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481.
Approaches in the determination of plant nutrient uptake and distribution in space flight conditions. 总被引:2,自引:0,他引:2
A G Heyenga A Forsman L S Stodieck A Hoehn M Kliss 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2000,26(2):299-302
The effective growth and development of vascular plants rely on the adequate availability of water and nutrients. Inefficiency in either the initial absorption, transportation, or distribution of these elements are factors which impinge on plant structure and metabolic integrity. The potential effect of space flight and microgravity conditions on the efficiency of these processes is unclear. Limitations in the available quantity of space-grown plant material and the sensitivity of routine analytical techniques have made an evaluation of these processes impractical. However, the recent introduction of new plant cultivating methodologies supporting the application of radionuclide elements and subsequent autoradiography techniques provides a highly sensitive investigative approach amenable to space flight studies. Experiments involving the use of gel based 'nutrient packs' and the radionuclides calcium-45 and iron-59 were conducted on the Shuttle mission STS-94. Uptake rates of the radionuclides between ground and flight plant material appeared comparable. 相似文献
482.
483.
A five-phase Power Management and Distribution System for a More-Electric Aircraft (MADMEL) program was awarded by the Air Force to Northrop/Grumman Military Aircraft Division in September 1991. The objective of the program is to design, develop, and demonstrate an advanced electrical power generation and distribution system for a More-Electric Aircraft (MEA). The MEA emphasizes the use of electrical power in place of hydraulics, pneumatic, and mechanical power to optimize the performance and life cycle cost of the aircraft. This paper presents an overview of the MADMEL program and a top-level summary of the program results, development and test of major components to date. In Phase I and Phase II studies, the electrical load requirements were established and the electrical power system architecture was defined for both near-term (NT-year 1996) and far-term (FT-year 2003) MEA application. The detailed design and specification for the Electrical Power System (EPS), its interface with the Vehicle Management System, and the test set-up were developed under Phase III. Phase IV, fabrication and testing of the subsystem level hardware, has been completed. Overall system level integration and testing, Phase V, is scheduled to be completed by September 1999 相似文献
484.
How do people describe the location of a target object to another? This task requires a reference object or frame and terms of reference. Traditional linguistic analyses have loosely organized perspectives around people, objects, or environments as reference objects, using reference terms based on a viewpoint or the intrinsic sides of an object, such as left, right, front, and back or based on the environment, such as north, south, east, and west. In actual communication, social, spatial, and cognitive factors may also affect perspective choice. We examine those factors by varying the spatial information (landmarks and cardinal directions), the communication task (relative cognitive burden to speakers and addressees), and the culture of participants (American and Japanese). Speakers used addressees' perspectives more when addressees had the greater cognitive burden. They also used landmarks and cardinal directions when they were available, especially to avoid difficult discriminations like left/right. Some cases appearing to be perspective taking can be interpreted as using a person as a landmark. Finally, terms like near indicating close proximity were preferred to far and to terms requiring projection of directions. Globally, perspective choices of American and Japanese samples were strikingly similar; that is, Japanese did not select addressees' perspectives more than Americans. The traditional linguistic analyses need to be enhanced to account for effects of cognitive, situational, and social factors. 相似文献
485.
It is well established that the prolonged and thorough mixing of numerous nucleosynthetic components that constitutes the
matter in the solar nebula resulted in an essential isotopic homogeneity of the solar system material. This may or may not
be true for the short-lived radionuclides which were injected into or formed within the solar nebula just prior to or during
solar system formation. Distinguishing between their heterogeneous or homogeneous distribution is important because the short-
lived radionuclides are now widely used for the relative chronology of various objects and processes in the early solar system
and as constraints for models of nucleosynthesis. The recent studies of the 53Mn-53Cr isotope system (half life of 53Mn is 3.7 Ma) in various solar system objects have shown that the relative abundance of radiogenic 53Cr is consistent with essentially homogeneous distribution of 53Mn in the asteroid belt. Thus, the relative 53Mn-53Cr chronometer can be directly used for dating samples which originated in the asteroid belt. Importantly, however, all meteorite
groups studied so far indicate a clear excess of 53Cr as compared to Earth and to a lunar sample, which exhibits also a terrestrial 53Cr/52Cr ratio. The results from the Martian (SNC) meteorites show that their 53Cr excesses are less than half of those found in the asteroid belt bodies. Thus, the characteristic 53Cr/52Cr ratio of Mars is intermediate between that of the Earth-Moon system and those of the other meteorites. If these 53Cr variations are viewed as a function of the heliocentric distance, the radial dependence of the relative abundances of radiogenic
53Cr is indicated. This observed gradient can be explained by either an early, volatility controlled, Mn/Cr fractionation within
the nebula or by an initial radial heterogeneous distribution of 53Mn. Although model calculations of the Mn/Cr ratios in the bulk terrestrial planets seem to be inconsistent with the volatility
driven scenario, the precision of these calculations is inadequate for eliminating this possibility. In contrast, recent studies
of the 53Mn-53Cr system in the enstatite chondrites indicate that, while their bulk Mn/Cr ratios are essentially the same as in ordinary
chondrites, the 53Cr excess in bulk enstatite chondrites is three times lower than that in the bulk ordinary chondrites. This difference cannot
be explained by a Mn/Cr fractionation and, thus, strongly suggests that a radial heterogeneous distribution of 53Mn must have existed in at least the early inner solar system. Using the observed gradient and the 53Cr/52Cr ratio of the bulk enstatite chondrites, their parent body(ies) formed at ∼1.4 AU or somewhat closer to the Sun.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
486.
Jui Tsun Hung Robertazzi T.G. 《IEEE transactions on aerospace and electronic systems》2004,40(3):968-982
Recursive analytical expressions for speedup and solution time for a multilevel tree sequentially processing a divisible load under cut through switching are developed. Such cut through switching is shown to be more efficient than store and forward switching. Aerospace applications include sensor networks, radar, and satellite imagery processing. 相似文献
487.
V. G. Fainshtein 《Cosmic Research》2007,45(5):384-392
A positive correlation is shown to exist between the angular sizes (in the sky plane) of eruptive prominences and/or post-eruptive arcades and coronal mass ejections (CMEs) associated with them. New regularities are found that characterize the relationship between the latitude of eruptive prominences (post-eruptive arcades) and the latitude position of CMEs associated with them in the field of view of a coronagraph. An interpretation of these regularities is given. 相似文献
488.
A numerical investigation of the melt flow and heat and mass transfer is carried out at the crystal growth under zero gravity, when the melt detachment from ampoule walls, crystal vibration, and various magnetic fields are active. Specific features of the melt flow are demonstrated depending on the size of a detachment zone adjacent to the crystallization boundary. The velocity of the averaged flow generated by crystal vibration is determined as a function of the vibration intensity. It is shown that the crystal vibration cannot compensate a thermal capillary flow (caused by detachment of the melt from the ampoule wall) and reduce the macrosegregation of impurities. It is shown that the application of steady and rotating magnetic fields are inefficient for all ampoule methods of crystal growth under microgravity conditions. 相似文献
489.
McEneaney W.M. Fitzpatrick B.G. Lauko I.G. 《IEEE transactions on aerospace and electronic systems》2004,40(4):1191-1216
A command and control (C/sup 2/) problem for military air operations is addressed. Specifically, we consider C/sup 2/ problems for air vehicles against ground-based targets and defensive systems. The problem is viewed as a stochastic game. We restrict our attention to the C/sup 2/ level where the problem may consist of a few unmanned combat air vehicles (UCAVs) or aircraft (or possibly teams of vehicles), less than say, a half-dozen enemy surface-to-air missile air defense units (SAMs), a few enemy assets (viewed as targets from our standpoint), and some enemy decoys (assumed to mimic SAM radar signatures). At this low level, some targets are mapped out and possible SAM sites that are unavoidably part of the situation are known. One may then employ a discrete stochastic game problem formulation to determine which of these SAMs should optimally be engaged (if any), and by what series of air vehicle operations. We provide analysis, numerical implementation, and simulation for full state-feedback and measurement feedback control within this C/sup 2/ context. Sensitivity to parameter uncertainty is discussed. Some insight into the structure of optimal and near-optimal strategies for C/sup 2/ is obtained. The analysis is extended to the case of observations which may be affected by adversarial inputs. A heuristic based on risk-sensitive control is applied, and it is found that this produces improved results over more standard approaches. 相似文献
490.
L. G. Blomberg J. A. Cumnock 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2004,33(12):2161-2165
Mercury has a small but intriguing magnetosphere. In this brief review, we discuss some similarities and differences between Mercury’s and Earth’s magnetospheres. In particular, we discuss how electric and magnetic field measurements can be used as a diagnostic tool to improve our understanding of the dynamics of Mercury’s magnetosphere. These points are of interest to the upcoming ESA-JAXA BepiColombo mission to Mercury. 相似文献