A revised shape model of asteroid (216) Kleopatra

Abstract

We used three different sets of Arecibo delay-Doppler radar images and five well-covered occultations to generate a revised three-dimensional shape model of asteroid (216) Kleopatra with a spatial resolution of ∼10 km. We find Kleopatra to be a bi-lobate contact binary of overall dimensions 276 × 94 × 78 km ± 15% and equivalent diameter Deq = 122 ± 30 km; our uncertainties are upper and lower bounds. Separated binary models are ruled out by multi-chord occultations. Our model is 27% longer than the “dog-bone” model originally published by Ostro et al. (2000) but is similar to their model in the minor and intermediate axes extents. Our model's dimensions are also consistent with more recent ones based on lightcurves, adaptive-optics, and interferometric imaging. We confirm a rotational period of P = 5.385280 h ± 0.000001 h and a rotation pole at ecliptic longitude and latitude (λ, β) = (74°, + 20°) ± 5°. Over its southern hemisphere (the one most frequently observed on Earth), Kleopatra's radar albedo is 0.43 ± 0.10, consistent with a high near-surface bulk density and, by inference, the high metal content expected for M-class asteroids. However, the radar albedo for equatorial observations is considerably lower and more typical of a dominantly silicate composition. This observation could readily be explained by a relatively thin (1–2 m) silicate mantle over equatorial latitudes. Kleopatra's surface is relatively smooth with a mean slope of 12° at the ∼10 km baseline scale. Analysis of its geopotential surface suggests loose material will preferentially migrate to the neck, and this is supported by our radar observations.