Opposition effect on S-type asteroid (25143) Itokawa

¹ Department of Physics and Astronomy, Seoul National University, 1 Gwanak, Seoul 08826, Republic of Korea
e-mail: mingyeong@kasi.re.kr; ishiguro@astro.snu.ac.kr
² University of Science & Technology, Daejeon 34113, Republic of Korea
³ Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea

Received: 21 January 2018
Accepted: 11 May 2018

Abstract

Aims. The opposition effect has been detected on solar system bodies such as asteroids and comets. Two mechanisms have been proposed to explain the effect: the shadow-hiding opposition effect (SHOE) and the coherent backscattering opposition effect (CBOE). The Hayabusa asteroid sample return mission provides a unique opportunity to investigate the opposition effect on disk-resolved images of the S-type asteroid (25413) Itokawa at very small phase angles α.

Methods. We made use of the data taken at α = 0.°04–2.°54 using the Asteroid Multi-band Imaging Camera (AMICA) on UT 2005 October 13. Comparing sets of two images taken at different phase angles, we derived the opposition slope parameter (S_OE) that characterizes a linear increase in the reflectance I∕F per unit phase angle.

Results. We found that (i) S_OE is less dependent on the incidence and emission angles; (ii) the reflectance increases nonlinearly toward the opposition at small angles with α ≲ 1.°4, showing a good correlation between mean I∕F and S_OE; and (iii) S_OE becomes nearly constant at α ≳ 1.°4 and shows no clear correlation between I∕F and S_OE.

Conclusions. From these results, we conjecture that CBOE is dominant at α ≲ 1.°4, while SHOE is dominant at α ≳ 1.°4.