全文获取类型
收费全文 | 7353篇 |
免费 | 16篇 |
国内免费 | 19篇 |
专业分类
航空 | 3362篇 |
航天技术 | 2417篇 |
综合类 | 22篇 |
航天 | 1587篇 |
出版年
2021年 | 73篇 |
2019年 | 45篇 |
2018年 | 241篇 |
2017年 | 172篇 |
2016年 | 125篇 |
2015年 | 60篇 |
2014年 | 180篇 |
2013年 | 225篇 |
2012年 | 232篇 |
2011年 | 381篇 |
2010年 | 281篇 |
2009年 | 378篇 |
2008年 | 398篇 |
2007年 | 280篇 |
2006年 | 156篇 |
2005年 | 213篇 |
2004年 | 186篇 |
2003年 | 207篇 |
2002年 | 145篇 |
2001年 | 222篇 |
2000年 | 128篇 |
1999年 | 162篇 |
1998年 | 192篇 |
1997年 | 110篇 |
1996年 | 172篇 |
1995年 | 203篇 |
1994年 | 189篇 |
1993年 | 124篇 |
1992年 | 150篇 |
1991年 | 47篇 |
1990年 | 50篇 |
1989年 | 139篇 |
1988年 | 59篇 |
1987年 | 57篇 |
1986年 | 63篇 |
1985年 | 185篇 |
1984年 | 145篇 |
1983年 | 107篇 |
1982年 | 116篇 |
1981年 | 214篇 |
1980年 | 49篇 |
1979年 | 45篇 |
1978年 | 51篇 |
1977年 | 42篇 |
1975年 | 50篇 |
1974年 | 39篇 |
1973年 | 32篇 |
1972年 | 35篇 |
1971年 | 35篇 |
1970年 | 37篇 |
排序方式: 共有7388条查询结果,搜索用时 15 毫秒
271.
A. Kiraga Z. K
os V. N. Oraevsky V. C. Dokukin S. A. Pulinets 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,15(12):21-24
There were several cases when modulated electron beam had been injected from APEX satellite into an otherwise unmodified ionospheric plasma. The beams were formed from 2 μs pulses repeated at a rate of 40kHz. Injections took place in the altitude range 400–1100km over Europe. The onboard receiver connected to the dipole antenna swept the frequency interval 1–10 MHz in 1 sec. Due to a relatively narrow receiver bandwidth (15 kHz), moderate frequency step (50 kHz), slow changes of fn and fc (local electron plasma and gyro frequencies) fine structure of excited emissions was detected. In many cases, a very prominent doublet could be convincingly identified as a (fn,fu=sqrt(fn*fn+fc*fc)) band, with fn practically equal to the local plasma frequency in the unperturbed ionosphere. Determination of plasma frequency is of prime importance in analysis of complex structures composed from various harmonics fn,fc,fu. Complexity is manifested as coalescing of various harmonics with maxima shifted from nominal frequencies or splitting them into components due to mixing of local and propagation effects. Despite the existence of strong emissions on frequencies characteristic for the unperturbed ionosphere, very strong broadcasting transmissions are frequently cut off, even when the beam is directed upward. Most typical spectra are discussed. 相似文献
272.
A Lazcano E Díaz-Villagómez T Mills J Oró 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,15(3):345-356
The most frequently invoked explanation for the origin of metabolic pathways is the retrograde evolution hypothesis. In contrast, according to the so-called "patchwork" theory, metabolism evolved by the recruitment of relatively inefficient small enzymes of broad specificity that could react with a wide range of chemically related substrates. In this paper it is argued that both sequence comparisons and experimental results on enzyme substrate specificity support the patchwork assembly theory. The available evidence supports previous suggestions that gene duplication events followed by a gradual neoDarwinian accumulation of mutations and other minute genetic changes lead to the narrowing and modification of enzyme function in at least some primordial metabolic pathways. 相似文献
273.
Palinkas LA 《Journal of spacecraft and rockets》1990,27(5):471-477
This paper examines the utility of remote, isolated Antarctic research stations as analogs for long-duration spaceflights from the perspective of psychosocial processes of adaptation and adjustment. Certain features of the physical and man-made environments found in Antarctica are similar to those that will be encountered in outer space. In both settings, men and women are likely to experience a number of physiological and psychological changes in response to the extreme environmental conditions and the prolonged isolation and confinement. Biomedical research in Antarctica provides an opportunity to study the causes of these changes and to develop strategies for reducing the risks to health and well-being before they pose a serious threat to crew safety and mission success. A number of lessons for long-duration spaceflight are examined, including screening and selection of personnel; training programs designed to facilitate individual adjustment and group adaptation and minimize group conflict; identification of optimal leadership characteristics for small, isolated groups; an understanding of social dynamics and group "microcultures" necessary for the organization and management of small but heterogeneous groups; organization of work activities; facility design; and support infrastructure. 相似文献
274.
King CB Butterfield AJ Hypes WD Nealy JE Simonsen LC 《Journal of spacecraft and rockets》1990,27(3):225-226
The space shuttle external tank, which consists of a liquid oxygen tank, an intertank structure, and a liquid hydrogen tank, is an expendable structure used for approximately 8.5 min during each launch. A concept for outfitting the liquid oxygen tank-intertank unit for a 12-person lunar habitat is described. The concept utilizes existing structures and openings for both man and equipment access without compromising the structural integrity of the tank. Living quarters, instrumentation, environmental control and life support, thermal control, and propulsion systems are installed at Space Station Freedom. The unmanned habitat is then transported to low lunar orbit and autonomously soft landed on the lunar surface. Design studies indicate that this concept is feasible by the year 2000 with concurrent development of a space transfer vehicle and manned cargo lander for crew changeover and resupply. 相似文献
275.
Paddison F.C. Shipley C.A. Maffett A.L. Dawson M.H. 《IEEE transactions on aerospace and electronic systems》1978,(1):27-34
A laboratory method to determine the magnitude and position of radar reflection sources on complex targets is described. In addition the method provides a way to measure the modification of the radar cross section (RCS) due to multipath. The method has application in modeling RCS for radar and electronic countermeasure (ECM) system performance analysis and in the study of the extent to which the signature of the target could be altered. The equipment described, termed MACROSCOPE, was developed for RCS studies by the U.S. Army and is described in limited distribution bution literature. The application to marine targets is new with this paper, as is the technique of measuring the RCS of parts of the target and analytically combining them to represent the whole. An illustration of the need for this type of laboratory equipment was illustrated by the extensive search for full scale data which could be compared to scale model data to validate the technique. 相似文献
276.
E. A. Botan 《Space Science Reviews》1964,3(5-6):715-723
277.
A. Ehmert 《Space Science Reviews》1964,3(1):2-4
278.
C. de Jager A. Ollongren Tj. de Graaf B. J. Levin M. Tucker 《Space Science Reviews》1969,9(5):740-742
279.
This is a concise review of physics of the substorm in the magnetotail. It consists of two parts. The first part summarizes historical developments in the early days of the space age (1960--1975) when the basic concepts such as magnetotail and reconnection were established and the leading model of the substorm was introduced. The second part is an overview of the research conducted in recent years (1995--2010) when very significant advances have been achieved in understanding the substorm physics by virtue of several major satellites missions that addressed the magnetotail physics intensively. 相似文献
280.
S. Mereghetti A. Tiengo P. Esposito G. Vianello A. De Luca D. Götz G. Weidenspointner A. von Kienlin G.L. Israel L. Stella N. Rea R. Turolla S. Zane 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
We describe the results obtained with Target of Opportunity observations of the galactic sources SGR 1627–41 and 1E 1547–5408. These two transients show several similarities supporting the interpretation of Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters as a single class of strongly magnetized neutron stars. 相似文献