Shape, thermal and surface properties determination of a candidate spacecraft target asteroid (175706) 1996 FG3

Abstract

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 light curves. The best-fitting orientation of the asteroid's spin axis is determined to be λ = 237| $_{.}^{\circ}$|7 and β = −83| $_{.}^{\circ}$|8 considering the observation uncertainties, and its rotation period is ∼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, so as to evaluate its surface properties. Assuming the primary and the secondary bear identical shape, albedo, thermal inertia and surface roughness, the best-fitting parameters are obtained from the observations. The geometric albedo and effective diameter of the asteroid are reckoned to be pv = 0.045 ± 0.002, |$D_{\rm eff}=1.69^{+0.05}_{-0.02}$| 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 Γ is derived to be a low value of 80 ± 40 Jm−2 s−0.5 K−1 with a roughness fraction fR 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 5–20 mm from the simulations of subsurface temperature distribution, indicating that 1996 FG3 could be a very suitable target for a sample return mission.