摘要：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 anglesα.Methods.We made use of the data taken atα= 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 (SOE) that characterizes a linear increase in the reflectanceI∕Fper unit phase angle.Results.We found that (i)SOEis less dependent on the incidence and emission angles; (ii) the reflectance increases nonlinearly toward the opposition at small angles withα≲ 1.°4, showing a good correlation between meanI∕FandSOE; and (iii)SOEbecomes nearly constant atα≳ 1.°4 and shows no clear correlation betweenI∕FandSOE.Conclusions.From these results, we conjecture that CBOE is dominant atα≲ 1.°4, while SHOE is dominant atα≳ 1.°4.
