NEOWISE Observations of the Potentially Hazardous Asteroid (99942) Apophis

Large potentially hazardous asteroids (PHAs) are capable of causing a global catastrophe if they collide with the Earth. It is, therefore, crucial to properly assess a potentially hazardous asteroid’s (or PHA's) degree of risk by estimating their physical characteristics quickly. NEA (99942) Apophis' close-approach in 2020-21 made it a suitable subject for a mock planetary defense exercise designed to evaluate the current systems and test the readiness of research groups to identify and rapidly characterize PHAs. We treated Apophis as a newly discovered asteroid and employed thermal and thermophysical modeling to determine its diameter, albedo, and thermal inertia using new data obtained from the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE; Mainzer et al. 2014, Wright et al. 2010). NEOWISE obtained seventeen detections of Apophis in the first epoch (December 2020) and eight additional ones during the second observing epoch (April 2021) from the two active bands (3.4 µm and 4.6 µm). Additionally, near-simultaneous visible photometry obtained using the SMARTS 1.0-meter telescope from the Cerro Tololo Inter-American Observatory was used to provide an improved constraint on the absolute visual magnitude. Two thermal models — the Near-Earth Asteroid Thermal Model (NEATM; Harris 1998) and the Spherical, Cratered, Rotating, Energy-conserving Asteroid Model (or simply, TPM; Wright 2007 and Koren et al. 2015) — were applied to derive the size and visual albedo estimates of Apophis. A range of thermal inertia values of the asteroid was also constrained using the TPM. Preliminary results suggest that (99942) Apophis is an elongated object with an effective diameter of 360 ± 60 m, an albedo of 0.35 ± 0.10, and thermal inertia in the range 500–3000 Jm^-2s^-0.5K^-1, and our estimates appear to be consistent with results put forward by Müller et al. (2014), Licandro et al. (2016), and Brozovic et al. (2018). We demonstrate that NEOWISE data and thermal modeling allows us to rapidly characterize an NEA’s diameter and albedo with high accuracy. We stress that such expeditious data processing could prove to be essential in rapidly assessing threats posed by PHAs. Conclusively, in the case of Apophis, the diameter derived from the NEOWISE “discovery” observations allowed exercise participants to quickly determine that the object would merely cause regional damage and not a global disaster.