Shape of asteroid 433 Eros from inversion of Goldstone radar Doppler spectra

We use new analysis techniques to constrain the shape of 433 Eros with Goldstone radar data obtained during the asteroid's close approach in 1975, A previous analysis of these data (Ostro, Rosema, and Jurgens, 1990, Icarus 84, 334-351) used estimates of the echo's spectral edge frequencies as a function of asteroid rotation phase to constrain the convex envelope of Eros' pole-on silhouette, Our approach makes use of the echo's full Doppler-frequency distribution (effectively similar to 15 times more echo data points) and is thus capable of constraining shape characteristics, such as concavities, within this convex envelope. The radar echoes are weak and north-south ambiguous, which limits the accuracy of our models, We present two different approaches, perturbations to an ellipsoid and successive approximations, that help to quantify the model uncertainties and identify features that are likely to be real, Both approaches yield models that are tapered along their lengths, with one or more prominent concavities on one side but not the other. We do not have sufficient information to determine the exact nature of the concavities, and in particular, whether they are craters, troughs, or bends in Eros' overall shape, The pole-on silhouette of the successive approximation model is shaped like a kidney bean, which resembles a nearly pole-on optical image derived from speckle interferometry (Drummond and Hege, 1989, in Asteroids II (R. P. Binzell, T. Gehrels, and M. S. Matthews, Eds.), PD 171-191, Univ. of Arizona Press, Tucson); however, eve cannot exclude shapes, such as the perturbation model, with more than one large concavity. Variations in the pyroxene/olivine ratio over Eros' surface have been inferred from visual and infrared observations (Murchie and Pieters, 1996, J. Geophys. Res, 101, 2201-2214). Correlating these variations with our shape information, we find that the side with concavities is relatively pr-rich compared with the more rounded opposing side. (C) 1998 Academic Press.