高等院校的研究推进维修业的发展

<正>一些高等院校在研的创新性先进技术,将使航空维修业发生巨变,这其中包括先进的监控系统、复合材料的修理和先进检测技术等。未来10年,飞机的监控、修理以及检测方法有可能发生根本性变化,它涉及机载系统的可靠性重新定义、复合材料问题的解决方案以及金属加工技术等领域。     

Terahertz circular dichroism spectroscopy: a potential approach to the in situ detection of life's metabolic and genetic machinery

We propose a terahertz (far-infrared) circular dichroism-based life-detection technology that may provide a universal and unequivocal spectroscopic signature of living systems regardless of their genesis. We argue that, irrespective of the specifics of their chemistry, all life forms will employ well-structured, chiral, stereochemically pure macromolecules (>500 atoms) as the catalysts with which they perform their metabolic and replicative functions. We also argue that nearly all such macromolecules will absorb strongly at terahertz frequencies and exhibit significant circular dichroism, and that this circular dichroism unambiguously distinguishes biological from abiological materials. Lastly, we describe several approaches to the fabrication of a terahertz circular dichroism spectrometer and provide preliminary experimental indications of their feasibility. Because terahertz circular dichroism signals arise from the molecular machinery necessary to carry out life's metabolic and genetic processes, this life-detection method differs fundamentally from more well-established approaches based on the detection of isotopic fractionation, "signature" carbon compounds, disequilibria, or other by-products of metabolism. Moreover, terahertz circular dichroism spectroscopy detects this machinery in a manner that makes few, if any, assumptions as to its chemical nature or the processes that it performs.     

Cenozoic bolide impacts and biotic change in North American mammals

North American mammals experienced a major mass extinction at the Cretaceous/Tertiary (K/T) boundary that is tied unambiguously to the Chicxulub impact event. Immediately afterwards, there was an immense adaptive radiation that greatly expanded taxonomic diversity and the range of body sizes and ecological strategies. However, ties between later, Cenozoic impact events and specific episodes in mammalian evolution cannot be demonstrated. A time series of maximum known crater sizes within 1.0-million-year-long temporal bins is shown not to cross-correlate with five separate measures of taxonomic turnover rate, one measure of change in relative taxonomic composition, and four measures of change in body mass distributions. The lack of correlation persists even after excluding the volatile Paleocene mammalian data, adding dummy data to represent intervals without known craters, or lagging the time series against each other for up to 5 million years. Furthermore, the data fail to support broad-brush correspondences between ages of major (>20 km in diameter) craters and the timing of five key, post-K/T biotic transitions, including medium-sized extinction episodes during the late Paleocene and latest Miocene. The results challenge the idea that extraterrestrial impacts drive all, most, or even many extinction and radiation episodes in terrestrial organisms, and add to other evidence that natural, long-term biotic changes are often independent of changes in the physical environment.     

Fluid inclusion studies of chemosynthetic carbonates: strategy for seeking life on Mars

Fluid inclusions in minerals hold the potential to provide important data on the chemistry of the ambient fluids during mineral precipitation. Especially interesting to astrobiologists are inclusions in low-temperature minerals that may have been precipitated in the presence of microorganisms. We demonstrate that it is possible to obtain data from inclusions in chemosynthetic carbonates that precipitated by the oxidation of organic carbon around methane-bearing seepages. Chemosynthetic carbonates have been identified as a target rock for astrobiological exploration. Other surficial rock types identified as targets for astrobiological exploration include hydrothermal deposits, speleothems, stromatolites, tufas, and evaporites, each of which can contain fluid inclusions. Fracture systems below impact craters would also contain precipitates of minerals with fluid inclusions. As fluid inclusions are sealed microchambers, they preserve fluids in regions where water is now absent, such as regions of the martian surface. Although most inclusions are < 5 microns, the possibility to obtain data from the fluids, including biosignatures and physical remains of life, underscores the advantages of technological advances in the study of fluid inclusions. The crushing of bulk samples could release inclusion waters for analysis, which could be undertaken in situ on Mars.     

Biogenesis and early life on Earth and Europa: favored by an alkaline ocean?

Recent discoveries about Europa--the probable existence of a sizeable ocean below its ice crust; the detection of hydrated sodium carbonates, among other salts; and the calculation of a net loss of sodium from the subsurface--suggest the existence of an alkaline ocean. Alkaline oceans (nicknamed "soda oceans" in analogy to terrestrial soda lakes) have been hypothesized also for early Earth and Mars on the basis of mass balance considerations involving total amounts of acids available for weathering and the composition of the early crust. Such an environment could be favorable to biogenesis since it may have provided for very low Ca2+ concentrations mandatory for the biochemical function of proteins. A rapid loss of CO2 from Europa's atmosphere may have led to freezing oceans. Alkaline brine bubbles embedded in ice in freezing and impact-thawing oceans could have provided a suitable environment for protocell formation and the large number of trials needed for biogenesis. Understanding these processes could be central to assessing the probability of life on Europa.     

Arm end-point trajectories under normal and micro-gravity environments

The purpose of the present experiment was to study the way in which the CNS represents gravitational force during vertical arm pointing movements. Movements in upward and downward directions were executed by two cosmonauts in normal-gravity and weightlessness. Analyses focused upon finger kinematics in the sagittal plane. In normal-gravity, downward direction movements showed smaller curvatures and greater relative times to peak velocity (AT/MT) when compared with upward direction movements. Data from the weightlessness experiments showed that whilst downward movements decreased their curvature during space flight, curvatures of upward movements changed slightly. Furthermore, AT/MT was modified during the first days in micro-gravity for both directions, recovering, however, to pre-flight values after 18 days in space. Results from the present study, provide evidence that gravitational force is centrally treated constituting an important component of the motor plan for vertical arm movements.     

美国TDRSS的发展及其新应用

本文将以1996年第二届TDRSS专题研讨会的内容为主,介绍美国TDRSS的现状,新研制星TDRS—H,I,J的主要技术指标改进,高精度定轨技术,TDRSS工作频段向Ka波段的过渡,几代用户转发器的发展情况,并较全面地介绍近几年来试验成功的若干TDRSS新应用,例如用于极区数据采集站高速数据中继,长期气球、无人飞行器和海上浮标的数据采集,运载火箭全程遥测数据的转发,遥感飞机高速图象数据和前向信息的传递以及正在研究的利用TDRSS对运载火箭发送靶场安全指令,为小卫星服务的按需接址业务等。     

NASA深空网（DSN）的现状及发展趋势

详细介绍了美国深空网的发展过程、目前的状况以及未来的主要发展计划。     

欧洲空间操作中心的精密轨道确定

目前,欧洲空间操作中心正在进行着一项需4年时间的开发计划,以支持未来的需要高精度轨道确定的ESA近地飞行任务。该工作是围绕着欧洲空间操作中心已开发的供轨道确定和误差分析用的一个软件系统进行的,该软件含有适合于各种轨道摄动和测量类型的先进模型。除了卫星轨道参数外,它还能对地球物理参数和测地参数进行估计和误差分析。     

哥达德航天中心天基、地基测控网的Ka频段过渡计划

1　工作于Ka频段的驱动因素NASA天基测控网 (SN)目前通过TDRSS卫星和白沙设施 (WSC)的地面设备以S和Ku频段为各种近地轨道科学卫星提供通信服务。同样 ,NASA地基测控网 (GN)利用分布于全球的地面站通过直接对地的S和X频段为科学卫星提供通信服务。按NASA预测许多科学卫星提出的测控需求 ,需要 1Gbit/s或更高的数传速率 ,这些快速上升的数据吞吐速率不可能以现用的Ku和X频段来满足。此外 ,NASA分配给TDRSSKu频段前、返向链路频率在国际电信联盟 (ITU)只是二级用户 ,固定卫星业务 (主用户 )地球站地球 -空间方向的发射会给…