NEOWISE Observations Of The Potentially Hazardous Asteroid (99942) Apophis

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 $\hat{I}¼$m and 4.6 $\hat{I}¼$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 $\hat{I}“$ $\sim$ 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.

19 Pages, 6 Figures, Accepted for publication in PSJ