全文获取类型
收费全文 | 6686篇 |
免费 | 268篇 |
国内免费 | 385篇 |
专业分类
航空 | 3543篇 |
航天技术 | 2139篇 |
综合类 | 296篇 |
航天 | 1361篇 |
出版年
2021年 | 54篇 |
2019年 | 55篇 |
2018年 | 83篇 |
2017年 | 56篇 |
2016年 | 51篇 |
2014年 | 101篇 |
2013年 | 202篇 |
2012年 | 179篇 |
2011年 | 233篇 |
2010年 | 138篇 |
2009年 | 223篇 |
2008年 | 320篇 |
2007年 | 155篇 |
2006年 | 164篇 |
2005年 | 184篇 |
2004年 | 239篇 |
2003年 | 282篇 |
2002年 | 265篇 |
2001年 | 367篇 |
2000年 | 177篇 |
1999年 | 176篇 |
1998年 | 188篇 |
1997年 | 139篇 |
1996年 | 178篇 |
1995年 | 218篇 |
1994年 | 187篇 |
1993年 | 156篇 |
1992年 | 166篇 |
1991年 | 212篇 |
1990年 | 172篇 |
1989年 | 251篇 |
1988年 | 79篇 |
1987年 | 68篇 |
1986年 | 60篇 |
1985年 | 195篇 |
1984年 | 150篇 |
1983年 | 130篇 |
1982年 | 133篇 |
1981年 | 177篇 |
1980年 | 59篇 |
1979年 | 42篇 |
1978年 | 52篇 |
1977年 | 53篇 |
1976年 | 38篇 |
1975年 | 58篇 |
1974年 | 38篇 |
1973年 | 42篇 |
1972年 | 51篇 |
1971年 | 42篇 |
1970年 | 45篇 |
排序方式: 共有7339条查询结果,搜索用时 0 毫秒
621.
J.S. Halekas D.A. BrainR.P. Lin J.G. LuhmannD.L. Mitchell 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(9):1347-1352
We investigate accelerated electrons observed by Mars Global Surveyor (MGS), using data from the Electron Reflectometer (ER) instrument. We find three different types of accelerated electron events. Current sheet events occur over regions with weak or no crustal fields, have the highest electron energy fluxes, and are likely located on draped magnetotail fields. Extended events occur over regions with moderate crustal magnetic fields, and are most often observed on closed magnetic field lines. Localized events have the lowest energy fluxes, occur in strong magnetic cusp regions, and are the most likely kind of event to be found on open magnetic field lines. Some localized events have clear signatures of field-aligned currents; these events have much higher electron fluxes, and are preferentially observed on radially oriented open magnetic field lines. Electron acceleration events, especially localized events, are similar in many ways to events observed in the terrestrial auroral zone. However, physical processes related to those found in the terrestrial cusp and/or plasmasheet could also be responsible for accelerating electrons at Mars. 相似文献
622.
623.
A major problem with operations of lifting reentry vehicle having an aft center-of-gravity location due to large engine mass at the rear is the required hypersonic trim to fight the desired trajectory. This condition is most severe for lifting maneuvers. As a first step toward analyzing this problem, this paper considers the lift requirement for some basic maneuvers in the plane of a great circle. Considerations are given to optimal lift control for achieving the maximization of either the final altitude, speed or range. For the maximum-range problem, phugoid oscillation along an optimal trajectory is less severe as compared to a glide with maximum lift-to-drag ratio. An explicit formula for the number of oscillations for an entry from orbital speed is proposed. 相似文献
624.
Comparing the improved Di Giovanni/Radicella model with sounding-based electron density profiles and with the IRI model 总被引:1,自引:0,他引:1
W. Singer J. Weiss J. Bremer 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(12):83-86
The Di Giovanni/Radicella model (DGR) /1/ determines a bottom side electron densty profile alone from the set of routinely scaled ionogram parameters foE, foF1, foF2 and M(3000)F2 and the total electron content; the smoothed sunspot number R12 appears in the calculation. Present designations are DGR2/2/ and DRR3 /3/ [see Appendix]; they are valid in the northern hemisphere. DGR is compared with electron density profiles derived from ionograms obtained at Juliusruh (54.6°N, 13.4°E), and with the (URSI-based) IRI90 at different conditiones. Experimental total electron content (TEC) data are compared to both models. At the considered station, the profiles obtained by both models are reasonably in agreement amongst themselves and with the experimental data.
The TEC derived from the DGR3 model is in good agreement with experimental TEC, whereas, at high solar activity, IRI90 gives too high TEC values, especially during daytime. 相似文献
625.
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
626.
G Horneck U Eschweiler G Reitz J Wehner R Willimek K Strauch 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,16(8):105-118
Spores of different strains of Bacillus subtilis and the Escherichia coli plasmid pUC19 were exposed to selected conditions of space (space vacuum and/or defined wavebands and intensities of solar ultraviolet radiation) in the experiment ER 161 "Exobiological Unit" of the Exobiology Radiation Assembly (ERA) on board of the European Retrievable Carrier (EURECA). After the approximately 11 months lasting mission, their responses were studied in terms of survival, mutagenesis in the his (B. subtilis) or lac locus (pUC19), induction of DNA strand breaks, efficiency of DNA repair systems, and the role of external protective agents. The data were compared with those of a simultaneously running ground control experiment. The survival of spores treated with the vacuum of space, however shielded against solar radiation, is substantially increased, if they are exposed in multilayers and/or in the presence of glucose as protective, whereas all spores in "artificial meteorites", i.e. embedded in clays or simulated Martian soil, are killed. Vacuum treatment leads to an increase of mutation frequency in spores, but not in plasmid DNA. Extraterrestrial solar ultraviolet radiation is mutagenic, induces strand breaks in the DNA and reduces survival substantially; however, even at the highest fluences, i.e. 3 x 10(8) J m-2, a small but significant fraction of spores survives the insolation. Action spectroscopy confirms results of previous space experiments of a synergistic action of space vacuum and solar UV radiation with DNA being the critical target. 相似文献
627.
628.
K. Miyamoto M. Oka R. Yamamoto Y. Masuda T. Hoson S. Kamisaka J. Ueda 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,23(12):2033-2036
Activity of auxin polar transport in inflorescence axes of Arabidopsis thaliana grown under simulated microgravity conditions was studied in relation to the growth and development. Seeds were germinated and allowed to grow on an agar medium in test tubes on a horizontal clinostat. Horizontal clinostat rotation substantially reduced the growth of inflorescence axes and the productivity of seeds of Arabidopsis thaliana (ecotypes Landsberg erecta and Columbia), although it little affected seed germination, development of rosette leaves and flowering. The activity of auxin polar transport in inflorescence axes decreased when Arabidopsis plants were grown on a horizontal clinostat from germination stage, being ca. 60% of 1 g control. On the other hand, the auxin polar transport in inflorescence axes of Arabidopsis grown in 1 g conditions was not affected when the segments were exposed to various gravistimuli, including 3-dimensional clinorotation, during transport experiments. Pin-formed mutant of Arabidopsis, having a unique structure of the inflorescence axis with no flower and extremely low levels of the activity of auxin polar transport in inflorescence axes and endogenous auxin, did not continue its vegetative growth under clinostat rotation. These facts suggest that the development of the system of auxin polar transport in Arabidopsis is affected by microgravity, resulting in the inhibition of growth and development, especially during reproductive growth. 相似文献
629.
P Todd J T Walker 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(10):187-197
High energy, high-Z (HZE) particles are present in high-altitude and high-inclination satellite orbits. Most of the HZE dose above LET = 200 keV/micrometer is due to Fe nuclei. Individual HZE particles can damage several cells adjacent to one another along the particle track in tissue. The outcome has been described as a \"microlesion\" by D. Grahn. The present study attempts to define conditions for microlesions in specific tissues, to seek biological evidence that microlesions are produced, and to evaluate the microlesion as a potentially useful unit of dose in assessing hazards to spaceworkers. Microlesions in individuals cells and hair follicles have been described. Microbial studies have provided some evidence for independent secondary electron action. Whether or not a few hundred microlesions would be damaging to the whole organism depends upon the nature of damage to critical tissues. For example, cancer may occur if microlesions kill several cells in a straight line and mutate other cells alongside the particle track. Fe particle irradiation of the mouse Harderian gland (Fry et al., this issue) produces tumors efficiently. Microlesions in the lens, cornea, and retina need to be considered. Further dialogue is required before a final decision can be made concerning the most appropriate way to assess the HZE hazard. 相似文献
630.
J. Dodion D. Fussen F. Vanhellemont C. Bingen N. Mateshvili K. Gilbert R. Skelton D. Turnbull S.D. McLeod C.D. Boone K.A. Walker P.F. Bernath 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008
Satellite-based limb occultation measurements are well suited for the detection and mapping of polar stratospheric clouds (PSCs) and cirrus clouds. PSCs are of fundamental importance for the formation of the Antarctic ozone hole that occurs every year since the early 1980s in Southern Hemisphere spring. Despite progress in the observation, modeling and understanding of PSCs in recent years, there are still important questions which remain to be resolved, e.g. PSC microphysics, composition, formation mechanisms and long-term changes in occurrence. In addition, it has recently become clear that cirrus clouds significantly affect the global energy balance and climate, due to their influence on atmospheric thermal structure. 相似文献