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   2488篇
  免费   3篇
  国内免费   5篇
航空   1221篇
航天技术   997篇
综合类   5篇
航天   273篇
  2021年   13篇
  2019年   14篇
  2018年   41篇
  2017年   34篇
  2016年   25篇
  2015年   11篇
  2014年   50篇
  2013年   62篇
  2012年   50篇
  2011年   87篇
  2010年   62篇
  2009年   105篇
  2008年   149篇
  2007年   62篇
  2006年   46篇
  2005年   62篇
  2004年   71篇
  2003年   91篇
  2002年   46篇
  2001年   101篇
  2000年   45篇
  1999年   86篇
  1998年   85篇
  1997年   62篇
  1996年   63篇
  1995年   88篇
  1994年   97篇
  1993年   37篇
  1992年   59篇
  1991年   22篇
  1990年   28篇
  1989年   56篇
  1988年   20篇
  1987年   35篇
  1986年   21篇
  1985年   74篇
  1984年   42篇
  1983年   47篇
  1982年   55篇
  1981年   77篇
  1980年   26篇
  1979年   24篇
  1978年   22篇
  1977年   18篇
  1976年   15篇
  1975年   18篇
  1974年   12篇
  1972年   17篇
  1969年   13篇
  1966年   8篇
排序方式: 共有2496条查询结果,搜索用时 15 毫秒
941.
    
Using data from dosimetry-radiometry system \"Liulin\" on board of \"Mir\"-space station the particle flux and doserate during September-October, 1989 has been studied. The orbit of the station was 379 km perigee, 410 km apogee and 51.6 degrees inclination. Special attention has been paid to the flux and doserate changes inside the station after intensive solar proton events (SPE) on 29 of September, 1989. The comparison between the doses before and after the solar flares shows increase of the calculated mean dose per day by factor of 10 to 200. During the SPE on the 29 of September the additional dose was 310 mrad. The results of the experiment are compared with the data for the solar proton fluxes obtained on the GOES-7 satellite.  相似文献   
942.
    
Results of in-flight tests of three modes of uncontrolled attitude motion of the Progress spacecraft are described. These proposed modes of experiments related to microgravity are as follows: (1) triaxial gravitational orientation, (2) gravitational orientation of the rotating satellite, and (3) spin-up in the plane of the orbit around the axis of the maximum moment of inertia. The tests were carried out from May 24 to June 1, 2004 onboard the spacecraft Progress M1-11. The actual motion of this spacecraft with respect to its center of mass, in the above-mentioned modes, was determined by telemetric information about an electric current tapped off from solar batteries. The values of the current obtained during a time interval of several hours were processed jointly using the least squares method by integration of the equations of the spacecraft’s attitude motion. The processing resulted in estimation of the initial conditions of motion and of the parameters of mathematical models used. For the obtained motions the quasi-static component of microaccelerations was computed at a point onboard, where installation of experimental equipment is possible.  相似文献   
943.
    
SVET Space Greenhouse (SG)--the first automated facility for growing of higher plants in microgravity was designed in the eighty years to be used for the future BLSS. The first successful experiment with vegetables was carried out in 1990 on the MIR Space Station (SS). The experiments in SVET SG were resumed in 1995, when an American Gas Exchange Measurement System (GEMS) was added. A three-month wheat experiment was carried out as part of MIR-SHUTTLE'95 program. SVET-2 SG Bulgarian equipment of a new generation with optimised characteristics was developed (financed by NASA). The new SVET-GEMS equipment was launched on board the MIR SS and a successful six-month experiments for growing up of two crops of wheat were conducted in 1996 - 97 as part of MIR-NASA-3 program. The first of these \"Greenhouse\" experiments (123 days) with the goal to grow wheat through a complete life cycle is described. Nearly 300 heads developed but no seeds were produced. A second crop of wheat was planted and after 42 days the plants were frozen for biochemical investigations. The main environmental parameters during the six-month experiments in SVET (substrate moisture and lighting period) are given. The results and the contribution to BLSS are discussed.  相似文献   
944.
Accumulation of tumor suppressor p53 in rat muscle after a space flight.   总被引:2,自引:0,他引:2  
Tumor suppressor p53 functions as a cell cycle checkpoint under stressful conditions. Early studies have shown that genotoxic stress activates p53 pathway. Recently, many kinds of non-genotoxic stress such as heat shock, cold shock, and low pH also have been found to activate p53 pathway. The effects on living organism remains to be explored. Here, we show that an 18-day space flight induced a 3.6 fold accumulation of p53 in rat skeletal muscle. This results suggests that the p53 pathway plays a role in safeguarding genomic stability against the stressful space environments and supports our previous observation of p53 accumulation in rat skin after a space flight.  相似文献   
945.
An ultraviolet sounding rocket telescope/spectrograph experiment observed Comet Halley on 26 February 1986, 17 days after perihelion. From the long-slit spectra, the production rates of O, C, and CO are calculated. The derived water production rate is a lower limit of 5.0 × 1029 s−1 and the volume mixing ratio of CO to H2O is 21%. The predicted brightness distribution from a radial outflow model with H2O and CO as parent molecules are in accordance with the measured spatial profiles of OI and CO emissions. The ratio of the production rates of CO to C is 2.7 which is consistent with the carbon source being the photodissociation of CO. However, the radial outflow model which best fits the CO data predicts significantly weaker CI emissions than was observed. A better fit to the carbon data is found when an inner coma source of C at a rate of 3% of the water production rate is included in the model.  相似文献   
946.
The weather radar system used in conjunction with Earth-satellite propagation experiments for quantifying rainfall rate in a unit volume, determined by the beamwidth and the pulselength, is described. Firstly, methods for deriving rainfall rate per unit volume are stated in detail with special emphasis on calibration to determine unknown parameters associated with propagation losses in waveguides and the atmosphere. The calibration method is somewhat different from a conventional one but is useful for routine observation. Secondly, in addition to a simple explanation of the hardware of this radar system, the data modes, developed for measuring rainfall rate along the propagation path and radar reflectivity factor in the horizontal and vertical cross sections around the station are described. These data modes are effectively combined by the operational scanning modes for routine observation of propagation experiments, while each data mode can also be operated independently. Examples obtained by each data mode are also given.  相似文献   
947.
An intercept mission with nuclear explosives is the most effective of the practical mitigation options against the impact threat of near-Earth objects (NEOs) with a short warning time (e.g., much less than 10 years). The existing penetrated subsurface nuclear explosion technology limits the intercept velocity to less than approximately 300 m/s. Consequently, an innovative concept of blending a hypervelocity kinetic impactor with a subsurface nuclear explosion has been developed for optimal penetration, fragmentation, and dispersion of the target NEO. A proposed hypervelocity asteroid intercept vehicle (HAIV) consists of a kinetic-impact leader spacecraft and a follower spacecraft carrying nuclear explosives. This paper describes the conceptual development and design of a baseline HAIV system and its flight validation mission architecture for three mission cost classifications (e.g., $500 M, $1 B, and $1.5 B).  相似文献   
948.
On the basis of data, obtained by means of the ground-based solar service RSTN (Radio Solar Telescope Network) and the geostationary satellite system GOES, the relationship between the solar cosmic rays (SCR) intensity I p with the proton energy E p > 1 MeV and parameters of meter-decameter type II radio bursts in the frequency range of 25–180 MHz is studied. The process of proton acceleration by shock waves was characterized by the frequency drift velocity of radio bursts V mII and the relative difference between radio emission frequencies at the first two harmonics b. It is shown that the coefficient of correlation between I p and b increases with E p growing from 0.40 to 0.70, while a similar coefficient between I p and V mII does not exceed 0.30. Indications in favor of the two-stage SCR acceleration model are obtained.  相似文献   
949.
The Space Environment Viewing and Analysis Network (SEVAN) aims to improve the fundamental research on particle acceleration in the vicinity of the sun, on space weather effects and on high-energy physics in the atmosphere and lightning initiation. This new type of a particle detector setup simultaneously measures fluxes of most species of secondary cosmic rays, thus being a powerful integrated device for exploration of solar modulation effects and electron acceleration in the thunderstorm atmosphere. The SEVAN modules are operating at the Aragats Space Environmental Center (ASEC) in Armenia, in Croatia, Bulgaria, Slovakia, the Czech Republic (from 2017) and in India. In this paper, we present the most interesting results of the SEVAN network operation during the last decade. We present this review on the occasion of the 10th anniversary of the International Heliophysical Year in 2007.  相似文献   
950.
Analytical and experimental considerations of the common-mode noise in a dc-to-dc connverterare discussed. Analyzing the mechanism of the common-mode noise generation by means of a high-frequency equivalent circuit, we estimate the output noise voltage generated during the transistor's turn-on time. This commonmode noise consists mainly of two frequency components distributed in several megahertz. The effects of parasitic parameters on the noise voltage and frequencies are clarified. In addition, the effect of a common-mode choke on the noise suppression is discussed experimentally.  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