Physical characterization of low delta-V asteroid (175706) 1996 FG3

Asteroid (175706) 1996 FG3 is a binary asteroid and the baseline target for the proposed MarcoPolo-R sample return mission. We present thermal-infrared photometry obtained with the European Southern Observatory (ESO) Very Large Telescope using the VISIR instrument, together with optical photometry obtained with the ESO New Technology Telescope using the EFOSC2 instrument. An absolute visual magnitude HV = 17.833 ± 0.024 and phase parameter G=−0.041 ± 0.005 are derived. The near-Earth asteroid thermal model has been fitted to the measured fluxes to derive a geometric visual albedo p v= 0.046 ± 0.014, effective diameter at the observed aspect D eff= 1.68 ± 0.25 km and beaming parameter η= 1.15 for phase angle α= 11°.7. The advanced thermophysical model (ATPM) has been fitted to the measured fluxes to derive a more accurate effective diameter D eff= 1.71 ± 0.07 km and albedo p v= 0.044 ± 0.004. Based on the ATPM results, assuming the same albedo for primary and secondary, we derive a primary mean spherical diameter D p= 1.69+0.18 − 0.12 km, secondary diameter D s= 0.51 ± 0.03 km and a secondary orbital semimajor axis a= 2.8+1.7 −0.7 km. A low surface thermal inertia Γ= 120 ± 50 J m−2 s−1/2 K−1 was also derived, suggesting a dusty surface and raising questions as to the binary formation mechanism of this asteroid. These physical properties are used to predict a Yarkovsky drift in semimajor axis of −60+31 −45 m yr−1.