Thermal infrared observations of asteroid (99942) Apophis with Herschel^⋆

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Giessenbachstraße, 85741 Garching, Germany
e-mail: tmueller@mpe.mpg.de
² Konkoly Observatory, Research Center for Astronomy and Earth Sciences, Hungarian Academy of Sciences ; Konkoly Thege 15-17, 1121 Budapest, Hungary
³ Astronomical Institute, Academy of Sciences of the Czech Republic, Fričova 1, 25165 Ondřejov, Czech Republic
⁴ European Space Astronomy Centre (ESAC), European Space Agency, Apartado de Correos 78, 28691 Villanueva de la Cañada, Madrid, Spain

Received: 19 March 2014
Accepted: 17 April 2014

Abstract

The near-Earth asteroid (99942) Apophis is a potentially hazardous asteroid. We obtained far-infrared observations of this asteroid with the Herschel Space Observatory PACS instrument at 70, 100, and 160 μm. These were taken at two epochs in January and March 2013 during a close-Earth encounter. These first thermal measurements of Apophis were taken at similar phase angles before and after opposition. We performed a detailed thermophysical model analysis by using the spin and shape model recently derived from applying a two-period Fourier series method to a large sample of well-calibrated photometric observations. We found that the tumbling asteroid Apophis has an elongated shape with a mean diameter of 375⁺¹⁴_-10 m (of an equal volume sphere) and a geometric V-band albedo of 0.30^+0.05_-0.06. We found a thermal inertia in the range 250–800 Jm^-2 s^-0.5 K^-1 (best solution at Γ = 600 Jm^-2 s^-0.5 K^-1), which can be explained by a mixture of low-conductivity fine regolith with larger rocks and boulders of high thermal inertia on the surface. The thermal inertia, and other similarities with (25143) Itokawa indicate that Apophis might also have a rubble-pile structure. If we combine the new size value with the assumption of an Itokawa-like density and porosity we estimate a mass between 4.4 and 6.2 × 10¹⁰ kg, which is more than 2–3 times larger than previous estimates. We expect that the newly derived properties will influence impact scenario studies and the long-term orbit predictions of Apophis.