2014/04/23 by T. G. Müller, C. Kiss, P. Scheirich +5 · 5 citations
Earth and Planetary Sciences · Physics and Astronomy · #Absolute magnitude #Albedo (alchemy) #Asteroid #Astro and Planetary Science #Geometric albedo #High-pressure geophysics and materials #Observatory #Orbit (dynamics) #Planetary Science and Exploration #Regolith #Thermal #astro-ph.EP
paper · pdf · doi:10.1051/0004-6361/201423841
Accepted for publication in Astronomy & Astrophysics, 21 pages, 8 figures, 2 tables
arxiv created 2014/04/23 · openalex publication_date 2014/04/29 · arxiv updated 2014/06/04 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
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.