11:30 〜 11:45
[MIS14-04] 軌道と自転から明らかにする小惑星Ryuguの力学進化
キーワード：小惑星、162173 リュウグウ、ヤルコフスキー効果、YORP効果、軌道進化、N体計算
Orbital evolution of small solar system bodies supplies near-Earth asteroids from the main asteroid belt. The inward migration of small bodies is also an important source of supply of meteorites to the Earth and transport of volatile materials in the solar system. In order to interpret the results of the coming analysis of Hayabusa2's sample, it is essential to understand the dynamical evolution processes of the target asteroid, 162173 Ryugu, in terms of its orbit and spin.
As well as the gravitational effect of the major planets, the thermal recoil force also affects the dynamics of an asteroid. The thermal recoil force and torque that secularly changes its orbit and spin are known as the Yarkovsky effect and Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect, respectively. The direction of the Yarkovsky drift, i. e., a change of the orbital semi-major axis depends on the direction of rotation. It is required to simulate the combined effect of Yarkovsky and YORP for understanding the evolutional path of Ryugu's orbit from the source region in the inner main belt.
First, we conducted a numerical simulation of Ryugu's spin alteration caused by the YORP effect (Kanamaru et al., submitted to JGR-Planets). Given the current near-Earth orbit and three-dimensional shape of Ryugu observed by Hayabusa2, it is estimated that the rotation of the asteroid is decreasing at a rate of (-0.42 to -6.3) × 10-6 deg/day2. The corresponding time scale of the spin-down is 0.58 to 8.7 million years. This "recent" spin-down is considered to be caused by a major resurfacing event on Ryugu such as the formation of the western bulge or the largest crater, Urashima. Our YORP simulation also indicates that the thermal torque has maintained the spin pole upright with respect to the orbital plane. The spin-pole stability with the retrograde rotation results in the inward migration of Ryugu by the Yarkovsky effect.
Second, we developed a N-body simulator for orbital evolution of small bodies, which incorporates the combined effect of the gravitational perturbation and above thermal force. We will present some possible paths of Ryugu's orbital evolution and its time scale from the source region in the main belt to the near-Earth orbit.
As well as the gravitational effect of the major planets, the thermal recoil force also affects the dynamics of an asteroid. The thermal recoil force and torque that secularly changes its orbit and spin are known as the Yarkovsky effect and Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect, respectively. The direction of the Yarkovsky drift, i. e., a change of the orbital semi-major axis depends on the direction of rotation. It is required to simulate the combined effect of Yarkovsky and YORP for understanding the evolutional path of Ryugu's orbit from the source region in the inner main belt.
First, we conducted a numerical simulation of Ryugu's spin alteration caused by the YORP effect (Kanamaru et al., submitted to JGR-Planets). Given the current near-Earth orbit and three-dimensional shape of Ryugu observed by Hayabusa2, it is estimated that the rotation of the asteroid is decreasing at a rate of (-0.42 to -6.3) × 10-6 deg/day2. The corresponding time scale of the spin-down is 0.58 to 8.7 million years. This "recent" spin-down is considered to be caused by a major resurfacing event on Ryugu such as the formation of the western bulge or the largest crater, Urashima. Our YORP simulation also indicates that the thermal torque has maintained the spin pole upright with respect to the orbital plane. The spin-pole stability with the retrograde rotation results in the inward migration of Ryugu by the Yarkovsky effect.
Second, we developed a N-body simulator for orbital evolution of small bodies, which incorporates the combined effect of the gravitational perturbation and above thermal force. We will present some possible paths of Ryugu's orbital evolution and its time scale from the source region in the main belt to the near-Earth orbit.