Origins for the near-Earth asteroids

Because of their short dynamical lifetimes, the population of near-Earth asteroids (NEAs) must be resupplied. Two sources have been hypothesized: main-belt asteroids and extinct comet nuclei. The difficulty of making physical measurements for similar sized (diameter D less than 5 kilometers) main-belt asteroids and comet nuclei has limited comparative tests for distinguishing between these alternatives. A new survey of physical properties for D < 5 kilometers main-belt asteroids reveals that their spin rate and shape distributions are similar to those of NEAs, as is fully consistent with a main-belt origin for most NEAs. Physical data on comet nuclei are limited. If the existing sample is representative of the comet population, analysis of the asteroid and comet samples constrains the fraction of comet nuclei to between 0 and 40 percent of the total NEA population.     

Adventures in near-Earth object exploration

Asteroids, because of the hazard they pose to Earth, are compelling targets for robotic and human space exploration. Yet because of their exotic low-gravity environment, simply landing on an asteroid appears to be much more challenging than we had appreciated 5 or 10 years ago. Thanks to a bold new mission from Japan that has made the first asteroid sample return attempt, this goal is now within our reach.     

Binary asteroids: evidence for their existence from lightcurves

The lightcurves of the asteroids 49 Pales and 171 Ophelia bear a striking resemblance to those of eclipsing binary stars. This evidence suggests that these asteroids are binary objects. Asteroids belonging to the Themis dynamical family have unusual lightcurves, possibly due to satellite events; these unusual lightcurves suggest that multiple objects may be formed during the disruption of asteroids in collisional events.     

Understanding the distribution of near-earth asteroids

We have deduced the orbital and size distributions of the near-Earth asteroids (NEAs) by (i) numerically integrating NEAs from their source regions to their observed orbits, (ii) estimating the observational biases and size distribution associated with asteroids on those orbits, and (iii) creating a model population that can be fit to the known NEAs. We predict that there are approximately 900 NEAs with absolute magnitude less than 18 (that is, kilometer-sized), of which 29, 65, and 6% reside on Amor, Apollo, and Aten orbits, respectively. These results suggest that roughly 40% of the kilometer-sized NEAs have been found. The remainder, on highly eccentric and inclined orbits, are more difficult to detect.     

Mining the apollo and amor asteroids

Earth-approaching asteroids could provide raw materials for space manufacturing. For certain asteroids the total energy per unit mass for the transfer of asteroidal resources to a manufacturing site in high Earth orbit is comparable to that for lunar materials. For logistical reasons the cost may be many times less. Optical studies suggest that these asteroids have compositions corresponding to those of carbonaceous and ordinary chondrites, with some containing large quantities of iron and nickel; others are thought to contain carbon, nitrogen, and hydrogen, elements that appear to be lacking on the moon. The prospect that several new candidate asteroids will be discovered over the next few years increases the likelihood that a variety of asteroidal resource materials can be retrieved on low-energy missions.     

Ancient asteroids enriched in refractory inclusions

Calcium- and aluminum-rich inclusions (CAIs) occur in all classes of chondritic meteorites and contain refractory minerals predicted to be the first condensates from the solar nebula. Near-infrared spectra of CAIs have strong 2-micrometer absorptions, attributed to iron oxide-bearing aluminous spinel. Similar absorptions are present in the telescopic spectra of several asteroids; modeling indicates that these contain approximately 30 +/- 10% CAIs (two to three times that of any meteorite). Survival of these undifferentiated, large (50- to 100-kilometer diameter) CAI-rich bodies suggests that they may have formed before the injection of radiogenic 26Al into the solar system. They have also experienced only modest post-accretionary alteration. Thus, these asteroids have higher concentrations of CAI material, appear less altered, and are more ancient than any known sample in our meteorite collection, making them prime candidates for sample return.     

Pallasitic meteorites: implications regarding the deep structure of asteroids

Olivine compositions in pallasites exhibit a bimodal distribution and indicate a high degree of internal equilibrium. Cooling rates measured in the metal phases are uniform and consistently lower than those of most iron meteorites. These factors suggest that the pallasites were derived from few parent bodies, and that they crystallized in a highly insulated site-presumably the core of their parent body. Most iron meteorites were derived either from isolated areas closer to the surface or from other parent bodies.     

双宾语结构再研究

在前人有关双宾语结构与与格结构关系研究的基础上,从句法理论、句法结构、语义制约等方面,论述双宾语结构与与格结构的关系,认为二者之间无转换生成关系。     

信号的二值化融合在农用车检测中的应用

以轴重信号的二值化处理和转换过程为例,说明了多通道信号转化为数字信号并编码后可实现在同一线路的融合传输.重点阐述了如何降低测试误差以及器件选取的理论基础和实践意义.结果表明,方法应用在农用车检测线中可简化工程布线、方便线路维修等,具有良好的应用价值.     

Gamma oscillations and object processing in the infant brain

An enduring controversy in neuroscience concerns how the brain "binds" together separately coded stimulus features to form unitary representations of objects. Recent evidence has indicated a close link between this binding process and 40-hertz (gamma-band) oscillations generated by localized neural circuits. In a separate line of research, the ability of young infants to perceive objects as unitary and bounded has become a central focus for debates about the mechanisms of perceptual development. Here we demonstrate that binding-related 40-hertz oscillations are evident in the infant brain around 8 months of age, which is the same age at which behavioral and event-related potential evidence indicates the onset of perceptual binding of spatially separated static visual features.     

关于二元函数极限的讨论

极限理论是微积分学的基础,极限的思想方法在许多领域有着广泛的应用.二元函数的极限与一元函数的极限含义相同,它研究的是平面上动点趋向某一定点时,相应的函数值的变化趋势.根据二元函数极限的定义,在点P0 (x0,y0) 的邻域内,动点P (x0,y0) 趋向于P0 (x0,y0) 的方式是任意的.因此,在判定二元函数极限是否存在以及极限的计算上都有一定难度.就二元函数的极限问题作了两个方面的探讨,以便提供一种解题思路.     

The provenances of asteroids, and their contributions to the volatile inventories of the terrestrial planets

Determining the source(s) of hydrogen, carbon, and nitrogen accreted by Earth is important for understanding the origins of water and life and for constraining dynamical processes that operated during planet formation. Chondritic meteorites are asteroidal fragments that retain records of the first few million years of solar system history. The deuterium/hydrogen (D/H) values of water in carbonaceous chondrites are distinct from those in comets and Saturn's moon Enceladus, implying that they formed in a different region of the solar system, contrary to predictions of recent dynamical models. The D/H values of water in carbonaceous chondrites also argue against an influx of water ice from the outer solar system, which has been invoked to explain the nonsolar oxygen isotopic composition of the inner solar system. The bulk hydrogen and nitrogen isotopic compositions of CI chondrites suggest that they were the principal source of Earth's volatiles.     

二叉树数据结构在植物检索中的应用

通过对植物检索表的编制和使用方法分析,并对其中异常结点进行深入研究,解决了因多父结点的存在致使二叉树查询不便的问题,在此基础上用计算机实现了植物检索,使"植物信息管理系统"的功能得到进一步完善。     

微卫星数据“01”化的研究

比较数据转化前后的遗传距离与聚类关系,发现“01”化后的数据因信息丢失引起遗传距离的减小,降低了聚类的分辨率与支持率。因此,建议不要将微卫星数据“01”化。     

Binary interaction dominates the evolution of massive stars

The presence of a nearby companion alters the evolution of massive stars in binary systems, leading to phenomena such as stellar mergers, x-ray binaries, and gamma-ray bursts. Unambiguous constraints on the fraction of massive stars affected by binary interaction were lacking. We simultaneously measured all relevant binary characteristics in a sample of Galactic massive O stars and quantified the frequency and nature of binary interactions. More than 70% of all massive stars will exchange mass with a companion, leading to a binary merger in one-third of the cases. These numbers greatly exceed previous estimates and imply that binary interaction dominates the evolution of massive stars, with implications for populations of massive stars and their supernovae.     

Evidence of thermal metamorphism on the C, g, B, and f asteroids

Reflectance spectra (0.3 to 2.6 micrometers) of 14 C, G, B, and F asteroids and 21 carbonaceous chondrite powders are compared in detail. Only three thermally metamorphosed CM-Cl chondrites that have a weak ultraviolet absorption are shown to have close counterparts among those asteroids. Reflectance spectra of heated Murchison CM2 chondrite are compared with the average C and G type asteroid spectra. Murchison heated at 600 degrees to 1000 degrees C exhibits a similar weak ultraviolet absorption and provides the best analog for those spectra. Comparison of ultraviolet absorption strengths between 160 C, G, B, and F asteroids and carbonaceous chondrites suggests that surface minerals of most of those asteroids are thermally metamorphosed at temperatures around 600 degrees to 1000 degrees C.     

双趾马脚部的解剖结构

本文首次报道了一匹胸肢和盆肢均为双趾的马第Ⅱ趾(指)的骨、关节、肌腱的结构,以及血管和神经的来源、走向和分支分布.同时对该马第Ⅱ趾(指)的形成原因进行了探讨.     

民族音乐学及其研究对象的嬗变

民族音乐学是在历史进程中,与文化学、人类学、地理学等各种学科的交融中逐渐形成和发展起来的。对民族音乐学的研究,不仅要考虑音乐形态产生的地理环境差异,而且还要考虑经济运行方式对音乐形成产生的影响,更要考察文化形态的变更对音乐发展产生的推动作用。     

数字农业研究中二进制CPFSK信号的产生及实现

相位连续的频移键控（CPFSK）是数字农业研究中为确保数据的可靠传输而广泛使用的一种调制方式。分析CPFSK原理及产生方法,利用直接数字频率合成器（DDS）建模,产生二进制CPFSK信号。结合多种仿真工具分别实现其系统级、功能和时序仿真,并将设计下载到Altera公司的FPGA器件EP1K30TC144-3中进行验证,达到以较少的硬件成本完成系统设计的目的。     

基于面向对象方法的网页设计

在网页设计过程中,经常需要对原有的代码进行编辑修改,但我们最多只能重复使用代码而不能重复使用组件,而使用原来的代码可能带来新的代码质量问题.采用面向对象的方法,通过构建和测试基本的对象,可以建立一个经过严格测试的代码库和可靠的组件,从而无需在以后的使用中对每个组件重新测试,而仅需对组件的整体进行测试.这种方法有助于加快开发的进度,提高代码的质量.