NEOWISE Observations Of The Potentially Hazardous Asteroid (99942) Apophis

TL;DR

This study uses NEOWISE infrared data to analyze asteroid Apophis during its 2020-2021 approach, providing key physical characteristics that inform its potential hazard level and regional impact risk.

Contribution

First thermal and physical characterization of Apophis during its close approach using NEOWISE data, informing hazard assessment.

Findings

Diameter approximately 340 meters

Albedo around 0.31

Thermal inertia estimated at 550 Jm$^{-2}$s$^{-0.5}$K$^{-1}$

Abstract

Large potentially hazardous asteroids (PHAs) are capable of causing a global catastrophe in the event of a planetary collision. Thus, rapid assessment of such an object's physical characteristics is crucial for determining its potential risk scale. We treated the near-Earth asteroid (99942) Apophis as a newly discovered object during its 2020-2021 close-approach as part of a mock planetary defense exercise. The object was detected by the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE), and data collected by the two active bands (3.4 ${\mu}$m and 4.6 ${\mu}$m) were analyzed using thermal and thermophysical modeling. Our results indicate that Apophis is an elongated object with an effective spherical diameter D$_{eff}$ = 340 $\pm$ 70 m, a geometric visual albedo p$_{V}$ = 0.31 $\pm$ 0.09, and a thermal inertia $\Gamma$ $\sim$ 150 - 2850 Jm$^{-2}$s$^{-0.5}$K$^{-1}$ with a best-fit value of 550 Jm$^{-2}$s$^{-0.5}$K$^{-1}$. NEOWISE "discovery" observations reveal that (99942) Apophis is a potentially hazardous asteroid that would likely cause damage at a regional level and not a global one.