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921.
The volatile species released in the coma are an important clue to the composition of the cometary nucleus ices. Their identification and the measurement of their abundances is possible by remote sensing. Considerable progress has been made recently using radio and infrared spectroscopy, especially with the observations of the two exceptional comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp).) 24 molecules likely to be parent molecules outgassed from the nucleus have now been identified. Significant upper limits exist for many other species, and the presence of unidentified lines suggests that further species are to be identified. In addition, isotopic varieties have been observed for hydrogen, carbon, nitrogen and sulphur. We will review these results with a special emphasis on the reliability of the identifications and of the molecular production rate determinations. A critical point is to assess whether a given species is a genuine parent molecule outgassed from nuclear ices, or is a secondary product coming from grains or from gas-phase photochemistry. Ground-based spectral imaging, such as radio interferometry, may help resolving this problem. This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
922.
Robert J. Rutten 《Space Science Reviews》1998,85(1-2):269-280
This “rapporteur” report discusses the solar photosphere and low chromosphere in the context of chemical composition studies. The highly dynamical nature of the photosphere does not seem to jeopardize precise determination of solar abundances in classical fashion. It is still an open question how the highly dynamical nature of the low chromosphere contributes to first ionization potential (FIP) fractionation. This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
923.
用自适应蚁群算法求解集中网站的WEB搜索优化策略 总被引:1,自引:1,他引:0
马力 《沈阳航空工业学院学报》2010,27(2):64-68
如何提高web信息搜索的效率,改善搜索的性能,是信息检索领域一个重要的研究课题。首先利用自适应蚁群算法求得针对特定类别的最小集中网站集,再通过网页聚类,找到能获得最完全而准确信息的网页序列,从而提高web搜索的速度和准确率。 相似文献
924.
925.
月球车热管理系统是为了确保车辆在恶劣的月球环境下正常工作的重要系统,质量是其重要的评价指标。通过对热管理系统模型的热网络分析,文章对影响月球车热管理系统质量的几个重要因素,如辐射器出口温度、热回路管径、冷媒流量、电控制元件、管路设计和阀门应用等分别进行计算,分析了这些因素对系统质量的影响规律;在此基础上,根据探月车辆模型环境参数和工作参数,利用搜索寻优法确定了使系统质量最小的最佳系统结构参数和操作参数,实现了热管理系统的初步质量优化分析。研究结果对于工程应用具有一定的参考意义。 相似文献
926.
为保证月球探测器进入姿态调整段时具有充分的高度与速度余量,本文提出一种基于控制变量参数化的月球探测器动力下降段最优轨迹求解方法。在三维探测器软着陆动力学模型基础上,将月球探测器软着陆制导律设计等效为燃料最优约束下的探测器俯仰角控制问题,利用控制变量参数化(Control Variables Parameterization, CVP)方法将该控制问题中的控制变量与约束条件转化为非线性规划问题求解,并引入时间尺度变换,将着陆时间序列加入待规划参数,进而求得满足精度的最优数值解。蒙特卡罗仿真实验表明,与传统的显式制导律相比,本文提出的参数化制导方法在动力下降段燃料更省,动力下降段的起始高度在±20%范围内波动时,仍能以高精度速度和高度指标完成末制导。 相似文献
927.
928.
《中国航空学报》2021,34(5):79-92
In this paper, the confinement characteristic of primary electrons in the non-axisymmetric discharge chamber of annular ion thruster is investigated by a three-dimensional (3D) non-self-consistent particle tracking model with Monte Carlo Collision (MCC) method. The results show that: The density of primary electrons upstream of ion optics on cathode axis is about 5–50 times higher than that of the other side, which means that the density of primary electrons is obviously non-axisymmetric. The channel width has a significant effect on the average density and uniformity of primary electrons. The average density can be increased by nearly 1.5 times under the appropriate channel width, meanwhile, the variance of density distribution can be reduced by more than 2 times. This is because that increasing the channel width can improve the average confinement length of primary electrons greatly. Furthermore, there is an infection point in the increase of primary electron average confinement length with the channel width, which caused by the significant change of magnetic field structure. Under the case after the inflection point, primary electron mainly (more than 50%) moves to the channel center, which makes the average confinement length of primary electrons and their number in the discharge chamber increase largely. 相似文献
929.
月球表面土壤温度仿真分析 总被引:1,自引:0,他引:1
文章尝试对月表一定深度的土壤的温度变化状况进行模拟分析。首先简要介绍了月壤热物性的探测结果,针对月球表面具有一定深度的土壤层建立了土壤温度变化的一维数学控制方程,探讨了数值求解的方法,并根据月壤探测样本的文献对热物性参数进行了归纳。最后对5个着陆区进行了月壤温度计算和结果分析,以协助深空探测工作的开展。 相似文献
930.