Thermal Infrared Observations of Asteroid (99942) Apophis with Herschel
T. G. M\"uller, C. Kiss, P. Scheirich, P. Pravec, L. O'Rourke, E., Vilenius, B. Altieri

TL;DR
This study presents the first thermal infrared measurements of asteroid Apophis using Herschel, revealing its size, shape, surface properties, and suggesting a rubble-pile structure, which impacts impact risk assessments and orbit predictions.
Contribution
It provides the first thermal measurements and detailed thermophysical modeling of Apophis, offering new insights into its physical properties and structure.
Findings
Diameter approximately 375 meters
Thermal inertia around 600 Jm$^{-2}$s$^{-0.5}$K$^{-1}$
Likely rubble-pile structure
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's PACS instrument at 70, 100, and 160 micron. 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 2-period Fourier series method to a large sample of well-calibrated photometric observations. We find that the tumbling asteroid Apophis has an elongated shape with a mean diameter of 375 m (of an equal volume sphere) and a geometric V-band albedo of 0.30. We find a thermal inertia in the range 250-800 JmsK…
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