上游太阳风中高能电子向同步轨道区的传输

通常认为，同步轨道区的电子通量增加是由于磁暴或者上游太阳风高速流的扰动所引起．近来的观测表明，起源于太阳活动的行星际高能电子也是引起同步轨道电子通量增加的重要原因之一．Zhao等在研究2000年7月14日太阳剧烈活动时发现，同步轨道区相对论电子通量巨幅增加时没有观察到上游太阳风高速流的扰动，并且磁暴发生在电子通量事件之后．采用解析磁场模型和实际磁场模型(T96模型)模拟来自太阳的相对论电子在磁尾中的运动特性．计算结果表明，当行星际磁场南向时，进入到磁尾的行星际相对论电子可以从较远的磁尾区域运动到同步轨道区域．这一研究结果从理论上论证了起源于太阳活动的高能电子可以对同步轨道区相对论电子通量的增加产生重要的作用．     

Domain evolution and functional diversification of sulfite reductases

Sulfite reductases are key enzymes of assimilatory and dissimilatory sulfur metabolism, which occur in diverse bacterial and archaeal lineages. They share a highly conserved domain "C-X5-C-n-C-X3-C" for binding siroheme and iron-sulfur clusters that facilitate electron transfer to the substrate. For each sulfite reductase cluster, the siroheme-binding domain is positioned slightly differently at the N-terminus of dsrA and dsrB, while in the assimilatory proteins the siroheme domain is located at the C-terminus. Our sequence and phylogenetic analysis of the siroheme-binding domain shows that sulfite reductase sequences diverged from a common ancestor into four separate clusters (aSir, alSir, dsr, and asrC) that are biochemically distinct; each serves a different assimilatory or dissimilatory role in sulfur metabolism. The phylogenetic distribution and functional grouping in sulfite reductase clusters (dsrA and dsrB vs. aSiR, asrC, and alSir) suggest that their functional diversification during evolution may have preceded the bacterial/archaeal divergence.     

Trojans in habitable zones

With the aid of numerical experiments we examined the dynamical stability of fictitious terrestrial planets in 1:1 mean motion resonance with Jovian-like planets of extrasolar planetary systems. In our stability study of the so-called "Trojan" planets in the habitable zone, we used the restricted three-body problem with different mass ratios of the primary bodies. The application of the three-body problem showed that even massive Trojan planets can be stable in the 1:1 mean motion resonance. From the 117 extrasolar planetary systems only 11 systems were found with one giant planet in the habitable zone. Out of this sample set we chose four planetary systems--HD17051, HD27442, HD28185, and HD108874--for further investigation. To study the orbital behavior of the stable zone in the different systems, we used direct numerical computations (Lie Integration Method) that allowed us to determine the escape times and the maximum eccentricity of the fictitious "Trojan planets."     

Beyond the principle of plentitude: a review of terrestrial planet habitability

We review recent work that directly or indirectly addresses the habitability of terrestrial (rocky) planets like the Earth. Habitability has been traditionally defined in terms of an orbital semimajor axis within a range known as the habitable zone, but it is also well known that the habitability of Earth is due to many other astrophysical, geological, and geochemical factors. We focus this review on (1) recent refinements to habitable zone calculations; (2) the formation and orbital stability of terrestrial planets; (3) the tempo and mode of geologic activity (e.g., plate tectonics) on terrestrial planets; (4) the delivery of water to terrestrial planets in the habitable zone; and (5) the acquisition and loss of terrestrial planet carbon and nitrogen, elements that constitute important atmospheric gases responsible for habitable conditions on Earth's surface as well as being the building blocks of the biosphere itself. Finally, we consider recent work on evidence for the earliest habitable environments and the appearance of life itself on our planet. Such evidence provides us with an important, if nominal, calibration point for our search for other habitable worlds.     

Toward optimizing lighting as a countermeasure to sleep and circadian disruption in space flight

Light is being used as a pre-launch countermeasure to circadian and sleep disruption in astronauts. The effect of light on the circadian system is readily monitored by measurement of plasma melatonin. Our group has established an action spectrum for human melatonin regulation and determined the region of 446-477 nm to be the most potent for suppressing plasma melatonin. The aim of this study was to compare the efficacy of 460 and 555 nm for suppressing melatonin using a within-subjects design. Subjects (N=12) were exposed to equal photon densities (7.18 x 10(12) photons/cm2/s) at 460 and 555 nm. Melatonin suppression was significantly stronger at 460 nm (p<0.02). An extension to the action spectrum showed that 420 nm light at 16 and 32 microW/cm2 significantly suppressed melatonin (p<0.04 and p<0.002). These studies will help optimize lighting countermeasures to circadian and sleep disruption during spaceflight.     

Physiological and biomechanical considerations for a human Mars mission

Evolving on Earth has made humans perfectly adapted, both physiologically and biomechanically, to its gravity and atmospheric conditions. Leaving the Earth and its protective environment, therefore, results in the degradation of a number of human systems. Long-duration stays on the International Space Station (ISS) are accompanied by significant effects on crew's cardiovascular, vestibular and musculoskeletal systems. Bone loss and muscle atrophy are experienced at a rate of 1-3% and 5% per month respectively, while VO2 (oxygen consumption) measurements are reduced by approximately 25% after a few weeks in space. If these figures are simply extrapolated, a future human mission to Mars will be seriously jeopardised and crews may find they cross the threshold of bone and muscle loss and aerobic fitness--ultimately with them being unable to return to Earth. When arriving on Mars, considerable biomechanical alterations will also occur. Optimum walking speeds will be approximately 30% lower and transitioning from a walk to a run will occur at a speed 25% slower. Peak vertical forces will be reduced by as much as 50%, while stride length, stride time and airborne time will all increase. On Mars, half as much energy will be required to travel the equivalent distance on Earth and it will be 65% more economical to run rather than to walk.     

全球定位系统用作洲际导弹制导系统鉴定工具的测量性能

八十年代初期,在两次民兵Ⅲ型导弹发射中(编号为PVM-18,19),试飞了一种四信道全球定位系统(GPS)接收机——弹载接收设备(MBRS)。这两次发射试验是称为“飞-2”系列飞行中的一部分。每次带两个NS20惯性测量装置。这两次试验的主要目的是在洲际导弹实际飞行环境中验证、测定全球定位系统的性能。本文概括了这两次飞行中GPS性能的分析结果。由此验证了如何利用这种质量的数据来高精度地鉴定制导误差,并用该数据来识别出飞行过程中各种制导误差。这两次试飞所得到的GPS数据证实了短期精度达到了预期能力。典型的随机误差为2英尺~*和0.01英尺/秒(一秒平均)。有了这一精度就可利用GPS作为外部测量仪器,对自由段中摆式积分陀螺加速度表(PIGA)的磁敏感效应进行直接观测。实现这种观测还是第一次。由于缺乏合适的标准,尚不容易进行绝对精度鉴定。显然,GPS的精度比同时参与导弹跟踪的雷达(多台)的好,拟合后残差非常一致,说明总精度很高。因此可得出结论,GPS的总精度达到了预期性能指标。 (本文常用缩略语AIRS——高级惯性基准球,BREW——民兵导弹上装的第二惯性测量装置,CSDL——Charles Stark Draper实验室,ECI——地心惯性座标系,GPS——全球定位系统,MBRS——导弹装载的GPS接收机,MISTRAM——米斯特拉姆,高精度无线电外测系统,PIGA——摆式积分陀螺加速度表,PVB——民兵导弹试验编号,TI——德克萨斯仪器公司)     

先进的航天发动机试验

本文报导一种航天发动机的高空模拟点火试验。这种发动机综合利用了几种先进技术:90%固体燃料的端羟基聚丁二烯/铝粉/过氯酸铵/奥克托金推进剂;在凯夫拉壳体内的头端满装药;无支撑的整体喉部和收敛段入口;特轻的喷管和用充气膨胀的作动筒展开的可延伸出口锥。这次试验结束了法国欧洲动力公司和美国联合工艺公司化学系统分部之间为期两年的分工协作的研究和试制工作。所达到的比冲值被认为是复合推进剂在样机发动机中的一个最高记录。