Shape, Thermal and Surface Properties determination of a Candidate Spacecraft Target Asteroid (175706) 1996 FG3

In this paper, a 3D convex shape model of (175706) 1996 FG3, which consists of 2040 triangle facets and 1022 vertices, is derived from the known lightcurves. The best-fit orientation of the asteroid's spin axis is determined to be $\lambda =237.7^\circ$ and $\beta=-83.8^{\circ}$ considering the observation uncertainties, and its rotation period is $\sim$ 3.5935 h . Using the derived shape model, we adopt the so-called advanced thermophysical model (ATPM) to fit three published sets of mid-infrared observations of 1996 FG3 \citep{Wolters2011,Walsh2012}, so as to evaluate its surface properties. Assuming the primary and the secondary bear identical shape, albedo, thermal inertia and surface roughness, the best-fit parameters are obtained from the observations. The geometric albedo and effective diameter of the asteroid are reckoned to be $p_{\rm v}=0.045\pm0.002$, $D_{\rm eff}=1.69^{+0.05}_{-0.02}$ km. The diameters of the primary and secondary are determined to be $D_{1}=1.63^{+0.04}_{-0.03}$ km and $D_{2}=0.45^{+0.04}_{-0.03}$ km, respectively. The surface thermal inertia $\Gamma$ is derived to be a low value of $80\pm40\rm Jm^{-2}s^{-0.5}K^{-1}$ with a roughness fraction $f_{\rm R}$ of $0.8^{+0.2}_{-0.4}$. This indicates that the primary possibly has a regolith layer on its surface, which is likely to be covered by a mixture of dust, fragmentary rocky debris and sand. The minimum regolith depth is estimated to be $5\sim20\rm mm$ from the simulations of subsurface temperature distribution, indicating that 1996 FG3 could be a very suitable target for a sample return mission.