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   6972篇
  免费   21篇
  国内免费   11篇
航空   3175篇
航天技术   2471篇
综合类   195篇
航天   1163篇
  2021年   56篇
  2019年   40篇
  2018年   139篇
  2017年   92篇
  2016年   96篇
  2015年   48篇
  2014年   174篇
  2013年   189篇
  2012年   198篇
  2011年   271篇
  2010年   180篇
  2009年   303篇
  2008年   393篇
  2007年   189篇
  2006年   167篇
  2005年   204篇
  2004年   181篇
  2003年   231篇
  2002年   234篇
  2001年   285篇
  2000年   106篇
  1999年   167篇
  1998年   190篇
  1997年   142篇
  1996年   172篇
  1995年   212篇
  1994年   187篇
  1993年   101篇
  1992年   152篇
  1991年   75篇
  1990年   60篇
  1989年   133篇
  1988年   59篇
  1987年   63篇
  1986年   63篇
  1985年   180篇
  1984年   162篇
  1983年   129篇
  1982年   155篇
  1981年   202篇
  1980年   64篇
  1979年   48篇
  1978年   50篇
  1977年   41篇
  1976年   41篇
  1975年   38篇
  1974年   47篇
  1972年   41篇
  1971年   40篇
  1970年   39篇
排序方式: 共有7004条查询结果,搜索用时 15 毫秒
391.
Red-cell mass determinations were performed before and after the first two Skylab missions. The data showed a 14% mean decrease in red-cell mass after the 28-day mission and a 12% mean decrease after the 59-day mission. The red-cell mass returned to premission levels more slowly after the shorter (28-day) than after the longer mission. Plasma volume decreases were found after each mission. with the crew from the longer mission showing the greater change (13% vs. 8.4%). Postmission decreases in red-cell mass and plasma volume have been a general finding in crewmen who return from short or long spaceflight.  相似文献   
392.
A method of technologic audit for technical re-equipment of enterprises is considered. The method developed is intended to verify technological solutions of projects of technical re-equipment of enterprises according to indicators of productivity.  相似文献   
393.
The problem of the optimal number of phased array faces for performing 360/spl deg/ horizon surveillance is considered. Assuming the detection performance is the same in all beam positions and the total number of T/R modules is constant, it is shown that the optimal number of array faces is three. This is true whether the arrays are operating simultaneously or sequentially. A parametric analysis is performed between the number of array faces operating simultaneously and the associated cost of simultaneous operation in terms of the size of the array.  相似文献   
394.
The Wave Experiment, F4, on the Swedish/German satelliteFreja, is designed to measure the electric wave fields up to 4 MHz, the magnetic wave fields up to 16 kHz and the plasma density and its relative variations up to 2 kHz. Six wave signals and four density probe signals can be measured simultaneously. The wave forms of all signals are transmitted to ground without any analysis onboard. The limited TM allocation does not allow continuous sampling of the wave signals, so normally the measurements are made in snapshots of various lengths dependent on sampling frequency, etc. Continuous sampling can be made for shorter time periods by using a 6 Mbyte memory as a buffer.  相似文献   
395.
    
Effects of variable thermal properties (specific heat and thermal conductivity) of the solid phase on the combustion of composite solid propellants are studied analytically. Both steady and unsteady burning are considered. It is shown that the effects of variable thermal properties are small and can be accounted for by choosing proper average values of specific heat and thermal conductivity.  相似文献   
396.
利用MMS观测数据,对磁层顶通量绳内离子惯性尺度(di)的结构进行分析研究.结果发现,许多不同尺度(约1di至数十di)的通量绳内都存在具有di尺度的电流 j m,其方向在磁层顶局地坐标系的-M方向,即与磁层顶查普曼-费拉罗电流同向,由电子在+M方向的运动( v em)携带.这些电流结构具有以下特征:磁鞘与磁层成分混合,磁场为开放形态;离子去磁化,电子与磁场冻结;N方向(即垂直于磁层顶电流片方向)的电场 E n显著增大,幅度达到约20mV·m-1,并伴有明显的尖峰状起伏,该增强和尖峰状起伏的电场对应于霍尔电场.分析表明,电流、电子与离子运动的偏离以及霍尔电场之间遵从广义欧姆定律,三者密切关联.进一步对磁层顶磁重联的探测数据进行分析发现,在很多重联区内也存在与通量绳内相似的结构,其尺度约为di量级,其中霍尔电场 E N、电流 j M和电子速度 v eM均与通量绳内对应物理量的方向相同且幅度相近.基于上述观测事实,采用经典FTE通量绳模型,对通量绳内电流、电子运动和霍尔电场的起源进行了初步探讨,认为其来源于磁层顶无碰撞磁重联区内的相应结构,并且后者在离子尺度通量绳的形成过程中起到重要作用.  相似文献   
397.
We describe the design and calibration of the Far-Infrared Photometer (FIRP), one of four focal plane instruments on the Infrared Telescope in Space (IRTS). The FIRP will provide absolute photometry in four bands centered at 150, 250, 400, and 700 μm with spectral resolution λ/Δλ ≈ 3 and spatial resolution ΔΘ = 0.5 degrees. High sensitivity is achieved by using bolometric detectors operated at 300 mK in an AC bridge circuit. The closed-cycle 3He refrigerator can be recycled in orbit. A 2 K shutter provides a zero reference for each field of view. More than 10% of the sky will be surveyed during the ≈3 week mission lifetime with a sensitivity of <10−13 W·cm−2·sr−1 per 0.5 degree pixel.  相似文献   
398.
    
Glavin DP  Bada JL 《Astrobiology》2001,1(3):259-269
The delivery of amino acids by micrometeorites to the early Earth during the period of heavy bombardment could have been a significant source of the Earth's prebiotic amino acid inventory provided that these organic compounds survived atmospheric entry heating. To investigate the sublimation of amino acids from a micrometeorite analog at elevated temperature, grains from the CM-type carbonaceous chondrite Murchison were heated to 550 degrees C inside a glass sublimation apparatus (SA) under reduced pressure. The sublimed residue that had collected on the cold finger of the SA after heating was analyzed for amino acids by HPLC. We found that when the temperature of the meteorite reached approximately 150 degrees C, a large fraction of the amino acid glycine had vaporized from the meteorite, recondensed onto the end of the SA cold finger, and survived as the rest of the grains heated to 550 degrees C. alpha-Aminoisobutryic acid and isovaline, which are two of the most abundant non-protein amino acids in Murchison, did not sublime from the meteorite and were completely destroyed during the heating experiment. Our experimental results suggest that sublimation of glycine present in micrometeorite grains may provide a way for this amino acid to survive atmospheric entry heating at temperatures > 550 degrees C; all other amino acids apparently are destroyed.  相似文献   
399.
Far ultraviolet imaging from the IMAGE spacecraft. 2. Wideband FUV imaging   总被引:3,自引:0,他引:3  
Mende  S.B.  Heetderks  H.  Frey  H.U.  Lampton  M.  Geller  S.P.  Abiad  R.  Siegmund  O.H.W.  Tremsin  A.S.  Spann  J.  Dougani  H.  Fuselier  S.A.  Magoncelli  A.L.  Bumala  M.B.  Murphree  S.  Trondsen  T. 《Space Science Reviews》2000,91(1-2):271-285
The Far Ultraviolet Wideband Imaging Camera (WIC) complements the magnetospheric images taken by the IMAGE satellite instruments with simultaneous global maps of the terrestrial aurora. Thus, a primary requirement of WIC is to image the total intensity of the aurora in wavelength regions most representative of the auroral source and least contaminated by dayglow, have sufficient field of view to cover the entire polar region from spacecraft apogee and have resolution that is sufficient to resolve auroras on a scale of 1 to 2 latitude degrees. The instrument is sensitive in the spectral region from 140–190 nm. The WIC is mounted on the rotating IMAGE spacecraft viewing radially outward and has a field of view of 17° in the direction parallel to the spacecraft spin axis. Its field of view is 30° in the direction perpendicular to the spin axis, although only a 17°×17° image of the Earth is recorded. The optics was an all-reflective, inverted Cassegrain Burch camera using concentric optics with a small convex primary and a large concave secondary mirror. The mirrors were coated by a special multi-layer coating, which has low reflectivity in the visible and near UV region. The detector consists of a MCP-intensified CCD. The MCP is curved to accommodate the focal surface of the concentric optics. The phosphor of the image intensifier is deposited on a concave fiberoptic window, which is then coupled to the CCD with a fiberoptic taper. The camera head operates in a fast frame transfer mode with the CCD being read approximately 30 full frames (512×256 pixel) per second with an exposure time of 0.033 s. The image motion due to the satellite spin is minimal during such a short exposure. Each image is electronically distortion corrected using the look up table scheme. An offset is added to each memory address that is proportional to the image shift due to satellite rotation, and the charge signal is digitally summed in memory. On orbit, approximately 300 frames will be added to produce one WIC image in memory. The advantage of the electronic motion compensation and distortion correction is that it is extremely flexible, permitting several kinds of corrections including motions parallel and perpendicular to the predicted axis of rotation. The instrument was calibrated by applying ultraviolet light through a vacuum monochromator and measuring the absolute responsivity of the instrument. To obtain the data for the distortion look up table, the camera was turned through various angles and the input angles corresponding to a pixel matrix were recorded. It was found that the spectral response peaked at 150 nm and fell off in either direction. The equivalent aperture of the camera, including mirror reflectivities and effective photocathode quantum efficiency, is about 0.04 cm2. Thus, a 100 Rayleigh aurora is expected to produce 23 equivalent counts per pixel per 10 s exposure at the peak of instrument response.  相似文献   
400.
We report on extensive BVRcIc photometry and low-resolution (λ/Δλ250) spectroscopy of the deep-space debris WT1190F, which impacted Earth offshore from Sri Lanka, on 2015 November 13. In spite of its likely artificial origin (as a relic of some past lunar mission), the case offered important points of discussion for its suggestive connection with the envisaged scenario for a (potentially far more dangerous) natural impactor, like an asteroid or a comet.Our observations indicate for WT1190F an absolute magnitude Rc=32.45±0.31, with a flat dependence of reflectance on the phase angle, such as dRc/d?0.007±2?mag?deg?1. The detected short-timescale variability suggests that the body was likely spinning with a period twice the nominal figure of Pflash=1.4547±0.0005s, as from the observed lightcurve. In the BVRcIc color domain, WT1190F closely resembled the Planck deep-space probe. This match, together with a depressed reflectance around 4000 and 8500 Å may be suggestive of a “grey” (aluminized) surface texture.The spinning pattern remained in place also along the object fiery entry in the atmosphere, a feature that may have partly shielded the body along its fireball phase perhaps leading a large fraction of its mass to survive intact, now lying underwater along a tight (1×80?km) strip of sea, at a depth of 1500?m or less.Under the assumption of Lambertian scatter, an inferred size of 216±30/α/0.1?cm is obtained for WT1190F. By accounting for non-gravitational dynamical perturbations, the Area-to-Mass ratio of the body was in the range (0.006?AMR?0.011)?m2?kg?1.Both these figures resulted compatible with the two prevailing candidates to WT1190F’s identity, namely the Athena II Trans-Lunar Injection Stage of the Lunar Prospector mission, and the ascent stage of the Apollo 10 lunar module, callsign “Snoopy”. Both candidates have been analyzed in some detail here through accurate 3D CAD design mockup modelling and BRDF reflectance rendering to derive the inherent photometric properties to be compared with the observations.  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