全文获取类型
收费全文 | 4101篇 |
免费 | 17篇 |
国内免费 | 12篇 |
专业分类
航空 | 2103篇 |
航天技术 | 1460篇 |
综合类 | 21篇 |
航天 | 546篇 |
出版年
2021年 | 30篇 |
2018年 | 60篇 |
2017年 | 37篇 |
2016年 | 34篇 |
2014年 | 69篇 |
2013年 | 94篇 |
2012年 | 71篇 |
2011年 | 129篇 |
2010年 | 98篇 |
2009年 | 135篇 |
2008年 | 196篇 |
2007年 | 80篇 |
2006年 | 68篇 |
2005年 | 89篇 |
2004年 | 112篇 |
2003年 | 128篇 |
2002年 | 66篇 |
2001年 | 105篇 |
2000年 | 78篇 |
1999年 | 69篇 |
1998年 | 128篇 |
1997年 | 83篇 |
1996年 | 105篇 |
1995年 | 138篇 |
1994年 | 120篇 |
1993年 | 96篇 |
1992年 | 123篇 |
1991年 | 56篇 |
1990年 | 48篇 |
1989年 | 108篇 |
1988年 | 42篇 |
1987年 | 47篇 |
1986年 | 64篇 |
1985年 | 155篇 |
1984年 | 100篇 |
1983年 | 107篇 |
1982年 | 113篇 |
1981年 | 126篇 |
1980年 | 52篇 |
1979年 | 50篇 |
1978年 | 48篇 |
1977年 | 41篇 |
1976年 | 32篇 |
1975年 | 42篇 |
1974年 | 38篇 |
1973年 | 33篇 |
1972年 | 34篇 |
1971年 | 36篇 |
1970年 | 33篇 |
1969年 | 41篇 |
排序方式: 共有4130条查询结果,搜索用时 0 毫秒
361.
With the arrival of private entrepreneurs and the appearance of such things as prizes to stimulate space initiatives, the space economy is changing rapidly from its initial government-driven character. A number of countries and territories, such as the Isle of Man, have understood the growing role of this new space economy and are preparing the appropriate boundary conditions to support its further development. This article shows why the Isle of Man may be uniquely placed to take advantage of recent developments in the space economy, thanks to its business-friendly tax and regulatory regime, and discusses the island’s strategy to attract space businesses. The establishment of the International Institute of Space Commerce, a recently formed think-tank, is an integral part of this strategy. 相似文献
362.
Preservation of microbial lipids in geothermal sinters 总被引:1,自引:0,他引:1
Lipid biomarkers are widely used to study the earliest life on Earth and have been invoked as potential astrobiological markers, but few studies have assessed their survival and persistence in geothermal settings. Here, we investigate lipid preservation in active and inactive geothermal silica sinters, with ages of up to 900 years, from Champagne Pool, Waiotapu, New Zealand. Analyses revealed a wide range of bacterial biomarkers, including free and bound fatty acids, 1,2-di-O-alkylglycerols (diethers), and various hopanoids. Dominant archaeal lipids include archaeol and glycerol dialkyl glycerol tetraethers (GDGTs). The predominance of generally similar biomarker groups in all sinters suggests a stable microbial community throughout Champagne Pool's history and indicates that incorporated lipids can be well preserved. Moreover, subtle differences in lipid distributions suggest that past changes in environmental conditions can be elucidated. In this case, higher archaeol abundances relative to the bacterial diethers, a greater proportion of cyclic GDGTs, the high average chain length of the bacterial diethers, and greater concentrations of hopanoic acids in the older sinters all suggest hotter conditions at Champagne Pool in the past. 相似文献
363.
Albi E Ambesi-Impiombato FS Peverini M Damaskopoulou E Fontanini E Lazzarini R Curcio F Perrella G 《Astrobiology》2011,11(1):57-64
The aim of this work was to analyze the possible alteration of thyrotropin (TSH) receptors in microgravity, which could explain the absence of thyroid cell proliferation in the space environment. Several forms of the TSH receptor are localized on the plasma membrane associated with caveolae and lipid rafts. The TSH regulates the fluidity of the cell membrane and the presence of its receptors in microdomains that are rich in sphingomyelin and cholesterol. TSH also stimulates cyclic adenosine monophosphate (cAMP) accumulation and cell proliferation. Reported here are the results of an experiment in which the FRTL-5 thyroid cell line was exposed to microgravity during the Texus-44 mission (launched February 7, 2008, from Kiruna, Sweden). When the parabolic flight brought the sounding rocket to an altitude of 264?km, the culture media were injected with or without TSH in the different samples, and weightlessness prevailed on board for 6 minutes and 19 seconds. Control experiments were performed, in parallel, in an onboard 1g centrifuge and on the ground in Kiruna laboratory. Cell morphology and function were analyzed. Results show that in microgravity conditions the cells do not respond to TSH treatment and present an irregular shape with condensed chromatin, a modification of the cell membrane with shedding of the TSH receptor in the culture medium, and an increase of sphingomyelin-synthase and Bax proteins. It is possible that real microgravity induces a rearrangement of specific sections of the cell membrane, which act as platforms for molecular receptors, thus influencing thyroid cell function in astronauts during space missions. 相似文献
364.
365.
H.O. Ruppe R.H. Schmucker W. Dittberner D. Hayn M. Braitinger 《Acta Astronautica》1979,6(12):1727-1743
The paper comprises an assessment of the design and the economics of so-called “low-cost” simple modular launch vehicles. It is shown that the performance is very marginal and that the cost per launch cannot compete with technically more advanced fully reusable vehicles. Especially a private-funded development cannot be amortized economically in case of an expendable launch vehicle. 相似文献
366.
Two bed rest analog studies of space flight were performed; one 14 d and the other 28 d in duration. Exercise response was studied in detail during the 28 d study and following both the 14 d and 28 d studies. This paper relates the results of these studies to physiologic changes noted during and following space flight. The most consistent change noted after both bed rest and space flight is an elevated heart rate during exercise. A second consistent finding is a postflight or postbed rest reduction in cardiac stroke volume. Cardiac output changes were variable. The inability to simulate inflight activity levels and personal exercise makes a direct comparison between bed rest and the results from specific space flights difficult. 相似文献
367.
L.M. Mukhin D.F. Nenarokov N.V. Porschnev V.B. Bondarev B.G. Gelman G. Israel F. Raulin J. Runavot R. Thomas 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1987,7(12):329-335
SIGMA - 3 gas chromatograph on board VEGA 1 and 2 landing probes has been operated successfully in the 60 - 50 km altitude range, providing several in - situ chemical analysis of the gas and the aerosols of Venus cloud layers. Post flight calibration required to derive atmospheric abundancies from gas chromatograms were carried out using the SIGMA - 3 spare model. A Venus atmospheric aerosol simulation chamber was used in which sulfuric acid droplets were generated. Preliminary results of these calibration experiments indicate that the concentration of sulfuric acid in the upper part of the clouds ( 60 to 55 km) is about 1 mg/m3 and suggest that an additional constituant must be present in noticeable amount in the aerosols. From these experiments the mixing ratio upper limits of SO2 is 100 ppmV and of H2S and COS is few 10 ppmV. 相似文献
368.
A. G. W. Cameron 《Space Science Reviews》1973,14(3-4):392-400
Certain fundamental scientific problems of a cosmological as well as cosmogonic character, may be solved by the insertion of entry probes into the atmospheres of the outer planets. It is recommended that attempts be made to determine the elemental and isotopic abundances of H, D, He3, He4, C, N, O, S, and the rare gas elements. These determinations should cast much light on the processes which participated in the assembly of the giant planets. This would give powerful boundary conditions on theories of the origin of the solar system, and would also give additional experimental information bearing on cosmology.This is one of the publications by the Science Advisory Group. 相似文献
369.
Bruckner J.M.H. Scott R.W. Rea F.G. 《IEEE transactions on aerospace and electronic systems》1973,(6):883-888
The estimation of a multimodal linear system whose mode-to-mode transitions are described by a finite-state Markov chain is described. The problem has application in studying separation standards in an air traffic control environment. An optimal solution is formulated which is computationally infeasible. A suboptimal estimator is then derived which closely approximates the optimal estimator. An example is presented to illustrate the technique. 相似文献
370.
F. Dehaeze G. N. Barakos A. N. Kusyumov S. A. Kusyumov S. A. Mikhailov 《Russian Aeronautics (Iz VUZ)》2018,61(1):40-47
This paper applies the Detached-Eddy Simulation (DES) method to resolve a larger part of the flow spectrum around rotor blades in hover and forward flight. A comparison between DES and Unsteady Reynolds–Averaged Navier–Stokes simulation was carried out for the case of a forward flying rotor suggesting that DES has great potential for rotor applications. 相似文献