Orientation and rotational parameters of asteroid 4179 Toutatis: new insights from Chang′e-2's close flyby

Abstract

In this paper, we investigate the rotational dynamics of the ginger-shaped near-Earth asteroid 4179 Toutatis, which was closely observed by Chang′e-2 at a distance of 770 ± 120 m from the asteroid's surface during the outbound flyby on 2012 December 13. A sequence of high-resolution images was acquired during the flyby mission. In combination with ground-based radar observations collected over the last two decades, we analyse these flyby images and determine the orientation of the asteroid at the flyby epoch. The 3–1–3 Euler angles of the conversion matrix from the J2000 ecliptic coordinate system to the body-fixed frame are evaluated to be −20<f>$_{.}^{\\circ}$</f>1 ± 1°, 27<f>$_{.}^{\\circ}$</f>6 ± 1° and 42<f>$_{.}^{\\circ}$</f>2 ± 1°, respectively. The least-squares method is utilized to determine the rotational parameters and spin state of Toutatis. The characteristics of the spin-state parameters and angular momentum variations are extensively studied using numerical simulations, which confirm those reported by Takahashi, Busch & Scheeres (<cross-ref type="bib" refid="bib20">2013</cross-ref>, AJ, 146, 95). The large amplitude of the precession of Toutatis is assumed to be responsible for its tumbling attitude as observed from Earth. The angular momentum orientation of Toutatis is determined to be described by <f>$\\lambda _{\\rm H} = 180 {^{\\circ}_{.}} 2^{+0{^{\\circ}_{.}} 2}_{-0{^{\\circ}_{.}} 3}$</f> and <f>$\\beta _{\\rm H} = -54{^{\\circ}_{.}} 75^{+0{^{\\circ}_{.}} 15}_{-0{^{\\circ}_{.}} 10}$</f>, implying that it has remained nearly unchanged for two decades. Furthermore, using Fourier analysis to explore the change in the orientation of the axes of Toutatis, we reveal that the two rotational periods are 5.38 and 7.40 d, respectively, consistent with the results of the earlier investigation. Hence, our investigation provides a clear understanding of the state of the rotational dynamics of Toutatis.