GTC/CanariCam observations of (99942) Apophis

Context. The potentially hazardous asteroid (PHA) (99942) Apophis is one of the most remarkable near-Earth asteroids (NEA) in terms of impact hazard. A good determination of its surface thermal inertia is very important in order to evaluate the Yarkovsky effect on its orbital evolution. Aims. We present thermal infrared observations obtained on January 29, 2013, with CanariCam mid-infrared camera/spectrograph attached to the Gran Telescopio CANARIAS (GTC, Roque de los Muchachos Observatory, La Palma, Spain) using the Si2-8.7, Si6-12.5, and Q1-17.65 filters with the aim of deriving Apophis' diameter (D), geometric albedo (pV), and thermal inertia (G). Methods. We performed a detailed thermophysical model analysis of the GTC data combined with previously published thermal data obtained using Herschel Space Observatory PACS instrument at 70, 100, and 160 mu m. Results. The thermophysical model fit of the data favors low surface roughness solutions (within a range of roughness slope angles rms between 0.1 and 0.5), and constrains the effective diameter, visible geometric albedo, and thermal inertia of Apophis to be D-eff = 380-393 m, p(V) = 0.24-0.33 (assuming absolute magnitude H = 19.09 +/- 0.19) and G = 50-500 Jm(-2) (-0.5) K-1, respectively.