全文获取类型
收费全文 | 345篇 |
免费 | 1篇 |
国内免费 | 5篇 |
专业分类
航空 | 150篇 |
航天技术 | 64篇 |
综合类 | 1篇 |
航天 | 136篇 |
出版年
2021年 | 6篇 |
2020年 | 4篇 |
2019年 | 5篇 |
2018年 | 11篇 |
2017年 | 6篇 |
2015年 | 2篇 |
2014年 | 7篇 |
2013年 | 9篇 |
2012年 | 23篇 |
2011年 | 36篇 |
2010年 | 17篇 |
2009年 | 23篇 |
2008年 | 18篇 |
2007年 | 21篇 |
2006年 | 16篇 |
2005年 | 25篇 |
2004年 | 3篇 |
2003年 | 14篇 |
2002年 | 12篇 |
2001年 | 4篇 |
2000年 | 5篇 |
1998年 | 6篇 |
1997年 | 7篇 |
1996年 | 3篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 4篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 7篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 4篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1968年 | 1篇 |
1967年 | 6篇 |
1966年 | 2篇 |
排序方式: 共有351条查询结果,搜索用时 15 毫秒
341.
Zhen Li Marek Ziebart Santosh Bhattarai David Harrison Stuart Grey 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2018,61(9):2352-2365
Physics based SRP (Solar Radiation Pressure) models using ray tracing methods are powerful tools when modelling the forces on complex real world space vehicles. Currently high resolution (1?mm) ray tracing with secondary intersections is done on high performance computers at UCL (University College London). This study introduces the BVH (Bounding Volume Hierarchy) into the ray tracing approach for physics based SRP modelling and makes it possible to run high resolution analysis on personal computers. The ray tracer is both general and efficient enough to cope with the complex shape of satellites and multiple reflections (three or more, with no upper limit). In this study, the traditional ray tracing technique is introduced in the first place and then the BVH is integrated into the ray tracing. Four aspects of the ray tracer were tested for investigating the performance including runtime, accuracy, the effects of multiple reflections and the effects of pixel array resolution.Test results in runtime on GPS IIR and Galileo IOV (In Orbit Validation) satellites show that the BVH can make the force model computation 30–50 times faster. The ray tracer has an absolute accuracy of several nanonewtons by comparing the test results for spheres and planes with the analytical computations. The multiple reflection effects are investigated both in the intersection number and acceleration on GPS IIR, Galileo IOV and Sentinel-1 spacecraft. Considering the number of intersections, the 3rd reflection can capture , and of the total reflections for GPS IIR, Galileo IOV satellite bus and the Sentinel-1 spacecraft respectively. In terms of the multiple reflection effects on the acceleration, the secondary reflection effect for Galileo IOV satellite and Sentinel-1 can reach 0.2? and 0.4? respectively. The error percentage in the accelerations magnitude results show that the 3rd reflection should be considered in order to make it less than . The pixel array resolution tests show that the dimensions of the components have to be considered when choosing the spacing of the pixel in order not to miss some components of the satellite in ray tracing. This paper presents the first systematic and quantitative study of the secondary and higher order intersection effects. It shows conclusively the effect is non-negligible for certain classes of misson. 相似文献
342.
Evidence of microbial life on Earth has been found in siliceous rock formations throughout the geological and fossil record. To understand the mechanisms of silicification and thus improve our search patterns for evidence of fossil microbial life in rocks, a series of controlled laboratory experiments were designed to simulate the silicification of microorganisms. The bacterial strains Pseudomonas fluorescens and Desulphovibrio indonensis were exposed to silicifying media. The experiments were designed to determine how exposure time to silicifying solutions and to silicifying solutions of different Si concentration affect the fossilization of microbial biofilms. The silicified biofilms were analyzed using transmission electron microscopy (TEM) in combination with energy-dispersive spectroscopy. Both bacterial species showed evidence of silicification after 24 h in 1,000 ppm silica solution, although D. indonensis was less prone to silicification. The degree of silicification of individual cells of the same sample varied, though such variations decreased with increasing exposure time. High Si concentration resulted in better preservation of cellular detail; the Si concentration was more important than the duration in Si solution. Even though no evidence of amorphous silica precipitation was observed, bacterial cells became permineralized. High-resolution TEM analysis revealed nanometer-sized crystallites characterized by lattice fringe-spacings that match the (10-11) d-spacing of quartz formed within bacterial cell walls after 1 week in 5,000 ppm silica solution. The mechanisms of silicification under controlled laboratory conditions and the implication for silicification in natural environments are discussed, along with the relevance of our findings in the search for early life on Earth and extraterrestrial life. 相似文献
343.
Modern stromatolites represent a significant resource for studying microbial ecology and evolution. A preliminary investigation was undertaken employing specific genetic probes to characterize the cyanobacteria responsible for stromatolite construction in a range of environments, including microbial mats found in Australia not previously examined with molecular methods. Isolates of cyanobacteria were collected from stromatolites in thermal springs, hypersaline lakes, and oceanic fringes on two continents. A polymerase chain reaction specific for DNA of cyanobacterial 16S rRNA was developed, the resulting products of the DNA amplification reaction were sequenced, and the data were used to infer relatedness between the isolates studied and other members of the cyanobacterial radiation. Complete sequence was generated for the region from position 27 to 408 for 13 strains of cyanobacteria associated with stromatolites. All stromatolite-derived sequences were most closely related to cyanobacteria, as indicated by local sequence alignment. It was possible to correlate genetic identity with morphological nomenclatures and to expand the phylogeny of benthic cyanobacteria. These inferences were also expanded to temporal variation in the dominant resident cyanobacterial species based on sampling of surface and core sinter laminations. Under the methods employed, only one cyanobacterial strain was detected in each sample, suggesting the possible dominance of a specific clonal population of cyanobacteria at any one time in the biota of the samples tested. The data indicate that internal core samples of a stromatolite at least 10 years old can be successfully analyzed by DNA-based methods to identify preserved cyanobacteria. 相似文献
344.
We describe the observational history and assess the current understanding of the magnetosheath and magnetotail of Venus, stressing recent developments. We make recommendations for research that can be done using existing observations, as well as desirable trajectory and instrumentation characteristics for future spacecraft missions. 相似文献
345.
Beaty DW Clifford SM Borg LE Catling DC Craddock RA Des Marais DJ Farmer JD Frey HV Haberle RM McKay CP Newsom HE Parker TJ Segura T Tanaka KL 《Astrobiology》2005,5(6):663-689
In October 2004, more than 130 terrestrial and planetary scientists met in Jackson Hole, WY, to discuss early Mars. The first billion years of martian geologic history is of particular interest because it is a period during which the planet was most active, after which a less dynamic period ensued that extends to the present day. The early activity left a fascinating geological record, which we are only beginning to unravel through direct observation and modeling. In considering this time period, questions outnumber answers, and one of the purposes of the meeting was to gather some of the best experts in the field to consider the current state of knowledge, ascertain which questions remain to be addressed, and identify the most promising approaches to addressing those questions. The purpose of this report is to document that discussion. Throughout the planet's first billion years, planetary-scale processes-including differentiation, hydrodynamic escape, volcanism, large impacts, erosion, and sedimentation-rapidly modified the atmosphere and crust. How did these processes operate, and what were their rates and interdependencies? The early environment was also characterized by both abundant liquid water and plentiful sources of energy, two of the most important conditions considered necessary for the origin of life. Where and when did the most habitable environments occur? Did life actually occupy them, and if so, has life persisted on Mars to the present? Our understanding of early Mars is critical to understanding how the planet we see today came to be. 相似文献
346.
The Cassini Science Management Plan was developed for NASA's Cassini mission to Saturn to assist the Science Instrument Manager in guiding the development of the spacecraft's science payload. This plan, unlike previous development approaches, allocated the entire mass, power, data rate and budget resources for the science instruments to the Principal Investigators. The result removed the Cassini Project from adjudicating and reallocating resources for instrument development problems. Instrument development problems that did occur were resolved by the Principal Investigators themselves through the use of a ‘resource exchange’. The exchange allowed Principal Investigators to submit ‘bids’ (i.e. a request for resources) to a database. Principal Investigators were allowed to barter their own resources with other investigators. The resulting exchange or multilateral trade allowed the investigators to reallocate their resource to ‘better’ their current position. 相似文献
347.
David W. Hughes 《Space Science Reviews》1992,61(3-4):275-299
Meteorites are being retrieved from the surface of our planet all the time, the number of located falls being at maximum in the range 0.3 to 1.0 yr-1 (106 km2)-1. Using the known flux of meteorite parent bodies to the top of the atmosphere this paper reviews suggested ablation processes and predicts the actual flux of meteorites to the Earth. It is found that this is some 70 to 300 times what is observed. 相似文献
348.
Three opportunities for missions to rendezvous ballistically with the Earth-crossing asteroid Anteros are studied to illustrate the requirements for a trip to a near-Earth minor planet. The rationale, sample payload, spacecraft requirements and trajectory characteristics of these opportunities are typical of a rendezvous mission to an accessible near-Earth object. Round trip ballistic trajectories to return small samples of the asteroid with launch dates between 1985 and 2000 are also presented. Contours of minimum total ΔV drawn in the space of launch and arrival true anomalies, given the designation Prime Rib curves, are introduced as a useful tool for mission design. 相似文献
349.
Many observations of geomagnetic cutoff phenomena and their implications with respect to the dynamics of charged particles in the geomagnetic field are discussed. Störmer's analytic treatment of the motion of charged particles in a dipole field is briefly reviewed, as are the approximate treatments of charged particle motions, first developed by Alfvén, which were to find successful application to the more complex fields now known to exist in the magnetosphere. In conclusion, the present understanding of geomagnetic cutoffs, together with some remaining areas of uncertainty are examined. 相似文献
350.
This paper reviews the first results of satellite experiments to measure magnetospheric convection electric fields using the double-probe technique.The earliest successful measurements were made with the low-altitude (680–2530 km) polar orbiting Injun-5 spacecraft (launched August, 1968). The Injun-5 data are discussed in detail. The Injun-5 results are compared with the initial findings of the electric field experiment on the polar orbiting OGO-6 satellite (400–1100 km, launched June, 1969).In addition to electric fields, the Injun-5 spacecraft also measures electric antenna impedance and thermal and energetic charged particle densities. Knowledge of these parameters makes possible a detailed investigation of the operation of the electric antenna system. We report on this investigation and discuss errors attributed to sunlight shadows on the probes, wake effects, and other factors. The Injun-5 experiment can generally determine electric fields to an accuracy of about ±30 mV m-1, and under favorable conditions, accuracies of ±10 mV m-1 can be obtained.Reversals in the electric field at auroral zone latitudes are the most significant convection electric field effect discovered in the Injun-5 data. Electric field magnitudes of typically 30 mV m-1, and sometimes 100 mV m-1, are associated with reversals. Electric field reversals occur on 36% of auroral zone traversals, at about 70° to 80° invariant latitude, at all local times, and in both hemispheres. The latitude of a reversal often changes markedly on time scales less than 2 h. Electric potentials of greater than 40 keV are associated with these high latitude electric fields. Reversals occur at the boundary of measurable intensities of >45 keV electrons and are coincident with inverted V type low energy electron precipitation events. In almost all cases the E×B/B
2 plasma convection velocities associated with reversals are directed east or west, with anti-sunward components at higher latitudes and sunward components at lower latitudes. Maximum convection velocities are typically 1.5 km s-1 and ordinarily occur at the auroral zone near the reversal.Two extreme (and many intermediate) configurations of anti-sunward plasma convection have been observed to occur on the high latitude side of electric field reversals: (1) Ordinarily, >0.75 kms-1 convection is limited to narrow (5° INV wide) zones adjacent to the reversal. (2) For 14% of reversals >0.75 km s-1 anti-sunward convection has been observed across the entire polar cap along the trajectory of the Injun-5 spacecraft. A summary pattern of >0.75 km s-1 polar thermal plasma convection is presented.Electric field measurements from the OGO-6 satellite have substantiated many of the initial Injun-5 observations with improved accuracy and sensitivity. The OGO-6 detector revealed the persistent occurrence of anti-sunward convection across the polar cap region at velocities (<0.75 km s-1) not generally detectable with the Injun-5 experiment. The OGO-6 observations also provided information indicating that the location of the electric field reversal shifts equatorward during periods of increased magnetic activity.The implications of the electric field measurements for magnetosphericand auroral structure are summarized, and a list of specific recommendations for improving future experiments is presented. 相似文献