loading page

YORP Effect on Asteroid 162173 Ryugu: Implications for the Dynamical History
  • +12
  • Masanori Kanamaru,
  • Sho Sasaki,
  • Tomokatsu Morota,
  • Yuichiro Cho,
  • Eri Tatsumi,
  • Masatoshi Hirabayashi,
  • Naru Hirata,
  • Hiroki Senshu,
  • Yuri Shimaki,
  • Naoya Sakatani,
  • Satoshi Tanaka,
  • Tatsuaki Okada,
  • Tomohiro Usui,
  • Seiji Sugita,
  • Sei-ichiro Watanabe
Masanori Kanamaru
Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA)

Corresponding Author:[email protected]

Author Profile
Sho Sasaki
Osaka University
Author Profile
Tomokatsu Morota
The University of Tokyo
Author Profile
Yuichiro Cho
The University of Tokyo
Author Profile
Eri Tatsumi
Instituto de Astrofisica de Canarias
Author Profile
Masatoshi Hirabayashi
Auburn University
Author Profile
Naru Hirata
University of Aizu
Author Profile
Hiroki Senshu
Chiba Institute of Technology
Author Profile
Yuri Shimaki
Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA)
Author Profile
Naoya Sakatani
Rikkyo University
Author Profile
Satoshi Tanaka
Author Profile
Tatsuaki Okada
Author Profile
Tomohiro Usui
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency
Author Profile
Seiji Sugita
University of Tokyo
Author Profile
Sei-ichiro Watanabe
Nagoya University
Author Profile


Asteroid 162173 (Ryugu) is a carbonaceous asteroid that was visited by Japan’s Hayabusa2 spacecraft in 2018. The formation mechanism of spinning-top shape of Ryugu is an essential clue to the dynamical history of the near-Earth asteroid. In this study, we address the spin-state evolution of Ryugu induced by the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, i.e., the thermal recoil torque that changes the rotation period and spin-pole direction.
Given the current orbit, spin state, and three-dimensional shape observed by Hayabusa2, we computed the YORP torque exerted on Ryugu using a simplified thermal model approximating zero thermal conductivity. Despite differences in meter-scaled topography, all 20 shape models that we examined indicate that the spin velocity of Ryugu is currently decreasing at a rate of (-0.42—6.3)*10-6 deg/day2. Our findings also suggest that the thermal torque on the asteroid is responsible for maintaining the spin pole upright with respect to the orbital plane.
Therefore, the YORP effect could explain the significant spin-down from a period of 3.5 h initially to 7.6 h currently. The corresponding time scale of the rotational deceleration is estimated to be 0.58–8.7 million years, depending on the input shape models. This time scale is comparable to e.g., the formation period of the largest crater, Urashima (5–12 Ma) or the western bulge (2–9 Ma) as derived from previous studies on crater statistics in Ryugu. It is considered that the rotation of the asteroid started to decelerate in the wake of the major crater formation or the resurfacing event on the western hemisphere.
Dec 2021Published in Journal of Geophysical Research: Planets volume 126 issue 12. 10.1029/2021JE006863