小行星YORP效应的可探测候选体

YORP (Yarkovsky-O''Keefe-Radzievskii-Paddack)效应是小行星长期动力学演化的机制之一. 与碰撞、引力摄动等因素相比, YORP效应作用量级小, 短时标观测效应不明显, 这给直接测量YORP效应带来了很大的困难. 利用小行星光变数据库中已知的小行星数据, 统计了小行星的自转速率分布, 使用核密度估计以及Kolmogorov-Smirnov检验分别分析了近地小行星和主带小行星自转速率的分布特性, 分别给出了在近地小行星和主带小行星中寻找受YORP效应影响减速自转的最佳样本群; 基于7颗已被探测到YORP旋转加速度的近地小行星, 利用YORP强度估计方法和光变探测条件建立了筛选模型, 给出了未来可直接通过光变数据探测\lk YORP效应的10颗近地小行星.     

近地小行星(10302) 1989 ML和(4660) Nereus的轨道特征及演化分析

近地小行星(10302) 1989 ML和(4660) Nereus作为下一代深空探测的候选目标一直备受关注. 在考虑太阳系主要天体的动力学背景下, 通过计算最大Lyapunov指数(MLE)及MEGNO (Mean Exponential Growth factor of Nearby Orbits)指数讨论它们的稳定性. 同时, 对每个小行星, 在其观测误差范围内按多元正态分布各选取1000个克隆粒子, 通过统计分析显示这两个小行星在10万年内可能的运动范围, 给出半长径-偏心率空间中的出现次数分布图, 并统计小行星与地球或其他大行星之间的密近交汇及碰撞的概率. 此外还对这两个小行星的标称轨道进行长期共振、Kozai共振及平运动共振的动力学分析. 综上得出结论, 1989 ML处在平运动共振主导的区域, 发生密近交汇的概率较小, 从而其轨道相对较稳定; 而Nereus处在地球的密近交汇区域, 轨道极不稳定.     

Flora小行星族自转特性研究

小行星族作为灾变碰撞的残留物,其基础物理性质提供了其母体以及后续演化信息.其中轨道以及自转特性分别反映了Yarkovsky效应以及Yarkovsky-O’Keefe-Radzievskii-Paddack效应(YORP效应)对于小行星族演化的影响.基于小行星光变数据库(Asteroid Lightcurve Database),通过对Flora小行星族自转速率分布进行研究,发现随着直径减小,族成员自转速率倾向于主要集中在3–5 d~(-1)的范围内.同时,可以注意到Flora小行星族整体表现出更倾向于顺行自转状态的现象,但对于轨道半长轴小于2.2au的成员来说,其顺行自转与逆行自转状态成员数目比接近于近地小行星中顺逆行自转状态源1:3的比例;此外,对于轨道半长轴大于2.2 au且具有顺行自转状态的部分族成员,在轨道半长轴-绝对星等分布中表现出聚集现象,并在聚集区域中有9颗成员展现出类似Slivan状态特征.     

天体运动的轨道偏心率研究概述---从恒星系统到行星系统

偏心率是描述天体运动轨道的重要参数之一, 能够为揭示天体的动力学演化提供重要线索, 进而帮助理解天体形成与演化的过程及背后的物理机制. 随着天文观测技术的不断发展, 人们对于天体运动轨道的研究已经走出太阳系, 包含的系统也从大质量端的恒星系统延伸到了低质量端的行星系统. 聚焦天体轨道偏心率研究, 回顾了目前在恒星系统(包括主序恒星、褐矮星以及致密星)和行星系统(包括太阳系外巨行星以及``超级地球''、``亚海王星''等小质量系外行星)方面取得的进展, 总结了不同尺度结构下偏心率研究的一些共同之处和待解决的问题. 并结合当下和未来的相关天文观测设备和项目, 对未来天体轨道偏心率方面的研究工作进行了展望.     

近地小行星发现数目与发现场景的统计分析

近地小行星是一类可能对地球安全造成潜在威胁的太阳系小天体, 目前绝大部分的近地小行星是由地基望远镜发现的, 且数目仍在不断增加. 为了对我国未来开展近地小行星发现监测提供参考和借鉴, 利用国际小行星中心公开的数据库对所有近地小行星首次发现时刻的观测资料开展了多维度统计分析. 发现望远镜探测能力的限制会对近地小行星的发现造成选择效应, 导致不同轨道类型近地小行星发现的相对比例逐年变化且与直径有关. 另外, 结合数值模拟获得的轨道数据, 对近地小行星首次发现时的观测场景进行了还原, 获得了发现时刻近地小行星位置在不同天球坐标系的分布, 分析了其分布特征与季节、测站纬度和小行星直径的依赖关系. 最后, 通过分析数据定量考察了太阳、月球和银道面对近地小行星发现的影响, 发现地基望远镜一般难以发现来自太阳方向90$^\circ$范围内直径140m以下的近地小行星, 并且随着小行星直径的减小该限制范围也将变大; 月光污染对近地小行星发现的影响也非常显著, 望月前后几天的观测限制可导致约29%的目标无法被发现, 而且分析表明农历上半月发现的目标一般比下半月发现的更难以被跟踪观测; 银道面特别是银心方向会对近地小行星发现产生影响, 使得黄道面附近存在与季节相关的观测``盲区''.     

半相接双星室女座UW轨道周期变化的物理机制研究

对大陵五型半相接双星室女座UW的轨道周期变化进行了分析.结果表明该星的轨道周期在长期快速增加(dP/dt=+1.37×10-6天/年)的同时也含有周期为62.3年的周期性变化.利用Brancewicz和Dworak在1980年给出的基本物理参量,对引起轨道周期变化的物理机制进行了分析研究.分析表明一个质量为Ms>0.94M⊙的第三天体的光时轨道效应能对轨道周期的周期性变化成份作出解释.由于在观测上没有发现这个第三天体存在的信息,它有可能是一个致密天体(如白矮星等).轨道周期的长期增加成份可解释为由次星到主星的物质交流引起(dM2/dt=1.43×10-7M⊙/年),这与该系统次星充满的半接几何结构是相一致的.但是,根据双星演化理沦,大陵五型半相接双星应该处于以次星的核反应时标进行物质交换的慢速物质交流演化阶段,而分析发现该星的轨道周期变化时标远小于次星的核反应时标,但接近于次星的热力学时标,揭示了(1)这颗双星处于以次星热力学时标进行物质交换的快速物质交流演化阶段;或(2)系统的星周物质要通过角动量交换对轨道周期的快速增加做贡献.     

关于北斗G2卫星的溯往追终

2022年1月, 失效的北斗G2卫星被实践21号卫星从地球静止轨道拖入了坟墓轨道. 为了这项捕获任务的安全实施, 需要预先确定北斗G2的旋转状态. 基于过去10 yr的测光观测数据展示了北斗G2卫星自转的演化过程. 根据北斗G2的自转速度和轨道的演化, 确认了在过去的10 yr里发生的6次异常事件. 据推测, 2012年的小碎片碰撞事件, 是随后几年燃油泄漏的导火索. 2017年之后剩余燃油完全释放, 再也没有出现转速异常. 将2014年太阳能帆板损坏和2016年的解体事件后建立的旋转动力学模型外推1 yr, 转轴的标准偏差小于3°, 转速标准偏差为0.11°· s^-1, 能够有效地满足捕获任务时刻旋转状态的精度要求.     

碰撞风险评估标准适用性分析

随着空间碎片数量的不断增加,空间目标碰撞预警技术已经受到人们越来越多的关注.早期普遍使用几何区域(BOX)判定法对空间目标的碰撞风险进行评估,但它是一种比较保守的方法,工程实用性低;90年代后开始广泛地研究和采用碰撞概率判定法,这种方法综合考虑空间目标交会时刻的多个参数,但是由于其数学模型对轨道误差数据的依赖程度很高,所以在某些情况下会导致预警结果不准确.通过仿真的方法,模拟两条确定交会的标准精密轨道,由程序生成随机误差加入到标准轨道生成有误差的轨道,计算最接近距离和碰撞概率,并分析二者之间的关系.结果表明,当轨道预报精度较差时,碰撞概率判定法可能会失效,需要结合几何区域判定法进行综合判定,以避免碰撞预警过程中的"误判"和"漏判".     

小行星族群热物理参数及分布特征研究

小行星热物理是近年来小行星研究领域的一个重要环节, 随着红外观测技术的进步, 该领域的研究取得了长足发展. 小行星发出的热辐射取决于小行星的尺寸、形状、反照率、热惯量(Gamma)、粗糙度等热物理参数. 研究小行星热物理特性的科学意义是多方面的, 比如能够帮助我们计算小行星的Yarkovsky效应和YORP (Yarkovsky-O’Keefe-Radzievskii-Paddackor)效应, 还能对小行星表面的表壤颗粒尺寸进行估算, 从而能更好地对小行星表面的物质成分特征进行研究. 另一方面, 研究小行星族群的热物理特性, 可进一步为研究小行星、小行星带乃至太阳系的形成和演化机制提供重要科学依据. 本文借助先进热物理模型(Advanced Thermophysical Model, ATPM), 结合相应的中红外观测资料计算了Vesta族群、Nysa-Polana族群、Pallas族群、Themis族群、 近地小行星(341843) 2008 EV5、 近地小行星(3200) Phaethon的热物理参数, 揭示了不同种类、不同族群的小行星之间热物理参数的差异和造成这些差异的原因, 以及相同族群中的小行星热物理参数的相似性, 并且基于这些差异和相似性对近地小行星和族群之间的联系及其轨道演化过程进行了讨论.
Vesta族群是由小行星(4) Vesta经历碰撞后产生的碎片形成的. 本文研究了该族群中的10颗小行星, 得到这10颗Vesta族群小行星的平均热惯量为42 $\rm Jm^{-2}\cdot s^{-1/2}\cdot K^{-1}$, 平均几何反照率大小0.328, 并发现与之对应的表面粗糙度普遍较低. 此外, 对已有的主带区域小行星几何反照率进行统计后, 发现Vesta族群小行星的几何反照率普遍偏大, 基于这些热物理参数, 我们进一步估算了这10颗小行星的表壤粒径尺寸范围在0.006--1.673 mm之间. Themis族群也是小行星带中重要的族群之一, 该族群小行星物质成分比较原始, 且成员中大部分可能都有水冰的存在, 对其成员小行星的热物理特性研究可为我们提供该族群母体小行星的内部信息. 我们借助WISE (Wide-field Infrared Survey Explorer)红外观测和ATPM对该族群中3颗体积较大的小行星(62) Erato、(171) Ophelia和(222) Lucia的热物理参数进行了计算, 发现3者之间的热参数大小非常接近, 从热物理的角度证明了这3颗小行星有可能是来自于同一个母体.
小行星(341843) 2008 EV5是一颗Aten型近地小行星(NEA), 光谱类型为C型, 具有潜在撞击地球的危险, 该小行星曾是欧洲空间局(ESA)的小行星探测任务Marco-Polo-R的基准探测目标, 我们借助ATPM和WISE红外观测得到2008 EV5的热惯量$\Gamma = 110_{-10}^{+30}$ $\rm Jm^{-2}\cdot s^{-1/2}\cdot K^{-1}$, 几何反照率$p_v= 0.095_{-0.003}^{+0.016}$, 有效直径$D_eff= 431_{-33}^{+6} \rm m$. 由于其热惯量相对大多数近地小行星较小, 我们推测其可能来自主带区域, 并对其1000条克隆轨道进行了逆向积分1 Myr, 发现其来自主带区域的概率为6.1%, 同时估算了表壤粒径尺寸为0.58--1.3 mm. 研究表明, 2008 EV5有可能来自于Nysa-Polana族群, 我们对这个族群中的小行星(135) Hertha的热参数进行了计算, 得到该小行星的$\Gamma = 30_{-21}^{+35}$ $\rm Jm^{-2}\cdot s^{-1/2}\cdot K^{-1}$, $p_v=0.135_{-0.034}^{+0.018}$, $D_eff=82.863_{-5.027}^{+12.937} \rm km$. 小行星(3200) Phaethon是日本航空航天局探测器(Japan Aerospace Exploration Agency, JAXA) DESTINY+ (Demonstration and Experiment of Space Technology for INterplanetary voYage Phaethon fLyby dUSt science)的探测目标, 其特殊的轨道形状(大偏心率、小近日点距离)导致在一个轨道周期内温度的变化幅度较大, 使之具有特殊的物理特性. 此外, 该小行星也是双子座流星雨的起源. 研究表明Phaethon起源于主带区域中的Pallas族群, 该族群是小行星带中B-type小行星的重要来源, 其成员数目不多, 但目前大部分的具有活动性的小行星均与Pallas族群相关. 本文中, 我们借助ATPM和WISE的红外观测得到Phaethon和Pallas族群小行星Zerlina的热惯量分别为: $\Gamma_Phaethon= 550_{-290}^{+920}$ $\rm Jm^{-2}\cdot s^{-1/2}\cdot K^{-1}$, $\Gamma_Zerlina=0_{-0}^{+34}$ $\rm Jm^{-2}\cdot s^{-1/2}\cdot K^{-1}$, 几何反照率分别为: $p_{v, Zerlina}=0.1435_{-0.0325}^{+0.0420}$, $p_{v, Phaethon}=0.1253_{-0.0020}^{+0.0034}$, 热参数上的差异可能是由于Phaethon较强的活动性, 当Phaethon的轨道演化至当前位置时, 其较高的近日点温度会使表面的物质发生变化, 同时观测也表明Phaethon有质量流失现象, 使得Phaethon与Pallas族群其他小行星相比, 其表面特性发生改变, 从而热物理参数也随之改变.     

快速旋转致密天体的平衡位形及其引力效应

本文用Harrison-Wheeler物态方程,通过“自洽场方法”,对Einstein场方程和广义相对论流体静力学平衡方程作数值求解,研究了快速旋转致密天体的平衡位形及其某些引力效应。结果表明:其平衡位形是扁的旋转椭球,当角速度大于3.0×10~2/秒时,偏心率和天体质量随角速度的增加而迅速增大,在极限情况下,偏心率可达0.7,质量增大可达10％—35％;旋转引起的天体表面引力红移的差异,光线顺逆旋转方向通过天体表面时的偏转角差异都是相当显著的。     

Yarkovsky origin of the unstable asteroids in the 2/1 mean motion resonance with Jupiter

The 2/1 mean motion resonance with Jupiter, intersecting the main asteroid belt at ≈3.27  au, contains a small population of objects. Numerical investigations have classified three groups within this population: asteroids residing on stable orbits (i.e. Zhongguos), those on marginally stable orbits with dynamical lifetimes of the order of 100 Myr (i.e. Griquas), and those on unstable orbits. In this paper, we reexamine the origin, evolution and survivability of objects in the 2/1 population. Using recent asteroid survey data, we have identified 100 new members since the last search, which increases the resonant population to 153. The most interesting new asteroids are those located in the theoretically predicted stable island A, which until now had been thought to be empty. We also investigate whether the population of objects residing on the unstable orbits could be resupplied by material from the edges of the 2/1 resonance by the thermal drag force known as the Yarkovsky effect (and by the YORP effect, which is related to the rotational dynamics). Using N -body simulations, we show that test particles pushed into the 2/1 resonance by the Yarkovsky effect visit the regions occupied by the unstable asteroids. We also find that our test bodies have dynamical lifetimes consistent with the integrated orbits of the unstable population. Using a semi-analytical Monte Carlo model, we compute the steady-state size distribution of magnitude   H < 14  asteroids on unstable orbits within the resonance. Our results provide a good match with the available observational data. Finally, we discuss whether some 2/1 objects may be temporarily captured Jupiter-family comets or near-Earth asteroids.     

Short-lived asteroids in the 7/3 Kirkwood gap and their relationship to the Koronis and Eos families

A population of 23 asteroids is currently observed in a very unstable region of the main belt, the 7/3 Kirkwood gap. The small size of these bodies—with the notable exception of (677) Aaltje (∼30 km)—as well as the computation of their dynamical lifetimes (3<T_D<172 Myr) shows that they cannot be on their primordial orbits, but were recently injected in the resonance. The distribution of inclinations appears to be bimodal, the two peaks being close to 2° and 10°. We argue that the resonant population is constantly being replenished by the slow leakage of asteroids from both the Koronis (I∼2°) and Eos (I∼10°) families, due to the drift of their semi-major axes, caused by the Yarkovsky effect. Assuming previously reported values for the Yarkovsky mean drift rate, we calculate the flux of family members needed to sustain the currently observed population in steady state. The number densities with respect to semi-major axis of the observed members of both families are in very good agreement with our calculations. The fact that (677) Aaltje is currently observed in the resonance is most likely an exceptional event. This asteroid should not be genetically related to any of the above families. Its size and the eccentricity of its orbit suggest that the Yarkovsky effect should have been less efficient in transporting this body to the resonance than close encounters with Ceres.     

Study on the Spin Characteristics of the Flora Asteroid Family

Asteroid families are the remnants of catastrophic collisions, and their fundamental physical properties provide us the information of their parent bodies and thereafter dynamical evolutions. Especially, the orbit and spin characteristics can reveal the influences of the Yarkovsky effect and the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect on the evolution of the asteroid family, respectively. Based on the Asteroid Lightcurve Database (LCDB), the spin rate distribution of the Flora asteroid family is studied, and a tendency that the spin rates of the small Flora family members concentrate primarily in the range of 3–5 d^?1 is found. The analysis on the spin states of the Flora family asteroids tells that most of these asteroid family members are in the prograde spinning state. However, for the Flora family members with an orbital semi-major axis smaller than 2.2 au, the ratio between the number of prograde spinning members and that of retrograde ones is close to that of the near-Earth asteroids, namely 1 : 3. Furthermore, for those prograde spinning Flora family asteroids with an orbital semi-major axis larger than 2.2 au, a portion of them exhibit the aggregation in the distribution of orbital semi-major axis against the absolute magnitude, and in which nine members show the features similar to the Slivan state.     

The Yarkovsky-driven origin of near-Earth asteroids

We investigate the relevance of the Yarkovsky effect for the origin of kilometer and multikilometer near-Earth asteroids (NEAs). The Yarkovsky effect causes a slow migration in semimajor axis of main belt asteroids, some of which are therefore captured into powerful resonances and transported to the NEA space. With an innovative simulation scheme, we determine that in the current steady-state situation 100-160 bodies with H < 18 (roughly larger than 1 km) enter the 3/1 resonance per million years and 40-60 enter the ν₆ resonance. The ranges are due to uncertainties on relevant simulation parameters such as the time scales for collisional disruption and reorientation, their size dependence, and the strength of the Yarkovsky and YORP effects. These flux rates to the resonances are consistent with those independently derived by Bottke et al. (2002, Icarus 156, 399-433) with considerations based only on the NEA orbital distribution and dynamical lifetime. Our results have been obtained assuming that the main belt contains 1,300,000 asteroids with H < 18 and linearly scale with this number. Assuming that the cumulative magnitude distribution of main belt asteroids is N(< H) ∝ 10^γ′H with γ′ = 0.25 in the 15.5 < H < 18 range (consistent with the results of the SDSS survey), we obtain that the bodies captured into the resonances should have a similar magnitude distribution, but with exponent coefficient γ = 0.33-0.40. The lowest value is obtained taking into account the YORP effect, while higher values correspond to a weakened YORP or to YORP-less cases. These values of γ are all compatible with the debiased magnitude distributions of the NEAs according to Rabinowitz et al. (2000, Nature 403, 165-166), Bottke et al. (2000b, Science 288, 2190-2194), and Stuart (2001, Science 294, 1691-1693). Hence the Yarkovsky and YORP effects allow us to understand why the magnitude distribution of NEAs is only moderately steeper than that of the main belt population. The steepest main belt distribution that would still be compatible with the NEA distribution has exponent coefficient γ′ ∼ 0.3.     

YORP-Induced Long-Term Evolution of the Spin State of Small Asteroids and Meteoroids: Rubincam's Approximation

The rotation states of small asteroids and meteoroids are determined primarily by their collisions, gravitational torques due to the Sun and planets (in the case of close encounters), and internal dissipative effects (that relax the free-precession energy toward the fundamental state of principal-axis rotation). Rubincam has recently pointed out that thermal reemission on irregular-shaped bodies also results in a torque that may secularly change both the rotation rate and the orientation of the spin axis (the so-called YORP effect). Here we pursue investigation of this effect. Keeping the zero thermal-relaxation approximation of Rubincam and the assumption of the principal-axis rotation, we study the YORP effect both for precisely determined shapes of near-Earth asteroids and also for a large statistical sample of automatically generated shapes by the Gaussian-sphere technique of Muinonen. We find that the asymptotic state of the YORP evolution is characterized by an arbitrary value of the obliquity, with higher but nearly equal likelihood of 0°/180° and 90° states. At the adopted approximation, the most typical feature of this end state of the YORP evolution is secular deceleration of the rotation rate, which means that at some instant collisions will randomize the rotation state. In a minority of cases, the final state of the obliquity evolution leads to a permanent acceleration of the body's rotation, eventually resulting in rotational fission. The YORP-induced slow evolution may also play an important role in driving the rotation state of small asteroids toward the resonances between the forced precession due to the solar torque and perturbations of the orbital node and inclination. We find that for small Themis asteroids these resonances are isolated in the relevant range of frequencies, and the YORP evolving rotation may be either temporarily captured or rapidly jump across these resonances. In contrast, the possible values of the forced precession for small Flora asteroids may be resonant with clustered, nonisolated lines of the orbital perturbation. The individual rotation histories of small Flora asteroids may be thus very complicated and basically unpredictable. We comment on possible astronomical consequences of these results.     

The YORP effect with finite thermal conductivity

The Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect has been recently suggested to significantly change, on a long-term, rotation state of small asteroids and meteoroids. Though YORP is closely related to the Yarkovsky (orbital) effect, it differs from the latter in two aspects: (i) YORP needs bodies of irregular shape to be effective, and (ii) YORP acts on bodies of zero surface thermal conductivity. To simplify computations, YORP has been so far investigated in the zero surface thermal conductivity limit only. Here we analyze the role of the surface conductivity and we find it substantially changes previous conclusions. Most importantly, unlike in the zero-conductivity limit, (i) YORP preferentially tilts obliquity toward two asymptotic states perpendicular to the orbital plane, and (ii) YORP asymptotically decelerates and accelerates rotation rate in about equal number of cases. Our work also indicates that direct detection of the YORP effect for a small asteroid may significantly constrain its mass.     

V-type asteroids in the middle main belt

V-type asteroids are bodies whose surfaces are constituted of basalt. In the Main Asteroid Belt, most of these asteroids are assumed to come from the basaltic crust of Asteroid (4) Vesta. This idea is mainly supported by (i) the fact that almost all the known V-type asteroids are in the same region of the belt as (4) Vesta, i.e., the inner belt (semi-major axis 2.1<a<2.5 AU), (ii) the existence of a dynamical asteroid family associated to (4) Vesta, and (iii) the observational evidence of at least one large craterization event on Vesta's surface. One V-type asteroid that is difficult to fit in this scenario is (1459) Magnya, located in the outer asteroid belt, i.e., too far away from (4) Vesta as to have a real possibility of coming from it. The recent discovery of the first V-type asteroid in the middle belt (2.5<a<2.8 AU), (21238) 1995WV7 [Binzel, R.P., Masi, G., Foglia, S., 2006. Bull. Am. Astron. Soc. 38, 627; Hammergren, M., Gyuk, G., Puckett, A., 2006. ArXiv e-print, astro-ph/0609420], located at ∼2.54 AU, raises the question of whether it came from (4) Vesta or not. In this paper, we present spectroscopic observations indicating the existence of another V-type asteroid at ∼2.53 AU, (40521) 1999RL95, and we investigate the possibility that these two asteroids evolved from the Vesta family to their present orbits by a semi-major axis drift due to the Yarkovsky effect. The main problem with this scenario is that the asteroids need to cross the 3/1 mean motion resonance with Jupiter, which is highly unstable. Combining N-body numerical simulations of the orbital evolution, that include the Yarkovsky effect, with Monte Carlo models, we compute the probability that an asteroid of a given diameter D evolves from the Vesta family and crosses over the 3/1 resonance, reaching a stable orbit in the middle belt. Our results indicate that an asteroid like (21238) 1995WV7 has a low probability (∼1%) of having evolved through this mechanism due to its large size (D∼5 km), because the Yarkovsky effect is not sufficiently efficient for such large asteroids. However, the mechanism might explain the orbits of smaller bodies like (40521) 1999RL95 (D∼3 km) with ∼70-100% probability, provided that we assume that the Vesta family formed ?3.5 Gy ago. We estimate the debiased population of V-type asteroids that might exist in the same region as (21238) and (40521) (2.5<a?2.62 AU) and conclude that about 10 to 30% of the V-type bodies with D>1 km may come from the Vesta family by crossing over the 3/1 resonance. The remaining 70-90% must have a different origin.     

Analysis of Orbital Characteristics and Evolution of Near-Earth Asteroids (10302) 1989 ML and (4660) Nereus

Near-Earth asteroids (10302) 1989 ML and (4660) Nereus have attracted much attention as candidates for the next generation of deep space explorations. In the study, the maximum Lyapunov exponent (MLE) and MEGNO (Mean Exponential Growth factor of Nearby Orbits) index are calculated after considering the effects of major objects in the Solar system, and the stabilities of these two asteroids are discussed. For each asteroid, 1000 clonal particles consistent with the observational uncertainties are generated from a multivariate normal distribution. Statistical results display probably emerging regions of each asteroid within 0.1 million years, and provide distributions of occurrence times in the phase space of semi-major axis versus eccentricity. We estimate the probability of close encounters and collisions between the asteroid and Earth or other planets. Furthermore, secular resonances, Kozai resonance, and mean motion resonances are analyzed for nominal orbits of the two asteroids. We conclude that 1989 ML is in the region dominated by mean motion resonances with terrestrial planets. The probability of close encounters with them is relatively small, therefore its orbit is relatively stable. Nereus is located in a region that can have close-encounters with the Earth, and it has an extremely unstable orbit.