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Physical Characteristics of near-Sun Binary Asteroid (66391) Moshup-Squannit: An Analog of the Didymos-Dimorphos System

Presentation #506.03 in the session Asteroids: Near-Earth Objects (Oral Presentation)

Published onOct 23, 2023
Physical Characteristics of near-Sun Binary Asteroid (66391) Moshup-Squannit: An Analog of the Didymos-Dimorphos System

The binary asteroid system (66391) 1999 KW4 consists of a rapidly-rotating primary (Moshup) and a synchronously-orbiting secondary (Squannit) as revealed by radar observations (Ostro et al., 2006. Science, 314, 1276—1280.). It is a potentially hazardous asteroid (PHA) with a perihelion distance of 0.20 au and will approach within 6.04 lunar distances to the Earth on 25 May 2036 – the closest approach since its discovery in 1999. Interestingly, the components of this system have a similar size ratio, shape, and spin characteristics to that of the Didymos-Dimorphos system (Naidu et al., 2020. Icarus, 348, 113777); a PHA and the target of the NASA Double Asteroid Redirection Test (DART). Physical characterization of Moshup and Squannit is therefore relevant for mitigating impact risks that PHAs could pose to Earth (Reddy et al., 2022. Planet. Sci. J. 3, 123). Comparison to Didymos and Dimorphos provides context for the future ESA Hera mission and subsequent investigations of the system.

Mutual events (eclipses and occultations of the primary by the secondary, and vice versa) have been serendipitously observed by the reactivated NEOWISE (Wide-Field Infrared Survey Explorer) survey (Mainzer et al., 2014. Astrophys J, 792, 30) on three separate occasions at wavelengths in which reflected light and thermal emission contribute to the total measured brightness. Using a thermophysical model that accurately accounts for light scattering between the two components, we estimate the surface properties of the primary and secondary. We use detailed shape models that are derived from radar observations (Ostro et al., 2006). Due to the observations of mutual events, it is possible to estimate the properties (i.e., the size, albedo, and thermal inertia) of the primary and secondary independently from the other. For assessing the energy exchange between the two components, we implement a Lommel-Seeliger scattering function and account for phase angle effects.

An orbital drift rate for Squannit has been determined using lightcurve photometry over a timespan of a few decades (Scheirich et al., 2021. Icarus, 360, 114321). Such an orbital drift is expected from the BYORP effect (binary YORP; McMahon and Scheeres, 2010. Icarus, 209, 494—509) caused by anisotropic thermal emission over the orbital trajectory of a satellite in a binary asteroid system. Using the orbital drift rate and physical properties constrained by the NEOWISE observations, we estimate the mass and bulk density of Squannit. Our results will be put into context regarding other near-Sun asteroids and binary near-Earth asteroids.

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