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   4650篇
  免费   28篇
  国内免费   9篇
航空   2125篇
航天技术   1763篇
综合类   26篇
航天   773篇
  2021年   37篇
  2019年   26篇
  2018年   61篇
  2017年   54篇
  2016年   51篇
  2014年   84篇
  2013年   121篇
  2012年   111篇
  2011年   183篇
  2010年   128篇
  2009年   186篇
  2008年   265篇
  2007年   130篇
  2006年   104篇
  2005年   149篇
  2004年   160篇
  2003年   150篇
  2002年   104篇
  2001年   161篇
  2000年   74篇
  1999年   114篇
  1998年   135篇
  1997年   87篇
  1996年   98篇
  1995年   125篇
  1994年   146篇
  1993年   79篇
  1992年   104篇
  1991年   40篇
  1990年   53篇
  1989年   91篇
  1988年   41篇
  1987年   33篇
  1986年   47篇
  1985年   143篇
  1984年   133篇
  1983年   110篇
  1982年   98篇
  1981年   173篇
  1980年   37篇
  1979年   38篇
  1978年   43篇
  1977年   39篇
  1976年   33篇
  1975年   37篇
  1974年   30篇
  1973年   32篇
  1972年   42篇
  1970年   25篇
  1969年   25篇
排序方式: 共有4687条查询结果,搜索用时 15 毫秒
861.
An estimate of the feasibility of the ultrahigh-energy cosmic ray and neutrino detection using a lunar satellite-borne radio receiver is presented. The data obtained in the proposed experiment will make resolving the current contradictions in the ultrahigh-energy cosmic ray spectra measured with the major ground-based instruments possible. Moreover, they will enable us to considerably extend the accessible energy range and to check predictions of various models of the origin of the highest-energy particles in the Universe. At the same time the lunar radio detector provides a means of searching for ultrahigh-energy neutrinos with a high sensitivity combined with a very large target effective mass.  相似文献   
862.
Laboratory impact tests have been performed on experimental versions of a proposed robotic sample collector for extraterrestrial samples. The collector consists of a retractable aluminum ring containing an impregnable silicone compound that is pressed into the surface of the body to be sampled. As part of a comprehensive program to evaluate this idea, we have performed tests to determine if the samples embedded in the collector medium can survive the impact forces experienced during direct reentry, such as that of the recent Genesis sample return mission. For the present study, samples of sand, rock, glass, and chalk were subjected to decelerations of 1440–2880 g using drop tests. We found that even the most fragile samples, chosen to be representative of a wide range of the types of materials found on the surface of asteroids that have currently been studied, can withstand impacts of the intensity experienced by a sample return capsule during direct reentry.  相似文献   
863.
    
A review is given of both observational and theoretical results concerning the latitudinal structure of the solar wind and interplanetary magnetic field. Observations are reported on the solar wind plasma and magnetic fields, obtained both from direct satellite measurements and indirect methods, such as the observation of comet tails, radio scintillations, the study of the polar geomagnetic field and the semi-annual variation of geomagnetic activity. Results of theoretical work, both on three-dimensional modelling of the solar wind and on gas-magnetic field interactions in the solar corona are summarized. Finally, an attempt is made to compare available observations and theories. This points to the open questions which, to be settled, will need direct observations of plasma and magnetic field at high heliographic latitudes.  相似文献   
864.
Feonychev  A. I.  Dolgikh  G. A. 《Cosmic Research》2004,42(2):117-128
A numerical investigation of the melt flow and heat and mass transfer is carried out at the crystal growth under zero gravity, when the melt detachment from ampoule walls, crystal vibration, and various magnetic fields are active. Specific features of the melt flow are demonstrated depending on the size of a detachment zone adjacent to the crystallization boundary. The velocity of the averaged flow generated by crystal vibration is determined as a function of the vibration intensity. It is shown that the crystal vibration cannot compensate a thermal capillary flow (caused by detachment of the melt from the ampoule wall) and reduce the macrosegregation of impurities. It is shown that the application of steady and rotating magnetic fields are inefficient for all ampoule methods of crystal growth under microgravity conditions.  相似文献   
865.
    
We examine the resonant non-linear interaction in the Earth's ionosphere of two powerful high frequency radio beams with frequencies f1 and f2 (both larger than the plasma frequency at F2max) and wave numbers k1 and k2 such that a whistler mode wave can be excited with a frequency f3 = f1f2 and a wave number k3 = k1k2. The feasibility of an effective ground based installation, sited at low latitudes, is discussed and the field strength of the wave emerging from a 10 km wide ionospheric region illuminated by the beams is evaluated for a range of transmitted frequencies, beam orientations and plasma frequencies in the interaction region. It is suggested that the longitude dependence of the enhancement of VLF noise bands detected by the Ariel 3 satellite may be due to a non-linear interaction of this type between any two or more medium wavelength signals from areas where there is a high concentration of commercial broadcasting stations, such as the NE region of the U.S.A.  相似文献   
866.
    
The result of an investigation of the effects of pulse shape and duty cycle on the probability of error are presented for a pulse-gated binary modulation (PGBM) laser communication system when the background irradiance has the important Lorentzian spectral shape.  相似文献   
867.
    
Increased sensitivity and dynamic range of the instrumental techniques used in conjunction with experiments on ballistic ranges have brought to the fore many problems arising from contamination in the ranges themselves. This is seldom discussed when experimental results are presented but is frequently the controlling limitation on the accuracy of the measurements. The authors discuss contamination due to dirt and debris resultant from gun operation, gaseous impurities, and projectile-borne impurities as they have occurred at the Re-entry Simulating Range of Lincoln Laboratory. The effects of these contaminants on measurements are discussed and illustrated, and measures for controlling them are outlined. Finally, a particular range operation is described from the standpoint of impurity control.  相似文献   
868.
In the 1990s, based on detailed studies of the structure of active regions (AR), the concept of the magnetosphere of the active region was proposed. This includes almost all known structures presented in the active region, ranging from the radio granulation up to noise storms, the radiation of which manifests on the radio waves. The magnetosphere concept, which, from a common point of view, considers the manifestations of the radio emission of the active region as a single active complex, allows one to shed light on the relation between stable and active processes and their interrelations. It is especially important to identify the basic ways of transforming nonthermal energy into thermal energy. A dominant role in all processes is attributed to the magnetic field, the measurement of which on the coronal levels can be performed by radio-astronomical techniques. The extension of the wavelength range and the introduction of new tools and advanced modeling capabilities makes it possible to analyze the physical properties of plasma structures in the AR magnetosphere and to evaluate the coronal magnetic fields at the levels of the chromosphere–corona transition zone and the lower corona. The features and characteristics of the transition region from the S component to the B component have been estimated.  相似文献   
869.
Lower-mesospheric inversion layers (MILs) were studied using the temperature profiles observed by TIMED/SABER over Cariri (7.5°S, 36.5°W), Brazil, in 2005. A total 175 MILs were identified with the maximum occurrence in April and October and the minimum in January and July. The lower MIL is located in a height region from 70 to 90 km, with the peak at around 83 ± 4 km with the temperature of 205 ± 5 K, and the thickness of 4–10 km. The results show large amplitudes of MILs during equinoxes and minimum in solstices, with a clear semiannual variation. A general feature of lower MIL in monthly mean profile was observed twice a year, one from February to May, and the other from August to October with a downward shift of the top level. These results suggest that formation and long persistence of MIL is an important factor to investigate propagation of atmospheric gravity waves in the mesosphere-lower thermosphere (MLT) region.  相似文献   
870.
Knowledge of the elemental composition of the interstellar gas is of fundamental importance for understanding galactic chemical evolution. In addition to spectroscopic determinations of certain element abundance ratios, measurements of the composition of interstellar pickup ions and Anomalous Cosmic Rays (ACRs) have provided the principal means to obtain this critical information. Recent advances in our understanding of particle acceleration processes in the heliosphere and measurements by the Voyagers of the energy spectra and composition of energetic particles in the heliosheath provide us with another means of determining the abundance of the neutral components of the local interstellar gas. Here we compare the composition at the termination shock of six elements obtained from measurements of (a) pickup ions at ~5 AU, (b) ACRs in the heliosphere at ~70 AU, and (c) energetic particles as well as (d) ACRs in the heliosheath at ~100 AU. We find consistency among these four sets of derived neutral abundances. The average interstellar neutral densities at the termination shock for H, N, O, Ne and Ar are found to be 0.055±0.021 cm?3, (1.44±0.45)×10?5 cm?3, (6.46±1.89)×10?5 cm?3, (8.5±3.3)×10?6 cm?3, and (1.08±0.49)×10?7 cm?3, respectively, assuming the He density is 0.0148±0.002 cm?3.  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