全文获取类型
收费全文 | 3819篇 |
免费 | 7篇 |
国内免费 | 8篇 |
专业分类
航空 | 1877篇 |
航天技术 | 1167篇 |
综合类 | 14篇 |
航天 | 776篇 |
出版年
2021年 | 23篇 |
2019年 | 27篇 |
2018年 | 187篇 |
2017年 | 146篇 |
2016年 | 72篇 |
2015年 | 35篇 |
2014年 | 63篇 |
2013年 | 78篇 |
2012年 | 106篇 |
2011年 | 218篇 |
2010年 | 191篇 |
2009年 | 236篇 |
2008年 | 271篇 |
2007年 | 208篇 |
2006年 | 71篇 |
2005年 | 117篇 |
2004年 | 88篇 |
2003年 | 93篇 |
2002年 | 64篇 |
2001年 | 90篇 |
2000年 | 55篇 |
1999年 | 60篇 |
1998年 | 77篇 |
1997年 | 52篇 |
1996年 | 60篇 |
1995年 | 85篇 |
1994年 | 86篇 |
1993年 | 55篇 |
1992年 | 71篇 |
1991年 | 36篇 |
1990年 | 30篇 |
1989年 | 68篇 |
1988年 | 24篇 |
1987年 | 30篇 |
1986年 | 28篇 |
1985年 | 102篇 |
1984年 | 73篇 |
1983年 | 66篇 |
1982年 | 77篇 |
1981年 | 88篇 |
1980年 | 29篇 |
1979年 | 39篇 |
1978年 | 25篇 |
1977年 | 17篇 |
1976年 | 12篇 |
1975年 | 11篇 |
1974年 | 14篇 |
1973年 | 15篇 |
1972年 | 14篇 |
1970年 | 12篇 |
排序方式: 共有3834条查询结果,搜索用时 15 毫秒
981.
982.
The determination of the chemical composition of solid cometary dust particles was one of the prime objectives of the three
missions to Comet Halley in 1986. The dust analysis was performed by time-of-flight mass-spectrometry. Within the experimental
uncertainty the mean abundances of the rock-forming elements in cometary dust particles are comparable to their abundances
in CI-chondrites and in the solar photosphere, i.e. they are cosmic. H, C, and N, on the other hand, in cometary dust are
significantly more abundant than in CI-chondrites, approach solar abundances, are to some extent related to O, and reside
in an omnipresent refractory organic component dubbed CHON. Element variations between individual dust grains are characterized
by correlations of Mg, Si, and O, and to a lesser extent of Fe and S. From particle-to-particle variations of the rock forming
elements information on the mineralogy of cometary dust can be obtained. Cluster analysis revealed certain groups that partly
match the classifications of stratospheric interplanetary dust particles. About half of Halley's analyzed particles are characterized
by anhydrous Fe-poor Mg-silicates, Fe-sulfides, and rarely Fe metal. The Fe-poor Mg-silicates link Halley's dust to that of
Hale-Bopp as shown by recent IR observations. No significant deviation from normal of the isotopic composition of the elements is unequivocally present with the notable exception carbon: 12C-rich grains with 12C/13C-ratios up to ≈ 5,000 link cometary dust to presolar circumstellar grains identified in certain chondrites.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
983.
V. G. Vil’ke 《Cosmic Research》2010,48(3):273-282
A mechanical system consisting from N deformable spheres interacting according to the law of gravity is considered as a model of planetary system. Deformations
of the viscoelastic spheres are described according to the model of the theory of elasticity of small deformations, the Kelvin-Voigt
model of viscous forces, and occur under the action of gravitational fields and fields of centrifugal forces. Approximate
equations describing motions of the centers of mass of the spheres and their rotations relative to the centers of mass are
constructed by the method of separation of motions on the basis of solving quasistatic problems of the theory of viscoelasticity
with allowance made for smallness of sphere deformations. Using the first integral of conservation of the angular momentum
of the system relative to its center of mass, the expression for the changed potential energy is obtained with the use of
the Routh method. An investigation of stationary rotations is carried out, and it is shown that all of them are unstable,
if the number of planets is more than two. 相似文献
984.
985.
Collecting Samples in Gale Crater, Mars; an Overview of the Mars Science Laboratory Sample Acquisition, Sample Processing and Handling System 总被引:1,自引:0,他引:1
R. C. Anderson L. Jandura A. B. Okon D. Sunshine C. Roumeliotis L. W. Beegle J. Hurowitz B. Kennedy D. Limonadi S. McCloskey M. Robinson C. Seybold K. Brown 《Space Science Reviews》2012,170(1-4):57-75
The Mars Science Laboratory Mission (MSL), scheduled to land on Mars in the summer of 2012, consists of a rover and a scientific payload designed to identify and assess the habitability, geological, and environmental histories of Gale crater. Unraveling the geologic history of the region and providing an assessment of present and past habitability requires an evaluation of the physical and chemical characteristics of the landing site; this includes providing an in-depth examination of the chemical and physical properties of Martian regolith and rocks. The MSL Sample Acquisition, Processing, and Handling (SA/SPaH) subsystem will be the first in-situ system designed to acquire interior rock and soil samples from Martian surface materials. These samples are processed and separated into fine particles and distributed to two onboard analytical science instruments SAM (Sample Analysis at Mars Instrument Suite) and CheMin (Chemistry and Mineralogy) or to a sample analysis tray for visual inspection. The SA/SPaH subsystem is also responsible for the placement of the two contact instruments, Alpha Particle X-Ray Spectrometer (APXS), and the Mars Hand Lens Imager (MAHLI), on rock and soil targets. Finally, there is a Dust Removal Tool (DRT) to remove dust particles from rock surfaces for subsequent analysis by the contact and or mast mounted instruments (e.g. Mast Cameras (MastCam) and the Chemistry and Micro-Imaging instruments (ChemCam)). 相似文献
986.
Thermal drill sampling system onboard high-velocity impactors for exploring the subsurface of Europa
P. Weiss K.L. Yung N. Kömle S.M. Ko E. Kaufmann G. Kargl 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
The planned Europa Jupiter System Mission (EJSM) will provide a unique opportunity to place scientific instruments onto the surface of Jupiter’s moon Europa in the late 2020s. After the Galileo mission, this will be a long awaited chance to have a close glimpse into some of the mysteries of this moon. Care must be taken in the choice of in-situ science that will be undertaken on the surface. 相似文献
987.
Bela G. Fejer 《Space Science Reviews》2011,158(1):145-166
The low latitude ionosphere is strongly affected by several highly variable electrodynamic processes. Over the last two decades
ground-based and satellite measurements and global numerical models have been extensively used to study the longitude-dependent
climatology of low latitude electric fields and currents. These electrodynamic processes and their ionospheric effects exhibit
large ranges of temporal and spatial variations during both geomagnetic quiet and disturbed conditions. Numerous recent studies
have investigated the short term response of equatorial electric fields and currents to lower atmospheric transport processes
and solar wind-magnetosphere driving mechanisms. This includes the large electric field and current perturbations associated
with arctic sudden stratospheric warming events during geomagnetic quiet times and highly variable storm time prompt penetration
and ionospheric disturbance dynamo effects. In this review, we initially describe recent experimental and numerical modeling
results of the global climatology and short term variability of quiet time low latitude electrodynamic plasma drifts. Then,
we examine the present understanding of equatorial electric field and current perturbation fields during periods of enhanced
geomagnetic activity. 相似文献
988.
T Ohnishi X Wang S Fukuda A Takahashi K Ohnishi S Nagaoka 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2000,25(10):2119-2122
Tumor suppressor p53 functions as a cell cycle checkpoint under stressful conditions. Early studies have shown that genotoxic stress activates p53 pathway. Recently, many kinds of non-genotoxic stress such as heat shock, cold shock, and low pH also have been found to activate p53 pathway. The effects on living organism remains to be explored. Here, we show that an 18-day space flight induced a 3.6 fold accumulation of p53 in rat skeletal muscle. This results suggests that the p53 pathway plays a role in safeguarding genomic stability against the stressful space environments and supports our previous observation of p53 accumulation in rat skin after a space flight. 相似文献
989.
A technological process of removing a complex-shaped ceramic core from the interior of GTE thin-walled blade castings is considered. We propose an aluminium oxide-based method for removing a core by leaching accompanied by process intensification with a carboxylic acid additive. 相似文献
990.
S. A. Danilova 《Russian Aeronautics (Iz VUZ)》2007,50(3):344-348
A problem on dynamic determination of a time interval in scheduling aperiodic requests in real-time systems is considered. It is shown that the application of a combined approach based on the combination of the probability method and the method of fuzzy sets provides an effective problem solution. 相似文献