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Physical Characteristics of Binary Asteroid (163693) Atira

Presentation #412.05 in the session Asteroids: Near-Earth Objects (Poster + Lightning Talk)

Published onOct 23, 2023
Physical Characteristics of Binary Asteroid (163693) Atira

Introduction: Asteroid (163693) Atira was discovered on February 11, 2003 by the LINEAR survey and radar observations from Arecibo Observatory in 2017 revealed Atira to be a binary system. Initial estimates found the primary to be 4.8 km ± 0.5 km in diameter and the secondary to be 1 km ± 0.5 km in diameter (Rivera-Valentin et al., 2017). The primary has been designated as S-type, and the secondary is assumed to be tidally locked. Two different rotation periods have been reported for Atira based on analyses of photometric data: 3.3984 ± 0.0006 h (Pravec, 2003), and 3.1532 ± 0.0001 h (Rondon et al., 2022). To gain understanding of how these binary NEA systems form and evolve, we place stronger constraints on the primary’s spin axis orientation, rotation period, size, and shape and we constrain the secondary’s physical and orbital characteristics.

Methods: We investigate the primary’s spin axis orientation and rotation period using SHAPE (Magri et al., 2007) with an iterative modeling procedure. We first test models using a simple ellipsoid approximation with spin axes distributed over the celestial sphere and rotation periods ranging from 2.8 h – 4.8 h. The overall best-fit ellipsoid models are used to initialize floating-vertex models to model prominent surface features. Reduced chi-squared values in conjunction with visual inspection of simulated delay-Doppler images and simulated lightcurves are used to determine overall best-fit models. We investigate the size and elongation of the secondary with a similar iterative modeling procedure. We test simple ellipsoid models by varying the ratios of the axial lengths of the ellipsoid where we assume the secondary’s spin axis orientation to be aligned with the primary. We use delay-Doppler images and mutual events detected in the lightcurve data to constrain the orbital parameters of the secondary.

Results: We report results on the orientation, rotation period, size, and shape of the primary along with physical and orbital characteristics of the secondary. Our work on modeling the primary finds two solutions for the average diameter of the primary depending on the orientation of the spin axis: 5.0 km for an orientation of ecliptic lat. -55° or 6.9 km for an orientation of ecliptic lat. -10°; we find both models to have a rotation period of ~3.39 h. Analysis of mutual events in the lightcurve data favors an average diameter of 5.0 km and a spin axis orientation of ecliptic lat. -55° as the more likely model of Atira.

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