The Resolved Asteroid Program – Size, shape, and pole of (52) Europa

Abstract

With the adaptive optics (AO) system on the 10 m Keck-II telescope, we acquired a high quality set of 84 images at 14 epochs of asteroid (52) Europa on 2005 January 20, when it was near opposition. The epochs covered its 5.63 h rotation period and, by following its changing shape and orientation on the plane of sky, we obtained its triaxial ellipsoid dimensions and spin pole location. An independent determination from images at three epochs obtained in 2007 is in good agreement with these results. By combining these two data sets, along with a single epoch data set obtained in 2003, we have derived a global fit for (52) Europa of diameters a  b  c = (379 x 330 x 249) ± (16 x 8 x 10) km, yielding a volume-equivalent spherical-diameter of (abc)1/3 = 315 +/- 7 km, and a prograde rotational pole within 7deg of [RA;Dec] = [257;+12] in an Equatorial J2000 reference frame (Ecliptic: 255;+35). Using the average of all mass determinations available for (52) Europa, we derive a density of 1.5 ± 0.4 g cm-3, typical of C-type asteroids. Comparing our images with the shape model of Michalowski et al. (2004, Astron. Astrophys. 416, 353), derived from optical lightcurves, illustrates excellent agreement, although several edge features visible in the images are not rendered by the model. We therefore derived a complete 3-D description of (52) Europa’s shape using the KOALA algorithm by combining our 18 AO imaging epochs with 4 stellar occultations and 49 lightcurves. We use this 3-D shape model to assess these departures from ellipsoidal shape. Flat facets (possible giant craters) appear to be less distinct on (52) Europa than on other C-types that have been imaged in detail, (253) Mathilde and (511) Davida. We show that fewer giant craters, or smaller largest-sized craters, is consistent with its expected impact history. Overall, asteroid (52) Europa is still well modeled as a smooth triaxial ellipsoid with dimensions constrained by observations obtained over several apparitions.