全文获取类型
收费全文 | 4429篇 |
免费 | 23篇 |
国内免费 | 7篇 |
专业分类
航空 | 2337篇 |
航天技术 | 1195篇 |
综合类 | 12篇 |
航天 | 915篇 |
出版年
2021年 | 30篇 |
2019年 | 27篇 |
2018年 | 179篇 |
2017年 | 154篇 |
2016年 | 77篇 |
2015年 | 45篇 |
2014年 | 68篇 |
2013年 | 85篇 |
2012年 | 118篇 |
2011年 | 235篇 |
2010年 | 205篇 |
2009年 | 252篇 |
2008年 | 265篇 |
2007年 | 215篇 |
2006年 | 78篇 |
2005年 | 141篇 |
2004年 | 96篇 |
2003年 | 113篇 |
2002年 | 70篇 |
2001年 | 125篇 |
2000年 | 73篇 |
1999年 | 83篇 |
1998年 | 106篇 |
1997年 | 70篇 |
1996年 | 95篇 |
1995年 | 131篇 |
1994年 | 102篇 |
1993年 | 67篇 |
1992年 | 91篇 |
1991年 | 34篇 |
1990年 | 39篇 |
1989年 | 86篇 |
1988年 | 43篇 |
1987年 | 37篇 |
1986年 | 35篇 |
1985年 | 96篇 |
1984年 | 92篇 |
1983年 | 82篇 |
1982年 | 82篇 |
1981年 | 90篇 |
1980年 | 27篇 |
1979年 | 27篇 |
1978年 | 29篇 |
1977年 | 27篇 |
1976年 | 19篇 |
1975年 | 35篇 |
1974年 | 25篇 |
1973年 | 18篇 |
1972年 | 33篇 |
1971年 | 18篇 |
排序方式: 共有4459条查询结果,搜索用时 31 毫秒
811.
S. A. Mikhailov T. A. Mukhametshin D. V. Nedel’ko A. V. Dvoryankin 《Russian Aeronautics (Iz VUZ)》2012,55(4):353-356
The methodical grounds for helicopter operation with respect to the technical condition for the state of the art of aeronautical engineering are analyzed and generalized. The most essential engineering solutions on developing the systems for monitoring the helicopter operating conditions are considered. The promising lines for further development of these systems are designated. 相似文献
812.
The martian surface environment exhibits extremes of salinity, temperature, desiccation, and radiation that would make it difficult for terrestrial microbes to survive. Recent evidence suggests that martian soils contain high concentrations of MgSO? minerals. Through warming of the soils, meltwater derived from subterranean ice-rich regolith may exist for an extended period of time and thus allow the propagation of terrestrial microbes and create significant bioburden at the near surface of Mars. The current report demonstrates that halotolerant bacteria from the Great Salt Plains (GSP) of Oklahoma are capable of growing at high concentrations of MgSO? in the form of 2 M solutions of epsomite. The epsotolerance of isolates in the GSP bacterial collection was determined, with 35% growing at 2 M MgSO?. There was a complex physiological response to mixtures of MgSO? and NaCl coupled with other environmental stressors. Growth also was measured at 1 M concentrations of other magnesium and sulfate salts. The complex responses may be partially explained by the pattern of chaotropicity observed for high-salt solutions as measured by agar gelation temperature. Select isolates could grow at the high salt concentrations and low temperatures found on Mars. Survival during repetitive freeze-thaw or drying-rewetting cycles was used as other measures of potential success on the martian surface. Our results indicate that terrestrial microbes might survive under the high-salt, low-temperature, anaerobic conditions on Mars and present significant potential for forward contamination. Stringent planetary protection requirements are needed for future life-detection missions to Mars. 相似文献
813.
P Tsou DE Brownlee CP McKay AD Anbar H Yano K Altwegg LW Beegle R Dissly NJ Strange I Kanik 《Astrobiology》2012,12(8):730-742
Abstract Life Investigation For Enceladus (LIFE) presents a low-cost sample return mission to Enceladus, a body with high astrobiological potential. There is ample evidence that liquid water exists under ice coverage in the form of active geysers in the "tiger stripes" area of the southern Enceladus hemisphere. This active plume consists of gas and ice particles and enables the sampling of fresh materials from the interior that may originate from a liquid water source. The particles consist mostly of water ice and are 1-10?μ in diameter. The plume composition shows H(2)O, CO(2), CH(4), NH(3), Ar, and evidence that more complex organic species might be present. Since life on Earth exists whenever liquid water, organics, and energy coexist, understanding the chemical components of the emanating ice particles could indicate whether life is potentially present on Enceladus. The icy worlds of the outer planets are testing grounds for some of the theories for the origin of life on Earth. The LIFE mission concept is envisioned in two parts: first, to orbit Saturn (in order to achieve lower sampling speeds, approaching 2 km/s, and thus enable a softer sample collection impact than Stardust, and to make possible multiple flybys of Enceladus); second, to sample Enceladus' plume, the E ring of Saturn, and the Titan upper atmosphere. With new findings from these samples, NASA could provide detailed chemical and isotopic and, potentially, biological compositional context of the plume. Since the duration of the Enceladus plume is unpredictable, it is imperative that these samples are captured at the earliest flight opportunity. If LIFE is launched before 2019, it could take advantage of a Jupiter gravity assist, which would thus reduce mission lifetimes and launch vehicle costs. The LIFE concept offers science returns comparable to those of a Flagship mission but at the measurably lower sample return costs of a Discovery-class mission. Key Words: Astrobiology-Habitability-Enceladus-Biosignatures. Astrobiology 12, 730-742. 相似文献
814.
Removing orbital debris with lasers 总被引:2,自引:0,他引:2
Claude R. Phipps Kevin L. Baker Stephen B. Libby Duane A. Liedahl Scot S. Olivier Lyn D. Pleasance Alexander Rubenchik James E. Trebes E. Victor George Bogdan Marcovici James P. Reilly Michael T. Valley 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2012
Orbital debris in low Earth orbit (LEO) are now sufficiently dense that the use of LEO space is threatened by runaway collision cascading. A problem predicted more than thirty years ago, the threat from debris larger than about 1 cm demands serious attention. A promising proposed solution uses a high power pulsed laser system on the Earth to make plasma jets on the objects, slowing them slightly, and causing them to re-enter and burn up in the atmosphere. In this paper, we reassess this approach in light of recent advances in low-cost, light-weight modular design for large mirrors, calculations of laser-induced orbit changes and in design of repetitive, multi-kilojoules lasers, that build on inertial fusion research. These advances now suggest that laser orbital debris removal (LODR) is the most cost-effective way to mitigate the debris problem. No other solutions have been proposed that address the whole problem of large and small debris. A LODR system will have multiple uses beyond debris removal. International cooperation will be essential for building and operating such a system. 相似文献
815.
Quasi-static microaccelerations are estimated for a satellite specially designed to perform space experiments in the field of microgravity. Three modes of attitude motion of the spacecraft are considered: passive gravitational orientation, orbital orientation, and semi-passive gravitational orientation. In these modes the lengthwise axis of the satellite is directed along the local vertical, while solar arrays lie in the orbit plane. The second and third modes are maintained using electromechanical executive devices: flywheel engines or gyrodynes. Estimations of residual microaccelerations are performed with the help of mathematical modeling of satellite’s attitude motion under the action of gravitational and aerodynamic moments, as well as the moment produced by the gyro system. It is demonstrated that all modes ensure rather low level of quasi-static microaccelerations on the satellite and provide for a fairly narrow region of variation for the vector of residual microacceleration. The semi-passive gravitational orientation ensures also a limited proper angular momentum of the gyro system. 相似文献
816.
In each polar cap (PC) we mark out “old PC” observed during quiet time before the event under consideration, and “new PC” that emerges during the substorm framing the old one and expanding the PC total area. Old and new PCs are the areas for the magnetosphere old and new tail lobes, respectively. The new lobe variable magnetic flux Ψ1 is usually assumed to be active, i.e. it provides the electromagnetic energy flux (Poynting flux) ɛ′ transport from solar wind (SW) into the magnetosphere. The old lobe magnetic flux Ψ2 is supposed to be passive, i.e. it remains constant during the disturbance and does not participate in the transporting process which would mean the old PC electric field absolute screening from the convection electric field created by the magnetopause reconnection. In fact, screening is observed, but far from absolute. We suggest a model of screening and determine its quantitative characteristics in the selected superstorm. The coefficient of a screening is the β = Ψ2/Ψ02, where Ψ02 = const is open magnetic flux through the old PC measured prior to the substorm, and Ψ2 is variable magnetic flux through the same area measured during the substorm. We consider three various regimes of disturbance. In each, the coefficient β decreased during the loading phase and increased at the unloading phase, but the rates and amplitudes of variations exhibited a strong dependence on the regime. We interpreted decrease in β as a result of involving the old PC magnetic flux Ψ2, which was considered to be constant earlier, in the Poynting flux ɛ′ transport process from solar wind into the magnetosphere. Transport process weakening at the subsequent unloading phase creates increase in β. Estimates showed that coefficient β during each regime and the computed Poynting flux ɛ′ varied manifolds. In general, unlike the existing substorm conception, the new scenario describes an unknown earlier of tail lobe activation process during a substorm growth phase that effectively increases the accumulated tail energy for the expansion and recovery phases. 相似文献
817.
We present the results of a cross-correlation analysis made on the basis of Spearman’s rank correlation method. The quantities
to correlate are daily values of the fluence of energetic electrons at a geosynchronous orbit, intensities of ground and interplanetary
ultra-low-frequency (ULF) oscillations in the Pc5 range, and parameters of the solar wind. The period under analysis is the
23rd cycle of solar activity, 1996–2006. Daily (from 6 h to 18 h of LT) magnetic data at two diametrically opposite observatories
of the Intermagnet network are taken as ground-based measurements. The fluxes of electrons with energies higher than 2 MeV
were measured by the geosynchronous GOES satellites. The data of magnetometers and plasma instruments installed on ACE and WIND spacecraft were used for analysis of the solar wind parameters and of the oscillations of the interplanetary magnetic field
(IMF). Some results elucidating the role played by interplanetary ULF waves in the processes of generation of magneospheric
oscillations and acceleration of energetic electrons are obtained. Among them are (i) high and stable correlation of ground
ULF oscillations with waves in the solar wind; (ii) closer link of mean daily amplitudes of both interplanetary and ground
oscillations with ‘tomorrow’ values of the solar wind velocity than with current values; and (iii) correlation of the intensity
of ULF waves in the solar wind, normalized to the IMF magnitude, with fluxes of relativistic electrons in the magnetosphere. 相似文献
818.
V. L. Varsegov 《Russian Aeronautics (Iz VUZ)》2010,53(2):212-220
An integral method is proposed for calculating a jet propagating from the turbofan engine thrust reverser and interacting
with a stream formed as a result of the after-landing aircraft run. The calculation results for the PS-90 engine are presented.
The calculation data obtained show that the mathematical model developed adequately describes a qualitative pattern of main
parameter variation in the sector jet propagating in a stream. 相似文献
819.
The application of the optimized dispersion relation preserving scheme (DRP-scheme) in combination with the explicit optimized
two-layer Runge-Kutta scheme is presented to solve a system of one-dimensional and quasi-one-dimensional Euler equations using
as an example the solution of four test problems, namely, discontinuity disintegration in a tube (Sod’s problem); transfer
of the lowamplitude Gaussian pulse; acoustic wave propagation through the transonic nozzle; acoustic wave-shock interaction.
Also given are the comparison of the calculation results using different schemes: DRP, CABARET, CE-SE and the standard Lax-Wendroff
schemes as well as the solutions obtained with the use of software packages. 相似文献
820.
M.D. Ngobeni M.S. Potgieter 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2010
Observations made with the two Voyager spacecraft confirmed that the solar wind decelerates to form the heliospheric termination shock and that it has begun its merger with the local interstellar medium. The compression ratio of this shock affects galactic cosmic rays when they enter the heliosphere. Hydrodynamic (HD) models show that the compression ratio can have a significant latitude dependence; with the largest value in the nose direction of the heliosphere, becoming significantly less towards the polar regions. The modulation effects of such large latitude dependence are studied, using a well-established numerical drift and shock modulation model. We focus on computing the modulated spectra for galactic protons with emphasis on the radial and polar gradients in the equatorial plane and at a polar angle of θ = 55°, corresponding to the heliolatitude of Voyager 1. Two sets of solutions are computed and compared each time; with and without a latitude dependence for the compression ratio. All computations are done for the two magnetic field polarity cycles assuming solar minimum conditions. Including the termination shock in the model allows the study of the re-acceleration of galactic protons in the outer heliosphere. We find that for the A < 0 polarity cycle the intensity between ∼200 MeV and ∼1 GeV in the vicinity of the shock in the heliospheric equatorial plane may exceed the local interstellar value specified at the heliopause. Unfortunately, at θ = 55°, the effect is reduced. This seems not possible during an A > 0 cycle because significant modulation is then predicted between the heliopause and the termination shock, depending on how strong global gradient and curvature drifts are in the heliosheath. The overall effect of the shock on galactic protons in the equatorial plane is to reduce the total modulation as a function of radial distance with respect to the interstellar spectrum. Making the compression ratio latitude dependent enhances these effects at energies E < 200 MeV in the equatorial plane. At larger heliolatitudes these effects are even more significant. The differences in the modulation between the two drift cycles are compelling when the compression ratio is made latitude dependent but at Earth this effect is insignificant. A general result is that the computed radial gradient changes for galactic protons at and close to the TS and that these changes are polarity dependent. In line with previous work, large polarity dependent effects are predicted for the inner heliosphere and also close to the shock’s position in the equatorial plane. In contrast, at θ = 55°, the largest polarity effect occurs in the middle heliosphere (50 AU), enhanced by the latitude dependence of the compression ratio. At this latitude, the amount of proton modulation between the heliopause and the termination shock is much reduced. If galactic cosmic rays were to experience some diffusive shock acceleration over the 100–1000 MeV range at the shock, the radial gradient should change its sign in the vicinity of the shock, how large, depends on the compression ratio and the amount of drifts taking place in the outer heliosphere. The effective polar gradient shows a strong polarity dependence at Earth but this dissipates at θ = 55°, especially with increasing radial distance. This tendency is enhanced by making the compression ratio latitude dependent. 相似文献