全文获取类型
收费全文 | 66714篇 |
免费 | 669篇 |
国内免费 | 1196篇 |
专业分类
测绘学 | 2325篇 |
大气科学 | 5040篇 |
地球物理 | 12826篇 |
地质学 | 28173篇 |
海洋学 | 4462篇 |
天文学 | 10288篇 |
综合类 | 2206篇 |
自然地理 | 3259篇 |
出版年
2020年 | 152篇 |
2019年 | 141篇 |
2018年 | 6996篇 |
2017年 | 6313篇 |
2016年 | 4071篇 |
2015年 | 599篇 |
2014年 | 532篇 |
2013年 | 1254篇 |
2012年 | 2290篇 |
2011年 | 5329篇 |
2010年 | 4663篇 |
2009年 | 5083篇 |
2008年 | 4188篇 |
2007年 | 5132篇 |
2006年 | 948篇 |
2005年 | 1408篇 |
2004年 | 1466篇 |
2003年 | 1504篇 |
2002年 | 1178篇 |
2001年 | 675篇 |
2000年 | 702篇 |
1999年 | 595篇 |
1998年 | 582篇 |
1997年 | 572篇 |
1996年 | 477篇 |
1995年 | 476篇 |
1994年 | 455篇 |
1993年 | 432篇 |
1992年 | 392篇 |
1991年 | 336篇 |
1990年 | 391篇 |
1989年 | 315篇 |
1988年 | 351篇 |
1987年 | 395篇 |
1986年 | 341篇 |
1985年 | 501篇 |
1984年 | 541篇 |
1983年 | 550篇 |
1982年 | 445篇 |
1981年 | 458篇 |
1980年 | 474篇 |
1979年 | 400篇 |
1978年 | 414篇 |
1977年 | 356篇 |
1976年 | 393篇 |
1975年 | 352篇 |
1974年 | 389篇 |
1973年 | 374篇 |
1972年 | 241篇 |
1971年 | 190篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
61.
Richard P. BINZEL Ettore PEROZZI Andrew S. RIVKIN Alessandro ROSSI Alan W. HARRIS Schelte J. BUS Giovanni B. VALSECCHI Stephen M. SLIVAN 《Meteoritics & planetary science》2004,39(3):351-366
Abstract— Using an H‐plot analysis, we identify 234 currently known near‐Earth objects that are accessible for rendezvous with a “best case” delta‐V of less than 7 km/s. We provide a preliminary compositional interpretation and assessment of these potential targets by summarizing the taxonomic properties for 44 objects. Results for one‐half (22) of this sample are based on new spectroscopic measurements presented here. Our approach provides an easy‐to‐update method for giving guidelines to both observers and mission analysts for focusing on objects for which actual mission opportunities are most likely to be found. Observing prospects are presented for categorizing the taxonomic properties of the most accessible targets that are not yet measured. 相似文献
62.
We discuss the difficulties encountered when the Heisenberg-Kolmogoroff model for turbulence is applied to the large-scale turbulence in: (A) molecular clouds (specifically the velocity vs size relationship) and (B) stars (specifically, the estimate of convective fluxes).A new model for large-scale turbulence is, therefore, needed. 相似文献
63.
64.
65.
66.
67.
S. W. Allen R. W. Schmidt A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2002,334(2):L11-L15
We present precise measurements of the X-ray gas mass fraction for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Parametrizing the total (luminous plus dark matter) mass profiles using the model of Navarro, Frenk & White, we show that the X-ray gas mass fractions in the clusters asymptote towards an approximately constant value at a radius r 2500 , where the mean interior density is 2500 times the critical density of the Universe at the redshifts of the clusters. Combining the Chandra results on the X-ray gas mass fraction and its apparent redshift dependence with recent measurements of the mean baryonic matter density in the Universe and the Hubble constant determined from the Hubble Key Project, we obtain a tight constraint on the mean total matter density of the Universe, , and measure a positive cosmological constant, . Our results are in good agreement with recent, independent findings based on analyses of anisotropies in the cosmic microwave background radiation, the properties of distant supernovae, and the large-scale distribution of galaxies. 相似文献
68.
69.
Long-range sidescan sonar can be used to map sediment distributions over wide expanses of deep ocean floor. Seven acoustic facies that arise from differing sediment or rock types have been mapped over the low-relief Saharan continental rise and Madeira abyssal plain. These have been calibrated with sampling, profiling and camera studies and the facies can be traced confidently on a regional scale using the sidescan data. The mapping of the sediment distribution shows that a complex interplay of turbidity current and debris flow processes can occur at a continental rise/abysaal plain transition over 1000 km from the nearest continental slope. 相似文献
70.
F. Goodarzi L.R. Snowdon P.R. Gunther W.A.M. Jenkins 《Marine and Petroleum Geology》1985,2(3):254-259
Samples of lower Palaeozoic bedrock from the Grand Banks of Newfoundland were examined using reflected light microscopy and Rock-Eval pyrolysis. These samples contained organic material which included bitumen (structureless organic material), acritarchs, chitinozoa, scolecodonts and graptolites. The reflectance of the organic material increased from acritarchs towards graptolites with bitumen showing a wide range of reflectance. Multiple phases of oil migration through one sample were inferred by examination of the bitumens: the lowest reflecting being more recent than the highest reflecting bitumen. Combined reflectance and Rock-Eval analysis indicate that these samples are mature to overmature. The two oldest samples (Arenig-Llanvirn, Ordovician) are assessed as mature and as having potential for generation of liquid hydrocarbons, but must be considered as mainly gas prone. 相似文献