Physical properties of ESA Rosetta target asteroid (21) Lutetia: Shape and flyby geometry

TL;DR

This study determines the spin state and shape of asteroid (21) Lutetia using a novel inversion algorithm, aiding in the preparation for ESA Rosetta's flyby by predicting its orientation and physical features.

Contribution

We applied a new KOALA inversion method combining lightcurves and images to accurately model Lutetia's physical properties and improve flyby observation predictions.

Findings

Lutetia's spin axis is near ({} = 52°, {} = -6°) in Ecliptic J2000.

The sidereal period is refined to 8.168270 hours.

The asteroid has a triangular cross-section with concavities, including a large impact-related feature.

Abstract

Aims. We determine the physical properties (spin state and shape) of asteroid (21) Lutetia, target of the ESA Rosetta mission, to help in preparing for observations during the flyby on 2010 July 10 by predicting the orientation of Lutetia as seen from Rosetta. Methods. We use our novel KOALA inversion algorithm to determine the physical properties of asteroids from a combination of optical lightcurves, disk-resolved images, and stellar occultations, although the latter are not available for (21) Lutetia. Results. We find the spin axis of (21) Lutetia to lie within 5 degrees of ({\lambda} = 52 deg., {\beta} = -6 deg.) in Ecliptic J2000 reference frame (equatorial {\alpha} = 52 deg., {\delta} = +12 deg.), and determine an improved sidereal period of 8.168 270 \pm 0.000 001 h. This pole solution implies the southern hemisphere of Lutetia will be in "seasonal" shadow at the time of the flyby. The apparent cross-section of Lutetia is triangular as seen "pole-on" and more rectangular as seen "equator-on". The best-fit model suggests the presence of several concavities. The largest of these is close to the north pole and may be associated with large impacts.