Hitoshi Beniyama (Doctoral Student, Department of Astronomy)

Shigeyuki Sakamukai (Associate Professor, Institute of Astronomy)

Ryo Osawa, Project Assistant Professor, Institute of Astronomy

Key points of the presentation

• We observed 60 near-Earth asteroids with diameters smaller than 100 m at two frames per second, and successfully estimated the rotation periods of 32 asteroids. This observation revealed the existence of an upper limit to the rotation period of asteroids.
• By performing immediate movie observation of the asteroids immediately after their discovery, we succeeded in capturing the short-term light intensity fluctuations associated with their high-speed rotation.
• It is expected to lead to the elucidation of the dynamical evolution of how asteroids come to the vicinity of the earth.

Summary of Presentation

A research group led by Graduate Student Hitoshi Beniyama of the Department of Astronomy, Graduate School of Science, the University of Tokyo, Associate Professor Shigeyuki Sakamukai of the Institute of Astronomy of the University of Tokyo, and Project Associate Professor Ryo Osawa has conducted video observations of 60 small asteroids with diameters less than 100 m using the visible light video camera Tomoegosen ^{(Note 1)} on the Kiso Schmidt Telescope at the University of Tokyo. The research group has successfully estimated the rotation periods of 32 of these objects by using the visible light movie camera Tomoegosen (Note 1) on the University of Tokyo's Kiso Schmidt Telescope. Asteroids, which appear to glow by reflecting sunlight, can be observed with high precision only during short periods of time, from a few hours to a few days, when they appear bright as they approach the Earth. Since most asteroids are not spherical, their brightness fluctuates in synchronization with the change in the area reflecting sunlight as the asteroid rotates around its axis. The research group conducted immediate moving image observations using Tomoegosen of the asteroid immediately after its discovery, and succeeded in estimating the rotation period of the asteroid by capturing the temporal changes in brightness. Thirteen of the asteroids were found to be rotating at high speed with a period of less than 60 seconds.

Through this observation, the research group discovered the existence of an upper limit (approximately 10 seconds) to the rotation period of asteroids, and proposed a dynamical model to explain the existence of this upper limit. This research is expected to lead to the elucidation of the dynamical evolution of asteroids approaching the Earth and how they come to be in the vicinity of the Earth.

Publication details

In recent years, many asteroids with near-Earth orbits have been discovered by sweeping sky observation projects around the world. Asteroids that are close to the Earth have the advantage of being easily accessible to spacecraft. They are also important observation targets from the viewpoint of protecting the Earth from asteroids that may cause damage to humans. Many near-Earth asteroids are thought to have evolved from the Mars-Jupiter asteroid belt to the Earth's vicinity over a long period of time. Asteroids rotate on their own axis like the Earth and other planets (Figure1). The rotation period is known to be affected by the Yope effect ^{(Note 2)}, which is caused by solar radiation. The yawp effect is known to be stronger for objects with smaller diameters. An asteroid whose rotation is accelerated by the Yope effect may be destroyed by centrifugal force when it reaches the critical rotation period allowed by its material strength. If there is a definite critical rotation period, we can determine the material strength of the asteroid from its rotation period distribution. However, since it is difficult to measure the rotation period of a small asteroid, there are many unknowns such as whether there is a critical rotation period determined by the strength of the asteroid or whether the rotation period distribution of the asteroid is consistent with the prediction of the Yope effect.

Figure 1: Imaginary image of a fast rotating near-Earth asteroid (Image credit: The University of Tokyo's Kiso Observatory)

The rotation period of an asteroid can be estimated by observing the asteroid multiple times as it approaches the Earth and examining the time variation of its brightness. This is because the apparent size of the asteroid as seen by the observer changes with its rotation. There are several research groups around the world that use large telescopes to observe the rotation period of asteroids, but they all use long-exposure observations, which flatten the brightness of the asteroid and may miss the variations in luminosity caused by the fast rotation of the asteroid. In addition, asteroids with orbits approaching the Earth are apparently brighter when they approach the Earth. Asteroids that approach within three times the Earth-moon distance can only be observed for a few hours to a few days at the time of closest approach. Because of these observational difficulties, only a small number of asteroids have had their rotation periods determined by follow-up observations. In order to estimate the rotation period of a small asteroid, it is necessary to make follow-up observations with high temporal resolution immediately after the discovery of the asteroid.

Figure 2: Observation image of asteroid 2022 _UQ6. The field of view is 1arcmin x 1 arcmin.

Figure 3: Temporal variation of the brightness of asteroid 2022 _UQ6(based on published journals ).

The research group conducted observations of a total of 60 asteroids, including 23 asteroids discovered by the research group itself, ^{(Note 3)}, using the visible light movie camera Tomoegosen in Nagano Prefecture, Japan. The difficulty of the time available for observation of the micro asteroids was overcome by immediately tracking and observing them within a few hours to a few days immediately after their discovery. Most of the observed asteroids were objects passing through the extremely near-Earth region within three times the Earth-moon distance. Figures 2 and 3 show the observed image of micro asteroid 2020 _UQ6 and its brightness variation with time. We successfully estimated the rotation periods of 32 asteroids by observing movies at two frames per second for about 20 minutes per object(Figure4). Thirteen of the 32 objects were found to be rotating at high speeds with periods of less than 60 seconds. However, considering the yawp effect, the rotation of small asteroids with diameters of 10 m or less should be accelerated to a rotation period of 10 s or less. However, only one asteroid with a rotation period of less than 10 s was found in this observation, which was inconsistent with the prediction of the Yope effect that the rotation period would accelerate to less than 10 s. The research group found that the rotation period of the asteroid was less than 10 s, which was consistent with the prediction of the Yope effect. The research group has tested several hypotheses that could explain the observed rotation period distribution, and has shown that the recently proposed tangential yawp effect (^(*4) ), which takes into account heat conduction along the asteroid's surface, can explain the observed results. The dashed line in Figure 4 is the expected rotation period of the asteroid when the tangential yawp effect is considered, which explains the observed results well.

Figure 4: Relationship between size and rotation period obtained from previous observations of asteroids. Red circles indicate the results of this study, and blue crosses indicate the results of previous studies. The dashed line is the upper limit of the predicted rotation period distribution of near-Earth asteroids when the tangential yawp effect is taken into account. The single-dashed line is the upper limit of the rotation period distribution of the near-Earth asteroid predicted by considering only the conventional yawp effect. Observations of previous studies were taken from The Asteroid Lightcurve Database by Warner et al. (2009, Icarus, 202, 134) (based on published journals).

In this study, we focused on micro asteroids, which have been difficult to observe in the past, and clarified their rotation period distribution by immediate moving image observation immediately after their discovery. The upper limit of the rotation period of micro asteroids suggests the existence of a mechanism that suppresses the rotation acceleration of micro asteroids. This study is expected to lead to the elucidation of the dynamical evolution of asteroids approaching the Earth and how they are acted upon as they come to the Earth's vicinity, based on the observation of the rotation state of asteroids.

This research has been supported by Grant-in-Aid for Scientific Research (project numbers: 21H04491, 20H04617, 18H05223, 18H01272, 18H01261, 18K13599, 17H06363, 16H06341, 16H02158, 26247074, 25103502), the Optical and Infrared Astronomy Inter-university Collaboration Program, Iwadate Scholarship Foundation, Hisao Iwai Memorial Tokyo Scholarship and Scholarship Fund, JST Support for Pioneering Research Initiated by the Next Generation Program JPMJSP2108, and UTEC University of Tokyo Scholarship.

Journals

Journal name	Publications of the Astronomical Society of Japan
Title of paper	Video Observations of tiny near-Earth objects with Tomo-e Gozen
Author(s)	Jin BENIYAMA*, Shigeyuki SAKO, Ryou OHSAWA, Satoshi TAKITA, Naoto KOBAYASHI, Shin-ichiro OKUMURA, Seitaro URAKAWA, Makoto YOSHIKAWA, Fumihiko USUI, Fumi YOSHIDA, Mamoru DOI, Yuu NIUI Fumi YOSHIDA, Mamoru DOI, Yuu NIINO, Toshikazu SHIGEYAMA, Masaomi TANAKA, Nozomu TOMINAGA, Tsutomu AOKI, Noriaki ARIMA, Ko ARIMATSU, Toshihiro KASUGA Sohei KONDO, Yuki MORI, Hidenori TAKAHASHI, and Jun-ichi WATANABE
DOI Number	10.1093/pasj/psac043
Abstract URL	https://academic.oup.com/pasj/advance-article/doi/10.1093/pasj/psac043/6638979

Terminology

Note 1: Visible light moving image camera tomoegosen

The world's first visible light wide-field movie camera developed mainly by The University of Tokyo for the 105 cm aperture Schmidt Telescope at the Kiso Observatory of the Institute of Astronomy ( Kiso County, Nagano Prefecture, Japan ). 84CMOS image sensors enable observation of a wide sky area with high temporal resolution. The camera is the world's first wide-field video camera for the 105 cm telescope.

(Image credit: The University of Tokyo,Kiso Observatory) https://tomoe.mtk.ioa.s.u-tokyo.ac.jp/ja/↑

Note 2: Yawp Effect

The yawp effect is a phenomenon in which the rotation state of a celestial body changes due to solar radiation. YORP is an acronym for Yarkovsky-O'Keefe-Radzievskii-Paddack and was named by David Parry Rubincam in 2000 (Rubicam 2000 , Icarus, 148, 2). Celestial bodies in the solar system receive sunlight on their surfaces, some of which is reflected and some of which is transferred to the interior of the body as heat. The thermal radiation and reflected light are emitted in different ways on each surface of the asteroid, and if the shape of the asteroid is axisymmetric with respect to the axis of the asteroid's rotation, they cancel out and no torque occurs. Real asteroids have asymmetric surfaces, which generate a torque that accelerates or decelerates the rotation. This torque changes the rotation state of the asteroid over a long period of time. ↑up

Note 3: Discovery of asteroids using tom oegosen

Tomoegosen has successfully discovered a total of 42 near-Earth asteroids from 2019 to June 30, 2022. For more information, please refer to the following website of Kiso Observatory, Institute of Astronomy Education and Research, Graduate School of Science, The University of Tokyo. ↑up

Note 4: Yope effect(tangential yope effect), which takes into account heat conduction along the asteroid's surface.

Since 2000, formulations of the yawp effect have considered heat conduction perpendicular to the asteroid's surface; in 2012, Golubov & Krugly (2012, ApJ, 752, L11) developed a heat conduction model for the case of structures on the asteroid's surface. The model calculations showed that the rotation period changes as strongly as or stronger than the previously considered yawp effect when heat conduction in the direction along the asteroid's surface is taken into account. ↑