全文获取类型
收费全文 | 315篇 |
免费 | 2篇 |
国内免费 | 2篇 |
专业分类
航空 | 164篇 |
航天技术 | 64篇 |
综合类 | 1篇 |
航天 | 90篇 |
出版年
2021年 | 8篇 |
2019年 | 4篇 |
2018年 | 10篇 |
2017年 | 1篇 |
2016年 | 2篇 |
2015年 | 3篇 |
2014年 | 5篇 |
2013年 | 15篇 |
2012年 | 14篇 |
2011年 | 29篇 |
2010年 | 10篇 |
2009年 | 15篇 |
2008年 | 22篇 |
2007年 | 20篇 |
2006年 | 11篇 |
2005年 | 18篇 |
2004年 | 9篇 |
2003年 | 15篇 |
2002年 | 4篇 |
2001年 | 5篇 |
2000年 | 6篇 |
1999年 | 7篇 |
1998年 | 4篇 |
1997年 | 3篇 |
1996年 | 3篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1989年 | 4篇 |
1988年 | 1篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1969年 | 1篇 |
1968年 | 3篇 |
1967年 | 9篇 |
1966年 | 11篇 |
排序方式: 共有319条查询结果,搜索用时 296 毫秒
41.
The measured D/H ratios in interstellar environments and in the solar system are reviewed. The two extreme D/H ratios in solar
system water - (720±120)×10−6 in clay minerals and (88±11)×10−6 in chondrules, both from LL3 chondritic meteorites - are interpreted as the result of a progressive isotopic exchange in
the solar nebula between deuterium-rich interstellar water and protosolar H2. According to a turbulent model describing the evolution of the nebula (Drouart et al., 1999), water in the solar system cannot be a product of thermal (neutral) reactions occurring in the solar nebula. Taking
720×10−6 as a face value for the isotopic composition of the interstellar water that predates the formation of the solar nebula, numerical
simulations show that the water D/H ratio decreases via an isotopic exchange with H2. During the course of this process, a D/H gradient was established in the nebula. This gradient was smoothed with time and
the isotopic homogenization of the solar nebula was completed in 106 years, reaching a D/H ratio of 88×10−6. In this model, cometary water should have also suffered a partial isotopic re-equilibration with H2. The isotopic heterogeneity observed in chondrites result from the turbulent mixing of grains, condensed at different epochs
and locations in the solar nebula. Recent isotopic determinations of water ice in cold interstellar clouds are in agreement
with these chondritic data and their interpretation (Texeira et al., 1999).
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
42.
Patrick J. Cannon Robert M. Enderson 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1985,5(1):17-20
After over twenty years there is a new film introduced in the United States for fabricating scientific balloons. The film was developed by Raven and is designated Astrofilm.The film is a result of a critical selection of both the resin and extruding parameters. The film's physical properties and results of a test flight are reported. Also reported are a proposed design modification and failure analysis. 相似文献
43.
Oxygen Isotopes in the Solar System 总被引:1,自引:0,他引:1
The oxygen three-isotope system has major advantages over the two-isotope systems of hydrogen, carbon, and nitrogen because
different fractionation laws govern intraplanetary and interplanetary processes. This permits discriminating between solar
nebular processes and parent-body processes. Oxygen isotopes also serve as a sensitive natural tracer for meteorite classification.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
44.
Dipak K. Srinivasan David Artis Ben Baker Robert Stilwell Robert Wallis 《Acta Astronautica》2009,65(11-12):1639-1649
The NASA Radiation Belt Storm Probes (RBSP) mission, currently in Phase B, is a two-spacecraft, Earth-orbiting mission, which will launch in 2012. The spacecraft's S-band radio frequency (RF) telecommunications subsystem has three primary functions: provide spacecraft command capability, provide spacecraft telemetry and science data return, and provide accurate Doppler data for navigation. The primary communications link to the ground is via the Johns Hopkins University Applied Physics Laboratory's (JHU/APL) 18 m dish, with secondary links to the NASA 13 m Ground Network and the Tracking and Data Relay Spacecraft System (TDRSS) in single-access mode. The on-board RF subsystem features the APL-built coherent transceiver and in-house builds of a solid-state power amplifier and conical bifilar helix broad-beam antennas. The coherent transceiver provides coherency digitally, and controls the downlink data rate and encoding within its field-programmable gate array (FPGA). The transceiver also provides a critical command decoder (CCD) function, which is used to protect against box-level upsets in the C&DH subsystem. Because RBSP is a spin-stabilized mission, the antennas must be symmetric about the spin axis. Two broad-beam antennas point along both ends of the spin axis, providing communication coverage from boresight to 70°. An RF splitter excites both antennas; therefore, the mission is designed such that no communications are required close to 90° from the spin axis due to the interferometer effect from the two antennas. To maximize the total downlink volume from the spacecraft, the CCSDS File Delivery Protocol (CFDP) has been baselined for the RBSP mission. During real-time ground contacts with the APL ground station, downlinked files are checked for errors. Handshaking between flight and ground CFDP software results in requests to retransmit only the file fragments lost due to dropouts. This allows minimization of RF link margins, thereby maximizing data rate and thus data volume. 相似文献
45.
尽管全球经济形势不断恶化,航空市场缩水,但中东地区丰富的石油资源和现金流,依然是航空维修业发展的沃土.中东各国政府、航空公司及其合作伙伴不惜花巨资开发和提升维修能力,成为飞机机体或发动机的维修中心.其中以迪拜的航空维修设施的发展最为抢眼. 相似文献
46.
航空维修业正处在发展的十字路口.一些航空公司将其维修业务剥离出去以增加现金流,独立的航空维修企业正在进行调整以应对不断改变的市场环境,更多的维修企业则在寻求新的投资和合作伙伴. 相似文献
47.
Kenneth R. Lang Robert F. Willson 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(7):105-110
Very Large Array (V.L.A.) measurements at 20 cm wavelength map emission from coronal loops with second-of-arc angular resolution at time intervals as short as 3.3 seconds. The total intensity of the 20 cm emission describes the evolution and structure of the hot plasma that is detected by satellite X-ray observations of coronal loops. The circular polarization of the 20 cm emission describes the evolution, strength and structure of the coronal magnetic field. Preburst heating and magnetic changes that precede burst emission on time scales of between 1 and 30 minutes are discussed. Simultaneous 20 cm and soft X-ray observations indicate an electron temperature and electron density during preburst heating in a coronal loop that was also associated with twisting of the entire loop in space. We also discuss the successive triggering of bursts from adjacent coronal loops; highly polarized emission from the legs of loops with large intensity changes over a 32 MHz change in observing frequency; and apparent motions of hot plasma within coronal loops at velocities V > 2,000 kilometerspersecond. 相似文献
48.
With the help of a very simple two zone model, we demonstrate the possibility of periodic thermal relaxation (limit cycle) oscillations in the helium burning envelope of accreting neutron stars. Physically reasonable model parameters can be chosen which yield agreement with the observed features of x-ray bursts and we suggest that this limit cycle is operative in neutron stars which have an accretion rate in a specific range. For hydrogen burning a similar cycle is possible, but it operates at such high temperatures that an unrealistically large accretion rate would be required. 相似文献
49.
Robert B. Decker 《Space Science Reviews》1988,48(3-4):195-262
We review the basic techniques and results of numerical codes used to model the acceleration of charged particles at oblique, fast-mode, collisionless shocks. The emphasis is upon models in which accelerated particles (ions) are treated as test particles, and particle dynamics is calculated by numerically integrating along exact phase-space orbits. We first review the case where ions are sufficiently energetic so that the shock can be approximated by a planar discontinuity, and where the electromagnetic fields on both sides of the shock are defined at the outset of each computer run. When the fields are uniform and static, particles are accelerated by the scatter-free drift acceleration process at a single shock encounter. We review the characteristics of scatter-free drift acceleration by considering how an incident particle distribution is modified by interacting with a shock. Next we discuss drift acceleration when magnetic fluctuations are introduced on both sides of the shock, and compare these results with those obtained under scatter-free conditions. We describe the modeling of multiple shock encounters, discuss specific applications, and compare the model predictions with theory. Finally, we review some recent numerical simulations that illustrate the importance of shock structure to both the ion injection process and to the acceleration of ions to high energies at quasi-perpendicular shocks. 相似文献
50.