全文获取类型
收费全文 | 2461篇 |
免费 | 29篇 |
国内免费 | 12篇 |
专业分类
航空 | 979篇 |
航天技术 | 908篇 |
综合类 | 12篇 |
航天 | 603篇 |
出版年
2022年 | 13篇 |
2021年 | 27篇 |
2019年 | 24篇 |
2018年 | 73篇 |
2017年 | 61篇 |
2016年 | 49篇 |
2015年 | 20篇 |
2014年 | 78篇 |
2013年 | 92篇 |
2012年 | 71篇 |
2011年 | 137篇 |
2010年 | 102篇 |
2009年 | 145篇 |
2008年 | 148篇 |
2007年 | 77篇 |
2006年 | 63篇 |
2005年 | 76篇 |
2004年 | 78篇 |
2003年 | 81篇 |
2002年 | 56篇 |
2001年 | 75篇 |
2000年 | 39篇 |
1999年 | 47篇 |
1998年 | 52篇 |
1997年 | 49篇 |
1996年 | 41篇 |
1995年 | 56篇 |
1994年 | 34篇 |
1993年 | 38篇 |
1992年 | 50篇 |
1991年 | 13篇 |
1990年 | 16篇 |
1989年 | 41篇 |
1988年 | 19篇 |
1987年 | 16篇 |
1986年 | 13篇 |
1985年 | 60篇 |
1984年 | 51篇 |
1983年 | 41篇 |
1982年 | 41篇 |
1981年 | 56篇 |
1980年 | 34篇 |
1979年 | 16篇 |
1978年 | 17篇 |
1977年 | 9篇 |
1976年 | 16篇 |
1975年 | 15篇 |
1974年 | 12篇 |
1971年 | 7篇 |
1969年 | 7篇 |
排序方式: 共有2502条查询结果,搜索用时 15 毫秒
341.
We describe the formation of hot intergalactic gas along with baryonic remnants in galaxy halos. In this scenario, the mass and metallicity of the hot intracluster and intragroup gas relates directly to the production of baryonic remnants during the collapse of galactic halos. We construct a schematic but self-consistent model in which early bursts of star formation lead to a large remnant population in the halo, and to the outflow of stellar ejecta into the halo and ultimately the Local Group. We consider local as well as high redshift constraints on this scenario. This study suggests that the microlensing objects in the Galactic halo may predominantly be 0.5M white dwarfs, assuming that the initial mass function for early star formation favored the formation of intermediate mass stars with m 1M. However, the bulk of the baryonic dark matter in this scenario is associated with the ejecta of the white dwarf progenitors, and resides in the hot intergalactic medium. 相似文献
342.
Space Science Reviews - 相似文献
343.
Harry N.A. Priem 《Space Science Reviews》1997,81(1-2):173-198
Climate is discussed as an integral part of System Earth, determined by a complex interplay of numerous geological, biological and solar processes. The historical and geological record of changing climate and atmospheric CO2 pressure does not support the current popular vision that this greenhouse gas is the dominant climate controlling agent. When empirically ante post tested against past global climate changes, the forecasts of the climate models mainly based on forcing by atmospheric CO2 are not borne out. On the other hand, recent studies show that solar variability rather than changing CO2 pressure is an important, probably the dominant climate forcing factor. 相似文献
344.
Battery charger design for the Columbus MTFF power system 总被引:1,自引:0,他引:1
van Dijk K. Klaassens J.B. Spruijt H.J.N. O'Sullivan D.M. 《IEEE transactions on aerospace and electronic systems》1997,33(1):29-37
A novel pulsewidth-modulated (PWM) dc-dc converter topology is proposed for the battery charge regulator (BCR) of the Columbus Man-Tended Free-Flyer (MTFF) power system. The system is a regulated bus system. Bus voltage control is implemented at the input of the BCR. The use of a conventional buck topology with PWM conductance control at the input results in a second-order behavior. A study of new PWM dc-dc converter topologies has been made to attain a suitable topology. The proposed converter topology is designed and a breadboard including the control loop has been built and tested. The experimental results show that the converter operates according to the design constraints. 相似文献
345.
A relative navigation system for formation flight 总被引:1,自引:0,他引:1
A relative navigation system based on both the Inertial Navigation System (INS) and the Global Positioning System (GPS) is developed to support situational awareness during formation flight. The architecture of the system requires an INS/GPS integration across two aircraft via a data link. A fault-tolerant federated filter is used to estimate the relative INS errors based on relative GPS measurements and a range measurement obtained from the data link. The filter is constructed based on a reduced-order model of the relative INS error process. A method for analyzing the filter performance is presented. A case involving two helicopters in formation flight is studied under three different night trajectories to account for the effect of vehicle motion on the INS state transition matrix. The results of the covariance analysis are compared with actual night results over an instrumented test range. 相似文献
346.
The problem of optimal data fusion in multiple detection systems is studied in the case where training examples are available, but no a priori information is available about the probability distributions of errors committed by the individual detectors. Earlier solutions to this problem require some knowledge of the error distributions of the detectors, for example, either in a parametric form or in a closed analytical form. Here we show that, given a sufficiently large training sample, an optimal fusion rule can be implemented with an arbitrary level of confidence. We first consider the classical cases of Bayesian rule and Neyman-Pearson test for a system of independent detectors. Then we show a general result that any test function with a suitable Lipschitz property can be implemented with arbitrary precision, based on a training sample whose size is a function of the Lipschitz constant, number of parameters, and empirical measures. The general case subsumes the cases of nonindependent and correlated detectors. 相似文献
347.
348.
We present our current knowledge of the solar chemical composition based on the recent significant downward revision of the solar photospheric abundances of the most abundant metals. These new solar abundances result from the use of a 3D hydrodynamic model of the solar atmosphere instead of the classical 1D hydrostatic models, accounting for departures from LTE, and improved atomic and molecular data. With these abundances, the new solar metallicity, Z, decreases to Z=0.012, almost a factor of two lower than earlier widely used values. We compare our values with data from other sources and analyse a number of impacts of these new photospheric abundances. While resolving a number of longstanding problems, the new 3D-based solar photospheric composition also poses serious challenges for the standard solar model as judged by helioseismology. 相似文献
349.
V. F. Pavlov V. A. Kirpichev N. I. Yakovenko D. V. Ivanov 《Russian Aeronautics (Iz VUZ)》2007,50(4):442-445
Residual stresses and endurance strength of specimens with stress concentrators are considered. The results obtained make it possible to predict the fatigue limit of strengthened parts with concentrators in the case of tension-compression. 相似文献
350.
Yuri N. Kulikov Helmut Lammer Herbert I. M. Lichtenegger Thomas Penz Doris Breuer Tilman Spohn Rickard Lundin Helfried K. Biernat 《Space Science Reviews》2007,129(1-3):207-243
Because the solar radiation and particle environment plays a major role in all atmospheric processes such as ionization, dissociation,
heating of the upper atmospheres, and thermal and non-thermal atmospheric loss processes, the long-time evolution of planetary
atmospheres and their water inventories can only be understood within the context of the evolving Sun. We compare the effect
of solar induced X-ray and EUV (XUV) heating on the upper atmospheres of Earth, Venus and Mars since the time when the Sun
arrived at the Zero-Age-Main-Sequence (ZAMS) about 4.6 Gyr ago. We apply a diffusive-gravitational equilibrium and thermal
balance model for studying heating of the early thermospheres by photodissociation and ionization processes, due to exothermic
chemical reactions and cooling by IR-radiating molecules like CO2, NO, OH, etc. Our model simulations result in extended thermospheres for early Earth, Venus and Mars. The exospheric temperatures
obtained for all the three planets during this time period lead to diffusion-limited hydrodynamic escape of atomic hydrogen
and high Jeans’ escape rates for heavier species like H2, He, C, N, O, etc. The duration of this blow-off phase for atomic hydrogen depends essentially on the mixing ratios of CO2, N2 and H2O in the atmospheres and could last from ∼100 to several hundred million years. Furthermore, we study the efficiency of various
non-thermal atmospheric loss processes on Venus and Mars and investigate the possible protecting effect of the early martian
magnetosphere against solar wind induced ion pick up erosion. We find that the early martian magnetic field could decrease
the ion-related non-thermal escape rates by a great amount. It is possible that non-magnetized early Mars could have lost
its whole atmosphere due to the combined effect of its extended upper atmosphere and a dense solar wind plasma flow of the
young Sun during about 200 Myr after the Sun arrived at the ZAMS. Depending on the solar wind parameters, our model simulations
for early Venus show that ion pick up by strong solar wind from a non-magnetized planet could erode up to an equivalent amount
of ∼250 bar of O+ ions during the first several hundred million years. This accumulated loss corresponds to an equivalent mass of ∼1 terrestrial
ocean (TO (1 TO ∼1.39×1024 g or expressed as partial pressure, about 265 bar, which corresponds to ∼2900 m average depth)). Finally, we discuss and
compare our findings with the results of preceding studies. 相似文献