Decision program on asteroid threat mitigation

The Association of Space Explorers Committee on Near-Earth Objects (NEOs) and its Panel on Asteroid Threat Mitigation have prepared a decision program to aid the international community in organizing a coordinated response to asteroid impact threats. The program is described in the ASE's report, Asteroid Threats: A Call for Global Response, which will be considered by the United Nations Committee on the Peaceful Uses of Outer Space in its 2009 sessions. The findings and recommendations of this report are presented here as well as some of the major implications of the complex decision-making involved in developing a coordinated international response to the challenge of protecting the Earth from NEO impacts.     

基于小波变换的单脉冲雷达目标RCS测量方法研究

随着现代信号处理技术的发展,对非平稳信号分析和处理的小波分析技术已成功应用于雷达目标特性分析领域,大功率单脉冲雷达作为我国航天测控网的主干设备,具有一定的目标特性识别能力。本文主要针对脉冲雷达RCS测量原理,讨论了基于小波变换的单脉冲雷达空间目标RCS测量方法,提出应发挥窄带低分辨率雷达在未来空间目标识别中的作用。     

近地尾向流和磁场——TC-1卫星观测结果

2004年10月12日,在01:30—04:30 UT期间,位于向阳侧磁层顶附近的Geotail卫星探测到行星际磁场为持续南向.此太阳风条件驱动了一个小磁暴,Sym-H指数在04:12 UT达到最小值-33 nT.在磁暴主相期间,AE指数维持在较高的水平,其最大值达400 nT.02:00—03:00 UT期间,TC-1卫星在近地磁尾(-10.6,3.2,-0.1)R_e处观测到明显的亚暴膨胀相特征和磁场偶极化过程.在偶极化前1 min,有较强的(v_x<-100 km/s)持续时间超过3 min的尾向流发生.分析发现该尾向流具有低温、高密度和沿磁场流动的特点,这说明尾向流具有来源于电离层风的特征.尾向流期间,TC-1观测的磁场分量B_x和总的磁场强度增加,磁倾角减小,磁场结构变成非偶极型,说明尾向流对磁场结构有一定的影响,文中尝试给出了相应的物理解释.观测表明,该事例中的近地磁尾尾向流可能对磁场偶极化过程的发生有重要意义.     

来自电离层的尾向流对近地磁场位形的影响

探测一号(TC-1)卫星的观测结果表明,尾向流能够拉伸近地磁尾的磁力线,从而导致磁场位形改变.尾向流具有垂直于磁场的速度分量,这种垂直磁场的速度分量会导致磁力线向尾向拉伸,磁场的结构由偶极型变为非偶极型.而随尾向流的终止,地向流的出现,磁场的结构由非偶极型变为偶极型,磁力线恢复原状.另外在磁场的结构由非偶极型变为偶极型的过程中,伴随磁能的释放热离子温度的迅速升高,温度由各向同性逐渐趋向各向异性.其次,观测结果显示来自电离层的尾向流对磁场By分量有重要的影响,能够引起磁场By分量的显著增强.上述分析结果表明来自电离层的尾向流对近地磁尾动力学过程有着重要的影响.      

Lunar apex–antapex cratering asymmetry as an impactor recorder in the Earth–Moon system

The degree of apex–antapex cratering asymmetry of a synchronously rotating satellite primarily depends on the mean encounter velocity of impactors with respect to the planetary system and the orbital velocity of the satellite. This means that we can estimate the mean encounter velocity of impactors by observing the apex–antapex cratering asymmetry, if the relationship between these is known. To apply this technique to the Moon, we attempt to derive the relationship between the mean encounter velocity of impactors and the degree of the lunar cratering asymmetry as a function of time, considering the temporal variation in the lunar orbital velocity during the last 4.0 Gyr. We used the cratering asymmetry of Zahnle et al. [Zahnle, K., Schenk, P., Sobieszczyk, S. et al. Differential cratering of synchronously rotating satellites by ecliptic comets. Icarus 153, 111–129, 2001] to obtain the relationship. Applying this relationship enables us to estimate the impactor’s velocity of the Earth–Moon system from an investigation of the spatial distribution of lunar craters. Furthermore, we re-evaluate the cratering asymmetry’s influence on lunar cratering chronology.     

A sample collector for robotic sample return missions I: Temperature effect on collected mass

Sample return is playing an increasingly important role in solar system exploration. Among the possible mission on the horizon, are sample return from asteroids, comets, the Moon and Mars. A collector initially intended for near-Earth asteroids is the touch-and-go-impregnable-pad (TGIP). Here we explore the effect of temperature on its collection capabilities. Temperatures expected on near-Earth asteroid mission targets range from −43 to 36 °C. Experiments were conducted at −75, −50, −25, 23, 65, and 105 °C. It was found that the mass of sample collected by the TGIP increased almost linearly to 23 °C and then leveled off at higher temperatures. We also found that the collector did not lose its ability to collect samples after being subjected to −75 °C temperatures (essentially frozen) and then thawed. These experiments have shown that the TGIP can operate effectively at temperatures expected on near-Earth asteroids, especially if collection is performed on the sunward side of the asteroid.     

Protostellar Winds and Chondritic Meteorites

We discuss the interaction between the magnetosphere of a young star and its surrounding accretion disk. We consider how an X-wind can be driven magnetocentrifugally from the inner edge of the disk where accreting gas is diverted onto stellar field lines either to flow onto the Sun or to be flung outwards with the wind. The X-wind satisfies many observational tests concerning optical jets, Herbig-Haro objects, and molecular outflows. Connections may exist between primitive solar system materials and X-winds. Chondrules and calcium-aluminum-rich inclusions (CAIs) experienced short melting events uncharacteristic of the asteroid belt where meteorites originate. The inner edge of the solar nebula has the shortest orbital timescale available to the system, a few days. Protosolar flares introduce another timescale, tens of minutes to hours. CAIs may form when solids are lifted from shaded portions of the disk close to the Sun and are exposed to its intense light for a day or so before they are flung by the X-wind to much larger distances. Chondrules were melted, perhaps many times, by flares at larger distances from the Sun before being launched and annealed, but not remelted, in the X-wind. Aerodynamic sorting explains the narrow range of sizes with which CAIs and chondrules are found in chondritic meteorites. Flare-generated cosmic-rays may induce spallation reactions that produce some of the short-lived radioactivities associated with primitive solar system rocks. This revised version was published online in June 2006 with corrections to the Cover Date.     

A sample collector for robotic sample return missions II: Radiation tests

Sample return from small solar system objects is playing an increasingly important part in solar system exploration. Critical to such missions is a robust, simple, and economic sample collector. We have developed a collector such as this for near-Earth asteroid sample return missions that we have termed the Touch-and-Go Impregnable Pad (TGIP). The collector utilizes a silicone substrate that is pushed into the dust and gravel surface layer of the asteroid. As part of a systematic evaluation of the TGIP, we have investigated the resilience of this substrate to ionizing radiations. Several miniature versions of the collector, containing typically ∼3 g of the collection substrate, were exposed to 0.564 MeV beta particles from a ⁹⁰Sr source and a 6 MeV electron beam in a linear accelerator to simulate the wide range of energies of solar and galactic ionizing radiation. Various radiation levels up to eight times greater than expected on a six-year asteroid mission (in the case of beta radiation) and 50 times greater than expected (in the case of the 6 MeV electron radiation) were administered to the substrate. After irradiation, the efficiency of the substrate in collecting samples of mock regolith was compared with that of collectors that had not been irradiated. No difference beyond experimental uncertainty was observed and we suggest that the operational TGIP will not be affected adversely by radiation doses expected during a typical six-year inner solar system mission.     

剖析用“Authorware”创作“MCAI”的几个问题

分析了用Authorware开发MCAI课件的几个问题。