全文获取类型
收费全文 | 6100篇 |
免费 | 51篇 |
国内免费 | 18篇 |
专业分类
航空 | 2969篇 |
航天技术 | 1869篇 |
综合类 | 24篇 |
航天 | 1307篇 |
出版年
2021年 | 52篇 |
2019年 | 46篇 |
2018年 | 207篇 |
2017年 | 174篇 |
2016年 | 105篇 |
2015年 | 55篇 |
2014年 | 134篇 |
2013年 | 151篇 |
2012年 | 167篇 |
2011年 | 308篇 |
2010年 | 244篇 |
2009年 | 335篇 |
2008年 | 356篇 |
2007年 | 248篇 |
2006年 | 125篇 |
2005年 | 182篇 |
2004年 | 151篇 |
2003年 | 181篇 |
2002年 | 115篇 |
2001年 | 186篇 |
2000年 | 100篇 |
1999年 | 124篇 |
1998年 | 145篇 |
1997年 | 110篇 |
1996年 | 123篇 |
1995年 | 168篇 |
1994年 | 129篇 |
1993年 | 93篇 |
1992年 | 129篇 |
1991年 | 44篇 |
1990年 | 54篇 |
1989年 | 120篇 |
1988年 | 59篇 |
1987年 | 52篇 |
1986年 | 46篇 |
1985年 | 145篇 |
1984年 | 126篇 |
1983年 | 109篇 |
1982年 | 114篇 |
1981年 | 140篇 |
1980年 | 57篇 |
1979年 | 42篇 |
1978年 | 43篇 |
1977年 | 35篇 |
1976年 | 35篇 |
1975年 | 48篇 |
1974年 | 34篇 |
1973年 | 24篇 |
1972年 | 38篇 |
1971年 | 23篇 |
排序方式: 共有6169条查询结果,搜索用时 15 毫秒
921.
M K Wallis N C Wickramasinghe F Hoyle 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1992,12(4):281-285
Comet Halley studies indicate most of the nucleus is covered by an insulating crust, presumed of pyrolysed organic material. The subcrust is warmed and percolated by gases within 2AU, so provides one habitat for primitive replicating organisms. Cracks and crevices within contaminated ice in the craters provides a habitat for photosynthesising organisms. Subsurface lakes on the Europa model, though insulated by some metres of ice, would require a trigger (perhaps meteorite impact and energy source (chemical or metabolic energy) to initiate and maintain a suitable-habitat on short period comets. Constraints on transfer between comets and other planetary bodies implies that radiation-resistant species with lengthy hibernation potential would be expected. 相似文献
922.
B.N. Swanenburg 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(4):71
The NASA Gamma-Ray Observatory, GRO, will carry two instruments for low energy gamma-ray astronomy. The ‘Oriented Scintillation Spectrometer Experiment - OSSE’ represents the latest step in the evolution of collimated detectors. A large detection area, simultaneous source and background observation and rigorous control over systematic errors yield significant improvements in sensitivity over earlier instruments. The ‘Imaging Compton Telescope - COMPTEL’ brings the proven concept of the Compton telescope to the state of the art level. Position sensitive scintillation detectors make it possible to generate sky images with a resolution of about 2° over a f.o.v. of about 1 sr. The complementary nature of these two experiments promises a first in-depth exploration of the sky in a wavelength range which covers the transition from the X-ray sky to the apparently unrelated high energy gamma-ray sky. Possible directions of further evolution of these experiments will be discussed. 相似文献
923.
924.
R. D. Blandford 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(13):39-48
Some recent observational results on extended and compact extragalactic radio sources are described. Theoretical interpretations are critically examined. 相似文献
925.
Direct initiation of detonations in gaseous mixtures of C2H2-O2, H2-O2 and H2-Cl2 in the pressure range of 10–150 torr using flash photolysis was studied. Similar to blast initiation using a concentrated powerful energy source, it was found that for photochemical initiation, there exists a certain threshold of flash intensity and energy for each mixture at any given initial pressure and composition below which a deflagration is formed. At the critical threshold, however, a fully developed detonation is rapidly formed in the immediate vicinity of the window of incident UV radiation. However, at super critical flash energies, the amplitude of the detonation formed decreases and combustion of the entire irradiated volume approaches a constant volume explosion. It was found that photo-chemical initiation requires both a certain minimum peak value of the free radical concentration generated by the photo-dissociation as well as an appropriate gradient of this free radical distribution. The minimum peak radical concentration permits rapid reaction rates for the generation of strong pressure waves, while the gradient is necessary for the amplification of the shock waves to a detonation. If the gradient is absent and the free radicals are uniformly distributed in the mixture, then the entire volume simply explodes as in a constant volume process. The present study reveals that the mechanism of photochemical initiation is one of proper temporal synchronization of the chemical energy release to the shock wave as it propagates through the mixture. In analogy to the LASER, the term SWACER is introduced to represent this mechanism of Shock Wave Amplication by Coherent Energy Release. There are strong indications that this SWACER mechanism is universal and plays the main role in the formation of detonations whenever a powerful concentrated external source is not used to generate a strong shock wave in the explosive. 相似文献
926.
The Galileo Probe Atmosphere Structure Instrument will make in-situ measurements of the temperature and pressure profiles of the atmosphere of Jupiter, starting at about 10-10 bar level, when the Probe enters the upper atmosphere at a velocity of 48 km s-1, and continuing through its parachute descent to the 16 bar level. The data should make possible a number of inferences relative to atmospheric and cloud physical processes, cloud location and internal state, and dynamics of the atmosphere. For example, atmospheric stability should be defined, from which the convective or stratified nature of the atmosphere at levels surveyed should be determined and characterized, as well as the presence of turbulence and/or gravity waves. Because this is a rare opportunity, sensors have been selected and evaluated with great care, making use of prior experience at Mars and Venus, but with an eye to special problems which could arise in the Jupiter environment. The temperature sensors are similar to those used on Pioneer Venus; pressure sensors are similar to those used in the Atmosphere Structure Experiment during descent of the Viking Landers (and by the Meteorology Experiment after landing on the surface); the accelerometers are a miniaturized version of the Viking accelerometers. The microprocessor controlled experiment electronics serve multiple functions, including the sequencing of experiment operation in three modes and performing some on-board data processing and data compression. 相似文献
927.
V. N. Oraevsky Yu. Ya. Ruzhin V. G. Korobeinikov A. S. Volokitin V. S. Skomarovsky 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1992,12(12):131-134
The paper presents the results and discussion of VLF wave measurements carried out in the course of two rocket experiments on injection of a dense cesium ion beam into the ionosphere at the heights of 165–240 km. The injection was accompanied by enhancement of the broad-band noise by several orders of magnitude. The wave measurements in the frequency range of 1–11 kHz revealed two pronounced frequencies somewhat exceeding that of the lower-hybrid resonance oscillations in the background plasma and the cesium beam. The oscillations were modulated by frequencies close to the ion-cyclotron frequencies of the background plasma. 相似文献
928.
We consider the distributed detection problem, in which a set of decision makers (DMs) receive observations of the environment and transmit finite-valued messages to other DMs according to prespecified communication protocols. A designated primary DM makes the find decision on one out of two alternative hypotheses. All DMs make decisions, in order to maximize a measure of organizational performance. We discuss three different types of decision rules (deterministic, independent randomization, and dependent randomization), and their implications on the organizational performance. Each DM is described by its individual receiver operating characteristic (ROC) curve, which is concave. We determine that concavity is not guaranteed for the case of the ROC curve of a team of DMs, even if the decision rules are perfectly continuous and if the individual ROC curves are strictly concave and smooth unless dependent randomization is allowed 相似文献
929.
D F Smart M A Shea 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1992,12(2-3):303-312
It is possible to model the time-intensity profile of solar particles expected in space after the occurrence of a significant solar flare on the sun. After the particles are accelerated in the flare process, if conditions are favorable, they may be released into the solar corona and then into space. The heliolongitudinal gradients observed in the inner heliosphere are extremely variable, reflecting the major magnetic structures in the solar corona which extend into space. These magnetic structures control the particle gradients in the inner heliosphere. The most extensive solar particle measurements are those observed by earth-orbiting spacecraft, and forecast and prediction procedures are best for the position of the earth. There is no consensus of how to extend the earth-based models to other locations in space. Local interplanetary conditions and structures exert considerable influence on the time-intensity profiles observed. The interplanetary shock may either reduce or enhance the particle intensity observed at a specific point in space and the observed effects are very dependent on energy. 相似文献
930.
This paper presents the results of medical investigations performed in the Salyut-7 8-month mission in which a professional physician took part. The paper contains anthropometric measurements, results of investigating the vestibular function, cardiovascular function at rest and in response to multi-step tests (with emphasis on echocardiographic measurements), metabolic parameters and hormonal status. It also discusses medical aspects of the extravehicular activity. The medical investigations, although some new methods were applied, provided the continuity of methodical approaches and data accumulated in previous missions. 相似文献