2011MNRAS.418.1246W

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 H_V= 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_–0.7km. A low surface thermal inertia Γ= 120±50 J/m2.s^1/2/K 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.

Based on observations performed at the ESO La Silla and Paranal Observatories in Chile (programme IDs 185.C-1033, 185.C-1034 and 184.C-1143).