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   2781篇
  免费   9篇
  国内免费   8篇
航空   1283篇
航天技术   963篇
综合类   13篇
航天   539篇
  2021年   22篇
  2019年   22篇
  2018年   65篇
  2017年   39篇
  2016年   40篇
  2015年   16篇
  2014年   63篇
  2013年   100篇
  2012年   65篇
  2011年   101篇
  2010年   80篇
  2009年   140篇
  2008年   145篇
  2007年   72篇
  2006年   70篇
  2005年   74篇
  2004年   72篇
  2003年   93篇
  2002年   50篇
  2001年   101篇
  2000年   54篇
  1999年   66篇
  1998年   74篇
  1997年   67篇
  1996年   70篇
  1995年   90篇
  1994年   88篇
  1993年   44篇
  1992年   52篇
  1991年   18篇
  1990年   25篇
  1989年   53篇
  1988年   32篇
  1987年   24篇
  1986年   31篇
  1985年   79篇
  1984年   72篇
  1983年   45篇
  1982年   71篇
  1981年   71篇
  1980年   19篇
  1978年   31篇
  1977年   13篇
  1975年   24篇
  1974年   17篇
  1973年   16篇
  1972年   17篇
  1969年   13篇
  1967年   15篇
  1966年   13篇
排序方式: 共有2798条查询结果,搜索用时 15 毫秒
731.
The first 8.5m infrared heterodyne spectrometer has been constructed using tuneable semiconductor (PbSe) diode lasers and was used to measure absorption line profiles of N2O in the laboratory and black body emission from the Moon and from Mars. Spectral information was recorded over a 200 MHz bandwidth using an 8-channel filter bank. The resolution was 25 MHz (6 × 10–6 m) and the minimum detectable (black-body) power was 1 × 10–16 W for 8 min of integration. The results demonstrate the usefulness of heterodyne spectroscopy for the study of remote and local sources in the infrared.We wish to thank Dr Jack Butler and Craig Simpson (Arthur D. Little, Inc.) for their continuing (and successful) efforts to grow better diode lasers during the course of this experiment. We thank the National Radio Astronomy Observatory for the loan of a 40-channel filter bank and associated mini-computer.We thank en masse the dozens of people who loaned equipment to us for the duration of this experiment, and the commercial firms (especially Harshaw, Inc.) which expedited orders for crucial equipment. We thank Dr Bertram Donn, Dr Norman Ness, and Dr George Pieper for administrative, scientific, and financial support; and we thank Mr James Faris for technical assistance and Dr Sol Glicker for preparation of sample gas cells. We thank Dr Thomas Clark for providing the Kohoutek ephemeris, sighting printout, and predicted molecular line positions and for advice and the loan of some equipment. Finally, we wish to acknowledge the strong support accorded us by our wives and families who scarcely saw us during this period.  相似文献   
732.
A critical appraisal is made of the hypothesis that power-line harmonic radiation can influence the Earth's radiation belts by triggering intense magnetospheric emissions which in turn resonantly scatter trapped electrons into the atmospheric loss cone. While such triggering may indeed occur, a combination of theoretical arguments supplemented by an indepth analysis of OGO-5 satellite data is employed to show that triggered waves comprise at best a small fraction of the total magnetospheric wave population. Previous claims to the contrary have been either based on erroneous statistical arguments or biased by the limited sample of ducted waves detectable by ground based receivers. The totality of satellite data is consistent with a predominantly natural origin for the two classes of electromagnetic waves (chorus and plasmaspheric hiss) which are known to interact strongly with energetic radiation belt electrons.  相似文献   
733.
A Retarding Potential Analyzer (RPA) is designed using a micro-channel plate in front of the collector as a flow amplifier, which makes it possible to detect ion species with very low concentration (<0,5 cm.?3). A technique is described for using the micro-channel plate at abnormally high pressure (>0.01 N/m2). This allows the precise determination of the mass composition of the low ionosphere. An adaptive retarding potential consisting of 2048 steps is used to increase the sensitivity. This retarding potential is applied in the range where dI/dU ≠ 0. The sensor is hermetically sealed and opens on command.  相似文献   
734.
Measurements of the precipitation of electrons and positive ions (in the keV to MeV range) detected aboard eight rockets launched from Northern Scandinavia are reported together with corresponding satellite data. The downgoing integral fluxes indicate the temporal fluctuations during each flight. Height profiles of the energy deposition into the atmosphere at different levels of geomagnetic disturbance are given.  相似文献   
735.
The asynchronous performance of a polyphase reluctance machine with rotor provided with axial slits is investigated for the first time on the basis of conductor boundary treated as a flux line. Theoretical and experimental investigations are presented for rotors with 1) many values of pole arc/pole pitch ratio, 2) varying slot depths, 3) slots on either pole faces or slot portions or both of them, and 4) with and without terminating end rings. Their comparison with predictions leads to interesting conclusions.  相似文献   
736.
The use of the 2 000 Å transmission window of the atmosphere permitted to observe for the first time from a balloon gondola the nearest galaxies ; 30 arc/sec. resolution images were obtained for M 31, spiral arms and nucleus, M 33, M 101, with their external spiral structure, M 82 and NGC 3 077. Nearly three hundred more distant galaxies have been detected up to the magnitude V = 13. Several HII regions of the Galaxy have been observed, chiefly NGC 7 000 and the Cyghus Loop. Numerous blue stars of the halo have been identified.  相似文献   
737.
We have tested the performance of the VISION System Blood Analyzer, produced by Abbott Laboratories, during parabolic flight on a KC-135 aircraft (NASA 930). This fully automated instrument performed flawlessly in these trials, demonstrating its potential for efficient, reliable use in a microgravity environment. In addition to instrument capability, we demonstrated that investigators could readily fill specially modified test packs with fluid during zero gravity, and that filled test packs could be easily loaded into VISION during an episode of microgravity.  相似文献   
738.
The ionic charge states of helium, carbon, oxygen, and iron have been determined for three solar particle enhancements by an electrostatic deflection analyzer, which is combined with a thin window proportional counter dE/dX vs. E system. The observations are obtained during the periods September 23 to 29, 1978, June 6 to 8, 1979, and September 15 to 26, 1979, with an instrument onboard the ISEE-3 spacecraft. The mean ionic charge states for He, C, and oxygen exhibit a high degree of ionization with values of Q = 2, 6, and 7.2, respectively. The charge state of iron is near 13 charge units. Variations from flare to flare and within the September 23, 1978 flare are small. The most surprising feature of the charge state measurement is the observation of a small (~10%) but finite contribution of singly ionized helium.  相似文献   
739.
The report presents some results on the cosmic radiation intensity research carried out with the high-altitude balloons for the period of 1977–1979. The intensity of gamma-radiation with the energy above 40 MeV was measured in two balloon flights at an altitude of 4–7 g/cm2 of residual atmosphere in the vicinity of the geomagnetic equator. A temporal analysis of the intensity to discover fluctuations with periods in the range of 4–60 min was made. Quasi-periodic fluctuations of gamma-radiation intensity with 5 min periods, amplitude ~20% and duration of several hours were discovered. Possible mechanisms of such fluctuation appearance are discussed.The report gives the results of measuring downward, upward and horizontal electron fluxes in the vicinity of the equator. The obtained data and the data provided by satellites are compared. The report discusses the prospects of further joint Soviet-Indian research of cosmic gamma-radiation.  相似文献   
740.
The combination of Voyager 1 (77.9 AU, 34.4° N) and Voyager 2 (61.2 AU, 24.5° S) at moderate heliolatitudes in the distant heliosphere and Ulysses with its unique latitudinal surveys in the inner heliosphere along with IMP 8 and other satellites at 1 AU constitutes a network of observatories that are ideally suited to study cosmic rays over the solar minimum of cycle 22 and the onset of solar activity and the long term cosmic ray modulation of cycle 23. Through 2000.7 there have been three well-defined step decreases in the cosmic ray intensity at 1 AU with the cumulative effect being in good agreement with the net decrease in cycle 21 at a comparable time in the solar cycle. Over this period the intensity changes at Ulysses are similar to those at 1 AU. In the distant heliosphere the initial decreases appear to be smaller than those at 1 AU. However the full effects of the interplanetary disturbances producing the most recent and largest step decrease in the inner heliosphere have not yet reached V-2. This revised version was published online in August 2006 with corrections to the Cover Date.  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