The NASA Astrobiology Roadmap

The NASA Astrobiology Roadmap provides guidance for research and technology development across the NASA enterprises that encompass the space, Earth, and biological sciences. The ongoing development of astrobiology roadmaps embodies the contributions of diverse scientists and technologists from government, universities, and private institutions. The Roadmap addresses three basic questions: how does life begin and evolve, does life exist elsewhere in the universe, and what is the future of life on Earth and beyond? Seven Science Goals outline the following key domains of investigation: understanding the nature and distribution of habitable environments in the universe, exploring for habitable environments and life in our own Solar System, understanding the emergence of life, determining how early life on Earth interacted and evolved with its changing environment, understanding the evolutionary mechanisms and environmental limits of life, determining the principles that will shape life in the future, and recognizing signatures of life on other worlds and on early Earth. For each of these goals, Science Objectives outline more specific high priority efforts for the next three to five years. These eighteen objectives are being integrated with NASA strategic planning.     

PDM核心业务基础和配置管理标准要求

配置管理是PDM的核心业务基础,本文从论述实现PDM应用中配置管理功能的重要性出发,介绍分析了国际上多种标准对配置管理的基本要求和方法.     

Advanced Space Missions and Computer Systems

Computer requirements for future space missions are developed and three different approaches to multiprocessing computer organizations are presented. These approaches are shown to have considerable advantages over conventional computers for advanced space missions. Reliability requirements also are assessed by a simulation method and shown to be attainable for long-duration missions.     

Impact of Geoid Improvement on Ocean Mass and Heat Transport Estimates

One long-standing difficulty in estimating the large-scale ocean circulation is the inability to observe absolute current velocities. Both conventional hydrographic measurements and altimetric measurements provide observations of currents relative to an unknown velocity at a reference depth in the case of hydrographic data, and relative to mean currents calculated over some averaging period in the case of altimetric data. Space gravity missions together with altimetric observations have the potential to overcome this difficulty by providing absolute estimates of the velocity of surface oceanic currents. The absolute surface velocity estimates will in turn provide the reference level velocities that are necessary to compute absolute velocities at any depth level from hydrographic data. Several studies have been carried out to quantify the improvements expected from ongoing and future space gravity missions. The results of these studies in terms of volume flux estimates (transport of water masses) and heat flux estimates (transport of heat by the ocean) are reviewed in this paper. The studies are based on ocean inverse modeling techniques that derive impact estimates solely from the geoid error budgets of forthcoming space gravity missions. Despite some differences in the assumptions made, the inverse modeling calculations all point to significant improvements in estimates of oceanic fluxes. These improvements, measured in terms of reductions of uncertainties, are expected to be as large as a factor of 2. New developments in autonomous ocean observing systems will complement the developments expected from space gravity missions. The synergies of in situ and satellite observing systems are considered in the conclusion of this paper. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of modified atmosphere on crop productivity and mineral content.

Wheat, potato, pea and tomato crops were cultivated from seeding to harvest in a controlled and confined growth chamber at elevated CO2 concentration (3700 microL L-1) to examine the effects on biomass production and edible part yields. Different responses to high CO2 were recorded, ranging from a decline in productivity for wheat, to slight stimulation for potatoes, moderate increase for tomatoes, and very large enhancement for pea. Mineral content in wheat and pea seeds was not greatly modified by the elevated CO2. Short-term experiments (17 d) were conducted on potato at high (3700 microL L-1) and very high (20,000 microL L-1) CO2 concentration and/or low O2 partial pressure (approximately 20,600 microL L-1 or 2 kPa). Low O2 was more effective than high CO2 in total biomass accumulation, but development was affected: Low O2 inhibited tuberization, while high CO2 significantly increased production of tubers.     

Investigation of low-energy proton effects on aptamer performance for astrobiological applications

Biochips are promising instruments for the search for organic molecules in planetary environments. Nucleic acid aptamers are powerful affinity receptors known for their high affinity and specificity, and therefore are of great interest for space biochip development. A wide variety of aptamers have already been selected toward targets of astrobiological interest (from amino acids to microorganisms). We present a first study to test the resistance of these receptors to the constraints of the space environment. The emphasis is on the effect of cosmic rays on the molecular recognition properties of DNA aptamers. Experiments on beam-line facilities have been conducted with 2 MeV protons and fluences much higher than expected for a typical mission to Mars. Our results show that this irradiation process did not affect the performances of DNA aptamers as molecular recognition tools.     

A formulation of multitarget tracking as an incomplete data problem

Traditional multihypothesis tracking methods rely upon an enumeration of all the assignments of measurements to tracks. Pruning and gating are used to retain only the most likely hypotheses in order to drastically limit the set of feasible associations. The main risk is to eliminate correct measurement sequences. The probabilistic multiple hypothesis tracking (PMHT) method has been developed by Streit and Luginbuhl in order to reduce the drawbacks of "strong" assignments. The PMHT method is presented in a general mixture densities perspective. The Expectation-Maximization (EM) algorithm is the basic ingredient for estimating mixture parameters. This approach is then extended and applied to multitarget tracking for nonlinear measurement models in the passive sonar perspective.     

高安全分区信息物理系统的仿真验证研究

信息物理系统(CPS)强调借助网络对物理世界的感知和控制,往往系统规模较大、结构复杂,通常重视对系统开发前、中期的验证工作,而高安全的CPS对验证的要求更高。在实现分区调度、通信等服务的基础上,通过构建Windows平台上的模拟分区环境,我们完成了某高安全信息物理系统的建模,检验系统设计的可调度性、故障恢复能力等行为特性,从而验证了研究方法的合理性和正确性。     

脉冲等离子体推力器羽流回流影响分析

将三维MHD双温入口模型的计算结果作为入口条件,运用DSMC(Direct Simulation Monte-Carlo)/PIC(Particle in Cell)流体混合算法,模拟实验室PPT样机羽流。验证计算显示,该模型具有模拟脉冲等离子体推力器羽流的能力。对不同初始电压和电容下的羽流场进行了模拟,给出了出口平面返流质量流率的变化情况和20μs羽流中离子和CEX离子的分布情况。计算结果显示,高能量状态对应高质量流率,高质量流率对应高动量,高动量离子和中性粒子对航天器撞击会造成更强影响。     

高速串行总线RapidIO与PCI Express协议分析比较

随着新一代嵌入式系统对数据传输能力要求的提高,高速串行总线有着越来越广泛的应用,RapidIO和PCI Express(亦称PCIe)是常见的选择。通过对这两种协议在整体工作机制及其特点、分层模型、拓扑结构、服务质量等方面的分析比较,为嵌入式系统设计中高速串行总线的选择提供参考。