全文获取类型
收费全文 | 4517篇 |
免费 | 14篇 |
国内免费 | 7篇 |
专业分类
航空 | 2111篇 |
航天技术 | 1616篇 |
综合类 | 18篇 |
航天 | 793篇 |
出版年
2021年 | 44篇 |
2019年 | 26篇 |
2018年 | 104篇 |
2017年 | 73篇 |
2016年 | 76篇 |
2015年 | 40篇 |
2014年 | 133篇 |
2013年 | 138篇 |
2012年 | 144篇 |
2011年 | 195篇 |
2010年 | 128篇 |
2009年 | 219篇 |
2008年 | 294篇 |
2007年 | 131篇 |
2006年 | 109篇 |
2005年 | 140篇 |
2004年 | 119篇 |
2003年 | 155篇 |
2002年 | 89篇 |
2001年 | 153篇 |
2000年 | 78篇 |
1999年 | 99篇 |
1998年 | 121篇 |
1997年 | 89篇 |
1996年 | 92篇 |
1995年 | 126篇 |
1994年 | 121篇 |
1993年 | 64篇 |
1992年 | 96篇 |
1991年 | 53篇 |
1990年 | 37篇 |
1989年 | 91篇 |
1988年 | 40篇 |
1987年 | 43篇 |
1986年 | 36篇 |
1985年 | 97篇 |
1984年 | 89篇 |
1983年 | 77篇 |
1982年 | 89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4538条查询结果,搜索用时 15 毫秒
771.
Alan P. Lightman 《Space Science Reviews》1982,33(3):335-357
We review some of the most important theoretical ideas and observations for quasars and the nuclei of active galaxies, and suggest areas of future research. Emphasis is on the nature of the power source, the radiation processes, and the mechanism for formation and collimation of jets. Phenomena that produce X-rays are of particular concern. Particular topics discussed are the observed and expected time variability, the gas supply mechanisms and luminosity evolution, thermal and nonthermal radiation processes, observed and theoretical spectra, criteria for thermalization of electrons and ions, effects of electron-positron pairs on relativistic plasmas, hydrodynamic, electrodynamic and inertial methods for producing and confining jets. We conclude with a list of needed observations.Based on a lecture given at the Goddard Workshop on X-ray Astronomy (October 1981). 相似文献
772.
773.
Margolis M.D. Messenger G.C. Fitch S.H. 《IEEE transactions on aerospace and electronic systems》1972,(6):771-779
A hardness assurance program suitable for a retaliatory or deterrent missile system is discussed. The program consists of three phases: the establishment of suitable controls and screens to ensure piece part hardness, a supplier data monitoring program to measure hardness related parameters in a timely fashion to obtain advanced warning of potential problems, and a hardness assurance verification testing (HAVT) program to periodically measure the radiation response of parts, circuits, subsystems, and systems on a sample basis to ensure proper functioning in simulated radiation environments. All of the data relevant to hardness assurance is stored in computer memory and routinely interrogated with programs which compare current results with the engineering baseline data and other data from preceding lots. Other computer programs enable statistical estimation of system response to the radiation environment with confidence based on the expanding data base provided by the HAVT program. 相似文献
774.
The BL Lac object Mkn 421 was observed by EXOSAT four times over a period of six days in February 1984. Significant X-ray variability was apparent on a timescale of less than a day, but with no accompanying spectral change. The source exhibited a very soft power law X-ray spectrum with an extremely low intrinsic column density (NH1020 cm–2). There was no evidence for an additional hard component attributable to synchrotron self-Compton emission. The observations when combined with other published data imply that significant changes occur in the form of the broad-band UV/X-ray continuum of this source. 相似文献
775.
The performance of the modulation cancellation altimeter is investigated. Several sources of error are considered, specifically: 1) terrain averaging error; 2) output noise fluctuations; 3) errors due to phase shifts; and 4) acquisition ambiguities. The predominant parameters affecting these errors are investigated with a view towards optimizing the performance of the altimeter. 相似文献
776.
777.
F. Fárník S. Fischer L. Křivský B. Valníček O.B. Likin V.N. Lutsenko 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(13):243-246
During the observation of solar cosmic rays on the Prognoz 6 and Helios 1 and 2 spacecrafts, several events with anomalous composition of accelerated particles (higher abundance of 3-He or Fe nuclei) occurred. We found seven such events from the period September to December 1977 for which data from the Prognoz 6 solar X-ray photometer are available. This material together with published optical and radio data from terrestrial observatories enabled us to identify more reliably the source flares and describe their characteristics. It turned out that the character of X-ray emission accompanying the emission of accelerated particles with anomalous composition shows no pronounced difference from other flares. No correlation has been found among the ratio 3-He/4-He and the angular distance between the field lines connected with the source flare and the place of observation. If a solar flare with anomalous ratio 3-He/4-He appears in a given active region, this region will probably produce other anomalous events. 相似文献
778.
G.S. Orton A.P. Ingersoll L. Froidevaux G. Neugebauer G. Münch S.C. Chase 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(8):179-182
The Pioneer 11 Infrared Radiometer instrument made observations of Saturn and its rings in broadband channels centered at 20 and 45 μm and obtained whole-disk information on Titan. A planetary average effective temperature of 96.5±2.5 K implies a total emission 2.8 times the absorbed sunlight. Correlation with radio science results implies that the molar fraction of H2 is 90±3% (assuming the rest is He). Temperatures at the 1 bar level are 137 to 140 K; regions appearing cooler may be overlain by a cloud acting as a 124 K blackbody surface. A minimum temperature averaging 87 K is reached near 0.06 bars. Ring boundaries and optical depths are consistent with those at optical wavelengths. Ring temperatures are 64–86 K on the south (illuminated) side, ~54 K on the north (unilluminated) side, and at least 67 K in Saturn's shadow. There is evidence for a south to north drop in ring temperatures. Titan's 45 μm brightness temperature is 75±5 K. 相似文献
779.
Mikael Stern 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(11):69-73
The accuracy of traditional censuses and the need for other census methods are discussed, and remote sensing methods for rough population estimates are described. 相似文献
780.
P S Haskins J E McKisson A G Weisenberger D W Ely T A Ballard C S Dyer P R Truscott R B Piercey A V Ramayya D C Camp 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1992,12(2-3):461-464
The Shuttle Activation Monitor (SAM) experiment was flown on the Space Shuttle Columbia (STS-28) from 8-13 August, 1989 in a 57 degrees, 300 km orbit. One objective of the SAM experiment was to determine the relative effect of different amounts of shielding on the gamma-ray backgrounds measured with similarly configured sodium iodide (NaI) and bismuth germante (BGO) detectors. To achieve this objective twenty-four hours of data were taken with each detector in the middeck of the Shuttle on the ceiling of the airlock (a high-shielding location) as well as on the sleep station wall (a low-shielding location). For the cosmic-ray induced background the results indicate an increased overall count rate in the 0.2 to 10 MeV energy range at the more highly shielded location, while in regions of trapped radiation the low shielding configuration gives higher rates at the low energy end of the spectrum. 相似文献