Photoelectric analysis of asteroid 216 Kleopatra: Implications for its shape

Abstract

In some recent theoretical papers it has been suggested that gravitationally bound “rubble piles” in hydrostatic equilibrium possibly exist among the asteroids. For a higher-than-critical value of the angular momentum acquired by such a body, the instability phenomena can produce fission into a binary system. S. J. Weidenschilling [ Icarus 44, 807–809 (1980)] suggested that 216 Kleopatra may represent a binary asteroid, since it has a large light curve amplitude (1.3–1.4 mag). In this paper new observations of Kleopatra are presented, suggesting the equal plausibility of the single triaxial ellipsoid model. Namely, when phase and aspect effects are taken into account, the actual maximum amplitude is reduced to about 0.9 mag at 90° of aspect which is close to the value predicted by theory for the instability limit. Moreover, multiple-scattering effects [M. Poutanen, E. Bowell, and K. Lumme, Bull. Amer. Astron. Soc. 13, 725 (1981)] can reduce the axial ratio a/ b even more. If the single-body model is adopted, the density of Kleopatra should be on the order of 1.7 g/cm 3. This low value seems reasonable for “rubble pile” models.