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   4466篇
  免费   13篇
  国内免费   7篇
航空   2086篇
航天技术   1604篇
综合类   16篇
航天   780篇
  2021年   44篇
  2019年   25篇
  2018年   104篇
  2017年   73篇
  2016年   76篇
  2015年   39篇
  2014年   130篇
  2013年   138篇
  2012年   143篇
  2011年   193篇
  2010年   127篇
  2009年   216篇
  2008年   293篇
  2007年   128篇
  2006年   107篇
  2005年   138篇
  2004年   117篇
  2003年   153篇
  2002年   89篇
  2001年   153篇
  2000年   74篇
  1999年   99篇
  1998年   120篇
  1997年   87篇
  1996年   90篇
  1995年   124篇
  1994年   121篇
  1993年   64篇
  1992年   95篇
  1991年   53篇
  1990年   36篇
  1989年   88篇
  1988年   40篇
  1987年   42篇
  1986年   36篇
  1985年   96篇
  1984年   86篇
  1983年   75篇
  1982年   86篇
  1981年   118篇
  1980年   41篇
  1979年   36篇
  1978年   32篇
  1977年   25篇
  1976年   28篇
  1975年   22篇
  1974年   22篇
  1973年   23篇
  1972年   23篇
  1971年   21篇
排序方式: 共有4486条查询结果,搜索用时 15 毫秒
101.
In each polar cap (PC) we mark out “old PC” observed during quiet time before the event under consideration, and “new PC” that emerges during the substorm framing the old one and expanding the PC total area. Old and new PCs are the areas for the magnetosphere old and new tail lobes, respectively. The new lobe variable magnetic flux Ψ1 is usually assumed to be active, i.e. it provides the electromagnetic energy flux (Poynting flux) ɛ′ transport from solar wind (SW) into the magnetosphere. The old lobe magnetic flux Ψ2 is supposed to be passive, i.e. it remains constant during the disturbance and does not participate in the transporting process which would mean the old PC electric field absolute screening from the convection electric field created by the magnetopause reconnection. In fact, screening is observed, but far from absolute. We suggest a model of screening and determine its quantitative characteristics in the selected superstorm. The coefficient of a screening is the β = Ψ202, where Ψ02 = const is open magnetic flux through the old PC measured prior to the substorm, and Ψ2 is variable magnetic flux through the same area measured during the substorm. We consider three various regimes of disturbance. In each, the coefficient β decreased during the loading phase and increased at the unloading phase, but the rates and amplitudes of variations exhibited a strong dependence on the regime. We interpreted decrease in β as a result of involving the old PC magnetic flux Ψ2, which was considered to be constant earlier, in the Poynting flux ɛ′ transport process from solar wind into the magnetosphere. Transport process weakening at the subsequent unloading phase creates increase in β. Estimates showed that coefficient β during each regime and the computed Poynting flux ɛ′ varied manifolds. In general, unlike the existing substorm conception, the new scenario describes an unknown earlier of tail lobe activation process during a substorm growth phase that effectively increases the accumulated tail energy for the expansion and recovery phases.  相似文献   
102.
The fluxes of hydrogen and helium isotopes in the solar wind are reconstructed over a long time scale since the present time up to 600 million years back. Abundances of helium isotopes, obtained in the helium isotopic analysis made for 8 lunar soil samples, were used as initial data in the reconstruction procedure. Samples were taken off from various levels of the 1.6-m core of lunar soil delivered by the automatic Luna-24 station in 1976. The data on modern hydrogen and helium fluxes were used as well. The developed reconstruction procedure allowed one to select various solar wind components in a ??gross?? composition. Proton flux variations over the interval of 600 million years do not exceed a value of 40 %. Helium flux variations reach a value of 1.5?C2 relative to the average value. Most likely, this circumstance is caused by considerable variations of a number of coronal mass ejections (CME) enriched by helium. The arguments in favor of solar activity polycyclicity on a long time scale are discussed.  相似文献   
103.
104.
The thermal resistance due to conductive heat transfer between two heat generating boxes mounted symmetrically on a thin mounting plate, one side of which is subjected to a constant heat flux and the opposite side is convectively cooled, is considered. The other two sides are maintained at constant temperature. A numerical solution for the temperature field is obtained and the heat transfer between the boxes is found by integrating between the critical heat flow lines. Various nondimensional parameters are identified and their influence on the thermal resistance is studied.  相似文献   
105.
At a time when scientific and commercial interest in the Moon is being reinvigorated it is becoming fashionable for ordinary individuals to ‘buy’ plots on the lunar surface, with the ‘vendors’ arguing that an absence of specific prohibition of individual private activity in space makes such action legal. It is therefore time for the legal community to address this situation by investigating just how legal such activity is—and bringing their findings to the attention of governments. This can be done through an examination of the relationship between national law and international space law, of the provisions of international space law—especially Article 2 of the Outer Space Treaty—and by answering any claims to private ownership of immovable property. Aside from the fact that individuals appear to be being duped, the pursuit of property claims on the Moon could impede future activities aimed at benefiting society.  相似文献   
106.
A quartz sensor of small accelerations with a capacitive transducer is designed and produced, allowing one to measure spacecraft accelerations with a resolution of 10–7 m/s2 in the range ±10–1 m/s2. The results of calibration of the sensor by the method of inclinations are presented.  相似文献   
107.
An investigation of the effects of drying and drying-rewetting cycles under vacuum conditions of 10(-2) torr at -30 degrees C on conidia of the atmospheric strain Penicillium chrysogenum from 48-77 km heights, collected by meteorological rockets found that survival and restoration of the initial physiological properties were possible through some very efficient protective cellular mechanisms. Such adaptive possibilities explain the widespread diffusion of microbial cells throughout the biosphere of our planet.  相似文献   
108.
The results of numerical simulation of the general circulation in the Titan’s atmosphere at heights from 0 to 250 km are presented, obtained using a new model based on numerical solution of complete equations of motion of viscous compressible gas at the temperature distribution given by an empirical model. The model uses no hydrostatic equation and, as compared with traditional models, has higher resolution in vertical and over horizon. The results presented differ from results of other models and agree with the vertical profile of the zonal component of wind velocity measured by the Huygens spacecraft. Interpretation of this profile is given, including its main peculiarity consisting in a nonmonotonic behavior at heights from 60 to 75 km.  相似文献   
109.
The possibility of an ocean within the icy shell of Jupiter's moon Europa has established that world as a primary candidate in the search for extraterrestrial life within our Solar System. This paper evaluates the potential to detect evidence for microbial life by comparing laboratory studies of terrestrial microorganisms with measurements from the Galileo Near Infrared Imaging Spectrometer (NIMS). If the interior of Europa at one time harbored life, some evidence may remain in the surface materials. Examination of laboratory spectra of terrestrial extremophiles measured at cryogenic temperatures reveals distorted, asymmetric nearinfrared absorption features due to water of hydration. The band centers, widths, and shapes of these features closely match those observed in the Europa spectra. These features are strongest in reddish-brown, disrupted terrains such as linea and chaos regions. Narrow spectral features due to amide bonds in the microbe proteins provide a means of constraining the abundances of such materials using the NIMS data. The NIMS data of disrupted terrains exhibit distorted, asymmetric near-infrared absorption features consistent with the presence of water ice, sulfuric acid octahydrate, hydrated salts, and possibly as much as 0.2 mg cm(-3) of carbonaceous material that could be of biological origin. However, inherent noise in the observations and limitations of spectral sampling must be taken into account when discussing these findings.  相似文献   
110.
Ugolnikov  O. S.  Maslov  I. A. 《Cosmic Research》2021,59(4):305-305
Cosmic Research - An Erratum to this paper has been published: https://doi.org/10.1134/S0010952521110010  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