全文获取类型
收费全文 | 7029篇 |
免费 | 12篇 |
国内免费 | 33篇 |
专业分类
航空 | 3056篇 |
航天技术 | 2613篇 |
综合类 | 23篇 |
航天 | 1382篇 |
出版年
2021年 | 65篇 |
2019年 | 44篇 |
2018年 | 161篇 |
2017年 | 108篇 |
2016年 | 100篇 |
2015年 | 41篇 |
2014年 | 169篇 |
2013年 | 207篇 |
2012年 | 190篇 |
2011年 | 290篇 |
2010年 | 199篇 |
2009年 | 337篇 |
2008年 | 395篇 |
2007年 | 218篇 |
2006年 | 176篇 |
2005年 | 218篇 |
2004年 | 215篇 |
2003年 | 250篇 |
2002年 | 162篇 |
2001年 | 245篇 |
2000年 | 148篇 |
1999年 | 166篇 |
1998年 | 184篇 |
1997年 | 145篇 |
1996年 | 176篇 |
1995年 | 226篇 |
1994年 | 202篇 |
1993年 | 106篇 |
1992年 | 165篇 |
1991年 | 56篇 |
1990年 | 61篇 |
1989年 | 146篇 |
1988年 | 49篇 |
1987年 | 54篇 |
1986年 | 71篇 |
1985年 | 211篇 |
1984年 | 150篇 |
1983年 | 121篇 |
1982年 | 152篇 |
1981年 | 197篇 |
1980年 | 54篇 |
1979年 | 35篇 |
1978年 | 42篇 |
1977年 | 43篇 |
1976年 | 37篇 |
1975年 | 37篇 |
1974年 | 44篇 |
1973年 | 32篇 |
1970年 | 31篇 |
1969年 | 26篇 |
排序方式: 共有7074条查询结果,搜索用时 359 毫秒
661.
Jodie M. Plumert 《Spatial Cognition & Computation》2013,13(4):259-263
ABSTRACTFictive motion in language (as in “the ridge went north”) is claimed to reflect the attention focus of the observer on the extension and spatial layout of an entity. This paper investigates fictive motion in alpine narratives, which describe the experience of moving in a very specifically structured space. We examine space properties that are highlighted through fictive motion in this specific context and describe how they go beyond spatial extension. We further report the communicative motivation behind the use of fictive motion, ranging from conveying the sense of place to encoding the full spatial footprint of a motion event. 相似文献
662.
R J Wassersug M Yamashita 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2000,25(10):2007-2013
Because of their rapid development, amphibians have been important model organisms in studies of how microgravity affects vertebrate growth and differentiation. Both urodele (salamanders) and anuran (frogs and toads) embryos have been raised in orbital flight, the latter several times. The most commonly reported and striking effects of microgravity on tadpoles are not in the vestibular system, as one might suppose, but in their lungs and tails. Pathological changes in these organs disrupt behavior and retard larval growth. What causes malformed (typically lordotic) tadpoles in microgravity is not known, nor have axial pathologies been reported in every flight experiment. Lung pathology, however, has been consistently observed and is understood to result from the failure of the animals to inflate their lungs in a timely and adequate fashion. We suggest that malformities in the axial skeleton of tadpoles raised in microgravity are secondary to problems in respiratory function. We have used high speed videography to investigate how tadpoles breathe air in the 1G environment. The video images reveal alternative species-specific mechanisms, that allow tadpoles to separate air from water in less that 150 ms. We observed nothing in the biomechanics of air-breathing in 1G that would preclude these same mechanisms from working in microgravity. Thus our kinematic results suggest that the failure of tadpoles to inflate their lungs properly in microgravity is due to the tadpoles' inability to locate the air-water interface and not a problem with the inhalation mechanism per se. 相似文献
663.
The paper describes the first results of all-sky polarization measurements of the twilight background started in central Russia in the very beginning of summer 2011. Time-frequency data of the sky intensity and polarization over a wide range of sky point zenith distances are used to separate single and multiple scattering and construct the altitude dependence of the scattering coefficient and polarization in the mesosphere (altitudes from 60 to 90 km) at different angles. The undisturbed structure of the mesosphere without noticeable aerosol stratification on observation days makes it possible to estimate the temperature of the atmosphere at these altitudes. 相似文献
664.
A. V. Tavrov O. I. Korablev A. V. Rodin I. I. Vinogradov A. Yu. Trokhimovsky A. Yu. Ivanov L. V. Ksanfomaliti D. A. Orlov 《Cosmic Research》2011,49(2):99-109
In order to observe exoplanets we propose a space-based achromatic stellar coronagraph combined with a 0.8–1.5 m telescope.
We develop an achromatic common path interferometer for observing an exoplanet (a faint off-axis source) on the background
of a hoste star (bright axial source). An image of the star and its copy acquire an achromatic phase shift by 180° and interfere
in antiphase. The achromatic phase shift is caused by geometric phase in the scheme of a three-dimensional interferometer.
The interference process divides spatially the dark and light fields of the star image redirecting them to the opposite sides
of a beam splitter. The interference process does not weaken the image of a planet, with equal intensities it is redirected
to both sides of a beam splitter. The suggested scheme of common path interferometer ensures mechanical stability. The background
signal is experimentally demonstrated to be reduced by six orders of magnitude. 相似文献
665.
I. A. Galiullin 《Cosmic Research》2011,49(2):175-178
The constraints imposed on parameters of a force field and of a symmetrical gyrostat in it are determined, at which regular
precession of this gyrostat demonstrate Lyapunov stability (or, respectively, instability). 相似文献
666.
Future space exploration may involve communications between spacecraft moving at relativistic velocities. One of the significant problems associated with such communication is spectral distortion of signals which are propagated between relativistic frames. This distortion is generated by both changing propagation distances and purely relativistic electromagnetic field transformations. In this paper a linear integral transformation is formulated for relating the Fourier spectra of the source antenna excitation current and the resulting incident electric field at the receiving antenna. The kernel of the transformation is evaluated for the case of a steerable source antenna tracking on the advanced receiver position. The transformation is then applied to the case of an ideal thin-wire half-wave dipole source antenna excited by a narrowband, double-sideband modulated current. The specific distortions of spectral spreading and translation are then related to increased bandwidth and upper cutoff frequency requirements of receiving systems in relativistic applications. 相似文献
667.
Possible consequences of collisions of natural cosmic bodies with the Earth’s atmosphere and surface are described. The methodological
basis of classification of consequences is the solution of meteor physics equations characterizing the trajectory of a body
in the atmosphere, namely, the dependence of the body’s velocity and mass on the flight altitude. The solution depends on
two dimensionless parameters characterizing the drag altitude and the role of mass loss by a meteoroid during its motion in
the atmosphere. Depending on values of these parameters, the degree of effect on the planetary surface considerably changes.
In particular, the conditions of cratering and meteorite fall on the planetary surface are obtained. The results are presented
in a simple analytical form. They quite match to the real events considered in the paper. Recommendations are given on further
investigations into the important problem of interaction of cosmic bodies with planetary atmospheres. 相似文献
668.
V. V. Semenov V. A. Volkov M. C. Kwon J. S. S. Sidhu 《Russian Aeronautics (Iz VUZ)》2016,59(2):211-217
The dependence of the wave resistance coefficients for planar periodic reliefs on the similarity parameters is investigated. It is proved that the wave resistance coefficients of the infinite reliefs and their finite analogs in the case of the whole wave numbers coincide, whereas in the case of the fractional wave numbers they differ. 相似文献
669.
I. Sh. Sharafeev 《Russian Aeronautics (Iz VUZ)》2008,51(3):339-343
Problems of data exchange between different systems of technological-purpose automation and tasks that challenge developers in choosing the integration schemes are considered. 相似文献
670.
I. De Moortel 《Space Science Reviews》2009,149(1-4):65-81
Outwardly propagating intensity disturbances are a common feature in large, quiescent coronal loop structures. In this paper, an overview is given of the observed properties and the theoretical modelling. As a large number of events have been observed and analysed, good statistical results on the estimated parameters have now been obtained. The theoretical modelling mainly focuses on two distinct aspects, namely the observed rapid damping of the perturbations, thought to be due to thermal conduction and the origin of the driver. Leakage of the solar surface p-modes is the main candidate to explain the observed periodicity, due to the strong correlation between loop position and period and the filamentary nature of the observed coronal intensity perturbations. Recent observational results appear to confirm the leakage and subsequent upward propagation of the solar surface 5 minute oscillations into the overlying atmospheric layers. 相似文献