Searching the pole solution of NEA 162173 (1999 JU3)

Near-Earth asteroid (NEA) 162173 (1999 JU3) (hereafter 1999 JU3) is the primary target of JAXA's Hayabusa 2 mission and also a backup target of NASA's OSIRIS-REx mission, not only because of its accessibility but also because it would be the first C-type asteroid for exploration missions. Knowing the information about spin status, such as rotational period, the ecliptic longitude and latitude of the pole is essential for the design of mission sequence such as the remote sensing observation. In order to get the physical properties of 1999 JU3, a total of 75 days ground-based observations at various geometries were carried out during 2007 - 2012 apparitions. Observations in the thermal infrared were also conducted with the Subaru, Akari, and Spitzer telescopes. We have analyzed the optical lightcurve of 1999 JU3, and derived a sidereal rotational period of 7.631 ± 0.001 hour. Using the lightcurve inversion method in conjunction with the thermal physical modeling, we determined a nearly spherical shape with a diameter of 823 ± 38 m, a geometric albedo of 0.058 ± 0.003, and a thermal inertia of 231 ± 76 J m-2 s-0.5 K-1. Although the pole orientation of 1999 JU3 is not strongly constrained, the best solution we derived is within 30 degree of (103,-20) in the ecliptic reference frame.