Lunar precursor missions for human exploration of Mars--III: studies of system reliability and maintenance

Discussions of future human expeditions into the solar system generally focus on whether the next explorers ought to go to the Moon or to Mars. The only mission scenario developed in any detail within NASA is an expedition to Mars with a 500-day stay at the surface. The technological capabilities and the operational experience base required for such a mission do not now exist nor has any self-consistent program plan been proposed to acquire them. In particular, the lack of an Abort-to-Earth capability implies that critical mission systems must perform reliably for 3 years or must be maintainable and repairable by the crew. As has been previously argued, a well-planned program of human exploration of the Moon would provide a context within which to develop the appropriate technologies because a lunar expedition incorporates many of the operational elements of a Mars expedition. Initial lunar expeditions can be carried out at scales consistent with the current experience base but can be expanded in any or all operational phases to produce an experience base necessary to successfully and safely conduct human exploration of Mars.     

Precipitating Protons and Their Role in Ionization of the Polar Ionosphere

Spatial distributions of the electron density in the latitude range 60°–90° N were calculated on the basis of a physical model of the E and lower Fregions of the high-latitude ionosphere using statistical models of auroral proton and electron precipitation. It is shown that precipitating protons can play the key role in the ionization of the Eregion in the dusk and midnight sectors of the auroral oval. However, quantitative estimates of the contribution of protons to the ionization depend on the used statistical models of electron precipitation. Comparison of the electron density profiles calculated for two incoherent scatter radars, EISCAT (Tromsö) and ESR (Svalbard), for simultaneous precipitation of electrons and protons and for electron precipitation only show that the influence of protons is the most significant in the dusk sector over the EISCAT radar and in the midnight sector over the ESR radar. The results presented indicate the need to take protons into account when radar data are used to derive precipitating electron spectra.     

Exobiology and future Mars missions: The search for Mars'' earliest biosphere

The primordial Mars may have possessed a thick carbon dioxide atmosphere, with liquid water common on the surface, similar in many ways to the primordial Earth. During this epoch, billions of years ago, the surface of Mars could have been conducive to the origin of life. It is possible that life evolved on Mars to be later eliminated as the atmospheric pressure dropped. Analysis of the surface of Mars for the traces of this early martian biota could provide many insights into the phenomenon of life and its coupling to planetary evolution.     

Theoretical consideration of the chemical pathways for radiation-induced strand breaks.

A theoretical approach to the understanding of the biochemical mechanisms of indirect action of ionizing radiation on SV40 DNA in aqueous solution is presented. The extent of OH attack on the sugar moiety and bases has been calculated. A realistic model for the DNA (in B form) based on available X-ray diffraction data is used and specific reaction sites for the OH radicals are obtained. A Monte Carlo scheme is used to follow the diffusion and reaction of the OH radicals. Effects of track structure have been considered and the single strand break D37 values for 14 MeV electrons (low-LET) and 670 MeV/u and 40 MeV/u neon particles are presented. Calculated results are in agreement with available experimental data. It has been found that regardless of the qualities of radiation, 80% of the OH attack on DNA is on the bases and 20% is on the deoxyribose. From probability considerations only, it appears that the number of double strand breaks varies linearly with dose.     

Gravity and perceptual stability during translational head movement on earth and in microgravity

We measured the amount of visual movement judged consistent with translational head movement under normal and microgravity conditions. Subjects wore a virtual reality helmet in which the ratio of the movement of the world to the movement of the head (visual gain) was variable. Using the method of adjustment under normal gravity 10 subjects adjusted the visual gain until the visual world appeared stable during head movements that were either parallel or orthogonal to gravity. Using the method of constant stimuli under normal gravity, seven subjects moved their heads and judged whether the virtual world appeared to move “with” or “against” their movement for several visual gains. One subject repeated the constant stimuli judgements in microgravity during parabolic flight. The accuracy of judgements appeared unaffected by the direction or absence of gravity. Only the variability appeared affected by the absence of gravity. These results are discussed in relation to discomfort during head movements in microgravity.     

Neutral buoyancy and sleep-deprived serum factors alter expression of cytokines regulating osteogenesis

We examined expression of genes associated with cytokine production, and genes implicated in regulating bone metabolism, in bone stromal and osteoblast cells incubated under standard ground conditions and under conditions of neutral buoyancy, and in the presence/absence of serum from normal or sleep-deprived mice. We observed a clear interaction between these two conditions (exposure to neutral buoyancy and serum stimulation) in promoting enhanced osteoclastogenesis. Both conditions independently altered expression of a number of cytokines implicated in the regulation of bone metabolism. However, using stromal cells from IL-1 and TNFx cytokine(r) KO mice, we concluded that the increased bone loss under microgravity conditions was not primarily cytokine mediated.     

基于哈希链表和时间链表的HTTP代理缓存机制的实现

随着互联网的快速发展，越来越多的用户需要访问Internet资源，局域网人的众多用户如何能够通过一条Internet连接进行快速的网络浏览？这涉及到WEB代理服务器的核心部分--HTTP缓存系统。HTTP代理服务器的缓存系统将用户浏览的网页保存在代理服务器中，一旦有用户浏览相同的网页并且该网页还没有更新，则代理服务器直接将网页传递给用户，无需重新下载。因此，缓存明显加快了浏览速度。本文首先介绍了代理服务器的缓存原理，然后对缓存机制的具体功能进行了划分，最后给出基于哈希链表和时间链表的缓存系统的实现方法。     

华东地区围护结构节能研究

根据南京市的典型年逐时气象参数，对于该市的某一座高层建筑物，应用自编的逐时动态负荷模拟程序，输入围护结构（窗户、墙体和屋顶）相关的物理参数及其与周围环境的关系（地点及朝向等），计算围护结构冬季采暖期散热量及夏季制冷得热量。结果表明，采用双层塑窗代替单层塑窗，使用节能墙体代替传统墙体能够使得冬季围护结构传散热量减少51％，夏季围护结构总得热量减少46％。因此，这一结果为华东地区建筑节能技术提供了理论依据。     

无穷观问题的研究(Ⅲ)——‘每一''与‘所有''

本组系列论文（I）－（Ⅴ）从数学和认识论的角度系统地研究了无穷观问题的历史发展和现状，确立了无穷观背景世界的三分法原则，指出了两种穷观相互排斥的局限性，形成了统一两种无穷于同一框架中的基本观点，并建立了一个统一实无限与潜无限于同一框架中的公理集合论系统APAS。