Opposition effect of Kuiper Belt objects: Preliminary estimations

Information on the surface structure of the Kuiper Belt objects can be obtained from studies of their opposition brightening. Although KBOs are observed at a very limited phase angle range they represent a unique opportunity to study the backscattering phenomenon down almost to zero phase angle. Preliminary estimations of the opposition effect amplitude and width based on composite phase curves of four KBOs and two Centaurs showed the existence of a very narrow opposition surge of about 0.1 - 0.2 mag at phase angles less than 0.1 - 0.2 deg. It may indicate a high porosity of the KBOs regoliths. Further observations are needed to confirm this phenomenon.