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141.
Droppert PM 《Journal of the British Interplanetary Society》1990,43(1):19-24
Exposure of astronauts to microgravity leads to the loss of calcium from weightbearing bones. Prolonged exposure, e.g., during a journey to Mars, may present problems on return to Earth, with increased risk of fractures and premature osteoporosis in later life. The precise mechanisms of calcium loss have yet to be determined although a key feature is the absence of mechanical loading. Countermeasures aimed at reducing calcium loss to acceptable levels include the use of exercise, drugs, dietary modifications and inertia suits such as the Soviet "Penguin" suit. Missions of a number of years may, however, require the development of artificial gravity on a spacecraft. The country that first solves the physiological problems of man in space and, in particular, skeletal calcium loss, will almost certainly be the first to be able to put a man on Mars. 相似文献
142.
Two bed rest analog studies of space flight were performed; one 14 d and the other 28 d in duration. Exercise response was studied in detail during the 28 d study and following both the 14 d and 28 d studies. This paper relates the results of these studies to physiologic changes noted during and following space flight. The most consistent change noted after both bed rest and space flight is an elevated heart rate during exercise. A second consistent finding is a postflight or postbed rest reduction in cardiac stroke volume. Cardiac output changes were variable. The inability to simulate inflight activity levels and personal exercise makes a direct comparison between bed rest and the results from specific space flights difficult. 相似文献
143.
Whedon GD Lutwak L Reid J Rambaut P Whittle M Smith M Leach C 《Acta Astronautica》1975,2(3-4):297-309
Prediction that the various stresses of flight, particularly weightlessness, would bring about significant derangements in the metabolism of the musculoskeletal system has been based on various observations of long-term immobilized or inactive bed rest. The only attempt at controlled measurement of metabolic changes in space prior to Skylab, a study during the 14-day Gemini VII flight, revealed rather modest losses of important elements. The three astronauts of Skylab II consumed a planned day-by-day, quite constant, dietary intake of major metabolic elements in mixed foods and beverages and provided virtually complete collections of excreta for 31 days preflight, during the 28 days inflight, and for 17 days postflight. Analyses showed that, in varying degree among the crewmen, urinary calcium increased gradually during flight in a pattern similar to that observed in bed-rest studies: the mean plateau peak of urinary calcium excretion in the latter part of flight was double preflight levels. Fecal calcium excretion did not change significantly, but calcium balance, owing to the urinary calcium rise, became either negative or less positive than in preflight measurement. Increased excretion and negative balance of nitrogen and phosphorus indicated appreciable loss of muscle tissue in all three crewmen. Significant losses also occurred inflight in potassium, sodium, and magnesium. Based on the similarity in pattern and degree between these observations and those in bed rest of the losses in calcium, phosphorus, and nitrogen, musculoskeletal integrity would not be threatened in space flights of up to at least 3 months. However, if similar changes occur, indicative of continuing losses of these elements, in the planned Skylab flights for considerably more than 28 days, concern for capable musculoskeletal function should be serious for flights of very many months' duration, and greater research attention will need to be given to development of protective counter-measures. 相似文献
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146.
最少燃料消耗的固定推力共面轨道变轨研究 总被引:10,自引:3,他引:10
本文研究了推力固定条件下,从圆轨道进入共面圆轨道一次入轨最节省燃料的推力方向控制策略。这类问题都可归结为两点边值问题,对自由初值的选取作了讨论,并采用打靶法迭代求解。计算了从停泊轨道到同步转移道以及两个过地圆轨道之间的最优转移,获得满意的结果。 相似文献
147.
蜂窝夹层结构常见制造缺陷分析 总被引:3,自引:0,他引:3
蜂窝夹层结构已成为卫星结构设计中广泛应用的重要材料。基于蜂窝夹层结构制造过程的复杂性和不可逆性,注重工艺控制对产品质量的保证是非常重要的。文章对蜂窝夹层结构制造过程中,由于工艺条件不当造成的缺陷和改进方法进行了分析。 相似文献
148.
在将纤维预制体视为“毛细管束”的基础上,采用达西定律建立了离心加速场中金属液浸渗纤维预制体的浸渗动力学模型。采用该模型分析了离心加速场中金属液浸渗纤维预制体的临界转速,考察了金属液浅注量、设备转速、铸件高度、金属液原如外半径以及纤维预制体孔隙率对离心加速场中金属 浸润纤维预制体的影响。结果表明,金属液浸润纤维预制体的临界转速只与临界压力、金属液的浇注量、铸件高度以及纤维预制休性质有关;增加金属液浇注量、设备转速及金属液原始外半径,降低铸件高度、预制体纤维体积分数,有利于金属液的浸渗。 相似文献
149.
随着成像光谱仪的发展,遥感数据量越来越大,这对电子学系统中的数据采集、传输和处理带来了一定的困难。基于目前技术和数据的实用性两方面考虑,采用编程选择波段的方法能够很好地解决数据量日益增大的问题,国内外的一些方案中已经对此有了一定的设想,即通过遥控指令改变波段数目、光谱取样间隔或波段的重组,这一切都要求有一个灵活性强、通用性好的数据采集系统,能够对不同的选择情况正确地收集用户所需的数据。文中对数据采集的可行性进行了研究,并提出了一个较为容易实现的方案。 相似文献
150.