The tumbling spin state of (99942) Apophis
The tumbling spin state of (99942) Apophis
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DOI：
10.1016/j.icarus.2014.01.026
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发表时间：
2014-05
期刊：
影响因子：
3.2
通讯作者：
P. Pravec;P. Scheirich;J. Ďurech;J. Pollock;P. Kušnirák;K. Hornoch;A. Galád;D. Vokrouhlický;A. Harris;E. Jehin;J. Manfroid;C. Opitom;M. Gillon;F. Colas;J. Oey;J. Vraštil;D. Reichart;K. Ivarsen;J. Haislip;A. Lacluyze
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作者：
P. Pravec;P. Scheirich;J. Ďurech;J. Pollock;P. Kušnirák;K. Hornoch;A. Galád;D. Vokrouhlický;A. Harris;E. Jehin;J. Manfroid;C. Opitom;M. Gillon;F. Colas;J. Oey;J. Vraštil;D. Reichart;K. Ivarsen;J. Haislip;A. Lacluyze
我们在2012年12月至2013年4月期间对小行星（99942）Apophis的测光观测显示，它处于非主轴旋转（翻滚）状态。我们建立了它的自旋和形状模型，发现它处于中等激发的短轴模（SAM）态，转动动能与基本自旋态能量之比E/E0 = 1.024±0.013. (All引用的不确定度对应于3 σ。）最大主惯性矩和中间主惯性矩几乎相同，I2/I3 = 0.965-0.015+ 0.009，但最小主惯性矩要小得多，I1/I3 = 0.61-0.08+ 0.11;小行星的动力学等效椭球体接近长椭球体。旋进和自转周期分别为P1 = 27.38±0.07 h和P2 = 263±6 h; SAM观测到的最强光变曲线振幅为P1 = P2 - 1-P2 - 1 - 1 = 30.56±0.01 h的二次谐波。自转是逆行的，角动量矢量的黄道经度和纬度分别为250°和-75 °（不确定区域近似是一个椭圆，长半轴和短半轴分别为27°和14°）。逆行旋转的一个暗示是阿波菲斯在2068年撞击的可能性有所增加，但它仍然非常小，巴勒莫尺度上的风险水平仍然远低于零。阿波菲斯是缓慢翻滚的小行星群中的一员。应用Breiter等人的小行星章动阻尼理论。（Breiter，S.，Rozelek，A.，Vokrouhlicksaud，D. [2012年]。Mon.没有R. Astron.Soc.427，755-769），我们发现缓慢翻滚的小行星在自旋率大小范围内占主导地位，在这个范围内，它们的估计阻尼时间大于约0.2 Gyr。PA/NPA转子转变线似乎遵循恒定阻尼时间线的外观可能是因为有两个或更多的小行星自旋演化机制在起作用，或者μ Q因子（弹性模量乘以品质因子）不是恒定的，但它可能会随着小行星尺寸的减小而减小，这将与碰撞年龄或激发时间的减小相反。
Our photometric observations of Asteroid (99942) Apophis from December 2012 to April 2013 revealed it to be in a state of non-principal axis rotation (tumbling). We constructed its spin and shape model and found that it is in a moderately excited Short Axis Mode (SAM) state with a ratio of the rotational kinetic energy to the basic spin state energy E/E 0= 1.024±0.013.(All quoted uncertainties correspond to 3 σ.) The greatest and intermediate principal moments of inertia are nearly the same with I 2/I 3= 0.965-0.015+ 0.009, but the smallest principal moment of inertia is substantially lower with I 1/I 3= 0.61-0.08+ 0.11; the asteroid’s dynamically equivalent ellipsoid is close to a prolate ellipsoid. The precession and rotation periods are P ϕ= 27.38±0.07 h and P ψ= 263±6 h, respectively; the strongest observed lightcurve amplitude for the SAM case is in the 2nd harmonic of P 1= P ϕ-1-P ψ-1-1= 30.56±0.01 h. The rotation is retrograde with the angular momentum vector’s ecliptic longitude and latitude of 250° and-75°(the uncertainty area is approximately an ellipse with the major and minor semiaxes of 27° and 14°, respectively). An implication of the retrograde rotation is a somewhat increased probability of the Apophis’ impact in 2068, but it is still very small with the risk level on the Palermo Scale remaining well below zero. Apophis is a member of the population of slowly tumbling asteroids. Applying the theory of asteroid nutational damping by Breiter et al.(Breiter, S., Rożek, A., Vokrouhlický, D.[2012]. Mon. Not. R. Astron. Soc. 427, 755–769), we found that slowly tumbling asteroids predominate in the spin rate–size range where their estimated damping times are greater than about 0.2 Gyr. The appearance that the PA/NPA rotators transition line seems to follow a line of constant damping time may be because there are two or more asteroid spin evolution mechanisms in play, or the factor of μ Q (the elastic modulus times the quality factor) is not constant but it may decrease with decreasing asteroid size, which would oppose the trend due to decreasing collisional age or excitation time.