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排序方式: 共有400条查询结果,搜索用时 31 毫秒
371.
372.
Rodger I. Thompson Jill Bechtold Daniel Eisenstein Xiaohui Fan David Arnett Carlos Martins Robert Kennicutt John Black 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008
Many theoretical models of dark energy invoke rolling scaler fields which in turn predict time varying values of the fundamental constants. Establishing the value of the fundamental constants at various times in the universe can probe and test the various dark energy theories. One of the constants that is predicted to vary is the ratio of the electron to proton mass μ. It was established early on that molecular spectra are sensitive to the value of μ and can be used as probes of that value. This article describes the use of the spectrum of molecular hydrogen in high redshift Damped Lyman Alpha systems (DLAs) as a sensitive probe of the time evolution of μ. 相似文献
373.
John M. Wilcox 《Space Science Reviews》1968,8(2):258-328
Many observations related to the large-scale structure of the interplanetary magnetic field, its solar origin and terrestrial effects are discussed. During the period observed by spacecraft the interplanetary field was dominated by a sector structure corotating with the sun in which the field is predominantly away from the sun (on the average in the Archimedes spiral direction) for several days (as observed near the earth), and then toward the sun for several days, etc. The average sector appears to be a coherent entity with internal structure such that its preceding portion is more active than its following portion. Cosmic rays corotate with the interplanetary field, and there are differential flows associated with the sector pattern. Profound effects on geomagnetic activity and the radiation belts are produced as the sector pattern rotates past the earth. The solar origin of the sector pattern is discussed. The solar source may be associated with the large-scale weak background photospheric fields observed with the solar magnetograph. It is suggested that there may be a rather continual relation between this solar structure and terrestrial responses, of which the recurring M-Region geomagnetic storms are just the most prominent example. 相似文献
374.
John Elliott 《Acta Astronautica》2011,68(3-4):418-424
The aim of HuLCC (the human language chorus corpus), is to provide a resource of sufficient size to facilitate inter-language analysis by incorporating languages from all the major language families: for the first time all aspects of typology will be incorporated within a single corpus, adhering to a consistent grammatical classification and granularity, which historically adopt a plethora of disparate schemes. An added feature will be the inclusion of a common text element, which will be translated across all languages, to provide a precise comparable thread for detailed linguistic analysis for translation strategies and a mechanism by which these mappings can be explicitly achieved. Methods developed to solve unambiguous mappings across these languages can then be adopted for any subsequent message authored by the SETI community. Initially, it is planned to provide at least 20,000 words for each chosen language, as this amount of text exceeds the point where randomly generated text can be disambiguated from natural language and is of sufficient size useful for message transmission [1] (Elliot, 2002). This paper details the design of this resource, which ultimately will be made available to SETI upon its completion, and discusses issues ‘core’ to any message construction. 相似文献
375.
376.
Solar Nebula Magnetohydrodynamics 总被引:1,自引:0,他引:1
The dynamical state of the solar nebula depends critically upon whether or not the gas is magnetically coupled. The presence
of a subthermal field will cause laminar flow to break down into turbulence. Magnetic coupling, in turn, depends upon the
ionization fraction of the gas. The inner most region of the nebula (≲0.1 AU) is magnetically well-coupled, as is the outermost
region (≳10 AU). The magnetic status of intermediate scales (∼1 AU) is less certain. It is plausible that there is a zone
adjacent to the inner disk in which turbulent heating self-consistently maintains the requisite ionization levels. But the
region adjacent to the active outer disk is likely to be magnetically ``dead.' Hall currents play a significant role in nebular
magnetohydrodynamics.
Though still occasionally argued in the literature, there is simply no evidence to support the once standard claim that differential
rotation in a Keplerian disk is prone to break down into shear turbulence by nonlinear instabilities. There is abundant evidence—numerical,
experimental, and analytic—in support of the stabilizing role of Coriolis forces. Hydrodynamical turbulence is almost certainly
not a source of enhanced turbulence in the solar nebula, or in any other astrophysical accretion disk.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
377.
Photographic mapping missions over planetary bodies present a number of interesting hardware, operational, data retrieval, and data analysis techniques. Broadly speaking, such a mapping mission can be conducted with present level state-of-the-art equipment. The space-craft and photographic equipment must perform over long periods of time unattended in the hostile space environment. Operations must support the mission and retrieve the photographic data. Maximum data are limited by photographic capacity, operations capacity, and analysis capacity. Data analysis techniques are under development and computer maps have been developed for RANGER analysis; these techniques can be especially suited to analysis of the video (analog or digital) signal transmitted back to Earth. 相似文献
378.
Apollo should not serve as a model for the many programs for lunar and planetary exploration currently making headway: it was a unilateral effort whose generous budget would be inconceivable today. Yet President Kennedy was an advocate of cooperation in major space missions, an attitude that makes perfect sense today, when so many agencies have ambitious exploration plans. The importance of President G.W. Bush's ‘Vision for Space Exploration’, by providing a focus for NASA and others, has been underestimated. It should give us a chance to find out whether a long-term objective of moving humans off the home planet is really feasible—surely the point of exploration. 相似文献
379.
The Lunar Orbiter Laser Altimeter Investigation on the Lunar Reconnaissance Orbiter Mission 总被引:3,自引:0,他引:3
David E. Smith Maria T. Zuber Glenn B. Jackson John F. Cavanaugh Gregory A. Neumann Haris Riris Xiaoli Sun Ronald S. Zellar Craig Coltharp Joseph Connelly Richard B. Katz Igor Kleyner Peter Liiva Adam Matuszeski Erwan M. Mazarico Jan F. McGarry Anne-Marie Novo-Gradac Melanie N. Ott Carlton Peters Luis A. Ramos-Izquierdo Lawrence Ramsey David D. Rowlands Stephen Schmidt V. Stanley Scott III George B. Shaw James C. Smith Joseph-Paul Swinski Mark H. Torrence Glenn Unger Anthony W. Yu Thomas W. Zagwodzki 《Space Science Reviews》2010,150(1-4):209-241
The Lunar Orbiter Laser Altimeter (LOLA) is an instrument on the payload of NASA’s Lunar Reconnaissance Orbiter spacecraft (LRO) (Chin et al., in Space Sci. Rev. 129:391–419, 2007). The instrument is designed to measure the shape of the Moon by measuring precisely the range from the spacecraft to the lunar surface, and incorporating precision orbit determination of LRO, referencing surface ranges to the Moon’s center of mass. LOLA has 5 beams and operates at 28 Hz, with a nominal accuracy of 10 cm. Its primary objective is to produce a global geodetic grid for the Moon to which all other observations can be precisely referenced. 相似文献
380.
对于许多微流体装置来说,流体的混合是至关重要的.为了实现微流体混合控制,对基于电磁流体动力学原理的微流道主动混合控制方法进行了研究.该方法中使用了一个混合室,在动态洛伦兹力的作用下,混合室内流体往复周期的运动.流体速度的周期性变化使流体分界面折叠,从而使流体接触面增加.用CCD记录设备记录了流体的快速混合过程,并对流体的混合程度进行定量分析. 相似文献