全文获取类型
收费全文 | 261417篇 |
免费 | 5204篇 |
国内免费 | 3413篇 |
专业分类
测绘学 | 6996篇 |
大气科学 | 19282篇 |
地球物理 | 54882篇 |
地质学 | 91352篇 |
海洋学 | 21743篇 |
天文学 | 56712篇 |
综合类 | 1026篇 |
自然地理 | 18041篇 |
出版年
2021年 | 2235篇 |
2020年 | 2623篇 |
2019年 | 2859篇 |
2018年 | 4018篇 |
2017年 | 3649篇 |
2016年 | 6002篇 |
2015年 | 4233篇 |
2014年 | 6948篇 |
2013年 | 14247篇 |
2012年 | 6664篇 |
2011年 | 8175篇 |
2010年 | 7184篇 |
2009年 | 9787篇 |
2008年 | 8659篇 |
2007年 | 8136篇 |
2006年 | 9698篇 |
2005年 | 7841篇 |
2004年 | 7740篇 |
2003年 | 7221篇 |
2002年 | 6792篇 |
2001年 | 6054篇 |
2000年 | 5975篇 |
1999年 | 5207篇 |
1998年 | 5245篇 |
1997年 | 5046篇 |
1996年 | 4684篇 |
1995年 | 4429篇 |
1994年 | 4118篇 |
1993年 | 3858篇 |
1992年 | 3644篇 |
1991年 | 3599篇 |
1990年 | 3763篇 |
1989年 | 3520篇 |
1988年 | 3303篇 |
1987年 | 3858篇 |
1986年 | 3422篇 |
1985年 | 4236篇 |
1984年 | 4744篇 |
1983年 | 4413篇 |
1982年 | 4319篇 |
1981年 | 3921篇 |
1980年 | 3646篇 |
1979年 | 3508篇 |
1978年 | 3487篇 |
1977年 | 3285篇 |
1976年 | 3055篇 |
1975年 | 2960篇 |
1974年 | 2917篇 |
1973年 | 3080篇 |
1972年 | 2024篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
411.
Florent Deleflie Gilles Métris Pierre Exertier 《Celestial Mechanics and Dynamical Astronomy》2006,94(1):83-104
This paper studies the long period variations of the eccentricity vector of the orbit of an artificial satellite, under the
influence of the gravity field of a central body. We use modified orbital elements which are non-singular at zero eccentricity.
We expand the long periodic part of the corresponding Lagrange equations as power series of the eccentricity. The coefficients
characterizing the differential system depend on the zonal coefficients of the geopotential, and on initial semi-major axis,
inclination, and eccentricity. The differential equations for the components of the eccentricity vector are then integrated
analytically, with a definition of the period of the perigee based on the notion of “free eccentricity”, and which is also
valid for circular orbits. The analytical solution is compared to a numerical integration. This study is a generalization
of (Cook, Planet. Space Sci., 14, 1966): first, the coefficients involved in the differential equations depend on all zonal coefficients (and not only on
the very first ones); second, our method applies to nearly circular orbits as well as to not too eccentric orbits. Except
for the critical inclination, our solution is valid for all kinds of long period motions of the perigee, i.e., circulations
or librations around an equilibrium point. 相似文献
412.
The large-scale structure associated with the 2′N HNCO peak in Sgr B2 [Minh, Y.C., Haikala, L., Hjalmarson, Å., Irvine, W.M., 1998. ApJ 498, 261 (Paper I)] has been investigated. A ring-like morphology of the HNCO emission has been found; this structure may be colliding with the Principal Cloud of Sgr B2. This “HNCO Ring” appears to be centered at (l,b) = (0.7°,−0.07°), with a radius of 5 pc and a total mass of 1.0 × 105 to 1.6 × 106 M. The expansion velocity of the Ring is estimated to be 30–40 km s−1, which gives an expansion time scale of 1.5 × 105 year. The morphology suggests that collision between the Ring and the Principal Cloud may be triggering the massive star formation in the Sgr B2 cloud sequentially, with the latest star formation taking place at the 2′N position. The chemistry related to HNCO is not certain yet, but if it forms mainly via reaction with the evaporated OCN− from icy grain mantles, the observed enhancement of the HNCO abundance can be understood as resulting from shocks associated with the collision between the Principal Cloud and the expanding HNCO Ring. 相似文献
413.
To derive a matched filter for detecting a weak target signal in a hyperspectral image, an estimate of the band-to-band covariance of the target-free background scene is required. We investigate the effects of including some of the target signal in the background scene. Although the covariance is contaminated by the presence of a target signal (there is increased variance in the direction of the target signature), we find that the matched filter is not necessarily affected. In fact, if the variation in plume strength is strictly uncorrelated with the variation in background spectra, the matched filter and its signal-to-clutter ratio (SCR) performance will not be impaired. While there is little a priori reason to expect significant correlation between the plume and the background, there usually is some residual correlation, and this correlation leads to a suppressing effect that limits the SCR obtainable even for strong plumes. These effects are described and quantified analytically, and the crucial role of this correlation is illustrated with some numerical examples using simulated plumes superimposed on real hyperspectral imagery. In one example, we observe an order-of-magnitude loss in SCR for a matched filter based on the contaminated covariance. 相似文献
414.
415.
416.
417.
418.
419.
Radiation exposure and Mission Strategies
for Interplanetary Manned Missions (REMSIM) 总被引:1,自引:0,他引:1
C. Cougnet N. B. Crosby S. Eckersley C. Foullon V. Guarnieri S. Guatelli D. Heynderickx A. Holmes-Siedle C. Lobascio S. Masiello P. Nieminen G. Parisi P. Parodi M. A. Perino M. G. Pia R. Rampini P. Spillantini V. Tamburini E. Tracino 《Earth, Moon, and Planets》2004,94(3-4):279-285
Cosmic radiation is an important problem for human interplanetary missions. The “Radiation Exposure and Mission Strategies for Interplanetary Manned Missions–REMSIM” study is summarised here. They are related to current strategies and countermeasures to ensure the protection of astronauts from radiation during interplanetary missions, with specific reference to: radiation environment and its variability; radiation effects on the crew; transfer trajectories and associated fluences; vehicle and surface habitat concepts; passive and active shielding concepts; space weather monitoring and warning systems. 相似文献
420.
We examine the potential contamination of cometary nuclei through impacts from asteroidal origin meteoroids. The paper uses
a simple model and has the goal of determining whether asteroidal contamination is potentially significant. We assume a meteoroid
power law mass distribution with index values in the range from s=1.83 to s=2.09. We used maximum and minimum models which we believe will bracket the true meteoroid mass distribution. We identify
those comets which are expected to be most significantly contaminated, and find values of up to 3.6 kg of asteroidal meteoroid
impact per square meter of the cometary surface per orbital revolution. This is less than the expected mass loss per perihelion
passage for most comets. Therefore any remnant effects of the contamination will depend on the penetration depth of the meteoroids
in the cometary nucleus, and possibly on the distribution of active and inactive areas on cometary nuclei. We present a simple
model which suggests that even small meteoroids will embed relatively deeply into a cometary nucleus. 相似文献