The Shape of Eros

Jet Propulsion Laboratory, California Institude of Technology, Pasadena, California 91109

Icarus 84, 334-351 (1990)

Abstract

The convex hull of Eros' polar silhouette, estimated from radar echo spectra obtained in 1975 by R. F. Jurgens and R. M. Goldstein (1976, Icarus 28, 1-15), provides new information about this asteroid's shape. Monte Carlo simulations are used to optimize the estimation, to explore the nature and severity of associated errors, and to guide bias-correction procedures. Eros' hull is shaped like a rounded trapezoid, whose long and short bases faced Earth during epochs of primary and secondary maxima, respectively, in the January 1975 optical lightcurves. The nonaxisymmetric shape helps to explain odd harmonics in Eros' echo spectral signature as a function of rotation phase, whose presence cannot be accounted for by homogeneous ellipsoid models. The extreme breadths of Eros' polar silhouette are within a few kilometers of 35 and 16 km. Additional constraints on Eros' figure are obtained by inverting an optical lightcurve to estimate the asteroid's "mean cross section," which is a two-dimensional average of the three-dimensional shape. Eros' mean cross section and polar silhouette have similar elongations. The hull estimate permits previously reported radar time-delay and Doppler-frequency measurements to be referenced directly to Eros' center of mass.