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 μ 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 ± 70 m, a geometric visual albedo p V =0.31 ± 0.09, and a thermal inertia Γ ∼ 150–2850 J m −2 s − 1 2 K −1 with a best-fit value of 550 J m −2 s − 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.