Phase relations of high albedo asteroids: The unusual opposition brightening of 44 Nysa and 64 Angelina

Abstract

Detailed photoelectric lightcurve observations are reported which were made for the purpose of defining the photometric phase relations of high albedo asteroids, particularly at low phase angles. We have observed 44 Nysa at one aspect, and 64 Angelina at both polar aspects, hence we have sampled three different surfaces. All three exhibit a remarkable opposition spike, or brightening, of about 0.25 magnitude, confined to within a few degrees of zero phase angle. Within the scatter of the observations, all three curves are identical, hence the opposition spike cannot be regarded as “anomalous” or unusual within that class of objects (both asteroids are of taxonomic class E). Since the effect is identical on both hemispheres of Angelina, we can conclude that it is a global characteristic. The observed phase curves are generally similar to those of the Uranian satellites, and of Saturn's rings, leading us to suggest that is it a normal property of moderate to high albedo atmosphereless surfaces. In particular, it appears that much of the opposition effect of the rings of Saturn may be due to the surface properties of the individual particles, rather than to interparticle shadowing, as has often been suggested.