Acceleration of the rotation of asteroid 1862 Apollo by radiation torques

The anisotropic reflection and thermal re-emission of sunlight from an asteroid's surface acts as a propulsion engine. The net propulsion force (Yarkovsky effect) changes the orbital dynamics of the body at a rate that depends on its physical properties; for irregularly shaped bodies, the propulsion causes a net torque (the Yarkovsky-O'Keefe-Radzievskii-Paddack or YORP effect) that can change the object's rotation period and the direction of its rotation axis. The Yarkovsky effect has been observed directly, and there is also indirect evidence of its role in the orbital evolution of asteroids over long time intervals. So far, however, only indirect evidence exists for the YORP effect through the clustering of the directions of rotation axes in asteroid families. Here we report a change in the rotation rate of the asteroid 1862 Apollo, which is best explained by the YORP mechanism. The change is fairly large and clearly visible in photometric lightcurves, amounting to one extra rotation cycle in just 40 years even though Apollo's size is well over one kilometre. This confirms the prediction that the YORP effect plays a significant part in the dynamical evolution of asteroids.     

Structure and thermal history of the H-chondrite parent asteroid revealed by thermochronometry

Our Solar System formed approximately 4.6 billion years ago from the collapse of a dense core inside an interstellar molecular cloud. The subsequent formation of solid bodies took place rapidly. The period of &<10 million years over which planetesimals were assembled can be investigated through the study of meteorites. Although some planetesimals differentiated and formed metallic cores like the larger terrestrial planets, the parent bodies of undifferentiated chondritic meteorites experienced comparatively mild thermal metamorphism that was insufficient to separate metal from silicate. There is debate about the nature of the heat source as well as the structure and cooling history of the parent bodies. Here we report a study of 244Pu fission-track and 40Ar-39Ar thermochronologies of unshocked H chondrites, which are presumed to have a common, single, parent body. We show that, after fast accretion, an internal heating source (most probably 26Al decay) resulted in a layered parent body that cooled relatively undisturbed: rocks in the outer shells reached lower maximum metamorphic temperatures and cooled faster than the more recrystallized and chemically equilibrated rocks from the centre, which needed approximately 160 Myr to reach 390K.     

Direct observation of the alignment of ferromagnetic spins by antiferromagnetic spins

The arrangement of spins at interfaces in a layered magnetic material often has an important effect on the properties of the material. One example of this is the directional coupling between the spins in an antiferromagnet and those in an adjacent ferromagnet, an effect first discovered in 1956 and referred to as exchange bias. Because of its technological importance for the development of advanced devices such as magnetic read heads and magnetic memory cells, this phenomenon has received much attention. Despite extensive studies, however, exchange bias is still poorly understood, largely due to the lack of techniques capable of providing detailed information about the arrangement of magnetic moments near interfaces. Here we present polarization-dependent X-ray magnetic dichroism spectro-microscopy that reveals the micromagnetic structure on both sides of a ferromagnetic-antiferromagnetic interface. Images of thin ferromagnetic Co films grown on antiferromagnetic LaFeO3 show a direct link between the arrangement of spins in each material. Remanent hysteresis loops, recorded for individual ferromagnetic domains, show a local exchange bias. Our results imply that the alignment of the ferromagnetic spins is determined, domain by domain, by the spin directions in the underlying antiferromagnetic layer.     

木材导热系数的支持向量回归预测

根据木材在不同影响因素(密度、含水率和比重)下沿横纹方向(包括径向和弦向)的导热系数的实测数据集,应用基于粒子群算法(PSO)寻优的支持向量回归(SVR)方法,建立了木材沿不同方向的导热系数的预测模型,并与通过类比法(ANA)导出的理论模型和BP神经网络(BPNN)模型进行了比较。结果表明:基于相同的训练样本和检验样本,木材导热系数的SVR模型比其ANA模型或BPNN模型具有更高的预测精度;增加训练样本数有助于提高SVR预测模型的泛化能力;基于留一交叉验证法(LOOCV)的SVR模型预测的最大绝对百分误差(MPE)、平均绝对误差(MAE)和平均绝对百分误差(MAPE)均为最小。因此,SVR是一种预测木材导热系数的有效方法。     

The landing of the NEAR-Shoemaker spacecraft on asteroid 433 Eros

The NEAR-Shoemaker spacecraft was designed to provide a comprehensive characterization of the S-type asteroid 433 Eros (refs 1,2,3), an irregularly shaped body with approximate dimensions of 34 x 13 x 13 km. Following the completion of its year-long investigation, the mission was terminated with a controlled descent to its surface, in order to provide extremely high resolution images. Here we report the results of the descent on 12 February 2001, during which 70 images were obtained. The landing area is marked by a paucity of small craters and an abundance of 'ejecta blocks'. The properties and distribution of ejecta blocks are discussed in a companion paper. The last sequence of images reveals a transition from the blocky surface to a smooth area, which we interpret as a 'pond'. Properties of the 'ponds' are discussed in a second companion paper. The closest image, from an altitude of 129 m, shows the interior of a 100-m-diameter crater at 1-cm resolution.     

Differentiation of the asteroid Ceres as revealed by its shape

The accretion of bodies in the asteroid belt was halted nearly 4.6 billion years ago by the gravitational influence of the newly formed giant planet Jupiter. The asteroid belt therefore preserves a record of both this earliest epoch of Solar System formation and variation of conditions within the solar nebula. Spectral features in reflected sunlight indicate that some asteroids have experienced sufficient thermal evolution to differentiate into layered structures. The second most massive asteroid--4 Vesta--has differentiated to a crust, mantle and core. 1 Ceres, the largest and most massive asteroid, has in contrast been presumed to be homogeneous, in part because of its low density, low albedo and relatively featureless visible reflectance spectrum, similar to carbonaceous meteorites that have suffered minimal thermal processing. Here we show that Ceres has a shape and smoothness indicative of a gravitationally relaxed object. Its shape is significantly less flattened than that expected for a homogeneous object, but is consistent with a central mass concentration indicative of differentiation. Possible interior configurations include water-ice-rich mantles over a rocky core.     

小行星碰撞地球对地球自转的影响

以小行星有可能碰撞地球为依据,分别在地球作近似定轴转动和定点转动的两种情况下,讨论了在碰撞的时候地球自转运动可能发生的变化.文章建立了与此相应的Euler方程,得到了该方程的解析解.由这些公式文章说明了SL9彗星的撞击,对木星的自转速率是没有影响的.基于Chicxulub陨石坑的一个模拟碰撞的数值计算中,文章获得结论:小行星对地球的大型碰撞有可能影响地球的自转运动.     

Annihilation of ecosystems by large asteroid impacts on the early Earth

Large asteroid impacts produced globally lethal conditions by evaporating large volumes of ocean water on the early Earth. The Earth may have been continuously habitable by ecosystems that did not depend on photosynthesis as early as 4.44 Gyr BP (before present). Only a brief interval after 3.8 Gyr exists between the time when obligate photosynthetic organisms could continuously evolve and the time when the palaeontological record indicates highly evolved photosynthetic ecosystems.     

The recent breakup of an asteroid in the main-belt region

The present population of asteroids in the main belt is largely the result of many past collisions. Ideally, the asteroid fragments resulting from each impact event could help us understand the large-scale collisions that shaped the planets during early epochs. Most known asteroid fragment families, however, are very old and have therefore undergone significant collisional and dynamical evolution since their formation. This evolution has masked the properties of the original collisions. Here we report the discovery of a family of asteroids that formed in a disruption event only 5.8 +/- 0.2 million years ago, and which has subsequently undergone little dynamical and collisional evolution. We identified 39 fragments, two of which are large and comparable in size (diameters of approximately 19 and approximately 14 km), with the remainder exhibiting a continuum of sizes in the range 2-7 km. The low measured ejection velocities suggest that gravitational re-accumulation after a collision may be a common feature of asteroid evolution. Moreover, these data can be used to check numerical models of larger-scale collisions.     

Seismic resurfacing by a single impact on the asteroid 433 Eros

Impact cratering creates a wide range of topography on small satellites and asteroids. The population of visible craters evolves with impacts, and because there are no competing endogenic processes to modify the surface, determining the various ways younger craters add to or subtract from the population is a fundamental aspect of small-body geology. Asteroid 433 Eros, the most closely studied small body, has regions of substantially different crater densities that remain unexplained. Here we show that the formation of a relatively young crater (7.6 km in diameter) resulted in the removal of other craters as large as 0.5 km over nearly 40 percent of the asteroid. Burial by ejecta cannot explain the observed pattern of crater removal. The limitation of reduced crater density to a zone within a particular straight-line distance through the asteroid from the centre of the large crater suggests degradation of the topography by seismic energy released during the impact. Our observations indicate that the interior of Eros is sufficiently cohesive to transmit seismic energy over many kilometres, and the outer several tens of metres of the asteroid must be composed of relatively non-cohesive material.     

无限非局域热非线性介质中矢量孤子的传输特性

利用变分法研究无限非局域热非线性介质中矢量孤子的传输特性.首先对试探解高斯-高斯光束进行变分分析,得到关于振幅、束宽、相位和波前曲率的演化方程;然后利用势函数对形成矢量孤子的束宽变化进行分析;最后对变分计算得到的结果进行数值验证.结果表明:当初始功率P0=2π时,高斯-高斯光束是矢量孤子很好的近似;而当P02π时,光束束宽先压缩后展宽,形成周期性振荡;当P02π时,光束束宽先展宽后压缩,形成周期性振荡.     

对313号小行星掩 HIP 47310的观测和研究?

对发生在世界时2013年10月21日19:29的313号小行星Chaldaea掩恒星HIP 47310进行了多站联测.对观测数据进行分析得出每个观测点的掩星时长,结合小行星轨道速度得出小行星剖面长度,用Occult 4软件对多站观测数据进行了椭圆拟合,最终得出该小行星的大小和形状以及轨道的偏差.同时,我们探讨了当今数码时代发动大量天文爱好者利用自身的业余观测器材取得较好的科研观测资料的可行性和优势.     

小行星“北师大星8050”的测光性质研究

为得到小行星"北师大星8050"的相关性质,使用中国科学院国家天文台兴隆观测站85 cm和云南天文台稻城选址点50 cm望远镜的观测数据,利用基于蒙特卡洛-马尔科夫链算法的盘积分Lommel-Seeliger椭球模型,对北师大星的相关参数进行了反演,得到北师大星的椭球半径长度分别为(5.140±0.010)、(1.698±0.004)、(1.077±0.004) km,自转周期为(5.397±0.003)h,几何反照率pv=0.195±0.001;在H-G1-G2星等系统下的相位因子G1=0.259±0.002、G2=0.373 ±0.001,自转轴指向的黄道坐标为(104.68.,±80.06.)或(284.68.,±80.06.).     

Coherent manipulation of single spins in semiconductors

During the past few years, researchers have gained unprecedented control over spins in the solid state. What was considered almost impossible a decade ago, in both conceptual and practical terms, is now a reality: single spins can be isolated, initialized, coherently manipulated and read out using both electrical and optical techniques. Progress has been made towards full control of the quantum states of single and coupled spins in a variety of semiconductors and nanostructures, and towards understanding the mechanisms through which spins lose coherence in these systems. These abilities will allow pioneering investigations of fundamental quantum-mechanical processes and provide pathways towards applications in quantum information processing.     

Dynamical decoupling of electron spins in phosphorus-doped silicon

Quantum coherence is an important enabling feature underpinning quantum computation. However, because of couplings with its noisy surrounding environment, qubits suffer from the decoherence effects. The dynamical decoupling (DD) technique uses pulse-induced qubit flips to effectively mitigate couplings between qubits and environment. Optimal DD eliminates dephasing up to a given order with the minimum number of pulses. In this paper, we first introduce our recent work on prolonging electron spin coherence in γ-irradiated malonic acid crystals and analyze different decoherence mechanisms in this solid system. Then we focus on electron spin relaxation properties in another system, phosphorous-doped silicon (Si:P) crystals. These properties have been investigated by pulse electron paramagnetic resonance (EPR). We also investigate the performance of the dynamical decoupling technique on this system. Using 8-pulse periodic DD, the coherence time can be extended to 296 μs compared with 112 μs with one-pulse control.     

Origin of asteroid rotation rates in catastrophic impacts

The rotation rates of asteroids, which are deduced from periodic fluctuations in their brightnesses, are controlled by mutual collisions. The link between asteroid spin and collision history is usually made with reference to impact experiments on centimetre-scale targets, where material strength governs the impact response. Recent work, however, indicates that for objects of the size of most observed asteroids (> or = 1 km in diameter), gravity rather than intrinsic strength controls the dynamic response to collisions. Here we explore this idea by modelling the effect of impacts on large gravitating bodies. We find that the fraction of a projectile's angular momentum that is retained by a target asteroid is both lower and more variable than expected from laboratory experiments, with spin evolution being dominated by 'catastrophic' collisions that eject approximately 50 per cent of the target's mass. The remnant of an initially non-rotating silicate asteroid that suffers such a collision rotates at a rate of approximately 2.9 per day, which is close to the observed mean asteroid rotation rate of approximately 2.5 d-1. Moreover, our calculations suggest that the observed trend in the mean spin frequency for different classes of asteroids (2.2 d-1 for C-type asteroids, 2.5 d-1 for S-type, and 4.0 d-1 for M-type) is due to increasing mean density, rather than increasing material strength.     

Retrograde spins of near-Earth asteroids from the Yarkovsky effect

Dynamical resonances in the asteroid belt are the gateway for the production of near-Earth asteroids (NEAs). To generate the observed number of NEAs, however, requires the injection of many asteroids into those resonant regions. Collisional processes have long been claimed as a possible source, but difficulties with that idea have led to the suggestion that orbital drift arising from the Yarkovsky effect dominates the injection process. (The Yarkovsky effect is a force arising from differential heating-the 'afternoon' side of an asteroid is warmer than the 'morning' side.) The two models predict different rotational properties of NEAs: the usual collisional theories are consistent with a nearly isotropic distribution of rotation vectors, whereas the 'Yarkovsky model' predicts an excess of retrograde rotations. Here we report that the spin vectors of NEAs show a strong and statistically significant excess of retrograde rotations, quantitatively consistent with the theoretical expectations of the Yarkovsky model.     

小行星群和Kirkwood空隙分布特征的探讨

引进小行星径向波函数Ｒｎｌ（ｒ）及划分小行星族的参考轨道根数（ａ，ｅ和ｉ），证明了小行星轨道半长径分布的３／２、４／３和１／１通约处是小行星径向概率最大的位置，而概率的零点与小行星轨道半长径分布的Ｋｉｒｋｗｏｏ空隙相对应，零点平移的位置，其平均运动的通约数为４／１、７／２、３／１、５／２、７／３、２／１。