Spectrogoniometer for measuring planetary surface materials at small phase angles.

The surfaces of airless bodies exhibit an anomalous increase in brightness as their faces become fully illuminated to the observer. This opposition effect is generally explained as the disappearance of mutual shadowing among the particles of the optically active portion of the regolith. Models suggest that the regolith's porosity and albedo are the primary factors which determine the effect's amplitude and angular dependence.