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761.
H K Charles T J Beck H S Feldmesser T C Magee T S Spisz V L Pisacane 《Acta Astronautica》2001,49(3-10):447-450
An advanced, multiple projection, dual energy x-ray absorptiometry (AMPDXA) scanner system is under development. The AMPDXA is designed to make precision bone and muscle loss measurements necessary to determine the deleterious effects of microgravity on astronauts as well as develop countermeasures to stem their bone and muscle loss. To date, a full size test system has been developed to verify principles and the results of computer simulations. Results indicate that accurate predictions of bone mechanical properties can be determined from as few as three projections, while more projections are needed for a complete, three-dimensional reconstruction. 相似文献
762.
I. V. Chashei V. I. Shishov S. A. Tyul’bashev A. V. Glyantsev I. A. Subaev 《Cosmic Research》2013,51(1):23-28
The results of observations of interplanetary scintillations of a statistical ensemble of radio sources in the period of 2007–2011 are presented. Observation were carried out in the monitoring regime with the BSA LPI radio telescope at the frequency 111 MHz. Fluctuations of radio emission flux of all sources (a few hundred in total) were recorded 24 hours a day. Those sources were investigated, which had a scintillating flux greater than 0.2 Jy and fell within the sky band of 8° width in declination, corresponding to radio telescope’s 16-beam system. The statistical ensemble of radio sources is characterized by the mean variance of a scintillating radiation flux, which is proportional to the squared scintillation index. It follows from the obtained data that the radial dependence of a mean scintillation index during a deep solar activity minimum of 2008–2009 occurs to be weaker than one could expect in the case of spherically symmetric geometry of the solar wind. Suppression of a radial dependence of the mean scintillation index is explained by the effect of the heliospheric current sheet, which reveals itself in a high density of solar wind’s turbulent plasma in the helioequator plane. It is shown that the level of scintillations, averaged over monthly series of observations, was changing synchronously with the solar activity level. 相似文献
763.
The electric field distribution around a charged satellite in a rarefied magnetospheric plasma influences greatly the densities and trajectories of particles measured by onboard instruments. The simulation of macroparameters of thermal plasma near the moving charged satellite, which is necessary for correction of experimental measurements, encounters considerable computational difficulties. In this work, two three-dimensional models of the electric field distribution around the satellite are considered under the conditions when the Debye length is comparable to the geometrical size of the spacecraft. In the first model a system of hydrodynamic equations of continuity and motion was used, which was solved jointly with the Poisson equation. In the second model the hydrodynamic equation of motion was used for analyzing the motion of large particles by means of the method of particles in a cell. The numerical algorithms and the results of calculations of the potential near the satellite, as well as the distributions of densities of electrons and ions and of volume charge, are considered. The results of test calculations for some situations in the ambient plasma are presented, and the influence of the spatial electric field distribution on the thermal plasma measurements is considered. 相似文献
764.
Z Nakhost M Karel V J Krukonis 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1987,7(4):29-38
Protein isolate obtained from green algae (Scenedesmus obliquus) cultivated under controlled conditions was characterized. Molecular weight determination of fractionated algal proteins using SDS-polyacrylamide gel electrophoresis revealed a wide spectrum of molecular weights ranging from 15,000 to 220,000. Isoelectric points of dissociated proteins were in the range of 3.95 to 6.20. Amino acid composition of protein isolate compared favorably with FAO standards. High content of essential amino acids leucine, valine, phenylalanine and lysine makes algal protein isolate a high quality component of CELSS diets. To optimize the removal of algal lipids and pigments supercritical carbon dioxide extraction (with and without ethanol as a co-solvent) was used. Addition of ethanol to supercritical CO2 resulted in more efficient removal of algal lipids and produced protein isolate with a good yield and protein recovery. The protein isolate extracted by the above mixture had an improved water solubility. 相似文献
765.
Ultrastructural and growth indices of Chlorella culture in multicomponent aquatic systems under space flight conditions. 总被引:2,自引:0,他引:2
A F Popova K M Sytnik E L Kordyum G I Meleshko V N Sychev M A Levinskykh 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1989,9(11):79-82
Submicroscopic organization of Chlorella cells cultivated under space flight conditions in three-component aquatic system has been studied. Comparison of the experimental cells with that of the controls revealed certain rearrangements of cell organelles particularly, a reduction in the amount of reserve polysaccharides in chloroplasts, increase of cell vacuolization and mitochondrion volume, complication configuration of plasmalemma evaginations and invaginations, and also disturbances in the process of cytokinesis. More over an increase in the number of Chlorella cells infected by bacteria was shown in the experimental variant. No considerable differences were established in the growth characteristics of the experimental and control populations. A comparative cytological analysis revealed general regularities of organelles in Chlorella cells cultivated under space flight condition in the uni- and multicomponent systems. 相似文献
766.
No two rocky bodies offer a better laboratory for exploring the conditions controlling interior dynamics than Venus and Earth. Their similarities in size, density, distance from the sun, and young surfaces would suggest comparable interior dynamics. Although the two planets exhibit some of the same processes, Venus lacks Earth’s dominant process for losing heat and cycling volatiles between the interior and the surface and atmosphere: plate tectonics. One commonality is the size and number of mantle plume features which are inferred to be active today and arise at the core mantle boundary. Such mantle plumes require heat loss from the core, yet Venus lacks a measurable interior dynamo. There is evidence for plume-induced subduction on Venus, but no apparent mosaic of moving plates. Absent plate tectonics, one essential question for interior dynamics is how did Venus obtain its young resurfacing age? Via catastrophic or equilibrium processes? Related questions are how does it lose heat via past periods of plate tectonics, has it always had a stagnant lid, or might it have an entirely different mode of heat loss? Although there has been no mission dedicated to surface and interior processes since the Magellan mission in 1990, near infrared surface emissivity data that provides information on the iron content of the surface mineralogy was obtained fortuitously from Venus Express. These data imply both the presence of continental-like crust, and thus formation in the presence of water, and recent volcanism at mantle hotspots. In addition, the study of interior dynamics for both Earth and exoplanets has led to new insights on the conditions required to initiate subduction and develop plate tectonics, including the possible role of high temperature lithosphere, and a renewed drive to reveal why Venus and Earth differ. Here we review current data that constrains the interior dynamics of Venus, new insights into its interior dynamics, and the data needed to resolve key questions. 相似文献
767.
William M. Folkner Véronique Dehant Sébastien Le Maistre Marie Yseboodt Attilio Rivoldini Tim Van Hoolst Sami W. Asmar Matthew P. Golombek 《Space Science Reviews》2018,214(5):100
The Rotation and Interior Structure Experiment (RISE) on-board the InSight mission will use the lander’s X-band (8 GHz) radio system in combination with tracking stations of the NASA Deep Space Network (DSN) to determine the rotation of Mars. RISE will measure the nutation of the Martian spin axis, detecting for the first time the effect of the liquid core of Mars and providing in turn new constraints on the core radius and density. RISE will also measure changes in the rotation rate of Mars on seasonal time-scales thereby constraining the atmospheric angular momentum budget. Finally, RISE will provide a superb tie between the cartographic and inertial reference frames. This paper describes the RISE scientific objectives and measurements, and provides the expected results of the experiment. 相似文献
768.
Layne CS Lange GW Pruett CJ McDonald PV Merkle LA Mulavara AP Smith SL Kozlovskaya IB Bloomberg JJ 《Acta Astronautica》1998,43(3-6):107-119
The precise neuromuscular control needed for optimal locomotion, particularly around heel strike and toe off, is known to he compromised after short duration (8- to 15-day) space flight. We hypothesized here that longer exposure to weightlessness would result in maladaptive neuromuscular activation during postflight treadmill walking. We also hypothesized that space flight would affect the ability of the sensory-motor control system to generate adaptive neuromuscular activation patterns in response to changes in visual target distance during postflight treadmill walking. Seven crewmembers, who completed 3- to 6-month missions, walked on a motorized treadmill while visually fixating on a target placed 30 cm (NEAR) or 2 m (FAR) from the subject's eyes. Electronic foot switch data and surface electromyography were collected from selected muscles of the right lower limb. Results indicate that the phasic features of neuromuscular activation were moderately affected and the relative amplitude of activity in the tibialis anterior and rectus femoris around toe off changed after space flight. Changes also were evident after space flight in how these muscles adapted to the shift in visual target distance. 相似文献
769.
The electromagnetic radio-frequency emission of the inner region of the Earth's plasmasphere discovered recently by the GEOTAIL satellite [4] and referred to as the kilometric continuum was observed by the INTERBALL-1 satellite (1995–2000) in the 100–500 kHz band in the AKR-X experiment. During a period of low solar activity (1995–1997), this continuum was found leaving the inner plasmasphere at geocentric distances of 2–4R
E as isolated pencil-like (1°–6°) beams located in the magnetic equator plane. During a time of high solar activity (1999–2000), the occurrence of the emission was extremely rare (it was observed only at a considerable fall of this activity). If detected, at the same geocentric distances (2–4R
E) the continuum demonstrated a strongly variable and perturbed character, as well as a considerably larger extension of the beam over the geomagnetic latitude (10°–20° and more). In addition, quasi-periodic (QP) signals, similar to the observed QP emissions of Jupiter, were sometimes detected in this period. The probable nature of the observed features of the kilometric continuum is briefly discussed. 相似文献
770.