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Shape and duplicity of Lucy Mission target Polymele from occultation observations

Presentation #512.03 in the session Trojan Asteroids: A horse of a different color.

Published onOct 20, 2022
Shape and duplicity of Lucy Mission target Polymele from occultation observations

We observed a third occultation of (15094) Polymele Trojan asteroid on 2022 Mar 27. This object is planned to be the second prime target of the Lucy Mission with an encounter in 2027. Our new observations are the most extensive by far. A core set of 21 stations were spaced out with a 1.8 km separation with 14 extra best-effort stations interspersed throughout the core pattern. We have 12 positive detections on Polymele, one with a secondary event closely associated with Polymele that is interpreted as a small topographic feature on the limb. An additional 2 positive detections were recorded that were offset by about 7 seconds. These extra detections were spatially coincident on the sky-plane at Polymele with an offset of about 200 km from the center of Polymele. The Polymele chords reveal a highly elliptical shape with projected major axes of 26.2 and 12.8 km. There are significant departures from the fitted ellipse, including the secondary event, that indicate complicated surface topography, only some of which is consistent with crater morphologies. The shape seen on this event can be reconciled with prior occultations and lightcurve observations if Polymele is a highly oblate object, very similar to the larger lobe of Arrokoth (in absolute size and shape). The extra two positive detections reveal the presence of a previously unknown satellite of Polymele that is ~5-6 km in diameter. Assuming a circular and equatorial orbit around Polymele, the semi-major axis of the orbit is 204 km. We will discuss these results, implications, and plans for additional followup observations including what may be learned from an planned occultation effort on 2022 Aug 26.

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