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Opposition effect on S-type asteroid (25143) Itokawa
Lee, Mingyeong,Ishiguro, Masateru Springer-Verlag 2018 Astronomy and astrophysics Vol.616 No.-
<P><I>Aims.</I> 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 <I>α</I>.</P><P><I>Methods.</I> We made use of the data taken at <I>α</I> = 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 (<I>S</I>OE) that characterizes a linear increase in the reflectance <I>I</I>∕<I>F</I> per unit phase angle.</P><P><I>Results.</I> We found that (i) <I>S</I>OE is less dependent on the incidence and emission angles; (ii) the reflectance increases nonlinearly toward the opposition at small angles with <I>α</I> ≲ 1.°4, showing a good correlation between mean <I>I</I>∕<I>F</I> and <I>S</I>OE; and (iii) <I>S</I>OE becomes nearly constant at <I>α</I> ≳ 1.°4 and shows no clear correlation between <I>I</I>∕<I>F</I> and <I>S</I>OE.</P><P><I>Conclusions.</I> From these results, we conjecture that CBOE is dominant at <I>α</I> ≲ 1.°4, while SHOE is dominant at <I>α</I> ≳ 1.°4.</P>