Opposition effect on S-type asteroid (25143) Itokawa

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 (SOE) that characterizes a linear increase in the reflectance I∕F per unit phase angle. Results. We found that (i) SOE 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 SOE; and (iii) SOE becomes nearly constant at α ≳ 1.°4 and shows no clear correlation between I∕F and SOE. Conclusions. From these results, we conjecture that CBOE is dominant at α ≲ 1.°4, while SHOE is dominant at α ≳ 1.°4.

Spectral type and geometric albedo of (98943) 2001 CC21, the Hayabusa2# mission target

“…The geometric albedo of an S-type asteroid, (25143) Itokaw a, w as estimated by polarimetry before the launch (Cellino et al 2005 ). Notably, the geometric albedo estimated by the polarization method matches the actual value derived by the in situ observation (Lee & Ishiguro 2018 ;Tatsumi et al 2018 ). Considering that CC21 is an S-type asteroid, our albedo value obtained by the same method as Cellino et al ( 2005 ) is also quite reliable.…”

The Geometric Albedo of (4179) Toutatis Estimated from KMTNet DEEP-South Observations

“…This was first conjectured observationally by Markov & Barabashev (1926) from lunar observations as mentioned in the appendix of Dollfus & Bowell (1971). It was confirmed by Lee & Ishiguro (2018) using a modern theoretical model: their Figure 4 shows that the radiance factor (defined in Section 3.2) is reduced only by ∼ 1 % or ∼ 3 % when the incidence angle is chagned from 0 • to 70 • or 90 • (the azimuthal angle and phase angle are fixed to 0). This means, if we correct the phase effect using Equation (3), the obtained H V will be linearly related to the logarithm of the geometrical cross-sectional area (Equation 5 below).…”

“…From the Vfilter result in Figure 7 of Tatsumi et al (2018), f 1 ∼ 1.3 for the asteroid (25143) Itokawa, an S-type asteroid. f 2 was determined for asteroid Itokawa in Lee & Ishiguro (2018). For Itokawa, they found f 2 ∼ 0.01 ± 0.01 for α > 1.4 • and f 2 ∼ 0.04±0.01 for α < 1.4 • (note that the sign convention is opposite from the one for their s OE ).…”

“…For Itokawa, they found f 2 ∼ 0.01 ± 0.01 for α > 1.4 • and f 2 ∼ 0.04±0.01 for α < 1.4 • (note that the sign convention is opposite from the one for their s OE ). We employed 0.7 • and 2.5 • because the phase angle difference is larger than 0.3 • as Lee & Ishiguro (2018) and this is the interpolated region. Both f 1 and f 2 coincide with the expected values from previous works, but it is difficult to arrive at further conclusions due to the uncertainties.…”