide clues to astrobiological processes? Are these materials available for sampling in large craters? Several lines of evidence strongly support the exploration of large impact craters to study deposits important for astrobiology. The great depth of impact craters, up to several kilometers relative to the surrounding terrain, can allow the breaching of local aquifers, providing a source of water for lakes and hydrothermal systems. Craters can also be filled with water from outflow channels and valley networks to form large lakes with accompanying sedimentation. Impact melt and uplifted basement heat sources in craters > 50 km in diameter should be sufficient to drive substantial hydrothermal activity and keep crater lakes from freezing for thousands of years, even under cold climatic conditions. Fluid flow in hydrothermal systems is focused at the edges of large planar impact melt sheets, suggesting that the edge of the melt sheets will have experienced substantial hydrothermal alteration and mineral deposition. Hydrothermal deposits, fine-grained lacustrine sediments, and playa evaporite deposits may preserve evidence for biogeochemical processes that occurred in the aquifers and craters. Therefore, large craters may represent giant Petri dishes for culturing preexisting life on Mars and promoting biogeochemical processes. Landing sites must be identified in craters where access to the buried lacustrine sediments and impact melt deposits is provided by processes such as erosion from outflow channels, faulting, aeolian erosion, or excavation by later superimposed cratering events. Very recent gully formation and small impacts within craters may allow surface sampling of organic materials exposed only recently to the harsh oxidizing surface environment. 相似文献