Determination of the pole orientation of an asteroid - The amplitude-aspect relation revisited

Near the opposition of a minor planet the geometrical approximation essentially constitutes a good representation of the Hapke-Irvine relation for describing the scattering properties of a surface layer, and the authors show that the normalized light curve of a three-axes ellipsoid model reduces to a straight line whose slope depends only on the aspect angle A and on the semi-axes ratios a/b, b/c of the ellipsoid. A set of non-linear equations is then solved by a least squares method in order to derive the four unknown parameters lambda[SUB]0[/SUB], beta[SUB]0[/SUB] (ecliptic coordinates of the pole) and a/b, b/c. The authors have applied this technique to published observations of two asteroids: For (624) Hektor two possible solutions are found; and for the case of (44) Nysa, they show that additional observations are needed in order to derive a self-consistent pole orientation.