全文获取类型
收费全文 | 7609篇 |
免费 | 641篇 |
国内免费 | 486篇 |
专业分类
航空 | 4115篇 |
航天技术 | 2113篇 |
综合类 | 436篇 |
航天 | 2072篇 |
出版年
2023年 | 42篇 |
2022年 | 111篇 |
2021年 | 172篇 |
2020年 | 129篇 |
2019年 | 123篇 |
2018年 | 278篇 |
2017年 | 267篇 |
2016年 | 188篇 |
2015年 | 188篇 |
2014年 | 242篇 |
2013年 | 274篇 |
2012年 | 310篇 |
2011年 | 450篇 |
2010年 | 456篇 |
2009年 | 498篇 |
2008年 | 449篇 |
2007年 | 386篇 |
2006年 | 230篇 |
2005年 | 286篇 |
2004年 | 219篇 |
2003年 | 229篇 |
2002年 | 231篇 |
2001年 | 220篇 |
2000年 | 162篇 |
1999年 | 185篇 |
1998年 | 174篇 |
1997年 | 154篇 |
1996年 | 132篇 |
1995年 | 177篇 |
1994年 | 176篇 |
1993年 | 105篇 |
1992年 | 116篇 |
1991年 | 66篇 |
1990年 | 68篇 |
1989年 | 84篇 |
1988年 | 48篇 |
1987年 | 34篇 |
1986年 | 48篇 |
1985年 | 137篇 |
1984年 | 116篇 |
1983年 | 93篇 |
1982年 | 85篇 |
1981年 | 164篇 |
1980年 | 33篇 |
1979年 | 36篇 |
1978年 | 39篇 |
1977年 | 35篇 |
1975年 | 34篇 |
1973年 | 31篇 |
1972年 | 42篇 |
排序方式: 共有8736条查询结果,搜索用时 15 毫秒
121.
根据固体火箭发动机内弹道计算模型,分析了使用大扩张比喷管地面静止试验性能与真空性能之间的联系,建立了由地面性能换算成真空性能的方法,计算出的真空推力、真空比冲等数据与实际相符。 相似文献
122.
In August, 1981, the Westerbork Synthesis Radio Telescope was used for 4 h to search for narrowband pulsing radio beacons in the direction of the Galactic Center. By using both the spatial discrimination and temporal stability available to an interferometric measurement, weak intermittent signals can be detected even in the face of the strong, naturally caused radiation from this region. A radio beacon within our bandwidth, centered on the 21 cm neutral hydrogen line, would be recognizable if it had a repetition period between 40 sec and 1/2 h. The rms sensitivity to point sources was approximately 50 mJy/cycle, and the detection limit was 500 mJy/cycle. The limit degrades for pulse widths < 0.02s. No repetitive signals were found. For a swept, narrow-band radio beacon constrained to the Galactic Disk (beamwidth = 0.02 rad), our detection limit corresponds to a transmitter power of 10(11) MW at the Galactic Center. 相似文献
123.
G. N. Boiko L. M. Erukhimov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,15(12):71-74
The creation of artificial plasma density irregularities (AI) with prescribed spectra in the ionospheric heating experiments is discussed. We show that periodic successions of powerful pulses, pumped into the F-region of the ionosphere lead to obtaining AI with the controlled stationary spatial spectrum and allow us to change power low spectral index 2β at least from 2β = 2.2 to 2β = 3.4 by changing of the powerful pulse time schedule. 相似文献
124.
Characteristics and formation of amino acids and hydroxy acids of the Murchison meteorite. 总被引:1,自引:0,他引:1
J R Cronin G W Cooper S Pizzarello 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,15(3):91-97
Eight characteristics of the unique suite of amino acids and hydroxy acids found in the Murchison meteorite can be recognized on the basis of detailed molecular and isotopic analyses. The marked structural correspondence between the alpha-amino acids and alpha-hydroxy acids and the high deuterium/hydrogen ratio argue persuasively for their formation by aqueous phase Strecker reactions in the meteorite parent body from presolar, i.e., interstellar, aldehydes, ketones, ammonia, and hydrogen cyanide. The characteristics of the meteoritic suite of amino acids and hydroxy acids are briefly enumerated and discussed with regard to their consonance with this interstellar-parent body formation hypothesis. The hypothesis has interesting implications for the organic composition of both the primitive parent body and the presolar nebula. 相似文献
125.
Engineering concepts for inflatable Mars surface greenhouses. 总被引:1,自引:0,他引:1
I Hublitz D L Henninger B G Drake P Eckart 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2004,34(7):1546-1551
A major challenge of designing a bioregenerative life support system for Mars is the reduction of the mass, volume, power, thermal and crew-time requirements. Structural mass of the greenhouse could be saved by operating the greenhouse at low atmospheric pressure. This paper investigates the feasibility of this concept. The method of equivalent system mass is used to compare greenhouses operated at high atmospheric pressure to greenhouses operated at low pressure for three different lighting methods: natural, artificial and hybrid lighting. 相似文献
126.
L. Balmaceda A. Dal Lago G. Stenborg C. Francile W.D. Gonzalez R. Schwenn 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2003,32(12):2625-2630
In this work we have tracked coronal mass ejections observed with the ground based Mirror Coronagraph for Argentina (MICA) and the Large Angle and Spectroscopic Coronagraph (LASCO) C2 and C3 on board of the Solar and Heliospheric Observatory (SOHO). The MICA telescope is located at El Leoncito (31.8 S, 69.3 W), San Juan (Argentina), since 1997 as part of a bilateral scientific project between Germany and Argentina. SOHO is a project of international cooperation between ESA and NASA. Together these instruments are able to observe the solar corona ranging from 1.05 to 32 solar radii. MICA images the Fe XIV emission line corona and LASCO coronagraphs observe the Thomson scattered white light corona. We have selected events for which there are observations from the three coronagraphs. Using the composite data we were able to obtain height-time diagrams for the corresponding dynamical coronal features traveling outwards in order to determine some of their kinematical properties, i.e., plane of sky velocity and acceleration. 相似文献
127.
董雷 《中国空间科学技术》1996,16(3):58-62
叙述了资源卫星动量轮的一般结构和开环测试原理。详细介绍了应用IEEE488总线构成的动量轮开环性能自动测试系统。说明了这个自动测试系统的系统构成以及测试项目、测试流程和获得的测试结果。并据此提出这个系统在卫星研制工作中具有推广价值。 相似文献
128.
R Beaujean J Kopp M Leicher G Reitz 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1996,18(12):179-182
Nuclear track detectors were used to measure the integral Linear Energy Transfer (LET) spectra above 1 GeV per cm water behind the complex material shielding inside a spacecraft. The measurements are compared with predictions of the contribution of high charge, high energy HZE particles of the galactic cosmic radiation taking into account the influence of solar and geomagnetic modulation and shielding by matter. 相似文献
129.
J-P de Vera G Horneck P Rettberg S Ott 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2004,33(8):1236-1243
Complementary to the already well-studied microorganisms, lichens, symbiotic organisms of the mycobiont (fungi) and the photobiont (algae), were used as "model systems" in which to examine the ecological potential to resist to extreme environments of outer space. Ascospores (sexual propagules of the mycobiont) of the lichens Fulgensia bracteata, Xanthoria elegans and Xanthoria parietina were exposed to selected space-simulating conditions (up to 16 h of space vacuum at 10(-3) Pa and UV radiation at 160 nm < or = lambda < or = 400 nm), while embedded in the lichen fruiting bodies. After exposure, the ascospores were discharged and their viability was tested as germination capacity on different culture media including those containing Mars regolith simulant. It was found that (i) the germination rate on media containing Mars regolith simulant was as high as on other mineral-containing media, (ii) if enclosed in the ascocarps, the ascospores survived the vacuum exposure, the UV-irradiation as well as the combined treatment of vacuum and UV to a high degree. In general, 50 % or more viable spores were recovered, with ascospores of X. elegans showing the highest survival. It is suggested that ascospores inside the ascocarps are well protected by the anatomical structure, the gelatinous layer and the pigments (parietin and carotene) against the space parameters tested. 相似文献
130.
应用数值计算的方法模拟了细长体截面绕流结构的演变过程.指出随着细长体背涡的发展,导致截面流场的拓扑结构发生变化,会出现一种临界流动状态.并用微分方程的定性理论分析了此时流场中出现的一种高阶奇点.这种高阶奇点的指数为-3/2,它是结构不稳定的,稍有扰动就会产生分叉,使流场的拓扑结构发生变化.得出了定常对称背涡流态下细长体的空间绕流结构图. 相似文献