全文获取类型
| 收费全文 | 46441篇 |
| 免费 | 7108篇 |
| 国内免费 | 10090篇 |
专业分类
| 测绘学 | 7429篇 |
| 大气科学 | 6805篇 |
| 地球物理 | 8465篇 |
| 地质学 | 19889篇 |
| 海洋学 | 5800篇 |
| 天文学 | 5700篇 |
| 综合类 | 3436篇 |
| 自然地理 | 6115篇 |
出版年
| 2024年 | 133篇 |
| 2023年 | 451篇 |
| 2022年 | 1309篇 |
| 2021年 | 1555篇 |
| 2020年 | 1749篇 |
| 2019年 | 1860篇 |
| 2018年 | 1654篇 |
| 2017年 | 1955篇 |
| 2016年 | 1987篇 |
| 2015年 | 2282篇 |
| 2014年 | 2698篇 |
| 2013年 | 2977篇 |
| 2012年 | 2973篇 |
| 2011年 | 3286篇 |
| 2010年 | 2803篇 |
| 2009年 | 3235篇 |
| 2008年 | 3262篇 |
| 2007年 | 3419篇 |
| 2006年 | 3216篇 |
| 2005年 | 2902篇 |
| 2004年 | 2562篇 |
| 2003年 | 2174篇 |
| 2002年 | 2007篇 |
| 2001年 | 1678篇 |
| 2000年 | 1538篇 |
| 1999年 | 1347篇 |
| 1998年 | 1220篇 |
| 1997年 | 956篇 |
| 1996年 | 839篇 |
| 1995年 | 750篇 |
| 1994年 | 690篇 |
| 1993年 | 562篇 |
| 1992年 | 349篇 |
| 1991年 | 305篇 |
| 1990年 | 220篇 |
| 1989年 | 182篇 |
| 1988年 | 143篇 |
| 1987年 | 91篇 |
| 1986年 | 60篇 |
| 1985年 | 63篇 |
| 1984年 | 40篇 |
| 1983年 | 18篇 |
| 1982年 | 29篇 |
| 1981年 | 15篇 |
| 1980年 | 22篇 |
| 1979年 | 14篇 |
| 1978年 | 17篇 |
| 1977年 | 18篇 |
| 1976年 | 4篇 |
| 1954年 | 13篇 |
排序方式： 共有10000条查询结果，搜索用时 906 毫秒
61.
Optimal deflection of NEOs en route of collision with the Earth 总被引：1，自引：0，他引：1
Recently, a method for the n-body analysis of the velocity change required to deflect a hazardous near-Earth object (NEO) was presented by Carusi et al. [Carusi, A., Valsecchi, G.B., D'Abramo, G., Boattini A., 2002. Icarus 159, 417-422]. We extent this method in order to optimize the velocity change vector instead of its along-track magnitude. From an application of both methods to a fictitious NEO we find Carusi's parallel approach to be reasonable for phases of unperturbed two-body motion. But, for orbit phases inhering third-body perturbations, i.e., for planetary close approaches or prior to a collision, the results obtained from the new method show the radial component of deflection impulse to play a major role. We show that a fivefold greater efficiency can be achieved by a deflection impulse being non-parallel to orbital velocity. The new method is applied to two possible 99942 Apophis impact trajectories in order to provide constraints for future Apophis deflection mission analysis. 相似文献
62.
在对嘉祥县石灰岩矿进行地质灾害危险性评估、矿山环境地质问题评价的基础上,建立了一套评价体系,其中地质灾害危险性评估分为2个评价指标,矿山环境地质问题评价分为2种要素3个指标,同时将矿山恢复治理难易程度也作为一个评价因子。评价过程中对各指标危害性大、中、小程度均单独赋值,然后将各指标的分值进行叠加,确定出矿山环境地质问题的等级,将概划出的13个评价单元分为极差、差、一般3个区。提出了强化矿山管理、植树造林、科学避让、修建拦水坝等保护与治理措施及建议。 相似文献
63.
Recent Doppler velocity measurements have revealed the existence of two planets orbiting the star HD 12661 on medium-eccentricity orbits. The inner planet has a period of 263.6 d and a mass of 2.3 Jupiter masses, and the outer planet has a period of 1444.5d and a mass of 1.57 Jupiter masses. The stability of this system requires the two planets to be in a state of mean-motion orbit resonances. By numerical method we have studied the orbit migration and stability of the system in its early ages under the action of the proto-stellar disk, and calculated the probabilities of the planets being captured into the mean -motion resonances during their migrations. It is found that at present the two planets are probably situated at the edge of the 11:2 mean-motion resonance and are in chaotic motions. This result may be helpful to clarify the arguments on the present configuration. Besides, it is indicated that very probably, after the formation of the system, the gaseous disk has almost disappeared before the planets migrated to the present configuration. 相似文献
64.
65.
66.
We present results from a new simulation code that accounts for the evolution of the reservoirs of carbon dioxide on Mars, from its early years to the present. We establish a baseline model parameter set that produces results compatible with the present (i.e., Patm?6.5 mbar with permanent CO2 ice cap) for a wide range of initial inventories. We find that the initial inventory of CO2 broadly determines the evolutionary course of the reservoirs of CO2. The reservoirs include the atmosphere, ice cap, adsorbed CO2 in the regolith, and carbonate rocks. We track the evolution of the free inventory: the atmosphere, ice cap and regolith. Simulations begin at 4.53 Gyr before present with a rapid loss of free inventory to space in the early Noachian. Models that assume a relatively small initial inventory (?5 bar) have pronounced minima in the free inventory of CO2 toward the end of the Noachian. Under baseline parameters, initial inventories below ∼4.5 bar result in a catastrophic loss of the free inventory to space. The current free inventory would be then determined by the balance between outgassing, sputtering losses and chemical weathering following the end of the late bombardment. We call these “thin” models. They generically predict small current free inventories in line with expectations of a small present CO2 ice cap. For “thick” models, with initial inventories ?5 bar, a surplus of 300-700 mbar of free CO2 remains during the late-Noachian. The histories of free inventory in time for thick models tend to converge within the last 3.5 Gyr toward a present with an ice cap plus atmospheric inventory of about 100 mbar. For thick models, the convergence is largely due to the effects of chemical weathering, which draws down higher free inventories more rapidly than the low. Thus, thick models have ?450 mbar carbonate reservoirs, while thin models have ?200 mbar. Though both thick and thin scenarios can reproduce the current atmospheric pressure, the thick models imply a relatively large current CO2 ice cap and thin models, little or none. While the sublimation of a massive cap at a high obliquity would create a climate swing of greenhouse warming for thick models, under the thin model, mean temperatures and pressures would be essentially unaffected by increases in obliquity. 相似文献
67.
68.
69.
70.