NEOWISE Observations of the Potentially Hazardous Asteroid (99942) Apophis

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 and 4.6 μm) were analyzed using thermal and thermophysical modeling. Our results indicate that Apophis is an elongated object with an effective spherical diameter D _eff = 340 ± 70 m, a geometric visual albedo p _V =0.31 ± 0.09, and a thermal inertia Γ ~ 150-2850 J m^-2 ${{\rm{s}}}^{-\tfrac{1}{2}}$ K^-1 with a best-fit value of 550 J m^-2 ${{\rm{s}}}^{-\tfrac{1}{2}}$ K^-1. NEOWISE "discovery" observations reveal that (99942) Apophis is a PHA that would likely cause damage at a regional level and not a global one.