Physical properties of OSIRIS-REx target asteroid (101955) 1999 RQ₃₆

Derived from Herschel, VLT/ VISIR, and Spitzer observations^⋆,⋆⋆

¹ Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse, 85748 Garching, Germany
e-mail: tmueller@mpe.mpg.de
² European Space Astronomy Centre (ESAC), European Space Agency, Apartado de Correos 78, 28691 Villanueva de la Cañada, Madrid, Spain
³ Observatoire de Paris, Laboratoire d’Études Spatiales et d’Instrumentation en Astrophysique (LESIA), 5 place Jules Janssen, 92195 Meudon Cedex, France
⁴ Konkoly Observatory, Research Center for Astronomy and Earth Sciences, Hungarian Academy of Sciences; Konkoly Thege 15–17, 1121 Budapest, Hungary
⁵ Ludwig-Maximilians-Universität München, Fakultät für Physik, Schellingstraße 4, 80799 München, Germany
⁶ INSA at European Space Astronomy Centre (ESAC), European Space Agency, Apartado de Correos 78, 28691 Villanueva de la Cañada, Madrid, Spain

Received: 20 July 2012
Accepted: 8 October 2012

Abstract

In September 2011, the Herschel Space Observatory performed an observation campaign with the PACS photometer observing the asteroid (101955) 1999 RQ₃₆ 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 511 m, 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 12 μm-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 = 650 Jm^-2 s^-0.5 K^-1 with a 3-σ confidence range of 350 to 950 Jm^-2 s^-0.5 K^-1, equivalent to what is observed for 25143 Itokawa and suggestive that 1999 RQ₃₆ has a similar surface texture and may also be a rubble-pile in nature. The low albedo indicates that 1999 RQ₃₆ 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.