2012A&A...548A..36M

In September 2011, the Herschel Space Observatory performed an observation campaign with the PACS photometer observing the asteroid (101955) 1999 RQ_36_ in the far infrared. The Herschel observations were analysed, together with ESO VLT/VISIR and Spitzer/IRS data, by means of a thermophysical model in order to derive the physical properties of 1999 RQ₃₆. We find the asteroid has an effective diameter in the range 480 to 511m, a slightly elongated shape with a semi-major axis ratio of a/b=1.04, a geometric albedo of 0.045^+0.015_–0.012, and a retrograde rotation with a spin vector between -70 and -90° ecliptic latitude. The thermal emission at wavelengths below 12µm-originating in the hot sub-solar region- shows that there may be large variations in roughness on the surface along the equatorial zone of 1999 RQ₃₆, but further measurements are required for final proof. We determine that the asteroid has a disk-averaged thermal inertia of Γ=650J/m2/s0.5/K with a 3-σ confidence range of 350 to 950 J/m2/s0.5/K, equivalent to what is observed for 25143 Itokawa and suggestive that 1999 RQ_36_ has a similar surface texture and may also be a rubble-pile in nature. The low albedo indicates that 1999 RQ_36_ very likely contains primitive volatile-rich material, consistent with its spectral type, and that it is an ideal target for the OSIRIS-REx sample return mission.