Optical Properties of (162173) 1999 JU3: In Preparation for the JAXA Hayabusa 2 Sample Return Mission

We investigated the magnitude-phase relation of (162173) 1999 JU3, a target asteroid for the JAXA Hayabusa 2 sample return mission. We initially employed the International Astronomical Union's H-G formalism but found that it fits less well using a single set of parameters. To improve the inadequate fit, we employed two photometric functions: the Shevchenko and Hapke functions. With the Shevchenko function, we found that the magnitude-phase relation exhibits linear behavior in a wide phase angle range (α = 5°-75°) and shows weak nonlinear opposition brightening at α < 5°, providing a more reliable absolute magnitude of H _V = 19.25 ± 0.03. The phase slope (0.039 ± 0.001 mag deg^-1) and opposition effect amplitude (parameterized by the ratio of intensity at α = 0.°3 to that at α = 5°, I(0.°3)/I(5°) = 1.31 ± 0.05) are consistent with those of typical C-type asteroids. We also attempted to determine the parameters for the Hapke model, which are applicable for constructing the surface reflectance map with the Hayabusa 2 onboard cameras. Although we could not constrain the full set of Hapke parameters, we obtained possible values, w = 0.041, g = -0.38, B ₀ = 1.43, and h = 0.050, assuming a surface roughness parameter \bar{θ } = 20°. By combining our photometric study with a thermal model of the asteroid, we obtained a geometric albedo of p _v = 0.047 ± 0.003, phase integral q = 0.32 ± 0.03, and Bond albedo A _B = 0.014 ± 0.002, which are commensurate with the values for common C-type asteroids.

This work was conducted as part of the activities of the JAXA Hayabusa 2 Ground-Based Observation Sub-Group.