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We seek the best size estimates of the asteroid (21) Lutetia, the directionof its spin axis, and its bulk density, assuming its shape is well described bya smooth featureless triaxial ellipsoid, and to evaluate the deviations fromthis assumption. Methods. We derive these quantities from the outlines of theasteroid in 307 images of its resolved apparent disk obtained with adaptiveoptics (AO) at Keck II and VLT, and combine these with recent massdeterminations to estimate a bulk density. Our best triaxial ellipsoiddiameters for Lutetia, based on our AO images alone, are a x b x c = 132 x 101x 93 km, with uncertainties of 4 x 3 x 13 km including estimated systematics,with a rotational pole within 5 deg. of ECJ2000 [long,lat] = [45, -7], orEQJ2000 [RA, DEC] = [44, +9]. The AO model fit itself has internal precisionsof 1 x 1 x 8 km, but it is evident, both from this model derived from limitedviewing aspects and the radius vector model given in a companion paper, thatLutetia has significant departures from an idealized ellipsoid. In particular,the long axis may be overestimated from the AO images alone by about 10 km.Therefore, we combine the best aspects of the radius vector and ellipsoid modelinto a hybrid ellipsoid model, as our final result, of 124 +/- 5 x 101 +/- 4 x93 +/- 13 km that can be used to estimate volumes, sizes, and projected areas.The adopted pole position is within 5 deg. of [long, lat] = [52, -6] or[RA DEC]= [52, +12]. Using two separately determined masses and the volume of ourhybrid model, we estimate a density of 3.5 +/- 1.1 or 4.3 +/- 0.8 g cm-3 . Fromthe density evidence alone, we argue that this favors an enstatite-chondritecomposition, although other compositions are formally allowed at the extremes(low-porosity CV/CO carbonaceous chondrite or high-porosity metallic). Wediscuss this in the context of other evidence.

The triaxial ellipsoid dimensions, rotational pole, and bulk density of ESA Rosetta target asteroid (21) Lutetia