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941.
This paper is devoted to the investigation of the role of multiply scattered light of different wavelengths in clear twilight, as well as to the influence of multiple scattering and scattering by atmospheric aerosol particles on the polarization of the twilight sky background during this period. The work is based on wide-angle polarimetric CCD-observations of the twilight sky near zenith in the U, B, V, and R bands, which were carried out in summer 2000 at the South Laboratory of the Moscow Shternberg Astronomical Institute. Based on the data obtained, we have determined the fraction of single-scattered light in clear twilight in the zenith for the U, B, and V bands and have estimated the lower limit of this value for the R band. The analysis of color and polarimetric evolution of the sky background was made taking into account single and multiple scattering properties. The characteristic height of the upper boundary of the tropospheric aerosol layer in the Earth's atmosphere was also obtained. 相似文献
942.
This paper analyzes superconducting (SC) coils technology to improve satellite electric propulsion. SC magnets can generate very high magnetic fields with extremely low ohmic losses, low electric power and PCU weight. Moreover, the weight of SC magnet coils is quite low compared with conventional or permanent magnets capable of producing the same field strength. For instance, to obtain a 0.4 T magnetic field strength, typical of applied field magnetoplasmadynamic thrusters (AF-MPD), based on the Lorentz force, and with thrust in the range of 1 N, this paper shows that only a 40 g coil SC weight is needed, whereas an equivalent field copper winding would weigh about 36 kg. Using SC technology it is shown that the limited magnetic induction provided by a permanent magnet may be raised and also that it is possible to obtain high magnetic fields (of order of several Tesla) with fewer turns, drastically reducing coil volume, weight and complexity compared to conventional winding.
Superconductivity at low temperature (LTSC) requires liquid helium at 4.2 K to produce very high current densities: the thermal analysis in this paper shows that, depending on satellite and thruster, high temperature superconductors (HTSC) wires are sometimes better than LTSC because these can support lower current densities but with a critical superconductivity temperature higher than for LHe, requiring LN2 at 77 K, easier to maintain during the whole mission.
Finally, this paper shows that implementing SC cryogenic technology on a satellite can be achieved with current active coolers technology. 相似文献
943.
This paper deals with the optimization of the ascent trajectories for single-stage-sub-orbit (SSSO), single-stage-to-orbit (SSTO), and two-stage-to-orbit (TSTO) rocket-powered spacecraft. The maximum payload weight problem is studied for different values of the engine specific impulse and spacecraft structural factor.The main conclusions are that: feasibility of SSSO spacecraft is guaranteed for all the parameter combinations considered; feasibility of SSTO spacecraft depends strongly on the parameter combination chosen; not only feasibility of TSTO spacecraft is guaranteed for all the parameter combinations considered, but the TSTO payload is several times the SSTO payload.Improvements in engine specific impulse and spacecraft structural factor are desirable and crucial for SSTO feasibility; indeed, aerodynamic improvements do not yield significant improvements in payload.For SSSO, SSTO, and TSTO spacecraft, simple engineering approximations are developed connecting the maximum payload weight to the engine specific impulse and spacecraft structural factor. With reference to the specific impulse/structural factor domain, these engineering approximations lead to the construction of zero-payload lines separating the feasibility region (positive payload) from the unfeasibility region (negative payload). 相似文献
944.
The major features of President Obama’s new US space policy are discussed and particular aspects are compared with those of the previous Bush policy. In many cases there are similarities of substance but the tone of the latest policy is more outward looking and inclusive, with a far greater emphasis on cooperation, both internationally and with the private sector. And while some complain that the policy does not sufficiently emphasize US leadership, a statement by President Obama on the day of its release makes clear that this remains a paramount goal. Serious questions remain, however, about implementation of the new policy, particularly where it requires substantial government funding such as human spaceflight and the restructured weather/environmental satellite programs. 相似文献
945.
Samuel Goroshin Francois-David Tang Andrew J. Higgins John H.S Lee 《Acta Astronautica》2011,68(7-8):656-666
The propagation of laminar dust flames in suspensions of iron in gaseous oxidizers was studied in a low-gravity environment onboard a parabolic flight aircraft. The reduction of buoyancy-induced convective flows and particle settling permitted the measurement of fundamental combustion parameters, such as the burning velocity and the flame quenching distance over a wide range of particle sizes and in different gaseous mixtures. Experimentally measured flame speeds and quenching distances were found in good agreement with theoretical predictions of a simplified analytical model that assumes particles burning in a diffusive mode. However, the comparison of flame speeds in oxygen–argon and oxygen–helium iron suspensions indicates the possibility that fine micron-sized particles burn in the kinetic mode. Furthermore, when the particle spacing is large compared to the scale of the reaction zone, a theoretical analysis suggests the existence of a new so-called discrete flame propagation regime. Discrete flames are strongly dependent on particle density fluctuations and demonstrate directed percolation behavior near flame propagation limits. The experimental observation of discrete flames in particle suspensions will require low levels of gravity over extended periods available only on orbital platforms. 相似文献
946.
A method of estimating the stability of the problem of orbit determination under disturbances caused by finite accuracy of calculation and geographical siting of observation stations is proposed. 相似文献
947.
948.
949.
S.V. Singh R.V. Reddy G.S. Lakhina 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009,43(12):1940-1944
Broadband electrostatic noise (BEN) is commonly observed in different regions of the Earth’s magnetosphere, eg., auroral region, plasma sheet boundary layer, etc. The frequency of these BENs lies in the range from lower hybrid to the local electron plasma frequency and sometimes even higher. Spacecraft observations suggest that the high and low-frequency parts of BEN appear to be two different wave modes. There is a well established theory for the high-frequency part which can be explained by electrostatic solitary waves, however, low-frequency part is yet to be fully understood. The linear theory of low-frequency waves is developed in a four-component magnetized plasma consisting of three types of electrons, namely cold background electron, warm electrons, warm electron beam and ions. The electrostatic dispersion relation is solved, both analytically and numerically. For the parameters relevant to the auroral region, our analysis predict excitation of electron acoustic waves in the frequency range of 17 Hz to 2.6 kHz with transverse wavelengths in range of (1–70) km. The results from this model may be applied to explain some features of the low-frequency part of the broadband electrostatic noise observed in other regions of the magnetosphere. 相似文献
950.
The results of modeling the thermal deformations of a space radio telescope’s reflecting surface are presented in the paper. Calculations were performed for the versions of the most unfavorable telescope illumination by the Sun. 相似文献