首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   973篇
  免费   2篇
  国内免费   1篇
航空   362篇
航天技术   368篇
综合类   2篇
航天   244篇
  2022年   5篇
  2021年   22篇
  2019年   9篇
  2018年   30篇
  2017年   22篇
  2016年   14篇
  2015年   8篇
  2014年   35篇
  2013年   47篇
  2012年   41篇
  2011年   42篇
  2010年   34篇
  2009年   60篇
  2008年   72篇
  2007年   22篇
  2006年   21篇
  2005年   31篇
  2004年   22篇
  2003年   28篇
  2002年   20篇
  2001年   36篇
  2000年   21篇
  1999年   12篇
  1998年   27篇
  1997年   11篇
  1996年   21篇
  1995年   18篇
  1994年   26篇
  1993年   11篇
  1992年   16篇
  1991年   4篇
  1989年   14篇
  1988年   7篇
  1987年   9篇
  1986年   3篇
  1985年   26篇
  1984年   18篇
  1983年   10篇
  1982年   11篇
  1981年   16篇
  1980年   5篇
  1979年   5篇
  1978年   5篇
  1976年   4篇
  1975年   6篇
  1974年   8篇
  1972年   8篇
  1968年   6篇
  1967年   5篇
  1966年   5篇
排序方式: 共有976条查询结果,搜索用时 46 毫秒
691.
Uri JJ  Lebedev ON 《Acta Astronautica》2001,48(5-12):845-851
The Phase 1 research program was unprecedented in its scope and ambitious in its objectives. The National Aeronautics and Space Administration committed to conducting a multidisciplinary long-duration research program on a platform whose capabilities were not well known, not to mention belonging to another country. For the United States, it provided the first opportunity to conduct research in a long-duration space flight environment since the Skylab program in the 1970's. Multiple technical as well as cultural challenges were successfully overcome through the dedicated efforts of a relatively small cadre of individuals. The program developed processes to successfully plan, train for and execute research in a long-duration environment, with significant differences identified from short-duration space flight science operations. Between August 1994 and June 1998, thousands of kilograms of research hardware was prepared and launched to Mir, and thousands of kilograms of hardware and data products were returned to Earth. More than 150 Principal Investigators from eight countries were involved in the program in seven major research disciplines: Advanced Technology; Earth Sciences; Fundamental Biology; Human Life Sciences; International Space Station Risk Mitigation; Microgravity; and Space Sciences. Approximately 75 long-duration investigations were completed on Mir, with additional investigations performed on the Shuttle flights that docked with Mir. The flight phase included the participation of seven US astronauts and 20 Russian cosmonauts. The successful completion of the Phase 1 research program not only resulted in high quality science return but also in numerous lessons learned to make the ISS experience more productive. The cooperation developed during the program was instrumental in its success.  相似文献   
692.
Monopotassium D, L-aspartate and monomagnesium D, L-aspartate, both in the solution of 500 mg in 100 ml of drinking water administered prior to, during and after the outset of prolonged continuous irradiation increased the survival of rats and mice and improved neuromuscular coordination and physical ability of mice.  相似文献   
693.
The colonization of space will depend on our ability to routinely provide for the metabolic needs (oxygen, water, and food) of a crew with minimal re-supply from Earth. On Earth, these functions are facilitated by the cultivation of plant crops, thus it is important to develop plant-based food production systems to sustain the presence of mankind in space. Farming practices on earth have evolved for thousands of years to meet both the demands of an ever-increasing population and the availability of scarce resources, and now these practices must adapt to accommodate the effects of global warming. Similar challenges are expected when earth-based agricultural practices are adapted for space-based agriculture. A key variable in space is gravity; planets (e.g. Mars, 1/3 g) and moons (e.g. Earth's moon, 1/6 g) differ from spacecraft orbiting the Earth (e.g. Space stations) or orbital transfer vehicles that are subject to microgravity. The movement of heat, water vapor, CO2 and O2 between plant surfaces and their environment is also affected by gravity. In microgravity, these processes may also be affected by reduced mass transport and thicker boundary layers around plant organs caused by the absence of buoyancy dependent convective transport. Future space farmers will have to adapt their practices to accommodate microgravity, high and low extremes in ambient temperatures, reduced atmospheric pressures, atmospheres containing high volatile organic carbon contents, and elevated to super-elevated CO2 concentrations. Farming in space must also be carried out within power-, volume-, and mass-limited life support systems and must share resources with manned crews. Improved lighting and sensor technologies will have to be developed and tested for use in space. These developments should also help make crop production in terrestrial controlled environments (plant growth chambers and greenhouses) more efficient and, therefore, make these alternative agricultural systems more economically feasible food production systems.  相似文献   
694.
Institute of Experimental Meteorology initiated investigations of anthropogenic contamination (AC) and its influence on the near-earth environment and orbiting vehicles. These investigations are based on rocket experiments on simulation of the effects of gas-dust fluxes at the rate of 7–8 km/s on vehicle optical elements under real space conditions. The fluxes are generated by rocket-borne explosive generators.  相似文献   
695.
Results of almost four years of continuous observations of the subauroral nonthermal radio emission (SANE) onboard the Interball-1 satellite are presented. The main features of SANE are described and discussed: the intensity and character of a signal, its appearance in time, beam directivity, propagation distance, and other observed parameters.  相似文献   
696.
We report new results obtained from the EXOSAT AO- 1 observation of the intermediate polar V1223 Sgr. The detection of a 12.4 minute period in the medium energy X-ray flux with an associated hardness ratio variation has been previously reported in Osborne et al. (1984a). Further work has revealed: a narrow dip at the phase zero in the folded medium energy light curve; 30% modulation in the low energy X-ray (3000 Lexan) flux; a count rate ratio from 3 filters which allow the presence of a bright low temperature blackbody component (kT = .05 –. 40 KeV); and a phase resolved ME spectrum which must have two or more components when the source is bright. New optical ephemerides show that the X-ray and optical pulses are in phase at an orbital phase of = 0.31.Affiliated to the Astrophysics Division, Space Science Dept., ESA  相似文献   
697.
This Note attempts to clear up the discussions relating to the interpretation of Tsiolkowskiy's equation vs Irving's solution.  相似文献   
698.
The barrier to low cost space programs has been identified, and we are it. Principal among the causes for escalation of space program costs is the ‘system’ which has evolved to control programs. The ‘system’ includes not only the procedures and documents that constitute the flow of paper, the reviews and approvals necessary to initiate actions, and the entire methodology of the decision-making and approval processes but, necessarily, the people, including political as well as industrial counterparts, who populate these environments. This complex ‘system’ has proliferated so that it now promotes time-taking routines, obstructs prompt action, inhibits decisions, extends schedules and escalates costs. Designed to aid and abet management by supplying information necessary to maintain cognizance of program status the ‘system’ has taken over the role of management. Problems and their solutions must now be addressed to the ‘system’ as aided and abetted by management.Most of the evident causes of program cost problems have long since been recognized. Attacking them will produce second-order effects until management is willing to face up to the internal cost driver.  相似文献   
699.
A central purpose of Viking was to search for evidence that life exists on Mars or may have existed in the past. The missions carried three biology experiments the prime purpose of which was to seek for existing microbial life. In addition the results of a number of the other experiments have biological implications: (1) The elemental analyses of the atmosphere and the regolith showed or implied that the elements generally considered essential to terrestrial biology are present. (2) But unexpectedly, no organic compounds were detected in Martian samples by an instrument that easily detected organic materials in the most barren of terrestrial soils. (3) Liquid water is believed to be an absolute requisite for life. Viking obtained direct evidence for the presence of water vapor and water ice, and it obtained strong inferential evidence for the existence of large amounts of subsurface permafrost now and in the Martain past. However it obtained no evidence for the current existence of liquid water possessing the high chemical potential required for at least terrestrial life, a result that is consistent with the known pressure-temperature relations on the planet's surface. On the other hand, the mission did obtain strong indications from both atmospheric analyses and orbital photographs that large quantities of liquid water flowed episodically on the Martian surface 0.5 to 2.5 G years ago.The three biology experiments produced clear evidence of chemical reactivity in soil samples, but it is becoming increasingly clear that the chemical reactions were nonbiological in origin. The unexpected release of oxygen by soil moistened with water vapor in the Gas Exchange experiment together with the negative findings of the organic analysis experiment lead to the conclusion that the surface contains powerful oxidants. This conclusion is consistent with models of the atmosphere. The oxidants appear also to have been responsible for the decarboxylation of the organic nutrients that were introduced in the Label Release experiment. The major results of the GEX and LR experiments have been simulated at least qualitatively on Earth. The third, Pyrolytic Release, experiment obtained evidence for organic synthesis by soil samples. Although the mechanism of the synthesis is obscure, the thermal stability of the reaction makes a biological explanation most unlikely. Furthermore, the response of soil samples in all three experiments to the addition of water is not consistent with a biological interpretation.The conditions now known to exist at and below the Martian surface are such that no known terrestrial organism could grow and function. Although the evidence does not absolutely rule out the existence of favourable oases, it renders their existence extremely unlikely. The limiting conditions for the functioning of terrestrial organisms are not the limits for conceivable life elsewhere, and accordingly one cannot exclude the possibility that indigenous life forms may currently exist somewhere on Mars or may have existed sometime in the past. Nevertheless, the available information about the present Martian environment puts severe constraints and presents formidable challenges to any putative Martian organisms. The Martian environment in the past, on the other hand, appears to have been considerably less hostile biologically, and it might possibly have permitted the origin and transient establishment of a biota.  相似文献   
700.
Voyager 2 data from the Plasma Science experiment, the Magnetometer experiment and the Planetary Radio Astronomy experiment were used to analyze the relationship between parameters of the solar wind/interplanetary medium and the nonthermal Saturn radiation. Solar wind and interplanetary magnetic field properties were combined to form quantities known to be important in controlling terrestrial magnetospheric processes.The Voyager 2 data set used in this investigation consists of 237 days of Saturn preencounter measurements. However, due to the immersion of Saturn and the Voyager 2 spacecraft into the extended Jupiter magnetic tail, substantial periods of the time series were lacking solar wind data. To cope with this problem a superposed epoch method (CHREE analysis) was used. The results indicate the superiority of the quantities containing the solar wind density in stimulating the radio emission of Saturn — a result found earlier using Voyager 1 data — and the minor importance of quantities incorporating the interplanetary magnetic field.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号