全文获取类型
收费全文 | 2663篇 |
免费 | 2篇 |
国内免费 | 19篇 |
专业分类
航空 | 1317篇 |
航天技术 | 1050篇 |
综合类 | 10篇 |
航天 | 307篇 |
出版年
2019年 | 18篇 |
2018年 | 24篇 |
2017年 | 18篇 |
2016年 | 17篇 |
2014年 | 47篇 |
2013年 | 58篇 |
2012年 | 51篇 |
2011年 | 82篇 |
2010年 | 60篇 |
2009年 | 104篇 |
2008年 | 157篇 |
2007年 | 63篇 |
2006年 | 66篇 |
2005年 | 69篇 |
2004年 | 79篇 |
2003年 | 80篇 |
2002年 | 51篇 |
2001年 | 75篇 |
2000年 | 49篇 |
1999年 | 63篇 |
1998年 | 80篇 |
1997年 | 49篇 |
1996年 | 61篇 |
1995年 | 78篇 |
1994年 | 76篇 |
1993年 | 49篇 |
1992年 | 61篇 |
1991年 | 31篇 |
1990年 | 30篇 |
1989年 | 70篇 |
1988年 | 26篇 |
1987年 | 28篇 |
1986年 | 30篇 |
1985年 | 119篇 |
1984年 | 68篇 |
1983年 | 57篇 |
1982年 | 58篇 |
1981年 | 103篇 |
1980年 | 34篇 |
1979年 | 26篇 |
1978年 | 24篇 |
1977年 | 28篇 |
1976年 | 18篇 |
1975年 | 31篇 |
1974年 | 19篇 |
1973年 | 25篇 |
1972年 | 20篇 |
1971年 | 16篇 |
1970年 | 24篇 |
1969年 | 26篇 |
排序方式: 共有2684条查询结果,搜索用时 117 毫秒
231.
The availability of water-ice at the surface in the Mars polar cap and within the top meter of the high-latitude regolith raises the question of whether liquid water can exist there under some circumstances and possibly support the existence of biota. We examine the minimum temperatures at which liquid water can exist at ice grain-dust grain and ice grain-ice grain contacts, the minimum subfreezing temperatures at which terrestrial organisms can grow or multiply, and the maximum temperatures that can occur in martian high-latitude and polar regions, to see if there is overlap. Liquid water can exist at grain contacts above about -20 degrees C. Measurements of growth in organisms isolated from Siberian permafrost indicate growth at -10 degrees C and metabolism at -20 degrees C. Mars polar and high-latitude temperatures rise above -20 degrees C at obliquities greater than ~40 degrees, and under some conditions rise above 0 degrees C. Thus, the environment in the Mars polar regions has overlapped habitable conditions within relatively recent epochs, and Mars appears to be on the edge of being habitable at present. The easy accessibility of the polar surface layer relative to the deep subsurface make these viable locations to search for evidence of life. 相似文献
232.
233.
S B Curtis E G Luebeck W D Hazelton S H Moolgavkar 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2002,30(4):937-944
When applied to the Colorado Plateau miner population, the two-stage clonal expansion (TSCE) model of radiation carcinogenesis predicts that radiation-induced promotion dominates radiation-induced initiation. Thus, according to the model, at least for alpha-particle radiation from inhaled radon daughters, lung cancer induction over long periods of protracted irradiation appears to be dominated by radiation-induced modification of the proliferation kinetics of already-initiated cells rather than by direct radiation-induced initiation (i.e., mutation) of normal cells. We explore the possible consequences of this result for radiation exposures to space travelers on long missions. Still unknown is the LET dependence of this effect. Speculations of the cause of this phenomenon include the suggestion that modification of cell kinetics is caused by a "bystander" effect, i.e., the traversal of normal cells by alpha particles, followed by the signaling of these cells to nearby initiated cells which then modify their proliferation kinetics. 相似文献
234.
235.
Education and public outreach are viewed by NASA as significant undertakings for all of its space missions. The IMAGE satellite is one of the first missions to explicitly include `E&PO in its original proposal to NASA in 1996. We will discuss what IMAGE has accomplished in this area to date, and what new activities it will conduct following a successful launch. 相似文献
236.
The goal of the Kinelite Project is to develop a space qualified motion analysis system to be used in space by the scientific community, mainly to support neuroscience protocols. The measurement principle of the Kinelite is to determine, by triangulation mean, the 3D position of small, lightweight, reflective markers positioned at the different points of interest. The scene is illuminated by Infra Red flashes and the reflected light is acquired by up to 8 precalibrated and synchronized CCD cameras. The main characteristics of the system are: Camera field of view: 45 degrees; Number of cameras: 2 to 8; Acquisition frequency: 25, 50, 100, or 200 Hz; CCD format: 256 x 256; Number of markers: up to 64; 3D accuracy: 2mm; Main dimensions: 45 cm x 45 cm x 30 cm; Mass: 23 kg; Power consumption: less than 200 W. The Kinelite will first fly aboard the NASA Spacelab; it will be used, during the NEUROLAB mission (4/98), to support the "Frames of References and Internal Models" (Principal Investigator: Pr. A. Berthoz, Co Investigators: J. McIntyre, F. Lacquaniti). 相似文献
237.
Jie Jiang Robert H. Cameron Dieter Schmitt Manfred Schüssler 《Space Science Reviews》2013,176(1-4):289-298
To reproduce the weak magnetic field on the polar caps of the Sun observed during the declining phase of cycle 23 poses a challenge to surface flux transport models since this cycle has not been particularly weak. We use a well-calibrated model to evaluate the parameter changes required to obtain simulated polar fields and open flux that are consistent with the observations. We find that the low polar field of cycle 23 could be reproduced by an increase of the meridional flow by 55% in the last cycle. Alternatively, a decrease of the mean tilt angle of sunspot groups by 28% would also lead to a similarly low polar field, but cause a delay of the polar field reversals by 1.5 years in comparison to the observations. 相似文献
238.
239.
240.
A robust satellite tracking antenna is designed to cope with the sensor imprecision and the highly noisy sea environment. Fuzzy logic is utilized for the controller imprecision and the highly noisy sea environment. Fuzzy logic is utilized for the controller design as well as inaccurate data interpretation. The fuzzy-rule based controller eliminates the need to model the nonlinear and noisy ship-mounted antenna system. With Global Positioning System and the tracking controller the antenna can be brought to a neighborhood of the desired orientation. Spiral search with signal power feedback can then servo the antenna to the true orientation. Computer simulations and antenna experiments verify our design is indeed robust and effective 相似文献