Albedo, Size, and Surface Characteristics of Hayabusa-2 Sample-Return Target 162173 1999 JU3 from AKARI and Subaru † Observations

Mid-infrared observations of the Apollo-type asteroid 162173 1999 JU3 were performed in 2007 May during its close Earth approach with the AKARI space telescope and in 2007 August with the Subaru telescope. A thermophysical model analysis of the obtained data resulted in a radiometric diameter of 0.92 $\pm$ 0.12km and a visual geometric albedo of 0.063 $^{+0.020}_{-0.015}$ . The geometric albedo is typical for C-type asteroids. The solutions for the thermal inertia of 1999 JU3's surface material were not as clear as for the asteroid 25143 Itokawa due to less-favorable observing conditions. But the most likely solutions are connected to high thermal inertia values, indicating that the surface is predominantly covered by boulders and bare rocks, while areas with thick dust regolith are less common.