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 ( 41 ; 39 ) , 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 _ { v } = 0.045 \pm 0.002 , D _ { 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 \pm 40 ~ { } Jm ^ { -2 } s ^ { -0.5 } K ^ { -1 } with a roughness fraction f _ { 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 \sim 20 ~ { } mm from the simulations of subsurface temperature distribution , indicating that 1996 FG3 could be a very suitable target for a sample return mission .