全文获取类型
收费全文 | 3701篇 |
免费 | 25篇 |
国内免费 | 10篇 |
专业分类
航空 | 1929篇 |
航天技术 | 1206篇 |
综合类 | 12篇 |
航天 | 589篇 |
出版年
2021年 | 34篇 |
2019年 | 27篇 |
2018年 | 82篇 |
2017年 | 58篇 |
2016年 | 63篇 |
2015年 | 32篇 |
2014年 | 81篇 |
2013年 | 101篇 |
2012年 | 97篇 |
2011年 | 134篇 |
2010年 | 107篇 |
2009年 | 153篇 |
2008年 | 201篇 |
2007年 | 108篇 |
2006年 | 81篇 |
2005年 | 100篇 |
2004年 | 89篇 |
2003年 | 113篇 |
2002年 | 70篇 |
2001年 | 123篇 |
2000年 | 73篇 |
1999年 | 83篇 |
1998年 | 106篇 |
1997年 | 70篇 |
1996年 | 95篇 |
1995年 | 132篇 |
1994年 | 103篇 |
1993年 | 62篇 |
1992年 | 90篇 |
1991年 | 34篇 |
1990年 | 39篇 |
1989年 | 86篇 |
1988年 | 43篇 |
1987年 | 37篇 |
1986年 | 35篇 |
1985年 | 97篇 |
1984年 | 93篇 |
1983年 | 82篇 |
1982年 | 82篇 |
1981年 | 90篇 |
1980年 | 27篇 |
1979年 | 27篇 |
1978年 | 29篇 |
1977年 | 27篇 |
1976年 | 19篇 |
1975年 | 35篇 |
1974年 | 25篇 |
1973年 | 18篇 |
1972年 | 33篇 |
1971年 | 18篇 |
排序方式: 共有3736条查询结果,搜索用时 15 毫秒
991.
SOHO/UVCS data indicate that minor ions in the corona are heated more than hydrogen, and that coronal heating results in T⊥ larger than T‖. Analogous behavior has been known from in situ measurements in solar wind for many years. Here we compare and contrast two
mechanisms which have been proposed to account for the above behavior: ion-cyclotron resonance and gravity damping.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
992.
We analyzed UVCS/SOHO data and compared the H I Lyα (121.6 nm) and O VI (103.2 nm, 103.7 nm) emission in the polar and equatorial
coronal holes. We found that the emission lines have similar characteristics in these two types of coronal holes. Both types
show evidence for superradially diverging boundaries. The latitudinal distribution of the O VI line ratio may indicate that
the equatorial coronal hole has O+5 outflow velocities lower than in the polar coronal holes.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
993.
994.
K M Prise M Folkard A M Malcolmson C H Pullar G Schettino A G Bowey B D Michael 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2000,25(10):2095-2101
Understanding the effects of single-particles from conventional radiation biology experiments is problematic due to the stochastics of particle tracks. This complicates the determinations of risk associated with low doses. We have developed a charged particle microbeam, which allows individually counted particles to be delivered to precise cellular locations. The system is capable of delivering a single charged particle with > 99% efficiency. Of these particles 90% are delivered with a resolution of +/- 2 micrometers and 96% with a resolution of +/- 5 micrometers. We have carried out preliminary studies in Chinese hamster V79 cells to monitor the effectiveness of low energy protons at inducing cytological damage. We have used the micronucleus assay as a measure of predominantly lethal chromosome damage. The effects of a single 3.2 MeV proton delivered individually to cells could be measured, with less than 2% of the exposed cells producing micronuclei 24 hours later. The yield of micronuclei formation was essentially linear up to the highest dose (30 particles per cell nucleus) delivered. Ultimately, the ability to target particles to different parts of the cell nucleus may start to impact on models available for chromosome aberration formation and chromosomal Organisation and mechanisms underlying genomic instability. 相似文献
995.
S B Jones D Or 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1998,22(10):1413-1418
An important and yet relatively under researched area of plant growth in microgravity, deals with the rooting environment of plants. A comprehensive approach for selecting the physical characteristics of root growth media which optimizes the dynamic availability of water and dissolved nutrients, and gases to plant roots was developed and tested. Physically-based and parametric models describing the relationship between content and fluxes of liquids and gases were used to cast a multi-objective optimization problem. This methodology reveals that a medium's ability to supply liquid and gas fluxes optimally is dependent upon physiological target values, system operation limits and root module design which dictate the medium's range of soil water characteristic and particle size distribution. Optimized media parameters designate a particle size distribution from which a particulated growth media was constructed and matched to the optimized media parameters. This methodology should improve the selection of optimal media properties for plant growth in microgravity as well as other porous media applications. 相似文献
996.
I. V. Mingalev N. M. Astaf’eva K. G. Orlov V. M. Chechetkin V. S. Mingalev O. V. Mingalev 《Cosmic Research》2012,50(3):233-248
Mechanisms of formation of cyclonic vortices in the tropical atmosphere of the Earth are investigated in the intratropical
zone of convergence using numerical simulation made with the complete system of equations of gas dynamics taking into account
transport of infrared radiation, phase transitions of water vapor into microdrops of water and ice particles, and sedimentation
of these drops and ice particles in the field of gravity force. Observational data on the structure of dominant air streams,
which are formed in the intratropical zone of convergence over the North Atlantic in the periods of its highest thermodynamic
intensity and instability, are used in the initial and boundary conditions of the model. Formation of cyclonic vortex flows
is obtained numerically at sufficiently strong bending of the intratropical zone of convergence. The results of numerical
modeling are compared with the data of satellite microwave monitoring: global radio thermal fields of the Earth from the electronic
collection GLOBAL-Field allowing one to study the structure of atmospheric motions in a wide range of space-time scales. 相似文献
997.
998.
Vesta and Ceres: Crossing the History of the Solar System 总被引:1,自引:0,他引:1
The evolution of the Solar System can be schematically divided into three different phases: the Solar Nebula, the Primordial Solar System and the Modern Solar System. These three periods were characterized by very different conditions, both from the point of view of the physical conditions and from that of the processes there were acting through them. Across the Solar Nebula phase, planetesimals and planetary embryos were forming and differentiating due to the decay of short-lived radionuclides. At the same time, giant planets formed their cores and accreted the nebular gas to reach their present masses. After the gas dispersal, the Primordial Solar System began its evolution. In the inner Solar System, planetary embryos formed the terrestrial planets and, in combination with the gravitational perturbations of the giant planets, depleted the residual population of planetesimals. In the outer Solar System, giant planets underwent a violent, chaotic phase of orbital rearrangement which caused the Late Heavy Bombardment. Then the rapid and fierce evolution of the young Solar System left place to the more regular secular evolution of the Modern Solar System. Vesta, through its connection with HED meteorites, and plausibly Ceres too were between the first bodies to form in the history of the Solar System. Here we discuss the timescale of their formation and evolution and how they would have been affected by their passage through the different phases of the history of the Solar System, in order to draw a reference framework to interpret the data that Dawn mission will supply on them. 相似文献
999.
R. M. Suleiman J.L. Kohl A. V. Panasyuk A. Ciaravella S.R. Cranmer L.D. Gardner R. Frazin R. Hauck P.L. Smith G. Noci 《Space Science Reviews》1999,87(1-2):327-330
The Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO) has been used to measure
spectral line profiles for H I Lyα in the south polar coronal hole at projected heliocentric heights from 3.5 to 6.0 R⊙ during 1998 January 5–11. Observations from 1.5 to 2.5 R⊙ were made for comparison. The H I Lyα profile is the only one observable with UVCS above 3.5 R⊙ in coronal holes. Within this region the outflowing coronal plasma becomes nearly collisionless and the ionization balance
is believed to become frozen.
In this paper, the 1/e half widths of the coronal velocity distributions are provided for the observed heights. The velocity
distributions include all motions contributing to the velocities along the line of sight (LOS). The observations have been
corrected for instrumental effects and interplanetary H I Lyα. The half widths were found to increase with projected heliographic
height from 1.5 to 2.5 R⊙ and decrease with height from 3.5 to 5 R⊙.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
1000.
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