全文获取类型
收费全文 | 3706篇 |
免费 | 60篇 |
国内免费 | 42篇 |
专业分类
航空 | 1757篇 |
航天技术 | 1343篇 |
综合类 | 71篇 |
航天 | 637篇 |
出版年
2022年 | 21篇 |
2021年 | 39篇 |
2018年 | 55篇 |
2017年 | 62篇 |
2016年 | 41篇 |
2015年 | 35篇 |
2014年 | 77篇 |
2013年 | 101篇 |
2012年 | 85篇 |
2011年 | 164篇 |
2010年 | 115篇 |
2009年 | 141篇 |
2008年 | 207篇 |
2007年 | 115篇 |
2006年 | 109篇 |
2005年 | 117篇 |
2004年 | 81篇 |
2003年 | 116篇 |
2002年 | 90篇 |
2001年 | 105篇 |
2000年 | 88篇 |
1999年 | 102篇 |
1998年 | 107篇 |
1997年 | 101篇 |
1996年 | 88篇 |
1995年 | 104篇 |
1994年 | 107篇 |
1993年 | 64篇 |
1992年 | 75篇 |
1991年 | 39篇 |
1990年 | 44篇 |
1989年 | 74篇 |
1988年 | 39篇 |
1987年 | 32篇 |
1986年 | 34篇 |
1985年 | 114篇 |
1984年 | 79篇 |
1983年 | 79篇 |
1982年 | 60篇 |
1981年 | 103篇 |
1980年 | 25篇 |
1979年 | 27篇 |
1978年 | 33篇 |
1977年 | 25篇 |
1976年 | 23篇 |
1975年 | 39篇 |
1972年 | 33篇 |
1971年 | 25篇 |
1969年 | 22篇 |
1967年 | 23篇 |
排序方式: 共有3808条查询结果,搜索用时 15 毫秒
861.
D E Schwartz R L Mancinelli M R White 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,15(3):193-197
An important question for exobiology is, did life evolve on Mars? To answer this question, experiments must be conducted on the martian surface. Given current mission constraints on mass, power, and volume, these experiments can only be performed using proposed analytical techniques such as: electron microscopy, X-ray fluorescence, X-ray diffraction, alpha-proton backscatter, gamma-ray spectrometry, differential thermal analysis, differential scanning calorimetry, pyrolysis gas chromatography, mass spectrometry, and specific element detectors. Using prepared test samples consisting of 1% organic matter (bovine serum albumin) in palagonite and a mixture of palagonite, clays, iron oxides, and evaporites, it was determined that a combination of X-ray diffraction and differential thermal analysis coupled with gas chromatography provides the best insight into the chemistry, mineralogy, and geological history of the samples. 相似文献
862.
M. K. Hudson W. Lotko C. A. Cattell R. L. Lysak I. Roth M. Temerin 《Space Science Reviews》1995,71(1-4):623-646
Intermediate or mesoscale processes mediate the transfer of mass, momentum, and energy across the dynamic solar wind-magnetosphere interface, and the propagation of this input through the system to the ionosphere and atmosphere. The Dartmouth-Berkeley-Minnesota theory team has identified a number of mesoscale phenomena to be investigated as part of the GGS program, including: (1) effects of upstream density fluctuations on magnetopause dynamics, (2) three-dimensional reconnection, (3) magnetopause depletion layer studies, (4) ring current interaction with Pc 1 and Pc 5 waves, (5) generation of ion Larmor-scale current layers in the near Earth plasmasheet, (6) test particle studies in the magnetotail, (7) simulation of magnetosphere- ionosphere coupling including effects of kinetic Alfvén waves and (8) auroral acceleration region studies of the effects of kinetic Alfvén waves on particle distribution functions. A broad range of techniques will be implemented including ideal and reduced MHD, two fluid, hybrid, particle-in-cell and test particle simulations. Detailed comparison of simulation results with GGS satellite and ground based data will be undertaken. 相似文献
863.
L.H. Brace R.F. Theis 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(1):89-91
Employing Atmsophere Explorer-C measurements made in 1974, just prior to solar minimum, Brace and Theis /1/ demonstrated that a remarkably consistent inverse relationship existed between the electron density Ne and temperature Te in the F-region. In this paper we use later data from AE-C, taken when solar activity was rising (1975–1978), and Dynamics Explorer-2 data taken at solar maximum (1981), to examine how the temperature and density relationship changes with solar activity. We find that the solar maximum Te is a factor of two larger than the solar minimum Te for the same values of Ne. Te does not necessarily increase with solar activity, however, because Ne increases enough to approximately cancel the effect of higher solar extreme ultraviolet heating. We find that the effect of solar activity can be accounted for by a simple function of the F10.7 cm index that multiplies the solar minimum equation of Brace and Theis /1/. 相似文献
864.
Rotman S.R. Tidhar G. Kowalczyk M.L. 《IEEE transactions on aerospace and electronic systems》1994,30(1):81-91
The problem of calculating an effective contrast for diffused targets in visual scenery is addressed. The effect of local variations of intensity in the vicinity of the target is modeled by considering the distribution of the intensity of the edges. The effect of global clutter is discussed from the vantage point of selecting the best model for background scenery. Generalized contrast measures are described and applied to real scenes 相似文献
865.
E.M. Apostolov L.F. Alberca D. Pancheva 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(12):47-50
In this paper we intend to enlarge the fof2 long-term prediction accuracy taking into account the systematic variations of the hysteresis and secular-time phenomena. 相似文献
866.
The Cloud Profiling Radar (CPR) for the upcoming CloudSat Mission is a spaceborne 94-GHz nadir-looking radar that measures the power backscattered by clouds as a function of distance from the radar. This sensor is expected to provide cloud measurements at a 500-m vertical resolution and a 1.5 km horizontal resolution. CPR will operate in a short-pulse mode and will yield measurements at a minimum detectable sensitivity of -28 dBZ. 相似文献
867.
A generic mission for an autonomous brilliant munition is presented and used to identify the functions that embedded signal processors must perform. Based on these functions and other operational factors such as weather, countermeasures, larger search areas, reduced false alarm rates, and increased munition maneuverability, the processing loads in bits/second messages/second, operations/second, and instructions/second are derived. An evaluation of general implementation issues such as the requirements for data fusion, distributed and parallel processing architectures, reliable software, and low-cost hardware is presented 相似文献
868.
Peter L. Biermann 《Space Science Reviews》1995,74(3-4):385-396
In the following we describe recent progress in our understanding of the origin of cosmic rays. We propose that cosmic rays originate mainly in three sites, a) normal supernova explosions into the interstellar medium, b) supernova explosions into stellar winds, and c) hot spots of powerful radio galaxies. The proposal depends on an assumption about the scaling of the turbulent diffusive transport in cosmic ray mediated shock regions; the proposal also uses a specific model for the interstellar transport of cosmic rays. The model has been investigated in some detail and compared to i) the radio data of OB stars, Wolf Rayet stars, radio supernovae, radio supernova remnants, Gamma-ray line and continuum emission from starforming regions, and the cosmic ray electron spectrum, ii) the Akeno air shower data over the particle energy range from 10 TeV to EeV, and iii) the Akeno and Fly's Eye air shower data from 0.1 EeV to above 100 EeV. 相似文献
869.
H.E Revercomb L.A Sromovsky V.E Suomi R.W Boese 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1985,5(9):81-84
Corrected thermal net radiation measurements from the four Pioneer Venus entry probes at latitudes of 60°N, 31°S, 27°S, and 4°N are presented. Three main conclusions can be drawn from comparisons of the corrected fluxes with radiative transfer calculations: (1) sounder probe net fluxes are consistent with the number density of large cloud particles (mode 3) measured on the same probe, but the IR measurements as a whole are most consistent with a significantly reduced mode 3 contribution to the cloud opacity; (2) at all probe sites, the fluxes imply that the upper cloud contains a yet undetected source of IR opacity; and (3) beneath the clouds the fluxes at a given altitude increase with latitude, suggesting greater IR cooling below the clouds at high latitudes and water vapor mixing ratios of about 2–5×10?5 near 60°, 2–5×10?4 near 30°, and >5×10?4 near the equator. 相似文献
870.
We review aspects of circumstellar habitable zones based on results reported at the First International Conference on Circumstellar Habitable Zones (held in 1994 at NASA Ames Research Center). Recent advances in atmospheric radiative transfer modeling have shown that circumstellar habitable zones are wider than previously thought. New considerations may allow a much shorter time scale both for the origin as well as the evolution of biological forms. The most abundant M dwarf stars, contrary to previous views, appear to be able to support the necessary conditions for a habitable zone around them. New planet formation models indicate that at least one planet should form within the circumstellar habitable zone of single dwarf stars regardless of mass. Biogenic materials also appear to be widespread and delivery to terrestrial planets via cometary impacts may be a viable mechanism. Finally, biology will modify a planet and provide positive feedback, in general, to increase the habitability of a planet. Overall, new astronomical, planetary, and biological considerations each seem to indicate that habitable zones around other stars may be both more widespread and more stable than previous research had indicated. 相似文献